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Senior Project Report
- 1. Boston University Departmentof Mechanical Engineering ENG ME461 A5 - Capstone Experience Regenerative Cooling Nozzle Final Report May 1st 2015 Team 11 Thomas Ransegnola Geoffrey McMahon Georgios Skoufalos Alexander Freedman Christian Tjia Customer Boston University Rocket Propulsion Group Advisor William Hauser
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- 4. 4 ii. Table of Contents Content Page 1.ReportSummary i. Abstract 3 ii. Table of Contents 4 iii. List of Figures 5 iv. List of Tables 6 v. List of Symbols 7 vi. Executive Summary 8 vii. Acknowledgements 10 2.BodyofReport I. Introduction and Background 12 II. Requirements 14 III. Engineering Specifications and Relevant Basic Physics 19 IV. Design Description 29 V. Design Decisions 47 VI. Design Evaluations 52 VII. Market / Cost Analysis 54 VIII. Conclusion and Recommendations 64 IX. References 66 3.Appendices A – Bill of Materials 68 B – Working Drawings 69 C – Gantt Chart 79 D – Analysis and Test Reports 80
- 5. 5 iii. List of Figures Body of Report Figures Figure Name Page 1 Hot wall contour of nozzle with entrance on top and exit on bottom 15 2 Close up view of throat 15 3 Heat transfer coefficients 16 4 Temperature of flow along the nozzle wall 16 5 Exaggerated top view of nozzle with nodes 20 6 Exaggerated top view of nozzle with heat travel 21 7 Exaggerated top view of nozzle with resistances 22 8 Thermal circuit of nozzle 23 9 Wall temperature plot example 26, 31 10 Coolant temperature plot example 26, 31 11 Outside wall stress plot example 27, 32 12 Nozzle temperature cross-section plot example 27, 32 13 Block diagram of MATLAB model user interface 29 14 Example user interface screen 30 15 User input module 33 16 Functional decomposition of regenerative cooling nozzle with results 35 17 Isometric view of nozzle body drawing 36 18 Cross sectional view of regenerative cooling nozzle 37 19 Exploded view of nozzle 38 20 Isometric view of upper manifold 39 21 Isometric view of lower manifold 39 22 Side view of coolant flow through nozzle 40 23 Indexing chuck (Left) and stage (Right) 42 24 Drilling fixture assembly 42 25 Detail of location pins and fastening bolts 43 26 Subsonic section and sealant ridge 44 27 The manifold undergoing the milling operation 45 28 Process flow chart 46 29 Map of the simulated hot wall imported convection coefficient 53 30 Map of the simulated temperature gradient of the nozzle 53 31 Map of the simulated equivalent stresses in the nozzle 54 32 Map of the simulated safety factor across the nozzle 54 33 Coolant temperature along the normalized position 56 34 Wall temperature along the normalized position 57 35 FEA Joint stress results 59 36 Learning curve in manufacturing 63
- 6. 6 Appendix Figures Figure Name Page A1 Rocket nozzle assembly drawing 69 A2 Rocket nozzle body drawing 70 A3 Rocket nozzle body drawing 2 71 A4 Rocket nozzle body isometric view 72 A5 Upper manifold drawing 73 A6 Upper manifold drawing 2 74 A7 Upper manifold isometric view 75 A8 Lower manifold drawing 76 A9 Lower manifold drawing 2 77 A10 Lower manifold isometric 78 A11 Gantt chart of major checkpoints 79 A12 Water flow data 1 84 A13 Water flow data 2 85 A14 FEA Joint stress results 1 86 A15 FEA Joint stress results 2 87 A16 Illustration of fixed bottom and pressure acting vertically on manifold 88
- 7. 7 iv. List of Tables Body of Report Tables Table Description Page 1 Project Success Points 9 2 Models developed during capstone experience 9 3 Collaborators and advisers 10 4 Weighted comparison of requirements 18 5 Nozzle specifications, tank conditions and combustion chamber conditions 28 6 N2O properties 28 7 Ammonia properties 28 8 Material properties 28 9 Morphological chart 47 10 Cooling channel pugh chart 48 11 Nozzle material pugh chart 49 12 Coolant pugh chart 50 13 Input parameters for the model 56 14 Shows the result of the model data compared to the test data 57 15 Complete project budget 60 16 Theoretical project budget for different materials 61 17 Process times for the first nozzle 62 18 Total production cost estimates for four nozzles 62 Appendix Tables Table Description Page A1 Bill of materials 68
- 8. 8 v. Executive Summary Project Problem The graphite nozzles that the Boston University Rocket Propulsion Group (BURPG) currently uses suffer extensive material damage due to high heat effects from the combustion process; this is a phenomenon that occurs in amateur and professional rocketry. Material from the inside hot wall of the nozzle melts, breaks off and deforms due to the heat it experiences. This damage prevents the user from using any nozzle for more than one operation. Design Solution One way to mitigate the heat damage to rocket nozzles is through regenerative cooling, which BURPG has decided to investigate through our team. By using regenerative cooling nozzles BURPG will improve the performance of their rockets while saving cost. Our goal is to develop a mathematical model that will allow BURPG to research different configurations and designs in the future. Our goal is also to suggest our own design, develop CAD models and build prototypes for BURPG to evaluate. Final State Our Team began by developing a user-friendly MATLAB model of a regenerative cooling nozzle with the help of BURPG and expert consultants. This model allows the user to test the effectiveness of different designs, through the use of coolants, materials and geometries, by evaluating inherent stresses and temperature / heat changes. Using this model we evaluated multiple designs and created CAD models to represent our designs. These CAD models were prototyped in order to evaluate how well they work and to evaluate the fluid distribution along the multiple channels through a water flow experiment. Finally we were able to compare our model to an FEA/CFD analysis, the “industry standard”, to show that our model is correct. We also compared our model to existing studies to reassure BURPG of its realistic results. Per the request of BURPG we specifically tested N2O and its viability as a coolant and found that it is not a reliable coolant. Finally FEA was done to the CAD model to ensure that our nozzle can withstand respective pressures.
- 9. 9 Below are the multiple success points we created with BURPG in order to make sure that we spent our time productively and in an organized manner. Success Point Status Create a user friendly model of the regenerative cooling nozzle Completed Test the validity of N2O as a coolant for BURPG’s nozzle Completed Create CAD models that can be used to manufacture a working prototype Completed Create and evaluate a working prototype to test on BURPG’s rocket Completed Table 1. Project Success Points Finally we have included a table to illustrate the models and prototypes we have created throughout the duration of the semester. Model Status User-friendly MATLAB Completed Multiple CAD Completed CFD / FEA for stresses and heat Completed FEA for bolt analysis Completed Acrylic and Aluminum Prototypes Completed Table 2. Models developed during capstone experience
- 10. 10 vi. Acknowledgements We would like to thank the following people for their cooperation, guidance and support in developing our models and prototypes. Robert Sjostrom and Joseph Estano for guidance in manufacturing. Gerald J. Fine for guidance in material selection. Ray Nagem for guidance in thermodynamics. Caleb Farny for guidance in testing procedure. William Hauser for management and organizational guidance. BURPG, and David Armor Harris, for guidance in project planning and for the opportunity to complete this project. There were no critical, unique or creative features involved in our project that cannot be credited to our design process and teamwork. Name Role Phone Number E-mail BURPG Customer group NA terrierrocket@gmail.com David A. Harris Principal BURPG Contact and Advisor NA dharris000@gmail.com armorharris@gmail.com William Hauser Section Instructor (617) 358-0663 wmhauser@bu.edu Ray Nagem Guidance in thermodynamic model (617) 353-5925 10jost@bu.edu Gerald J. Fine Guidance in material selection (617) 353-6373 gjfine@bu.edu Caleb Farny Guidance in test procedure (617) 353-8664 farny@bu.edu Joseph Estano Guidance in manufacturing and prototyping (617) 353-6653 jestano@bu.edu Robert Sjostrom Guidance in manufacturing and prototyping (617) 353-4246 10jostrom@bu.edu Table 3. Collaborators and advisers
- 11. 11 2. Body of Report I. Introduction … pg. 12 II. Requirements … pg. 14 III. Engineering Specifications and Relevant Basic Physics … pg. 19 IV. Design Description … pg. 29 V. Design Decisions … pg. 47 VI. Design Evaluations … 52 VII. Market / Cost Analysis … 58 VIII. Conclusions and Recommendations … 64 IX. References … 66
- 12. 12 I. Introduction and Background Problem Definition and Prior Art Regenerative cooling was introduced to nozzles and rockets in the early 1900s, followed by significant advancements in the 1930s by Italian and Russian engineers. Regenerative cooling in nozzles was introduced in order to combat the high amounts of heat that the nozzle interior becomes subject to. This high amount of heat causes material damage to the hot wall of the nozzle and can make nozzles unusable after just one run. BURPG has also encountered this problem, experiencing significant damage to parts of their rockets due to heat. While regenerative cooling has been explored by BURPG, no serious study has been conducted to implement regenerative cooling technology to nozzles. The hope of this project is to aid BURPG in acquiring functioning regenerative cooling nozzles that can be used multiple times due to their ability to combat high heat effects. Regenerative cooling has gained its name because the coolant is often also part of the combustion process and is used as coolant and oxidizer or propellant. The theory behind regenerative cooling in nozzles is that by exposing the nozzle to a cooling fluid, the nozzle will experience cooler temperatures and experience less damage due to thermal stresses. There are various methods of achieving this result such as varying channel profiles, types of coolants and nozzle materials. Essentially a fluid flows next to the hot wall (inside wall), through channels, to take away heat. In regards to channel profiles, there are four major design options used by various groups depending on manufacturing capabilities and cost. The first option is to run tubes of coolant up and down the wall of the nozzle. This can be done through one pass or two passes (both down and up). A popular improvement to this method is to use square channels instead of circular profiles. This allows for more contact with the hot wall and therefore better heat conduction. The limitations inherent with this technique are both cost and manufacturing. This complex geometry is more expensive and requires more complicated manufacturing techniques.
- 13. 13 Another popular method of regenerative cooling is wrapping a tube around the nozzle and allowing the fluid to travel down in a spiraling manner. Aleksei Mihailovich Isaev developed and innovative approach in which he was able to use corrugated steel to create passageways for the fluid to travel. When it comes to coolants, a popular technique is to use the oxidizer from the rocket. This means that the oxidizer first acts as a coolant and then is used in the combustion chamber. The benefit of this technique is that no additional weight has to be added to the rocket for coolant and it improves combustion efficiency by warming up the oxidizer. Additionally the oxidizers have advantageous fluid properties, which makes this an even better technique. Regenerative cooling in nozzles gains its name from this ability to use the oxidizer or fuel as a coolant. In order to compare ideas, methods, and results we have researched existing products, from professional institutions such as NASA to hobbyist projects and we have collected test data from real cases in order to compare our results with the real world. We have also read extensively on the technology including readings such as Rocket Propulsion Elements by Sutton, Effect of Tube Geometry on Regenerative Cooling Performance by Daniel K. Paris and D. Brian Landrum as well as Nitrous Oxide Cooling in Hybrid Rocket Nozzles by Patrick Lemieux. This is in order to assure the costumer, and provide a double check for our team, that our MATLAB model is accurate and realistic. The data we used for comparison is sited below. The principal method used in the industry is comparing to an elaborate CFD / FEA model, which we also did and discuss in section VI Design Evaluations As our customer is a student group, manufacturability is limited to the resources available in the EPIC facility. While a corrugated steel sheet may have its advantageous it is also more expensive and difficult to manufacture. Our aim is to find a balance with effectiveness and feasibility. References found in section IX. References
- 14. 14 II. Requirements The underlying goal, set by Mr. Harris and BURPG, is to advance BURPG’s use of Regenerative Cooling Nozzles. Because this goal is very open ended we have developed restrictions and success points with the guidance of Mr. Harris. Our research is to focus on Regenerative Cooling for a nozzle whose hot wall geometry was designed by BURPG. Mr. Harris provided us with the initial conditions of the possible fluids, heat transfer coefficients as a function of position along the nozzle, the nozzle hot wall geometry and the driving pressure. II. A Nozzle specifications provided by BURPG Chamber Pressure: 500 psi Flame Temperature: 5,530 degrees F Coolant: N2O preferably Fuel: HTPB O/F: 5.5 to 5.8 Throat Diameter: 0.582 inches Expansion Ratio: 7 Profile: 15 degree exit cone Thrust: 200 lbs Burn Time: 20 seconds
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- 17. 17 II. B Financial specifications While our Senior Capstone budget is limited to $400 plus extra funding from other groups (which we will most likely not need) we also have a theoretical $2000 from BURPG. If our final CAD models were to be fabricated they should not cost more than $2000. II. C Deliverable specifications BURPG will be given a report with our findings, CAD models for the nozzle and any accompanying parts, a user-friendly MATLAB model of the model we have developed in order to test new materials, coolants and geometries and a report of the financial breakdown. Finally, if appropriate, we will be delivering our working prototype. II. D Timeline specifications Our timeline is limited to the timeline set by the Department of Engineering (early May), and we do not have any immediate or critical dates to meet for BURPG. II. E Safety and testing specifications While we anticipated testing our prototype on BURPG rockets, we were not able to achieve this due to the lack of time. Therefore the only safety and testing specifications we have to worry about revolve around the water flow test that was performed, which provided no real safety concerns.
- 18. 18 II. G Weighted Comparison of Requirements 0 – Little importance 1- Medium importance 2 – Critical importance Requirement Weight Description Nozzle Specifications 2 The nozzle specifications provided by BURPG are final and cannot be changed to fit our needs. Therefore we must work around them and not against them. Financial Requirement 1 While we are well aware of our budget we did not find any difficulties meeting this budget. The theoretical $2000 budget is a lot more than our real $400 budget. Deliverable Requirement 1 Our deliverables are important but are due at the end of the project. Therefore we did not have any intermediate or time sensitive deliverables to worry about. Timeline Requirement 1 While timing and time management are important we did not have to meet any critical dates beyond the final due date at the end of the semester. Safety and Testing Requirement 0 As discussed above, our only testing did not pose safety concerns. Table 4. Weighted comparison of requirements
- 19. 19 III. Engineering Specifications and Relevant Basic Physics References available in section IX. References. III. A Relevant Basic Physics In order to understand the MATLAB model, as well as the nozzle itself please consider the following representations of the nozzle. These are to be viewed as a cross sectional slice of the nozzle from the top view. The viewer sees only one channel (the small circle) as well as the inside hot wall (smaller diameter curve) and the wall exposed to the atmosphere (larger diameter curve). The circle that represents our channels has ammonia (our coolant of choice) traveling through it (into the page). These drawings are not drawn to scale, but are exaggerated in order to aid in understanding of our model. The first image we consider is Figure 5. Figure 5 displays a slice of the nozzle with nodes that represent known and unknown temperatures that must be considered. Figure 5 also displays the thermal circuit (explained in more detail below) that was used to solve for unknown temperatures.
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- 21. 21 the heat can travel through. These arrows follow the directions illustrated by the thermal circuit in Figure 5. Since heat travels from hot to cold we can see that the atmosphere, the colder outside wall and the coolant draw heat into them. Figure 6. Exaggerated top view of nozzle with heat direction Finally consider Figure 7, which displays the same slice, but this time it includes the resistances from the thermal circuit. Thermal circuits and thermal resistances are explained in more detail below. Briefly, thermal circuits can be used to represent how fast heat travels through matter. The resistances are determined through properties of the matter they represent. These resistances can be used to solve for unknown temperatures. The known temperatures are the atmospheric temperature, the temperature of the coolant (from initial conditions), and the temperature of the hot wall without any cooling (provided by BURPG). The circuit can be evaluated using MATLAB in order to solve for the remaining nodes. The temperatures vary along the height of
- 22. 22 the nozzle, which we account for by using Euler’s method (explained more below) and using the new coolant temperature for each new differential slice of the nozzle. These slices look like thin cylinders stacked on top of each other to form a nozzle. These slices can be made to be very small using MATLAB in order to find all temperatures along the nozzle. Figure 7. Exaggerated top view of nozzle with resistances In summary, this thermal circuit can be used in conjunction with the known temperatures and thermal resistances to solve for unknown temperatures. Convective and conductive methods of heat transfer must be considered. Euler’s method of iteration is used to travel along the height of the nozzle and solve for all the temperatures in the system. Below are explanations of relevant basic physics and how they apply to the system.
- 23. 23 Thermal Circuits As described above, thermal circuits can be used to evaluate heat transfer. A thermal circuit is based off of electrical circuits and uses known parameters to solve for temperatures at different nodes. The current becomes the rate of heat transfer, electrical resistance becomes thermal resistance, and the potential difference V becomes the temperature difference between each node. By considering the material properties of the nozzle, the fluid properties of the coolant, and the geometry of the channels the thermal resistances can be solved for. The rate of heat transfer depends of whether conduction or convection is taking place and gives the temperature at each node. The circuit is reproduced below. Figure 8. Thermal circuit of nozzle Equation 1 represents the heat transfer as a function of resistance and temperature difference. 𝑞 = ∆𝑇 𝑅!! (1) Conduction Conduction is one of two types of heat transfer present in the system. Conduction is a form of heat transfer that occurs between two adjacent materials. The rate of conduction is governed by the difference in temperature, the distance between the two areas in question, and the thermal conductivity of the material. Some materials conduct heat better than others, thus have higher thermal conductivities. For many materials these thermal conductivities also change in relation to temperature, with different materials exhibiting different relations.
- 24. 24 In the regenerative cooling rocket nozzle, heat transfer due to conduction plays a leading role in determining the way heat is passed from the hot wall of the nozzle to either the border of the internal cooling channels or the cold wall, on the exterior of the nozzle. At these locations the energy is dissipated through other methods. Equation 2 represents the thermal resistance considered in areas experiencing conduction. L represents the characteristic length; k represents the thermal conductivity and A represents the area through which the heat is traveling. 𝑅!"#$%!&'"# = 𝐿 𝑘𝐴 (2) Forced and Free Convection Convection is the second type of heat transfer present in the system. In convection, heat is transferred between a solid and a fluid. There are two kinds of convection: forced and free convection. Free convection is a type of heat transfer in which the fluid motion is controlled by the density differences between the fluid and the solid. For the regenerative cooling rocket nozzle, natural convection happens between the outer wall and the air around it. Forced convection is similar to free convection except that the fluid motion is generated by an external source like a pump or a fan. The forced fluid flow happens within the cooling channel, as the coolant is pumped through the channels before being combusted. The thermal resistance used to represent convection is given below, where h is the convection heat transfer coefficient and A is the solid surface area. 𝑅!"#$%!&'"# = 1 ℎ𝐴! (3) Euler’s Method Euler’s method, described by Equations 4 and 5, was used to solve for the temperatures at all points along the nozzle. In this case, Euler’s method uses the results from one circuit to calculate the input conditions for the next circuit. The first thermal circuit can be solved using the initial conditions of the coolant in the tank. The heat transfer rate that this first circuit provides can be used to find the temperature increase in the nozzle, which is then the only unknown in the next equation. This increase in temperature is
- 25. 25 added to the temperature of the slice in order to be used as the coolant temperature in the next differential slice. In this system, Euler’s method consists of the following equations: 𝑞 = 𝑚𝑐! 𝑑𝑇 → 𝑑𝑇 = 𝑞 𝑚𝑐! (4) 𝑇 𝑠 + 𝑑𝑠 = 𝑇 𝑠 + 𝑑𝑇 (5) The reason this method simplifies the calculation is because the thermal resistances of the system are a function only of the material properties. This means that all of these values are known from either experimental relations or basic material and fluid properties. Thermal Stresses Another important consideration in the system is the thermal stress caused by changes in temperature and thermal expansion. As a metallic alloy heats up, it expands. Seeing as the nozzle will have a temperature gradient from an internal hot wall to an external cold wall, it is imperative to ensure the expansion of the hot internal portion of the nozzle will not cause the colder, external wall to crack or yield, destroying the rocket. This thermal stress caused by the inner material heating disproportionally to the outer material is the largest contributor to overall stress in the nozzle, greater than that caused by mechanically confining the gas flow. This is modeled using a “temperature gradient across a thick walled cylinder”, with a thick hot ring inside a thin cold ring, similar to a “shrink fit” equation. The demarcation line between the hot ring and cold ring was chosen closer to the outer edge than the center in order to present a worst case scenario, ensuring the proposed “cold ring” would not include any interference with the cooling channels as well as minimizing the “cold” area available to contain the inner ring. Example Results Finally we have included some example results of our model to illustrate how our analysis is displayed.
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- 27. 27 Figure 11. Outside wall stress plot example Figure 12. Nozzle temperature cross-section plot example III. B Engineering Specifications Below is a list of the engineering specifications considered in the system. This list does not include the specifications and requirements discussed in section II Requirements that are not directly related to engineering and analysis. Included are tabulated values of the information provided by BURPG from their ANSYS analysis in Appendix D.
- 28. 28 Nozzle Specifications Value Area Ratio 7 to 1 Throat Diameter 0.582 inch Flame Temperature 5,530 deg F Tank Conditions Pressurized TCP Temperature -30 C TCP Pressure 700 PSI Combustion Chamber Conditions, Pressurized CCCP Pressure 500 PSI Other Burn time 20 sec Table 5. Nozzle specifications, tank conditions and combustion chamber conditions N2O Properties ρ (kg/m^3) 1050 µ (Pa*s) 0.000325 cp (J/kgK) 1720 cv (K/kgK) 913.1 k (W/mK) 0.01805 T start (K) 243.15 T crit (K) 363.15 Ptank (psi) 700 Pchamb (psi) 500 ΔP (Pa) 1379000 Table 6. N2O properties Ammonia Properties Density (kg/m^3) 0.696 Dynamic Viscocity (Pa s) 0.00001009 Thermal Conductivity (W/mK) 0.02449 Specific Heat Cp (J/kg K) 2061 Tank Temperature Ti (K) 273 Critical Temperature Tcr (K) 405.55 Table 7. Ammonia properties Name Copper 6061 Aluminum Melting Temperature (K) 1353.15 890.15 Young's Modulus (Pa) 1.17E+11 6.89E+10 Poisson's Ratio 0.34 0.33 Table 8. Material properties
- 29. 29 IV. Design Description There are two major deliverables required by the BURPG: the user-friendly MATLAB model of regenerative cooled nozzles and the manufactured prototype. This section is split into two sub- sections in order to thoroughly describe the systems. IV. A MATLAB Model Description This MATLAB Model is different from commercially available simulation tools in that it is has a simple, user-friendly interface and is capable of accurately modeling regenerative cooled nozzles. The user interface makes it easy to analyze designs with different specifications such as coolant type, nozzle material, and thrust parameters. Below is a block diagram of the model, followed by images of the model and explanations of its components. Figure 13. Block diagram of MATLAB model user interface
- 30. 30 From Figure 13 we can see that this model is relatively simple. The user inputs important information into an input panel, the model then calculates the necessary information and plots it into 4 plots. The user has the choice to export the data into tabular form. Below is an image of the model, as it would appear on the screen. For an explanation of how the model conducts the analysis refer to section III A Relevant Basic Physics. Figure 14. Example user interface screen As can be seen from Figure 14 the user interface (UI) provides two important sections. On the left hand are four plots. Starting on the top left and going clockwise we find a plot of the wall temperatures as a function of nozzle position, the oxidizer temperature as a function of nozzle position, a heat map of the nozzle cross section and finally the wall stress as a function of nozzle position. These plots are reproduced below individually to show more detail.
- 31. 31 Figure 9. Wall temperature plot example The wall temperature plot includes a variety of important temperatures along the cross section of the differential slice displayed in Figure 4. It also includes a dashed line to represent 2/3 of the melting temperature of the chosen material. 2/3 of the melting temperature, are displayed because it is the recommended temperature safety factor used in literature regarding rocket components. Figure 10. Coolant temperature plot example
- 32. 32 The coolant temperature is displayed along with the critical temperature and the temperature it reaches, depending on the input. Figure 11. Outside wall stress plot example The wall stresses are also calculated and displayed in graphical form. We are able to include the maximum stress reached as well as the yield stress of the nozzle at each point. Figure 12. Nozzle temperature cross-section plot example Finally we have created a heat map to display the temperature distribution along the cross section of the nozzle.
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- 34. 34 The inputs panel allows the user to test different designs by changing input parameters. As can be seen from Figure 15, the user can choose coolants, nozzle body material and geometries as well as channel geometries (including square channels) and the number of tubes. Additionally information can be exported in tabular form to excel. The user also has the ability to add new materials and coolants of their choice into a memory bank that can draw upon them on later dates. The number of tubes that is chosen can either be done by writing in a number or by choosing “Use Maximum”. The maximum number of tubes is calculated by fitting as many channels as possible with no interference and then taking 2/3 of that number as the maximum. Finally, this model has been “idiot proofed” as much as possible. This is done in order to assure that no errors or impossible inputs are being entered into the model. Through this proofing we enhance the usability of the model. IV. B Nozzle Description The regenerative cooling nozzle is made of aluminum and uses ammonia (NH3) as its coolant. The reasons for these decisions are covered more extensively in section V Design Decisions. Briefly, using our MATLAB model we decided that aluminum provides the cheapest functioning material that can also be easily machined. Ammonia was chosen due to its effectiveness as a coolant as well as its availability to BURPG. To provide a sense of size, the nozzle is roughly 2.5 inches in diameter and roughly 3 inches tall. Dimensions are provided in the drawings in Appendix B. Below is a functional decomposition of the nozzle system, including everything the nozzle needs to accomplish as well as the solutions we have chosen. The justification for these choices is provided in section V Design Decisions.
- 35. 35 Figure 16. Functional decomposition of regenerative cooling nozzle with results There are two major components to this design. The first is the nozzle body. The nozzle body is made up of the hot wall as well as the channels that run along the hot wall. The CAD below displays two views of the nozzle body, including a whole view and a cross sectional, isometric view. For dimensioned drawings refer to Appendix B. Regenerative Cooling Nozzle Accelerate propellant from combustion chamber to supersonic Converging / Diverging hot wall geometry provided by BURPG Connect to rocket components NPT port Cool hot wall Ammonia as a coolant Aluminum body Circular channels along hot wall
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- 37. 37 Figure 18. Cross sectional view of regenerative cooling nozzle The next major component is the two manifolds, connected to the top and bottom of the nozzle body, also seen in the above cross-sectional view. The purposes of the top manifold are to intake coolant from an NPT (National Pipe Thread) port and distribute it into the manifold. The coolant can then flow down the channels of the nozzle body. The bottom manifold serves a similar purpose but instead it collects the coolant from the channels and sends it out through another NPT port back into the rocket. The coolant motion is all driven by a pressure gradient provided by the rocket. Below are images of the manifolds and finally and image of the whole system. The manifolds and nozzle body are held together by the use of screws as displayed by the exploded view. ME461TEAM11BURPGNOZZ01 ME461TEAM11BURPGNOZZ03 ME461TEAM11BURPGNOZZ02
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- 40. 40 Figure 22. Side view of coolant flow through nozzle In Figure 22 we see that the coolant enters through the side, in from an NPT port, and spreads throughout the upper manifold. It then flows down the 12 channels that are distributed around the nozzle. Once it reaches the end of the channel, it falls into the lower manifold and exits through another NPT port. O-rings and sealant are not displayed in the CAD but are included in the design in order to prevent coolant leakages from occurring during operation.
- 41. 41 In terms of operating with this apparatus there are a few precautions that must be taken. As this nozzle is meant for use with rockets it is crucial to review safety instructions set by BURPG in testing. Before running any test please contact BURPG for procedures and safety factors in testing. In terms of the actual part it is important to follow the following instructions every time the nozzle is tested. After each test, having waited considerable time before handling due to high temperatures, please inspect the nozzle for damage. Damage may include charring, oxidizing, melting and deformation. Before the next run make sure to remove the current O-rings and replace them with new rings. If O-rings are not providing enough of a seal (i.e. there are gaps between the rings and nozzle) use RTV sealant to complete the operation. Before running a test make sure the bolts are screwed on as tightly as possible to prevent the manifolds from dislocating from the nozzle body. In addition we have included a summary of our manufacturing process as well as recommendations in order to aid the reproduction of this nozzle. IV. C Manufacturing Process 1. Nozzle Body Fixturing and Set-Up The most complex part of the manufacturing process for the rocket nozzle is the custom fixturing set-up required to machine the cooling channels. Since the EPIC facility is equipped with only three-axis mills, the nozzle must be rotated about the y-axis in order to cut the angled channels. To perform this operation, two key fixturing tools must be used: an indexing chuck and a stage with adjustable angles (Figure 23). These two tools can be combined to build a fixture, which rotates about the y-axis and the z-axis, allowing all channels to be drilled accurately (Figure 24). Location pins are used in order to ensure that the drilled holes line up on both sides of the nozzle. By using the four holes where the manifolds will be bolted to the nozzle body, the nozzle body can be placed into the fixture in the same orientation every time. Rather than placing the nozzle
- 42. 42 directly into the chuck, a base plate with the location pins is fastened into the chuck and the nozzle is slid onto the location pins and tightened down with bolts (Figure 25). The custom fixturing required for the nozzle turning and drilling operations is the most significant cost factor in the production of the first nozzle. Figure 23. Indexing chuck (Left) and stage (Right) Figure 24. Drilling fixture assembly As part of the turning operation of the nozzle body, two end caps are necessary to hold the nozzle body while in the lathe. These end caps perform three important functions: they allow the lathe’s chuck to securely attach to a wider base and prevent direct contact damage to the material
- 43. 43 while also providing physical references for the width of the flanges located on either side of the nozzle body. Figure 25. Detail of location pins and fastening bolts Processing The manufacture of the nozzle body begins with sizing the raw material to the correct height. First, a piece of roughly the correct dimensions is cut from the larger stock rod using a circulating saw. This rough piece is then placed into a lathe and is faced to the correct length. From this point, the workpiece is handled as two halves of a larger whole. The first half, or the subsonic portion of the nozzle, is first turned to the desired outer diameter of the flange and given the seal ridge. The inner contour of the nozzle is then turned up to the widest part of the nozzle’s throat, from where the tool turns a hole of constant diameter through to the bottom of the workpiece (Figure 26). The workpiece is then flipped around in the lathe and the supersonic contour is turned. Precision in the turning of the flanges requires the use of two, quarter inch end caps so the workpiece can be held by both ends and so that the thickness of the flanges can be more accurately determined. Each nozzle spends approximately one hour and thirty minutes being processed in the lathe.
- 44. 44 Figure 26. Subsonic section and sealant ridge The nozzle is then brought to one of the six manual mills. The operation of drilling the cooling channels requires extremely high precision in order to ensure that the holes line up and that the drill does not break through the walls of the nozzle. First, the angle on the indexing chuck must be set using the spacer bars. A 45-degree angle must be set for the subsonic channels and a 15- degree angle must be set for the supersonic channels. When the angle has been set, the nozzle can be inserted into the 4 locator pins, and the securing bolts can be tightened (Figure 25). In order to give the drill bit a flat surface to cut into to prevent deflection, all twelve channels must be spot faced using an end mill. Before drilling the first hole, the Z depth must be set on the machine by lowering the drill bit to the surface of the nozzle. The Z stop is then set using the cylindrical spacers. After all twelve holes have been drilled, the nozzle can be flipped and the process is repeated. Between the setup time, tool changes, and actual run time, the cooling channels require approximately three hours for processing. 2. Manifolds Processing The milling & turning operations of the nozzle manifolds do not require custom fixtures or special set-up to perform. First, slices of approximately one inch in thickness are cut from the raw material. The rough cut is then placed into a vertical chuck and secured in the CNC mill. The mill first uses a fly-cutting tool to remove a small amount of surface material from the stock. This is done to ensure a smooth surface and to establish a datum from which the other cutting
- 45. 45 operations will be performed. The CNC machine then mills the various slots and channels into the manifold. When the milling operation has completed, the manifold must be cut from the excess stock material with which the chuck held the workpiece in the machine. Figure 27. The manifold undergoing the milling operation To complete the manifold, the workpiece is placed into a lathe with the excess material facing away from the chuck. During the milling process, the stock was cut down to the desired diameter, which means that the work piece is essentially a cylinder with two diameters. The edge between these two diameters serves as a datum for the turning process. With this datum, the process can be completed much more quickly, as a very large portion of the excess material can be quickly cut away. After turning, the hose connection is drilled into the side of the manifold. This operation is performed on a vertical manual mill and requires NPT drill bits. The process is identical for the upper and lower manifolds, with the only exception being the use of different NCF codes during milling. 3. Process Analysis Figure 28 shows the process flow chart for the rocket nozzle assembly. From this diagram it is immediately clear that the cycle time, defined as the time between completions of an assembly, is around three hours. The process bottleneck is the drilling operation used to create the coolant channels along the nozzle contour. The throughput time, or the time that an assembly spends
- 46. 46 being processed, is around six hours and fifteen minutes. Note that the assembly time includes time to allow sealant to begin setting. Figure 28. Process flow chart Fully utilizing the capacity of the EPIC facility would be the quickest way improve the system’s cycle time. By adding an additional vertical mill to the system, the capacity of that process doubles, reducing cycle time to one and a half hours at that workstation. Increasing the drilling operations capacity then moves the system’s bottleneck to the turning operation of the nozzle body. Further improvements would be fairly costly, as increasing capacity at the turning workstation would require the purchase of an addition CNC lathe.
- 47. 47 V. Design Decisions Once again, the MATLAB model and the actual nozzle design are our areas of focus. In regards to the MATLAB model we had to decide how to best represent the nozzle system in accurate thermodynamics and heat transfer terms. In respect to the nozzle we found design flexibility in the material of the nozzle, the cooling channels and the coolant fluid. Below are analyses of our decisions throughout the project. Appendix D provides any supporting material that needs to be considered regarding our decisions. We have also included a morphological chart to illustrate our choices and decisions, which are explained below. Regenerative Cooling Nozzle Morphological Chart Heat Exchange Model Lumped Pipe Thermal Circuit Cooling Channels Circular Tube Square Tube Spiraling Tube Corrugated Jacket Nozzle Material 6061 Aluminum 110 Copper 360 Brass Coolant N2O Ammonia CO2 Table 9. Morphological chart The criteria we considered vary with each component but are based around physical properties/limitations and cost/manufacturing. Refer to section II Requirements for specific requirements. Briefly, BURPG has limitations in manufacturing capabilities due to the fact that they operate at BU. They also provided us with a theoretical budget of $2000, as an upper bound, to how expensive our design could when manufactured. Our project was also heavily based on thermodynamic and heat transfer analysis, which dictated many of our choices. This analysis was conducted through our MATLAB model.
- 48. 48 V. A MATLAB Model As has been mentioned above, a long amount of time was dedicated to our MATLAB model. A nozzle is a complicated system to model and we wanted to make sure our model appropriately portrays what happens within the nozzle during operation. We began by considering a lumped pipe model. The idea behind this is that we can model the channel as a pipe with a fluid running through it. We had to graduate to our final choice of using a thermal circuit because of the limitations of the lumped model. We found that the temperature difference from the hottest side of the channel to the opposite was considerable enough to matter. Our final model is the thermal circuit model, which takes into account the variation in temperature as well as the different paths heat can travel through. V. B Cooling Channels The cooling channels of the nozzle are meant to transfer the fluid in order to cool the nozzle. The following Pugh Chart illustrates our criteria and decision regarding the different channel geometries. For more information on the channels refer to section I Introduction and Background. Criteria Weight Circular Tube Square Tube Spiraling Tube Corrugated Jacket Efficiency in Heat Transfer 1 0 1 0 1 Stays Below Supercritical Temperature 2 0 0 -1 0 Cost 1 0 -1 -1 -1 Manufacturability 1 0 -1 -1 -1 Total 0 -1 -4 -1 Table 10. Cooling channel pugh chart
- 49. 49 Through a Pugh Chart analysis, illustrated by Table 10, it is evident that while the corrugated jacket and square tube may be the better methods of cooling, they are more expensive and difficult to fabricate through BU manufacturing facilities. The spiraling tube, while novel, is inefficient in that it exposes the fluid for too long. Through our MATLAB model we found that the coolant is exposed to the heat for too long and reaches supercritical temperature, which in turn causes it to act unexpectedly and lose efficiency in cooling. V. C Nozzle Material The nozzle material must withstand the melting effects of the combustion process while also being strong enough to withstand thermal stresses. The material must also be easily accessible, relatively cheap and easy to work with at BU manufacturing facilities. Criteria Weight 6061 Aluminum 110 Copper 360 Brass Cost 1 0 -1 -1 Manufacturability 1 0 0 0 Does Not Deform Under Temperatures of Rocket 1 0 0 0 Weight 1 0 -1 -1 Availability 1 0 -1 0 Can Withstand Thermal Stresses 1 0 0 0 Total 0 -3 -2 Table 11. Nozzle material pugh chart A Pugh Chart analysis shows that while all these materials can perform in the nozzle application, aluminum is cheapest and thus provides an important benefit, on top of being lighter.
- 50. 50 V. D Coolant The coolant must withstand heat effects and remain well below the critical temperature, be accessible to BURPG and must be either an oxidizer or a fuel. Criteria Weight N2O Ammonia CO2 Remains Below Super Critical Temperature 2 0 1 1 Is Used by BURPG 1 0 0 -1 Oxidizer / Fuel 1 0 0 0 Total 0 2 1 Table 12. Coolant pugh chart This last Pugh Chart shows that Ammonia is the best coolant to use. Additionally it was recommended by BURPG as an alternative to N2O. After analysis through our MATLAB model we found that N2O reaches a supercritical temperature and cannot be used. Ammonia on the other hand, performed well and was chosen as the coolant for the final design. V. E Runner-Up Design The most prominent runner up design to our final design includes circular tubes along the height of the nozzle but instead uses N2O. We worked with this design for a long time until we found that a decrease in mass flow of the coolant resulted in N2O heating up too much. While N2O was the preferred coolant, by BURPG, we were forced to switch it out for ammonia due to this significant factor. V. F Future Design Iterations Moving forward there are a few suggestions we can make regarding the design of the nozzle. After interacting with our first prototype we found four areas of possible improvement. The first
- 51. 51 area of improvement is in the O-ring. While our O-rings worked and fit properly, into the nozzle, we believe that a gasket is the next step in improving the sealing mechanism in our nozzle. The second area of improvement also depends on BURPG. We would like to design an interface between the MK IIB rocket and our nozzle that is more appropriate than our current design. Our current design does not take into account the rocket because the rocket designs had not been finalized in time. The third improvement is that we would like to investigate whether there is an optimal outer diameter for the rocket nozzle that would consider both efficient heat transfer and aerodynamics. The theory behind this is that we can decrease the mass by finding the perfect amount of material that is necessary for keeping the nozzle cool. At the same time we can consider aerodynamics to provide a more efficient shape for the rocket during flight. This step will have to take place in the future, after the nozzle can be tested in a static test by BURPG. Finally, while we believe the manifold will distribute coolant evenly to all the channels, it must be investigated further and possibly redesigned if it is found to be too small or too large.
- 52. 52 VI. Design Evaluations VI. A FEA / CFD An FEA / CFD analysis was conducted on Ansys in order to verify the MATLAB results. This is the “industry practice” in verifying models. Through this analysis it was concluded that the MATLAB model develops accurate results, appropriate for its application and purpose. The most appropriate method for simulating this nozzle would be to perform a conjugate heat transfer (CHT) analysis using either of Ansys’ computational fluid dynamics suites, then feed the resulting temperature gradient and pressures within the aluminum nozzle to an Ansys Structural analysis, to test for thermal stress. A CHT simulation would simultaneously model the flow of propellant through the nozzle, the flow of liquid ammonia through the cooling channels, the thermal interaction between both of these flows and their respective interfaces with the aluminum nozzle, as well as the thermal gradient in the aluminum. It does this by mapping/matching the nodes on either side of any material interface, in order to more closely calculate the heat fluxes across this boundary. Unfortunately, due to the geometry of the nozzle and the detail needed to accurately simulate this heat transfer, the mesh required for a CHT analysis of the nozzle contains more nodes than is compatible with the Student Edition of the Ansys Solvers, and runs into a convergence error when meshed more coarsely. Licensing and installation problems prevented the use of Boston University’s copy of Ansys 15, thus a Student Edition had to be made to work. In order to lower the number of nodes in the CFD mesh to levels compatible with Ansys Student Edition, the simulation was broken up into multiple sub-simulations. First, the flow of combustion products and liquid ammonia was simulated in Ansys Fluent using a mesh based on curvature. This simulation applied an energy model, supersonic flow model, and k-epsilon RNG turbulence model. From this solution, the convection coefficients for both the hot wall interface and cold wall interfaces of the nozzle were imported into an Ansys Mechanical simulation. An example of one of these coefficients is illustrated below.
- 53. 53 Figure 29. Map of the simulated hot wall imported convection coefficient Using these convection coefficients, a transient thermal analysis was performed in order to determine the thermal gradient in the aluminum nozzle body. The results of this thermal analysis are illustrated below. Figure 30. Map of the simulated temperature gradient of the nozzle Lastly, this thermal gradient is fed into a static structural analysis along with the nozzle and cooling channel pressures calculated in the original Fluent analysis. The model is solved then for
- 54. 54 stress and strain. The contours of the stress map and safety factor map can be seen in the figures below. Figure 31. Map of the simulated equivalent stresses in the nozzle Figure 32. Map of the simulated safety factor across the nozzle The temperature gradient of the nozzle was found to be extremely similar in shape to what was predicted by the MATLAB model. Despite this similarity, the maximum temperature was 19.3 percent lower than expected, at 363 degrees kelvin. This can be attributed to the lack of a realistic heat transfer model between the hot and cold fluids in the Fluent simulation (due to the
- 55. 55 not using a CHT approach) as well as slight differences in hot fluid flow in the nozzle due to an updated, easier to manufacture, bell profile. The stresses were also found to be of similar pattern to the MATLAB model. The maximum stress in the Ansys model was found to be 9.3 percent higher than predicted. This small discrepancy can be partially attributed to the fact that the force of pressure upon the cold walls was accounted for in the computer simulations. This stress maximum was also extremely localized and not symmetric around the nozzle, with the average stress in the area being lower and more in line with the MATLAB code’s predictions, suggesting that the majority of this discrepancy is most likely due to a meshing anomaly. The minimum safety factor of 2.16 was found to be at the throat of the nozzle, as it was expected to be in the MATLAB model. This was determined to be an acceptable level, as this is a performance application in an extremely weight sensitive field. The implemented multi-simulation method, despite being an alternative to the more appropriate CHT, is very similar to the method currently practiced by BURPG to simulate the thermal stresses in their graphite nozzles. This was determined to be a useful approach, regardless of the absence of enough computing power to perform a CHT analysis, as it is fitting with the goal of delivering a “user friendly” cooled nozzle design toolset. This model should be only marginally more complex than the existing BURPG models, and has been labeled for ease of modification, allowing future BURPG projects to quickly utilize this for future designs. VI. B Live Comparison As an additional measure to verify the validity of our MATLAB model, we conducted a study using existing results. The paper used for this goal was Analysis of Regenerative Cooling in Liquid Propellant Rocket Engines by Mustafa Emre Boysan with additional design data from Comparison of High Aspect Ratio Cooling Channel Designs for a Rocket Combustion Chamber with Development of an Optimized Design by Mary F. Wadel, in collaboration with NASA (referenced in section IX References).
- 56. 56 First, the initial design conditions provided for the rocket nozzle had to be applied to the relevant Excel spread sheets, these included: string length, contour geometry and heat flux along the normalized position. To make sure the adding and editing functions of the model worked, the coolant data was added through the GUI, instead of directly from the Excel sheet, the required data was: density, dynamic viscosity, thermal conductivity, specific heat, tank (initial) temperature and critical temperature. Input Parameter Value String Length (m) 1.16 Wall Thickness (m) 0.424 Channel Shape Rectangle Channel Height (m) 0.004 Channel Width (m) 0.003 Number of Channels 100 Table 13. Input parameters for the model Results Figure 33. Coolant temperature along the normalized position
- 57. 57 Figure 34. Shows the Wall Temperature along the Normalized Position The maximum temperature for the coolant and the hot wall was compared to the results from the analysis. The results are shown in the table below. Parameter Maximum Coolant Temperature (K) Maximum Wall Temperature (K) Model Data 652.9 852.2 Test Data 724.0 872.5 Error 9.82% 2.33% Table 14. Shows the result of the model data compared to the test data From the results it is observed that the maximum coolant temperature is within 10% of the test data, and the maximum wall temperature is within 3%. This shows that the MATLAB model is highly accurate in this comparison.
- 58. 58 Some sources of error with this model are that the heat flux and nozzle inner radius along the normalized position did not have as high resolution as the test data, and so the interpolated data would be less accurate. VI. C Water Flow Testing A design concern that had to be addressed was whether or not the manifold would distribute the fluid evenly to all the channels around the nozzle. The major concern was that the fluid could enter from the NPT port and only flow down the closest channels, never reaching the far end of the manifold and therefore failing to cool the nozzle evenly. In order to assess this issue we decided to conduct water flow testing in which water would enter through the NPT port at a certain flow-rate and then be collected from each channel individually to test whether the amount of water coming from each channel is the same. This was to be done at different time increments between 0 and 20 seconds (the burn time). In order to get accurate results, the Reynolds number had to be matched between the water and ammonia (our coolant of choice). We used the flow rate of ammonia, as well as properties of ammonia, to calculate the Reynolds number. We then matched this Reynolds number for water and solved for the flow rate necessary to reproduce legitimate results. This analysis is included in Appendix D. We found that approximately 3 Gallons per second of water were needed to simulate the ammonia flowing through the channels at our flow-rate. Planning to use the Fluid’s lab water pump / pipe system we found that it could not support such values. Additionally acquiring pipe adapters for this experiment would put us above our project budget. The water flow test was not conducted because of these reasons. VI. D FEA Joint Stress A finite element analysis (FEA) was performed on the manifold-nozzle body interface in order to test the validity of the design. In particular, there was some concern that the flange on the nozzle body and the screws, which hold the assembly together may not be able to withstand the fluid pressure in the manifolds. Since the design was already modeled in Solidworks, the FEA analysis was performed using the Solidworks Simulation add-in. For the purposes of this study,
- 59. 59 the pressure was assumed to be a constant value throughout the upper manifold. Pressures acting radially on the walls of the manifold sum to zero due to symmetry, allowing the pressure to be modeled as acting vertically on the manifold. The screws can be modeled using the Solidworks “connections” tool, preventing geometry interferences, which would cause the system to be unsolvable if CAD models of the screws were used instead. The result of the simulation shows that the stresses at the manifold-nozzle body interface reach values of around one order of magnitude smaller than the yield strength of aluminum 6061-T6. Moreover, using Equation (6) , the pre load of each screw can be determined. The pre load of each screw was found to exceed the force caused by the fluid pressures in the manifold by over one thousand pounds. As such, this analysis demonstrated the mechanical feasibility of the manifold-nozzle body interface. Below is an image to illustrate the results of the FEA Analysis. Figure 35. FEA Joint stress results
- 60. 60 VII. Cost Analysis In terms of cost we have included 2 analyses. The first illustrates the actual budget used throughout this project. The second analysis is a production cost breakdown and it shows the cost of manufacturing the regenerative cooling nozzle, as well as the learning curve and change in cost over time. VII. A Budget Analysis Table 13 displays the distribution of our budget throughout this project. Product Name Dimensions Part Number Quantity Price Shipping Optically Clear Cast Acrylic Rod 3.5" Diameter, 1 ft. Length 8528K45 1 Rod $115.45 $9.10 6061 Aluminum Rod 3.5" Diameter, 1 ft. Length 1615T78 1 Rod $94.08 $10.79 Square Buna-N O- Rings 1/16" Frational Width, Dash Number 029 4061T134 1 Pack $10.82 $16.44 Square Buna-N O- Rings 1/16" Frational Width, Dash Number 034 4061T139 1 Pack $13.62 Square Buna-N O- Rings 1/16" Frational Width, Dash Number 037 4061T142 1 Pack $13.50 Square Buna-N O- Rings 1/16" Frational Width, Dash Number 040 4061T145 1 Pack $14.14 Silicon O-Rings 3/32" Frational Width, Dash Number 128 9396K148 1 Pack $9.30 $8.21 Silicon O-Rings 3/32" Frational Width, Dash Number 137 9396K158 1 Pack $5.26 Silicon O-Rings 3/32" Frational Width, Dash Number 144 9396K164 1 Pack $6.67 Silicon O-Rings 3/32" Frational Width, Dash Number 149 9396K169 1 Pack $7.09 3M Instant Bonding Adhesives 0.07 oz. Tube 75445A71 1 Each $6.47 Permatex Gasket Maker General Purpose Blue RTV, 3 oz. Tube 7535A12 1 Each $4.72 Total $301.12 $44.54 Total Cost $345.66 Table 15. Complete project budget
- 61. 61 As can be seen from Table 13, we were able to stay below our budget throughout this project. Pennies are included in Table 13 because the purchases have already been made and we can report an exact amount. Our choice of aluminum allowed us to remain below the budget cut-off, while brass and copper would have pushed us beyond the $400 limit. The table below displays this phenomenon. Product Name Dimensions Part Number Quantity Price Total Cost 110 Copper Rod 3.5" Diameter, 1 ft. Length 8966K79 1 Rod $364 $615 360 Brass Rod 3.5" Diameter, 1 ft. Length 2572T72 1 Rod $274 $525 Table 16. Theoretical project budget for different materials Table 14 displays the total cost if we were to choose either copper or brass. As can be seen, the estimated total cost surpasses the $400 limit in both cases. VII. B Production Cost Analysis and Learning Curve In order to provide a realistic estimate of our production cost we created a production analysis using some assumptions. This production cost analysis shows that even with labor cost we are able to keep the production of this nozzle below the theoretical $2000 limit provided by BURPG. The difference between this analysis and the previous is that in the production cost analysis we are considering the exact amount of material needed and not the total that has been acquired. The costs that factor into the production of the rocket nozzle assembly can be broken down into two major categories: processing and materials. By far the larger contribution of the two is the processing cost of each nozzle. The processing cost for the nozzle is simply the processing time multiplied by an estimated “shop rate” of $75.00/hour. For the first nozzle, the time taken to build custom fixtures and the time taken to create the GibbsCAM NC codes must be taken into account. For subsequent nozzles, these costs are taken to be zero and as such the cost to process the nozzle decreases significantly from the first to the second iteration. The operations and their estimated time for the first nozzle can be seen in Table 15. The processing cost of the first
- 62. 62 nozzle plus the cost of raw materials used in that nozzle result in a total production cost of $1,063.00. The cost of the second nozzle is less than half of the first, at about $576.00. Process Body Manifold Turn Drill Mill Turn Fixturing 4.00 0.00 Setup 1.00 2.00 0.25 0.25 Run Time 1.00 1.00 0.50 0.75 GibbsCAM Solidworks Edits 0.50 2.00 TOTAL TIME 9.50 3.75 13.25 Table 17. Process times for the first nozzle. All values are in hours Cost Nozzle # Processing Materials Total Aluminum O-Rings 1 $1,275 $30.00 $2.00 $1,307 2 $675 $30.00 $2.00 $707 3 $534 $30.00 $2.00 $565 4 $459 $30.00 $2.00 $491 Table 18. Total production cost estimates for four nozzles At first glance, this would indicate a significant learning curve of over fifty percent. However, to estimate a more accurate and long term learning curve, the manufacture of a third and fourth is considered. In this particular situation, learning can be expected to take place most significantly in the setup times for each operation. The decrease in setup time from the second nozzle to the third nozzle was taken to be the setup time from the second nozzle multiplied by the ratio of the
- 63. 63 first and second nozzles’ processing time (around fifty two percent). This rough estimate was made based on the assumption that the decrease in total process time between the first two nozzles is a good estimation for the decrease in setup times for subsequent nozzles. The same calculation was made to find the processing costs for the fourth nozzle. A learning factor of about 77% was then calculated from the total cost of the fourth and second nozzle and extrapolated up to the sixteenth nozzle. The learning curve can be seen in Figure 34. Figure 36. Learning curve in manufacturing
- 64. 64 VIII. Conclusions and Recommendations BURPG’s graphite nozzles experience damage due to thermal stresses and high temperatures. Regenerative cooling nozzles allow BURPG to use nozzles more than once, saving cost, while also improving the combustion efficiency due to the heated coolant. In order to aid BURPG in its pursuit of regenerative cooling nozzles we created a user friendly MATLAB model to test various designs, tested the viability of N2O as a coolant, created CAD models of our recommendation and built 2 prototypes to illustrate our results. The MATLAB model is user-friendly and includes an input module that allows the user to alter coolant properties, nozzle material and size, channel profiles (including square profiles) and size as well as the number of tubes. The purpose of this nozzle is to allow the user to quickly assess various regenerative cooling nozzle designs in order to then verify the findings through a more complex computational tool, before manufacturing the nozzle design. Per the request of BURPG N2O was specifically tested as a coolant and was found to be unsatisfactory for BURPG’s intended purpose. CAD models were created in order to provide BURPG with files from which to deviate or replicate in the future. The CAD models are based off of findings from the MATLAB model. These CAD models were then built in the form of an acrylic and aluminum prototype. The acrylic prototype was built first in order to test the fixturing set up and manufacturing protocol. The aluminum prototype was then built, after success with the acrylic prototype. In order to evaluate the MATLAB model as well as the prototypes, a series of comparisons and tests were conducted. The MATLAB model results were compared to a CFD analysis using the CAD model, as is the standard for this sort of project. The evaluation showed similar results, assuring us that the MATLAB model is accurate and can be trusted for its purpose. Additionally we compared our MATLAB results to completed studies in order to show the validity of the model in comparison to “real life” examples. The next evaluation was an FEA to ensure that the bolts and manifold will stay in tact under the pressure of the incoming coolant. Once again it was
- 65. 65 found that the forces and stresses experienced by the bolts are well below any critical or significant amount. Our recommendations for the future are as follows. Our FEA / CFD model can be improved by running a CHT simulation instead of separate CFD and thermal simulations. This would help model the heat transfer across the nozzle even better than currently at the expense of increasing computational requirements and user complexity. In terms of redesign and iteration from the first prototypes, there are four important areas of improvement. Firstly, the O-rings should be replaced with gaskets to create more efficient seals. Second, BURPG will need to finalize rocket designs in order to create a better interface, other than the existing NPT port, between the rocket and nozzle. There should also be an investigation in the optimal diameter of the nozzle body as well as an investigation in aerodynamics for the nozzle body. This step will need to take place when the rocket is ready to take flight. Finally the manifold should also be evaluated in order to confirm its ability to evenly distribute the coolant to all channels. In terms of the MATLAB model, it is recommended that it be used as a tool to quickly investigate and compare designs. It should not be the final step to an investigation of a certain design. This is because, while the MATLAB model is accurate, higher computational tools such as ANSYS are more capable of providing exact results before moving onto manufacturing. Throughout the duration of this semester we were able to meet all of our success points and evaluate our models extensively to ensure their legitimacy.
- 66. 66 IX. References Literature and Benchmarking References from I. Introduction and Background Sloop, John. "LIQUID HYDROGEN AS A PROPULSION FUEL,1945-1959, Hydrogen through the Nineteenth Century." History.nasa.gov. NASA. Web. 1 Jan. 2015. <http://history.nasa.gov/SP-4404/app-a1.htm>. kaszeta “What is Film Cooling” me.unm.edu . UNM. Web. Jan. 2015. <http://www.me.umn.edu/labs/tcht/measurements/what.html> Andrew Nowicki “Curtain cooling with malten salt” sci.tech-archive.net. Sci-Tech. Web. Jan. 2015. <http://sci.tech-archive.net/Archive/sci.space.tech/2005-01/0059.html> Xiaoying Zhang “Coupled simulation of heat transfer and temperature of the composite rocket nozzle wall” sciencedirect.net. Science Direct. Web. Jan. 2015 <http://www.sciencedirect.com/science/article/pii/S1270963810001306> Robert Watzlavick “100 lbf Regenerative cooled nozzle” watzlavick.com/Robert. Web. Jan. 2015 <http://watzlavick.com/robert/rocket/regenChamber/index.html> Sutton. "Ch 8. Thrust Chambers." Rocket Propulsion Elements. 268-338. Print. Sutton, George. "History of Liquid-Propellant Rocket Engines in Russia, Formerly the Soviet Union." Journal of Propulsion and Power 19.6 (2003): 1008-037. Print. Huzel. Design of Liquid Propellant Rocket Engines. 2nd ed. National Space and Aeronautics Administration. Print. Boysan, Mustafa. "Analysis of Regenerative Cooling in Liquid Rocket Engines." (2008). Print. Literature References for section III. Engineering Specifications and Relevant Basic Physics Cengel, Yunus, and Afshin Ghajar. Heat and Mass Transfer Fundamentals & Applications. 4th ed. New York: McGraw-Hill Companies, 2011. Print.
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- 68. 68 Appendix A – Bill of Materials Part Number Description Supplier Quantity ME461TEAM11BURPGNOZZ01 ROCKET NOZZLE BODY Custom 1 ME461TEAM11BURPGNOZZ02 UPPER MANIFOLD 1 ME461TEAM11BURPGNOZZ03 LOWER MANIFOLD 1 92200A642 #5-40 SCREW McMaster-Carr 8 9396K169 OUTER O-RING, UPPER MANIFOLD 1 9396K158 INNER O-RING, UPPER MANIFOLD 1 9396K164 OUTER O-RING, LOWER MANIFOLD 1 9396K148 INNER O-RING, LOWER MANIFOLD 1 Table A1. Bill of materials
- 69. 69 Appendix B – Working Drawings Figure A1. Rocket nozzle assembly drawing 2 3 1 4 NOTES: 1. NOTE THAT O-RINGS ARE NOT PICTURED BOM TABLE ITEM NO. PART NUMBER DESCRIPTION QTY. 1 ME461TEAM11BURPGNOZZ01 NOZZLE BODY 1 2 92200A642 #5-40 SCREW 8 3 ME461TEAM11BURPGNOZZ02 UPPER MANIFOLD 1 4 ME461TEAM11BURPGNOZZ03 LOWER MANIFOLD 1 5 9396K169 OUTER O-RING, UPPER MANIFOLD 1 6 9396K158 INNER O-RING, UPPER MANIFOLD 1 7 9396K164 OUTER O-RING, LOWER MANIFOLD 1 8 9396K148 INNER O-RING, LOWER MANIFOLD 134 1 OF 1 REVSIZE C PART NUMBERSHEET SCALEDATEDRAWN BY 2 3 2 14 A B C D D C B GEOFFREY MCMAHON 4/17/15 1:1 - BOSTON UNIVERSITY COLLEGE OF ENGINEERING ROCKET NOZZLE ASSEMBLY ME461TEAM11BURPGNOZZ00 DIMENSIONS IN INCHES TOLERANCES EXCEPT AS NOTED: FRACTIONS 1/32 X.X 0.030 X.XX 0.015 X.XXX 0.005 ANGLES 0 30' A TITLE
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- 80. 80 Appendix D – Analysis and Test Report Appendix D MATLAB Model User Manual Below is the user manual attached to the MATLAB model. For code please refer to the archives. *NOTE THIS FILE MUST ALSO BE IN THE SAME LOCATION AS THE GUI WORKBOOKS -If you do not wish to use the add/edit GUI’s within this main GUI, coolant and material properties can be edited from within the excel sheets that store their information. You still must interact with the “Settings.xlxs” workbook, however. W.1) Settings Workbook Because there is no built in GUI for the Combustion Settings workbook, it must be edited within Excel. Because this sheet contains all of the information on the combustion conditions of the propellant, IT IS VERY IMPORTANT THAT YOU MAKE SURE THIS SETTINGS SHEET IS CORRECT BEFORE MOVING ON TO THE REST OF THE FUNCTION. If the wrong “Settings” conformation is used, the results of this GUI will be misleading. *See the help tab of the workbook for an in-depth look at how it is used. W.2) Coolant/Material Worksheet In the case of these properties worksheets, you do not necessarily need to use them. If you find that using the built in MATLAB GUI’s is easier, the result will be the same. If you would like to use them however, see their help tabs for an in-depth look at how it is used. GUI Interaction 1) GUI Basics -All that is required to “install” this program is to unzip the contents, and make sure that the entire contents of the zipped folder are all in the SAME FOLDER. DO NOT RENAME THE COOLANT, MATERIAL, SETTINGS, README.PDF. You may rename the main function “model_gui” as you wish, but not any of the sub functions. The name and location of this information does not matter. -To Run, simply make sure that the GUI is in the path of MATLAB, and call the main function without any input. If you would like to have the function output the figure handles of the GUI, simply give the function a single
- 81. 81 output argument. -To the right are the inputs of the function. You must fill in all of the sections outlined here in order to ensure a successful analysis of your nozzle. Note that this function will try its best to error check the inputs given to it, but it is only so effective. It is likely that strange inputs (eg massflow=NaN) may be able to “sneak by” and error this code out. If this is the case, simply close the GUI and restart it. 2) Coolant 2.1) Select which of the coolants you would like to use in your channels. 2.2) If you wish to edit or add a new coolant, do so by pressing the corresponding button within the “Coolant” panel. Your options for step 2.1) will automatically update after you are finished. *Look to section 9) for more information on how to edit or add a coolant. 2.3) Input the mass flow of the coolant. Note that there are two options for the units of this value. 3) Nozzle Body 3.1) Select which of the material you would like the nozzle to be made of. 3.2) If you wish to edit or add a new material, do so by pressing the corresponding button within the “Nozzle Body” panel. Your options for step 3.1) will automatically update after you are finished. *Look to section 10) for more information on how to edit or add a material. 3.3) Input the “width” of the nozzle. Note that there are two options. -If the “Outer Radius” option is selected, then the solver will treat the outer contour of the nozzle as a constant diameter cylinder, defined by the input box below. -If the “Offset Width” option is selected, then the solver will treat the outer contour of the nozzle as having a constant width, defined by the input box below. To help visualize, it should be known that if the inner and outer radiuses we to be plotted, they would be the same shape, but the outer contour would be a distance away from the inner contour. 4) Channel Geometry 3.1) Select which cross-section type you would like the channels to have. 3.2) Input the geometry of the cross-section. Note that there are two options. -If the “Circular Cross-Section” option is selected, then the solver will solve for the use of circular cross- sections. The input D then
- 82. 82 corresponds to the diameter of this circle. -If the “Rectangular Cross-Section” option is selected, then the solver will solve for the use of rectangular cross- sections. The input a and b then corresponds to the width and height respectively. Note that the side a is in the radial direction with respect to the center line of the nozzle, where b is tangential. The figure below shows what is meant here. *To control the offset of the channel with respect to the inner contour, refer to the settings workbook. An example of this is included in the figure below. Note that it is called “d offset” in the figure. 5) Number of Channels 5.1) Decide if you would like the code to decide how many channels it can fit, or if you wish to input the number yourself. - If you allow the code to calculate, solver calculates how many channels could be fit end to end on the throat. Because this is the smallest portion of the nozzle, this value corresponds to the maximum number of channels that could be fit. It then takes 2/3 of that amount to be the number of channels, in order to leave material to keep the structure of the nozzle sound. 6) Solve 6.1) If all of the inputs are filling in and acceptable, then the solver will run and populate the axes of the GUI with the results. These results can be saved to a “.mat” file by selecting export. Before solving, make sure that you have the right “Settings” workbook for the rocket you wish to analyze. 7) Export 7.1) If the solver has been run, and the results of that solver have not yet been saved, then pushing this button will make this function export the workspace that solver used to calculate the results found. These results can be accessed easily using the “load” function. -When solver is
- 83. 83 run, a temporary file will be saved in case you want to export it. If this is exported, it will be renamed as follows, otherwise the temp file will be deleted when the GUI is closed. The name will take the form: “NozzleMaterial-Coolant-Crosssection-dateofexport.mat”. 8) Help 8.1) Push to open this pdf for guidance. 9) Add/Edit Coolant When in the add/edit coolant GUI, the following procedure should be taken. -To Add a New Coolant: 1) Input the name of the material you wish to add to the list. The GUI will not let you repeat a name. 2) Fill in the requested properties. Note the units of these values, as the solver will assume these units. -To Edit an Existing Coolant: 1) Simply edit the cell's values as you wish. The GUI will not let you repeat a name. *If anything is unclear, go to the “Coolant.xlxs” workbook. The Help tab will give you an in-depth explanation of the information requested. 10) Add/Edit Material When in the add/edit material GUI, the following procedure should be taken. -To Add a New Material: 1) Input the name of the material you wish to add to the list. The GUI will not let you repeat a name. 2) Fill in the requested properties. Note the units of these values, as the solver will assume these units. -To Edit an Existing Material: 1) Simply edit the cell's values as you wish. The GUI will not let you repeat a name. *If anything is unclear, go to the “Material.xlxs” workbook. The Help tab will give you an in-depth explanation of the information requested. 11) Run into an error? In development, I found that sometimes MATLAB gets overwhelmed with the sheer size of this code and all the processing that it requires. This is especially true because the code requires files
- 84. 84 to be loaded and saved, so it is possible for internal errors to propagate. I would recommend: 1) Close out of the function, and try to run it again. 2) If this does not work, close out of MATLAB entirely and try again. 3) If this does not work, it is possible that some part of the code or excel workbooks were accidentally edited in a way that they shouldn't have been. Try to use the backup workbooks, or go back to the original zip and reinstall the code. 4) If none of these steps fix the problem, email transegn@gmail.com with the error code and any relevant information about what you were trying to do, and I will work to fix it on the code end. Appendix D Water Flow Test Below are the numbers used to determine the volumetric flow of water during a water test. As discussed above, the Reynolds number of water and ammonia was matched using known ammonia initial conditions and properties. Figure A12. Water flow data 1
- 85. 85 Figure A13. Water flow data 2 Appendix D FEA Results and Supporting Images Pre-Load Equation: 𝐹 = 𝑐𝐴! 𝑆! (6) Where c = correction factor (0.89 for permanent connections) At = Tensile shear area Sp = proof load of bolt Steps taken for FEA 1. Enable Simulation Addin 2. Begin new static study 3. Define component material 4. Define contact surface between components 5. Define screw connections 6. Define pressure and acting surface 7. Define fixed surface 8. Mesh & Run
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