We’re experiencing an emergence in space systems. When space exploration began, a computer was the size of an entire room, but now each one of you has one that fits in the palm of your hand.
It should be no surprise, then, that satellites are getting smaller as well. Certainly there is still use for satellites which are of a large scale. OSIRIS-REx just launched for Bennu, and it required over 1,000,000 lbf of thrust to achieve the velocities required to travel there.
But that is not the case for ALL satellites.
Indeed there is a place for satellites which are far smaller in scale. We know this place to be NewSpace. A space driven by cost, and full of easy access to custom orbits, not just the 200 mile orbit we get when Chris Cassidy tosses your CubeSat out the space station window.
So people have managed to get pretty good at reducing the size of satellite hardware, but we haven’t done much to reduce the size of the rockets CARRYING CubeSats. So the main goal of NewSpace remains unfulfilled: access is still very limited. This is because costs remain high. It costs $100,000 per cube to piggyback to an orbit of someone else’s choosing.
The trouble is that we’re building big rockets, but there is room to build smaller.
That’s where the Portland State Aerospace Society, or PSAS comes in.
The mission for PSAS is to push forward the development ultra-low cost orbital space systems. Of course a lot of people are attempting to do that. The difference here is that PSAS is attempting to open-source everything, and because there are a lot of enthusiastic computer science and electrical engineers who are really interested in donating their time (and degrees) to space; PSAS happens to be very high quality. You could think of it as Professional by amateurs.
Of course they’re amateurs with a lofty goal. The declared goal is to put a CubeSat into orbit.
Now, I say that with my tongue planted firmly in my cheek. We are all aware that this goal is currently next to impossible. But it’s with this in mind that all of the systems are designed. For PSAS, it’s just not okay to use cheep mechanical tricks that have 90% success rates to pop parachutes. For one, they’re not interested in cleaning up any more rockets off the ground of the Oregon desert; but for two that’s just not the kind of system that would exist on an orbital rocket, so it’s not the kind of system which will exist on their rocket.
The emphasis is honestly on showing the world that just because space is hard, doesn’t mean it’s impossible, especially when you’re taking the position that we can collectively solve huge problems when we cooperate in an open-source environment
There are still some issues with meeting that goal. They don’t have a pool of talented mechanical engineers. PSAS has been launching its rockets with off the shelf solid motors, and has buying the biggest engines available which are still reasonable. Unfortunately they’re just not big enough. Solid motors just won’t fly.
Hybrid motors aren’t much different. They don’t seem to have the impulses required to create an orbital system.
The truth is that the answer is something that all the big kids have been using the whole time we’ve been going to space routinely: It’s time to develop a liquid fuel engine.
Of course there is a big problem with this. Engine design takes a specialized knowledge so vast that it begs a trope that most of you have probably heard way too much:
“it’s not rocket science.”
It’s not very accurate, maybe propulsion engineering would be better. Still - that’s used to describe something that could be hard, but isn’t designing rockets.
Unfortunately, this is designing rockets. Designs are difficult, and mathematically cumbersome; there are way too many variables to just solve for any particular one, so getting them to converge is difficult and takes many iterations.
So designing an engine is HARD.
Iterating an engine is HARD.
Liquid fuel presents even more new challenges too,
So designing liquid fuel engines is HARDER
And iterating a liquid fuel engine is harder, too.
That’s why we say the same thing every single time something goes wrong on the way to space.
Space is hard.
The goals for PSAS then were simple,
They needed to develop tools to make engine design easier
They needed to develop a process which is easy and quick to iterate
Of course a pretty big problem still remained. - Where were all the mechanical engineers?
That’s where we came in. A team of 6 of us were feeling pretty ambitious about finishing our undergraduate degrees at Portland State with a bang, and we were approached by PSAS with a capstone, the magnitude of which probably didn’t really resonate with us.
Our goal was to develop a liquid fuel engine and to heavily document the design process. This started in roughly October of 2015.
Since then we have developed a tool which allows for the design of engines with a minimal knowledge of the mathematical specifics of engine design, we called it the Liquid Fuel Rocket Engine design document, or LFRE because we love acronyms as much as the next engineer and we had spent our creativity elsewhere.
we’ve developed a similar document to aid in the sizing and design of a straightforward pintle injector,
We’ve 3D printed an actual engine
And we’re excited to test our engine soon, so that we may also test the validity of our design documents. It’s not quite as sexy to say that designing plumbing for a test stand fuel delivery system is hard, but I’m sure you’re all aware that real engineering starts after the excel spreadsheets shut down, and that the devil is really in the details of these systems. You should definitely be keeping an eye out to the PSAS twitter feed, the test will be exciting whether we created an engine or a bomb.
Still, it shouldn’t be missed that all of this is open sourced.
Every single one of these design documents is available, right now, to you and everyone with an internet connection and access to GitHub.com
All of the design documents were written in Jupyter notebooks, which are just python driven documents with the main feature of being able to execute code live.
This means that all of our work is open source, accessible and editable. You can use this right now to make an engine yourself, you could even edit it yourself and show us how it’s really done.
The main design document is the Liquid Fuel Rocket Engine or LFRE document. With this tool a designer can input some parameters, things like a desired thrust, and a chamber pressure; as well as some properties that are dependent on the propellant combination chosen.
These parameters can be obtained by some tools that NASA engineers have made public.
And then, with a single click, a rocket engine can be generated based on these inputs.
LFRE does this by first taking the information provided by the user to determine basic geometry and flow parameters; things like the areas of the throat and exit of the nozzle.
It then uses these new parameters to determine the heat being passed through the nozzle throat, which allows for a reasonable calculation of the wall thickness.
And here’s where it gets interesting. After finding a wall thickness, parametric equations are constructed which justify the entire nozzle. The whole contour of the nozzle, the inner wall, the inner cooling channel wall, the outer cooling channel wall, and the outer surface of the nozzle are all represented by these equations.
And since the nozzle is uniquely defined, a finite element analysis is conducted to determine whether or not the coolant will be boiling before it makes its way through the cooling channels, as well as determine all of the properties of the flow along the length of the nozzle. This gives a designer a huge wealth of information with which they may be able to tweak subsequent iterations.
It lets a designer know that, if they don’t have enough heat transfer, they need to start tweaking their initial assumptions in order to find more. Perhaps shortening the nozzle, and sacrificing some efficiency in order to lower the nozzle surface area which must be cooled.
It means that each iteration happens seamlessly. There is no reworking a design from the ground up because new geometries have to be tested.
LFRE is kind of like the amateur spreadsheet on steroids. A designer can see the results of their edits immediately, they can see the changing stresses with each subsequent iteration.
And when they’re done iterating, and have decided that they want to make their engine a reality, the parametric equations can quickly be transferred into drafting software, and a print can be made.
Design flows like this used to be on the scale of years. Regenerative cooling channels were enormously constrained by the machining methods available to create them. An extraordinarily skilled laborer would be required to braze channels together, and mistakes were costly.
But now the design geometry is trivial. It’s silly to NOT include these features. 3D printing is truly revolutionary, and because the machining is no longer such a resource heavy endeavor; a tool like LFRE isn’t just convenient, it’s necessary.
The result of all of this, is something like this…
This is a cross sectional rendering of the engine we 3D printed.
It is designed to produce 500 lbf of thrust.
It uses a liquid oxygen / ethanol combination for propellants.
It was printed out of a high silicon, high temperature aluminum alloy.
It uses supplemental film cooling.
It features 82 regenerative cooling channels.
The studied rocket design engineers among you are probable shouting in your heads right now: “why is the cooling channel thicker at the throat! The heat transfer is highest there!”
But upon closer inspection you can see that the aspect ratio of the channels is changing.
At the throat, the channels are tall and thin. As the nozzle expands outward, they get short and wide.
This isn’t a common design for regenerative engines, imagine the process that has to occur in order to create this geometry. Previous regeneratively cooled engines relied on varying the distance between each channel, with a relatively uniform channel cross section. Drawing processes could mitigate this, but compromised a uniform wall thickness in the process.
Accounting for the effect of the machining process took costly electroplating procedures.
The fixes were just patches to cover up limitation in the PROCESS. The machining process was just not ready to realize the kinds of designs that could lead to real breakthroughs in efficiencies. Certainly the kind of machining required was extraordinarily cost prohibitive for an engine of relatively small magnitudes.
But now we can confidently say that we are no longer limited by geometry. If there are any current limitations, they lie in the resolution of our 3D printers, which will certainly only become better with time.
Do you want to build complex flow geometries within the cooling jacket? Your imagination is the only limitation. Creating the geometry itself is TRIVIAL.
For us, our imagination took us toward these varying aspect ratios, which are automatically calculated for a designer.
The main feature of the parametric equations is that they ensure a constant cross sectional flow area for the coolant. Meaning that the velocity through the channels is roughly uniform.
Of course if we consider the SHAPE of the cooling channels at any particular point, the hydraulic diameter may be increasing or decreasing, resulting in varying flow velocities. Future versions of the tool indeed may include equations with the property of constant HYDRAULIC diameter, rather than constant cross sectional area, although this is quite a large mathematical lift.
Nonetheless, this allows for a designer to simply scale these equations in order to directly control the velocity of the coolant.
Left unchanged, the hydraulic diameter will decrease at the throat, leading to higher (and more desirable) velocities there.
The scope of our project also included an injection system. We chose a pintle injector because they were a straightforward, interchangeable design which allowed us to easily avoid extra manufacturing or the welding associated with injector plates. Other injection methods can be used though.
The document inputs are simply the number of holes, the diameter of those holes, the propellant properties, and the some outputs from the LFRE document.
Basically, after the initial design of the engine, there won’t much complicated sizing work in order to find out what orifice sizes to use for the pintle.
It also becomes easy to find the momentum ratio of the impinging propellants, as well as the blockage factor.
Here you can see the pintle mated to the combustion chamber. The main concerns with this design are the machining tolerances for the annulus gap, and the size of the impinging orifices, which become much more difficult to control the smaller the engine gets, especially since research concerning pintle design seems to be pretty difficult to find.
This is our final product.
A 3D printed, aluminum, regeneratively cooled engine, designed to produce 500 lbf of thrust and ready for testing.
Don’t forget to click the picture to see the live 3D model
So what does this all mean?
For our team, it means that we can generate a design quickly.
It means we can iterate that design quickly.
And it means we can machine that design quickly.
And because none of us really believe that any of our models do much more than get us into the ballpark when it comes to designing rockets, iteration and testing is the name of the game. We can churn through 100’s of designs in a short amount of time just by playing with single parameters. So now that it’s fast and cheap to design and machine, there is new space for these smaller scale engines which didn’t exist before.
And - It doesn’t stop here. Testing is the next step. It’s what turns this from a speculative exercise, into reality. A lot of people fantasize about being real deal in this field but don’t possess the willpower or skill to make anything out of it. The testing and PROOF of concept is where the rubber meets the road.
But it means much more than that.
It means that ENTREPRENUERS can now find their way over to NewSpace. Certainly aerospace engineers are important - a simple spreadsheet on steroids doesn’t replace the kind of talent here in this room; but designing a space system just can’t take a team of THOUSANDS of engineers in order for NewSpace development to be cost effective, or enticing to people with great ideas but no team to back them up.
Because engines can now be developed on a small scale reliably by small teams, we really believe that development costs can be brought down by an order of magnitude. We designed this entire utility, and printed our first engine on a budget of less than $10,000. In fact, even without donations, we would be able to print an engine for roughly 2-3 thousand dollars, all the while using geometries that would have cost millions of dollars using traditional machining methods.
Which means that professional quality, is now obtainable by amateurs. We did not have any engine design background before embarking on this. We were just a team of students with a lot of drive and just enough recklessness to try something really big.
And if we can do it, then so can you. I don’t mean the Aerospace Engineers already specializing in propulsion in the room, I mean you; the person with doubts over whether or not they’re good enough to do this.
And you can finally say, with confidence to your buddies when you’re out drinking beers; the very coveted phrase once limited to only the PhDs:
“It’s not rocket science”
LIQUID FUEL ENGINES
NEW SPACE | CUBESATS
Equipment is getting smaller
Focuses on low cost access to space
Cubes are the unit of NewSpace
Current costs are roughly $100,000 per unit
CubeSats need a reasonable delivery system
PORTLAND STATE AEROSPACE
SOCIETY (PSAS): #GOALS
Develop ultra-low cost space systems
Open source everything
Professional by amateurs
Place a satellite into orbit
Severe mechanical engineering volunteer deficiency
Off the shelf solid motors just won’t fly
Hybrid motors won’t be much different
Solution: Develop a liquid fuel engine
Engine design is hard
Iterating an engine design is hard
Liquid fuel engines are harder
Iterating a liquid fuel engine design is harder
Develop tools to make engine design easier
Develop a process which is easy and quick to iterate
Goal: Develop a liquid fuel engine and document the design process
1. Liquid Fuel Rocket Engine (LFRE) design document
2. Pintle Injector design document
3. 3D Printed engine
4. (Coming Soon) Test and verification of design document accuracy
GITHUB | JUPYTER NOTEBOOKS
GitHub is used for version control and sharing of open source
Design documents are hosted on GitHub (github.com/psas)
Design documents are driven by Jupyter Notebooks
Jupyter Notebooks are python driven documents with the main
feature of being able to execute code live
Everything is open source, everything is accessible and editable
LIQUID FUEL ROCKET ENGINE
Utilizes parameters obtainable by NASA’s CEArun tool
Calculates heat transfer parameters
Determines wall thickness based on heat transfer at throat
Utilizes wall thickness to construct parametric equations
FEA analysis of the final geometry to determine if boiling will occur
3D PRINTED ENGINE
500 lbf thrust
High temperature aluminum alloy
82 regenerative cooling channels
SO WHAT DOES THIS MEAN?
Engine design is not limited only to aerospace engineers
The successful utilization of these kind of technologies can bring
space flight costs down by an order of magnitude
Professional quality can be obtained by amateurs
You can be a design engineer now
“It’s not rocket science”
Machine Sciences Corporation
Derek Tretheway (Portland State University)
Robert Watzlavick (Rocket Moonlighting)
Armor Harris (Space X)
Oregon National Guard