Space craft structural and functional analysis

1. Engineering MechanicsEngineering Mechanics
Presented by-
Ashruth Vallabdhas
Chandan Kumar Sinha
Prathamesh Deshmukh
Vineeth Bodapati
Chandan Yadav
Term Project
Semester II­2014
Structural and Force Analysis of RocketStructural and Force Analysis of Rocket

2. Force AnalysisForce Analysis­ Basic Forces­ Basic Forces
● Weight ­ Gravitational forces acting on
various parts of rocket.
● Thrust ­ Due to exhaust of gases making
rocket a variable mass system.
● Drag and Lift – Aerodynamic forces due to
relative movement between fluid and solid body.
(Acts on Centre of Pressure of which 'drag' and
'lift' are the components along and perpendicular
to flight movement respectively)
Source- http://exploration.grc.nasa.gov/education
Trivia- For rocket aligned with flight path, lift
force is zero.

3. Weight & Centre of GravityWeight & Centre of Gravity
●
The weight of the system
keeps on changing due to
exhaust of gases.
●
As the consequence, the
centre of gravity keeps
shifting.
●
Internal arrangement of rocket
is kept such that the centre of
gravity is as close to the nose
as possible.
Source- nasa.gov/force.model.rocket.pdf

4. Rocket Equation-Rocket Equation-
A variable mass systemA variable mass system
Source- http://www.real-world-physics-
problems.com
=Tangential component of external forces
(gravity+drag)
= Relative velocity of exhaust wrt.
rocket (const. at any time t)
(1)
● Thrust force acts in the opposite direction when the
mass is leaving the system.
If the gravity & drag is neglected, then
In that case-

5. Aerodynamic aspectsAerodynamic aspects ­ ­
● Lift and Drag forces are net forces normal and
parallel to air flow respectively.
Rocket Drag equation­
Dynamic
pressure
CD : Drag coefficient. Contains all complex dependencies
like air compressibility, viscosity body shape and
angle-of-attack.
A : Reference area, typically the base diameter of the nose.
Different A, affect the value of CD.
: Density of the atmosphere of consideration
(typically.23kg/m3
for air at sea-level).

6. The Force Factor-The Force Factor-
(A comparison between airplane and rocket)(A comparison between airplane and rocket)
Well, when we look into these two modern technologies- airplane & rocket, a novice
thought is all they do is just flying in sky consisting of almost similar forces, but there are
some significant differences in the application of forces:
Aspects AeroplaneAeroplane RocketRocket
Lift forces Used to overcome the weight.
Lift is used to stabilize and control the
direction of flight. Thrust is used in
opposition to weight.
Aerodynamic
Forces
Generated by wings and tail
surfaces.
generated by the fins, nose cone, and
body tube.
Lift to drag ratio High Low, drag is usually much greater
than the lift.
Acting Forces Magnitude and direction remains
fairly constant.
Changes dramatically during a typical
flight.

7. Center of Pressure (CCenter of Pressure (Cpp)-)-
Source- tir-33.pdf by centuri
●
Its the point where all
the air pressure acting
on rocket seem to be
concentrated. Air
pressure force
distributed on the rocket
ahead of Cp is same as
there behind it.
●
Normal air pressure
force mainly determines
the Cp while axial forces
contributes to
aerodynamic drag,
important for calculating
altitude performance of
rocket.
No role in determining

8. Center of Pressure ­Center of Pressure ­
(Theoretical calculation)(Theoretical calculation)
Assumptions-
● Rocket align itself parallely
to the air flow (angle of
attack 0)
● Speed is low as compared to
speed of sound ( mach
number<0.6)
● The structure is ideal (thin
compared to length, tip-
pointed nose, axial
symmetry, flat plates fins).
Normal force acting on rocket­
= Coefficient of normal force
accounting for shape of rocket.
= Refrence area indicating size.
Generally equals cross sectional
area at the base of nose.
= Angle of attack
● In general,the normal force on the
nose is identical for all shapes while location
on nose varies with each different nose shape.
Source- tir-33.pdf by centuri

9. Stability Criteria-Stability Criteria-
● As told earlier, the lift force is zero
when rocket is aligned along flight
path.
● But what if it isn't?
● Now the 'angle of attack' comes into
role affecting the size and shape of
the normal force distribution.
● The consequent normal force
developed acting on centre of
pressure produces a torque about
centre of gravity in order to
decrease .
● The distance between and is called
'static margin'. It should be larger in order
to bring to zero proportionally faster.

10. Stability CriteriaStability Criteria
.-cont.-cont
● The important conclusion drawn
from it­ Centre of pressure should be
behind the Center of gravity.
● As rocket's angle of attack increases,
the moves forward, simultaneously
causing unstability in the rocket.
● But why the produced torque act
about Centre of gravity only?
● In free flight, any body rotates about
its centre of gravity.
Source- tir-33.pdf by centuri
Source- https://encrypted-
tbn3.gstatic.com/images

11. More about structural analysis­More about structural analysis­
(Fuselage structural strength) (Fuselage structural strength)
● Why do the shape of fuselage cylindrical mainly?
– High strength of thin-walled aluminium cylinder
loaded primarily in axial compression.
– structurally efficient, carrying the applied load in
such a manner that the load is evenly distributed,
resulting in an even stress level throughout the
wall.
– The applied stress in cylinder walls is given by-
P=applied load (lbs), D=avg. Fuselage diameter,
t=wall thickness.
– With L/D ratio > 15, bukling failure occurs.
Source-http://www.nakka-
rocketry.net/fusestru.html

12. An Example ­An Example ­
http://www.nakka-
rocketry.net/fusestru.html
So, if we take a model rocket with specifications-
L= 20 inch, wall thickness t = 0.016 inch,
the average diameter D = 2.234 inch
● Fuselage strength-
If the fuselage is made up of aluminium, then
compressive yield strength is about 15000psi.
So max. allowable axial load that can be handled-
● Fuselage loading-
1.Calculating the aerodynamic force using earlier
formula-
2.Additionally, inertial force acts here with m=2.0lbs &
gmax
=50 g's
Total compressive force,
Which is much less than previously allowableWhich is much less than previously allowable
compressive foce, thus having wide margin of safety.compressive foce, thus having wide margin of safety.