Theory for Exam Flashcards
What are the three main components of a typical rocket motor ( brief explanation)
- Injector- injects propellants into combustion chamber in right proportions and conditions to yield an efficient, stable combustion process
- Combustion chamber- place where the burning of propellants takes place. The function of chemical energy into thermal energy; obtaining high pressure and temperature
- Converging- diverging nozzle: where combustion gases are expanded and accelerated. Convert thermal energy generated in the combustion chamber into kinetic energy the flow
Numerate types of propulsion systems
- Booster
- Main
- Upper stage engines
- Satellite and altitude control systems
Explain term “pogo oscillations”
Self-excited vibration in liquid rocket engines caused by combustion instability.
The unstable combustion results in variations of engine thrust therefore variations of acceleration on vehicles flexible structure which in causes variations in propellant pressure and flow rate, closing self-excitation cycle.
How is specific impulse defined ?
Indicates how many kilograms of thrust is obtained by the consumption of one kilogram of weight propellant per second:
Isp = T/m_dot_p * g0
Why does the parameter contain the word impulse ?
Can also be defined via impulse of force. Specific impulse equals the impulse of thrust delivered to rocket in time interval delta-t divided by propellant weight consumed in time interval.
Why is the specific impulse an important characteristic of a thruster ?
Measure of how efficiently a rocket engine is using propellant to create thrust
What does “effective exhaust velocity” mean and how it is related with specific impulse ?
Defined as a thrust divided by propellant mass flow rate.
C= T/m = ve+ Ae/m_dot_p (pe-pa)
Can be expressed through specific impulse c= ispg0
Thrust T of a rocket motor can be described by following equation
M dv/dt = m_dot ve+ (pe-pa) A
Name all parameters in this expression
M dv/dt = thrust force
M = mass flow rate of propellant
Pe = pressure at nozzle edge
Pa = atmospheric pressure
A = nozzle area
V = rocket velocity
Ve = expelled propellant mass velocity ~ exhaust velocity
Under what nozzle condition is the thrust T of rocket motor maximised
It is under condition pe=pa that the thrust is maximised and the nozzle is said to be at its optimum expansion
The change in velocity for a single stage rocket is written as delta-v = Ispg0 ln(mo/mf)- delta-vD - delta-vG. What are the parameters and what do they physically represent ?
ISP- specific impulse of rocket- defined as thrust per sea level weight rate of propellant consumption
Delta-vD- D dt- drag loss
Delta-vG- mgcosy dt- gravity loss
M0 = initial mass of rocket
Mf = final mass of rocket
Explain term “sounding rocket”
(Research rocket) Instrument carrying rocker designed to take measurements and perform scientific experiments during its sub-orbital flight
Describe main launch phases of a single stage rocket
- Boost phase - burning propellant
Max velocity achieved and height achieved at burnout - Coast phase- post propellant burnout
Time taken for the rocket to achieve zero velocity
Distance travelled during deceleration - Recovery- return to earth
What actions are taken to minimise the effects of dynamic pressure and induced lifting forces?
Throttle back through q-max - all rockets throttle back through q-max to minimise aerodynamic loads
Implement gravity turn manoeuvre so the area presented to atmosphere is just cross section of rocket - frontal area.
Explain term “gravity turn”
Manoeuvre used in launching spacecraft into an orbit around.
Trajectory optimisation that uses gravity to steer vehicle into its desired trajectory.
Thrust does not have to be used to change spacecrafts direction thus more of it is used to accelerate vehicle into orbit.
During initial ascent phase , vehicle can maintain low or zero aoa. This minimises aerodynamic stress on the launch vehicle thus allowing for lighter launch.
Explain why gravity turn manoeuvre is used as the ascent strategy for rocket launch into space
Designed to be strong in lengthwise compression , like a column.
To save weight, made relatively weak in bending, which are loads induced by lifting forces.
Holding aoa constant during early part of flight reduces lift forces on rocket this reduces velocity losses due to drag- maximising velocity gain.
This means rocket is axial this rocket is presented just the cross section due to velocity vector being axial. As aoa is zero , negative lift forces tend toward zero significantly reducing aerodynamic stress on rocket .
