propulsion and launch systems

Question 1

define primary propulsive system

Answer 1

launch vehicle motor

Question 2

define secondary propulsive system

Answer 2

systems induced on the payload or satellite intended for in-orbit operations

Question 3

what are apogee boost systems used for?

Answer 3

used to move satellite to the orbit

Question 4

what is the basic principle behind rocket motors

Answer 4

momentum exchange

Question 5

when is the ideal rocket equation valid?

Answer 5

• high thrust to weight ratio
• little to no external forces
• thrust duration is negligible in comparison to overall flight time (impulsive)
• nozzle exit flow ideally expanded
• thrust vector = flight vector (no gimballing)

Question 6

define impulsive burn

Answer 6

burn time negligible compared to flight time

Question 7

what do we need to consider for a rocket rising through the atmosphere?

Answer 7

additional forces acting on rocket from atmospheric pressure and atmospheric gas drag

Question 8

why do we want the exhaust plume exiting the rocket to be fully expanded?

Answer 8

if it is not fully expanded, we have an additional force term due to pressure differential between exhaust and atmosphere

Question 9

i.e. Pe < Pa, Pe = Pa, Pe > Pa

describe expansion conditions

Answer 9

• Pe = Pa: fully expanded = all exhaust flow exits in direction of thrust vector
• Pe < Pa: over expanded = some exhaust flow exits in a direction off axis to the thrust
• Pe > Pa: under expanded = vector resulting in a reduction in the overall thrust for the motor

Question 10

why is the total impulse useful?

Answer 10

assessing if a propulsion system can provide the energy required by the mission profile

Question 11

define specific impulse

Answer 11

ratio of total impulse of the system to the weights of the fuel expanded

Question 12

fill the missing word: the higher the specific impulse of a rocket motor, the ——— the mass flow rate of the propellant

Answer 12

lower

Question 13

define propulsive efficiency

Answer 13

ratio of power of the vehicle, relates the rate at which work is done propelling the vehicle to the rate at which kinetic energy is added to the exit flow of the rocket motor

Question 14

list the losses and gains experienced by a prograde orbit

Answer 14

• gravity loss
• drag loss
• earth spin gain

Question 15

true or false: velocity increments can be summed to give total velocity increment required?

Answer 15

true

Question 16

define multistage rockets

Answer 16

series of individual vehicles or stages, each with their own structure, tanks, engines, that accelerate the payload before being detached

Question 17

disadvantages of multistage rockets?

Answer 17

• requires vehicle to lift motors and tanks that are not used until later
• entire rocket system much more complex = greater risks and higher costs

Question 18

advantages of multistage rockets?

Answer 18

• by jettisoning the empty stages, the mass is subsequently decreased, allowing the thrust of the remaining stages to accelerate the rocket more easily to the final required velocity
• for a given rocket mass, a multistage system is more efficient than single stage

Question 19

define launch window

Answer 19

a given launch site is only capable of launching directly into an inclination which is greater than or equal to the latitude of the launch site

Question 20

if L > i….

list the number of possible launch windows

Answer 20

• if L > i: no launch windows
• if L = i: one launch window
• if L < i: two launch windows