Fundamentals of Propulsion

Question 1

What are the 4 propulsion technologies?

Answer 1

Cold-gas, monopropellant, bipropellant, solid motor

Question 2

Advantage of Cold-Gas?

Answer 2

simplest and least expensive. provide multiple restarts and pulsing.

Question 3

Disadvantage of Cold-Gas?

Answer 3

Low specific impulse, low thrust levels, high weight

Question 4

Advantage of Monopropellant Systems?

Answer 4

Supply Pulsing or steady-state thrust. Typically Hydrazine.

Question 5

Advantage of Bipropellant System?

Answer 5

High specific impulse and wide range of thrust capability. Pulsing and steady-state modes.

Question 6

Disadvantage of Monopropellant System?

Answer 6

high total impulse with restart

Question 7

Advantage of Solid Motor Systems?

Answer 7

All of the impulse is to be used in a single burn and impulse can be accurately calculated. simple, medium performance, & costs are comparable to monopropellant.

Question 8

What is thrust?

Answer 8

Generated by momentum exchange between the exhaust and the vehicle and by the pressure imbalance at the nozzle exit

Question 9

What is the Thrust equation due to momentum?

Answer 9

Fm=mdot(ve-vo)
mdot: mass flow rate of propellant or exhaust gas slugs/s
Ve= average velocity of exhaust gas, fps
v0= initial velocity of gasses fps

Question 10

What is the thrust equation due to pressure?

Answer 10

if ambient pressure is not zero: Fp=(Pe-Pa)Ae
Fp: Thrust due to exit plane pressure
Pe: static pressure in exhaust gas
Pa: ambient static pressure
Ae: are of the nozzle

Question 11

Fundamental Thrust equation?

Answer 11

Thrust= Thrust due to Momentum + thrust due to Pressure

Question 12

What are the 7 assumptions that need to be defined to determine theoretical performance?

Answer 12

1. Propellant gasses are homogeneous
2. Propellant gasses follow the perfect gas laws
3. No friction at nozzle walls or no boundary layer
   4.There is no heat transfer at nozzle wall
4. Flow is steady and constant
5. All gasses leave the engine axially
6. Gas velocity if uniform across and section normal to nozzle axis

Question 13

What are the 11 performance parameters you can find using theoretical performance assumptions.

Answer 13

1. Conditions at Any Nozzle Section
2. Critical Pressure Ratio
3. Gas Velocity
4. Specific Impulse, Isp
5. Total Impulse, I
6. Weight Flow Rate
7. Area Ratio
8. Characteristic Velocity, C*
9. Thrust Coefficient, Cf
10. Mixture Ratio, MR
11. Bulk Density

Question 14

What are the conditions at any nozzle section tell you?

Answer 14

Velocity of gas, Absolute temperature of gas, area of the nozzle, & absolute pressure of the gas

Question 15

What is critical pressure ratio?

Answer 15

the pressure ratio required for sonic, or choked, flow at the throat. The equation shows that critical pressure ratio is a pure function of specific heat ratio. Example, For an engine operating at sea level, a chamber pressure of about 35 psia is adequate to assure supersonic flow in the nozzle. The higher the altitude, the lower the chamber pressure for sonic flow.

Question 16

What is gas velocity?

Answer 16

an expression for velocity at any plane in the expansion section of the
nozzle in addition the equation show that exit velocity, and hence thrust, increases as chamber temperature and gas constant increase. The pressure ratio and specific heat ratio are minor influences.

Question 17

What is specific Impulse Isp?

Answer 17

rocket performance. It is defined as the thrust per unit weight flow rate of propellant. conventional method of comparing propellants, propellant combinations, and the efficiency of rocket engines. Usually measured in seconds
Isp= F/mdot*g

Question 18

What 5 things do you need to establish Isp?

Answer 18

1. Chamber pressure
2. Area Ratio
3. Ambient Pressure
4. Shifting or frozen equilibriums
5. is it real or theoretical

Question 19

What is total impulse?

Answer 19

Change in momentum caused by a force acting over time, I=Ft

Question 20

What is weight flow rate?

Answer 20

mass of a substance which passes per unit of time. The equation shows, for a given propellant and stagnation temperature, the
flow rate through a nozzle is proportional to chamber pressure and throat area and those parameters only

Question 21

What is area ratio?

Answer 21

the ratio of the exit area to the throat area or the measure of the gas expansion

Question 22

What is the characteristic velocity C*

Answer 22

rocket parameter used to separate the thermochemical performance of propellants from the performance of a particular engine. it measures combustion performance by indicating how many pounds of propellant must be burned to maintain chamber pressure. It is independent of the performance of the nozzle and does not vary with ambient pressure

Question 23

What is Thrust coefficient Ct?

Answer 23

proportional constant between thrust and the product of chamber pressure and throat area. improvement in thrust provided by the nozzle is characterized by thrust coefficient

Question 24

What is Mixture ratio? MR

Answer 24

ratio of oxidizer to fuel flow rate on a weight basis. Each propellant combination has an optimum mixture ratio that produces maximum
Isp.

Question 25

What is Bulk Density?

Answer 25

Bulk density is the mass of a unit volume of a propellant combination “mixed” at the appropriate mixture ratio