Rocket Propulsion - Definitions and Fundamentals Quiz(MCQ)

1 .

Which of the following is not an air-breathing engine?

A)

Rocket

B)

Turbojet

C)

Turbofan

D)

Scramjet

Correct Answer : Rocket

Explanation : Unlike turbojet, turbofan, scramjet, ramjet or turboprop engines, rockets are not air-breathing engines. It carries both fuel and oxidizer along with it, while the other engines suck in air to undergo combustion with the stored fuel.

2 .

Which of the following is not a valid classification of rocket engines based on their energy source?

A)

Solar

B)

Nuclear

C)

Chemical

D)

Geothermal

Correct Answer : Geothermal

Explanation : Chemical, solar, nuclear, electric are all different types of rocket engines based on their energy sources. But geothermal energy source is limited locally on earth and can’t be harnessed for the whole course of the rocket’s journey.

3 .

Consider two rocket engines A and B with A having a specific impulse of 242s and B having a specific impulse of 150s. What can be said about their efficiency?

A)

B is more efficient than A

B)

A is more efficient than B

C)

Both A and B are equally efficient

D)

Efficiency doesn’t depend on specific impulse

Correct Answer : A is more efficient than B

Explaination : Higher specific impulse will result in better efficiency. Rocket A has a higher specific impulse than rocket B. Hence it is more efficient.

4 .

Given that the total impulse is 25000 Ns and specific impulse is 300s, find out the effective propellant mass for a burn duration of 8s.

A)

8.5 kg

B)

17 kg

C)

34 kg

D)

68 kg

Correct Answer : 8.5 kg

Explaination : Given Isp = 300s and I_t = 25000 Ns.
Using Isp = I_t/m_pg_o, with g_o = 9.8 m/s².
m_p = 25000 / (9.8 x 300) = 8.5 kg.

5 .

For a rocket, the specific impulse was found to be 250s. Given that the thrust force is constant and assuming gravity-free and drag-free conditions, what will the thrust force be for a mass flow rate of the form (4t²-3t) kg/s and a burn duration of 3s?

A)

12426 N

B)

15595 N

C)

18375 N

D)

25234 N

Correct Answer : 18375 N

Explaination : Specific impulse Isp = ∫Fdt/(g_o∫mdt) where m is mass flow rate, go is standard acceleration due to gravity at sea level.
Then Isp = Ft_b/g_o(4t_b³/3 – 3t_b²/2).
Using t_b = 3s, Isp = 250s, g_o = 9.8 m/s²,
F = 250 x 9.8 x 22.5/3 = 18375 N.

6 .

What is the range of temperature of reaction product gases in a rocket engine?

A)

500 to 1700 °C

B)

1700 to 2500 °C

C)

2500 to 4100 °C

D)

4100 to 6900 °C

Correct Answer : 2500 to 4100 °C

Explaination : The temperature at which product gases of the chemical reaction taking place within the combustion chamber is too high. It is even higher than the melting point of some of the more commonly used materials in aircraft industry like aluminum (660 °C). Appropriate cooling mechanisms are set up to ensure that this high temperature doesn’t transfer over to the rocket components and that it doesn’t lead to the development of thermal stresses.

7 .

Which of the following pressure feed systems is mostly used in attitude control of liquid propellant rocket engine with more than one thrust chamber per engine?

A)

Pump-fed system

B)

No feed system is used

C)

Gravity assisted feed system

D)

The Gas pressure feed system

Correct Answer : The Gas pressure feed system

Explanation : For attitude control of such vehicles, the thrust, as well as the total energy of the propulsion system should be low. Gas pressure feed system is suitable in such circumstances.

8 .

What is the typical liquid propellant feed system used in space launch vehicles?

A)

Pump-fed system

B)

Gravity assisted feed system

C)

The Gas pressure feed system

D)

No feed system is used

Correct Answer : Pump-fed system

Explanation : In vehicles used for space launch, the thrust required and the overall energy of propulsion required is too high. In such cases, the rate of supply of propellants to the combustion chamber will be high. A pump fed system is capable of generating a large mass flow rate of the propellants. Hence it is used here.

9 .

For a constant thrust of 6000 N, find the total impulse for the burning time of 3s.

A)

0 Ns

B)

9000 Ns

C)

18000 Ns

D)

36000 Ns

Correct Answer : 18000 Ns

Explanation : Here we assume that the start and stop transients are negligible during the operation of the system. Then, for a constant thrust (F), and for the total burning time of t seconds, the total impulse I_t = Ft.
I_t = 6000 x 3 = 18000 Ns

10 .

Suppose the thrust force of a rocket varies with time t as F = 250(3t² + 2t). Then find out the total impulse for a burn duration of 4s.

A)

10000 Ns

B)

20000 Ns

C)

40000 Ns

D)

80000 Ns

Correct Answer : 20000 Ns

Explaination : The total impulse can be obtained by integrating the force F with respect to time t from 0 to 3. The total impulse for a varying force F is obtained by I_t = ∫Fdt.
I_t = 250 x (t_b³ + t_b²) = 250 x (64 + 16) = 20,000 Ns.

11 .

If the exit pressure is thrice the atmospheric pressure for a rocket A and if it is twice for rocket B, what will be the ratio of pressure thrust of rocket A to that of B?

A)

1

B)

2

C)

3

D)

4

Correct Answer : 2

Explaination : Pressure thrust is the difference between nozzle exit pressure and ambient pressure multiplied by nozzle exit area.
P_e = 3P_a for rocket A
P_e = 2P_a for rocket B
Pressure thrust T_p = (P_e-P_a)A_e
So T_pA/T_pB = 2P_a/P_a

12 .

In the expression of pressure thrust, what does the area (A) represent?

A)

The cross-sectional area of the throat

B)

The cross-sectional area of the nozzle exit

C)

The total surface area of the rocket casing

D)

The total surface area of the combustion chamber

Correct Answer : The cross-sectional area of the nozzle exit

Explanation : Area (A) represents the nozzle exit cross-sectional area. The expression for pressure thrust is (P_e-P_a)A, where P_e is the exit pressure and P_a is the ambient pressure.

13 .

What will happen to thrust and specific impulse as the rocket is propelled to higher altitudes?

A)

Thrust decreases, specific impulse increases

B)

Thrust increases, specific impulse increases

C)

Thrust increases, specific impulse decreases

D)

Thrust decreases, specific impulse decreases

Correct Answer : Thrust increases, specific impulse increases

Explanation : Atmospheric pressure decreases with increasing altitude. So the pressure thrust part of the total thrust increases and hence the total thrust increases. Specific impulse is impulse per unit weight of the propellant and proportional to thrust, so it too will increase.

14 .

The rocket nozzle is usually so designed such that the exhaust pressure is ____ the ambient fluid pressure.

A)

equal to

B)

less than

C)

greater than

D)

equal to or slightly higher than

Correct Answer : equal to or slightly higher than

Explanation : Exhaust pressure should be such that it is equal to or slightly greater than the atmospheric pressure. Else the pressure thrust will be negative and lead to low overall thrust.

15 .

How does the total thrust vary with an extendable nozzle?

A)

Keeps on increasing

B)

Keeps on decreasing

C)

First increases, then decrease

D)

First decreases, then increase

Correct Answer : First increases, then decrease

Explanation : Initially as the nozzle is extended, thrust keeps increasing. This is because of the rate of increase in exhaust velocity being high compared to the decrease in exit pressure. This increase happens until the point where exit pressure equalizes atmospheric pressure. Afterward, the decrease in pressure thrust is more significant and as a result of this, the total thrust also decreases.

16 .

Consider a rocket engine A and a jet engine B. Which of them will have higher specific impulse?

A)

A

B)

B

C)

A doesn�t have a specific impulse

D)

B doesn�t have a specific impulse

Correct Answer : B

Explanation : B will have much higher specific impulse because it uses large amounts of outside air for combustion. Since specific impulse is impulse per unit amount of propellant and the stored propellant in jet engines is only the fuel (and not both fuel and oxidizer like in rocket engines), it will have a higher specific impulse.

17 .

If exhaust pressure is P_e and surrounding fluid pressure is P_a, what is the condition for the optimum expansion ratio of a rocket nozzle?

A)

P_e=P_a

B)

P_e<P_a

C)

P_e>P_a

D)

P_e=0.5*P_a

Correct Answer : P_e=P_a

Explaination : For the optimum expansion of the nozzle, exhaust pressure should be equal to the ambient pressure. When that happens, the size of the rocket exhaust plume matches the size of the nozzle and losses are minimum.

18 .

A rocket produces a total thrust of 20,000 N. If the mass flow rate of propellants is 16 kg/s, what will be effective exhaust velocity?

A)

625 m/s

B)

1250 m/s

C)

2500 m/s

D)

5000 m/s

Correct Answer : 1250 m/s

Explanation : Total Thrust = mass flow rate x effective exhaust velocity. Here we assume that the mass flow rate is constant, flow is axially directed and frictional effects are neglected.
So, 20,000 = 16 x u_eff
u_eff = 20,000/16 = 1250 m/s.

19 .

What is the nozzle throat diameter of a rocket having chamber pressure (Pc) of 3 MPa, mass flow rate (m) of 20 kg/s and characteristic velocity (c*) of 1500 m/s?

A)

10 cm

B)

11.3 cm

C)

12.8 cm

D)

25 cm

Correct Answer : 11.3 cm

Explaination : From the expression c* = Pc A_t / m, throat area A_t can be determined.
A_t = 1500 x 20/(3 x 10⁶) = 0.01 m²
A_t = πd²/4

20 .

Which of the following is not part of a rocket nozzle?

A)

Throat section

B)

Divergent section

C)

Convergent section

D)

Staggering section

Correct Answer : Staggering section

Explanation : Main elements of a nozzle include convergent, divergent and throat sections. Staggered nozzle sections are uncommon and unfavorable for the smooth flow of exhaust gases.

21 .

What constitutes the exhaust gas at the nozzle exit of a chemical rocket engine?

A)

Propellants

B)

Surrounding air

C)

combustion products of the propellants

D)

A mixture of combustion products and surrounding air

Correct Answer : combustion products of the propellants

Explanation : The exhaust gas at the nozzle exit of a chemical rocket engine consists of combustion products of the propellants. If the combustion were inefficient, it would also contain unburnt propellants. Downstream of the nozzle exit, the exhaust jet entrains the surrounding air and becomes a mixture of both.

22 .

If C denotes effective exhaust velocity and Isp is specific impulse, then which of them is a better performance parameter?

A)

C

B)

Isp

C)

Both of them are equal

D)

Both of them are equivalent

Correct Answer : Both of them are equivalent

Explanation : Isp = C/go, where go is the standard acceleration due to gravity at sea level. Since Isp and c differ only by a constant factor, they are equivalent.

23 .

Which of the following variables is not useful in the determination of rocket effective exhaust velocity?

A)

Rocket velocity

B)

Exit area

C)

Mass flow rate

D)

Exhaust velocity

Correct Answer : Rocket velocity

Explanation : In the formulation of effective exhaust velocity, mass flow rate, exhaust velocity, exit area, nozzle exit pressure, ambient pressure are all relevant variables. But the rocket velocity doesn’t come into picture in the final expression.

24 .

Characteristic velocity is a performance parameter that relates to the efficiency of _____

A)

combustion chamber

B)

casing

C)

nozzle

D)

injector

Correct Answer : combustion chamber

Explanation : Characteristic velocity can be used to compare the performance of combustion chambers in chemical rocket engines. It can also be used to compare different propellants and propulsion systems.

25 .

Which form of energy is most useful for propulsion?

A)

The kinetic energy of ejected matter

B)

The electrical energy of the rocket control system

C)

The chemical energy of combustion of propellants

D)

The potential energy of the rocket at some altitude

Correct Answer : The kinetic energy of ejected matter

Explanation : At the nozzle exit, the exhaust gases are ejected with some kinetic energy. This is the most useful form of energy crucial to propulsion. Chemical energy produced from the combustion of propellants is of no use unless it can be harnessed for propelling the rocket forward by some means.

26 .

For a constant gas ejection velocity (v) of 1440 m/s and a mass flow rate (m) of 15 kg/s, find the power of the jet (Pjet).

A)

7.78 MW

B)

15.55 MW

C)

31.10 MW

D)

62.21 MW

Correct Answer : 15.55 MW

Explanation : Power of the jet is 1/2mv². It is the time rate of expenditure of ejected jet’s kinetic energy.
Pjet = 1/2 x 15 x 1440²
= 15.55 MW.

27 .

For a rocket having 95% combustion efficiency, total chemical power of 17MW, a mass flow rate of propellants of 15 kg/s, and effective jet exhaust velocity of 1443 m/s, find the internal efficiency of rocket propulsion.

A)

91%

B)

93%

C)

95%

D)

97%

Correct Answer : 97%

Explaination : Internal efficiency of rocket propulsion is the ratio of kinetic power in the jet to the available chemical power. It is an indicator of how much the system is capable of converting input energy into useful kinetic energy.
P_{av chem} = η_comb x P_{tot chem}
= 0.95 x 17 x 106 W
η_int = P_kinetic / P_{av chem}
= 1/2μ_e² / P_chem
= 0.5 x 15 x 1443² / (0.95 x 17 x 10⁶)
≅ 0.966.

28 .

Which of the following is not a way of losing energy in a chemical rocket?

A)

Poor mixing

B)

Incomplete burning

C)

Heat loss to the walls

D)

Radial combustion propagation

Correct Answer : Radial combustion propagation

Explanation : Axial and radial combustion propagation are two different kinds of burning propagation. Poor mixing and incomplete burning constitute combustion losses.

29 .

If a propulsion system produces a thrust of 5 kN, exhaust velocity of 4500 m/s, and vehicle velocity of 3000 m/s, what is the power transmitted to the vehicle?

A)

10 MW

B)

15 MW

C)

75 MW

D)

225 MW

Correct Answer : 15 MW

Explanation : Power transmitted to the vehicle is the product of vehicle velocity and the thrust produced by the propulsion system. It is not the same as exhaust jet power.
P_trans = T v
= 5 x 10³ x 3000
= 15 MW.

30 .

For a propulsive device, if power available is denoted by P1, power input by P2, combustion efficiency by e, then which of the following expression is correct?

A)

P1=P2*e

B)

P1=P2/e

C)

P1=P2/e²

D)

P1=P2*e²

Correct Answer : P1=P2*e

Explaination : P1=P2*e is the correct expression. Input power will be reduced by a factor of combustion efficiency to become power available.

31 .

Which of the following Is the maximum energy available per unit mass of chemical propellants?

A)

enthalpy of fusion

B)

enthalpy of atomization

C)

heat of combustion

D)

enthalpy of vaporization

Correct Answer : heat of combustion

Explanation : The heat of combustion is the maximum energy per unit mass that can be extracted from chemical propellants. It can be of two types – higher or lower heating value. Higher heating value is determined by taking all the propellants to their pre-combustion temperature and letting all the vapor produced in the reaction to condense.

32 .

What is the way of production of energy in a chemical rocket?

A)

Expulsion of propellants

B)

Ionization of propellants

C)

Combustion of propellants

D)

Atomization of propellants

Correct Answer : Combustion of propellants

Explanation : Propellants are mixed together and combusted to create energy for propulsion. Expulsion of propellants after combustion converts this energy into a more useful form (kinetic energy).

33 .

If a chemical rocket has a jet power of 18 MW, inert mass of 50 kg, a propellant mass of 1000 kg, then what is its specific power?

A)

8.91 kW/kg

B)

17.14 kW/kg

C)

25.43 kW/kg

D)

34.28 kW/kg

Correct Answer : 17.14 kW/kg

Explanation : Specific power is jet power divided by the total mass of that the propulsion system prior to its launch. It is lower for electric propulsion system when compared to chemical rockets. This is because of the heavier and relatively inefficient power source that it has to carry.
P_spec = P_jet / m_total
m_total = m_inert + m_prop
= 50 + 1000 = 1050 kg
∴ P_spec = 18 x 10⁶ / 1050
= 17.14 kW/kg.

34 .

Which of the following is not true about chemical rockets?

A)

Very low specific power

B)

Relatively light machinery

C)

Very high thrust capability

D)

Relatively low values of specific impulse

Correct Answer : Very low specific power

Explanation : Chemical rockets have very high thrust capability. Because of this, chemical rockets have high specific power and high acceleration.

35 .

Which of the following vehicles is used where the flight time is long?

A)

Solid propulsion system

B)

Liquid propulsion system

C)

Hybrid propulsion system

D)

Electrical propulsion unit

Correct Answer : Electrical propulsion unit

Explanation : Electrical propulsion units have low thrust values. This means that it is a less viable option during take-offs and landing. Because of its very low acceleration potential, it takes a long period of time to accelerate and hence is suited for missions that are time intensive.

36 .

What is a sensible specific impulse value for a nuclear fission type rocket engine among the given options?

A)

300 s

B)

600 s

C)

800 s

D)

2000 s

Correct Answer : 800 s

Explanation : Chemical rocket engines have relatively lower values of specific impulse. Ion-electrostatic type of rocket engines have very high specific impulses, but they lag in their acceleration potential. Nuclear fission type rocket engines have typical values of the specific impulse around 600 s, while the arc-electro thermal ones have it around 800 s.

37 .

A rocket engine moving at 500 m/s produces a total thrust of 9563 N, consuming propellants at the rate of 4.55 kg/s. The energy content of the propellants is 5.35 MJ/kg. Find the effective exhaust velocity.

A)

1400 m/s

B)

1555 m/s

C)

2100 m/s

D)

3936 m/s

Correct Answer : 2100 m/s

Explaination : Rocket thrust is the product of mass flow rate and effective exhaust velocity. Using this,
u_e = T/m
= 9563/4.55
≈ 2100 m/s.

38 .

Which of the following types of engines use Xe as a typical working fluid?

A)

Hall effect

B)

Resistojets

C)

Nuclear fission

D)

Electro thermal

Correct Answer : Hall effect

Explanation : Hall effect type of rocket engines uses Xe as a typical working fluid. Resistojet rockets typically use H₂ or N₂H₄ as working fluid, while nuclear fission uses H₂ and electro thermal one uses NH₃, N₂H₄ or H₂.

39 .

What is the typical order of magnitude of mass flow rate (in kg/s) for a chemical rocket engine?

A)

0.01

B)

0.1

C)

1

D)

10

Correct Answer : 0.01

Explanation : Typical value of the mass flow rate for a chemical rocket engine is about 0.03. Nuclear fission and arc-electro thermal types of engines have it around the same order of magnitude.

40 .

Which of the following engines can achieve the maximum specific impulse of 5000 s?

A)

Hall effect

B)

Ion-electrostatic

C)

Solar heating

D)

Nuclear fission

Correct Answer : Ion-electrostatic

Explanation : Ion-electrostatic engine has a specific impulse of 1200-5000 s. Nuclear fission type engines have Isp of 500-860 s, while solar heating engines have it around 400-700 s.

41 .

What is the propulsion duration of resistojets?

A)

Few weeks

B)

Few days

C)

Few minutes

D)

Few seconds

Correct Answer : Few days

Explanation : Resistojets have a propulsion duration spanning a few days. The chemical rocket engines have a propulsion duration lasting a few seconds to a few minutes, while for ion-electrostatic engines, it lasts for months.

42 .

In which of the propulsion systems is the maximum temperature achieved?

A)

Nuclear

B)

Chemical

C)

Arc heating

D)

Solar heating

Correct Answer : Arc heating

Explanation : Arc heating can lead to temperatures of about 20,000 deg C. Chemical propulsion systems have a maximum temperature ranging from 2500-4100 deg C, while nuclear fission leads to temperatures of about 2700 deg C.