250+ TOP MCQs on Liquid Propellant Rocket Engine – Flow and Pressure Balance and Answers

Rocket Propulsion MCQs on “Liquid Propellant Rocket Engine – Flow and Pressure Balance”.

1. The oxidiser flow (m_o) through the gas generator is ____________ if its flow rate through the pump is 10 kg/s and through the thrust chamber is 5 kg/s.
a) 5 kg/s
b) 15 kg/s
c) 20 kg/s
d) 25 kg/s
Answer: a
Clarification: m_o = (m_o)_gg + (m_o)_c
So, (m_o)_gg = 10 – 5 = 5 kg/s.

2. The fuel flow (m_f) through the gas generator is ____________ if its flow rate through the pump is 18 kg/s and through the thrust chamber is 10 kg/s.
a) 18 kg/s
b) 10 kg/s
c) 8 kg/s
d) 28 kg/s
Answer: c
Clarification: m_f = (m_f)_gg + (m_f)_c
So, (m_f)_gg = 18 – 10 = 8 kg/s.

3. What is the total mass flow rate through the gas generator if the oxidizer flow through it is 15 kg/s and fuel flow is 18 kg/s?
a) 3 kg/s
b) 48 kg/s
c) 53 kg/s
d) 33 kg/s
Answer: d
Clarification: Total mass flow rate through the gas generator m_gg = (m_gg)_o + (m_gg)_f
m_gg = 15 + 18 = 33 kg/s.

4. In a turbopump, what are are the gear ratios for a_o and a_f (for oxidizer pump and fuel pump respectively) if no gears are used?
a) 0 for both
b) 1 for both
c) 0 for a_o and 1 for a_f
d) 0 for a_f and 1 for a_o
Answer: b
Clarification: If no gears are used, a_o = a_f = 1. For sha_ft speed N, a_oN_o = a_fN_f, where the subscript o and f denote oxidizer and fuel respectively.

5. If the gear ratio for fuel pump is thrice that of the oxidizer pump, the sha_ft speed of fuel pump is ____________ the oxidizer pump.
a) half
b) twice
c) thrice
d) one-third
Answer: d
Clarification: One-third because it follows from the relation, a_oN_o = a_fN_f.
Given, a_f=3 x a_o.
Using this, N_f = 1/3 x N_o.

6. If power of turbines equals P_T, power consumed by pumps is P_P and the power consumed by auxiliaries is P_A, what is the expression for auxiliary power?
a) P_A = P_P – P_T
b) P_A = P_P + P_T
c) P_A = P_T – P_P
d) P_A = P_T x P_P
Answer: c
Clarification: Turbine power produced is consumed by pumps as well as the auxiliaries. This is called as the power balance for turbopumps.

7. The power is expressed as the product of torque and ____________
a) vehicle speed
b) shaft speed
c) exhaust velocity
d) volume flow rate per unit cross-sectional area
Answer: b
Clarification: Torque L x Shaft speed N is a form of representation of the power P. Part of the power produced goes to waste by bearing, seal, transmission, and friction losses.

8. Which of the following expressions is true when there are no gears in a particular turbopump?
a) N_T = N_o ≠ N_f
b) N_T ≠ N_o = N_f
c) N_T ≠ N_o ≠ N_f
d) N_T = N_o = N_f
Answer: d
Clarification: If there are no gears in a particular turbopump, then all the shaft speeds are same. So N_T = N_o = N_f is the correct expression.

9. If L represents torque, and the subscripts T, o, f, b denotes the turbopump, oxidizer pump, fuel pump and bearings respectively, then which of the following expressions is true when there are no gears in a particular turbopump?
a) L_T = L_o + L_f + L_b
b) L_T = L_o x L_f x L_b
c) L_T = L_o = L_f = L_b
d) L_T = L_o = L_f ≠ L_b
Answer: a
Clarification: The torque of the turbopump will then be the sum of all the other torques. This is evident from the expression, P_T = L_TN_T = L_oN_o + L_fN_f + P_b. When there are no gears, all the N terms will be equal and the expression simplifies to L_T = L_o + L_f + L_b.

10. If the pressure rise ΔP across a pump is half the discharge pressure, then which of the following relates the suction pressure P_s to discharge pressure p_d?
a) P_s = p_d
b) P_s = 2 x P_d
c) 2 x P_s = P_d
d) P_s = 4 x P_d
Answer: c
Clarification: p_d = P_s +ΔP.
Since ΔP = 1/2 P_d, P_d = 2P_s.