# Rocket Propulsion Questions and Answers – Gravity Free Drag Free Space Flight Performance

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This set of Rocket Propulsion Assessment Questions and Answers focuses on “Gravity Free Drag Free Space Flight Performance”.

1. What is the nature of the trajectory of rockets in gravity-free, drag-free environment?
a) Two-dimensional, curved path
b) One dimensional, curved path
c) Two-dimensional, straight line path
d) One dimensional, straight line path

Explanation: One dimensional, straight-line acceleration path is followed in such an environment as the only force acting on the rocket is its thrust and it acts in the flight direction. Under the influence of gravity, the flight path will become curved.

2. How will propellant mass flow and thrust change with burn duration in a gravity-free, drag-free environment?
a) Mass flow and thrust remains constant
b) Mass flow increases, thrust decreases
c) Mass flow decreases, thrust increases
d) Mass flow decreases, thrust decreases

Explanation: Both mass flow and thrust remain constant in such an environment. Then for a given burning time tb, total mass expelled from the rocket can be determined from mass flow rate m = m/tb equation.

3. Where do we account for residual propellant mass in a rocket engine?
a) Propellant mass
b) Inert mass
d) Structural mass

Explanation: We account for residual propellant mass in inert mass section. Inert mass includes the masses of the engine system, like that of the nozzles, tanks, cases or unused, residual propellants. It doesn’t come under the useful propellant mass consumed for propulsion.

4. What is the nature of the effect of the propellant mass fraction on the vehicle velocity?
a) Exponential
b) Logarithmic
c) Linear
d) Parabolic

Explanation: Propellant mass fraction has a logarithmic effect on the vehicle velocity.
up = ∆u = -c ln(1-ζ).

5. Keeping the system inert mass constant, which of the following changes will not result in flight velocity increment?
a) Use of better propellants
b) A more favorable nozzle area ratio
c) Higher chamber pressure
d) Better nozzle cooling

Explanation: Because doing any of these except better nozzle cooling will result in better Isp. Velocity increment is directly proportional to Isp, so more Isp means more velocity increment.

6. For given average effective exhaust velocity(c), which of the following values of propellant mass fractions(ζ) gives maximum vehicle velocity (up)?
a) 0.6
b) 0.7
c) 0.8
d) 0.9

Explanation: 0.9 is the correct answer. Because of the relation up = ∆u = -c ln(1-ζ), we can see that higher ζ gives higher up.

7. What is the effect of increasing mass ratio (mo/mb, where mo is the initial mass and mb is the burnout mass) on Isp?
a) Increases
b) Decreases
c) No effect
d) Increases only after a limit

Explanation: Isp increases because of the relation up = c ln(mo/mb). Here c is the exhaust velocity of the vehicle and ln denotes natural logarithm.

8. Which of the following will decrease the effective propellant fraction?
a) Higher system inert mass
b) More favorable nozzle area ratio
c) Higher chamber pressure
d) Higher temperature at the inlet of the nozzle

Explanation: This is because the effective propellant fraction is the ratio of the mass of propellants to the initial mass of the system. If inert mass(mf) increases, then the total initial mass(mo) will also increase as mo = mp + mf.

9. Which of these is a reasonable value of the mass ratio (Initial mass/inert mass) for single-stage vehicles for gravitation free drag-free space flight?
a) 100
b) 1000
c) 200
d) 20

Explanation: Single-stage rocket vehicles can have mass ratio up to about 20. To minimize weights and lateral loads, spherical shape of the rocket is desirable, although it may not be the easiest to manufacture.

10. Which of these is a reasonable value of the mass ratio (Initial mass/inert mass) for multistage vehicles for gravitation free drag-free space flight?
a) 100
b) 1000
c) 200
d) 20 