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# Rocket Propulsion – Gravity Free Drag Free Space Flight Performance

1 - Question

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

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Answer: dExplanation: 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 - Question

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

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Answer: aExplanation: 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 - Question

Where do we account for residual propellant mass in a rocket engine?

a) Propellant mass

b) Inert mass

c) Payload mass

d) Structural mass

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Answer: bExplanation: 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 - Question

What is the nature of the effect of the propellant mass fraction on the vehicle velocity?

a) Exponential

b) Logarithmic

c) Linear

d) Parabolic

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

5 - Question

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

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Answer: dExplanation: 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 - Question

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

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Answer: dExplanation: 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 - Question

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

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Answer: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 - Question

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

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Answer: aExplanation: 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 - Question

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

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Answer: dExplanation: 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 - Question

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

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Answer: cExplanation: Multistage vehicles can have mass ratios that exceed 200. In this case, the rocket is segmented into multiple segments in which each of the segment is a separate propulsive system.