Discussion
Consider the following statements about propulsion:
1. All propulsion systems generate thrust by accelerating a fluid rearward.
2. Air breathing engines take their oxidiser from the atmosphere.
3. A rocket's thrust falls with altitude like any other engine.
Which of the statements given above is or are correct?
Answer: B
Correct answer: 1 and 2 only
Statements 1 and 2 are correct: all propulsion works by Newton’s third law on some accelerated fluid, and an air breathing engine takes its oxygen from the atmosphere. Statement 3 is inverted - a rocket’s thrust *rises* with altitude, because \( F = \dot{m}V_e + (p_e-p_a)A_e \) and the falling ambient pressure increases the pressure term. Only an air breather loses thrust with altitude, and it does so because the air thins.
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