CAPF AC 2022 Q47: Science & Technology – Space Technology & Astronomy
Consider the following statements:
1. Aluminium powder is used as a solid fuel for rocket engines.
2. Compared to liquid fuel propelled rocket engines, the thrust per amount of fuel burned is higher in solid fuel propelled rocket engines.
Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer: (a) 1 only
Explanation
Solid and liquid rocket fuels
How to think: check each statement on its own.
Solid rocket boosters do burn aluminium powder.
But for efficiency, liquid engines are better.
| Statement | Verdict | Reason |
|---|---|---|
| 1. Aluminium powder is a solid rocket fuel | Correct | It is the fuel; ammonium perchlorate is the oxidiser and HTPB the binder |
| 2. Solid engines give more thrust per amount of fuel than liquid engines | Wrong | Liquid engines have the higher specific impulse |
Solid and liquid engines compared:
| Feature | Solid | Liquid |
|---|---|---|
| Efficiency (specific impulse) | Lower | Higher |
| Thrust at lift-off | Very high | Lower |
| Control | Cannot be shut down once lit | Can be throttled and restarted |
| Indian example | S139 stage and strap-ons of PSLV | Vikas engine, cryogenic CE-20 |
Answer: (a) 1 only.
Why each option is right or wrong
- (a) CORRECT. Only statement 1 is true: aluminium powder is a solid rocket fuel.
- (b) Incorrect. Statement 2 is false; liquid engines give more thrust per unit of fuel.
- (c) Incorrect. Statement 2 is false, so both cannot be right.
- (d) Incorrect. Statement 1 is true.
Key point
Solid propellant: aluminium powder (fuel) + ammonium perchlorate (oxidiser) + HTPB (binder). Specific impulse: cryogenic > liquid > solid. PSLV: four stages, solid-liquid-solid-liquid.
Remember
Solid: aluminium + ammonium perchlorate. Liquid: more efficient.
Common mistakes
- Confusing high thrust at lift-off (solid) with high efficiency (liquid).
- Not knowing that a metal powder such as aluminium can be a fuel.
- Thinking solid engines can be throttled or shut down like liquid ones.
Exam tip
Educated guess: if solid fuel were both simpler and more efficient, nobody would build complicated liquid and cryogenic engines. So statement 2 must be false.
Sources
- Indian Space Research Organisation — PSLV: vehicle description (solid and liquid stages, HTPB propellant)
- NASA — Solid Rocket Boosters: propellant composition (aluminium powder and ammonium perchlorate)