Fuel economy for rockets
Specific impulse Isp (seconds) measures how efficiently a propulsion system uses propellant mass. Higher Isp means more Δv per kilogram of propellant.
Typical values
| System | Isp (s) | Notes |
|---|---|---|
| Solid (APCP) | 220-280 | Simple, reliable, low Isp |
| Kerolox (RP-1 + LOX) | 260-340 | Dense, storable oxidiser |
| Methalox (CH4 + LOX) | 320-370 | Balanced, reusable-friendly |
| Hydrolox (LH2 + LOX) | 430-465 | Best chemical but low density |
| Monoprop hydrazine | 220 | Simple thrusters |
| Hall-effect thrusters | 1,500-3,000 | Low thrust but very efficient |
| Gridded ion | 2,500-9,000 | Station-keeping, deep space |
| Nuclear thermal | 800-900 | Research, hot H₂ |
The Isp vs thrust tradeoff
Electric propulsion has incredible Isp but tiny thrust. Chemical has strong thrust but mediocre Isp. Mission profile drives the choice: escape burns need thrust (chemical); long-duration plane changes favour efficiency (electric).