Fusion Rockets
Even in the fusion era, rockets are the blunt tool that wins schedules: dump heat into light propellant and throw it out the back. “Rocketry” here means anything that expels reaction mass directly—from heater-fed water jets to true fusion-thermal H₂ drives. They burn prop far faster than ions but deliver the decisive high-g pushes ions can’t.
- What they are
- Thermal rocket (heater-fed): The main reactor superheats propellant (H₂ best, H₂O acceptable) via lasers, RF/MHD, or a closed gas-core loop, then expands it through a magnetic nozzle.
- Fusion-thermal (proper): Same idea, but with reactor temperatures and heat flux high enough to push H₂ exhaust velocity into the 15–30 km/s band.
- (Anything “torchier” than that is rare, ruinously expensive, and not line-service kit.)
- Performance bands (order of magnitude)
| Class |
Heat source |
Typical prop |
Exhaust velocity (v_e) |
Sustained accel window
|
| Heater-fed thermal |
Laser/RF/MHD exchanger |
H₂O / H₂ |
8–15 km/s |
0.02–0.1 g for hours
|
| Fusion-thermal (closed loop) |
Gas-core / advanced exchanger |
H₂ |
15–22 km/s |
0.05–0.3 g for tens of minutes–hours
|
| Hot fusion-thermal (open-ish) |
Very high flux exchanger |
H₂ |
22–30+ km/s |
0.1–1.0 g for minutes
|
- Propellant math you actually need
Prop fraction for a single impulsive burn:
- prop% = 100 × (1 − e^(−Δv / v_e))
| Target Δv |
v_e = 15 km/s |
v_e = 20 km/s |
v_e = 30 km/s
|
| 5 km/s |
28% |
22% |
15%
|
| 10 km/s |
49% |
39% |
28%
|
| 15 km/s |
63% |
53% |
39%
|
- How you use them (doctrine)
- Kick, then cruise: Do the big burn at perigee/perihelion to exploit the Oberth effect (0.1–1 g for minutes→tens of minutes). Then let ions handle weeks of trim and transfer.
- Arrive on purpose: Pay an arrival burn (another 5–10 km/s) or use moon flybys/assist capture. If you only need a flyby, you don’t decelerate—fast transit, cheap prop, zero loiter.
- Spin vs thrust: Milli-g ions let you keep spin gravity. Fusion-thermal doesn’t: despin/lock the ring and strap in along the thrust axis for any burn ≥~0.05 g.
- Tactics: In combat or tight rendezvous, thermal gets you the rapid vector change; ions don’t. Budget prop like ammo.
- Propellant choice (no nonsense)
- H₂ = best mass efficiency (highest v_e), hateful tanks. Use for fast schedules or long legs.
- H₂O = easy tanks/dual-use mass, lower v_e. Use when logistics and simplicity beat raw Δv.
- NH₃ (ammonia) = great volumetric density and easy tanks; crack to H₂ when you must, or run it “dirty” with lower Isp.
- Signatures & safety
- Bright plumes, easy to spot. Periapse burns light you up across the system; everyone sees your timetable.
- Thermal load limits. Radiators and nozzles, not just prop stock, cap how long you can hold high-g.
- Aegis fields: Fusion-thermal can coexist; fields will aurora and you’ll scatter some plume. Keep hundreds of meters from other shields/structures; drop shields on final approach.
- Worked example (sanity check)
- Want a fast Earth→Jupiter arc? A single 10 km/s kick at Earth with a v_e≈20 km/s fusion-thermal costs ~39% of departure mass in prop, delivered in ~17–34 minutes at 1.0–0.5 g (or ~2.8 h at 0.1 g). You’re now on a high-energy trajectory ions can actually work with.