Talk:Icarus Fall

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.