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.

Spin Habitats

From cislunar space outward, spin habitats are the dominant human living spaces. This chapter covers both free‑space stations and spin habitats in gravity (buried on worlds). Most legacy stations are rotating cylinders in stable orbits — usually at Lagrange points or in heliocentric drift. Artificial gravity comes from spin: the farther from the central axis, the stronger the gravity. People live on the interior of the rotating tube, often in stacked levels. Spacecraft dock at the zero‑G ends.

Free‑Space Spin Habitats

Habitats are built from the center outward. "Down" means toward the hull. A glowing central light tube provides illumination, supports plant growth, and doubles as a transit tunnel for rapid movement along the station’s length. The outermost interior layer holds parks, farms, and elite estates. Below that are habitation levels, then industry, then the bottom layers: warehouses, water tanks, and radiation shielding. In poor habitats, slums cling to the shell — where gravity is highest, shielding thinnest, and life harshest.

Larger habitats can afford to spin slowly, making Earthlike gravity stable and comfortable. Smaller ones must spin faster, causing steep gravity gradients and disorienting motion — dropped objects curve unpredictably as the habitat rotates beneath them. In large, safe habitats, gliding from the zero‑G center is a popular sport.

Here are examples of typical habitats and their living conditions. Small ones are harsh. Large ones support leisure and autonomy. There are exceptions — but this is the usual pattern.

200-meter diameter, 1,200 meters long

3.0 RPM, 2.0 G at hull

750,000 m² living space

Population: ~25,000 up to 100,000

Vertigo is constant. Gravity gradients are extreme. Survival depends on repression or fanaticism. Many such settlements cram people much tighter to house more workers, lower levels are barely usable, especially not for habitation. The earliest habitats were like this, held together by dedication to the cause of colonization.

400-meter diameter, 1,400 meters long

2.2 RPM, 1.5 G at hull

1.8 million m² living space

Population: ~60,000

Still unpleasant. Gravity is tolerable, but vertigo remains common. The smallest model that can sustain a society. Slums likely. Low quality of life.

600-meter diameter, 1,600 meters long

1.8 RPM, 1.3 G at hull

3 million m² living space

Population: ~100,000

Livable. Most residents have a decent life. Gravity is only mildly uncomfortable. Slums exist, but there’s room for stability and choice.

1,000-meter diameter, 2,000 meters long

1.3 RPM, 1.2 G at hull

6 million m² living space

Population: ~200,000

Comfortable and affluent. If slums exist, they are maintained by policy, not necessity. Life is good unless the regime is harsh.

1,600-meter diameter, 2,600 meters long

1.0 RPM, 1.12 G at hull

13 million m² living space

Population: ~400,000

Spacious and stable. Gravity differences are gentle. Inequality is political, not structural. Open cultures thrive here.

2,000-meter diameter, 3,000 meters long

0.85 RPM, 1.1 G at hull

20 million m² living space

Population: ~600,000

Opulent. Large, slow‑spinning, and easy to manage. Most inhabitants live in comfort.

Smaller habitats focus on resource extraction. Mid‑sized ones support industry. Large ones host researchers, merchants, and middle‑class life. All habitats have managers who rule and control resources.

A habitat can always be made shorter with only a proportional loss of size and population, preserving comfort but potentially making it less economically viable. Longer habitats can also be built, with increased risk of structural instability. There’s no hard line between a habitat and a large spaceship. Most are stationary, but some mount ion drives for slow migration or repositioning. These are often short wide habitat rings connected to vast cargo arrays that do not rotate and remain in zero‑G.

Many habitats become self‑sufficient city‑states over time. Their isolation make them perfect for cults — religious, political, or corporate. Whoever controls the air, water, and access to zero‑G controls everything. Other habitats embrace openness. Some cluster in chains, sharing transit and infrastructure. These tend to be prosperous, and diverse.

Building Spin Habitats

Building a space habitat is relatively easy — just gather materials and 3D‑print the structure. The hardest resource to source is carbon, needed for the biotope on the top level. Without enough carbon, soil is shallow or absent, stunting plant growth and removing the environment's natural stress relief. Many spacers would kill for carbon if their habitat is running low.

Spin Habitat Terrain

Habitats present terrain unlike either gravity or microgravity. As long as you're standing on the inner surface or floating near the center, things behave predictably. But with spin‑induced gravity and confusing flight dynamics, two terrain types are unique to rotating space habitats:

The first is the central zero‑gravity region. From here, you can jump to nearly any point on the inner surface — if your aim and braking are good. Get it wrong, and you’ll either drift helplessly or slam into the wrong spot.

The second is the outer hull. Gravity here seems to pull away from the hull, but if you let go, you’re flung along a tangent — not straight out. As you approach the hull, it slides sideways beneath you, making it hard to grab hold. Staying close requires constant acceleration toward the surface.

This is why many Jovian personal vehicles are humanoid in form, equipped with magnetic or grappling feet to stay anchored on the outside of a habitat.

Spin Habitats in Gravity (On‑World Rings)

Spin habitats are also built in gravity wells — buried, vertical‑axis rings on moons and planets. Inside, "down" is the vector sum of centrifugal force and local gravity. Key differences from free‑space:

Bank angle: Floors and walls are tilted inward so they’re perpendicular to the effective "down." Example: on Ganymede a 1 g ring is banked ~8.4° inward; on Callisto ~7.2°.

Usable width: Keep a single flat floor to ~0.2–0.3 × radius to hold 0.85–1.15 g across the district. To grow, add another ring and cross‑link; you don’t “stretch” a buried ring.

Metrics to publish: radius, RPM, bank angle, rim speed (also the peri‑rail boarding speed), perimeter, and flat‑floor width.

Entry/exit: Two standard methods:

Radial lifts from the center (native g) outward — you feel gravity strengthen and tilt slightly.

Peri‑rail boarding: a ground maglev accelerates in a covered trench to rim speed (e.g., ~98.5 m/s for r=1,000 m at 1 g on Ganymede), rolls to the bank angle, and docks to a rotating collar for smooth, full‑g entry.

Environment control: Covered trenches and hard vacuum keep dust out, stabilize temperature, and prevent HV arcing on rails and mass drivers.

Mirror‑sky parks: Derotated optical periscopes feed real starlight/Jupiter‑light to a 1 g park without visual wheel‑spin — a prestige feature in major ground cities.

Degrav/Regrav: Ground rings host Regrav wards (0.8–1.2 g). Field crews commute back for daily Regrav; long‑term recovery lives here.

Practically: surface rings are modular and stacked by era (old worker wheels, newer market wheels, prestige sky wheels). Expansion means new rings, not longer ones.