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Space Combat Physics

Vacuum removes wind, drop and drag, and replaces them with light-lag, enormous flight times, and the impossibility of hiding.

Light-lag and flight time

Sensors show where a target was. Drift is what a target can actually achieve given a delta-v budget, not what a sustained 1 g burn would produce — an earlier version tabulated 1 g for months, which at the far end silently required 0.82c of delta-v.

Drift below assumes a 15 km/s budget spent evading, which is a well-found warship.

The qualitative result stands and is now honestly derived: past a few thousand kilometres a target drifts further than its own length during flight time, so unguided fire stops being aimed at a ship. Guided weapons dominate at range, point defence is the real battle, and closing is a deliberate tactic.

Firing a 6 km/s slug across an AU is not a tactic anyone would attempt; the far rows exist to show where the approach dies, not to suggest using it.

There is no stealth in space

A ship must radiate waste heat or cook its crew.

Assumes an isotropic radiator at emissivity 1 against a dedicated cryogenic sensor. Directional radiators buy a real reduction toward a known threat axis, and an all-sky search takes time — which is exactly why the achievable goal is ambiguity, not invisibility. Being one contact among many, hiding your vector, or looking like cargo all work. Being invisible does not.

Delta-v is the currency

dv = Isp g ln(wet mass / dry mass)