Spider Stellar Engine Thrust Calculator

Vidal 2024: pulsed companion ablation generates directed thrust — compute the propulsive force, system acceleration, and galactic travel timescales for any spider pulsar

🔬 Established physics (rocket equation, pulsar spin-down) ⚠ Theoretical (spider engine propulsion mechanism) ✦ Engineering fiction (galactic travel)
Vidal (2024) proposes that spider pulsars — binaries where the pulsar wind ablates a low-mass companion — are naturally configured as stellar engines. By ejecting companion mass preferentially during the "backward" orbital phase (when the companion trails the desired direction of motion), the system generates net thrust. The companion is both fuel and propellant; when it is exhausted the engine stops. This tool computes the propulsive budget for any pulsar–companion binary.
Pulsar Engine Parameters
Spin-down luminosity + pulsed ejection efficiency → ablation rate → thrust
Period P (ms)1.607
Period derivative Ṗ (s/s)1.68×10⁻²⁰
Ejection efficiency η_ej (fraction of L_sd)0.01
0.01%1%30%
Pulsed fraction f_pulse0.20
5% (tight beam)50% (half orbit)
Companion & System
Companion provides propellant; ejection velocity sets thrust per unit mass rate
Companion mass M_c (M☉)0.025
Companion radius R_c (R☉)0.14
Ejection velocity v_ej (× v_esc)1.0
0.5× escape5× escape
Propulsive Budget
Spin-down power → ablation rate → thrust → acceleration
Spin-down luminosity L_sd
—
W
Ablation power L_abl
—
W (η_ej × L_sd)
Companion escape velocity
—
km/s
Ejection velocity v_ej
—
km/s
Mass ablation rate Ṁ
—
kg/s
Thrust F = Ṁ × v_ej × f_pulse
—
N
System mass M_sys
—
kg
Acceleration a_sys
—
m/s² (= F/M)
Companion fuel lifetime
—
yr (M_c / Ṁ)
Total Δv over companion lifetime
—
km/s
Journey Time Estimator ✦
Constant-acceleration travel under thrust F for duration T_fuel (companion lifetime)
Travel time to destination (constant acceleration)
Omega Centauri (17,900 ly)—
Galactic centre (26,000 ly)—
Milky Way diameter (100,000 ly)—
Andromeda (2.5 Mly)—
—

What this tool does

This tool implements the propulsion budget from Vidal (2024, JBIS 77:156). A spider pulsar irradiates its companion star with its wind at luminosity L_int. A fraction η_ej of that power accelerates companion material to ejection velocity v_ej. If ejection is pulsed — occurring only during the fraction f_pulse of the orbit when the companion is "behind" relative to the desired direction of travel — the momentum impulse is directed, producing net thrust.

Key formulas

Ablation power: L_abl = η_ej × L_sd (the fraction of spin-down power that actually accelerates companion mass)

Mass ablation rate: Ṁ = 2L_abl / v_ej² (from kinetic energy balance: L_abl = ½Ṁv_ej²)

Thrust: F = Ṁ × v_ej × f_pulse (momentum per second, directed by pulsing)

Companion escape velocity: v_esc = √(2GM_c/R_c) — sets the natural ejection floor

System acceleration: a = F / M_sys where M_sys = M_pulsar (1.4 M☉) + M_companion

Fuel lifetime: T_fuel = M_c / Ṁ

Travel distance at constant acceleration: d = ½ a T_fuel² (non-relativistic; formula breaks down for Δv > 0.1c)

Caveats

The ejection efficiency η_ej is the largest source of uncertainty — Vidal (2024) does not give a single value. Typical estimates range from 0.1% to 10% of L_sd. The pulsed fraction f_pulse depends on the orbital geometry and the beaming of the pulsar wind. For maximum thrust, f_pulse ≈ 0.1–0.2 (narrowly collimated ejection window). Travel times assume constant acceleration throughout the companion lifetime, which is an overestimate for massive companions (the system mass decreases as fuel is consumed).

References

Vidal, C. (2024). "The Spider Stellar Engine." JBIS 77:156. arXiv:2411.05038 Vidal, C. (2016). "Stellivore Extraterrestrials?" Acta Astronautica 128:251. DOI: 10.1016/j.actaastro.2016.06.038

v1.0 — 2026-06-02