Nathan Geffen Space Travel Calculator
How efficient does a rocket engine need to be to travel interstellar distances in a reasonable amount of time? Explore fuel conversion rates, wet mass, and relativistic effects.
Mission Parameters
Fraction of fuel mass converted to energy. 100% = perfect matter-antimatter annihilation. ~0.7% = hydrogen fusion.
Velocity & Relativity
Fuel & Mass
Length Contraction
At peak velocity, an outside observer would see the spacecraft compressed to 9.79% of its rest length.
At a fuel conversion rate of 100.00%, you need 41.43 thousand tonnes of fuel for a 100.00 tonnes ship.
A mass ratio of 415:1 is very demanding.
About This Calculator
This calculator is inspired by Nathan Geffen's space travel model, which asks: "How efficient does a rocket engine need to be to travel interstellar distances in a reasonable amount of time?"
Fuel Conversion Rate
The key metric here is fuel conversion rate — the fraction of fuel mass that gets converted into usable energy (and therefore momentum). This is technology-agnostic: it doesn't matter whether you're using fusion, fission, or antimatter annihilation. What matters is how much mass becomes thrust.
- Nuclear fission: ~0.08% conversion rate
- Hydrogen fusion: ~0.7% conversion rate
- Matter-antimatter annihilation: 100% (theoretical maximum)
Why Mass Ratio Matters
The mass ratio (wet mass / dry mass) determines feasibility. A ratio of 10:1 means 90% of your starting mass is fuel. Ratios above ~100:1 become engineering nightmares, and ratios above 10⁶ are physically absurd. Higher fuel conversion rates dramatically reduce the required mass ratio.
Length Contraction
At relativistic speeds, an outside observer sees the spacecraft compressed along its direction of travel by a factor of γ. This is a real physical effect predicted by special relativity — though the crew onboard wouldn't notice any change to their ship.