Physics

Why Ion Thrusters Are Feeble but Unstoppable: Specific Impulse vs Thrust-to-Power

By Lucian — builder & engineer, LK Forge

An ion thruster accelerates propellant through an electric field instead of burning it, reaching exhaust velocities tens of times higher than any chemical engine — but only a sliver of mass flows through the beam at once, so instantaneous thrust stays in the millinewtons. We swept the Ion Thruster Lab's model across beam voltage and propellant species to measure exactly how raising voltage trades thrust-to-power for specific impulse, why lighter propellant reaches higher exhaust velocity, and how a modest xenon load turns into a large Δv over a long burn.

 ·  6 min read  ·  measured from the voltage/propellant tradeoff benchmark

3738.6 s
specific impulse at the default 1200 V beam voltage, versus 2158.5 s at 400 V
99.78 mN
thrust at 1200 V — a fraction of a newton, even with a fully charged beam
39.3 mN/kW
thrust-to-power at 1200 V, falling to 24.8 mN/kW at 3000 V
6.684 km/s
Δv from 100 kg of xenon burned over 442.3 days at the 1200 V default

Beam voltage trades thrust-to-power for specific impulse

Exhaust velocity, and with it specific impulse, rises only with the square root of beam voltage, but the electrical power needed to drive the beam rises linearly with voltage. Thrust itself scales with exhaust velocity, so it climbs more slowly than the power spent making it — which is exactly why thrust-to-power falls as voltage climbs, even while specific impulse keeps rising.

exhaust velocity ∝ √(beam voltage)   —   beam power ∝ beam voltage

At a fixed 1.8 A beam current, moving from 400 V to 3000 V takes specific impulse from 2158.5 s to 5911.2 s, while thrust-to-power falls from 68 to 24.8 mN/kW.

Specific impulse versus thrust-to-power, across beam voltage

Specific impulse (blue, left axis) climbs steadily while thrust-to-power (orange, right axis) falls, for the 1.8 A beam-current thruster swept from 400 V to 3000 V.

0 1500 3000 4500 6000 020406080 050010001500200025003000 beam voltage (V) Isp (s) mN/kW

● specific impulse (left axis)    ● thrust-to-power (right axis)

Lighter propellant reaches higher exhaust velocity

At the same beam voltage and current, exhaust velocity depends only on propellant ion mass — a lighter ion accelerates through the same electric field to a higher speed. Xenon (131.293 u) is favored in real thrusters for reasons beyond this model (it is easy to store and ionize), but swapping in a lighter noble gas at the 1200 V default shows the underlying physics directly: krypton (83.798 u) reaches 52.568 km/s, and argon (39.948 u) reaches 76.136 km/s, versus 41.997 km/s for xenon. Thrust moves the opposite way, because thrust also depends on the ionized mass flow, which is heavier for xenon at a fixed beam current.

Propellant Ion mass Exhaust velocity Isp Thrust
Xenon 131.293 u 41.997 km/s 3738.6 s 99.78 mN
Krypton 83.798 u 52.568 km/s 4679.6 s 79.71 mN
Argon 39.948 u 76.136 km/s 6777.7 s 55.04 mN

A little xenon, spread over a long burn

The rocket equation says Δv equals exhaust velocity times the natural log of the mass ratio — mass before the burn over mass after. With a 500 kg dry spacecraft carrying 100 kg of xenon, the mass ratio is only 1.20, far smaller than a chemical rocket typically needs, but exhaust velocity here is so high that the product still reaches 6.684 km/s. The trade is time: at 99.78 mN of thrust, burning through that propellant load takes 442.3 days of continuous operation.

Engine: 1200 V beam voltage, 1.8 A beam current, xenon propellant. Mission: 500 kg dry mass, 100 kg propellant load. Generated 2026-09-11.

Try it yourself

Open the lab, set your own beam voltage, beam current, propellant ion mass, and efficiency factors, and watch thrust, specific impulse, thrust-to-power, and mission Δv move in real time.

Open the Ion Thruster Lab →
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Common questions

Why does an ion thruster only produce millinewtons of thrust?

Thrust is the ionized mass flow times exhaust velocity, and the ionized mass flow is tiny — it is set directly by the beam current (1.8 A here) divided by the charge on a single ion, so only a few milligrams per second of propellant ever leave the beam. Exhaust velocity is enormous (tens of kilometers per second), but multiplying a minuscule mass flow by that speed still lands at 99.78 mN of thrust at the 1200 V default in this sweep — a push you could hold back with a finger, delivered continuously for months.

Why is specific impulse so high on an ion thruster?

Specific impulse is exhaust velocity divided by standard gravity (times the efficiency factors that survive to the beam), and exhaust velocity here comes from accelerating ions through an electric field rather than from a chemical reaction, so it easily reaches tens of kilometers per second — versus a few kilometers per second for the fastest chemical engines. That is why this sweep runs from 2158.5 s at 400 V up to 5911.2 s at 3000 V, an order of magnitude beyond chemical propulsion.

Why does raising beam voltage trade thrust-to-power for specific impulse?

Exhaust velocity grows with the square root of beam voltage, and specific impulse rises the same way — but the power needed to drive the beam grows linearly with voltage. So thrust, which scales with exhaust velocity, climbs more slowly than the power spent to produce it, and thrust-to-power falls as voltage climbs: from 68 mN/kW at 400 V down to 24.8 mN/kW at 3000 V in this sweep, even as specific impulse rises from 2158.5 s to 5911.2 s over the same range.

How does 100 kg of xenon give kilometers per second of Δv?

The rocket equation says Δv equals exhaust velocity times the natural log of the mass ratio — the spacecraft's mass before the burn over its mass after. With a 500 kg dry spacecraft and 100 kg of xenon, that mass ratio is only 1.20, small next to a chemical rocket's, but exhaust velocity here is so high that the product still reaches 6.684 km/s. It takes 442.3 days of continuous low thrust to burn through that xenon at the 1200 V default, which is exactly the trade an ion thruster makes: a small mass ratio spread over a long, patient burn instead of a large mass ratio spent in minutes.