Fun With Science / Globe Deconstruction / Rockets in Vacuum / Thrust, Measured in Flight
Rockets firing in real vacuum, audited by people who had no stake in this argument.
Claim #2 (p. 2) says thrust from gas “must have something to push off, and an infinite vacuum does not provide that.” The main answer makes the physics case and its §9 gathers corroboration at three scales — but all of it except the missile intercepts is measured on the ground, and a chamber can always be accused of not being empty enough. Flight removes the objection: a vacuum many orders of magnitude harder than any chamber, no wall anywhere near, and the same firing measured by several unrelated physical routes at once. None of the work below was done to settle a debate about whether rockets work; it was done because mission planners needed the number.
Audited repeatedlyBudgets close
What the flight record shows
In 1993 NASA published a cross-check of the ways ion-engine thrust had been measured in orbit — an onboard accelerometer, the change in the spacecraft's orbit, the momentum of its reaction wheels (steering flywheels) — against electrical telemetry calibrated on a ground thrust stand. They agree to within a few per cent. Deep Space 1's thrust was measured from the Doppler shift of its radio signal to better than 0.5 mN and plotted against ground-chamber data. Dawn's published propellant consumption and its measured velocity change close via the rocket equation to about 3%.
There is a real vacuum-chamber artefact in all this, and it is measured too: about one per cent, in the direction that makes chambers flatter engines slightly. The claim needs it to be one hundred per cent — thrust in vacuum going to zero rather than to ninety-eight per cent of its chamber value. The limit of this page is that every measurement is an agency's; what it adds is physical independence, not institutional independence (§5).
The most useful document is a 1993 NASA technical memorandum comparing the ways in-flight thrust had been determined on ion-propulsion flights — housekeeping, by engineers wanting to know which method to trust. (A millinewton, mN, is a thousandth of a newton, roughly the weight of a paperclip; ion engines are gentle, which is why their thrust needs careful measuring.)
SERT II (launched 1970, 1,524 kg, in a roughly 1,000 km orbit) carried a mercury ion thruster, whose thrust was determined three ways in flight and a fourth on the ground:
| how it was measured | thrust | what it depends on |
|---|---|---|
| Electrical telemetry, corrected by a ground thrust stand | 26.8 mN | beam current and voltage; a pendulum calibration in a chamber |
| Onboard accelerometer (MESA) | 27.4 mN | a mass on a spring, in space |
| Change in the spacecraft's orbit | 28 mN | Newtonian gravity and 34 days of tracking |
Sovey & Rawlin, NASA TM-106283 (1993). The paper reports the orbit-derived figure as 3.7–4.5% higher than the telemetry-plus-ground-stand value, and the accelerometer as within 3% of it.
The orbital route uses nothing but geometry and gravity. The altitude rose 20 km over 34 days of thrusting from an initial orbit radius of 7,400 km; the rate at which an orbit grows under continuous thrust is fixed by the tangential force, the mass and Earth's gravity. Working it from those three published numbers gives a tangential component of 5.1 mN against the paper's 4.9 mN, and with the thrust vector 10° off, about 30 mN against their 28 — rounding in the inputs. A reader can recover the thrust of a rocket firing in space without trusting any thrust measurement at all.
A flat-plane reader will object that the orbital figure assumes the globe and Newtonian gravity, and it does. The other channels do not: the accelerometer is a mass on a spring, reading the push on the spacecraft whatever it is orbiting; a reaction wheel is a steering flywheel whose spin changes under the engine's off-axis push at a rate set by the wheel's inertia and nothing outside the vehicle. Those are the channels for a reader who rejects orbital mechanics, and they give the same number.
ETS-III, a 382 kg Japanese satellite with a 5 cm ion thruster, gives the same picture from a different agency's hardware: electrical telemetry plus ground calibration gave 1.76 and 1.87 mN for its two thrusters, reaction-wheel diagnostics gave 1.92 and 1.99 mN, and raising the orbit 5.7 km over 110 hours of thrusting gave 2.05 mN — about 9.6% above the electrical figure.
Modern deep-space missions do not carry thrust sensors at all. Dawn's chief engineer: “Because the thrust is so gentle, there are no sensors on board that directly measure it.” What is measured is the frequency of the spacecraft's radio carrier, which shifts with its velocity — the Doppler effect anyone can hear on a passing ambulance. Velocity change is read off the shift; thrust follows from that and the mass. So the thrust figure is not a specification reported back; it is a measurement of the spacecraft's motion, taken from Earth. On Deep Space 1 the Doppler data gave the thrust “with an uncertainty of less than 0.5 mN” on an engine producing 20–92 mN. Dawn's navigation could detect velocity changes “smaller than 0.5 millimetres per second.”
Dawn is the cleanest worked case: the two halves of the sum were published separately by the same team.
Put those in the rocket equation — Δv = ve ln(m0/m1) — and 71.7 kg thrown from 1,215 kg at an exhaust velocity of 3,100 × 9.81 m/s predicts 1.85 km/s. The measured figure was 1.80. The budget closes to within 3%, with the residual inside the rounding on every input.
Over the whole mission the sum is consistent rather than precise: 401 kg of xenon and 11.14 km/s of measured Δv from a 1,218 kg spacecraft implies a mission-average specific impulse of about 2,845 s, where it should sit for a mission that ran near full power early and throttled heavily at Ceres. Dawn's propellant bookkeeping was itself cross-checked two ways — flow-orifice pressure against tank pressure and temperature — and agreed “to within approximately 5 kg, or approximately one percent of the total xenon used.”
A sceptic's best line against the ground-based evidence is that a vacuum chamber still contains gas and the engine is quietly pushing against it. The propulsion literature agrees it is a real effect, and the same 1993 memorandum quantifies it. Residual chamber gas is drawn into a thruster and adds to its propellant flow: about 1% of the input flow rate for a 5 kW xenon thruster tested at 1.3 mPa (a millipascal is about a hundred-millionth of atmospheric pressure), which is why the paper recommends test pressures below 2 mPa for flight qualification. It runs in the direction a sceptic would expect — the chamber slightly helps.
The flight data shows the same sign. Deep Space 1's engine, measured by Doppler in interplanetary space, came out marginally worse than the ground chamber predicted: specific impulse about 60 s low at the start of the mission, efficiency 2 to 2.5 percentage points low, thrust up to 1.6 mN below the calculated value at intermediate power. JPL plotted flight against ground thrust-balance data in one figure: they “agree well… at low powers, but are lower at intermediate powers.”
The usual reply is that space is not empty either — the engine pushes on the solar wind. The densities settle it. The chamber gas whose whole contribution measured one per cent, xenon at 1.3 mPa and room temperature, is about 7×10−8 kg/m³; the solar wind at Earth's distance, about five protons per cubic centimetre, is roughly 10−20 kg/m³ — some ten trillion times thinner — and Deep Space 1 in it delivered ninety-eight per cent of its chamber thrust. Thinning the medium by thirteen orders of magnitude cost two per cent.
The claim's structure is that Newton's third law was “abused … to sell humanity on the idea of space travel” (p. 2), which needs the error to be undetected. But the propulsion literature is a record of engineers measuring the same quantity several ways because they trusted no single method, publishing the disagreements to the nearest per cent, and quantifying the one artefact that would flatter their own results. An engine whose thrust depended on ambient gas would have failed that audit in 1970, on SERT II, where the orbit itself was the instrument.
Some widely quoted figures did not survive checking and are not used above. A Dawn orbit-transfer budget in the literature implies a specific impulse about 5% below the engine's stated floor; every input is quoted as approximate, so it is most likely rounding, but it does not close and is not cited. Deep Space 1's frequently repeated mission totals could not be confirmed in a primary source, so the figures used stop where the papers do. The orbit recomputation in §1 and the rocket-equation arithmetic in §3 are original and reproducible from the quoted inputs; the density comparison in §4 takes 1.3 mPa of xenon at 293 K and a solar wind of 5 protons per cubic centimetre.