Science1 publisher2 min readPublished
A Stuttgart doctoral thesis ran a plasma thruster on simulated very-low-orbit air
Francesco Romano's design collects the residual air that drags satellites down and turns it into plasma, and his models put indefinite flight between 190 and 250 kilometres inside a standard satellite's power budget.
The Scientist · Science desk
What happened
- Francesco Romano's doctoral thesis at the University of Stuttgart, available on arXiv, designs a satellite engine that scoops the residual air causing drag in very low Earth orbit and ionises it for thrust.
- Of three intake geometries he tested, a parabolic mirror coated with graphite or silicon dioxide collected about 94.3% of incoming particles in a wind tunnel and tolerated misalignment best.
- The thruster is driven by a birdcage antenna of the kind used in MRI machines, which put 99% of the delivered electrical power into the thruster instead of losing some to a wire coil's reactance.
- In a vacuum chamber set to VLEO-like concentrations of atomic oxygen, argon and nitrogen, the engine held steady plasma on 50 to 60 W of radio-frequency power.
- Applied to case studies including GOCE, which flew a xenon ion thruster in VLEO and eventually ran out of fuel, the thesis calculates indefinite operation between 190 and 250 km on under 1.6 kW.
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Why it matters
- capability Sharper cameras and lower-power radio links are the reasons to fly at a few hundred kilometres, and an engine fed by the surrounding air removes the tank that has capped how long a satellite can stay there.
- constraint The modelled indefinite-flight band covers 60 km of the 350 km the article calls VLEO, so the denser air below 190 km, where drag is worst and the fuel is most abundant, sits outside the result.
- decision For anyone sizing a VLEO constellation, the design question moves from how much xenon each satellite carries to whether its bus can deliver kilowatts of continuous power to propulsion for years.
- exposure Atomic oxygen still lands on the intake mirror, and its collection figure describes a wind tunnel campaign, so how the graphite or silica coating degrades in orbit remains untested in the reported work.
Atomic oxygen is the reason air-breathing propulsion has been hard to build. Ultraviolet light in the upper atmosphere splits O2 into a single-atom form that corrodes metal electrodes, acceleration grids and the cathodes used in Hall thrusters and other ion engines [6]. The most exposed part is the cathode in the electron gun that neutralises the spacecraft. Without it the vehicle charges up and pulls its own exhaust ions back, cancelling the thrust they were meant to provide [7]. Romano's answer was to delete that part: the thruster is contactless and neutraliser-less [9], and a solenoid wrapped around it pushes positive and negative particles out together in a quasi-neutral jet [14].
Pointing tolerance matters because a scoop only collects what it faces. Tilting the specular intake 15 degrees cost 8% of its collection efficiency [12]. As a relative loss, the mirror still takes in about 86.8% of what reaches it; as percentage points, 86.3% [2].
The two power figures do different jobs. The chamber number is what it took to strike and hold a plasma in a fixed gas mix [15]. The thesis figure is what a satellite would draw to keep itself up between 190 and 250 km [17], and it is 27 to 32 times larger [1]. Both are described as within what standard spacecraft solar panels generate [15][17].
The intake was characterised in a wind tunnel and the thruster in a vacuum chamber, and the thesis does not report an integrated test of the two producing measured thrust in excess of measured drag [18]. Net thrust against drag is what a mission planner needs. Modelled cases stand in for it. GOCE is the most instructive of them, because it failed in the way this design is meant to prevent: it launched into VLEO with a xenon ion thruster and eventually ran out of fuel [16].
Xenon is expensive, and a satellite's endurance down there is set by how much of it went up [3]. Romano's design puts a continuous electrical demand in place of a tank. An operator would have to price that against array mass and array area, both of which add drag.
The chamber also held the atmosphere still. Density and composition at these altitudes change with the day-night cycle, with latitude and with solar activity, and according to the phys.org account, making an engine operate continually across all those conditions has proven difficult so far [8].
What to watch
- An integrated test of intake plus thruster measuring net thrust against measured drag at simulated 200 km densities.
- Erosion data showing whether the graphite or silicon dioxide mirror coating holds its collection efficiency after long atomic-oxygen exposure.
- Whether any VLEO operator commits to a bus that can deliver kilowatts continuously to propulsion, and at what array mass.