Uncle Sam explores satellites that can create propellant out of thin air Electric propulsion systems that generate power from the scant air in the outer edges of the atmosphere could drive satellites in very low Earth orbit (VLEO), without the need for conventional propellants – at least in theory. Armed with $400,000 in funding from DARPA, boffins at the US Department of Energy's Princeton Plasma …
yeap, sometimes your fuel is your reaction mass, chemical rockets for example, but as the design in the article is setup, it's electrical input for the energy, which is your "fuel" (it's just weird to say electricity is a fuel, but in this case it does full fill that role), and the air for the reaction mass. *nods*
Maybe the government should award the contact to people who know what they are doing. This problem was solved in the 1960's and is used on every ion thruster built since, including the ~4000 Starlink LEO satellites presently in orbit.
...or perhaps they do know what they are doing. There is a reason existing designs use noble gases generally (reduces induced dipoles by orders of magnitude) and xenon in particular (high molar mass).
But once again, an anonymous El Reg commentard knows better than those spending their entire careers in sector.
On The Other Hand, dipping into the sticky stuff causes drag and slows the sat (in fact All LEO sats suffer this to some extent, that's what defines LEO). Also, the ionization energy you put in gets wasted as heat on recombination. So you have to scoop-eject extra propellant to maintain orbital speed. And on both counts you have to have a beefier leccy supply, not forgetting that solar panels would cause even more drag so it has to be all-onboard.
Not saying the balance can't be tipped in favour of a net efficiency gain, but it's no easy ride.
I guess the best way to look at it is as an electric ramjet.
On top of which, the drag up there at the top of the atmosphere isn't constant. That'd be too easy. The drag depends on how much material is arriving from space -- IIRC chiefly via the "solar wind" -- which is anything but constant. Doesn't mean this scheme can't be made to work. But getting it to work is likely to be challenging.
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Nice idea, but I really don’t think this is going to be practical (except, see last paragraph). If you are flying low enough to be collecting reaction mass, you experience drag from the medium in exactly the same ratio. The drag is proportional your velocity through the medium, orbital velocity 7.8km/s, while the thrust is proportional to the ion engine exhaust velocity, typically 30km/s. From first principles, for thrust to match the drag, the mass collector must intercept at least 25% of the frontal area. That’s difficult, as spacecraft are usually designed totally un-aerodynamic. But it might be just marginally possible. Except…. Solar panels.
You would have to make the solar panels point aerodynamically edge-on to your velocity, since they are about 5x the area of everything else put together. Otherwise, they’ll drag the whole thing down.That is what is shown in the diagrams for this, so they do understand that. However, it is normally critical for the solar panels to be motor-pointed and locked onto the Sun position, for continuous power delivery. The spacecraft is going round the Earth, so constantly turning away from the Sun. This is a huge problem…..especially as the power from the solar panels is exactly what is driving the ion engine thrust. If you turn the solar panels away from the Sun, there’s zero thrust…..
So, why are they doing this, are they stupid? No, I think this is a military program in disguise. The solar panel issue goes away if you power the spacecraft with a Nuclear Thermal Battery, which civilian programs aren’t allowed to launch, instead of solar panels. As noted, the thrust scales in exact ratio with the drag, so long as you have the power. This means that you can fly as low as you like, ie as dense as you like (within limits). If you remove the solar panels from the satellite, and cover it in stealth material, it becomes nearly invisible. The plume from an ion thruster is visible in a darkened room, but that’s about all. So I think this is for military systems.
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