Sky skimmers: The race to fly satellites at the lowest orbits yet
22 comments
·February 9, 2025mcswell
phire
I would suspect that the conditions behind the VLEO satellite are close enough to a vacuum that vacuum chamber results are actually relevant.
throwup238
Getting to ultra high vacuum like in higher orbits requires a multistage vacuum pump. To get the right conditions for VLEO they only need to skip the final stage turbomolecular vacuum pump or run it slower to decrease its effectiveness - it's basically just a bunch of fan blades that use transfer of momentum to bump molecules out of the vacuum chamber and keep them out.
I think the harder part is simulating the relative velocities of the molecules of air. They can try to use the same mechanism as a turbomolecular pump to accelerate the remaining molecules at the test setup, but AFAIK the maximum velocities in a turbomolecular pump are on the order of 1km/s, not 8km/s like in VLEO. Quick back of the napkin math says it'd need tip velocities of 4km/s which translates to over 380,000 RPM with a 4 in blade radius. I'm not sure there any materials capable of withstanding those forces. At 0.5m diameter it's a more reasonable ~76,000 RPM but that's basically the size of a commercial jet engine that probably caps out at 10-20,000 RPM.
missedthecue
Why not just a large constellation of weather balloons with a satellite payload? You have to deal with wind patterns, but that's probably a solvable problem with enough of them. Much cheaper to launch and they don't regularly and necessarily burn to ashes!
throwup238
A VLEO satellite orbits the earth in under two hours, giving it visual coverage of the planet along the satellite ground track for a dozen or more orbits per day. Launching enough balloons to get that kind of coverage, especially without the ability to control their position would cost an astronomical amount. You're not going to get anywhere near the same coverage and they'll probably all end up blown by the same wind system into a vortex and get stuck there anyway.
Weather balloons are inexpensive because the radiosonde payload is very cheap and light, not requiring much power or other infrastructure. Putting a proper surveillance payload on it would dramatically increase the price mostly because it'd then have to power the payload. That role is currently mostly done by UAVs.
the8472
Alas, not the tropopause-skimming Solid Osmium Sphere Satellite.
https://www.researchgate.net/publication/326153433_The_edge_...
LorenDB
This also has the benefit of guaranteeing that your rocket's second stage will be deorbited in a timely manner.
Havoc
> Theoretically, if you can generate a thrust that is the same as your drag
Is it just me or does that make no sense at all. Surely the friction will always be significant higher.
So at best such tech lets you go a little lower not go near indefinitely as article suggests. Sorta like how an energy recovery system extends a cars range
pavon
Because it is using power from solar panels to accelerate the ions, that's what makes it not a perpetual motion machine. We know we can build ion thrusters that generate enough thrust to overcome the drag at these orbits. But that is with different propellant. The question is if you can do the same using air as the propellant, and with even higher thrust to account for the additional drag a scoop would have over an aerodynamic design.
throwup238
The air molecules used by the ion engine as propellant go through channels in the satellite. The molecules aren't "caught" but instead pass through it and once they're ionized, the electromagnetic repulsion between the propellant and the satellite pushes them apart.
As long as the solar panels can supply enough energy, I don't see a problem. It isn't actually a perpetual motion machine as there's a constant energy input and there's no violation of conservation of momentum that I can see. The problem I think is more of an optimization - how big can they make the solar panels without adding too much drag? That will set a bound for how low these things can fly.
null
uSoldering
I like how the article glazes over the fact that this is drone swarm delivery tech. You are naive to think otherwise.
throwup238
What kind of drones are you thinking, other than just good ol' missiles?
Anything even remotely drone shaped would have to be covered in so many aerogel reentry tiles and thermal insulation I'm not sure there would be any usable weight left for a payload.
mechagodzilla
Why would I want a drone swarm in VLEO? That seems like a really hard place to effectively use drones from compared to pretty much any alternative.
uSoldering
You are missing the forest for the trees. This is an aerospace vehicle capable of flying in the air indefinitely. How many drones can you fit on a vehicle? How many vehicles can you fly at once? 60 miles is already reachable by the cheapest and most deadly weapon class of short-range drones. And now all they need to do is fall down. At freefall that is 140 seconds until it reaches the ground.
ianburrell
The drones would burn up. The satellites are in space and moving at space speeds. If the satellite drops anything, it stays in orbit and slowly decays. Dropping anything requires a large rocket to get out of orbit. The payload would still be going fast and deal with re-entry.
Also, the article doesn't say that they are at 60mi, just that VLEO goes down to 60mi. The satellites are likely to be higher because there is less drag.
pavon
They would just need to fall, they would need to shed 30,000 km/hr of velocity.
brookst
You come across as fairly confrontational.
But I don’t understand the point. Satellites already fly indefinitely and can drop payloads of kinetic energy weapons. Why all the complexity of VLEO and drones? What could you that’s impossible with other, simpler, already-extant weapons systems?
tomrod
Quick deployment into a battlefield or disaster zone anywhere on earth is one idea.
Connection, remote sensing, and mesh networking are useful.
ianburrell
There is no difference between these and higher satellites. It still super expensive to get them down because have to stop orbital speed. If anything, VLEO is worse cause they are going faster. Not to mention the cost of getting into space in first place. Plus, need lots of satellites to support dropping things anywhere on world.
Also, it is rare to have anything that needs 30 min response time. Most things can be handled by local resources or wait for an airplane. Good example is that the US is working on hypersonic missiles and deploying them around the world.
Sort of like a Bussard ramjet, but using solar power rather than nuclear fusion.
Something that bothered me, until I looked up some numbers: the air a VLEO satellite scoops up is presumably more or less at rest wrt the Earth below it--there could be wind and effects of the Earth's magnetic field, and so forth, but for practical purposes it would be coming at the VLEO satellite at about 18,000 mph, or 8 km/s. In order to produce thrust, it has to exit the back of the satellite at a significantly higher speed. It turns out that ion thrusters have an exit speed of 20--50 km/s. So I guess it's feasible.
More info in this Wikipedia article: https://en.wikipedia.org/wiki/Atmosphere-breathing_electric_... --which says the exit speed of this kind of ion thruster is 55 km/s (as tested in a vacuum chamber).