Hacker News Reader

Comment by mcswell | Hacker News Reader

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).

View on HN

> In order to produce thrust, it has to exit the back of the satellite at a significantly higher speed.

Why? It can exit at 1 m/s and still be thrust. And as long as momentum from engine pushing is larger than momentum of stopping atmosphere -- it will accelerate.

In other words -- chemical engines have exit speeds way lower than 8 km/s, but are able to accelerate spaceships to and above 8 km/s.

I suppose because they are catching them when they are at ~0m/s in the atmosphere so they're worth -8km/s worth of momentum to begin with. So you would have to fire them out a >8km/s to get a net +ve change in momentum.

> It can exit at 1 m/s and still be thrust

But it won't be enough thrust to counterbalance the force of scooping up air that collides at 8 km/s

In order to produce net thrust, it has to be more than that.

The mass of propellant being expelled is many times higher than the mass of air colliding with the rocket.

If that is the case then yes, agree, but there was no indication that is the case. The air is very thin out there.

In fact, it's linearly proportional, because momentum us m*v, so for 20 km/s propellant exhaust seed and 8 km/s air it needs to spend exactly 8/20=0.4 times the propellant to not slow down.

I understood that they want to used scoop up air to use as propellant. Only a fraction of the air it encounters will be able to be sucked in by the rocket.

You're overlooking the conservation of mass constraint.

I would suspect that the conditions behind the VLEO satellite are close enough to a vacuum that vacuum chamber results are actually relevant.

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.

I know more about turbines then I do vacuum pumps, and do agree that about 1000 m/s tip velocity is pretty much the material limit. I suppose something like the solid disc in a tesla turbine might exceed that however I don't know if that's useful or if you'll get even close to 8km/s out of that.

Going beyond spinning discs there's the USAFs "Hypervelocity Wind Tunnel 9", that can hit mach 14 or 4800 m/s, which, still not good enough.

Turbomolecular pumps are basically turbines except they’ve got more blades in a different configuration, they’re driven by a motor instead of extracting energy, they have no method for convective cooling, and can’t use any lubricants or cooling fluid because they would vaporize and contaminate the vacuum.

I’ve got more experience with turbojet engines than turbines so I’m not sure how that changes the calculus on the last two points. Turbomolecular pumps use active electromagnetic bearings and require very precise manufacturing to match blade radius with the chassis and balance the blades. The material science for the blades is mostly the same.

I’m sure they’d could get plenty of useful data at mach 14 but sadly I doubt the USAF wind tunnel machinery would work in a vacuum environment without melting immediately, and it’s probably way bigger than even the biggest vacuum chambers NASA has. I’ve never heard of the tesla turbine before so I don’t know how the angular momentum translates to the molecules.

Presumably you'd have an array of 'slow' ion thrusters upstream of your test article in your vacuum chamber to generate your flow. Matching temperature and velocity profile exactly is probably fairly tricky, but producing nominally 8km/s of very dilute gas shouldn't be particularly difficult.

I suppose something like the solid disc in a Tesla turbine might exceed that

It’s been a few years since I studied these types of issues, but I assume the relevant mechanical deformation from these speeds (creep) is a problem no matter the geometry.

Reply on news.ycombinator.com