Sky skimmers: The race to fly satellites at the lowest orbits yet
bbc.com
bbc.com
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).
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.
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.
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 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.
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.
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.
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.
Starlink direct-to-cell satellites are already in a VLEO orbit shell, distinct from the rest of the constellation. They use electric propulsion (like all Starlinks) to counter drag, though it is not air-breathing electric propulsion (the theme of the OP).
- "The higher luminosity of these DTCs compared to regular Starlinks is partly because they circle Earth at just 217 miles (350 kilometers) above the surface, which is lower than traditional Starlink internet satellites, whose altitude is 340 miles (550 kilometers), the study reported. [...] There are now over 100 DTC satellites in low Earth orbit, including 13 that were launched last week. Following successful testing of the first batch of DTCs, in March SpaceX requested an amendment to their license with the U.S. Federal Communications Commission that would allow them to operate up to 7,500 DTCs in LEO."
https://www.space.com/spacex-starlink-direct-to-cell-satelli...
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.
Weirdly, I'm probably the one person who's worked extensively with both paradigms...
There are several players in the balloon market spinning up. It's becoming very hot very fast.
The balloons in question are much smarter that radiosondes now. They are fully capable of autonomous navigation within certain limits and can even orbit a point for days at a time via some clever weather mechanics. They're also very easy to launch. Two guys and a truck can launch over a dozen in a day and they're all aggregated and flown remotely via various links.
On the payload side it's actually much easier to do optics on a balloon, you have all the same pressure and thermal issues as space but you can iterate much faster when your cost to first pixels is thousands instead of tens of millions for the same ground resolution (GSD, drives necessary aperture size). I'm not going to spell out the details on either because that's the secret sauce that pays my mortgage but it suffices to say, if you're 10x lower, you need a lens that weights a little less. It's also worth noting that a VLEO sat will only be overhead for a few minutes, balloons can stream live video as long as you want, right now (no future tech or constellations needed).
The class of UAV that can begin to compete with a balloon is two to three orders of magnitude more expensive and still has nowhere near the endurance.
There are also other sensors and phenomenologies that are wildly more capable on a balloon platform than any type of satellite or UAV but I'll get yelled at if I spell any of those out...
There was recently an article about the 'UFOs' actually being 'just' Chinese balloons designed to record everything about the radars poking them to learn about capabilities of US defense systems (including F-35 in action). Guess we'll see more of them soon.
LEO is cheap but you need a massive aperture, measured in meters, to get equivalent GSD to even the crappiest balloon imager, so the price tag suddenly jumps. These aren't cubesats, they're suddenly the size of a bus, see Worldview Legion for a recent comparison, and it has a much worse nadir GSD than balloons.
VLEO is still extremely expensive because you need a very robust propulsion system and there are other design considerations like atomic oxygen corrosion. The optics to match a balloon also put you into the mini-fridge to refrigerator sized optics assembly class which means while you can rideshare, it's not cheap to build or launch, see Albedo space:
https://albedo.com/post/upcoming-launch-of-clarity-1-and-alb...
Also the regulatory issues are still massive, you need to get a NOAA license for imagery and once you go under a certain GSD limit they become very difficult to obtain.
The can sometimes leak slightly but they make up for it by dispensing small amounts of ballast material.
They're generally more environmentally friendly too, worst case you have to go ask a farmer to retrieve the thing out of their field.
Please tell me, in the industry, this is called a "rapid unscheduled descent".
If it isn't, it should be! ;)
_In principle_, a VLEO satellite could stay up forever (or, well, realistically until the engine wore out).
Weather balloons also don't last forever.
https://www.researchgate.net/publication/326153433_The_edge_...
The classic LEO, roughly 500km-1000km, is already quite full, and placing a lot of new satellites there increases the risk of triggering a chain reaction of collisions with orbital debris (Kessler syndrome). At these altitudes the atmosphere is thin enough that even small pieces of debris are long-lived, thus potentially creating a long-term problem for all kinds of human and non-human spaceflight.
In the VLEO band, on the other hand, the thicker atmosphere makes maintaining orbit more difficult, but also makes debris very short-lived. Due to this, it may be reasonable to risk placing a million-satellite communications network somewhere between 200-300km, which would be utterly impossible further up. Data rates, latency, mesh size, etc. also improve of course.
Despite advancements in technology, as on today it is not possible to get near real time information of flooding.
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
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.
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.
Connection, remote sensing, and mesh networking are useful.
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.
VLEO has its uses.
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.
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?
Assuming the tech works, you can move the sat to any orbital plane, making it significantly harder for an adversary to track all of your vleo assets.
Are you thinking of something like this? https://en.wikipedia.org/wiki/Kinetic_bombardment#2003_Unite...
Again, you probably wouldn't bother with VLEO for that, too much extra complexity with no obvious advantage vs VLEO.
That's, ah, a huge 'all' :) They'd be moving at about 8-10 kilometres per second. The vehicle is not _really_ 'flying in the air', it just looks a bit like that. It's in orbit at terrifying speeds. Your 'drones' would require very extensive heat shielding. They would not be at all cheap. And why use this rather than a normal, cheaper, less complex LEO satellite? Or an ICBM. If you want to arbitrarily drop heavily shielded gadgets on someone's head at silly speeds, the normal tool is an ICBM (the gadgets usually contain a nuke, not a drone, mind you).
Even if this were practical, why use weird expensive highly-visible VLEO satellites rather than conventional LEO satellites? I can't see what advantage the VLEO stuff would give you here.
Based af