It’s not like we are doing any mining for exotic materials on the Moon or Mars, so I suspect that this interesting intellectual capital might sit on a shelf for a few decades.
SpinLaunch is playing with a different, electric model for mass launching to orbit. It is trying to throw mass into space, but there are challenges.
In October, a company called SpinLaunch threw a 10-meter long dart at the sky, reaching roughly 10,000 meters in altitude. So what, you might ask. Well, it did it in a novel and interesting way, which one day might actually be useful for throwing stuff into orbit using electricity instead of rocket fuel.
What’s novel about it? Well, the launcher is a giant solid sling inside a vacuum chamber. It has a big counterweight on a short arm at one end, and a long end that holds the payload at the other. Over 90 minutes or so, it uses electricity to bring the rotating arm with the dart on it up to absurd revolutions per second, about 10,000 gravities of centripetal force.
Then, at exactly the right microsecond, they let the dart go. It goes up through a tube with a light plastic sheet keeping the vacuum in and air out, and continues upward under its own inertia for 10 kilometers right now.
Their goal is to get the device up to the 200 kilogram range and throw satellites with final stage rockets into orbit. This is obviously very interesting, which is why they’ve received $75 million in funding so far. $38 million of that went to build this sub-scale prototype, which managed to be the biggest vacuum chamber in the world.
So, it was a successful technical demonstration of an electrically powered, zero-emission satellite launcher. That’s pretty good. It doesn’t require kilometers of track pointing upward, as proposed linear accelerators do, and the vacuum chamber means that it avoids air resistance until it’s already going at absurd speeds.
The intent is to craft a sabot — a surrounding aerodynamic shell — which wraps around a non-aerodynamic thruster, fuel tanks, and payload. The sabot is attached to the long arm inside the spinning accelerator, and thrown at orbit. Up in orbit, or near orbit, the sabot would pop apart, leaving the simple space vehicle to deliver the payload to its final orbit before it presumably has its own orbit degrade and becomes a brief flash of light in the sky somewhere.
However, there are a lot of challenges to overcome before SpinLaunch might be considered a competitor to SpaceX, never mind the much easier target of Blue Origin. The parts I have concerns about are the following:
First, while the demonstrator is amazing, as a prototype it’s well below the rule of thumb of quarter-scale by volume for mechanical system prototypes. They assert that it’s a 3rd scale, but that’s by diameter, not 3-dimensionally. As such, it’s a great demonstrator of the principal and as impressive as any piece of awesome engineering that cost $38 million to build, but doesn’t derisk nearly enough of the major technical challenges in my opinion. This is a fairly constant challenge in aerospace, as actual quarter-scale prototypes are wickedly expensive. I wrote about this challenge in 2014 regarding Google Makani, the now defunct airborne wind energy firm, in that their 29 kW prototype was too small to adequately derisk the 600 kW target machine. Along with many other challenges inherent to airborne wind energy, this proved to be the death knell for Makani
Side note, I spent a couple of hours recently talking to Damon Vander Lind, the guy who ran Makani for a few years, and then moved to Kitty Hawk to design their electric vtol. He’d read my recent pieces on urban air mobility and electric vtols and reached out to talk through our intersecting perspectives on both airborne wind energy and urban air mobility. He was remarkably pleasant given I’d published harsh critiques of his two major roles of the past 13 years. Of course, that was probably helped by apparently not having seen the Makani pieces, and no longer being with Kitty Hawk. Part of our conversation was devoted to his current focus on delivering a worthwhile product that moved the needle on climate change.
The key point here is that the successful demonstration opens the way for funding for a bigger prototype, one which will actually start to test some of the weirder aspects, and perhaps equal Blue Origin in throwing mass past the notional 100 km altitude where space is considered to begin — not humans, though, as the centripetal force would turn squishy humans into red jam on the inside of the sabot long before launch.
The second challenge is that the sabot, enclosed orbital vehicle, and payload have to be able to survive not only 10,000 G lateral forces, but the orbital vehicle and payload have to manage the rocket forces when they kick in. The sabot is shed by that point, but it’s much easier to build something that will survive extreme forces in one direction than something that will survive extreme forces at right angles to one another.
Third, the gripping component of the spinning arm has to be able to support the sabot at 10,000 gs and also release it in a microsecond without causing any wobble. That’s an extreme engineering edge case by itself.
Just preventing the sabot from crumpling under the stress at the attachment point, or even folding in half is also seriously non-trivial engineering. 10,000 gs at what is necessarily a small set of attachment points around the center of gravity of the sabot leaves dangling sabot under serious strain at either end. The more gs you pile on, the more attachment points you need, and the less ability you have to release them instantly.
A 1,000 kg total package for a 200 kg payload at 10,000 gs is equivalent to 10 million kg of weight on earth. Electromagnets are absurdly strong, but a 3 Tesla magnet only puts out 522 psi, and the strongest electromagnet is 35 Tesla. That degree of magnetic field will also fry a lot of things. It’s unclear to me what their attachment solution is intended to be, but it’s expected to do an absurd job.
Fourth, the rotating arm’s moment of inertia is going to change radically and instantly at release. The buildup of velocity takes 90 minutes, so it’s easy to balance, but the release is instant, with a couple of tons of mass at 10,000 gs disappearing at the long end of the arm. Getting the mechanics of that right is another extreme engineering case by itself. I can posit a couple of approaches to that, but it’s going to be a long, hard slog to get it to not disintegrate.
Fifth, atmospheric buffeting at release will be non-linear. Hypersonic speeds in the bottom parts of Earth’s atmosphere are non-trivial, which is engineering speak for really hard. To hit orbit, it will be at serious multiples of Mach speed at ground level. So, also very, very noisy. Not a good neighbor.
My intuition — and it’s only a somewhat informed guess — suggests that the combination might not be surmountable on Earth. However, on the Moon or Mars, a lot of things become much simpler. No atmosphere or an incredibly thin atmosphere both eliminate or reduce the need to create a vacuum chamber in the first place, and make the hypersonic sabot’s interaction with the atmosphere immaterial. Much lower orbital velocity requirements mean that the issues related to 10,000 gs aren’t there, just a smaller but still absurd number of gs. There are still issues with the robustness of the sabot and payload, release and moment of inertia, but more manageable ones. It’s easy to see throwing Helium 3 from the Moon into a slow orbit that will intercept with Earth’s orbit.
Of course, it’s not like we are doing any mining for exotic materials on the Moon or Mars, so I suspect that this interesting intellectual capital might sit on a shelf for a few decades. I could be wrong, however, and the team appears deeply talented and bright, so perhaps it can be used on Earth. It will be interesting to find out.