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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.
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. //
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. //
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.
The payload has to be able to survive both as well, which means that the engineering and packaging of the payload has just become harder. We’re not going to throw iron bars into space for processing with orbital solar smelters. Non-compressible liquids are possible, but liquids like to slosh, so the sudden change of forces would be really difficult to dampen. //
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. //
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.