Rockets of Today


Stoke Space, a startup in the Pacific Northwest with a bunch of SpaceX and Blue Origin veterans in it, looked like just another in a long line of small me-too rocket companies that I figured had no chance of catching up to the leaders, but now I think they ought to be taken seriously. They are coming up with unique and innovative approaches that have some potential to seriously disrupt the smallsat launch business. And they’ve gotten it off the ground.

The idea is to make both stages fully reusable, and since this has yet to be achieved with an upper stage, they’re working on that first. (A lot of small outfits start with the upper, but usually that’s just because they don’t have the facilities to do anything big.) Here’s their idea: make the bottom of the stage a heat shield, and make the engine into a ring with many nozzles firing over the edge. Because the heat shield is dome shaped, the exhausts of the nozzles tend, in the upper atmosphere, to pull together in the center in a sort of aerospike effect. And during reentry they produce a somewhat similar effect with a cold gas orifice in the center, which helps put a protective layer of vapor in front of the shield surface. That surface is also cooled from behind by the cryogenic propellants, with the boiloff providing the cold gas. And the low density of mostly empty tanks means it should lose speed quickly. The result, they hope, is that the shield can be bare steel, and be quickly reusable with little or no refurbishment. I have grave doubts about the survivability of a bare steel shield — some additional protective layer will probably be unavoidable.

That upper stage ring motor burns hydrogen, and the bottom stage will burn methane. Probably. Their plan is to go for 100% ambition and see if they can achieve full-flow staged combustion — the most advanced and complex type of rocket engine there is, which only SpaceX has successfully managed to build so far for their Starship. They’ve gotten far enough that they’ve gotten a prototype to start up and run for a couple of seconds.

That first stage exists so far only as an artist’s conception, with a stubby body slightly narrower than the upper stage heat shield, four fold-out legs, and seven engines. But the upper stage has been built and flown as a hopper prototype. This test article has only fifteen nozzles around the ring motor — half as many as they plan to use in the real thing, but it’s enough to do hover tests with. They can control the thrust of the individual nozzles by using the pintle injectors as throttles, so nothing needs to gimbal. The nozzles share a single expander-cycle turbopump. The heat shield is slightly tilted off horizontal, producing an aerodynamic effect similar to the way traditional capsules are asymmetrically weighted, helping them stay upright and reenter at a shallower angle. It has three straight legs, which in the real version will retract pistonwise up onto the sides. The plan is to have the fairing hinge open and stay attached.

There are no specs announced, but it looks like the Nova will be pretty substantial for a first rocket, with a lift capacity of several tons and a diameter fatter than a Falcon 9. Aiming for full reusability means the rocket will use substantially more fuel than an expendable one, and therefore be bulkier in size, but it should still be far cheaper to fly. So cheap that small rockets would probably be unable to compete with it. But that all depends on whether they really can make a stainless steel heat shield survive reentry. There isn’t any way to simulate reentry conditions on the ground, and there also isn’t any good way to get data back from a failing reentry, as all radio communication is blocked. So if it fails, it will be very hard to diagnose why.

Nova: Mass unknown, diam ~4 m, thrust unknown, imp unknown, full-flow staged combustion? (methane), payload ~7 t (expendable)?, cost unknown.