In the end I’ve been told fuel crossfeed is a lot more dangerous than a layman like me would expect. Crazy idea, but it was fun.

Running the numbers with likely propellant combinations gives very promising results.

I suppose a baseline non-cryogenic design would use 98% peroxide and 1,2-butadiene as propellants. The chamber would run at about 120bar and the ground nozzle would only expand it about 16:1, for an expected sea level ISP around 275-285. During the first exo-atmospheric rendezvous, which would occur about 80km directly above the launch pads, you would also deploy a nozzle extender, increasing the expansion ratio to about 100:1, giving you an expected vacuum ISP of 330-340. I foresee difficulties trying to loft a payload bigger than about half the dry mass of the plane module, but that should be enough to make the concept profitable.

It does seem that the rendezvous operation itself would have to be exceptionally low probability of failure, but I see no reason to expect that the required reliability would be impossible.

Also, the question of where the upper high-velocity sub-orbital stages would be able to land would become a significant issue. I don’t believe that they could all fly directly back to the launch site, unless they were refueled in mid-air.

What I like about it most is that it scales directly from sub-orbital test flights to orbital missions without changing the design at all.

Start with Dave Salts’ 4 ton payload vehicles that have to be refueled leo before delivering a geo bird. After Dave has proved that he can quickly dock and move fuel in orbit, Just start moving the refueling process back toward the release from the lower stages. After a dozen or so launches, the mating could be early enough to seriously add to the fuel remaining in orbit. Two ships delivering a satelite and 6 tons of fuel instead of a satelite and 4 tons of fuel.

It also allows the tanker stage to be completely drained with no requirement for unusable reserve. He could eventually deliver 10 ton satelites instead of 4 ton ones.

This could provide early use with minor changes in the upper stages. If this adds something useful, then I dub it Partnered Upper Stages. Probably just use the three letter acronym.

That sounds like a good honeymoon activity for people, not just a way to test machine coupling.

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One can probably get some of the efficiency back for the upper atmosphere ISP by having half the fleet optimized to be going onward.

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I think that there is some built in redundancy here for all but the payload module; about half of them will need male and female components and of those, they are pretty interchangeable so you only net one of the wet lock directions to take. Or one can have the plumbing be an interstage to save some weight.

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I think that if you miss the wet lock but have a dry lock, one can just burn all the fuel in one of the pair. That is, instead of propellant transfer, some of the matchups can just be momentum transfer which should provide a way to get much of the use of the propellant if not optimal.

So it’s just subterminal automatic RW with cold gas thrusters. Piece of cake

Recharge the air bottles & batteries, hoist ‘em up, and do it again fifteen minutes later.

Doug Jones, wistful former skydiver

Under those controled conditions, it should be possible to dock in 10 seconds or less. A couple thousand feet of altitude should do it. Chute recovery of the models.

When talking about proximity-operations testing, what occurred to me as a way to experiment with this quickly and cheaply, would be to simplfy it to 2D by having all the robots driving up the inside of a freely-rotating drum. (like a very wide hamster wheel). That would at least let you do a first-order approximation of your necessary routines.

– Carl.