[ERPS] Trajectory, schmajectory

[Ian Woollard]

I've been doing some modelling of a two stage to orbit vehicle, it's interesting. I've been cooptimising the vehicle and trajectory to try to find small vehicles.

And at one point my simulation gave a 4 tonne (wet) vehicle, but that was non reusable, and very skinny. Incidentally, I've got nothing against SSTO, but I'm looking for a really tiny vehicle, and aerolosses seem to eat my lunch at the very smallest sizes, so I wimped out and went TSTO. I want payload.

Anyway I changed the aspect ratio to give a more realistic blunt reentry shape (say, 5:1 height:width) and added some reentry shielding, but now my wet vehicle mass has jumped by almost an order of magnitude to 28 tonnes or so. The problem seems to be mainly the aerodynamics.

The optimised trajectory is interesting though- it goes subsonic, and very nearly vertical up to about 30km and then separates and the second stage burns briefly for a lob up to about 200km and then it restarts its engines and does its orbital insertion burn. I figure the first stage could land back at the takeoff site with hardly much fiddling- DC-X style; so it looked about as good as a TSTO is ever going to be. Using HTP for this might be good, because then the engines restart easily.

But the first stage really bugs me.  I mean it works, mostly, but I'm taking off vertically at about 1/2 the speed of sound and I stay there as my fuel drops from 27 tonnes down to 4 tonnes, and I haven't even gone supersonic ;-(

Darned air gets in the way; if I try to go faster I burn all my fuel too soon. If I go slower, then gravity losses do me. Argh.

It would be nice to airbreath, but that sounds rather too complex and lox and tanks are dirt cheap, and you need them above 10km or so anyway. Now John Carmack and Gary Hudson had tried a rocket powered rotor; but both had abandoned them. And I wasn't entirely clear why that would help, what's the theory?

Anyway, woke up this morning, and suddenly spotted the theory of why the HMX had the rotating rocket tipped wings at takeoff . It reduces the effective ISP of the engine.That's bad isn't it? Actually no, the optimal energy efficiency of a rocket engine is when the velocity of the exhaust is about the same speed as the vehicle moves at. Since the rocket tips are mainly supplying the energy, and the propellent is mostly air you are using their energy more efficiently, so you need less fuel. Carrying the rotor blades round for a first stage is no big deal, as they stop well short of orbit. Nice.

Cool. I checked USENET and found that this was known ~7 years ago, and a lot longer than that with jets (fan bypass or something).

I think HMX abandoned this idea because of weight growth didn't they? Anyone?