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Thursday, November 8th, 2007

Time Event
Rocketplane Thoughts
My former employer made the news again, this time for a good reason. They've announced a new design of their tourist rocketplane. This is three designs past where it was when I left. I helped produce this concept:

That was the first stage of a cargo launcher. The grey compartment in the middle opened to release a satellite and upper stage before reentering. This was right when Iridium was going bankrupt and the market for LEO commsats went *poof*. So I went back to my old job and the rest of the company converted it into a tourist vehicle. When the new guard took over they announced a new design based on a Learjet fuselage. That's out in the latest one, they're doing a from scratch structure, and I'm not at all surprised. Their design does have a few features that intrigue me.

The first bit that catches my eye is the canards (fins by the nose) and T-tail. The old crew had worked under the assumption that flat pieces smaller than the wings couldn't handle reentry. Looks like all that wind-tunnel testing says differently.

The jet engines are now afterburning and the rocket is ignited at 40,000 feet. They say this increases total thrust-to-weight by 50% so I suspect they increased the ignition altitude so they could use a larger nozzle without flow separation issues. The engine is a modification of an Atlas vernier, which is braver than the old design. We'd worked to the rule that any engine development would go over schedule so we only designed to off-the-shelf parts.

Another big concern for us was making sure the jet engines wouldn't have to endure the heat of reentry. The easiest way to tell the difference between design iterations was to check the location of the engine inlet, which always had a solid door blocking it during reentry. But the new team says they "Don't need to protect inlets during reentry". If the engines can handle the maximum reentry heat and pressure I'm impressed. Though it may be that they've positioned the engines in the lee of the wings so the reentry loads will be reduced to what they can handle.

The article says the trajectory would be "nearly vertical" which surprises me quite a bit. I can't help wondering if the reporter got it wrong. Wings can give you a good boost on the way up by counteracting gravity, but you need to have a relatively shallow trajectory to get the best use of them (it's a complicated optimization problem). I'd gone round on this once with a professor who'd done an analysis of the launch vehicle concepts out there and (in my opinion unfairly) ranked assisted-horizontal-take-off very low. Turned out he was constraining the first stage to be vertical at 2nd stage separation and that hurt performance. The 2nd stage benefited from all the horizontal velocity the rocketplane had as orbital velocity needs to be horizontal anyway. Then again, that's not an issue for tourists. What a vertical trajectory does for tourists is maximize the peak Gs on reentry. That's acceptable if the peak isn't too high.

I'll be watching Rocketplane as they build this design and I wish them the best of luck.

Current Mood: hopeful

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