Karl Gallagher (selenite) wrote,
Karl Gallagher
selenite

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The escape hatch opens a little more

Burt Rutan has announced his design to win the X-prize. It's a nice design, I like it, and I think it'll work to win him the prize. Nobody else looks likely to beat him to it.



There's obviously a lot of work in the design to keep it simple. He's using a hybrid engine with a room-temperature propellant. The fuel section looks like it can be just yanked out and replaced after each flight without a lot of work or detailed inspections. Hybrids are about as safe as a rocket engine can get, so that's going to save him a lot of time and money on design details and operations procedures. He's even limited the throttle to on/off. The SS1 doesn't need to hit a precise altitude and flight path angle like a satellite delivery vehicle, so overshooting a bit is fine. This way they only have to worry about a simple valve and control instead of testing a complex one.

No space suits is another part of the simplicity. They're going to have a pressure-tight cockpit and get it right. I suspect this is mostly driven to conserve weight and volume. Putting the three crew in pressure suits would probably double the size of the cockpit. As it is they may need shoehorns to get them all in.

The most innovative part of the design is the "shuttlecock" reentry stabilization. I had to stare at that for a while to be convinced of it. A completely passive attitude system is pretty unusual for RLVs. But I see how it works, at least as long as they keep the CG in a limited range. They have an interesting trade between sticking them out far enough to get good control authority and not being out of the wake enough that they overheat.

That brings me to the dog that didn't bark-the thermal protection system. There's no mention of it at all. The trajectory plot makes it look like it's going to come down like an elevator, so they'll have minimum speed, but it's still going to have enough energy to reach 1400 m/s and that's going to get warm. Keeping the wingtips protected might be more crucial than the fuselage-the more heat it can stand the more control they get.

The two worries I have are holes in the flight envelope if they go short of the maximum apogee. If they shut down the engine early on a max-load flight the center of gravity is going to be way back of nominal. That could put the nozzle on the windward side of the ship, leading to instability or damage. The other problem is an abort or test flight to the top of the atmosphere. Chuck Yeager's famous NF-104 crash happened because he got into a region where there wasn't enough atmosphere to use the aerodynamic controls but there was enough to keep the RCS rockets from moving the ship to the proper attitude. If SS1 comes back in at the wrong attitude it could wind up tumbling. Both of these would probably be dealt with operationally by defining some no-abort windows during the ascent. So the engine could be turned off during the first (say) fifteen seconds, but once you're past that you have to keep it until 55 seconds in no matter how many red lights go on . . . .

Controlling the center of gravity is going to be a major issue during the ascent. The engine looks to be fixed to the structure so the CG must be on the thrust line to prevent torques. Any deviation-such as a passenger having the wrong weight-will require the attitude control system to correct for the torques. While in atmosphere that'll be easy, but once the RCS has to make the corrections they could rapidly exhaust their fuel. Balancing the ship will probably require a different ballast load for every flight, or an "active" ballast system that can correct for an improper CG.

I'm pretty sure Rutan has all those bases covered. Then again, this is the guy who said "I think that if no one dies going after this prize, why then we're not really going out and truly searching for new ideas."

The SS1 isn't going to lead directly to an orbital vehicle. It'd need a lot more delta-V and a real TPS, and the "shuttlecock" reentry won't work at all-the wingtips would burn off right away. So it can't just be scaled up. It will have two benefits-market and psychology.

Once the ship is flying anyone interested in a tourist flight can see that they can get it for real, and if Rutan won't sell tickets someone else will build a vehicle that can. If you have a thousand willing customers waving cash, the bank will make a loan. This will also give a good estimate of the size of the market for orbital tourism and people will start working out business cases and designs for that.

The psychological impact may be even greater. A successful SS1 flight will cross into a domain where only the government has been before, and do it for 1/1000 the cost of the Orbital Space plane. Some senator will notice that Rutan got 1/4 the altitude at 1/1000 the price and start asking questions. And NASA will give the same answers it usually does, minus the ones about how difficult rocket engines and operating in a vacuum are. Then people outside NASA will realize, "Hey, you can do this with a small group of engineers and techs instead of an army of tens of thousands" and investors will have a lot more faith in rocket entrepreneurs. A lot of possibilities will open up. We may want to keep an eye out for NASA complaining about new ventures being a safety hazard, they've done that to some other projects to try to get them cancelled.

I'm pretty sure Rutan's going to win the X-prize if he can keep his schedule. It's a good design and he's not trying to overengineer it to do anything more than the prize requires. Unless somebody comes along and buys it off him to start a tourist service it'll probably do two full-up flights and then go to the Smithsonian. But showing that it can be done is going to have a big impact on the rocket field, bigger than DCX. Here's wishing him the best of luck.

After building the Voyager round-the-world aircraft Rutan said, "See what free men can do." If he puts SpaceShipOne into space he'll have made an even greater demonstration of the power of freedom.

Tags: engineering, projects, space
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