Monday, September 12, 2005

The cold equations of spaceflight

Jeffrey F. Bell writes in his usual colourful fashion in his newest spacedaily op-ed, The Cold Equations of Spaceflight about the unrealistic nature of a lot of past space development and the "fanboys" too.

I've come to some of the same conclusions myself. I think his best line is:
The Cold Equations dictate that rockets need to look like oil storage tanks, not the sleek spaceships of science fiction.
Yeah, that says it the most elegantly. It's funny how political processes so often can without worries fly in the face of physics, like the X-30 or X-33 projects. There are probably many things currently (though in my opinion the American ballistic missile defence is not so clearly one of them) that are being done similarly, money being wasted while almost everyone involved in the real engineering or science part understands that there will never be clear results to show. It might even be quite true about the global warming.

The one thing about rocket development that outlines the difficulties, is precisely the mass ratio. On a speculated single-stage-to-orbit vehicle using the most energy-efficient propellants, Bell mentions that the mass ratio becomes 92% fuel, 8% everything else.

Then your payload can be maybe only 12% of the "everything else" part, making the mass breakdown to 92% fuel, 7% rocket, 1% payload. (Out of that rocket, a lot of weight is on engines and plumbing, so the tanks are probably only 5% of the total mass.) Since it's really hard to make containers that can sustain many G:s of acceleration and hold 18 times their own weight in fuel (especially with light fuels like hydrogen), you might run into unexpected problems and increase the weight of the tanks and engines by a modest amount like 6%. Have fun, since you have just halved your payload! The absurd mass ratios make margins very thin and the design and development extremely sensitive.

Seems like some have buried those dreams of reusable single-stage-to-orbit vehicles or high-energy yield hydrogen and concentrate on leanly manufactured good quality hydrocarbon rockets with two stages and in-house built engines. I'm talking about SpaceX of course. With their first smallish rocket to launch before year's end, I'm looking forward to it as the most sensible plan in the medium term to send stuff into space.

To return to Bell's article once more, I think he is being a bit unfair. The fancy ships like the gliding lifting body x-33 or the vertically with a rocket engine landing dc-x were only prototypes and their mass ratios and other properties shouldn't be directly compared to refined rockets.
I'm still not exactly sure though, if those approaches would have some role as first or second stages in rockets or even payloads.


Anonymous Anonymous said...



Tue Sep 13, 11:08:00 AM GMT  
Anonymous Zwiffle said...

Yeah I read an article about SpaceX, fascinating design plan. Small company acting like a big dog, and from what I've seen they're likely to steal business away from everyone else. Incredibly small rocket designs with reusable hulls and ultimately only a fraction of the cost to send something into space as other companies. (I think $1.6 million compared to like $30 million as the next smallest competitor iirc.) And the guy who started the company is whacko/genius too, really thinking outside the box on this one. Internet millionaire turned rocket-entreprenuer.

Anyway, that's the only part of the blog I knew about, so I had to say hello. :)

Tue Sep 13, 12:15:00 PM GMT  

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