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Old July 22 2012, 12:46 AM   #172
Location: Kentucky
Re: SpaceX is a go for April 30th: 1st commercial launch to space stat

sojourner wrote: View Post
This is not entirely true. You would get a data set of 81 initial burns. Not the same data set as firing the same engine 81 times. The former would be of limited use for reliability testing of reusables.
Oops. An unstated assumption was that the 81 engine rocket wasn't built from completely new hardware, but assembled from existing re-used Falcon-9 stages or their engines. That's another benefit of the idea over a clean sheet design, because you already have a pretty good idea about the system reliability going in.

The Falcon 9 actually has engine out capability only after the first 30 seconds of launch. Before that the T/W prevents 8 engines from providing enough lift.
Yep. It's close! Just a few more engines...

It's probably moot anyway. If NASA asked SpaceX fora rocket with that large a payload They would clean sheet it with a larger core diameter and bigger main engine just to avoid the functional nightmare of a frankesteinian 81 engine, 9 core system.
Very true, and even Elon is doing that with his Merlin II design, with a thrust of 1.9 million lbf. It's like nobody will get to the other side of that statistical hump in the failure rate.

Still, the idea is a pretty trivial one. To multiply the payload by X, use X times as many of everything - in parallel. And the same system covers all configurations of (X - n), so if X is large, you've got a good granuarity to match launchers to payloads without designing anything new. Of course the logic only pays off if all those stages fly back for re-use, otherwise you're just throwing away X times as many stages.
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