[AR] Re: Falcon Heavy use cases
- From: John Schilling <john.schilling@xxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Wed, 7 Feb 2018 21:31:06 -0800
On 2/7/2018 9:13 PM, Henry Vanderbilt wrote:
On 2/7/2018 7:32 PM, John Schilling wrote:
The Falcon Heavy upper stage is not reusable, so you'd need a new one
of those every time - and they represent a larger fraction of the
total system cost than their relative size would suggest.
The most expensive single component of an F9 booster is likely the
Merlin engines. 28 in an F9H, 27 recovered, one expended in the upper
stage. So, 3.6% of the overall vehicle cost there.
Careful, the Merlin Vacuum isn't the same engine as the Merlin 1D. Even
accounting for the shared hardware, being produced (and more importantly
being /tested/) on a much smaller scale, is going to make the upper
stage engine significantly more expensive than one of the booster engines.
Tankage and structure are cheap compared to engines, but yes, 2nd
stage tankage is likely more than 1/28th the total. And of course
there's the guidance system, attitude thrusters, etc.
I'd still be very surprised if the F9H upper stage is as much as 10%
of the overall vehicle manufacturing cost.
Which all begs the real question, as far as I'm concerned: F9H cries
out for a high-energy upper stage.
In the best of all worlds, an ACES stage scaled for the F9H would
provide very, very interesting capabilities.
Yes, and now that you've suggested it, I'm going to have to model that
combination. I'll let you handle the politics of a ULA-SpaceX joint
venture.
Also, Schilling's three rules of space launch propulsion:
1. It is foolish to use anything but cheap, dense propellants in your
Earth launch stage. You need thrust against gravity, and you shouldn't
much care about weight when it's just cheap rocket fuel and sheet metal.
2. It is foolish to use anything but LOX/LH2 for your orbital insertion
stage. You need Isp to build delta-V, and every pound of "cheap"
propellant has to be lofted halfway to orbit by an expensive booster.
3. It is foolish to use different propellants on different stages of
your rocket, because that makes every bit of hardware and every
operational procedure a complete duplication of effort.
Now go design a not-foolish space launch vehicle. Elon has made his
choice, and in my experience most rocket scientists are fairly stubborn
about which of the three rules is "obviously" wrong or at least less
important than the other two. Rocket plumbers may be more pragmatic, of
course, but I don't take Elon to be a plumber.
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