John:
Your rules appear to solve to a tripropellant SSTO....
Bill
On Wed, Feb 7, 2018 at 10:31 PM John Schilling <
john.schilling@xxxxxxxxxxxxxx> wrote:
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.