SSTO is hard no matter how you do it, but whenever I try to make a
concept close I do find tripropellant an attractive choice. And judging
by the number of published studies, I'm not the only one by far. The
performance gains are marginal, but at the point where the margin is
between 1% payload fraction and -1%...
John Schilling
john.schilling@xxxxxxxxxxxxxx
(661) 718-0955
On 2/8/2018 7:33 AM, William Claybaugh wrote:
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 <mailto: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.