[AR] rocket size (was Re: Nothing to do with rockets.)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sun, 3 Oct 2021 21:09:32 -0400 (EDT)
On Sun, 3 Oct 2021, Rand Simberg wrote:
If you really want to get a LOT of mass to orbit cheaply, it's probably
better to do it with a larger than a smaller rocket, just to reduce the
total number of launches required...
There's no fundamental reason why per-launch overhead has to be large
enough to be a major cost. The only *essential* per-launch activities
that take significant time are to position the vehicle on the pad, put a
cargo in it, and fill the tanks. The Zenit pads at Baikonur were built
for a 90-minute launch cycle.
Ignoring complications like development cost and facilities limits, the
ideal size is probably one where the reasonably-likely payload stream can
keep a modest fleet -- say a dozen vehicles -- busy.
If the fleet is smaller than perhaps a dozen, then anything which takes a
vehicle out of service -- permanently due to a crash, or temporarily due
to repairs or maintenance -- becomes a substantial disruption. (Spacex
has already had to scramble, at least once, because they lost a first
stage that was already booked for later flights.) Such outages *will*
happen; four or five vehicles is too few. And vehicles sitting around
waiting for payloads are wasted investment. "The 747 is a good airliner,
but if you don't keep it in the air at least 13 hours a day, it will eat
your lunch."
I calculated the other day that we could deliver 300,000 tons to
equatorial LEO with four flights per day of Starship/Superheavy.
If the turnaround time for a single vehicle is several days, that could
make sense. If it's more like an advanced aircraft -- usually maybe a few
hours of checkout and touchup -- then four launches a day is way too few,
it wants to be more like 20-30. A short-turnaround SS/SH is grossly
oversized for that payload flow.
Henry
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