In article , says...
I might add that it appears from what I last read that Elon plans to
attach the vacuum Raptors on Starship to the "airframe",

https://twitter.com/elonmusk/status/1131433322276483072

with the sea level Raptors being able to gimbal but the vacuum ones do
not. So real complexity there to try to add extensions to the sea level
Raptors which need to also move. Also consider cost. It might actually
cost *more* to put extensions on the SL Raptors than just add Raptors
dedicated to vacuum operation given the fact that the engine mfg. is
already vertically integrated into your company and therefore by
definition your are obtaining the engines *at cost*. As I understand it
these engines are somewhat cheaper to build for SpaceX anyway. So it
makes sense to me to just use more...

I think that SpaceX didn't do much optimisation of its rockets.

This is a loaded statement right off the bat. Firstly, what are you
optimizing for? When people start talking extending nozzles and the
like, they're likely talking about optimizing performance. O.k., so
we're back to minimizing propellant mass and/or minimizing dry mass like
an expendable? Why?

Someone did the calculations to figure out the cost of propellants for a
single Starship/Super Booster flight. The final cost was on the order
of $1 million. Think about that. Why would you try to minimize
propellant mass? Let's say you shave off 10% of that mass. You've
saved on the order of $100,000 per flight. But at what cost? And by
cost, I mean money. If it's by adding an extensible nozzle, what costs
does that add in terms of cost of the added complexity in terms of the
initial build and in terms of ongoing maintenance and testing (because
it's got to work right because you don't want to risk a failed extension
damaging a nearby engine).

Secondly, as I've said on this group before, we're looking at Mk1 and
Mk2 right now. Musk has already admitted they *aren't* optimized. Why?
Because they're the early test vehicles used to gather data which will
be used to refine the design! Actual flight data is always a *good
thing* to have.

NASA was able to use flight data from Columbia to refine the designs of
the later orbiters by making them lighter. Same thing will happen with
Starship/Super Booster but I'd imagine, based on Falcon experience, that
we'll see far more iterations of the design than the space shuttle
orbiters that only saw minor tweaks to the design as a result of the
data gathered by Columbia.

They
could be improved quite a lot. As you say it isn't obvious that
extending the nozzle in flight is the way to go, but there is a good
chance that it would improve performance.

And there is a certainty that it will increase costs because you're
adding moving components that must work and not fail, increasing
complexity, and increasing costs. Will those increased costs cover the
cost of propellant saved? Doubtful, IMHO, since we've already figured
out propellant costs and they're still small compared to all other costs
involved in launching and maintaining Starship/Super Booster.

There are many other things
that could be done to improve performance. And I think that such work
will be done in the not too far future.

This I agree with. But, I seriously doubt extensible nozzles will be
one of them.

Now don't read me wrong. I'm not complaining that SpaceX didn't do it
the right way. The main problem in rocketry was that rockets were used
once. The important thing to do was to make them reusable. If cars were
thrown away once the fuel in the gas tank is all used, the important
thing to do wouldn't be to make a hybrid electric/gas car that can run
longer on that single tankful of gas. You don't make the car more
expensive so it can run longer on the limited fuel. You make the car
reusable. But once that is done, yes making the car more fuel efficient
is important. Until recently, the cost of fuel in a rocket launch was
irrelevant, something like 0.1%. SpaceX is now making it relevant
because all other costs have gone way down.

I'd argue we're still a long way from propellant costs being a dominant
cost for orbital launch.

Still, SpaceX chose a propellant combination that's relatively cheap and
has several advantages over the alternatives. Liquid hydrogen is more
expensive to buy, handle, and store (because of its very low bulk
density) than liquid methane. Liquid methane results in a smaller
launch vehicle, compared to liquid hydrogen, because it's more dense
than liquid hydrogen. Merlin's kerosene is cheap, but it is harder in
terms of inspection and maintenance due to its ability to coke in
cooling passages and the like in a liquid fueled rocket engine. So,
liquid methane is likely a "sweet spot" for rocket engine fuel,
especially for lower stages.

Jeff
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