On Oct/9/2019 at 11:09, David Spain wrote :
On 2019-10-09 7:50 AM, Jeff Findley wrote:
In article ,
says...

On Thursday, October 3, 2019 at 6:49:33 AM UTC-7, David Spain wrote:

Nozzle design helps to. It would appear to have sea level *and* vacuum
optimized nozzles is the win over aerospikes at least for a TSTO
vehicle. Though Elon remains open to any "gifts", in this regard.

Dave


What happened to nozzle extensions?

Added complexity.Â* It's non-trivial to extend while firing the engine.
For a vehicle like Starship/Super Booster which stages "early" compared
to its expendable counterparts, I'd imagine it's harder than designing
an extension for a fully vacuum optimized engine.Â* For example, the RL-
10 engines with extensions like this extend *before* the engine starts
firing.Â* This takes a bit of time, so wouldn't be a good trade for an
upper stage that stages early due to gravity losses during the nozzle
extension period.

All IMHO of course.

Jeff


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. 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. 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.

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.


Alain Fournier