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Old October 9th 19, 04:09 PM posted to sci.space.policy
David Spain
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Default A conversation with Elon Musk

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. 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. For example, the RL-
10 engines with extensions like this extend *before* the engine starts
firing. 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...

Dave