In article ,
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On 2019-09-21 07:22, Jeff Findley wrote:


Yes. All Raptors so far are all sea level. They don't need a vacuum
optimized engine yet. Eventually I'd expect them to put some vacuum
engines on Starship, but they'll still need sea level engines too, for
landing on earth.

Shuttle engines were sea level, right? yet they worked in vacuum. Would
the oppposite apply with a vacuum optimised engine still working at sea
level (just not as efficient) or would the presence of air disrupt flow
so much that the engine would be useless?

O.k., let us get technical. Technically "sea level" engines are a tad
over-expanded when operating at actual sea level atmospheric pressure.
You just can't over-expand them too much or you get flow separation
which is a "bad thing". That causes thrust that isn't even, or
vibrations, or even structural failure of the nozzle. So, you simply
can't operate a vacuum optimized engine with a bell nozzle at sea level
pressure or "very bad things" will happen.

The RS-25, like Raptor, operates at a very high chamber pressure, which
compensates a bit for the change in external, atmospheric, pressure as
it ascends. It's not technically altitude compensating though, it's an
engineering compromise. It's a tad over expanded at sea level and
definitely under expanded in vacuum, but the high pressure of the
combustion chamber means the losses aren't terribly great.

Super Booster is needed for orbital flight. But so is Starship. It's a
TSTO system, not SSTO.

I was thinking of early scenarios with BFR used as first stage and
conventional second stage/payload to have BFR start generating revenue
befrore the cargo version of Starship is complete.

Again, why would SpaceX do that? Falcon does this, and throws away an
expensive upper stage on every flight. Also, it would divert
development money, and time, from Starship. Time that SpaceX doesn't
want to waste. They have a crap ton of Starlink satellites to launch
and doing it all with Falcon would be more expensive than with
Starship/Super Booster.

Hopper is the only vehicle as you describe. The two Starship prototypes
being built may not look pretty, but then again neither do most aircraft
that have mirrored surfaces when you get up close to them. Their skin
is "wrinkly" as well.

For a hopper flight, shape not a problem. But for the real McCoy, don't
they need very precise "non wrinkly" shapes for aerodynamic purposes and
ensure the boundary layers kieep plasma away from direct contact with
the steel?

No. Go look at some actual aircraft up close. Their skin is wrinkly
and they were built indoors. Such tiny imperfections really don't
matter all that much.

Now you're asking about details only SpaceX knows.

OK, so basically, those "protocypes" being built are still just "hopper"
vehicles if the final details of Starship aren't done. If they don't
know where the doors will be and how big they will be and how they open,
they can't really build a prototype of the Starship.

I didn't say that. You're assuming facts not in evidence. This month,
Elon Musk will update the public on the progress of Starship/Super
Booster. I'm sure much has already changed and they're finalizing the
design. But again, only SpaceX knows those details at the current time.

Again, you're asking about details only SpaceX knows.

Do we know that SpaceX actually has selected and validated/tested a tile
material that will work?

Only SpaceX knows for sure, but they've released pictures of the
hexagonal tiles being thermal tested in a lab. And there were 7 of them
attached to Starship Hopper when it flew. Beyond that, we know nothing
concrete.

SpaceX announced it would drop composited in favour of heavy steel
because the weight of tiles to protect compites was more than the weight
of steel.

Yes.

But now they are adding tiles to steel as well, perhaps not
all the way around. But if the tiles haven't been finalized yet, how
could they have made the initial decision since you can't compare one
model against another based on tile weight when the tiles themselves
aren't final.

I'm pretty sure they're close to "finalized" if they were flying them on
Hopper. Still, like the space shuttle TPS, details may change over time
as actual flight data is gathered. Remember that Space Shuttle Columbia
flew with an instrument pod on the tip of its vertical stabilizer that
contained an infrared camera to gather data. Also, its structure was
outfitted with thermocouples, strain gauges, and other sensors when it
flew the first orbital test flights. That data was used to optimize the
TPS of later vehicles and also led to developing other materials and
techniques (e.g. the so called thermal blankets used in areas of lighter
heating). Starship will no doubt go through a similar optimization
process as they gain actual flight data.

To me, this smells fishy, and I wouldn't be surprised if the steel
hoppers are really to get something tested ASAP, and SpaceX hasn't
actually finalized the actual Starship.

Good to know actual engineering "smells fishy" to you because the space
shuttle program was actually quite similar.

Or is it expected that a landed Starship will have to go through a
maintenance cycle that will "refresh" the heat shield?

Again, you're asking about details only SpaceX knows.

They bragged about turning it around in 30 minutes of some very small
amount of time. So thir magic tiles would be expected to last a number
of launches/landings. It could be that they air for 2 flighst after
which the tiles have to be totally stropped off and new tiles added, or
that tiles really can last 100 flights without any maintenance.

It seems to me that the credibility of this project depends on SpaceX's
selection of heat shield. And so far, they don't really have relased any
info on it. A bit like some politician promising some totally new health
care package without giving any details.

Whatever. It's doubtful they'd turn one around that quickly in the
first few years anyway (the flight rate simply won't require it), so be
prepared to be disappointed.

Jeff
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