Starship usefulness ?

In article ,
says...

On 2019-09-15 10:32, Jeff Findley wrote:

Nope. Starship is the most technically challenging. A good program
manager focuses on retiring risk. Put another way, Starship is the long
pole in the tent for the launch vehicle, so it needs to be started on
first.

Fair point.

Fai to assume that the Raptor engine(s) being used for "hopper" tests
are really BFR engines with sea level engine bells/configurations?

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.

I had been told that Musk intended to fund development of this "system"
by using it to launch commercial payloads including his own Starlink.
So I would have assumed BFR readyness would be higher priority.

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

The "hopper" prototypes may be shaped and pitched as Starship
prototypes, but in my opinion their build quality points to them being
more generic prototypes than Sharship prototypes.

Aka: a test bench to test engines and other components as opposed to
being the "ship" prototype.

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. People keep looking at the wrinkles in the
prototype Starship's skin and think that's somehow "sub-par". It's not.
They're mostly propellant tanks. They have to work properly, pretty
simply isn't necessary for proper operation.

and flying a Super Booster without a Starship on top is kind of
pointless.

Has the cargo version of Starship been finalized? I read "cargo door".
Are we talking single door, two doors like on Shuttle? And how will
satellites be launched form inside Starship? similar system to when
Shuttle launched satellites? Are are springs enough to push the
satellite out of the belly and into its final orbit?

Now you're asking about details only SpaceX knows.

Any chance that early cargo launches would have BFR on top of which are
recoverable fairings and single use conventional stage2/payload ?

Extremely doubtful. What you're describing is how Falcon operates. The
pitfall is you're throwing away a very expensive upper stage with every
flight. Full reuse is the entire point of Starship/Super Booster. That
an in orbit refueling allows Starship to travel to the moon, Mars, and
perhaps other places as well.

Hexagonal heat shield tiles. Starship Hopper flew with some of them
attached near its base.

Not going to happen. Shuttle tiles needed an obscene amount of
inspection and maintenance between flights.

Had the shuttle been rolled out the day before launch, would this
maintenance have been dramatically reduced compared to having the
shuttle out on the beach for weeks prior to launch?

Possibly, but given how humid and unpredictable Florida weather can be,
it would be a really bad idea anyway. Pop-up thunderstorms are a thing.

And where tiles will be used on Starship, what material now exists to
allow for re-usablility/quick turn around and is light? I was thihking
the odds are this will look a lot like what the shuttle used (but
obviously machined to different sizes/shapes)

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

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.

SpaceX is a private company. They have trade secrets.

Jeff

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On Sep/21/2019 at 15:45, JF Mezei wrote :
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?

Engines optimised for sea level will always work in vacuum, but not at
their optimal performance. In most cases, engines optimised for vacuum
will work at sea level but not at their optimal performance. But for
some vacuum optimised engines operating at sea level, you can have
instability problems and in some cases, the engine might go kaboum.

The space shuttle main engine were high pressure engines, in that case,
the difference between sea level optimised and vacuum optimised is very
small. Therefore they could use those engines at sea level and in space
with little performance penalty (but there usually is a cost penalty in
going high pressure).

Anyway that's my understanding of it, but I'm not a rocket engine
specialist.


Alain Fournier

In article ,
says...

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
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.