From: tl@none.invalid
JF Mezei wrote:
>On 2021-01-24 00:10, Snidely wrote:
>Sylvia Else mentioned number of engines. Was SN8 at its mass limit
>with 3 engines only able to lift very light load of fuel, thus limiting
>altitude to 12.5km? (the condensation rings showed the tanks were
>perhaps 1/4 full and that is generous).
The calculation below suggest they could have put quite a bit more
fuel into it if they had wanted. SpaceX's comments about throttling
down and shutting down engines as they couldn't throtttle deep enough
suggests the same.
>If 3 engines are only able to lift barely filled tanks, would addition
>of 3 vacuum engines give it the additinal thrust needed to take off with
>full tanks?
>
>At sea level, is there a huge difference in thrust between the sea level
>and vaccum engines? Will the SNxx tests end up having 6 sea level
>engines to get the thing up to orbit to test re-entry/heat shield?
IIRC the fixed engines on SuperHeavy will be slightly more powerful
(10%?) but have no throttling (it'll also have a few gimball/throttle
engines like the center ones on Starship).
I *think* the outer engines on Starship be similar but if it's just
10% it doesn't change the calculus that much.
The fixed engines are often called vacuum-optimized but AFAIK while
the expansion-ratio is higher (IE larger nozzle) for increased
efficiency at lower pressures they're small enough to still be safe to
run at ground level (just) without needing advance nozzle geometries
like the SSME needed.
This is because the fairly large Raptor combustion chamber means a
fully vacuum optimized nozzle would be far too large to fit 3 of them
in there, even without the center 3 gimballed engines! Likewise the
inner ones can't be that as large as the outer fixed engines and still
be able to gimball sufficiently (if they'd even fit as fixed).
As an upper-stage Starship can get away with a less impressive TWR, in
fact going below 1.0 can often increase the payload if you have a
powerful first stage (which it does)! From the figures Google find I'm
guessing it's probably around 1.0-1.1 for a fully fueled Starship with
all 6 engines running (and increasing as fuel burns off).
For ground take-off going close to (but above) 1.0 is pretty wasteful
(the payload increase is VERY small) but IIRC a number of actual
rockets starts around 1.3-1.35 TWR so lets use that as a starting
point. This is also a common recommendation in KSP tutorials for
liquid fuelled rockets for maximizing payload.
That suggest a maximum realistic loadout for ground launching of
Starship of perhaps 35% with just the three center engines and 75%
with 3 center and 3 fixed engines.
Not sure SpaceX have any of these fixed configurations Raptor fully
tested yet though? IIRC we've seen at least one tested but while it
had the larger bell it still had remnants of gimbal hardware so might
not be ready for flight just yet.
Height clearly wasn't the goal for SN8 anyway, I strongly suspect that
with a more aggressive launch profile SN8 would likely have gone to at
least 20k with the same fuel and still had enough for a landing but
they'd have gone MUCH faster which wasn't the profile they wanted.
Even if they have the fixed configuration Raptors available I doubt it
would be worth risking additional engines, even if SpaceX does seem to
have a LOT of Raptors now (OTOH SuperHeavy will eat up a lot of
engines).
It's likely they feel that going higher wouldn't give them much extra
usable data but expose them to unnecessary risks. High-level winds
have been mentioned as one possibility.
Note that Spaceship would be FAR more vulnerable to winds during a
test of this nature than during a real launch or landing. It will by
necessity be at very low speeds near the peak altitude while in an
real scenarios it'd always be going *FAST* through those altitudes
which sharply reduce the impact of whinds.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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