I have watched the SpaceX website video a couple of times now, and seen some other things from other sources. This test flight was mostly successful, especially the performance of the heat shield. I saw nothing to indicate any hinge line burn-throughs at the aft flaps, seen in some of the earlier test flights. And the heat shield looked to be in still-usable condition at the time of the test flight splashdown.
This flight achieved most,
but not all, of the intended
objectives. The two main shortfalls were
the Superheavy booster boost-back, and
the engine relight test aboard Starship in space. Plus,
Starship flew its mission with one engine out. The Superheavy was prevented from doing its
boost-back by the loss of almost all its engines. There was only one still working at its
downrange splashdown, which appeared to
hit at about the speed of sound.
The proximity in time of that engine-out on the second stage
Starship and all the lost engines on the Superheavy first stage booster, to the hot staging event itself, raises the possibility these problems are
related somehow to that hot stage event.
This author is not an insider to SpaceX,
so he does not know that to be true,
but the close timing is very suspicious.
It is a good place to start looking.
Previous flights of the Version 2 configurations that
included hot staging, did exhibit some
degree of upper stage rocket blast damage to the grid fins on the lower
stage. Those boosters had 4 grid
fins, equally-spaced around the
circumference. The chance was pretty
high that one grid fin might see some rocket blast during the hot stage event. Unlike the propellant tank walls, these grid fins were not cooled by contact
with cold propellant vapors. Rocket
blast damage happens very rapidly to uncooled structures.
For version 3, there
were changes to both stages. Both were
fitted with the new, higher-pressure
Raptor-3 engines, including 3 vacuum
Raptors in the second stage Starship.
The first stage Superheavy had 3 grid fins 90 degrees apart, each about 50% larger than before, but not equally spaced! The “missing fin” spot on the Superheavy
booster lined up with the mid-line of the belly heat shield on the upper stage
Starship, in pre-launch images.
Hot staging involves starting the upper stage engines while
still in contact with the lower booster stage,
which in turn is still slightly-thrusted to keep the propellants down in
the aftermost ends of the tanks where the pump suctions are located. The upper stage has to accelerate away from
the lower stage at a higher acceleration than the lower stage can achieve, even without the heavy upper stage still
attached. If not achieved this way, they will collide catastrophically.
The lower stage acceleration still has to be enough to keep
the propellants settled in the tanks, so
that the lower stage engine pumps can maintain a suction on only liquids. Drawing vapor into the pumps of an operating
rocket engine is also very catastrophic.
That complicated thrust balance is why something like only 3
to 5 engines, out of 33 on the
booster, are all that are used during
hot staging. These are also strongly
thrust-vectored to help produce the flipping action that points the stage back
toward the launch site for the boost-back event.
The rocket blast from the upper stage hits a piece of armor
atop the forward tank of the lower stage,
otherwise that rocket blast would likely burn holes into that propellant
tank, another quite likely-catastrophic
event.
The pushback from that upper stage rocket blast cannot
exceed the thrust of the few lower stage engines used during the staging
event, or the lower stage gets
accelerated net rearward, with the
propellants moving to the forward ends of the tanks! That sends vapors instead of liquids to the
lower stage engines, which is likely
catastrophic, in that their turbopumps
likely will explode, the same way an
aircraft jet engine comes apart if it swallows too big a bird, or swallows something truly hard of any significant
size at all.
This pushback can even be directed more to one side than the
other, by varying the open areas of the
interstage, circumferentially. That way the upper stage rocket blast
pushback force also helps the vectored booster thrust to flip the booster
around toward the launch site faster. But
not too fast!
If the flip-around spin rate is too high, the propellant in the booster forwardmost
tank (in this case liquid methane), gets
flung forward, letting the booster
engine turbopumps suck vapor methane instead of liquid. Leading again to turbopump explosions. Catastrophic!
In other words, if
you do not do hot staging “right”,
within some rather narrow limits,
then truly bad things will almost inevitably happen!
Based on previous grid fin rocket blast damage, and the “missing fin” location in the version
3 Starship/Superheavy, I expected to see
the stage do its flip in the plane of the missing grid fin, so that the other grid fins are farther from
the upper stage rocket blast during the flip,
and thus avoid damage to them.
This is depicted as the “EXPECTED” geometry shown in Figure 1.
What I saw in the video was the “OBSERVED” pattern in the
figure: a flip just about 90 degrees
away from the direction I expected to see!
I have to conclude that the hot staging event did not happen
according to plan!
And so I must also wonder if that deviation had
anything to do with the lost engine on the upper stage Starship, and with the multiple engine failures seen on
the Superheavy booster as the flip proceeded!
Not keeping the propellants properly settled in the after ends of the
tanks would very probably cause engine failures on the Superheavy, which is exactly what we all saw in the
video. I am less sure about the
engine-out in the Starship.
Figure 1 – Author’s Observations About Hot Staging on Flight
12
Now the Raptor-3 version of these engines, whether sea level or vacuum, operates at a higher chamber pressure, and a significantly-increased thrust
level, than the previous version 2
Raptors used in earlier test flights.
The earlier flights used 3 engines on the booster during the flip, Flight 12 used 5. And the Starship second stage thrust at full
throttle is also significantly higher.
So, the hot staging event forces
are all higher.
That means the materials are most likely being pushed “right
to the limits”, in order to stay as
lightweight as possible. And these new
Raptor-3 engines reportedly use more 3-D-printed metal parts than even the previous
Version-2 Raptors used.
This author is an old retired engineer (and teacher). His knowledge may be obsolete, but he does remember that very early on, 3-D-printed metal parts were weaker than
their forged counterparts. Later
on, printed-part strengths improved to
equal forged values, although ductile
plastic elongation capability still fell short of that of forged parts. This is depicted in Figure 2. Whether that is still true is not known to
this author.
It is the plastic elongation capability that confers
toughness against shock load forces and impact forces! The part survives, although it distorts, if the elongation at failure is large. If elongation is insufficient, the part still fails suddenly, in a sort-of brittle fashion.
Figure 2 – Typical Metal Alloy Stress-Strain Behaviors
This author has to wonder if the shock loads from the
violent hot staging process, especially
one not proceeding to plan, might
not have cracked something on the one vacuum Raptor that failed on the upper
stage Starship.
Telemetry said it lit and immediately shut back down. There was some sort of “smoke” seen
persisting about the chamber end of the engine,
plus a reddish glow at one point near its exit, plus another reddish glow across the way on
the engine bay skirt. That “smoke” was most
probably leaking propellant vapors.
Were those red glows the result of a fire in the engine bay,
from propellants leaking from the feed
piping or turbopump shells near the head end of that engine? It would have to be both fuel (liquid
methane) and oxidizer (liquid oxygen) leaking, for there to be an engine bay fire at all, because staging takes place where the
atmosphere is so thin, that it is first
cousin to vacuum! No one has said
so, but it sure looked like a fire to
this old engineer.
If there were leaking propellants, that might also explain why SpaceX cancelled
the in-space engine re-light test. With
leaks happening at the shut-down engine,
there might not have been enough propellants left aboard, after engine cutoff, to do both that test and make a landing burn
at splashdown!
The task facing SpaceX is to look at all these results, and figure out what actually happened, because there were definitely things about
this flight test that did not go according to plan! Then they have to fix whatever went
wrong. Finding and fixing troubles is
what testing is all about, as this old retired
engineer knows all too well!
I wish SpaceX well doing these things. And I heartily congratulate them yet again, for the bulk of the test flight, which went right!
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