Update 6-3-2025: A close version of this article ran in today's Waco "Tribune-Herald" as a column on the opinion page.
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The recent Flight 9 test resulted in the loss of both the
upper stage spaceship and the lower stage booster. A lot of stories class this as just another
failure in a series of three, but it is
a little more complicated than that.
The “Superheavy” booster portion of this test was actually a
lot more successful than many believe.
They never intended to recover this one,
instead choosing to stress it possibly to destruction, on a return flight profile that was likely
fatal. And that is exactly what
happened.
On the plus side,
this was the very first re-flight of the reusable “Superheavy”
booster. This very same stage launched
the Flight 7 “Starship” test, with most
of those 33 engines reused on this Flight 9 test. What was “new” on Flight 9 was twofold: (1) the deliberate overstress return, and (2) the demonstration of a “quick flip”
during hot staging.
The “quick flip” was entirely successful! Until this flight, after staging, the booster would flip slowly using only
vectored thrust from a few of its engines,
which also settled the propellants in the tanks so that the engine pumps
could maintain a good suction.
This time around, the
hot stage ring that protects the forward end of “Superheavy” from the hot blast
of “Starship’s” engines during staging,
was built to an asymmetric shape,
so that “Starship’s” exhaust was deflected out one side only. The force of that asymmetric exhaust
deflection flipped “Superheavy” around much faster, saving propellant for the landing. That was completely successful!
The stressful return involved flying the booster at an angle
to the oncoming wind, instead of
streamlining straight in, tail
first. That being off-angle increases
the aerodynamic drag, so that less
braking propellant might be used.
But, there are structural limits
to how much off-angle is survivable.
They found that limit experimentally, essentially breaking up that “Superheavy”
with too much wind load coming from the side,
where it is nowhere near as strong.
That is something very important to know “for sure”. It is just like pulling too many gees that can
rip the wings off an airplane! You must
know your limits!
The Flight 9 “Starship” upper stage spacecraft did not
experience engine failures during the ascent burn from staging to orbital
insertion, unlike what happened on
Flights 7 and 8. It appears that they
may have fixed at least a part of what was going wrong on those two
flights.
However, there was a
slow loss of propellant tank pressure,
due a leak (or leaks) somewhere.
Their attitude thrusters are cold gas thrusters powered by the
propellant tank pressure. And there
must be at least some pressure to successfully ignite an engine.
Whether this loss of propellant tank pressure had anything
to do with opening the payload door is unknown to me, but not long after that door failed to open, is when the vehicle went into an uncontrolled
spin for lack of attitude thrusters. And
they called off the engine relight test because of it, too.
Not having attitude control is why the ship broke up upon
reentry over the Indian Ocean. The heat
shielding is only on the belly and around the nose, so that if you are pointed in the wrong
direction, you are unprotected from the
extreme heating of orbital reentry. The
vehicle was spinning completely out of control as it entered.
SpaceX is going to have to figure out what caused the leak
and loss of tank pressure. That won’t be
easy. But it has to be done, and I hope they obtained the necessary data
to figure out that problem, from this
test.
Getting a reentry survival is crucial toward verifying their
heat shield design. I would suggest that
SpaceX add some independent attitude thrusters to these flight tests, so that loss of tank pressure does not mean
loss of attitude control. The Draco
attitude thrusters on their Dragon capsules would be a good choice, with lots of real experience behind them.
I do not know for sure, but had Flight 9 been equipped with independent attitude thrusters, that particular “Starship” might have made it back through reentry successfully. That would have been a huge “plus” for this mission! They will still have to find and fix the propellant tank pressure leak, in order to make the final landing burn, however.
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