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Old January 19th 20, 03:57 PM posted to sci.space.policy
Jeff Findley[_6_]
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Default SpaceX Dragon 2 In Flight Abort Test

In article ,
says...

On 2020-01-18 10:17, Jeff Findley wrote:

NASA knows the details of this for sure. But I believe that the Super
Draco engines are sized to pull Dragon 2 away from Falcon 9 at the same
time the engines are shutting down.


Media articles tend to "Readers Digest" what is given to them by
authorities which are already a Reders Digest version of reality.

Say an abort happens for causes other then engines. If engines are
already giving astronauts a 3G acceleration, and you fire the Super
Dracos that give 4G, the crew would go from 3G to 7G and drop to 4G.


That's not how acceleration works. In the case you state, Falcon 9
would continue to accelerate a 3G, but the Dragon 2 will accelerate at
4G. So, Dragon 2 still pulls away from the Falcon 9.

But this *can't happen*. If Dragon 2 aborts, the engines will stop.
That's how the abort system is designed.

If the capsule ejects after engines have ended thrust, then crew go from
3G to 0G to 4G.


These numbers are correct.

So it would seem to me that the sequencing becomes important. And I
suspect that a command to shut down engines doesn't result in immediate
end of thrust.


Not immediate, but quite quick. However fast the input valves leading
to the gas generator can be shut dictates how quickly the engines will
stop. No gas generator means no power for the pumps.

Since these engines are designed to not get damaged when
stopped, I would assume there is a smooth progressive shutdown as
opposed to abruptly ending fuel/oxydizer flow or abruptly putting the
space equivalent of a pump through a bike wheel's to stop the turbopumps.


Firstly, Merlin engines are designed to throttle. They can already
adjust thrust quickly. Shutting down simply uses the same
actuators/valves as throttling. It therefore follows you can shut them
down very quickly as well.

Also, in an abort situation, you don't care one bit if you destroy the
engines (i.e. turbopumps) in the process of quickly shutting them down.
If you're in an abort situation, you're *already* having a "bad day" and
are trying to save the crew's lives! So, you don't care that the Falcon
9 first stage will be lost. This simply isn't the same situation as
carefully shutting down the Merlins when landing a Falcon 9 in order to
prevent engine damage.

This is akin to ejecting from a fighter jet. No one gives a damn about
the fighter jet "surviving" an ejection. Same thing with a Falcon 9
during an abort.

that would no doubt be sufficient acceleration to pull away from a
Falcon 9 that's still firing (even though that won't ever happen).


And if Stage 2 is exploding while Stage 1 is intact, then super Dracos
firing ASAP is more important than waiting for Stage 1 to shut down.


Regardless of the situation, Dragon 2 will leave *immediately*. Which
is to say it will start pressurizing the tanks for the Super Dracos the
instant that it receives the abort signal. Once the pressure is
sufficient, the Super Dracos fire. This happens in fractions of a
second, but it's all carefully sequenced.

Simultaneous with the above, Falcon 9's engines will be shut down.

Since the Super Dracos have variable thrust, I wonder if at ejection
time, the initial thrust would be adjusted based on how many G2 the crew
are experiencing already and trhen quickly increase to max in order to
make the "jolt" more survivable.


Dragon 2 under power of the Super Dracos will generate more than 4 Gs of
acceleration. The idea is to save the crew's lives. The human body can
take that jerk to 4 Gs. Look up the USAF ejection seat tests done
decades ago. This acceleration is quite tame compared to what e-seats
expose USAF pilots to.

I posted another article on Falcon 9's automatic abort system. This
system eliminates the need for anyone on the ground to do anything as it
will terminate thrust and destruct the vehicle.


When you had aexplained it I though you refered onto to abort, didn't
realise it also included range safety.


Obviously the two systems are related. If you abort, obviously range
safety comes next. But in many cases, like this abort test, the abort
will result in the destruction of the Falcon 9 launch vehicle before the
range safety system attempts to fire.

No one in their right mind would destruct a vehicle with a crewed
capsule still attached (automated or manual). But to verify, you'd have
to ask NASA.


NASA wouldn't admit to it. But consider a rocket headed directly to
Disneyworld. Two crew abord, abort fails. Thousands on the ground.
Someone would have to make a hard decision. pretty sure NASA would have
to press the big red button.


This situation simply *can't* happen. Dragon 2 would abort *long*
before "imminent danger" to anyone on the ground. Also, this is why
Falcon 9 flies *east* out over the ocean. Nothing launched from KSC
flies towards Orlando.

making sure the Abort works is this very very important to avoid NASA
ever having to make that decision.


Which is why the abort system needs to be as simple as possible. This
is why both Dragon 2 and Starliner use a pressure fed system using
hypergolic propellants. It's about as simple as it can be.

telling Dragon 2 to abort). Destruction of Falcon 9 would happen

later.
But destruction of Falcon 9 will likely be moot since aerodynamic forces
will rip it apart before the deliberate destruct happens.


Range safety had been initiated for a Cygnus launch as its engine
failed seconds after clearing tower. At that moment, there woudln't be
aerodynamic forces to break the stage apart.


This is why both Dragon 2 and Starliner have already performed pad abort
tests (both passed). The pad abort test covers the other extreme, which
is the one where you don't even clear the tower when the abort is
initiated.

And say Stage1 engines shut down prematurely 2 minutes before planned
MECO. High enough that aeronynamic forces no longer "Lethal" but the
inert stage might fall over Spain in a purely ballistic course. So a
decision to detonate it might be made by human.


True, but again, Dragon 2 will have aborted long before the destruct
command would need to be sent to Falcon 9. Falcon 9 destruct would
always happen after Falcon 9 thrust termination and Dragon 2 abort.

Remember Challenger. It was ripped apart *very* quickly by

aerodynamic
forces. Falcon 9 will face the exact same fate.


But only a portion of flight sees lethal aerodynamic forces. Before
that portion, not enough speed, and after it, atmosphere too thin.


True, which is why there is a huge exclusion zone during *any* launch
from KSC/Cape Canaveral. This zone is miles around the pad and
continues beneath the flight path. So even in the case of an abort
where a deliberate destruct will happen, destruct still happens after
Falcon 9 thrust termination and Dragon 2 abort.

When thrust terminates, the stack is still on a ballistic trajectory.
Thrust would be terminated long before the launch vehicle deviates from
its path enough to be outside the launch exclusion zone. That's why the
launch exclusion zone is there in the first place.

You'd have to ask SpaceX. But suffice it to say that there are enough
sensors such that there are many ways you'd verify the engine is working
properly.


Would have expected a simple answer.


It is a simple answer. Either your sensors indicate the engine is
working properly, or they don't.

It would seem to me that in a
context where software gets to decide to initiate abort, situational
awareness of health/power generated by each engine is very important.


If the sensors say the engine is going to go FUBAR, it is shut down. On
a Falcon 9 first stage, that doesn't mean the flight is over. It means
the other 8 engines take over for the one that's shut down. Shutting
down one of eight engines does not automatically trigger an abort.

Decising that loss of an engine is survivable depends on knowing the
thrust of other engines is nominal, and that loss of 2 engines does not
result in inability to keep balanced thrust to stay on course. And there
is also a decision on whether to continue with reduced thrust and
deposit Dragon and its stage2 in a sub-suborbital place where they can
then plan and prepare for de-orbit vs initiating abort.


Merlins can throttle and gimbal. Shutting down one engine doesn't mean
you shut down the opposite engine. When an SSME was shut down on a
shuttle flight (due to a faulty sensor), they didn't have to shut down
any other engines. In fact, they inhibited the faulty sensors on the
other two engines and did an Abort To Orbit.

And from a business point of view, if there is loss of engines that
prevent continuation of flight, but engines needed to land are still
healthy, it means that while the mission may have to be aborted, the
stage could land safely and Dragon perform controlled re-entry/landing.


That doesn't apply to crewed flight. If the crew needs to abort, no one
gives a rat's ass about trying to recover the Falcon 9 first stage.

Innertial measurement can be made at top of rocket to measure its
acceleration, but that doesn't give health info for each engine, just
overall thrust being generated.


Merlins have enough sensors to monitor the health of the engine. It's
how the computers decide whether or not to shut down an ailing engine.

Jeff
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