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Old January 18th 20, 04:17 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 2020-01-16 07:55, Jeff Findley wrote:

No. In the case of any abort during first stage burn, the Falcon 9
simultaneously shuts down the engines and sends the "GTFO" signal to
Dragon 2 to initiate its abort.


Pedantic question: You had mentioned shutdown of engines and GTFO happen
simultaneously. Is that absolutely at same time, or is there a short
pause between sending command to engines and sending command to Dragon?
(to give engines time to initiate shutdown). If not, wouldn't Drago
essentiall launch off Falcon9 with Falcon9 engines still at full thrust?


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.

Cite:

SpaceX delays dramatic Crew Dragon abort test due to high winds and
rough seas. BY WILLIAM HARWOOD, JANUARY 18, 2020 / 7:05 AM / CBS NEWS
https://www.cbsnews.com/news/spacex-...nch-in-flight-
abort-test-to-prove-crew-ship-safe-delay-2020-01-18/

From the article:

The Falcon's engines will shut down as the Crew Dragon's
eight Super Draco abort engines ignite, pushing the capsule
away from the now-powerless booster with an acceleration a
race car driver would envy. Within just a few seconds, the
spacecraft will reach a relative velocity of some 430 mph,
subjecting two heavily instrumented astronaut test dummies
to more than four times the normal force of gravity.

Sounds simultaneous to me. Also, shutting down the liquid fueled
engines should only take a fraction of a second anyway. Also note the
acceleration of more than 4 Gs that Dragon 2 will experience while the
Super Dracos are firing at full thrust. This is much more acceleration
than would normally be experienced during that part of the flight, so
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).

Also, in cases where the rocket veers off course, is range safety
totally eparate from abort and only triggered from ground? or can the
stage 1 initiate it by itself?


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.

This article also said NASA was still evaluating its use on crewed
flights. So you'd have to ask NASA.

And theoretically, can ground initiate range safety with the crewed
capsule still attached? or would range safety command, ujpon reception
by the Falcon 9, initiate the abort if it wasn't fonr already and then
light up the fireworks?


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.

or is range safety such that once triggered, Falcon9 will detect
anomaly, initiate the abort and Dragon 2 will be out before the
fireworks have reached the top of the stack?


Thrust termination and destruction are two separate events. No doubt on
a crewed flight thrust termination would come first (simultaneous with
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.

Remember Challenger. It was ripped apart *very* quickly by aerodynamic
forces. Falcon 9 will face the exact same fate.

Out of curiosity, how do they measure how much thrust is produced by
each engine? A sensor that measures pressure against the wall of the
engine bell? An electronic scale measuring "weight" of engine against
the mounting brackets of the stage?

Or is thrust assumed from speed of turbopumps (fuel flow)?


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. When something goes wrong with a first stage engine, the
Falcon 9 computers will know this and shut down that one engine,
compensate for loss of thrust with the other engines, and the flight
will continue. An upper stage engine failure is different since there
is only one Vacuum Merlin on the upper stage.

Abort for Dragon 2 during first stage burn (which is what this abort
simulates) will only happen for some sort of massive failure.

I assume that thrust from each engine is a huge component for the "abort
scenario" software in terms of not only detecting non-nominal engine,
but also deciding if flight can continue? ex: loss of 2 engines on
same side vs loss of 2 engines opposite each other).


You'd have to ask SpaceX. Since there has only been loss of thrust on
one Merlin engine in flight on all of the flights of Falcon 9, I'd say
the demonstrated reliability of the engine (and the armor between them)
makes failure of more than one engine on the first stage *very*
unlikely. Still, the Dragon 2 abort system is there in case of such an
unlikely event.

That's one advantage of having a liquid
fueled stage rather than SRBs (which are harder to shut down reliably
without a lot of transient forces being sent through the stack).


I take it the SLS SRBs have "zipper" explosives similar to shuttle?


For range safety, one would hope so.

Once
SLS is put out of its misery, is it safe to bet that we won't see SRBs
used for manned flight anymore, or would NASA still be open to the idea?


Note that Atlas V will use solids on the crewed configuration used for
Starliner launches. They're much smaller solids, but they are still
there.

Again, the 2nd stage isn't designed to handle the loads of being in the
atmosphere without *something* on top of it to handle the aerodynamic
forces. It will likely shred.


I hadn't realised there was a 2nd stage. Thansk for clarification. I
though it was Dragon, its service module and Falcon 9 first stage.


Test as you fly; fly as you test. Not having an upper stage on this
flight would violate that principle. The mass and aerodynamics of the
launch vehicle would be all wrong and would have to be compensated for.
Because of this, it's just easier to stick a real upper stage on the
stack and fully fuel it, just like a real commercial crew launch.

From the animatiosn I have seen, Dragon ejects with its service module
still attached (apparently for aerodynamic purposes), and only at apogee
does it detach, capsule uses thrusters to set descent attitude , deploys
parachutes


Yes. The fins on the trunk aren't there to look pretty. They keep the
capsule stable during the abort, up to max altitude (essentially zero
velocity).

If there is a second stage, would this be a dummy one just for mass
equivalence, or would they have a real McCoy that is destroyed without
having done anything ?


It's a real, fully fueled, Falcon 9 upper stage. Only the Vacuum Merlin
is a "dummy" mass simulator on this abort test since the upper stage
won't need to be fired on this flight.

Again, test as you fly; fly as you test.

Jeff
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