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Old January 16th 20, 01:55 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 2020-01-15 07:39, Jeff Findley wrote:

I understand that the abort will actually happen at the time of maximum
drag, which isn't exactly maximum aerodynamic pressure.


is drag different because it includes drag alongside the stage whereas
MaxQ only measures pressure at the tip end of rocket ?


Pretty much.

Absolutely not. It will be destroyed by the test. The first stage has
no grid fins or landing legs.
The Falcon 9 first stage will be simulating an abort, which means that
its engines will all shut down.


Thanks. So at the time the Super Dracos lift Dragon off the stack, the
stack will no longer be producing thrust but still get a lot of drag.


Correct.

From the point of view of testing the worse case scenario, shouldn't the
capsule eject be tested at MaxQ while engines still producing thrust?
Or do all failure scenarios conjured up always involve capsule eject at
time of or after loss of thrust from lower stage?


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. 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).

So it will have little control
authority when Dragon 2 aborts.


I realize this is moot because any actual abort scenario implicitely
means the lower stage has something very wrong with it, but
**theoretically**, could fins located at the top (front) control
trajectory to keep it "flying" until it has slowed enough to flip and
start to descent engine first? (theoretically)


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. That will make for quite an
"interesting" environment for the first stage.

Also, if something is "horribly wrong" with the first stage, trying to
land it, either on land in Florida or on the ADS, is "not a good idea".

Or is the speed/air density at that point make it impossible to even
deploy the fins ?


That too. The grid fins on the first stage are designed to deploy in
vacuum *before* first stage reentry. No doubt the actuators don't have
the strength to deploy. Also, even if they did deploy, they'd likely
rip right off anyway.

This will expose the (blunt) top of the
2nd stage directly to airflow, including exhaust from the Super Dracos.


Assuming for a second a more or less intact Stage 1, would the exhaust
from the Super Dracos compromise the top of Stage 1's tank and cause
some pretty fireworks? Or is the direction of the escape engines exhaust
such that impingement onto the top of Stage 1's tanks won't happen?


Again, you're forgetting there is a fully fueled *second stage* between
the two. Falcon 9 ain't an SSTO. Remember how Challenger was ripped
apart by aerodynamic forces? That's what's going to happen on this
abort to both the first and second stages of Falcon 9.

I don't believe they're going to "detonate" stage 1 because it won't
have a chance to do so.


If the trigger is shutdown of engines, then there is no need to detonate
it. Hopefully we get nice footage of the stage as it starts to veer off
course and eventually breaks off.


I'd imagine that events will unfold very quickly. Again, remember what
happened to Challenger. It ripped apart in seconds.

In the event of rocket veering off course and aiming for the White
House, when it leaves it cone of normal flight, what order are commands?

Shutdown engines, initiate eject, initiate range safety? (with shutdown
of engines possibly not happening)


First two happen simultaneously. Range safety likely comes later. NASA
and SpaceX would know the details (likely dictated by NASA for crew
safety).

Also, engine shutdown *will* happen. If you rip the engine control
module from a modern car, the engine doesn't keep running. It's a
necessary component to keep it going. A liquid fueled rocket engine
should be much the same. Absent being commanded to "keep going", I'd
imagine valves are designed to "fail safe", which means shutting the
thing down.

NASA's not going to crew rate any vehicle that doesn't have a way to
reliably terminate thrust. For liquids, that's relatively easy to
design into the system.

Shutdown engines, initiate range safety, and initiate capsule eject?


No.

Or would it be just capsule eject and then range safety ?


Yes.

Jeff
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