SpaceX F9 : Crew Dragon In-Flight Abort Test : Jan. 19, 2020 : Discussion

[TheRadicalModerate]

Edit: based on more recent info, it does seem that the abort was initiated by reaching a set speed (and not by detecting loss of thrust as had been speculated),

It's not speculation. The 2 astronauts were interviewed (posted elsewhere on this site) after Elon and Jim and the sequence was discussed as one of the questions. I think it was Bob who said that the detection of loss of thrust was what triggered the abort. And Dragon did not command the Merlins to shut down. That was programmed into the flight computer, located on the upper stage of the Falcon 9.

Do you have a link to the video?  I can't find it.

I have to say that the more I think about this, the less sense it makes to trigger the abort with out-of-band code.  We know that F9 engine shutdown has to be part of the abort sequence, and we also know that there are likely many different criteria for triggering an abort.  Triggering it specifically on a low thrust condition doesn't buy you very much.  It's a lot easier to insert a dummy criterion into the abort monitoring system that it is to hot-wire engine shutdown code through a back door.  It's a lot cleaner, and less likely to generate some useless artifact in the software that could mess up a very expensive test for no purpose.

[Raul]

Do you have a link to the video?  I can't find it.

Doug Hurley at 8:00 in this video

.. For this type of scenario there are several automated abort triggers, for this particular example it was a loss of thrust of the booster - meant that was the trigger for this abort...

[mn]

So we have conflicting seemingly 'definitive' statements on the abort trigger. (Can someone with a twitter account ask Irene (https://twitter.com/Free_Space/status/1220394820574818304 - also posted in the update thread) what is her source)

(considering how this seems so hard to pin down, I would not be surprised if the astronauts didn't have the latest most accurate info)

[LandingZone-1]

How is this conflicting? This "special configuration of min-acceleration" lines up with how Elon said it was specifically set up to be sensitive to loss of thrust (at a specific time) so that as soon as the engines started to shut down, the abort would start.

[mn]

How is this conflicting? This "special configuration of min-acceleration" lines up with how Elon said it was specifically set up to be sensitive to loss of thrust (at a specific time) so that as soon as the engines started to shut down, the abort would start.

I understood that to mean that reaching the minimum speed would trigger an abort (which would trigger engine shutdown AND SD activation at the same time).

If it meant that at this speed it will trigger engine shutdown, which will then trigger the abort, I guess I misunderstood.

(and we do have the video which seems to show the SD's firing while the engines seem to still be running).

[LandingZone-1]

minimum speed

at this speed

It's not speed, it's acceleration, as in the engines shut down so the acceleration lowers until it hits a minimum defined for the test.

(and we do have the video which seems to show the SD's firing while the engines seem to still be running).

The shutdown sequence is not instantaneous. Just because the engines are still producing "fire" doesn't mean they are performing at 100%.

[Rondaz]

@TheFavoritist(Brady Kenniston): It looks like SpaceX implemented Crew Access Arm (CAA) throwback for the In-Flight Abort Test!

https://twitter.com/TheFavoritist/status/1220731762361413638

[mn]

minimum speed

at this speed

It's not speed, it's acceleration, as in the engines shut down so the acceleration lowers until it hits a minimum defined for the test.

(and we do have the video which seems to show the SD's firing while the engines seem to still be running).

The shutdown sequence is not instantaneous. Just because the engines are still producing "fire" doesn't mean they are performing at 100%.

The tweet says "acceleration trigger at 536 m/s", I know it says 'acceleration' that number has to be the speed, (the video doesn't show speed at time of abort, but the last number on the screen is 1497 km/h (aprox 416 m/s)

[king1999]

@TheFavoritist(Brady Kenniston): It looks like SpaceX implemented Crew Access Arm (CAA) throwback for the In-Flight Abort Test!

https://twitter.com/TheFavoritist/status/1220731762361413638

It doesn't make sense. I think it was just vision illusion or somehow the video got distorted a bit by the air temperature. The CAA can be moved in advance to the end and doesn't need the throwback. It is not like the strongback which has cables connected to the rocket until the last second.

Added: it is totally visual effect. You can see the whole access tower and lightning rod tilting back a bit as the rocket climbed.

[Tommyboy]

Added: it is totally visual effect. You can see the whole access tower and lightning rod tilting back a bit as the rocket climbed.

Are you high? Even in the SpaceX webcast it's clear as day that the CAA is moving. Just cover up the other moving stuff (rocket, TE, smoke) and you can see it swinging away.

[TheRadicalModerate]

So we have conflicting seemingly 'definitive' statements on the abort trigger. (Can someone with a twitter account ask Irene (https://twitter.com/Free_Space/status/1220394820574818304 - also posted in the update thread) what is her source)

(considering how this seems so hard to pin down, I would not be surprised if the astronauts didn't have the latest most accurate info)

This is pure speculation on my part, but if I were doing this, I'd have a hunk of software that reads and evaluates a series of criteria, which would all be in some form of logical meta-expression.  For example:

(parameter #1) < (value #1) AND (parameter #27) >= (value #2)
OR
(parameter #5) > (value #3) AND (parameter #83) == (value #4) AND (parameter #1) > (value #5)
OR
...

An interpreter would simply read through these and if any of them evaluated to TRUE, the abort would be initiated.  This is a nice, simple way of configuring mission rules, and even changing them from mission to mission, based on payload, fairing, and trajectory requirements.  For example, you could just have a list of permissible accelerations for various ranges of speeds.

If this is close to right, then it would be easy to add in:

(airspeed) >= 536 m/s AND (thrust) < 1000 MN

Since thrust will never be that high, this will trigger an abort as soon as airspeed hits 536 m/s.

This sorta-kinda reconciles the two conflicting pieces of information:  Yes, this will definitely be a "loss of thrust" trigger, but in reality it's just reconfiguring the thrust criterion to a ridiculous number once your target speed is hit.

[king1999]

Added: it is totally visual effect. You can see the whole access tower and lightning rod tilting back a bit as the rocket climbed.

Are you high? Even in the SpaceX webcast it's clear as day that the CAA is moving. Just cover up the other moving stuff (rocket, TE, smoke) and you can see it swinging away.
https://www.youtube.com/watch?v=mhrkdHshb3E?t=1080

Wish I were high Check this out. No movement.


There is absolutely no reason for CAA to swing at the last moment.

[LandingZone-1]

There is absolutely no reason for CAA to swing at the last moment.

Maybe to keep the arm as close as possible until the last moment? If you watch the SpaceX webcast, it is very obvious that it swings as the Falcon lifts off.

[quagmire]

There is absolutely no reason for CAA to swing at the last moment.

Maybe for an aborted launch, it could swing back to the capsule much like the Shuttle crew access arm did for when the Shuttle had its pad aborts, but isn't necessary to trigger the Dragon abort system. But if the Falcon actually goes, swings back for further protection from launch.

[Semmel]

Added: it is totally visual effect. You can see the whole access tower and lightning rod tilting back a bit as the rocket climbed.

Are you high? Even in the SpaceX webcast it's clear as day that the CAA is moving. Just cover up the other moving stuff (rocket, TE, smoke) and you can see it swinging away.
https://www.youtube.com/watch?v=mhrkdHshb3E?t=1080

Wish I were high Check this out. No movement.


There is absolutely no reason for CAA to swing at the last moment.

That was one of the best sounds I ever heard from a rocket launch. Fantastic! Whish it wsant late at night here and I could turn up the volume..

[floron]

It sure looks like it moves to me.

[abaddon]

Same here.  I guess we're all high?

[HVM]

"Wish I were high Check this out. No movement."
From that same video:

[thirtyone]

"Wish I were high Check this out. No movement."
From that same video:

Yeah, really hard to tell because the apparent percent linear change in projected length varies greatly by camera angle wrt arm, and it's really not obvious from this particular camera angle. It's almost a bit of a visual illusion that hides the movement. When I watched the video at full screen on PC monitor, it's a lot more obvious that there's some movement of the arm a bit after liftoff.

[gongora]

This tweet from Brady had a short video showing the movement:
https://twitter.com/TheFavoritist/status/1220731762361413638