SpaceX Falcon 9/Dragon CRS SpX-1 MISSION GENERAL DISCUSSION

[edkyle99]

It's been over a month, and I was really hoping we'd have heard something by now from the investigation into the engine failure.

I was hoping there wouldn't have been any failure in the first place, but that didn't pan out, did it?

The fact this issue (whatever it was) wasn't picked up by extended engine burns on the ground and subsequent inspections suggested from the start that this isn't going to have a quick resolution. I wouldn't be at all surprised if no smoking gun is found in the end and that it ends up being something everyone's scratching their head over.

A thorough investigation should, it seems likely to me, provide some good clues if not an absolute answer.  They'll have substantial telemetry, manufacturing data, and ground test data to review.  They may take apart an engine or two on the production line.  They should be able to do failure mode modeling to pin down potential culprits.  Steps to reduce the odds of future failures will be recommended.  Test procedures are going to be part of this mix, no doubt. 

The fact that this occurred in the vicinity of Max-Q, a condition not duplicated during the extensive static ground test firings, provides a very big fat clue, I think.  https://docs.google.com/viewer?a=v&q=cache:7y5GymBuP6EJ:www.math.utah.edu/~gustafso/2250resonance.pdf+&hl=en&gl=us&pid=bl&srcid=ADGEEShaUzsIqSMLXk0UnJUuKhY5-hHEBacFExeV0pR3fvzuhfHKFFOPVZpG4jET44Gs1BY_e-utcSoJ94wMEEIcc46YXd5WAvznL82deYphQvdzZjeIfRpFJIVbJ_7B33Y8pEeSgnao&sig=AHIEtbTW6gRCU8VX4NqAyv14mxAz2T7xSg

 - Ed Kyle

[Comga]

The fact that this occurred in the vicinity of Max-Q, a condition not duplicated during the extensive static ground test firings, provides a very big fat clue, I think. 
 - Ed Kyle

Did you mean to link to this reference on Resonance?

[edkyle99]

The fact that this occurred in the vicinity of Max-Q, a condition not duplicated during the extensive static ground test firings, provides a very big fat clue, I think. 
 - Ed Kyle

Did you mean to link to this reference on Resonance?

That looks like the one.

If only they could throttle down through Max-Q ....

 - Ed Kyle

[Prober]

The fact that this occurred in the vicinity of Max-Q, a condition not duplicated during the extensive static ground test firings, provides a very big fat clue, I think. 
 - Ed Kyle

Did you mean to link to this reference on Resonance?

That looks like the one.

If only they could throttle down through Max-Q ....

 - Ed Kyle

all the better reason to reallocate resources and get F1.1 flying out west.

[mmeijeri]

If only they could throttle down through Max-Q ....

They could adjust their trajectory instead.

[edkyle99]

If only they could throttle down through Max-Q ....

They could adjust their trajectory instead.

There might be a trade there:  accepting additional gravity losses to reduce Max-Q.  I wonder if another method could be to "detune" the engine cluster slightly, by modifying the thrust settings.  All of this still assuming a Max-Q related resonance condition as a cause.

 - Ed Kyle

[mmeijeri]

Then again, throttling down also increases gravity losses, though perhaps less so.

[LouScheffer]

Resonance conditions strong enough to shake/break engines apart show up very clearly on the accelerometers on the vehicle, and are typically easy to diagnose.  For example, on the Delta-III, a resonance condition caused the hydraulics to run out of fluid trying to suppress a rolling motion (when in fact they were the cause of this motion).  Within 2 weeks they knew the problem http://www.boeing.com/news/releases/1998/news_release_980908a.html and the final report was a few weeks later http://www.boeing.com/news/releases/1998/news_release_981015a.html.  Since the review board seems to be struggling to find the cause in this case, it's likely not something as simple as resonance.

The fact that this occurred in the vicinity of Max-Q, a condition not duplicated during the extensive static ground test firings, provides a very big fat clue, I think. 
 - Ed Kyle

Did you mean to link to this reference on Resonance?

[Jim]

all the better reason to reallocate resources and get F1.1 flying out west.

No, because V1.1 is not much different

[BobCarver]

Resonance conditions strong enough to shake/break engines apart show up very clearly on the accelerometers on the vehicle, and are typically easy to diagnose.  For example, on the Delta-III, a resonance condition caused the hydraulics to run out of fluid trying to suppress a rolling motion (when in fact they were the cause of this motion).  Within 2 weeks they knew the problem http://www.boeing.com/news/releases/1998/news_release_980908a.html and the final report was a few weeks later http://www.boeing.com/news/releases/1998/news_release_981015a.html.  Since the review board seems to be struggling to find the cause in this case, it's likely not something as simple as resonance.

Vibration wouldn't have to be high enough to literally shake the engine apart. Metal fatigue may have played a factor since these engines are test-fired several times before flight. A more complete inspection process may be needed, even on Merlin-1D, especially if the basic parts haven't changed much from the 1C.

[Jim]

Resonance conditions strong enough to shake/break engines apart show up very clearly on the accelerometers on the vehicle, and are typically easy to diagnose.  For example, on the Delta-III, a resonance condition caused the hydraulics to run out of fluid trying to suppress a rolling motion (when in fact they were the cause of this motion).

That was a flight control  resonance and not a structural resonance

[cambrianera]

Resonance conditions strong enough to shake/break engines apart show up very clearly on the accelerometers on the vehicle, and are typically easy to diagnose. 

Resonance is a strange beast, still dangerous at low power levels.
F14 original hydraulic system was designed without accumulators and the pulses from the pump were in resonance with the system; as a result the lines broke during first test flight.
Obviously in that case recovering the wreckage was easy, and the broken pipes quickly led to the cause.

[LouScheffer]

Resonance conditions strong enough to shake/break engines apart show up very clearly on the accelerometers on the vehicle, and are typically easy to diagnose.  For example, on the Delta-III, a resonance condition caused the hydraulics to run out of fluid trying to suppress a rolling motion (when in fact they were the cause of this motion).

That was a flight control  resonance and not a structural resonance

The resonance was structural, a 4 Hz roll mode that developed as the stage mass decreased.  The flight system should have ignored it, but was not programmed to do so, and so tried to "correct" it, which it could not do  (presumably not fast enough).  Attempting to do so, it ran out of fluid...

From the report "The 4 hertz roll mode was caused by the three airlit solid-rocket motors rocking back and forth in unison. As the ground lit SRMs burned and lost weight, the rocking air-lit solids had a greater influence on the vehicle. The mode became significant 40 to 50 seconds into flight. Once the control system recognized the mode, it attempted to correct the roll oscillation. In doing so, the hydraulic fluid used to move the nozzles on the solid-rocket motors with TVCs was depleted."

[Jim]

The resonance was structural, a 4 Hz roll mode that developed as the stage mass decreased.

Huh?  You say it right there "roll mode", that is flight dynamics, which is a control issue.  Reread the report that you linked.
"roll instability which led to the Delta III failure can be corrected by a change to our control software"
The fixed was not strengthening the SRM attachments but a flight software fix.   That is why the Delta problem is not relevant to the Spacex problem.  If the Spacex program is any related "Resonance conditions strong enough to shake/break engines apart", that has nothing to do with flight dynamics and everything to do with structures.  It will be a hardware fix and not a software fix.

[LouScheffer]

Resonance conditions strong enough to shake/break engines apart show up very clearly on the accelerometers on the vehicle, and are typically easy to diagnose. 

Resonance is a strange beast, still dangerous at low power levels.
F14 original hydraulic system was designed without accumulators and the pulses from the pump were in resonance with the system; as a result the lines broke during first test flight.
Obviously in that case recovering the wreckage was easy, and the broken pipes quickly led to the cause.

AS-502 (an Apollo test flight) had a similar problem.  A resonance in the bellows sections of a fuel line caused them to rupture in flight.  It was not detected in ground testing since when tested in air, air liquified on the bellows, which were full of liquid hydrogen.  The liquid air added enough damping to keep the resonance from destructive levels.  In a vacuum there was no additional damping and the vibration grew to destruction.  (This supports your point - the vibrations were not strong enough to show on the vehicle accelerometers.  However other indications pointed to a bellows failure, a vacuum test reproduced it, and then the cause was found.)

[Jim]

(This supports your point - the vibrations were not strong enough to show on the vehicle accelerometers.  However other indications pointed to a bellows failure, a vacuum test reproduced it, and then the cause was found.)

There are many accelerometers, not just those in the guidance system.  Don't know exactly what Spacex does, but there are usually accels on the payload adapter, on the guidance system structure, on the thrust structure, on the engines themselves or gimbal blocks. 

[LouScheffer]

The resonance was structural, a 4 Hz roll mode that developed as the stage mass decreased.

Huh?  You say it right there "roll mode", that is flight dynamics, which is a control issue.  Reread the report that you linked.
"roll instability which led to the Delta III failure can be corrected by a change to our control software"
The fixed was not strengthening the SRM attachments but a flight software fix.   That is why the Delta problem is not relevant to the Spacex problem.  If the Spacex program is any related "Resonance conditions strong enough to shake/break engines apart", that has nothing to do with flight dynamics and everything to do with structures.  It will be a hardware fix and not a software fix.

My reading of the report was that there was a 4 Hz structural resonance about the roll axis, and the control system could not handle this.  From Ed Kyle's report http://www.spacelaunchreport.com/thorh13.html "The roll oscillation was created when the RS27A and three of the ground-lit solid motors vectored in a way that amplified a natural 4 Hertz resonance of the vehicle.  As the system fought the resonance, it rapidly used up its hydraulic fluid.  At T+65 seconds, the fluid ran out..."

So in this case the software made a structural resonance worse, so the answer was to fix the software and not the structure.

Of course, if the resonance has nothing to do with the software (as in the AS-502 bellows resonance failure) then you need to fix the structure.

[Jim]

So in this case the software made a structural resonance worse, so the answer was to fix the software and not the structure.

No, it is not make the "structural resonance worse".  The structure remained intact and it did not break apart from destructive resonance.  The mode was the vehicle rolling back and forth, that became worse.  It was not the SRMs rocking back and forth and becoming detached. It made the control system fight itself and it resulted in hydraulic fluid depletion.  The 4 hz roll mode along with 56 others were known preflight, it was just thought that it was a secondary and not a primary mode.

[LouScheffer]

So in this case the software made a structural resonance worse, so the answer was to fix the software and not the structure.

No, it is not make the "structural resonance worse".  The structure remained intact and it did not break apart from destructive resonance.  The mode was the vehicle rolling back and forth, that became worse.  It was not the SRMs rocking back and forth and becoming detached. It made the control system fight itself and it resulted in hydraulic fluid depletion.  The 4 hz roll mode along with 56 others were known preflight, it was just thought that it was a secondary and not a primary mode.

We agree on what happened; the (root) problem, a resonance that was not correctly accounted for, did not directly cause a structural failure.   Instead, it caused steering fluid depletion, which caused the rocket to not point into the wind, which did cause structural failure.  I'm just considering it as an example of  "problem caused by a resonance".  I agree the resonance itself did not break the rocket, or any part of the rocket, directly.

By the way, the same thing very nearly happened on Mariner 10.  Fortunately, it was in real time communication, controllers saw the cold gas supply dropping, they figured it must be an oscillation, and turned the gyros off to stop it.  But it lost a good chunk of its cold gas supply in an hour, and almost cost the mission.

[Prober]

I want to believe, but I am still having trouble buying into the 27 engine thing.

 - Ed Kyle

Care to expand of this Ed?