FAILURE: SpaceX Falcon I Launch III - August 2

[nacnud]

Could anyone elaborate on the F1 RCS? Is it enough to keep the stack orientated while any burps or transients occur during first stage shut down? Does it rely on either the first stage or second stage engine being in operation. I couldn't find the info on the spacex website.

[Patchouli]

3.  It is not needed.  The D-IV doesn't have them.  "Making sure" is doing your design right and not adding unneeded hardware.  That is called rocket science.

4.  Ares I has retro and ullage rockets

Talk about contradicting one's self yes I know that Ares has separation motors but it also has one big unknown the five segment SRB also an RSRM has never been staged in that manner either so there might be some surprises.

By RCS I meant a full three axis RCS vs just roll control and a steerable main engine.
 
BTW the Saturn did use separation motors and ullage rockets Von Braun preferred to error on the side of caution.
Having some margin in a design is called sound engineering he was a real rocket scientist.

[Jim]

Talk about contradicting one's self yes I know that Ares has separation motors but it also has one big unknown the five segment SRB also an RSRM has never been staged in that manner either so there might be some surprises.
 
BTW the Saturn did use separation motors and ullage rockets Von Braun preferred to error on the side of caution.
Having some margin in a design is called sound engineering he was a real rocket scientist.

There is no contradiction.   They are complely different designs  Ares is an SRM and therefore needs one.

1.  Von Braun didn't design the Saturn V, he was only in charge of the people that did.  He had no involvement in sep and ullage motors.

2.  Extra margin is called waste and not sound engineering.   

Using sep and ullage motors when not needed is plain stupid.

[Jim]

Could anyone elaborate on the F1 RCS? Is it enough to keep the stack orientated while any burps or transients occur during first stage shut down? Does it rely on either the first stage or second stage engine being in operation. I couldn't find the info on the spacex website.

The 2nd stage RCS is not uncovered (available) until after staging.

[nacnud]

Ok so between MECO and second stage engine start the F1 is unguided, as i suspect most rockets are.

[Lee Jay]

I know there are some pretty big vacuum chambers out there.  Is there any which could handle the test of shutting down an engine the size of the F1 (after a second or two of burn) to measure any residual thrust or reactions in a near vacuum environment?  My first thought is that even one or two seconds of firing would put enough pressure in the chamber to negate any testing.

not needed.  other first stage engines are not tested in vacuum chambers.   Also it is not just the engine, it is the propulsion system, which includes the stage.

I thought these big engine test stands were capable of simulating low pressure environments using some method involving water that I don't understand.

[Jim]

Delta II is a perfect example of sound and efficent engineering.  It uses no sep or ullage motors.  Just push off springs.  The 2nd stage is "submerged" in the interstage more that 60% of it length.

[A_M_Swallow]

That's a very good question, a poster (or a couple) on this forum saw this as a potential problem looking at the video of the test.  Why wouldn't a bunch of professional engineers identify this as a problem.  (perhaps they did, but were given no time to solve it?)

I suspect that the underlying problem is there is no process for modifying the software when the hardware design is changed (or the process is inadequate).

Remember this from page 3 of the Falcon 1-002 launch report
"Stage 1 Trajectory Performance

This anomaly is two-fold. First, an incorrect propellant utilization file was loaded into the engine
computer. This error caused the engine mixture ratio to be lean on lift-off and rich at altitude. Therefore,
thrust was slightly lower than intended early in the flight, resulting in increased gravity losses and causing
the first stage trajectory to be slightly lower and slower than predicted. SpaceX has adjusted its
configuration management system to ensure that this will not recur.
"
http://spacex.com/F1-DemoFlight2-Flight-Review.pdf

Translation: SpaceX changed to hardware to use a different fuel with out changing the software.

On the third flight the programmers were not ordered to change the software to allow for the longer time the new engines take to shut down.

p.s.  Does every hardware and specification change need reviewing to see if a software change is needed?

[Jim]

Ok so between MECO and second stage engine start the F1 is unguided, as i suspect most rockets are.

Delta II and Atlas I used vernier motors.

[Jim]

I thought these big engine test stands were capable of simulating low pressure environments using some method involving water that I don't understand.

For upperstage engines, not large first stage engines

[Lee Jay]

I thought these big engine test stands were capable of simulating low pressure environments using some method involving water that I don't understand.

For upperstage engines, not large first stage engines

Okay, but isn't the Merlin around 1/3 the size of the J2x?

[Jim]

By RCS I meant a full three axis RCS vs just roll control and a steerable main engine.

Duh!  That is SOP, every non spinning upperstage has a three axis RCS.    There would be no way of pointing the spacecraft and stage once on orbit.   

[Jim]

1.  I suspect that the underlying problem is there is no process for modifying the software when the hardware design is changed (or the process is inadequate).

2.  SpaceX has adjusted its configuration management system to ensure that this will not recur.
Translation: SpaceX changed to hardware to use a different fuel with out changing the software.

3.  On the third flight the programmers were not ordered to change the software to allow for the longer time the new engines take to shut down.

4.  p.s.  Does every hardware and specification change need reviewing to see if a software change is needed?

1.  There is a process

2.  Wrong translation.  No hardware changes were made. This was a procedural change to ensure that proper software version is used and loaded on the vehicle. 

3.  Wrong and absurd interpretation.   The programmers wouldn't know to make the change unless someone told them, so they couldn't be "ordered" not to make change.  It would be a flight design person would come up with the change.

5.  System Engineering dictates that all systems and processes are reviewed when there is a change.

[jongoff]

You don't need to measure it in a vacuum.  As noted in my post above, ambient conditions are just part of the equations used in the design process, and those conditions change with every millisecond of flight.  Experienced designers realize this without even really thinking about it.  They know cutoff conditions for any kind of trajectory will be different from launch conditions and design accordingly.

The problem is that you need to know what the pressure was in order to estimate how much of a delay you need.  In other words, you need some sort of measurement.  You can't just do the math without a measurement.  And Elon's point was that with ambient pressure, it screwed up the measurements enough that it was hard to get a real number that you could use in those equations.  Also, the big fireball was due to the fact that you have a fuel rich exhaust that's shooting into air, which down where we live provides a lot more oxidizer than up at 35km altitude.  Once again, it isn't entirely obvious that they had enough data to really know.  Sure, I agree in hindsight that with the fact that the shutdown transients were qualitatively different, that the fact they didn't have detailed data should still have led them to conservatism, but I don't think this is as much a case of ameteurism as you're claiming.

I respect what you're saying, but have to disagree.

~Jon

[A_M_Swallow]

2.  Wrong translation.  No hardware changes were made. This was a procedural change to ensure that proper software version is used and loaded on the vehicle. 

At some point SpaceX changed the fuel, that is a big hardware/requirements change.  Several years later the software was still trying to burn the original fuel.

3.  Wrong and absurd interpretation.   The programmers wouldn't know to make the change unless someone told them, so they couldn't be "ordered" not to make change.

Re-read exactly what I wrote.

  It would be a flight design person would come up with the change.

True.  Flight design or integration person.

5.  System Engineering dictates that all systems and processes are reviewed when there is a change.

[Lee Jay]

Should this type of "burp" happen during engine shut down?  What I mean is, is this type of engine behavior considered acceptable industry practice, or should things have been changed in the shutdown sequence to eliminate this behavior?

[Herb Schaltegger]

You don't need to measure it in a vacuum.  As noted in my post above, ambient conditions are just part of the equations used in the design process, and those conditions change with every millisecond of flight.  Experienced designers realize this without even really thinking about it.  They know cutoff conditions for any kind of trajectory will be different from launch conditions and design accordingly.

The problem is that you need to know what the pressure was in order to estimate how much of a delay you need.  In other words, you need some sort of measurement.  You can't just do the math without a measurement.

I know what you're saying but I sort of disagree, but let me explain.  A nominal first stage trajectory will put the second stage into a condition where it can complete the mission.  Sure there is a targeted "perfect" altitude and for that altitude there will be a standard-atmosphere set of conditions.  Of course, the reality is that the first stage may over- or under-perform to some degree and still fall within the performance range of the second stage to complete the mission.  If the first stage under-performance is too great, the mission fails because it cannot reach orbit.  If the first stage over-performs, the second stage guidance should be able to compensate and still complete the mission.  In either case, however, the standard atmosphere model will give very close estimates of the ambient conditions to be expected at staging altitude. 

Prudent engineering means quantifying your level of uncertainty in each case and designing to deal with it.  The design may or may not require ambient condition measurements at shutdown in order to dynamically calculate the delay before staging but there are myriad practical difficulties in getting accurate measurements in the wake of a supersonic vehicle at altitude.  Robust engineering practices would be to recognize the "burp" phenomenon and plan for the range of separation conditions which could reasonably exist within the relatively narrow window of acceptable first stage performance.  The range of conditions outside that window don't matter because the stage will have performed so far out of spec that the second stage cannot over come the shortfall.

[Jim]

1.  At some point SpaceX changed the fuel, that is a big hardware/requirements change.  Several years later the software was still trying to burn the original fuel.

2.  Re-read exactly what I wrote.

1.  there ha been no change in fuel .  It has been always RP-1.   Incorrect  software was loaded (propellant utilization file) which adjusts the mixture ratio.   There was no change in hardware.  Just a wrong (older) version of software was loaded. 

2.  I reread it and it didn't change that you are wrong.  Put it this way, no one at spacex knew enough to change the parameter for staging delay, least of all, the programmers

[Gov't Seagull]

Should this type of "burp" happen during engine shut down?  What I mean is, is this type of engine behavior considered acceptable industry practice, or should things have been changed in the shutdown sequence to eliminate this behavior?

Shutdown transients are common, and if they are properly characterized there are well-known methods for ensuring they do not doom the mission.

[Jim]

Should this type of "burp" happen during engine shut down?  What I mean is, is this type of engine behavior considered acceptable industry practice, or should things have been changed in the shutdown sequence to eliminate this behavior?

All engines have a shutdown transient.   The process is to wait until things settle down.