Discovery ascent trajectory anomaly noted on STS-124 IFA review

[Chris Bergin]

http://www.nasaspaceflight.com/content/?cid=5465 - part one of two from the STS-124 IFA presentations on L2.

[Orbiter]

Great story as always Chris.
Let me get this strait, This has never happened before?

[Lawntonlookirs]

I don't remember seeing or hearing any comments on this during lift off.  Of course, .88 seconds would be hard to notice unless you were looking for it.  With all of the solid fuel in both SRB, and expecting them to burn out at the same time is amaxing in itself, talking about a 0.88 second difference.

[Lee Jay]

Great article, Chris.

[psloss]

I don't remember seeing or hearing any comments on this during lift off.  Of course, .88 seconds would be hard to notice unless you were looking for it.  With all of the solid fuel in both SRB, and expecting them to burn out at the same time is amaxing in itself, talking about a 0.88 second difference.

It would have been hard to get a comment in about this with all the discussion about ET foam during and immediately after powered flight.

The propellant in the motor segments is cast from the same batch in order to match performance.

[Justin Space]

Watch the ET cam video from just before separation to separation of the boosters, you can see a roll of about five degrees, which is then corrected after sep.

[Ford Mustang]

You know, you can barely see it, but I spotted it in the SRB cams, the one was closer to the shuttle than the other one...  It came off just a tick slower, but it looked different to me.

Good article, Chris.

[Chris Bergin]

Great story as always Chris.
Let me get this strait, This has never happened before?

Thanks and don't know. I was making a point that there's no reference to historical on the two IFA presentations it's noted on.

[Chris Bergin]

Watch the ET cam video from just before separation to separation of the boosters, you can see a roll of about five degrees, which is then corrected after sep.

Could be, but they do tend to move around a bit during staging. Here's the tailoff and sep from 124's ET cam. Certainly is a bit of roll and a correction (especially after sep).

[Justin Space]

Yep, that's what I was talking about.

[Jim]

You know, you can barely see it, but I spotted it in the SRB cams, the one was closer to the shuttle than the other one...  It came off just a tick slower, but it looked different to me.

This wouldn't have affected SRB sep.  They still come off at the same time

[Jim]

Could be, but they do tend to move around a bit during staging. Here's the tailoff and sep from 124's ET cam. Certainly is a bit of roll and a correction (especially after sep).

At 23 sec's in the video clip, there is a puff of N2O4 (reddish gas).  When did OMS assist initiate for this flight?  I am thinking the puff is from the OMS, if not, what then?   RCS for added roll authority?

[Lee Jay]

Could be, but they do tend to move around a bit during staging. Here's the tailoff and sep from 124's ET cam. Certainly is a bit of roll and a correction (especially after sep).

At 23 sec's in the video clip, there is a puff of N2O4 (reddish gas).  When did OMS assist initiate for this flight?  I am thinking the puff is from the OMS, if not, what then?   RCS for added roll authority?

Just after the launch, I mentioned this "puff" and we went through an exercise that convinced me that it was the OMS assist - it happened within a half-second or so of when the OMS assist was scheduled to start, and it was aft of the vehicle (I initially thought it was from the FRCS, but slowing down the video to frame-by-frame convinced me otherwise).

[JSC Phil]

Could be, but they do tend to move around a bit during staging. Here's the tailoff and sep from 124's ET cam. Certainly is a bit of roll and a correction (especially after sep).

At 23 sec's in the video clip, there is a puff of N2O4 (reddish gas).  When did OMS assist initiate for this flight?  I am thinking the puff is from the OMS, if not, what then?   RCS for added roll authority?

That's a good call, but I believe this would be OMS assist kicking in for that stage of flight.

[Launch Fan]

Do we have the same clip from STS-122, STS-123 to compare? Impressive how the orbiter's SSMEs can smoothly steer like you can see on the video.

[Shuttle Scapegoat]

That's so cool. So if there's wind shere earlier or something like this later, she corrects herself? The pilot or commander don't have to press anything or use the stick?

[Martin FL]

That's so cool. So if there's wind shere earlier or something like this later, she corrects herself? The pilot or commander don't have to press anything or use the stick?

It's automatic. The stick is only used during landing.

[psloss]

As the following references note, the vehicle is in attitude hold at SRB sep and for a few seconds until guidance converges:
http://spaceflight.nasa.gov/shuttle/reference/shutref/events/1stage/
http://spaceflight.nasa.gov/shuttle/reference/shutref/events/2stage/

This turned out to provide a visual indicator of that.

[DaveJSC]

The event was an anomoly and is being investigated. The shuttle can cope with such things, as the article correctly noted.

Could be, but they do tend to move around a bit during staging. Here's the tailoff and sep from 124's ET cam. Certainly is a bit of roll and a correction (especially after sep).

At 23 sec's in the video clip, there is a puff of N2O4 (reddish gas).  When did OMS assist initiate for this flight?  I am thinking the puff is from the OMS, if not, what then?   RCS for added roll authority?

OMS assist.

[Firehawk153]

Does anyone (Jim?) know how much thrust the SRB's are producing at burnout? 

[Jim]

"SRB separation is initiated when the three solid rocket motor chamber pressure transducers are processed in the redundancy management middle value select and the head- end chamber pressure of both SRBs is less than or equal to 50 psi. A backup cue is the time elapsed from booster ignition.

The separation sequence is initiated, commanding the thrust vector control actuators to the null position and putting the main propulsion system into a second-stage configuration (0.8 second from sequence initialization), which ensures the thrust of each SRB is less than 100,000 pounds. Orbiter yaw attitude is held for four seconds, and SRB thrust drops to less than 60,000 pounds.

The SRBs separate from the external tank within 30 milliseconds of the ordnance firing command. "

[Captain Scarlet]

Was this a very serious event?

[Chris Bergin]

Was this a very serious event?

Someone didn't read the article No, not at all. "Within spec".

Wrote it up as it's interesting and part of the IFA review I usually do after each mission.

[Captain Scarlet]

Oops, yes. Read it now, and feel a bit silly Great article as always!

[Orbiter]

That's so cool. So if there's wind shere earlier or something like this later, she corrects herself? The pilot or commander don't have to press anything or use the stick?

Yep, and super strong wind shear has hit a Space Shuttle Before.
You all know which one.

[wingod]

"SRB separation is initiated when the three solid rocket motor chamber pressure transducers are processed in the redundancy management middle value select and the head- end chamber pressure of both SRBs is less than or equal to 50 psi. A backup cue is the time elapsed from booster ignition.

The separation sequence is initiated, commanding the thrust vector control actuators to the null position and putting the main propulsion system into a second-stage configuration (0.8 second from sequence initialization), which ensures the thrust of each SRB is less than 100,000 pounds. Orbiter yaw attitude is held for four seconds, and SRB thrust drops to less than 60,000 pounds.

The SRBs separate from the external tank within 30 milliseconds of the ordnance firing command. "

Jim, am I the only one that is completely freaked out by this?  Something that according to the data, has never happened in the program before and they call this highly successful?  Watching for trends as we get to the end of the program is absolutely essential to keep the birds flying right and getting crews home in one piece.

Does anyone know how much of the SSME gimbal capacity was used in correcting this thrust misalignment?

Just remember, only the paranoid survive.

An Ares 1 potential issue as well.  What would happen if the Ares 1 SRB thrust tailed off 0.88 seconds early?  What is that in the total impulse equation?  It has to be significant.

[DaveS]

Does anyone know how much of the SSME gimbal capacity was used in correcting this thrust misalignment?

Not much! Not anywhere near the max either!

The RTHU is far more demanding on SSME TVC than this little roll excursion. It only took 3 seconds for the guidance to correct this roll error.

[apos]

The roll is obvious and i dont remember seeing anything like that before although the spacecraft dynamics are not always 100% predictable. I might be wrong on this one, but If you take a look at the SRB flare reflections off the shuttle belly you can see the left reflection (corresponding to the right SRB) trailing of quicker than the right one. 0.8 sec is enough to notice it

[jeffgibson]

"SRB separation is initiated when the three solid rocket motor chamber pressure transducers are processed in the redundancy management middle value select and the head- end chamber pressure of both SRBs is less than or equal to 50 psi. A backup cue is the time elapsed from booster ignition.

The separation sequence is initiated, commanding the thrust vector control actuators to the null position and putting the main propulsion system into a second-stage configuration (0.8 second from sequence initialization), which ensures the thrust of each SRB is less than 100,000 pounds. Orbiter yaw attitude is held for four seconds, and SRB thrust drops to less than 60,000 pounds.

The SRBs separate from the external tank within 30 milliseconds of the ordnance firing command. "

Jim, am I the only one that is completely freaked out by this?  Something that according to the data, has never happened in the program before and they call this highly successful?  Watching for trends as we get to the end of the program is absolutely essential to keep the birds flying right and getting crews home in one piece.

Does anyone know how much of the SSME gimbal capacity was used in correcting this thrust misalignment?

Just remember, only the paranoid survive.

An Ares 1 potential issue as well.  What would happen if the Ares 1 SRB thrust tailed off 0.88 seconds early?  What is that in the total impulse equation?  It has to be significant.

Per the quote...

"The separation sequence is initiated, commanding the thrust vector control actuators to the null position"

I wonder if they are talking about the SSME's actuators going to null, or the SRB's actuators going to null.

If the SRB actuator is supposed to go to null (I know the SRB's can control some form of the stearing), I'm wondering if one of the SRB's didn't go null.  Maybe that could have caused the stack to roll the amount it did.

Just thinking outloud here.

Jeff Gibson
Nashville, TN

[Jim]

I wonder if they are talking about the SSME's actuators going to null, or the SRB's actuators going to null.

Both

[Jim]

Jim, am I the only one that is completely freaked out by this?  Something that according to the data, has never happened in the program before and they call this highly successful?  Watching for trends as we get to the end of the program is absolutely essential to keep the birds flying right and getting crews home in one piece.

Does anyone know how much of the SSME gimbal capacity was used in correcting this thrust misalignment?

Just remember, only the paranoid survive.

An Ares 1 potential issue as well.  What would happen if the Ares 1 SRB thrust tailed off 0.88 seconds early?  What is that in the total impulse equation?  It has to be significant.

This is bothersome to me, in that it only happen this late in the program. 

It has huge Ares implications.  1.  They don't know what is going on with their 'safe" SRM and 2.  the peformance hit

[mkirk]

Does anyone (Jim?) know how much thrust the SRB's are producing at burnout? 

PC<50 (meaning the SRB Chamber Pressure is below 50 psi) equates to nearly 200,000 lbs of thrust per booster. 

By the time the SEP Command is issued (not to be confused with the start of the SEP Sequence) this thrust should be less than 80,000 lbs.  Time delays within the SEP Sequence itslef are designed to ensure that happens.

Mark Kirkman

[mkirk]

An Ares 1 potential issue as well.  What would happen if the Ares 1 SRB thrust tailed off 0.88 seconds early?  What is that in the total impulse equation?  It has to be significant.

Wouldn’t be overly significant at all (in my opinion) and should be bounded well within the flight design constraints.  Ares-I second stage guidance would compensate for dispersions the same the Space Shuttle does.  Second stage guidance on the Shuttle assumes SRB dispersions will exist and targets MECO accordingly.  In other words the Ares upper stage propellant margins will account for dispersions in the first stage motor. 

What makes this IFA significant (and in my opinion it is not OVERLY alarming) is that the Left SRB reached PC<50 .88 seconds sooner than the right and guidance had to adapt with additional steering commands.  In my training the number always quoted to me for allowable difference between SRB during tail-off was 5 seconds.

For the Shuttle, the Sep Sequence is initiated by a mission elapsed timer and begins operating at MET of 1 minute 40 seconds.  At that time the software mid-value selects the PC transducers on each SRB, if the Left and Right Boosters do not reach PC<50 within 5 seconds of each other then SRB SEP is delayed to the backup cue which occurs at an MET of 2 minutes 11.3 seconds.  This ensures that both SRBs have indeed burned out.  Of course a "Delayed Sep" would be a performance hit - please don't ask me to quantify that since I have been away from the program way too long to recall.

Anyway the SSP (Space Shuttle Program) is not accustomed to seeing such a difference within the flight history – although this IFA was within spec – because great effort is made to ensure an even burn – such as using the same propellant batch for all of the segments in a given SRM pair.

Mark Kirkman

[Martin FL]

Mark, would they have felt/noticed this roll and the correction on the flight deck, visually and/or via readouts, or would they have been busy with other items and wouldn't of noticed?

I remember seeing flight deck launch videos and the windows are covered in checklists, so that's why I'm asking.

[mkirk]

Mark, would they have felt/noticed this roll and the correction on the flight deck, visually and/or via readouts, or would they have been busy with other items and wouldn't of noticed?

I remember seeing flight deck launch videos and the windows are covered in checklists, so that's why I'm asking.

I really don't know.

I doubt it.  If they did feel any motion I am sure they would not have recognized it as anything unusual.

The crew is doing their public debrief tomorrow at the Space Center Houston Visitor Center.  If anyone in the forum is going I am sure the crew would answer the question but I expect the answer would be as I listed above.

Mark Kirkman

[Chris Bergin]

Part 2 IFA review of STS-124 (On ET-128) included here:
http://www.nasaspaceflight.com/content/?cid=5468

[kevin-rf]

Saw that and imediately wondered if there was a large temperature differential between the SRB's? Since the solid fuel temperature has some influence on burn rate, where the winds blowing just right to chill one of the boosters more than normal?

[Stowbridge]

Any updates on this?

[JSC Phil]

Any updates on this?

It's with ATK last I heard.

[psloss]

I really don't know.

I doubt it.  If they did feel any motion I am sure they would not have recognized it as anything unusual.

The crew is doing their public debrief tomorrow at the Space Center Houston Visitor Center.  If anyone in the forum is going I am sure the crew would answer the question but I expect the answer would be as I listed above.

FWIW, based on the ICOM audio provided in the crew's recent post flight presentation (link to John44's video post), I believe the flight deck crew commented on it as you say.  But not sure.

[Paul Howard]

Watch the ET cam video from just before separation to separation of the boosters, you can see a roll of about five degrees, which is then corrected after sep.

Could be, but they do tend to move around a bit during staging. Here's the tailoff and sep from 124's ET cam. Certainly is a bit of roll and a correction (especially after sep).

Looked at some other ET cams, and they do move around, but mainly up an down, rather than rolling.

[Orbiter]

Watch the ET cam video from just before separation to separation of the boosters, you can see a roll of about five degrees, which is then corrected after sep.

Could be, but they do tend to move around a bit during staging. Here's the tailoff and sep from 124's ET cam. Certainly is a bit of roll and a correction (especially after sep).

Looked at some other ET cams, and they do move around, but mainly up an down, rather than rolling.

Ya, thats what I saw, It look to me more of an up and down motion.

[Patchouli]

I wonder why this happened a slightly bad or miss matched batch of fuel segments?
Also how big of an issue would this be to Ares and Direct launcher?

I think Direct and Ares V likely would handle it the same as the shuttle.
 The Ares V core has so much mass and six steerable RS68s it likely would roll less then the shuttle did.
 While Ares I would depend on the upper stage being able to deal with the first stage coming up a little short on impulse and being able to react by staging sooner.

[Energy Jobs]

Do we have the same clip from STS-122, STS-123 to compare? Impressive how the orbiter's SSMEs can smoothly steer like you can see on the video.

I have got my answer. can you explain SSMEs steering..

Regards

[Chris Bergin]

Do we have the same clip from STS-122, STS-123 to compare? Impressive how the orbiter's SSMEs can smoothly steer like you can see on the video.

I have got my answer. can you explain SSMEs steering..

Regards

Welcome to the site's forum.

Each SSME can gimbal via bearing bolted to the main injector and dome assembly. You can see this during main ignition where they center to launch position. The flight computers use the steering for numerous stages powered flight, such as for the roll, ascent and after SRB sep to position the orbiter on the correct trajectory. You'll also see them gimbal to a drain position after landing.

In the situation noted on the thread, they automatically countered the roll by gimballing slightly. All automatic.

[ugordan]

I think Direct and Ares V likely would handle it the same as the shuttle.
 The Ares V core has so much mass and six steerable RS68s it likely would roll less then the shuttle did.

Another point is would there be any induced roll moment at all? Both Ares V and Jupiter are inline configurations with the center of mass being located along the thrust vector, while the Shuttle's center of mass is displaced toward the orbiter.

Am I correct in assuming a difference in tail-off would thus induce just a yaw rate and virtually no roll in inline configurations?