Author Topic: SSME Throttle settings  (Read 23728 times)

Offline Jim

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SSME Throttle settings
« on: 03/04/2006 11:12 pm »
Many threads have asked the question:  How can you have more than 100% thrust (much like the Spinal Tap volume setting of 11)? 

The SSME was designed to provide a thrust of 375,000 pounds at sea level.  This is the rate power level.  It is similar to how jet engines are rated.  This means that there are design margins that allow the engine to operate at this level and still maintain safety limits, temp limits, design life, reusability, etc.  0nce the SSME was designed and tested it was found that there was excess margins and this meant that the engine could be operated at a higher power level  and still maintain safe margins.  104 % was the first increment and has been used for every mission since it was approved (STS-X).   109%  is another story.

Offline Rob in KC

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RE: SSME Throttle settings
« Reply #1 on: 03/04/2006 11:22 pm »
I've seen many a school teacher wince in pain when George Diller mentions "All three engines back at 104 percent of rated performance"

I'm surprised they didn't recalibrate the performance of the 109 to 100 percent.

Offline mkirk

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RE: SSME Throttle settings
« Reply #2 on: 03/05/2006 12:05 am »
Quote
Jim - 4/3/2006  6:12 PM

Many threads have asked the question:  How can you have more than 100% thrust (much like the Spinal Tap volume setting of 11)? 

The SSME was designed to provide a thrust of 375,000 pounds at sea level.  This is the rate power level.  It is similar to how jet engines are rated.  This means that there are design margins that allow the engine to operate at this level and still maintain safety limits, temp limits, design life, reusability, etc.  0nce the SSME was designed and tested it was found that there was excess margins and this meant that the engine could be operated at a higher power level  and still maintain safe margins.  104 % was the first increment and has been used for every mission since it was approved (STS-X).   109%  is another story.


Hey Jim-

I hope you dont mind me adding my 2 cents:

To make things more confusing, here is how the shuttle program defines the various power settings:

Full Power Level (FPL)                               109%

High Power Level (HPL)                             106%

Typical Mission Power Level                       104.5%

Rated Power Level (RPL)                           100%

Minimum Power Level (MPL)                      65%  (67% is used operationally as idle power)

104% was used for quite a while until a few years ago when it was changed to 104.5%...a slight performance enhancement.  

109% would be used in some two engine out scenarios in order to keep the shuttle from drooping below 265,000 feet at relatively high velocities.  At that altitude and a high enough speed the shuttle could violate external tank heating constraints resulting in a very bad day and loss of the vehicle and crew.

The 109% setting is what they are refering to when you here the call "Single Engine OPS 3, 109" usually around 5 minutes and 20 seconds after launch.  

Also, a few years ago the orbiter software was updated with a change request (CR) that would allow selection of 106% for TAL (transoceanic abort landings) and ATO (abort to orbit) but would continue to use 104.5 for RTLS and late uphill aborts.  As yet this has not been operationally implemented.  

Another CR incorporated but never used allows the use of 106% in second stage but keeps the old setting for first stage...the idea was this would allow a performance gain of about 1500 pounds on the weight critical space station missions.  Early studies showed a concern for excessive structural loads if 106% was used during first stage.

They have looked at settings of 115% and even higher.  106% would probably have been implemented but funding of the SSME envelope expansion was dropped post Columbia.


Mark Kirkman
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Offline Launch Fan

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RE: SSME Throttle settings
« Reply #3 on: 03/05/2006 12:10 am »
You guys might be able to help with this:

http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=656&start=1

Video and audio feed from inside the flight deck on launch. Maybe the comments on that particular launch could go on that thread - while this continues on the actual SSME throttle settings.

Offline mkirk

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RE: SSME Throttle settings
« Reply #4 on: 03/05/2006 12:15 am »
Quote
Launch Fan - 4/3/2006  7:10 PM

You guys might be able to help with this:

http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=656&start=1

Video and audio feed from inside the flight deck on launch. Maybe the comments on that particular launch could go on that thread - while this continues on the actual SSME throttle settings.

Actually I had planned to post a "quasi-transcript" of a typical launch on this web site.  While I may or may not be able to decipher the poor audio on that video I can tell you exactly what is supposed to be said and by whom as this is standardized.  The internal call outs are standard but some crews will make subtle changes and of course they will also make remarks and comments to each other that are not part of the call outs.

I promise to do this soon.

Mark Kirkman

Mark Kirkman

Offline lmike

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RE: SSME Throttle settings
« Reply #5 on: 03/05/2006 12:15 am »
Thanks for this thread.  I've been curioius about this.  How is the 'extra throttle' implemeted physically?  Is it simply more propelants (per unit time) injected into the chamber, higher turbopump speeds, ...?  Or is there 'extra' hardware to implement the 100+% throttle?  Or is the engine hardware basically the same prior to 100% and after they found they could go beyond?  Does the physical throttle 'lever' have more settings, or just the lever step size re-calibrated to signify larger increases in output (to the engines)  Sorry for naive questions.

Offline STS Tony

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RE: SSME Throttle settings
« Reply #6 on: 03/05/2006 12:19 am »
Awesome thread. Another reason why this is hands-down the best site for launch fans.

Offline SRBseparama

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RE: SSME Throttle settings
« Reply #7 on: 03/05/2006 12:21 am »
Great thread. A 'what happens during a launch' type thread would be really cool. This is a good standalone thread too.

Offline Launch Fan

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RE: SSME Throttle settings
« Reply #8 on: 03/05/2006 12:21 am »
Quote
mkirk - 4/3/2006  7:15 PM

Quote
Launch Fan - 4/3/2006  7:10 PM

You guys might be able to help with this:

http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=656&start=1

Video and audio feed from inside the flight deck on launch. Maybe the comments on that particular launch could go on that thread - while this continues on the actual SSME throttle settings.

Actually I had planned to post a "quasi-transcript" of a typical launch on this web site.  While I may or may not be able to decipher the poor audio on that video I can tell you exactly what is supposed to be said and by whom as this is standardized.  The internal call outs are standard but some crews will make subtle changes and of course they will also make remarks and comments to each other that are not part of the call outs.

I promise to do this soon.

Mark Kirkman


Superb! Thanks Mark, that would be so very great of you! :)

Offline Jamie Young

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RE: SSME Throttle settings
« Reply #9 on: 03/05/2006 12:22 am »
Quote
Launch Fan - 4/3/2006  7:10 PM

You guys might be able to help with this:

http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=656&start=1

Video and audio feed from inside the flight deck on launch. Maybe the comments on that particular launch could go on that thread - while this continues on the actual SSME throttle settings.

That was a really interesting video as there's not only the "momentary out flag" call, but the way the Commander or Pilot sound so relaxed!!

Offline psloss

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RE: SSME Throttle settings
« Reply #10 on: 03/05/2006 12:24 am »
In the mostly useless trivia department, 100% was used a few times after the introduction of the 104% power level on Challenger's debut, STS-6.  I seem to recall that 100% was used pre-51L to reduce wear and tear (or something along those lines) on launches in which 104 wasn't necessarily needed...anyway, Jenkins 3rd. Edition shows the following flights after STS-6 used 100%:

STS-8
STS-11/41-B
STS-17/41-G
STS-23/51-D

(The first five flights all flew at 100%.)

Philip Sloss

Offline mkirk

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RE: SSME Throttle settings
« Reply #11 on: 03/05/2006 12:32 am »
Quote
lmike - 4/3/2006  7:15 PM

Thanks for this thread.  I've been curioius about this.  How is the 'extra throttle' implemeted physically?  Is it simply more propelants (per unit time) injected into the chamber, higher turbopump speeds, ...?  Or is there 'extra' hardware to implement the 100+% throttle?  Or is the engine hardware basically the same prior to 100% and after they found they could go beyond?  Does the physical throttle 'lever' have more settings, or just the lever step size re-calibrated to signify larger increases in output (to the engines)  Sorry for naive questions.

Okay I'll try to keep this basic:

Yes there have been hardware changes that improved the overall safety margins of the engines themselves.  The shuttle program is currently flying what is called the block II engine with a slightly larger throat and improved tubopumps.

As far as changing the actual throttle settings this is done via commands sent from the General Purpose Computers to the main engines which control the operation of five hydraulically operated valves.  Two of those valves in particular do the throttling...the Oxidizer Preburner Oxidizer Valve (OPOV) is the primary thrust control...and the Fuel Preburner Oxidizer Valve (FPOV) works as the mixture control.  In some of the PAO reference material you can see these valves labled on the schematic

Try this link and select the file called main propulsion system...if you are a true space geek:)
http://www.shuttlepresskit.com/STS-105/scom.htm


The throttle lever in the cockpit is the speed brake handle on the center console next to the Pilot (right seat), the commander's handle only works for the speedbrake, he/she can not throttle the engines.  Normally all throttle changes are done automatically with the PLT checking the chamber pressure meters, helium consumption, and manifold pressures to verify the changes indeed occured.  Manual throttle is done by pressing a red button on the Speedbrake handle and moving the lever to the desired setting, once at the correct setting he releases the button.   The throttle can be returned to auto by pressing the auto throttle button on the eyebrow panel (dash board).  The actual range of motion is adjusted by the computer software...for instance full forward is 104.5% unless max throttles are enabled for an abort situation in which case full forward would now be 109%.

Mark Kirkman
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Offline Rocket Guy

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RE: SSME Throttle settings
« Reply #12 on: 03/05/2006 01:06 am »
Quote
Rob in KC - 4/3/2006  7:22 PM

I've seen many a school teacher wince in pain when George Diller mentions "All three engines back at 104 percent of rated performance"


Regardless of the topic, you are referring to one of the JSC announcers, not Diller. KSC is done as soon as it clears the tower. All the commentators speak the same lingo as well, so no point in singling them out.

There really isn't something 'wrong' with saying 104% as has been stated already. It's all relative.

Offline Rob in KC

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RE: SSME Throttle settings
« Reply #13 on: 03/05/2006 01:12 am »
I was being humorous Ben!

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RE: SSME Throttle settings
« Reply #14 on: 03/05/2006 01:19 am »
Thank you very much for the explanation and the link!  This answers my questions.  The preburner valves!

Offline Rocket Guy

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RE: SSME Throttle settings
« Reply #15 on: 03/05/2006 01:32 am »
Quote
Rob in KC - 4/3/2006  9:12 PM

I was being humorous Ben!

Sorry, didn't realize. Just wanted to make the off-topic point of who's commentating when :-) There are probably others here who don't realize too.

Offline Jim

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RE: SSME Throttle settings
« Reply #16 on: 03/05/2006 01:59 am »
Quote
lmike - 4/3/2006  8:19 PMThank you very much for the explanation and the link!  This answers my questions.  The preburner valves!

Another SSME lesson

The SSME use a mixture ratio of 6:1,which is fuel rich and combustion is limited by the oxidizer. ( the ratio of water of 8:1)

The amount of oxidizer coming out of the oxidizer turbopump determines the thrust level and the H2 turbopump provides the appropriate amount of H2 to maintain the mixture ratio.

Likewise, the Preburner oxidizers valves control the amount of combustion in the preburners (which are near 1:1), which controls the speed of the turbopumps.




Offline FransonUK

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RE: SSME Throttle settings
« Reply #17 on: 03/05/2006 11:53 am »
Quote
mkirk - 4/3/2006  7:15 PM

Quote
Launch Fan - 4/3/2006  7:10 PM

You guys might be able to help with this:

http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=656&start=1

Video and audio feed from inside the flight deck on launch. Maybe the comments on that particular launch could go on that thread - while this continues on the actual SSME throttle settings.

Actually I had planned to post a "quasi-transcript" of a typical launch on this web site.  While I may or may not be able to decipher the poor audio on that video I can tell you exactly what is supposed to be said and by whom as this is standardized.  The internal call outs are standard but some crews will make subtle changes and of course they will also make remarks and comments to each other that are not part of the call outs.

I promise to do this soon.

Mark Kirkman


That would be very nice of your Mark. The video linked above is very impressive, but hard to understand as we've never heard what goes on inside the flight deck.
Don't ya wish your spaceship was hot like me

Offline mkirk

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RE: SSME Throttle settings
« Reply #18 on: 03/06/2006 01:02 am »
Here is another lesson on main engine power settings:)

The space shuttle main engines are started at T-6.6 seconds in reverse order, 3-2-1, and staggered by an interval of 120 milliseconds.  Ignition occurs with the nozzles in the start position to minimize shock loads and then they are gimbaled to the launch position (this is the movement you see on TV close-ups).

The engines are commanded to start at 100% of there rated power level (see earlier posts) and must be above 90% within 4.6 seconds of the first start command…i.e. by T-2 seconds.  If this does not occur than an automatic shutdown is initiated…this is called an RSLS (redundant set launch sequencer) Abort.

If all goes well then at T-0 the SRBs ignite and liftoff occurs.  At T+3 seconds, or more precisely, when the vehicle reaches a velocity of 60 feet per second (40 miles per hour), all three engines are commanded to the mission power level which is currently 104.5%.  The engines are not started at this level because of the high shock and acoustical loads that power level would impose while the vehicle is so close to the launch pad.

As the shuttle approaches the MAX Q (maximum dynamic pressure) region the engines will be throttled back to minimize vehicle stress.  This is called the throttle bucket because of the way a graph of the power settings would appear on a chart.  On average this occurs about 30 seconds into the flight.  Usually this throttle bucket is normally planned to occur in a single movement at a predetermined velocity.  For example on STS-114 the pre flight design had the engines throttling back to 72% at mach .96 and then returning to 104.5% after at mach 1.24.  However, on launch day actual atmospheric conditions and upper level winds are factored into the guidance computations which may or may not affect the planned throttle settings.  In my notes for launch day on STS-114, I show the crew was given new settings of 72% at mach .81 and back to 104.5% at mach 1.27

During flight, usually at about 20 seconds after launch, the guidance system evaluates the actual performance of the SRBs and based on that it may change the throttle bucket settings.  If for example the boosters were burning a “little hot” (such as on a hotter than normal day at the Cape) then the engines would be throttled in a two step fashion, such as down to 80% then 67% before throttling back up to 104.5%.

After “throttle up” to the mission power level of 104.5% the engines will remain there until about 7 ½ minutes into the flight when 3G throttling occurs.  This is a gradual reduction in power setting that keeps the vehicle from being overstressed by G forces…the limit is 3Gs.  Just before MECO (main engine cut off) the engines should be at idle power which is 67%.  MECO occurs when the vehicle achieves a predetermined flight specific velocity, usually around 8 ½ minutes after launch.  For STS-114 MECO was at a velocity of 25,819 feet per second (17,603 mile per hour) at 8:29 mission elapsed time.

A NOTE: “throttle up” is usually complete by about 56 (for station trajectories) seconds into the flight…the call made by Houston, “Go At Throttle Up” is not a command but rather a status check/comm check performed at that point…think of it as “you are go at (this point in time)”…The engines are already throttled back up and Houston has looked at all of the data an they like what they see.


Mark Kirkman
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RE: SSME Throttle settings
« Reply #19 on: 03/06/2006 05:51 pm »
Mark,

I know the engines are started before the SRBs to get the twang effect out of the way, but why the staggered startup? Why can't the three main engines be ignited simultaneously?

Thanks

Ben

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