Shuttle Q&A Part 5

• #2660 by sivodave on 19 Aug, 2012 15:42
• Hi all.
  
  I'm looking for documents explains the start and shutdown sequences of the SSMEs. I've got all the handbooks, workbooks available on L2 and I've also searched on NTRS but I haven't found anythig that explains in details what happen instant by instant during SSME start and shutdown. They all explains very well how they worked during flight but not at the begining or end.
  
  Thanks very much for your help.
  
  Davide
• #2661 by DMeader on 19 Aug, 2012 16:45
• Quote from: sivodave on 19 Aug, 2012 15:42

  Hi all.
  
  I'm looking for documents explains the start and shutdown sequences of the SSMEs. I've got all the handbooks, workbooks available on L2 and I've also searched on NTRS but I haven't found anythig that explains in details what happen instant by instant during SSME start and shutdown. They all explains very well how they worked during flight but not at the begining or end.
  
  Thanks very much for your help.
  
  Davide
  

  
  This is rather good...
  
  http://www.enginehistory.org/SSME/SSME3.pdf
• #2662 by sivodave on 19 Aug, 2012 17:48
• Thanks very much...that's exactly what I wanted!
• #2663 by jnc on 19 Aug, 2012 18:41
• Quote from: DMeader on 19 Aug, 2012 16:45

  This is rather good...
  

  
  Wow. Very cool site. Thanks for the pointer.
  
  Noel
  
• #2664 by sivodave on 20 Aug, 2012 20:22
• Hi all.
  
  Questions on the SSMEs. Which was the order in which the engines were ignited? And why that sequence was chosen?
  Which was the reason for igniting the engine in a staggered pattern (120 millisec if I'm right)?
  
  Thanks very much
  
  Davide
• #2665 by jnc on 20 Aug, 2012 20:55
• Quote from: sivodave on 20 Aug, 2012 20:22

  Which was the reason for igniting the engine in a staggered pattern (120 millisec if I'm right)?
  

  
  This part I seem to recall. There's this thing the Shuttle stack does that's called 'twang', which is when the liquids light off, since they are offcenter, the whole stack sways back and forth (quite a bit - I think it's like 2m at the nose). So I would guess that the offset ignition is to minimize the shock loading to the stack's structure from liquid ignition - if they all lit simultaneously, it would be even greater.
  
  Noel
  
• #2666 by psloss on 20 Aug, 2012 22:29
• Quote from: sivodave on 20 Aug, 2012 20:22

  Which was the order in which the engines were ignited?
  

  3-2-1 (right-left-center).
  
• #2667 by sivodave on 20 Aug, 2012 22:37
• Quote from: jnc

  This part I seem to recall. There's this thing the Shuttle stack does that's called 'twang', which is when the liquids light off, since they are offcenter, the whole stack sways back and forth (quite a bit - I think it's like 2m at the nose). So I would guess that the offset ignition is to minimize the shock loading to the stack's structure from liquid ignition - if they all lit simultaneously, it would be even greater

  
  Thanks jnc...it makes sense.
  
  

  Quote from: psloss

  3-2-1 (right-left-center)

  
  Thanks psloss...do you know why this sequence was chosen? I mean, why not the opposite for example?
  
  Thanks again
  
  Davide
• #2668 by wolfpack on 21 Aug, 2012 15:44
• Quote from: sivodave on 20 Aug, 2012 22:37

  Thanks psloss...do you know why this sequence was chosen? I mean, why not the opposite for example?
  
  Thanks again
  
  Davide
  

  
  I've wondered this myself (not so much the sequence but rather the staggered start). Plausible reasons include:
  
  1) Fluid dynamics in the 17" feed lines. What happens to turbopump inlet pressures if all 3 start at the same time? Would that cause too much of a drop?
  
  2) Nozzles resonate at startup, so best to not have all 3 resonating in phase with one another.
  
  I suspect "twang" of the stack, or loads on the thrust structure have little to do with it. The twang period is measured in seconds (~6 seconds), as is thrust buildup to 100% RPL (~3 seconds). The staggered start is measured in milliseconds (~120 ms). Turbopump overspeeds and overtemps are the only thermodynamic effects I can think of that have time constants of similar magnitude.
• #2669 by Jim on 21 Aug, 2012 15:52
• Quote from: wolfpack on 21 Aug, 2012 15:44

  
  I suspect "twang" of the stack, or loads on the thrust structure have little to do with it.
  

  
  Yes, it does also have to do with the structure.
• #2670 by jnc on 21 Aug, 2012 20:41
• Quote from: wolfpack on 21 Aug, 2012 15:44

  1) Fluid dynamics in the 17" feed lines. What happens to turbopump inlet pressures if all 3 start at the same time? Would that cause too much of a drop?
  2) Nozzles resonate at startup, so best to not have all 3 resonating in phase with one another.
  
  I suspect "twang" of the stack, or loads on the thrust structure have little to do with it. The twang period is measured in seconds (~6 seconds), as is thrust buildup to 100% RPL (~3 seconds). The staggered start is measured in milliseconds (~120 ms).
  

  
  Sorry, my post wasn't as clear as it could/should have been. I wasn't meaning to imply that the stagger had to do with the twang directly, I was more mentioning the twang to give a sense of the magnitude of the effects on the structure of the engines' thrust.
  
  To the two excellent points you mention, let me add a third suggestion (well, actually, it's another way of looking at your first), which is the inertia of the propellant in the lines. Once things are up and running, the entire line is filled with fuel/oxidizer moving at constant (basically) speed. But of course it starts with all that fluid at rest. So starting the engines with slight delays allows a longer transition from 'no flow' to 'full flow'. (Of course, these inertial effects are what cause the potential temporary pressure drop you mentioned.)
  
  Noel
  
• #2671 by wolfpack on 21 Aug, 2012 21:00
• Quote from: jnc on 21 Aug, 2012 20:41

  Sorry, my post wasn't as clear as it could/should have been. I wasn't meaning to imply that the stagger had to do with the twang directly, I was more mentioning the twang to give a sense of the magnitude of the effects on the structure of the engines' thrust.
  

  
  I would think that the greatest stress on the thrust structure occurs after liftoff, when the engines are running at 104% and the stack is approaching max Q (Orbiter getting "pushed" by 3 SSME's and "squashed" by oncoming air). But, heck, I am just an electrical engineer guessing at mechanical engineering problems. I'm lucky to know how to spell thermodynamics. 
  
  There's a pretty well known fellow around here who got his start at NASA working Shuttle propulsion at JSC. I'll bet you he knows the answer (or answers, as the case usually is with complex engineering problems). 
• #2672 by sivodave on 22 Aug, 2012 22:06
• hi guys.
  
  thanks for your answers...they're all very interesting.
  
  

  Quote from: jnc

  To the two excellent points you mention, let me add a third suggestion (well, actually, it's another way of looking at your first), which is the inertia of the propellant in the lines. Once things are up and running, the entire line is filled with fuel/oxidizer moving at constant (basically) speed. But of course it starts with all that fluid at rest. So starting the engines with slight delays allows a longer transition from 'no flow' to 'full flow'. (Of course, these inertial effects are what cause the potential temporary pressure drop you mentioned.)
  

  
  Regarding this point, however, I have kind of my doubts. Reason being that starting with loading of the ET around 6 hours before lift off, also SSMEs thermal conditionig started and this consisted in having propellants going around the engines' components. In particular both propellants flowing up to the high-pressure turbopumps. So the propellants were never still.
  
  I've been thinking that the reason for the staggered ignition sequence was due to the possible water hammer that could occur in the hydrogen feedline coming from the ET.
  
  What I mean it's the following: during thermal conditioning the hydrogen flowed up to the high-pressure turbopump but didn't pass inside the coolant channels of the nozzle and combustion chamber. At ignition when the main fuel valve in each engine is open, the hydrogen passing in the coolant channels of the combustion chamber and nozzle flashed to gaseous due to the high difference between the hydrogen temperature (cryogenic) and combustion chamber/nozzle (room temperature). This instantanous formation of gaseous hydrogen caused oscillations in the fuel line, which also cause a lot trouble in developping a safe ignition sequence.
  
  My guess is that these oscillations propagated upstreams, creating kind of water hammer effect. If the three engines where ignited all together at the same time, then these oscillations could sum up together creating disraptions in the flow of fuel delivered from the ET.
  
  For the starting sequence, I've to say that it made sense to have the center engine to be the last one to be ignited since in this way there would have been less time for the twang. Well just few hundreads of milliseconds but I suppose that with the tremendous forces in place, few milliseconds less of twang were welcomed by the structure.
  
  What do you think? Do my reasons make sense?
  
  Thanks very much
  
  Davide
  
  
• #2673 by SiameseCat on 23 Aug, 2012 03:23
• I have a question about manual throttling during ascent. The crew can take over SSME throttling by pressing the takeover switch on the throttle and moving the throttle to within 4% of the commanded value. It seems to me that this would cause the commanded throttle value to change slightly - for example, if the GPCs were commanding 104% thrust, takeover would occur when the throttle was moved to the 100% position, and the SSMEs would be set to 100% thrust. Is this acceptable, or am I missing something here?
• #2674 by Jim on 23 Aug, 2012 03:29
• Quote from: SiameseCat on 23 Aug, 2012 03:23

  I have a question about manual throttling during ascent. The crew can take over SSME throttling by pressing the takeover switch on the throttle and moving the throttle to within 4% of the commanded value. It seems to me that this would cause the commanded throttle value to change slightly - for example, if the GPCs were commanding 104% thrust, takeover would occur when the throttle was moved to the 100% position, and the SSMEs would be set to 100% thrust. Is this acceptable, or am I missing something here?
  

  
  They are going to change the setting anyways so it shouldn't matter
• #2675 by sivodave on 27 Aug, 2012 07:17
• Hi all.
  
  A question about the ET. Why the propellant loading started with a slow filling followed after 5% of propellant loaded by a fast loading?
  
  My guess is that when the tanks started to be filled, they were still "hot", at least at room temperature, and therefore upon contact the propellant started to boil. So to avoid a large mass of propellant boiling (possibly quite violently), the filling started at slow rate first so that the tank had time for chilling down and so allowing a faster loading without having everything boiling as cooking pot.
  
  Am I correct?
  
  Thanks very much
  
  Davide
• #2676 by Jim on 27 Aug, 2012 13:26
• Quote from: sivodave on 27 Aug, 2012 07:17

  Hi all.
  
  A question about the ET. Why the propellant loading started with a slow filling followed after 5% of propellant loaded by a fast loading?
  
  My guess is that when the tanks started to be filled, they were still "hot", at least at room temperature, and therefore upon contact the propellant started to boil. So to avoid a large mass of propellant boiling (possibly quite violently), the filling started at slow rate first so that the tank had time for chilling down and so allowing a faster loading without having everything boiling as cooking pot.
  
  Am I correct?
  
  Thanks very much
  
  Davide
  

  
  
  Standard process used by most cryogenic vehicles.  It is not just the boil off but also conditioning the vehicle to take the thermal shock
• #2677 by spacecane on 29 Aug, 2012 02:19
• I know that this won't happen but my question is CAN it happen...
  
  Could the retired orbiters ever be made spaceworthy again or were they basically turned into 1:1 scale models of themselves? 
• #2678 by Jim on 29 Aug, 2012 02:45
• No,
  
  The pad systems are gone,
  The LCC systems are gone,
  The industrial capability (suppliers, documentation, personnel, etc) are gone
• #2679 by sivodave on 29 Aug, 2012 16:59
• Hi All.
  
  Quick one: I know that Endeavour was the first Orbiter to have the GPS system on board. Had also all the other Orbiters (Columbia included) received the GPS systems?
  
  Thanks very much
  
  Davide