Shuttle Questions Q and A

• #480 by Orbiter Obvious on 03 Mar, 2006 22:00
• Quote

  Seattle Dave - 3/3/2006  4:14 PM
  
  Excellent thread.
  
  Q) Why do they keep powering up and down the Orbiters even out of OMM periods. Such as Atlantis is powered down on the latest status report. Is Atlantis completely turned off and when on, is it a case of her computer screen displays all being on etc?

  
  Good question!!
• #481 by Jim on 04 Mar, 2006 15:10
• Quote

  Orbiter Obvious - 3/3/2006  5:00 PM

  Quote

  Seattle Dave - 3/3/2006  4:14 PMExcellent thread.Q) Why do they keep powering up and down the Orbiters even out of OMM periods. Such as Atlantis is powered down on the latest status report. Is Atlantis completely turned off and when on, is it a case of her computer screen displays all being on etc?

  Good question!!

  
  The orbiters are not required to powered for all the prep work.  A lot of the work is mechanical:  tile repair, payload attach fittings, OMS and RCS pod installlations, middeck reconfiguring.  The orbiters are only powered for specifc tests:  to check out systems, verify wriring and payload installations.  Once it gets to pad, it is only powered up for pad verifications, payload installations, interface test, end to end tests.
  
• #482 by Super George on 05 Mar, 2006 18:56
• Are the APUs basically like big batteries? I remember seeing one launch video where they had a problem on the pad and had a specific time limit to get the launch going, with reference to how long they had left on the APUs.
  
  Will try and find it as the launch team at Kennedy were awesome in solving the problem and picked up the count at T-31 and got her off.
• #483 by mkirk on 05 Mar, 2006 19:03
• Quote

  Super George - 5/3/2006  1:56 PM
  
  Are the APUs basically like big batteries? I remember seeing one launch video where they had a problem on the pad and had a specific time limit to get the launch going, with reference to how long they had left on the APUs.

  Quote

  
  It's funny I just discussed the APUs for another news organization I work for.
  
  The APUs are turbine devices fueled by hydrazine that provide the mechanical power to operate the hydraulic pumps on the orbiter.  The orbiter uses hydraulic power to move the aerodynamic control surfaces (elevons, rudder, body flap etc..), gimbal the engine nozzles, operate the engine valves, retract the external tank umbilical plates, lower the landing gear, and to operate the brakes and nose wheel steering systems.
  
  
  Mark Kirkman
• #484 by mkirk on 05 Mar, 2006 19:14
• Quote

  Super George - 5/3/2006  1:56 PM
  
  Are the APUs basically like big batteries? I remember seeing one launch video where they had a problem on the pad and had a specific time limit to get the launch going, with reference to how long they had left on the APUs.
  
  Will try and find it as the launch team at Kennedy were awesome in solving the problem and picked up the count at T-31 and got her off.

  
  
  I think the launch you are referring to was STS-88.  It was actually a launch attempt that was scrubbed because they ran out of time while discussing the issue.  It was very dramatic to listen to but it actually opened a lot of eyes when it comes to dealing with problems late in the count.  Mission Control in Houston and the Launch Control Team at the Cape were not on the same page when it came to evaluating the APU data for that discrepency.
  
  At the last second as the hold time ran out KSC started to pick up the count but it was too late and Houston Flight (John Shannon) called out "we are no go for launch!!!!!!!"
  
  I was at the Cape on NASA business not long after that and I ran into the Ascent Flight Directors (Wayne Hale, Leroy Cain, and John Shannon) in a restaurant.  After exchanging pleasantries John remarked that they were in Florida to meet with the NTDs (nasa test directors)...these are the folks that manage the launch count.  I replied "Oh...trying to avoid another STS-88" he rolled his eyes and said "exactly".
  
  I tell you that only to get the point accross that many of us in the mission ops and launch ops communities were not happy with the way that went down...I think it is fair to say it scared us.  You don't like to have a vehicle all powered up and loaded with fuel and not all agree that you are go for launch...and you don't want to be running around with your hair on fire trying to trouble shoot an issue.
  
  
  Mark Kirkman
• #485 by Chris Bergin on 05 Mar, 2006 19:32
• I should know this stuff off by heart, but something tells me you're talking about STS-31 with Discovery - the HST launch?
  
  Didn't they hold at T-31 due to the LO2 fill and drain valve showing open, rather than closed. They recycled the proceedure, while holding on the pad, a load of people crowded around a screen in the LCC, then a cheer goes up, they pick up the count and they get her off. Sound fimilar George, Mark?
• #486 by mkirk on 05 Mar, 2006 19:37
• Quote

  Chris Bergin - 5/3/2006  2:32 PM
  
  I should know this stuff off by heart, but something tells me you're talking about STS-31 with Discovery - the HST launch?
  
  Didn't they hold at T-31 due to the LO2 fill and drain valve showing open instead of closed. They recycled the proceedure, while holding on the pad, a load of people crowded around a screen in the LCC, then a cheer goes up, they pick up the count and they get her off. Sound fimilar George, Mark?

  
  I was just trying to answer his specifc question about an APU issue.  He and I may be remembering two different things.  We have had many launches where we threaded the needle in trying to launch within the window.  APUs are often an issue once they are started at T-5 minutes because of the limited fuel supply they have.  Once they are started you need to launch quickly or scrub.
  
  As far as an issue related specifically to the APUs themselves the most recent memory I have was the STS-88 scrub.
  
  Mark Kirman
• #487 by Chris Bergin on 05 Mar, 2006 19:47
• Oh I know, Mark. And your reply was far more relevant and appreciated.
• #488 by Launch Fan on 05 Mar, 2006 20:25
• Interesting. How much hold time is built in before the APUs would cause a scrub?
• #489 by mkirk on 05 Mar, 2006 20:57
• Quote

  Launch Fan - 5/3/2006  3:25 PM
  
  Interesting. How much hold time is built in before the APUs would cause a scrub?

  
  Actully I guess I mis-spoke when I implied that the APUs have been a contraint...this is true for long launch windows but really isn't a player for the short space station launch windows of 5 to 10 minutes.  The APUs are limited to 24 minutes of ground run time...so if you subtract the 5 minutes from the time they are strated during the count (T-5 minutes to T-zero) that leaves you with an APU hold time of 19 minutes.  The ground run time limit ensures the APUs have enough fuel for launch and entry during the mission.
  
  For typical launches to the space station you are limited after T-5 minutes to a hold time of the launch window itself or something we call drainback hold time.  This ususally equates to about 5 ninutes or less of hold time after T-5.
  
  Sorry for making it sound so complicated but there are a lot of variables late in the countown.
  
  Mark Kirkman
• #490 by Jim on 05 Mar, 2006 21:33
• Quote

  Launch Fan - 5/3/2006  3:25 PMInteresting. How much hold time is built in before the APUs would cause a scrub?

  
  
  On non-station flights, T-5 minutes was the last hold point they could stay at for any length of time. 
  
• #491 by psloss on 05 Mar, 2006 22:52
• Quote

  mkirk - 5/3/2006  3:14 PM
  
  I think the launch you are referring to was STS-88.  It was actually a launch attempt that was scrubbed because they ran out of time while discussing the issue.  It was very dramatic to listen to but it actually opened a lot of eyes when it comes to dealing with problems late in the count.  Mission Control in Houston and the Launch Control Team at the Cape were not on the same page when it came to evaluating the APU data for that discrepency.
  
  At the last second as the hold time ran out KSC started to pick up the count but it was too late and Houston Flight (John Shannon) called out "we are no go for launch!!!!!!!"
  
  I was at the Cape on NASA business not long after that and I ran into the Ascent Flight Directors (Wayne Hale, Leroy Cain, and John Shannon) in a restaurant.  After exchanging pleasantries John remarked that they were in Florida to meet with the NTDs (nasa test directors)...these are the folks that manage the launch count.  I replied "Oh...trying to avoid another STS-88" he rolled his eyes and said "exactly".
  
  I tell you that only to get the point accross that many of us in the mission ops and launch ops communities were not happy with the way that went down...I think it is fair to say it scared us.  You don't like to have a vehicle all powered up and loaded with fuel and not all agree that you are go for launch...and you don't want to be running around with your hair on fire trying to trouble shoot an issue.

  Hi Mark,
  
  It's really neat to hear these stories -- I should probably just go back and look at all the launch campaigns and throw out questions.  That STS-88 launch attempt was quite memorable, just listening to the main loop (channel 212), real-time.
  
  I remember John Shannon (Houston Flight on the main loop) saying just before they ran out of time at T-31, "gotta pick it up, NTD," just after saying that they were go for launch and the ISL? console (someone help me out with the console and acronym!) "concurs," but the shuttle project engineer console (SPE, I'm a little familiar with that one) still had questions when NTD came back to them.
  
  Listening to the inflections of the voices, even though the situation was downplayed to the press, it always sounded like there was something else going on there.  To me, it sounded like there were differing senses of urgency between JSC and KSC.
  
  The countdowns after T-9 minutes usually go so smoothly that I'm more interested in the unusual cases because they reveal more about the procedures and all the things that the teams have to deal with than when everything is nominal.  That count was unusual for stopping at a couple of GLS milestones, with T-4 minutes being a little more unusual (though in the cases where they've stopped there, it's been something with the hydraulics in general or the APUs specifically).
  
  Thanks again,
  
  Philip Sloss
  
• #492 by psloss on 05 Mar, 2006 23:06
• Quote

  Chris Bergin - 5/3/2006  3:32 PM
  
  I should know this stuff off by heart, but something tells me you're talking about STS-31 with Discovery - the HST launch?
  
  Didn't they hold at T-31 due to the LO2 fill and drain valve showing open, rather than closed. They recycled the proceedure, while holding on the pad, a load of people crowded around a screen in the LCC, then a cheer goes up, they pick up the count and they get her off. Sound fimilar George, Mark?

  Coincidentally, the first launch attempt (10 April 1990) was scrubbed due to one APU (#1) that ran OK in high-speed, but not in normal speed; and they stopped at T-4 like the STS-88 attempt.  The crew apparently took the APU to high-speed because it wasn't controlling the speed in normal.
  
  Even though these were different issues, as with STS-88, they were going to continue down to T-31, but it sounds like just before they were going to tell the GLS console operator to pick up, the APU console comes back and says that they are no go..."the violation is LCC 6.6-6"
  
  Philip Sloss
  
• #493 by Maverick on 05 Mar, 2006 23:11
• Great stories, great site. Thanks for everything here. Wish I had this over the years, but now do
• #494 by psloss on 05 Mar, 2006 23:20
• Quote

  mkirk - 5/3/2006  4:57 PM
  
  For typical launches to the space station you are limited after T-5 minutes to a hold time of the launch window itself or something we call drainback hold time.  This ususally equates to about 5 ninutes or less of hold time after T-5.

  If I'm around soon enough before launch, I always try to keep my ears open for the LOX drainback hold time, because that does seem to me to usually be the limiter.  (The last nail-biter I recall was STS-110, where I think they got off with 10-11 seconds of LOX drainback hold time left.)
  
  Perhaps you or Jim or one of the MPS folks can answer this question: for a couple of instances (pre-station) where the launch was scrubbed after holding at T-31 seconds for a long time (many minutes), the scrub has been attributed to what I've heard described as "getting outside engine start box."  (One example would be the 25 Feb 1990 launch attempt for STS-36.  In that case, the reason the clock was held unrelated to the engines -- a range safety computer crashed.)
  
  If I'm not just making that up, can someone explain that, and more to the point, what's the time limit there?  Is it the same thing as the LOX drainback hold time, related to LOX drainback, or something else?
  
  Thanks,
  
  Philip Sloss
  
• #495 by mkirk on 06 Mar, 2006 01:13
• Quote

  psloss - 5/3/2006  6:20 PM
  
  If I'm not just making that up, can someone explain that, and more to the point, what's the time limit there?  Is it the same thing as the LOX drainback hold time, related to LOX drainback, or something else?
  
  Thanks,
  
  Philip Sloss
  

  
  
  Although I have had to do so many times these constraints are not easy to explain quickly...I will post a summary of the launch drainback and start box limit here tomorrow.  These are two differnet limits and the shorter of the two is used to determine maximum hold time between T-5 minutes and T-31 seconds.
  
  I will probably just quote the text book on this issue...so I warn you it will not be easy reading for everyone.
  
  Mark Kirkman
• #496 by psloss on 06 Mar, 2006 01:37
• Quote

  mkirk - 5/3/2006  9:13 PM
  
  I will probably just quote the text book on this issue...so I warn you it will not be easy reading for everyone.

  I'm OK with that.
  
  
• #497 by SRBseparama on 06 Mar, 2006 02:20
• Me too. I'll wear my clever hat tomorrow and suck it up, as they say
• #498 by mkirk on 06 Mar, 2006 02:32
• Okay here is a rundown of the Drain Back Hold Time constraint that you here so much about during a shuttle count, as I warned you most of this is straight from the manual...I don't know how to say it any simpler;)
  
  Definitions:
  
  Down Comer - this is the 17 in liquid oxygen feedline that runs up the lenght of the external tank.
  
  Start Box - the engine starting temperature regime (i.e. upper and lower temperature/pressure limits for starting the engine).  A graph of temperature and pressure for the start conditions would draw a box hence "start box"
  
  Performance Margine -  The external fuel tank is always filled to the 100% level for both LO2 (liquid oxygen) and LH2 (liquid hydrogen).  The ET loading is not flight specific; only the MPS (main propulsion system) performance margin changes from flight to flight.  The 100% level of LH2 provides more than enough fuel to complete any shuttle mission.  The mission specific requirements determine how much LO2 is needed.  If less than 100% of the LO2 is required for a particular mission, then that mission will have some performance margin
  
  NOTE: The shuttle is fueled by the ground support equipment by sending the propellants from storage tanks (we will call this the ground facility) to the launch platform and into the tail service masts (TSMs).  The propellant enters the orbiter from the TSM and flows thru the internal plumbing to the umbilical interface with the external tank.  For the liquid oxygen, it then travels up the down comer (17 inch diameter feedline) to the liquid oxygen portion of the external tank and fills the tank from the bottom up.  Once fueling is completed the ground facility contiues to replenish the oxygen and hydrogen that boils off during the count.  This "stable replenish mode" continues until the last minutes of the countdown.
  
  The attached pictures show the TSM attached to the Orbiter aft compartment, the orbiter interface with the feedline, and then a wider angle showing the 17 inch feedline that runs up the side of the tank.
  
  
  DRAIN BACK HOLD TIME EXPLAINED:
  
  After the APUs are started at T - 5:00 minutes, the LO2 outboard fill/drain valve closes at T - 4:55. No more LO2 comes in from the ground facility, but the overboard bleed valve is still open, so the LO2 in the tank "drains back" to the facility at 18 lb/sec. “Drain Back Hold Time” is defined as the time past the planned T - 0 that the launch sequence can be held during LO2 drain back until all the performance margin has drained away.
  
  For example, 16,200 lbs of LO2 margin starts draining at approximately T - 5 minutes and takes 15 minutes to drain at 18 lbs/sec, leaving 10 minutes of drain back hold time. For this much performance margin, holding for more than 10 minutes past the planned T - 0 time would result in draining too much LO2 and not having enough to make the nominal MECO target. Drain back hold time is computed before the mission and computed in real time based on day-of ¬launch loading information from KSC. Drain back hold time is directly related to the amount of performance margin.
  
  The LO2 in the down comer is warmer than the LO2 in the tank and is warmer than the LO2 coming from the ground facility. When drain back starts, the LO2 engine inlet temperature starts to rise as the warm LO2 from the down comer flows through the engine. When all the warm LO2 from the down comer has drained away, the LO2 engine inlet temperature starts to decrease as cold LO2 from the tank flows through the engine. If drain back continues too long, the LO2 engine inlet temperature goes below the limit for a successful engine start. When this happens, the Launch Processing System issues a launch hold. Therefore, the maximum value of drain back hold time is the length of time required to drain the LO2 down comer. For example, the amount of LO2 in the down comer is calculated by multiplying the density of LO2 by the volume of the down comer (a cylinder, 100 feet long, 17 inches in diameter). This amount of LO2 (11,186 lbs) takes approxi¬mately 10 minutes to drain at 18 1bs/sec, giving the maximum of 5 minutes of drain back hold time. If the performance margin drain back hold time is more than 5 minutes, the LO2 inlet temperature limit is violated before the performance margin limit. If the performance margin drain back hold time is less than 5 minutes, the performance margin limit is violated before the LO2 inlet temperature limit.
  
  The smaller of these values is what determines the hold capability after T-4:55.
  
  Another point on performance margin...during a hold the earth rotates the state of Florida and thus the Launch Pad away from the plane of the Space Stations orbit.  This means a launch past the "in-plane targetd launch time" will require more gas for the shuttle to get to the correct orbit and thus each second of unplanned hold time eats up the available performance margin.
  
  Mark Kirkman

  ---

  

  

  
• #499 by SRBseparama on 06 Mar, 2006 03:01
• I think I get it.
  
  So basically, if the Shuttle was sat on the pad too long, with the fill and drain valve closed, the LO2 would keep boiling off to the point there wouldn't be enough LO2 to supply the SSMEs for the performance needed?