Shuttle Questions Q & A (Part 3)

• Shuttle Questions Q & A (Part 3) by Skinny on 14 Jan, 2007 00:42
• 
  
  To keep everything at readable proportions, this is part 4 of the highly informative thread. Below are the links to previous parts. Please use the search function to see if your question has been answered before. Have Fun!
  
  Shuttle Q&A Part 1
  Shuttle Q&A Part 2
  Shuttle Q&A Part 3
  Shuttle Q&A Part 4
  Shuttle Q&A Part 5
• #1 by trebloc on 14 Jan, 2007 22:46
• A few quick questions about Shuttle launches.
  
  1. After the SSME ignition the stack moves towards the ET. I understand this because the SSME's are offset from the CoG of the stack. This is called the "twang" effect I think?? SRB ignition happens when the stack is back in the upright again...makes sense..my question however is how does the stack get back to the upright, if the SSME's are still running at full trottle, should the stack not be left pointing away from the Orbiter?
  
  2. How is the roll manouver controlled? Is it by the SSME's or by the Orbiter aero surfaces?
  
  Thanks in advance
• #2 by Jim on 14 Jan, 2007 22:46
• 1.  There is a spring back effect.  That's why it is called "twang". If the shuttle was kept on the pad, the stack would vibrate back and forth, but eventually end up displaced from vertical
  
  2.  SRB's and SSME's
• #3 by Austin on 14 Jan, 2007 22:46
• There is also a connection extending from the launch gantry to the rear of the ET that is blown away at T-O.
  
  As Jim stated, the SRB's and SSME's control the roll and pitch of the vehicle.  Both the main engine and SRB nozzles gimble, in effect steering the vehicle and positioning it in the proper launch plane.  These patterns are preprogrammed and controlled by the computers, of course.
  
  During the next countdown, watch for the main engine gimble check at the T minus 3 minutes, 55 second mark and you will see them moving through their ascent flight profile.
  
  The final gimble check of the SRB nozzles occurs at T minus 21 seconds, but this will not be visible.
• #4 by Jim on 14 Jan, 2007 22:46
• Quote

  Austin - 14/1/2007  3:18 PM
  
  There is also a connection extending from the launch gantry to the rear of the ET that is blown away at T-O.
  

  
  There is no connection at the aft of the ET.  The only T-0 connection is at the intertank and it is not structural
• #5 by spaceflight101 on 14 Jan, 2007 22:46
• After one of the launches last year, NASA TV showed replays of the launch from all of the many, many cameras they have. The ones from the pad are perhaps the most interesting. They appear to show the "stack" lifting a few feet as the SSMEs ignite, then settling back down. I read somewhere that the front of the "stack" actually moves some three feet in response to the applied thrust before the SRBs ignite.
  If you want to have some fun during a liftoff, and landing, try this:
  Get a copy of Boy Meets Girl's "Waiting For a Star To Fall", and begin playing it at T-16 seconds. Play it all the way through, and listen for the major "events" in the song which correspond to main engine ignition, and SRB separation. When I pointed this out to George and Shannon, they sent me a copy of their unreleased album "New Dream", and asked me to listen to "Flying The Cosmos" and tell them what I thought of it.
  Any song that begins and ends with an SR-71 flyby is a big hit with me.
  On the way home, I cue up Kavin Hoo's "On The Wings of an Angel" from his "Above the Clouds" instrumental album. There is a pause at 4:50 that just coincides with that special point when the rear wheels first touch down.
  At that point, they have made it home...on the wings of an angel.
  
• #6 by Austin on 14 Jan, 2007 22:46
• Quote

  Jim - 14/1/2007  12:40 PM
  
  

  Quote

  Austin - 14/1/2007  3:18 PM
  
  There is also a connection extending from the launch gantry to the rear of the ET that is blown away at T-O.
  

  
  There is no connection at the aft of the ET.  The only T-0 connection is at the intertank and it is not structural

  
  I know that it is not structural (as in fixed) but it is connected to the ET.  The 51I launch video (www.insideksc.com) provides a good view of it being released as it falls away at T-0.  Also, in pictures from behind the ET it is clearly visible.
• #7 by spaceshuttle on 14 Jan, 2007 23:21
• Quote

  Austin - 14/1/2007  5:29 PM
  
  There is also a connection extending from the launch gantry to the rear of the ET that is blown away at T-O.
  
  As Jim stated, the SRB's and SSME's control the roll and pitch of the vehicle.  Both the main engine and SRB nozzles gimble, in effect steering the vehicle and positioning it in the proper launch plane.  These patterns are preprogrammed and controlled by the computers, of course.
  
  During the next countdown, watch for the main engine gimble check at the T minus 3 minutes, 55 second mark and you will see them moving through their ascent flight profile.
  
  The final gimble check of the SRB nozzles occurs at T minus 21 seconds, but this will not be visible.

  
  Looking at some daytime VAB Roof launch replays, it looked like the SRB exhaust criss-crossed, so (I if my observation is correct) you can see how the roll actualy happens from that view. The flame also spreads out.
• #8 by Austin on 15 Jan, 2007 00:42
• Quote

  spaceshuttle - 14/1/2007  4:04 PM
  
  Looking at some daytime VAB Roof launch replays, it looked like the SRB exhaust criss-crossed, so (I if my observation is correct) you can see how the roll actualy happens from that view. The flame also spreads out.

  
  The VAB roof launch view has always been my favorite.  Very nice perspective of a launch.
• #9 by elmarko on 15 Jan, 2007 10:53
• Skinny, you have your links the wrong way round in your first post
• #10 by Skinny on 15 Jan, 2007 12:34
• Quote

  elmarko - 15/1/2007 5:36 AM Skinny, you have your links the wrong way round in your first post

  
  Thanks for pointing that out ! :-)
• #11 by Radioheaded on 15 Jan, 2007 14:31
• After watching the excellent Diary of Discovery posted by Gordo, I noticed that in one part of that program they very quickly showed the crew working an RTLS scenario in the simulator.  I was curious (considering how quickly they moved to a different scene) how often are the RTLS performed successfully in the simulator?  It's my understanding that were this abort to actually occur that it might take some good fortune (perhaps even divine intervention) to pull it off. It just made me wonder how crews handle it during their simulator time, and what would be the % of successfull landings versus not.
• #12 by mkirk on 15 Jan, 2007 15:26
• Quote

  Radioheaded - 15/1/2007  9:14 AM
  
  After watching the excellent Diary of Discovery posted by Gordo, I noticed that in one part of that program they very quickly showed the crew working an RTLS scenario in the simulator.  I was curious (considering how quickly they moved to a different scene) how often are the RTLS performed successfully in the simulator?  It's my understanding that were this abort to actually occur that it might take some good fortune (perhaps even divine intervention) to pull it off. It just made me wonder how crews handle it during their simulator time, and what would be the % of successfull landings versus not.

  
  
  That is really a loaded question.  The problem with giving an easy answer is there are too many variables.  For instance is the RTLS being performed because of a Systems problem such as with the APUs, Fuel Cells & Electrical Power Busses, Cabin Leaks, OMS/RCS Propellant Leak, or was the Abort called because of a Performance Problem such as the loss of an engine?  Then you have to consider what if anything happens during the Abort Profile.  Did additional engines fail, are there flight control system issues, is Guidance and Nav good or degraded?
  
  During flight specific training a crew will receive plenty of exposure to RTLS aborts with varying levels of complexity.  In fact just about every 4 hour training session of Ascent/Abort procedures will have at least one run that results in an RTLS (a typical session has 4 to 5 ascent runs).  
  
  Despite the best effort of the training team to “kill” their crew, it is not something that happens very often. Believe me the instructors work very hard to come up with creative ways to make life hard for the Crew in training, however, if the crew dies during a training session it is a very big deal within the Mission Operations Community.  Many folks will look over every detail of what happened to determine if the reason for the simulated “loss of the crew” was related to a real world problem with the shuttle and its flight regime, a flaw in the simulator hardware or software, a mistake by the training team, or a mistake by the crew (or mission control team for integrated simulations).
  
  My point is the intent of training is to work the crew very hard by giving them multiple problems to deal with in an effort to increase the stress levels.  However, it is counter productive to train them with situations they can not get out of.  If crews are dying in the sim then something is wrong somewhere and it needs to be addressed.
  
  Back to your specific question; RTLS is a certified abort profile and is expected to be survivable.  It ws not invented to give the crew something to occupy their time until they hit the dirt.  Being a "certified abort" means that it has been studied to a high level of confidence and with much more rigor than other less likely aborts such as contingency cases.  For the loss of a single engine or many of the potential system failures I personally see no reason RTLS will not work as advertised.  This is not to say that it is not an ambitious profile and that it does not “push the performance envelope”.  There are points in the profile where the vehicle is flying backwards thru its own exhaust plume at high mach numbers (~ Mach 5 – 7) and relatively high alpha (angle of attack), the vehicle has to perform an aggressive pitch around maneuver (while mated to the ET), there is a point where the mated stack is falling vertically at around Mach 1 as it zeros out its downrange velocity and begins to head back to the Cape, the orbiter has to get off the external fuel tank (ET), the ET doors have to get closed, and the entry flight software has to get loaded – these are just some of the reasons RTLS is ambitious.
  
  In the simplest of RTLS scenarios, I would have to say my biggest concern would be at ET SEP.  You have to make sure the orbiter is at the right alpha (angle of attack) of -2 degrees, and with minimal remaining propellant (i.e. less than 2%) to ensure the tank does not re-contact the orbiter…at least that is what the models say.
  
  Survival both in the sim and real world will depend on just how bad a day the crew is having and how many things are going wrong.  I really cant give you the answer in a percentage…
  
  Personally I thought flying RTLS in the simulator was a hell of a lot of fun and the more complicated the scenario the better.  In fact in order for me (or anyone) to be certified in the simulator as a so called “motion pilot” – meaning you are allowed to use the simulator on your own (without a safety instructor) with full motion on – I had to fly the entire RTLS profile manually without the use of the HUD (heads up display) while an instructor evaluates you and asks lots of questions so you can’t concentrate.  Compared to the more complex RTLS scenarios this was actually pretty easy.  
  
  The reason they make you do this for the certification is because it demonstrates your ability to maneuver the shuttle (simulator) in a manner that will not damage the sim or its occupants.  The motion based simulator can move aggressively and they don’t want you hurting people or damaging expensive equipment.  They also don’t want you hurting some VIP (such as the President, Congressman, movie star, or Journalist) that has managed to get a sim ride.
  
  
  
  Mark Kirkman
  
  P.S.
  
  Last year Craig Covault of Aviation Week and Space Technology sat in on a suited Ascent/Abort sim with the STS-115 crew.  He wrote an aritcle about it last fall which gives a good overview of the RTLS profile and a narative of a typical TAL Abort.  If you have access to the magazine this will give you some good insight into what a training session is like.  He is much better at translating the technical stuff than I am.
  
• #13 by DMeader on 15 Jan, 2007 15:56
• Somewhere I think I saw a quote from a crewmember referring to the RTLS abort as "two miracles followed by an act of God".
  
  Briefly on another topic, the Shuttle image used in the headline of the "Shuttle fleet on schedule ahead of NASA summit" article is one of the most stunning launch photographs I have ever seen. Can someone point me to a full-size high-quality copy somewhere I can download, perhaps on L2? I'm not an L2 member yet, but I believe I would join on the spot for that image.
  
  Thank you!
• #14 by jabe on 15 Jan, 2007 19:03
• slightly off topic..can't seem to find the info..I teach physics and do  lessons on rocketry.  my search skills seem to be lacking today ..  
  how much mass is the shuttle losing each second at the start of the launch?  Can do some rough estimates on the masses before and after but curious the actual values.  I under stand it is several tonnes per second..
  is the there a "chart" that discusses it ..  I can find the thrust profile discussed but never the mass loss..  those numbers seem to imprsee kids more than the thrust.  Mass loss seems to be a better "visual" for them than thrust
• #15 by mkirk on 15 Jan, 2007 19:28
• Quote

  jabe - 15/1/2007  1:46 PM
  
  slightly off topic..can't seem to find the info..I teach physics and do  lessons on rocketry.  my search skills seem to be lacking today ..  
  how much mass is the shuttle losing each second at the start of the launch?  Can do some rough estimates on the masses before and after but curious the actual values.  I under stand it is several tonnes per second..
  is the there a "chart" that discusses it ..  I can find the thrust profile discussed but never the mass loss..  those numbers seem to imprsee kids more than the thrust.  Mass loss seems to be a better "visual" for them than thrust

  
  Well of course it does vary with power setting for the SSME’s (space shuttle main engines) and the burn pattern for the SRB (solid rocket booster).
  
  The SSME’s consume about 1,084 pounds of propellant per engine (3 engines total) per second at the mission throttle setting of 104.5%.  I will have to look up the burn rate for the SRB and get back to you if someone else doesn't provide it sooner.
  
  Mark Kirkman
  
• #16 by psloss on 15 Jan, 2007 20:23
• Quote

  mkirk - 15/1/2007  11:09 AM
  
  P.S.
  
  Last year Craig Covault of Aviation Week and Space Technology sat in on a suited Ascent/Abort sim with the STS-115 crew.  He wrote an aritcle about it last fall which gives a good overview of the RTLS profile and a narative of a typical TAL Abort.  If you have access to the magazine this will give you some good insight into what a training session is like.  He is much better at translating the technical stuff than I am.
  

  Thanks for the heads up.  I believe it was Mr. Covault who wrote one of the first published articles on shuttle abort scenarios for AW&ST back in 1979 (something I didn't find until several years later).  Sounds like an interesting read.
  
• #17 by Radioheaded on 15 Jan, 2007 21:12
• Thanks very much Mark,  that was precisely the kind of info that I was after (though my question may have been poorly worded).  I really appreciate you taking the time to answer questions from rookies like me.....  
• #18 by mkirk on 16 Jan, 2007 00:59
• Quote

  jabe - 15/1/2007  1:46 PM
  
  slightly off topic..can't seem to find the info..I teach physics and do  lessons on rocketry.  my search skills seem to be lacking today ..  
  how much mass is the shuttle losing each second at the start of the launch?  Can do some rough estimates on the masses before and after but curious the actual values.  I under stand it is several tonnes per second..
  is the there a "chart" that discusses it ..  I can find the thrust profile discussed but never the mass loss..  those numbers seem to imprsee kids more than the thrust.  Mass loss seems to be a better "visual" for them than thrust

  
  I haven’t had a chance to look up the SRB burn rates as I promised in my previous post, but I just realized for your purposes it would be much easier to look at this chart compiled by Bill Harwood.  It is from the Ascent Flight Design Package for STS-121.  It gives you the total vehicle mass (l2nd to last column) in one second intervals from liftoff to MECO (main engine cutoff – zero thrust).  MECO mass includes the still attached ET (external fuel tank).
  
  Mark Kirkman
  
• #19 by Chris Bergin on 16 Jan, 2007 01:58
• Quote

  DMeader - 15/1/2007  4:39 PM
  
  Somewhere I think I saw a quote from a crewmember referring to the RTLS abort as "two miracles followed by an act of God".
  
  Briefly on another topic, the Shuttle image used in the headline of the "Shuttle fleet on schedule ahead of NASA summit" article is one of the most stunning launch photographs I have ever seen. Can someone point me to a full-size high-quality copy somewhere I can download, perhaps on L2? I'm not an L2 member yet, but I believe I would join on the spot for that image.
  
  Thank you!

  
  http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=6171&posts=8&start=1 for different options. I took the image I used from one of the StarScapes presentations on L2, which usually have some stunning images in them. Although L2 is not about images, that's usually a side bonus if that.