Shuttle Questions Q and A

• #520 by mkirk on 18 Mar, 2006 16:01
• Quote

  simcosmos - 17/3/2006  7:25 AM
  
  Hello,
  
  Can we talk a little more about specific STS (Orbiter >> OMS/RCS) performance numbers (or estimatives)?
  I tried to search but was not able to find too much precise info.
  
  I mean, could someone please share what are the expected minimum / maximum values for total OMS/RCS propellant quantity in the following steps of a space shuttle mission?
  
  1. After completing orbital insertion,
  2. Before starting the final transfer burn for rendezvous (supposing here ISS),
  3. At station keeping distance (before going for dock),
  4. After docking,
  5. Just before starting the de-orbit burn,
  6. After de-orbit burn.
  
  
  Related with the last point: after(?) de-orbit burn some of the remaining OMS/RCS propellant is dumped, right? How much is the minimum needed for reentry (RCS control)?
  
  

  
  Here are a couple of links that will allow you to do some detective work and derive some of the numbers you are looking for.  For the early shuttle flights the flight plan actually had prop usage charts in it but that is not the case.  I know that the shuttle prop folks would be able to answer this and I don't hav emy notes with me so I can't answer this in the detail you want.
  
  In taining I never really saw the big picture on prop usage from start to finish.  If I were involved in Rendeavous/Docking training I would start with XX amount of prop and finish with YY.  The goal was always to use as little as possible.  Crew members are evaluated among other things on how well they manage prop usage during maneuvers.  The basic technique is to move the stick...wait for the response and then move it again if needed.  In other words give the input time to respond.
  
  You also have to distinguish between RCS and OMS quantities.  What makes this confusing is for small maneuvers less than 6 feet per second the plan is to use the RCS rather than the OMS.  Also the RCS jets can use propellant from the OMS tanks but the OMS can not use RCS propellant.
  
  With that in mind you can look at this link to the STS-114 execute packages. For instance in the FD (flight Day 2) package on page 14 you will see the expected quatities after the NC2 burn.  Each package will likley have a page some where in it that mentions prop quatities before and after a set of maneuvers.
  
  http://www1.nasa.gov/returntoflight/crew/mission_docs/execute_packages.html
  
  This link here will give you the STS-114 Flight Plan...go down the page and select under 114 Flight Data Files the Rev A version of the Flight Plan.  If you go to the attitude timeline you will see some of the Burn data and in the last column you will see on some lines a DV for Delta V in feet per second.  If you know the ball park usage of RCS and OMS propellant needed for a specific delta V then you can determine propellant quatities.  Example: 1% of OMS propellant is equivalent to 6 Feet Per Second, 80 LBS of oxidizer and 50 LBS of Fuel.  !% of RCS Prop is equal to 1 Foot Per Second of Delta V and about 22 LBS of Propellant.
  
  http://www.nasa.gov/centers/johnson/news/flightdatafiles/index.html
  
  Like I said you will have to do your homework unless a Shuttle Prop Controller happens to answer this thread or until I get home in a few months and can look it up in my notes.
• #521 by simcosmos on 19 Mar, 2006 10:34
• Thanks for all the info and research directions: already had a first quick glance at them and I should be able to extract what I wish although it will require a little of number crunching (conversions, etc). Another thing that I must have in mind are the current limitations of the simulated items being used here: just to give one or two examples, I still have to double check things such as all components masses, engine's performance as well as propellant quantities, etc.
  
  And talking about OMS/RCS quantities, in the current simulation, it seems that there is only a single "resource/tank" for the OMS/RCS, I mean: any burn from the reaction control system AND from the orbital manoeuvring system will drain from that unique tank. This alone might by itself lead to some improvement in performance given that there is no differentiation / constraints regarding source tanks and related management issues this to not talk about the simulation of additional things that happen in real life (that would require an immense extra work)
  
  Another thing I also have to research is about propellant transfers. As far as I know / confirmed above:
  - both OMS pods can crossfeed resources between them
  - OMS can transfer propellant to the RCS (the other way around is not possible)
  - Buran had plans for additional propellant on the cargo bay: I also seem to remember that this was at least considered somewhere on STS design phase but not implemented in the orbiter (another think that have to check)
  
  I must now also research if the OMS >> RCS transfers are valid for all RCS thrusters, I mean: if, for example, the frontal RCS can receive from the OMS pods or if this is only valid for the rear RCS. Lots to research but little time
  
  I guess that will leave all this work/googling around, etc for a later occasion (need time now for other projects). On that later occasion I might come back with a few more specific questions regarding this and hypothetical shuttle missions like, for example, the polar ones from Vandenberg (I have already saw the related threads in this forum, my questions are more about what would happen after the launch) and stuff like that.
  
  One of the objectives of these questions is to understand a little more this incredible system's capabilities, limitations and perhaps compare those with expected performances, now that we know that STS will start its last operational period. At the same time, the space simulator and available add-ons (spacecraft, sats, tools such as MFD and automatic "computers", EVA, Virtual Mission Control, etc) give a funny and interesting way to have a small impression about how it looks like to be in a past / present STS mission and, at the same time, to have a first hint / experience about hypothetical scenarios.
  
  Thanks again,
  António
  
• #522 by Jim on 19 Mar, 2006 12:26
• Forward RCS can not use OMS propellant; there are no propellant lines interconnecting them. 
  
  The payload bay OMS tanks were not required once the shuttle program developed direct insertion.
  
• #523 by DaveS on 19 Mar, 2006 13:28
• Quote

  Jim - 19/3/2006  2:26 PM
  
  Forward RCS can not use OMS propellant; there are no propellant lines interconnecting them. 

  Well, I read in Jenkin's 3rd edition of his "Space Shuttle" book and it does mention a Forward RCS Interconnect System that would connect the FRCS module propellant tanks with the ARCS propellant tanks and that a test implementation of the FCIS had been done on Enterprise in the late 1990's(could be early 2000, don't have the book near me at this time).
  
  And also noted that it had been approved for implementation.
  
  Any truths in this?
• #524 by Jim on 19 Mar, 2006 13:53
• Quote

  DaveS - 19/3/2006  8:28 AM

  Quote

  Jim - 19/3/2006  2:26 PMForward RCS can not use OMS propellant; there are no propellant lines interconnecting them.

  Well, I read in Jenkin's 3rd edition of his "Space Shuttle" book and it does mention a Forward RCS Interconnect System that would connect the FRCS module propellant tanks with the ARCS propellant tanks and that a test implementation of the FCIS had been done on Enterprise in the late 1990's(could be early 2000, don't have the book near me at this time).And also noted that it had been approved for implementation.Any truths in this?

  
  Never was done, one of many improvements (Electric APU's, etc) shelved
  
• #525 by mkirk on 20 Mar, 2006 14:57
• Quote

  simcosmos - 19/3/2006  5:34 AM
  
  Another thing I also have to research is about propellant transfers. As far as I know / confirmed above:
  - both OMS pods can crossfeed resources between them
  - OMS can transfer propellant to the RCS (the other way around is not possible)
  - Buran had plans for additional propellant on the cargo bay: I also seem to remember that this was at least considered somewhere on STS design phase but not implemented in the orbiter (another think that have to check)
  
  

  
  Crossfeed Options:
  
  -You can feed each OMS engine with the propellants from its own pod
  
  -You can feed an OMS engine with one or both propellants from the other pod (oxidizer, fuel, or both)
  
  -You can feed the AFT RCS JETS with OMS propellant from either pod.
  
  -You can NOT feed the OMS engines with the RCS fuel tanks...it is mechanically possible if you leave the crossfeed switches in the wrong position, but you don't want to feed them this way because the OMS engines would suck the small RCS tanks dry and damage the screens that line the RCS tank walls.  The screens are used to direct the fuel out of the tank in zero gravity by the use of surface tension/capillary action...this is because there are no fuel pumps and pumps wouldn't be very effective in zero g if you did have them.
  
  -There are still switches in the orbiter cockpit for the OMS KIT but as Jim said the concept was never utilized. The OMS KIT was a pallet that would be installed in the payload bay to provide additional fuel, oxidizer, helium, and nitrogen for the OMS engines.
  
  -The modification to add interconnect lines between the Forward and Aft RCS pods was to be incorporated...and in fact the modification got farther than most of the other shuttle modifications that were planned because it really provides alot of redundancy and flexibility...but given the short life span of the program it does not make sense to spend the money.
  
  
  
  
• #526 by TheMadCap on 20 Mar, 2006 22:14
• Greetings All! Fabulous site you have here. I have been monitoring you for quite a while and am very impressed. You seem to have some really knowledgeable personage on this site, some people who actually work at KSC and on the Shuttle itself. I can say you guys are living my dream! I am a chemist, but have always had an affinity for all things related to space ships. Took me a while to catch up on this thread, I knew the Shuttle was the complex machine ever built, but I am starting to get a much better understanding of how much is really involved in making the program happen. Anyways, I have a question:
  
  How does the Beanie Cap actually work? Is the LOX tank under pressure? If so, does the cap depress a spring clip to access the gas vent and use vacuum pumps to draw off the gas? Any close up photos of the arm in action would be wonderful.
  
  Keep up the great work on the site!
  
  Rick aka TheMadCap
• #527 by Jim on 20 Mar, 2006 23:20

• Quote

  TheMadCap - 20/3/2006 5:14 PM Greetings All! Fabulous site you have here. I have been monitoring you for quite a while and am very impressed. You seem to have some really knowledgeable personage on this site, some people who actually work at KSC and on the Shuttle itself. I can say you guys are living my dream! I am a chemist, but have always had an affinity for all things related to space ships. Took me a while to catch up on this thread, I knew the Shuttle was the complex machine ever built, but I am starting to get a much better understanding of how much is really involved in making the program happen. Anyways, I have a question: How does the Beanie Cap actually work? Is the LOX tank under pressure? If so, does the cap depress a spring clip to access the gas vent and use vacuum pumps to draw off the gas? Any close up photos of the arm in action would be wonderful. Keep up the great work on the site! Rick aka TheMadCap

  The LO2 tank is not under pressure during the countdown and the vent valve is open to allow the LO2 to boil off and maintain the proper density.  At T-7:30 (somebody correct me if I have the time wrong) the vent valve is closed and Beanie cap is retracted.  The vent valve will maintain a pressure in the LO2 of xx psi and release GO2 if it reaches yy psi.  

  The Beanie cap draws off the GO2

• #528 by Shuttle Man on 20 Mar, 2006 23:48
• Welcome to the site Rick.
  
  On Jim's answer. Retract Orbiter Access Arm is at T-7:30 seconds. Retract Gaseous Oxygen Vent Arm, or "beanie cap" is at T-2:55 seconds.
• #529 by psloss on 21 Mar, 2006 01:25
• I have kind of ancillary questions regarding the GOX vent hood/arm...perhaps I'm not looking in the right places, but I can't find any references to dock seals (similar, I suppose, to the white room).  During the STS-1 launch campaign, I believe the seal didn't work -- at least during the FRF and launch countdowns (this is another case where I wish I could refer to the Aviation Week editions of the day).  If you review video of the STS-1 launch count, on launch day, the "beanie cap" was retracted during the hold at T-9 minutes (I think), well before the normal time.  I believe that was fixed by the next launch and I don't recall that happening since.
  
  Anyway, my questions are, first, if anyone knows of documentation about the dock seal for the beanie cap...and then my second question regards the retraction of the cap and the arm...looking at launch video, it looks like the cap comes back off the nose of the ET at T-2 minutes, 30 seconds.   Does the "break down" of the dock seals in the cap start at T-2 min, 55 seconds after LOX tank pressurization starts or is it between T-2:55 and T-2:30?
  
  Thanks,
  
  Philip Sloss
  
• #530 by mkirk on 21 Mar, 2006 05:10
• Quote

  psloss - 20/3/2006  8:25 PM
  
  I have kind of ancillary questions regarding the GOX vent hood/arm...perhaps I'm not looking in the right places, but I can't find any references to dock seals (similar, I suppose, to the white room).  During the STS-1 launch campaign, I believe the seal didn't work -- at least during the FRF and launch countdowns (this is another case where I wish I could refer to the Aviation Week editions of the day).  If you review video of the STS-1 launch count, on launch day, the "beanie cap" was retracted during the hold at T-9 minutes (I think), well before the normal time.  I believe that was fixed by the next launch and I don't recall that happening since.
  
  Anyway, my questions are, first, if anyone knows of documentation about the dock seal for the beanie cap...and then my second question regards the retraction of the cap and the arm...looking at launch video, it looks like the cap comes back off the nose of the ET at T-2 minutes, 30 seconds.   Does the "break down" of the dock seals in the cap start at T-2 min, 55 seconds after LOX tank pressurization starts or is it between T-2:55 and T-2:30?
  
  Thanks,
  
  Philip Sloss
  

  
  
  Here is the scoop as I know it:
  
  The Gaseous Oxygen Vent Arm allows the 13 foot diameter Vent Hood (beanie cap) to drop over the top of the external tank so that heated nitrogen can be pumped in to prevent the formation of ice on the liquid oxygen tank vent louvers.  Inside the hood are inflated seals that cover the lovers and direct the oxygen vapors away from the tank and out two ducts that extend downward around the midpoint of the arm.  The flow rate during "boiloff" is about 1 LB per second.
  
  For STS-1 the countdown procedure was to retract the GOX Vent Arm prior to coming out of the T-9 minute hold.  It looked pretty cool to watch because even though the arm was retracted the tank continued to vent until the vent valve was closed at T-2:55.  So for STS-1 you got to see the oxygen venting directly off the top of the then white external tank after T-9 minutes.  
  
  My guess is prior to STS-1 they felt that between T-9 and liftoff there would not be enough time for significant ice formation on the vent louvers.  Well as most of you know that turned out not to be the case and ice debris actually damaged some tiles on the orbiter, most notably the OMS pods.
  
  Starting with STS-2 the Arm & Hood were modified and the procedure was changed to retract the hood after the vent valve is closed for pressurization at T-2:55.
  
  The current procedure is to start pressurization at T-2:55 and the GLS (ground launch sequencer) issues the commands to terminate the Vent Hood GN2 Purge and start the Vent Hood/Arm retract sequence at T-2:50.  I guess the actual time it takes to run the sequence results in the visible movement of the hood from the tank at around T-2:30.
  
  During the countdown while the “stable replenish” or “boil off” is underway the liquid oxygen tank ullage pressure will range from 0 to 17.1 psig.  When the tank is pressurized with helium from the ground support equipment at T-2:55 it will the ullage pressure will range from 19.3 to 22.5 psig.
  
  The picture I attached is of the Vent Arm testing conducted prior to STS-2 at the "arm farm" by the Operations and Checkout building at the Kennedy Space Center.  Notice the smaller ducting than what is on the arm today.  The second picture is one I took of the Vent hood prior to the 114 launch.
  

  ---

  

  

  
• #531 by psloss on 21 Mar, 2006 11:22
• Hi Mark,
  
  Thanks, as usual, for the details.  Regarding the GOX vent hood/arm on STS-1, I was too young at the time for that to have any significance and don't remember seeing the retraction "live"; I do have some beat up copies of some Aviation Week articles made back when I was in school in the late 80s that I'll try to dig up...I don't know if they retracted the arm early for the FRF or not, but that's where I remember the issue with the dock seal being mentioned...I've since seen some video of the FRF, but also have to dig it up.
  
  One other thing that's a bit noticeable is that the vent lines were longer, running pretty much all the way back to the FSS tower.  Not sure if that was also modified between the first two flights, though sneaking a peak at a shot of STS-2 at the pad, it appears so.
  
  Philip Sloss
  
• #532 by astrobrian on 21 Mar, 2006 12:24
• I was just shy of turning 5 when sts-! went up, but for somereason I remember the suspense of them pulling that back so soon. Not sure why I remember that.  Great info as usual  
• #533 by British NASA on 21 Mar, 2006 14:17
• About three years ago I thought the arm and beanie cap were there for stability of the stack. Shows what you can learn by reading threads like this!
• #534 by Jim on 21 Mar, 2006 16:21

• Quote

  British NASA - 21/3/2006 9:17 AM About three years ago I thought the arm and beanie cap were there for stability of the stack. Shows what you can learn by reading threads like this!

  It is only put on for the countdown.  Most of the time it is retracted.  I am going to diverge here a second.

  For outsiders, they have a mind's eye view of the shuttle (and the oldtimers, Saturn V) of it (them) sitting at the pad in launch configuration.  When in reality, it is covered by the RSS most of the time (the Saturn V was covered by the MSS).  The shuttle is "exposed" only 3-4 short times during the stay at the pad.  They are:

  1.  When it arrives at the pad

  2.  the RSS is retracted for payload installation

  3.  The day before launch

  4.  sometimes for APU hot fires.

• #535 by Jim on 21 Mar, 2006 16:21

• Quote

  British NASA - 21/3/2006 9:17 AM About three years ago I thought the arm and beanie cap were there for stability of the stack. Shows what you can learn by reading threads like this!

  It is only put on for the countdown.  Most of the time it is retracted.  I am going to diverge here a second.

  For outsiders, they have a mind's eye view of the shuttle (and the oldtimers, Saturn V) of it (them) sitting at the pad in launch configuration.  When in reality, it is covered by the RSS most of the time (the Saturn V was covered by the MSS).  The shuttle is "exposed" only 3-4 short times during the stay at the pad.  They are:

  1.  When it arrives at the pad

  2.  the RSS is retracted for payload installation

  3.  The day before launch

  4.  sometimes for APU hot fires.

• #536 by Jamie Young on 21 Mar, 2006 18:11
• Do they need to replace the beanie cap after each launch? I ask as it must be in a direct line of fire of the SRBs?
• #537 by Jim on 21 Mar, 2006 19:46

• Quote

  Jamie Young - 21/3/2006 1:11 PM Do they need to replace the beanie cap after each launch? I ask as it must be in a direct line of fire of the SRBs?

  INo, it is retracted out of the way just like the orbiter access arm.

• #538 by psloss on 21 Mar, 2006 20:04
• Also, for a few seconds after liftoff, the vehicle drifts to the north due to the canted thrust of the main engines (one of the pros will be able to say at what point that changes...it's shortly after liftoff).  Even though it doesn't take the pad tower out of the line of fire, perhaps it mitigates those effects somewhat...
  
  If you're not already familiar with that, next time you see video of a shuttle liftoff from the east camera site (facing west), check it out...
  
• #539 by TheMadCap on 21 Mar, 2006 21:49
• Thanks for the wonderfull pics and explaination on the beanie cap.
  
  Another dumb question:
  
  why route the incoming cryogenic fuel and oxidizer through the shuttle into the tanks during fueling? Is it to check the flow path to ensure there are no leaks? It would seem easier to fill the tanks themselves. My apologies if I missed this in any of the previous 36 pages.