Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!telesoft!garym From: garym@telesoft.com (Gary Morris @wayward) Newsgroups: sci.space.shuttle Subject: Re: Shuttle Status for 04/22/91 (Forwarded) Message-ID: <1244@telesoft.com> Date: 26 Apr 91 23:04:49 GMT References: <1991Apr22.234907.29805@news.arc.nasa.gov> <1991Apr23.133522.21767@zoo.toronto.edu> <1991Apr24.163541.21584@elroy.jpl.nasa.gov> Reply-To: garym@telesoft.com Organization: TeleSoft, San Diego, CA, USA Lines: 45 In <1991Apr24.163541.21584@elroy.jpl.nasa.gov> pjs@euclid.jpl.nasa.gov (Peter Scott) writes: >In article , mvk@aix01.aix.rpi.edu (Michael V. Kent) writes: >> In article <1991Apr23.133522.21767@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes: >> >In article rose@beowulf.ucsd.edu (Dan Rose) writes: >> >>This may be a naive question, but . . . Why do they have these planned >> >>countdown holds? >> > >> >Well, the reason for having pauses in the sequence is to give some slack >> >in case minor problems develop (which does happen with some frequency). >> >> Not to correct the famous Henry Spencer :) but I believe the holds are there >> on purpose. The reason is that it is psychologically easier to extend a >> planned hold than to stop the countdown in progress. >I believe that there is also a second clock running during the >countdown that has holds figured into it, and therefore does not stop. >I heard them mentioning it during a launch sequence once, but I forget >what it was called. There are two clocks running the L- (L minus) and T- (T minus) clock, one stops during holds the other includes planned holding time, but I get them mixed up :-) I think T- is the time remaining not counting any holds, and L- is the time to launch assuming scheduled hold time. The reason for scheduled holds is to have everything in a known state that can stay that way for a period of time to allow for fixing problems. Most of the schedules/checklists are written using T- time so that you can tell exactly where you are supposed to be irregardless of how long the scheduled holds have been. When you get to a hold point, you usually don't want various people to go on to the next steps until everything else is ready. There are time limits on many things, for example you wouldn't want to start the APUs (which can only run for a few minutes before you have to start over) while you are working out a problem somewhere else. In general, when you go past a hold point, you have a certain amount of time in which to launch or you have to scrub until the next day (or move back to an earlier hold point), you want to make sure everything is ok so far before leaving a hold. --GaryM -- Gary Morris Internet: garym@telesoft.com KK6YB (was N5QWC) UUCP: ucsd!telesoft!garym TeleSoft, San Diego, CA, USA Phone: +1 619-457-2700