Date: Fri, 4 Dec 92 10:01:31 From: Space Digest maintainer Reply-To: Space-request@isu.isunet.edu Subject: Space Digest V15 #505 To: Space Digest Readers Precedence: bulk Space Digest Fri, 4 Dec 92 Volume 15 : Issue 505 Today's Topics: Pop in space Shuttle downtime Welcome to the Space Digest!! Please send your messages to "space@isu.isunet.edu", and (un)subscription requests of the form "Subscribe Space " to one of these addresses: listserv@uga (BITNET), rice::boyle (SPAN/NSInet), utadnx::utspan::rice::boyle (THENET), or space-REQUEST@isu.isunet.edu (Internet). ---------------------------------------------------------------------- Date: Fri, 4 Dec 1992 11:51:40 GMT From: Tor Houghton Subject: Pop in space Newsgroups: sci.space I don't know - didn't Coke or Pepsi device special cans for the Space Shuttle crew? :) Cheers, Tor -- ---------------------------------------------------------------------- email: torh@cogs.susx.ac.uk "Then we will wonder if machines will steal each others dreams." ------------------------------ Date: Fri, 4 Dec 92 09:01:49 EST From: John Roberts Subject: Shuttle downtime -From: prb@access.digex.com (Pat) -Subject: Re: shuttle downtime -Date: 3 Dec 92 19:04:39 GMT -Organization: UDSI -On the other hand. shortly after the STS 51-l loss, the new calculations -indicated an expected loss rate of 1/25 for the shuttle fleet. in fact this -was a major grounds for criticizing and stagerring SSF launches, and was a -driver for looking at using HLV's to lift SSF. That's the number I heard. Prior to Challenger, the NASA estimate was one in hundreds - afterwards, NASA was under considerable pressure to come up with a more accurate number. -If the three year down, has helped push down this loss rate then i am all for it -in fact, we are at flight 52 and no sign of blowing up one. if this means -we have cut the loss rate, we are in good shape. -Henry, you'd be the expert. has the expected loss rate of orbiters dropped -due to the system improvements, or is it still sitting at 1/25? The number calculated by NASA for the post-Challenger Shuttle system was one orbiter loss per 78 flights. Truly didn't like to use that number, but Goldin has used it several times lately, notably at the town meetings. It appears that the greatest remaining risks are the SSME turbopumps (which NASA would like to improve), and landing accidents. There are also many risks of lesser magnitude. It should be noted that the number relates to the entire launch *system*, which includes the protocol for launch preparation and flight. By changing the protocol (i.e. more vigorous testing of SRB joint seals and not launching in cold weather), it's possible that the odds could have been improved to better than one in 25 with the old hardware - I don't know how much of the improvement is due to hardware changes. (I know that the SRB chambers are now pressure tested to check the seals.) Also, some of the more probable scenarios (such as a landing accident that damages the orbiter beyond repair) would not necessarily kill the crew - not really significant from an economic viewpoint, but survival of the crew would be better from a public relations viewpoint. I note that the estimate has not changed since the first post-Challenger flight, during which time several new safety features have been implemented. My guess would be that NASA wants to remain slightly conservative on the estimate - they don't want to portray the Shuttle as being extremely dangerous to use, but the "nightmare scenario" would be an analysis following a future accident showing that the actual risk had been greater than the claimed estimate. There have been some relaxations in flight rules over the last few years. The explanation given is that the rules were made extremely conservative after Challenger, and they are being reviewed one at a time to determine whether any of them were made more stringent than needed (the rules on cross-winds for takeoff have been modified, for instance, partly as a result of improved ability to modify the launch control parameters in response to existing conditions). Before the launch of STS-53, there was talk of a review (which has been going on for a year or more) on whether the rule for the combination of low temperature and ground wind speed at launch could be relaxed slightly. It was stated that if no decision was reached, the old rule would continue to apply for STS-53. Does anybody know whether a decision was reached? (As it turned out, there was excessive frost buildup on the cryogenic portions of the launcher at the beginning of the launch window, and the launch team waited until the sun came up and melted/sublimed the excess ice before proceeding with the launch. The SRB joints were apparently not the main concern, as they are always electrically heated until a few seconds before launch - the principle concern was chunks of ice breaking off and damaging the orbiter tiles.) John Roberts roberts@cmr.ncsl.nist.gov ------------------------------ End of Space Digest Volume 15 : Issue 505 ------------------------------