UUCP@ofa123.fidonet.org (UUCP) (05/02/90)
From: karl@sugar.hackercorp.com The following is extracted from RISKS-Forum Digest Volume 9 Issue 88 ------------------------------ Subject: Software Bug Causes Shuttle Countdown Hold at T-31 Seconds According to Aviation Week (April 30, 1990, pg. 24), a software problem caused a three minute hold at T-31 during the launch countdown of the shuttle mission that orbited the Hubble Space Telescope on April 24th. At T-48 seconds, newly written software detected that the outboard external tank liquid oxygen fill and drain valve was open when it should have been closed. The ground launch sequencer (GLS) stopped the countdown clock at T-31 seconds. George T. (Ted) Sasseen, director of shuttle engineering, said that the software changes were made after an incident that occurred on April 2nd, where a pipe burst and sprayed water over a 4,000 volt motor control assembly, shorting it and causing the launch processing system (LPS) control room to go down, prompting concern that the LPS could lose power in the last few seconds of the countdown. Sasseen said that unless oxygen is drained within nine minutes after the flow is stopped, a phenomenon called "geysering" could rupture plumbing and destroy the tank. "So the fix was to put a purge in that [liquid oxygen] line and to put a small gas pad in it. We did that by hand after the inboard valve was closed and before the outboard valve was closed. The GLS sent its 'close outboard' command at 48 sec." He said that about 10 sec. after this happened, "everyone looked around and said, 'Oh boy. We were dumb.'..." Sasseen said the processing team violated one of its own cardinal rules that says: Never make a software change unless you can run it many, many times in simulations and tests. He said, "There are oddities about software changes that you don't always get the first time through. And the basic rule we violated is that it wasn't tested enough." He said the purge/gas pad software may be removed because it is unlikely that there would be a total launch processing system launch between T-31 sec. and T-0. "But I want to emphasize that the software safed the system as it was supposed to." To their credit, the system engineers were able to determine what the problem was and fix it within three minutes. A more cautious approach might have been to scrub the launch until a more careful analysis and review was performed. ------------------------------ Comments from our usually well informed contributors? --- Opus-CBCS 1.12 * Origin: Universal Electronics, Inc. (1:103/302.0) -- uucp: UUCP Internet: UUCP@ofa123.fidonet.org BBS: 714 544-0934 2400/1200/300