Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site brl-tgr.ARPA Path: utzoo!watmath!clyde!burl!mgnetp!ihnp4!houxm!vax135!cornell!uw-beaver!tektronix!hplabs!hao!seismo!brl-tgr!jcp From: jcp@brl-tgr.ARPA (Joe Pistritto ) Newsgroups: net.columbia Subject: Re: Discovery's 1st stage performance low? Message-ID: <4351@brl-tgr.ARPA> Date: Fri, 7-Sep-84 02:23:59 EDT Article-I.D.: brl-tgr.4351 Posted: Fri Sep 7 02:23:59 1984 Date-Received: Sat, 1-Sep-84 14:56:18 EDT References: <890@ut-ngp.UUCP> Organization: Ballistics Research Lab Lines: 18 The 'SRB Performance Low' call you heard has occurred on almost every shuttle mission, and represents the Solids not achieving nominal thrust parameters. Due to the way solid rocket engines work, their performance is much more variable than equivalent liquids. The Discovery automatically compensates for low SRB performance by boosting thrust on the Space Shuttle Main Engines, or moving the throttle-back point slightly, (the region of maximum dynamic pressure). Typical 'low' values are 3 to 7 percent below nominal, I believe. Sufficient reserve fuel is always carried to compensate for low solid performance, as the shuttle must reach a fairly precise 'window in space' at the time of Main Engine shutdown in order to the OMS system, (which is much smaller than the SSMEs) to execute the orbit circularization burn successfully. None of the shuttle flights have had their orbital parameters significantly affected by low 1st stage performance. Most of this I learned from articles on the subject in Aviation Week -JCP-