Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!gem.mps.ohio-state.edu!lavaca.uh.edu!jane!cheehh From: CHEEHH@jane.uh.edu (Rikhit Arora) Newsgroups: sci.space.shuttle Subject: SSME ignition in the launch timeline. Message-ID: <5091@jane.uh.edu> Date: 1 Dec 89 00:00:40 GMT Organization: University of Houston Lines: 40 This is in response to Phil Jansen's mailing regarding the sequence of SSME start and SRB ignition and liftoff. The exact number are as follows: . . T-2 minutes, 55 seconds Pressurize liquid oxygen tank for flight and retract gaseous oxygen vent hood. T-1 minute, 57 seconds Pressurize liquid hydrogen tank. T-31 seconds "Go" from ground computer for orbiter computers to start the automatic launch sequence. T-28 seconds Start SRB hydraulic power units. T-21 seconds Start SRB gimbal profile test. T-6.6 seconds Main engine start. T-3 seconds Main engines at 90 percent thrust. T-0 SRB ignition, holddown post release and liftoff. T+7 seconds Shuttle clears launch tower and control switches to JSC. As can be seen, it takes 3.6 sec for the main engines to get to 90% thrust and 3 sec thereafter to go all the way to 104% and the autosequence computers to verify that the SSME's are delivering the expected thrust. This probably is the primary reason that the main engines ignite before the SRB's. -- ------------------------------------------------------------------------------- ............. | Rikhit Arora And, while with silent, lifting mind I've trod | cheehh@uhupvm1.bitnet The high untrespassed sanctity of space, | cheehh@jane.uh.edu Put out my hand, and touched the the face of God.| ~~~~~~~~~~~~~~~~~~ -------------------------------------------------------------------------------