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  Main engines "vapor" before shuttle launch

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Author Topic:   Main engines "vapor" before shuttle launch
fryson
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posted 02-28-2011 02:26 AM     Click Here to See the Profile for fryson   Click Here to Email fryson     Edit/Delete Message   Reply w/Quote
I've always wondered what the "vapor" is that is coming from the nozzles (or whatever the actually are) that are on the outside of the SSMEs? Seems like I remember hearing what exactly it was and it's purpose but I've forgotten.

It also seems like it occurs up near the top of the external tank but it's always readily apparent near the SSME.

Ben
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From: Cape Canaveral, FL
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posted 02-28-2011 02:32 AM     Click Here to See the Profile for Ben   Click Here to Email Ben     Edit/Delete Message   Reply w/Quote
Liquid oxygen venting.

fryson
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posted 02-28-2011 03:02 AM     Click Here to See the Profile for fryson   Click Here to Email fryson     Edit/Delete Message   Reply w/Quote
Awesome, I really appreciate the quick reply.

garymilgrom
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posted 02-28-2011 05:53 AM     Click Here to See the Profile for garymilgrom   Click Here to Email garymilgrom     Edit/Delete Message   Reply w/Quote
I've read it took *years* to develop the 6 second starting sequence for these engines. I think even us geeks don't realize how complicated these systems are.

marshallspacerx
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From: Conway, AR
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posted 02-28-2011 05:54 AM     Click Here to See the Profile for marshallspacerx   Click Here to Email marshallspacerx     Edit/Delete Message   Reply w/Quote
Roger, copy You're venting. (Apollo 13 movie)

Byeman
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posted 02-28-2011 06:15 AM     Click Here to See the Profile for Byeman   Click Here to Email Byeman     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by garymilgrom:
I've read it took *years* to develop the 6 second starting sequence for these engines.
Actually it is more like 3 seconds. The remaining time is for the vehicle to rebound from the twang.

The SSME is unique in that it is one of few engines to use a "head" start, that is only relying on the pressure of the propellant at the inlets. Most pump fed engines use solid cartridges or gas spin systems to start the turbo pumps spinning.

garymilgrom
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From: Atlanta, GA
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posted 02-28-2011 06:46 AM     Click Here to See the Profile for garymilgrom   Click Here to Email garymilgrom     Edit/Delete Message   Reply w/Quote
NASA gives main engine start at T-6.6 seconds in the countdown and the thrust from the engines initiates the twang. The three engines start 100 milliseconds apart to reduce loads on the orbiter.

The following is from "Space Shuttle Main Engine: The First Ten Years" by Robert E. Biggs which is a PDF available on the web:

Start and Shutdown
The first hurdle that had to be overcome in the engine test program was to learn how to safely start and shut down the engine. Five years of analysis had produced sophisticated computer models that attempted to predict the transient behavior of the propellants and engine hardware during start and shutdown.

All times are CT (US)

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