Thrust
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Joined: Wed Sep 17, 2003 12:17 pm

Space Shuttle Engine Power Questions

Sat Jun 25, 2011 3:58 am

Hi there. I've been trying to figure out several things about the shuttle stack lately whose answers have proven elusive to me. First, I've gotten varying results as to the total thrust each solid rocket booster produces at maximum. NASA claims it is 2.8 million pounds increasing to 3.3 million pounds each. Other sources I've found claim they produce 3.1 million pounds each. Which of these is correct, or are neither correct? Also, is the 104% rated performance 104% of the 1.2 million pounds the orbiter's engines produce, or did they produce less than 1.2 million pounds total at one time? I read that at some point their power was increased from what it was in the early days.

Second, I'm wondering if the thrust of the shuttle stack increases after max Q. The SI-C stage of the Saturn V increased to over 9 million pounds after passing through max Q, that's why I'm wondering. If in fact the shuttle stack does have greater total thrust after max Q, does anyone have an actual number for that? Thanks.
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zanl188
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RE: Space Shuttle Engine Power Questions

Sat Jun 25, 2011 12:00 pm

Define the conditions and you'll get better answers I think, but generally as atmospheric pressure decreases tthrust increases.

SSME rated thrust @ 100% = 375000lbs at sea level, 470000lbs in a vacuum.

Shuttle SSMEs throttle down thru max Q. SRBs are also designed to produce less thrust thru max Q. You may recall the infamous last words from Challenger "Go at Throttle Up!!" as Dick Scobee saw thrust levels increase post max Q.

I don't believe the F-1s on the S-1C could be throttled. More likely the increase in S-1C thrust was due to change in atmospheric pressure.

Check out some footage of the S-1C in flight. Note that at sea level the edge of the flame front is pretty much attached to the engine bell. But as atmospheric pressure decreases & aerodynamics change the edge of the flame front will move up towards the front of the RP-1 tank!

For good info on the shuttle check out Dennis R Jenkins book "SPACE SHUTTLE: The History of the National Space Transportation System". I understand a new edition will out within a year or two.
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rwessel
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RE: Space Shuttle Engine Power Questions

Sat Jun 25, 2011 7:24 pm

Quoting Thrust (Thread starter):
Also, is the 104% rated performance 104% of the 1.2 million pounds the orbiter's engines produce, or did they produce less than 1.2 million pounds total at one time?

The 100% number was based on the original design, and design improvements increased the available thrust, but they never changed the scale. At one point there were hopes of increasing the normal power to 110%, but NASA could never get that to work reliably, and 109% is the limit even for emergency use.

Quoting Thrust (Thread starter):
The SA) and Svea Flyg (Sweden)">SI-C stage of the Saturn V increased to over 9 million pounds after passing through max Q

The rated sea level thrust of the base F1 was 1,500,000lbs, the vacuum thrust was 1,740,000lbs. The uprated engine on SA-503 upped that a bit, and a bit more on the second uprate on SA-510 (which was rated 1.522Mlbs). Actual performance was a bit higher (the 9.1Mlbs figure comes from working the acceleration of the vehicle backwards on SA-510*). The F-1's expansion bell was sized for near sea level conditions, where thrust was most critical for the S-1C. Contrast the proportionately much larger bells on the J-2s and the AJ-10 (-137) on the Apollo Service Module, which were both optimized to work in vacuum.

Maximum thrust was achieved just before the center engine shutdown (which is about 55s after Max-Q, so it's not really related to that). The center engine was shut down first on both the S-IC and S-II to keep maximum acceleration below the vehicle limits.

* http://en.wikipedia.org/wiki/File:SaturnVThrust2.jpg

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