104% thrust

asterix · April 28, 2009, 12:08:25 PM

Can someone please explain why during ascent, the SSME's are throttled at 104% of rated thrust and not 100%. i.e. why isn't maximum at 100% like in most worlwide applications of the figure. (Just a small mathematical ponderance).

uri_ba · April 28, 2009, 12:23:11 PM

I'm just hypothesizing there, but I think it's like "War power" on props (and jets)
you have maximum rated thrust without time limitations (100%). but you can spool up the engine to a higher thrust with a time limit. for example, if I remember correctly (working the memory of something I read 15 years ago), a J52 engine (A4 and A6) could be operated at 102% for 30 seconds and up to 106% momentary

lets say that SSMEs are rated for 10 minutes @ 104%, but because SSMEs only burn for about 8.5 minutes you have no problems operating at 104% for the entire duration of the powered flight.
I guess they can be spooled up even higher with a tighter time restrictions, say 110% for 30 seconds.. but that is something that you will only know for a fact after digging deep into the SSME tech manual.

uri_ba · April 28, 2009, 12:40:39 PM

found this
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930012456_1993012456.pdf

pages 10 and 11 deal with the subject.

apparently 100% is the "rated thrust". and later during engine development they have discovered the engine can safely operate at a higher thrust, but they have decided to keep the 100% where it is to avoid confussion.

apperantly this operation (running the engine in higher thrust) increases the probability of an engine problem.
at a 100% inflight premature shutdown probability is once every 120 flights. at 104% it's a shutdown every 45 flights and at 109% chances of a in-flight premature engine shutdown jumps to an unacceptable rate of a failiure every 8 flights.

Nephi · April 28, 2009, 03:25:36 PM

It is like the nuclear reactor of a submarine : it can be used above 100% but with increased danger.

Dee-Jay · April 28, 2009, 04:04:25 PM

And like an F-16's engine ... but without danger

davidrobinsonjr · April 30, 2009, 02:54:40 AM

We run our TFE-731's at 101.5% with no time limits.

asterix · April 30, 2009, 01:25:42 PM

Thanks guys for your answers, makes sense now.

Nephi, are you a fellow sun-dodger perchance???

Nephi · April 30, 2009, 01:41:23 PM

QuoteNephi, are you a fellow sun-dodger perchance???

Used to be

One of the most wonderful experience in my life.

elmarko · April 30, 2009, 05:28:39 PM

Quote from: Nephi on April 30, 2009, 01:41:23 PM
QuoteNephi, are you a fellow sun-dodger perchance???
Used to be One of the most wonderful experience in my life.

You just became 104% more awesome

I thought it wasn't so much that the engines were investigated and were found to be able to operate at 104% and 109%, but more that there were constant upgrades and improvements carried out as the years have gone on.

Firecapt_32 · May 01, 2009, 02:42:44 PM

If anything it'll shorten the time between inspections and/or determine the TTBOH of the engine.

psowen · May 11, 2009, 08:13:13 PM

I noticed during today's launch that the built in hold at the t-9 minute point in the count was considerably longer than in the sim. In the sim it was only 10 minutes.

Admin · May 11, 2009, 09:06:37 PM

Correct. We went for 10 minutes to ease the waiting period on the gamers, but that is usually mission-dependent and can be usually twice as that. We may add the correct historical hold times and add them to the sim once we get hold of that information.

For STS-125 the time assumed by the PAO was 23 min but the actual one was around 45 min.

/Admin