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philkarn (November 30, 1999 at 12:00 am)
I was thinking about getting the maximum energy out of the fuels, but you're quite right -- only exhaust velocity matters. It's a lucky coincidence that excess H2 is a lot less corrosive than excess O2.The S-II J-2 got a few more seconds of Isp after the mixture ratio shift. Thrust dropped quite a bit, so they did it toward the end of the burn when they had to limit acceleration anyway.
MrFireAss (November 30, 1999 at 12:00 am)
Even if i have to drive 30 minutes a day to get to work, and have to start off pushing a broom all day while @ work, i am going to see if i can get in @ Space X in McGregor, Texas.Its been a life-long dream of mine to work in the Rocket Industry, like my father did.
MrFireAss (November 30, 1999 at 12:00 am)
yessir, i was drinking a beer on my back porch, about 15 miles from McGregor, and all of a sudden there was a loud "thump" followed by a sustained "roar", that shook my house/windows.i think the locals that were against them are coming around, now that they are increasing from 40 employees to 200, and are going to disclose further test times to the public, instead of scaring them 1/2 to death in the middle of the night.The people of McGregor are genuine good ppl, but many arent "up to speed"
ugowar (November 30, 1999 at 12:00 am)
Did you hear and see the Nov 22 test? :) Judging by the comments, a majority of the people are actually against SpaceX there, I guess that's just ignorance on their part. I myself would be thrilled to have a test stand near my house!
ugowar (November 30, 1999 at 12:00 am)
Yes, H2 is much lighter than H2O and the only thing that counts in the rocket equation is exhaust velocity. For a given temperature, lighter molecules have higher velocity. I believe PU shift works because the Laval nozzle isn't 100% efficient in converting all energy to kinetic, there's a bit of useless thermal random movement of the molecules. That's supposedly where PU shift comes in, the exact mechanism is unclear to me too.I think S-II shift was primarily to increase fuel efficiency.
philkarn (November 30, 1999 at 12:00 am)
I'm still trying to understand PU shift. The S-II decreased O2 mass flow and H2 flow was about constant. Thrust went down but Isp increased slightly. Is that due to the lower molecular weight of the exhaust? Is H2 so light that there's little penalty to not burning it all?Was this done late in the burn to limit acceleration, with the slight Isp gain as a bonus?
MrFireAss (November 30, 1999 at 12:00 am)
I live nearby their McGregor ,Texas , Test Facility, and im happy to announce a 1.6 billion dollar contract just being awarded to them. Space - X has done a lot for the community of McGregor, and brings a lot of Aerospace jobs to our county, we wish them nothing but the best.
1143443 (November 30, 1999 at 12:00 am)
someone smart please help mecomment me back eW
ugowar (November 30, 1999 at 12:00 am)
I guess it depends on the geometric arrangement of the turbopump assembly and also total stage dry weight. It could be that they were designed with this in mind and Falcon 1 design didn't take this into consideration. Most other rockets I've seen just start a slight roll after shutdown. Note Delta II has its vernier engines burning for a couple of seconds after MECO, probably to keep attitude.
ugowar (November 30, 1999 at 12:00 am)
Yes, that's fuel to oxidizer ratio. A incorrect value was used in the software in case of F1 launch 2. The PU shift on Saturn V S-II was to maximize Isp, increasing H2 consumption and noticeably lowering thrust as a result. I don't think it was due to depletion requirements.I don't know about Merlin, but it could be shifted as well. |
