NASA has postponed the Discovery STS-119 mission to the International Space Station while it ponders a possibly troublesome hydrogen flow control valve. The agency explains that the shuttles have "three flow control valves that channel gaseous hydrogen from the main engines to the external fuel tank" - one of which was damaged …
I'm no rocket scientist
...but why would you want to move hydrogen from the engines to the fuel tanks?
Because the when the fuel is pumped into the external tank, it goes through the shuttle's engine bay to do so.
Re: I'm no rocket scientist.
To make it go BANG, obviously.
If they couldn't make it go BANG occasionally, they'd lose all that TV coverage and public interest that keeps the tax dollar largesse flowing their way.
The gaseous H2 is aslo used to keep the LH2 tank pressurised through the powered flight. At launch the tank will be pressurised by the natural boil-off of the fuel and this helps force the LH2 to the pumps for ignition. During flight the tank needs to be kept pressurised to stop the tank being damaged (imploding) as the fuel is sucked out of the tank, hence the feedback of gas to the tank.
As I understand it from reading the NASA article:
As the external tank empties a vacuum is created above the liquid hydrogen left in the tank this cannot be vented to air for obvious reasons. The engines convert some of the liquid hydrogen fuel to gaseous hydrogen and return it to the tank to equalise the pressure.
No Really .. I'm Not a Rocket Scientist
But even I know that if you want the liquid to come out you need to put a gas back in... Glug Glug.. and in the case of liquid Hydrogen, you really dont want to let the air in.. warmed up Hydrogen as gas from the engines is best!
Whatever happened to...?
astronauts with some balls (figuratively or not)?
If we worried that ZOMG! We might DIE! all the bloody time, we would never have left the oceans, let alone made it to the trees.
Why not just pray to God; that always works, doesn't it? Isn't that the American way of fixing things?
You are the first folks to actually post a link to the PDFs, including several sites I read which are space-only. And maybe the 2nd non-space site to not go "OMG THE SHUTTLE'S GONNA ASPLODE!!!111oneoneone". Good on ya.
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