Why does the LV-N need oxidizer?

[Brotoro]

You're right - it's not relevant. But neither is the rest your reply... because nobody (other Russian ICBM/SLBM operators) puts liquid fuels (other than maybe in a sealed RCS system, *maybe*) onboard a launch vehicle "days or weeks" ahead of time, refrigerated or not. It's unsafe as hell to have that much explosive sitting around essentially unshielded. (And depending on the liquid fuel/oxidizer involved, rough on the equipment too.)

Spacecraft are fueled as late in the launch sequence as possible, because once fueled, nobody sensible wants to be within miles of a fueled (read:explosive) booster.

I have noticed in multiple threads that DerekL1963 speaks about real-life rockets with knowledge and wisdom... I just thought I'd mention that.

[RoboRay]

You're right - it's not relevant.

It's extraordinarily relevant.

We are not discussing just "shoot them and go home" launch vehicles (where no, it isn't relevant for the reasons you mentioned), but also vehicles that will remain in space for extended periods of time. If you fuel them with hydrogen, the tank will be significantly depleted when they arrive at their destination and need to use the engine again.

Edited May 18, 2013 by RoboRay

[DerekL1963]

It's extraordinarily relevant.

We are not discussing just "shoot them and go home" launch vehicles (where no, it isn't relevant for the reasons you mentioned), but also vehicles that will remain in space for extended periods of time. If you fuel them with hydrogen, the tank will be significantly depleted when they arrive at their destination and need to use the engine again.

The message I replied to wasn't about vehicles in space for an extended period - it was about vehicles on the launch pad for an extended period. So yes, his comment is irrelevant. So is yours.

[Temstar]

With the shuttle, since they often have to sit on the pad for ages waiting for the all clear to lift off NASA simply vent away the hydrogen vapour that has boiled off and then constantly refile the ET with more liquid hydrogen to ensure it's topped up. The refile line is only retracted shortly before lift off.

Even if you could solve the boil off issue by installing some kind of cryogenic refrigeration planet on your spacecraft you still can't hold a tank of hydrogen indefinitely. Hydrogen atoms are so small that every now a then some hydrogen atoms seep into the metal wall of the tank and travel some distance into the metal. When these atoms meet each other two hydrogen atoms then recombine into a hydrogen molecule. A hydrogen molecule is much bigger than two hydrogen atoms by themselves so the result molecule greatly expand in volume and pushes the metal atom in the metal lattice away, thus disrupting the structure of the metal and making it weaker, causing it to fail eventually.

[RoboRay]

The message I replied to wasn't about vehicles in space for an extended period - it was about vehicles on the launch pad for an extended period. So yes, his comment is irrelevant. So is yours.

If you want to confine yourself to a tiny part of a larger discussion, feel free. But don't pretend that there's not more being discussed, especially when making absolute statements that don't apply absolutely.

Edited May 18, 2013 by RoboRay

[The Ideal Gas Lawyer]

As I understand it, the F-18 (or w/e) has a very loose fitting fuel tank, such that it actually leaks fuel while on the runway; this tank will expand into the body once the plane reaches supersonic speeds, but until then it will constantly lose fuel.

ARGH! Please stop repeating this. It is NOT true for the F/A-18 or any other aircraft. IIRC he myth started because a SR-71 prototype leaked fuel on the ground but they fixed that in later models by using a elastic bladder as a fuel tank liner to allow for expansion. Sorry, pet peeve of mine.

/nerd rage.

Hydrogen atoms are so small that every now a then some hydrogen atoms seep into the metal wall of the tank and travel some distance into the metal. When these atoms meet each other two hydrogen atoms then recombine into a hydrogen molecule. A hydrogen molecule is much bigger than two hydrogen atoms by themselves so the result molecule greatly expand in volume and pushes the metal atom in the metal lattice away, thus disrupting the structure of the metal and making it weaker, causing it to fail eventually.

Got a source on that? I wouldn't think that LH2 would spontaneously disassociate allowing individual atoms to seep into the metal.

Edited May 18, 2013 by The Ideal Gas Lawyer

[NovaSilisko]

I see this thread about a week ago; Nova answers it in like two seconds.

1 week later 4 pages. AWAHHAH?!?!

I had the same reaction...

[Temstar]

Got a source on that? I wouldn't think that LH2 would spontaneously disassociate allowing individual atoms to seep into the metal.

http://www.uni-saarland.de/fak8/wwm/research/phd_barnoush/hydrogen.pdf

[Pbhead]

material scientist here. Let me tell you that hydrogen embrittlement of metals is a HUGE thing we have to deal with and/or be mindful of.

And if you dont de-gas all the hydrogen out of molten aluminum before you cast it, it ends up looking like swiss cheese.

[The Ideal Gas Lawyer]

material scientist here. Let me tell you that hydrogen embrittlement of metals is a HUGE thing we have to deal with and/or be mindful of.

And if you dont de-gas all the hydrogen out of molten aluminum before you cast it, it ends up looking like swiss cheese.

Oh I know it's a big deal. I'm just surprised/skeptical that you can get hydrogen inclusions from a piece of metal being immersed in liquid diatomic hydrogen. I get monoatomic atomic hydrogen making its way into a workpiece in casting or welding since the high temperature would disassociate the H2 but cryogenic LH2 seems unlikely to behave the same way.

*shrug* I'm not an expert on the subject. That's why I'm asking. My background is straight chem with only a basic familiarity with MSE

Edited May 18, 2013 by The Ideal Gas Lawyer

[Fel]

ARGH! Please stop repeating this. It is NOT true for the F/A-18 or any other aircraft. IIRC he myth started because a SR-71 prototype leaked fuel on the ground but they fixed that in later models by using a elastic bladder as a fuel tank liner to allow for expansion. Sorry, pet peeve of mine.

/nerd rage.

Meh on the nerd rage, and more meh on the myths. I remember something that I was told from who knows where a decade ago, and it turns out it is untrue People should stop making myths, I say.

So you all are saying that liquid hydrogen is so uncontainable that it does not matter how big your sponge is, you're still going to lose more than you can afford... and even with the thicker lining of the shuttle's ET; it still is profusely leaking as is?

[rosenkranz]

My bad for using wiki as a source. But that was some of what I read looking that stuff up weeks ago.