#111
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fun with expendable SSTOs (was The 100/10/1 Rule.)
In article ,
Pat Flannery wrote: The orthodox approach is to pressurize with helium... Was Atlas helium pressurized prior to launch, or was the LOX boil-off used? Classical Atlas -- in fact, the modern ones too -- pressurized with helium throughout. I assume the latter, as it was venting pretty much right up till launch. Even pressurizing with helium, you do still get LOX boiling off and having to be vented (and yes, some of your helium goes with it). In fact, in this shot it looks like it's venting after lifting off... That *might* be leftover gas in the vent system; not sure. -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#112
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fun with expendable SSTOs (was The 100/10/1 Rule.)
In article ,
Pat Flannery wrote: The LOX still boils off to some extent due to contact with the tank walls, which are warmer than it is. you simply put a pressure relief valve on the tank set to the amount of internal pressure you want in it, and it vents the LOX if the pressure goes above that value in the LOX tank. However, this doesn't work if you want subcooled LOX. The whole point of subcooled LOX is that it's below its boiling point, and therefore is not boiling off. (And yes, to keep it that way on the pad, you probably have to actively refrigerate it.) -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#113
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fun with expendable SSTOs (was The 100/10/1 Rule.)
On Mar 13, 5:47 am, (Henry Spencer) wrote:
In article , Pat Flannery wrote: The LOX still boils off to some extent due to contact with the tank walls, which are warmer than it is. you simply put a pressure relief valve on the tank set to the amount of internal pressure you want in it, and it vents the LOX if the pressure goes above that value in the LOX tank. However, this doesn't work if you want subcooled LOX. The whole point of subcooled LOX is that it's below its boiling point, and therefore is not boiling off. (And yes, to keep it that way on the pad, you probably have to actively refrigerate it.) Surely subcooled LOX will still be evaporating (the vapor pressure isn't going to be zero, after all), and so to keep a constant-ish pressure as the temperature increased you'd still have to vent gas. - jake |
#114
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fun with expendable SSTOs (was The 100/10/1 Rule.)
"Jake McGuire" wrote in message ups.com... On Mar 13, 5:47 am, (Henry Spencer) wrote: In article , Pat Flannery wrote: The LOX still boils off to some extent due to contact with the tank walls, which are warmer than it is. you simply put a pressure relief valve on the tank set to the amount of internal pressure you want in it, and it vents the LOX if the pressure goes above that value in the LOX tank. However, this doesn't work if you want subcooled LOX. The whole point of subcooled LOX is that it's below its boiling point, and therefore is not boiling off. (And yes, to keep it that way on the pad, you probably have to actively refrigerate it.) Surely subcooled LOX will still be evaporating (the vapor pressure isn't going to be zero, after all), and so to keep a constant-ish pressure as the temperature increased you'd still have to vent gas. True, but to prevent cavitation in the turbopump intakes (or at least make the design of the turbopumps easier), it's a good thing to have the LOX under some pressure. That and many tank designs require the pressurization to insure structural integrity of the tank under flight loads. With (early) Atlas, the balloon tank design required tank pressurization to keep the tank from collapsing while it was on the ground. :-O Jeff -- "They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety" - B. Franklin, Bartlett's Familiar Quotations (1919) |
#115
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fun with expendable SSTOs (was The 100/10/1 Rule.)
Henry Spencer wrote: Even pressurizing with helium, you do still get LOX boiling off and having to be vented (and yes, some of your helium goes with it). You'd think most of the helium would go with it, given the position of the vent at the top of the Lox tank. Or do they keep adding helium as it sits on the pad? Is the fuel tank pressurized by helium or nitrogen? I Googled around, and found references to something being pressurized by nitrogen in both Atlas and Delta stages. Pat |
#116
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fun with expendable SSTOs (was The 100/10/1 Rule.)
Henry Spencer wrote: However, this doesn't work if you want subcooled LOX. The whole point of subcooled LOX is that it's below its boiling point, and therefore is not boiling off. (And yes, to keep it that way on the pad, you probably have to actively refrigerate it.) I always remember shooting the Viking rocket with the rifle story. I've seen video of a Atlas sitting on a pad when suddenly its LOX tank ruptures and the whole thing falls apart like it was made of aluminum foil. In that case it didn't look like the tank had any LOX in it, so I don't know what went wrong. Pat |
#117
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fun with expendable SSTOs (was The 100/10/1 Rule.)
In article ,
Pat Flannery wrote: Even pressurizing with helium, you do still get LOX boiling off and having to be vented (and yes, some of your helium goes with it). You'd think most of the helium would go with it, given the position of the vent at the top of the Lox tank. Or do they keep adding helium as it sits on the pad? They do, I believe. Is the fuel tank pressurized by helium or nitrogen? The references I have on hand -- good but not absolutely definitive -- agree that Atlas pressurization was all helium. I Googled around, and found references to something being pressurized by nitrogen in both Atlas and Delta stages. No nitrogen on Atlas that I know of... with one slightly-odd exception. On later big rockets like the Saturns, you'll often find helium storage placed *inside* a cryogenic propellant tank, both because cold helium is denser than warm helium, and because cold metal is stronger than warm metal and hence can take higher pressures. Atlas didn't do that... but it *did* have LN2 jackets around its helium tanks to chill them; the LN2 was dumped just before launch. -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#118
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fun with expendable SSTOs (was The 100/10/1 Rule.)
I wrote:
In fact, in this shot it looks like it's venting after lifting off... That *might* be leftover gas in the vent system; not sure. Investigating pressurization, I got an answer on this one: on the early Atlases, the boiloff vent *remained open* during ascent. Later versions closed it so the boiloff would help maintain pressure. (Possibly the original designers were worried that rapid boiloff might overpressurize the tank during ascent.) -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#119
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fun with expendable SSTOs (was The 100/10/1 Rule.)
In article . com,
Jake McGuire wrote: However, this doesn't work if you want subcooled LOX. The whole point of subcooled LOX is that it's below its boiling point, and therefore is not boiling off. (And yes, to keep it that way on the pad, you probably have to actively refrigerate it.) Surely subcooled LOX will still be evaporating (the vapor pressure isn't going to be zero, after all)... Like any normal non-boiling liquid, it'll evaporate until the partial pressure of GOX above it equals the vapor pressure, and then stop. You get continuous boiloff from a liquid at constant temperature only if its vapor pressure exceeds the ambient pressure (so it cannot reach a gas-liquid equilibrium). and so to keep a constant-ish pressure as the temperature increased you'd still have to vent gas. To maintain LOX subcooled, you don't let the temperature increase. See comment above about active refrigeration. -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#120
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fun with expendable SSTOs (was The 100/10/1 Rule.)
"Henry Spencer" wrote in message ... In article , Pat Flannery wrote: The LOX still boils off to some extent due to contact with the tank walls, which are warmer than it is. you simply put a pressure relief valve on the tank set to the amount of internal pressure you want in it, and it vents the LOX if the pressure goes above that value in the LOX tank. However, this doesn't work if you want subcooled LOX. The whole point of subcooled LOX is that it's below its boiling point, and therefore is not boiling off. (And yes, to keep it that way on the pad, you probably have to actively refrigerate it.) It would be interesting to have refrigerant coils low in the tank and heating coils high such that warm GOX could pressurize subcooled LOX. Same on the fuel side to avoid helium entirely. It would be a pain with some possibility of payoff. -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
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