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.
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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.)
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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

"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)

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

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

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.
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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.)
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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.
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spsystems.net is temporarily off the air; | Henry Spencer
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"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.
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spsystems.net is temporarily off the air; | Henry Spencer
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