Apollo return fuel requirements?

[bknight]

Did I imply it was with the CSM?  The original comments between Bob and myself were centered on the CSM, but with AS13 the damage to it with the ruptured oxygen disk made using the CSM engine risky.

EDITED: Added CSM statement

[Zakalwe]

Did I imply it was with the CSM?

Well, as you were discussing the SM, it's fuel tanks and how they are pressurised, I would say yes.

There were two fuel tanks and two oxidizer tanks for the SPS. One tank fed into the other, which fed the engine. The one in the middle was called the "sump" tank, and was depleted only after the corresponding storage tank was emptied.

I'm not sure why this was done, but I'd guess it was to maintain good mass properties of the stack as the propellants were depleted.

That seems inefficient, unless one pump fed the first tank into the second and finally into the rocket engine.  However it was constructed it did the job required.

There were no pumps, only helium pressure in the tanks to drive the fuel into the engine.

Of course how stupid of me.  But I remember a conversation in another thread that the helium disk burst on AS13 requiring the RCS to make the final flight correction.

[bknight]

Please see my edit, as you were quickly quoting me.

[Bob B.]

I don't think the SPS propellant tanks had bladders. The +X RCS engines were fired for a few seconds to settle the SPS propellants before the SPS engine was fired; the burn pads make reference to "ullage", the length of this pre-burn. It was not necessary when the tanks were full.

What do you suppose the device is that is shown inside the fuel tank in the cut out view?

http://www.braeunig.us/space/specs/drwgs/apollo.htm

[bknight]

Here is a link that has more complete diagrams.
http://history.nasa.gov/afj/aoh/aoh-v1-2-04-sps.pdf

It looks like the structure in in the oxidizer/fuel sump tanks.  I haven't looked at all the drawings and am unsure of many of the functions of each piece.

EDIT:  Perhaps it is a retention reservoir as depicted on page three of the document.

[VQ]

What do you suppose the device is that is shown inside the fuel tank in the cut out view?

http://www.braeunig.us/space/specs/drwgs/apollo.htm

Heater? Bubble eliminator?

[ka9q]

EDIT:  Perhaps it is a retention reservoir as depicted on page three of the document.

I looked them over again and I think this is correct. The pressurizing helium is fed to the top of the storage tank. The bottom of the storage tank is connected to a standpipe that runs to the top of the sump tank. The bottom of the sump tank has the retention mechanism, which then feeds the engine. That's probably what you're seeing in that diagram.

As I see it, the storage tank empties first, filling with helium while keeping the sump tank full. Once the storage tank empties, helium blows through it into the sump tank, which then empties as well.

The text says that the retention mechanism avoids the need for ullage burns when the tanks are more than about 56% full, except after a docked LPS burn by the LM.

The 56% figure is probably the point where the storage tank empties and the sump tank begins to form ullage. A docked LPS burn by the LM would tend to move the ullages in the SPS tanks to the bottom, which is probably the reason for requiring a SPS ullage burn even when the tanks are more than 56% full. This is to avoid having a helium bubble sucked into the sump tank before the storage tank is actually empty.