Newsgroups: sci.space.history
From: henry@spsystems.net (Henry Spencer)
Subject: Re: LM ullage
Date: Mon, 31 May 1999 14:35:50 GMT

In article <37526DAA.BDEEDF94@bigpond.com>,
Julian Bordas  <JBordas@bigpond.com> wrote:
>        I've read that the LM RCS was used to push the fuel and oxidiser
>to the bottom of their tanks prior to firing the descent engine.

Correct.  The tanks did have baffles at their outlets, but that was more
to ensure that the feed lines stayed liquid-filled during maneuvering.
The system wasn't capable of feeding reliably without RCS settling burns
first.

>what I do not yet under stand is how did the fuel oxidiser get to the
>bottom of the tanks that feed the RCS?

The RCS tanks had bladders separating pressurant gas from propellant
liquid, so that the RCS feed lines always got liquid.  (This is difficult
to do in large tanks, but works just fine for small ones.)

>...How did the throttle work? I know that they injected
>Helium into the fuel and oxidiser lines...

No, wrong, that was the system proposed in the *losing* bid for that
engine (although you will find confusion about this in some normally-
reliable sources).  The system actually used was throttling valves in the
propellant lines, plus a variable-geometry injector mechanically linked to
the valves.

>...Why did NASA go
>with the fixed flow rate Helium injection over the variable flow rate as
>ameans of throttling the decsent engine?  Why go to all the extra weight
>and increased complexity that Helium injection throttling entailed?

They didn't.  See above.

The attractive aspect of helium injection was that it avoided having to
vary the injector geometry, something which had never been tried before.
(Without a variable injector, the lower flow rates would result in very
small injection pressure drops, which is asking for stability problems.)
However, the variable injector worked out all right, and it is probably
just as well that it was chosen, since helium injection might have had
really nasty stability problems.

>Is
>this covered in Chariots for Apollo? I'm skimming thrugh this but I seem
>to get more questions than answers.

CfA (I assume we are talking about the NASA History book, rather than the
Pellgrino&Stoff book) is more of an organizational history than a technical
one.  There is a distinct lack of a good technical history of Apollo, alas.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.history
From: henry@spsystems.net (Henry Spencer)
Subject: Re: LM ullage
Date: Tue, 1 Jun 1999 14:17:00 GMT

In article <3753D0B3.CA9E78A1@bigpond.com>,
Julian Bordas  <JBordas@bigpond.com> wrote:
>> ...The system actually used was throttling valves in the
>> propellant lines, plus a variable-geometry injector mechanically linked to
>> the valves.
>
>Thanks for the response.  The variable geometry injector, did that alter the
>size of the nozzle holes?

No, it was cuter than that.  Oversimplifying slightly...  The injector,
rather than being a flat plate closing the top of the chamber, was a
cylinder sticking down into it.  The oxidizer flowed down the center, hit
a plate on the end, and sprayed outward radically in a flat circular
sheet.  The fuel came down a passage just under the outer surface, and
emerged more or less axially as a cylindrical sheet (the cross section
being a ring), which struck the oxidizer sheet.  Separating the oxidizer
passage from the fuel passage was a movable sleeve, and careful shaping of
the sleeve, the oxidizer plate, and the outer casing of the injector meant
that as the sleeve moved downward, both passages were narrowed in the same
proportion.  The lever that actuated the sleeve was mechanically connected
to the throttling valves, so that flow rates and injection areas changed
together.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)