From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: Slot High Density Injectors
Date: Thu, 19 Mar 1998 21:46:31 GMT

In article <19980315151801.KAA05418@ladder03.news.aol.com>,
JHare10079 <jhare10079@aol.com> wrote:
>Most injector drawings that I have seen are round and limited to
>impinging, non impinging, and axial types. The other types, which
>I get the impression are obsolete, are impinging sheets...

Actually, there are a number of other types, but this is an area where
people have stuck with a few solutions that worked, and haven't put a
lot of effort into investigating alternatives.  (At least, in the West.)

>I have read
>that unlike impinging gives best performance and is least stable.

There is frequently a conflict between improving stability and improving
performance.  Unlike-impinging designs produce quick and rapid mixing,
which is good for combustion efficiency.  But the impingement pattern is
easily disrupted, perhaps because the impinging jets are of different
fluids and hence respond differently to disturbances, and that definitely
hurts stability.  (On the other hand, because of the more effective
mixing, you can afford to make the individual injection elements bigger,
and *that* definitely helps stability.  The tradeoffs here have not been
examined well.)

>Axial is most stable but unsuited to heavy fuels.

Not true, the Russians have used coaxial injectors with LOX/kerosene,
and quite successfully too -- the upper-stage engine of the Soyuz booster
is built that way.  And they had some stability problems; there's a very
interesting chapter in the recent AIAA combustion-stability book about it.

The US traditionally uses coaxial injectors only with LOX/LH2 (well, more
precisely, LOX/LH2 in the J-2 and LOX/GH2 in the RL10 and SSME), but they
are useful for other combinations too.  Their appearance of being highly
stable is probably at least partly due to the fuel combination rather than
the injector; in particular, LOX/GH2 will make almost any injector look
good, because injecting one component as a cryogenic liquid and the other
as a high-velocity warm gas is much better than injecting both as liquids.

>Is it possible that alternating slots of fuel and oxidiser could give
>a stable, simple, high density injection system.

Possibly.  The LM descent engine used a scheme along those lines, with
only a single injector element (which was mechanically adjustable to give
varying flow rates for throttling).  It did have some stability problems
early on, as I recall.

>I'm sure this has been tried somewhere and found wanting. Does
>anyone know of this being tried, and what is wrong with it?

Even aside from the LM engine, sheet injection has certainly been tried --
some of the books mention it -- but I haven't heard anything much about
how well it worked.
--
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         | henry@zoo.toronto.edu

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Propane/LOX
Date: Sun, 2 Jan 2000 19:52:54 GMT

In article <386EA1FF.620BBCB@Gramlich.Net>,
Wayne Gramlich  <NoSpam@Gramlich.Net> wrote:
>Are Propane and LOX hypergolic at LOX boiloff temperatures?

No, they aren't.  Only the most aggressive fuels, like diborane, are
hypergolic with LOX, and conversely, only the most aggressive oxidizers,
like fluorine, are hypergolic with the (relatively mild-mannered)
hydrocarbons.

There was some work done with FLOX/propane, FLOX being a fluorine/oxygen
mixture.  *That* combination is hypergolic, and also gives higher
performance than LOX/propane, at the cost of adding a number of technical
hassles.  (One unobvious one is that performance peaks essentially right
at the stoichiometric mixture ratio, which means that performance falls
off very sharply with even small mixture-ratio errors, making injector
design tricky.)

An alternative is to dissolve a trace of O3F2 in the LOX, which makes it
hypergolic with nearly anything (including, unfortunately, many materials
that are compatible with normal LOX).

>The reason for asking is because it may be possible to simplify
>the combustion chamber injector plate by premixing the LOX
>and propane before injection.

Premixing injectors were tried, in the early days; the usual result was
explosions.

Even disregarding that, there is a problem in that LOX and propane are
immiscible at anything like typical rocket mixture ratios.  The most you
could do would be a suspension of propane droplets in LOX.  (You can
dissolve a little bit of propane in LOX, or vice versa, but not a lot.)

A better approach is to use the propane for regenerative cooling, perhaps
even running an expander cycle for turbine drive, and inject it as a gas.
Gas-liquid injectors give far more efficient mixing than liquid-liquid
designs, because the gas velocity is quite high and that shreds the liquid
streams into droplets very effectively.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)