From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: primitive rockets (was Re: continuous-feed solid fuel rocket)
Date: Sun, 2 Apr 2000 17:35:43 GMT

In article <8c5rq8$qei@crl3.crl.com>,
George Herbert <gherbert@crl3.crl.com> wrote:
>I've been wondering if a staged combustion motor with something
>hypergolic for the first combustion stage, then injecting the non-hypergol
>fuel and rest of the oxidizer in, might be an interesting way to make
>a liquid engine work well.  Haven't had time to work out any detailed
>designs yet, though.

Such things have been proposed, and even done, although for other reasons.
For example, the lithium-fluorine-hydrogen work (which gets mentioned here
occasionally) used a staged-combustion scheme to get around the problems
of lithium injection:  burning some of the hydrogen with the fluorine
first made it possible to inject the lithium into a high-velocity hot-gas
stream, greatly simplifying atomization.  (They ended up burning all of
the hydrogen with the fluorine first, in fact, because lithium reacts
rapidly and completely with HF to liberate the hydrogen again, and chamber
temperatures are a lot lower if you avoid a direct Li-F reaction.)
--
"Be careful not to step                 |  Henry Spencer   henry@spsystems.net
in the Microsoft."  -- John Denker      |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Li-F-H (was Re: Stuff)
Date: Fri, 10 Sep 1999 15:01:19 GMT

In article <936380480.2192.0.nnrp-03.9e98d142@news.demon.co.uk>,
Ian Stirling  <root@mauve.demon.co.uk> wrote:
>In the case of more exotic chemicals, for example Li/F/H, the fluorine
>is there because it's a powerfull oxidiser, the Li, as it reacts strongly
>with the fluorine, and the hydrogen to make the exhaust practical. (straight
>Li/F would I think produce either solid particles, or a liquid, not good
>for rocketry)

Straight Li/F actually works okay, because LiF has a relatively low boiling
point.  Measured Isp 458s, although with a mind-numbing flame temperature
that made engine design difficult.  (One advantage of adding hydrogen is
that it not only raises performance, it takes the temperature way down.)
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Stuff
Date: Fri, 10 Sep 1999 20:00:00 PST

In article <7qmgmn$2fj$1@news-int.gatech.edu>,
Allen  <allen@havoc.gtf.org> wrote:
]:>    6)  What is your favorite liquid propellant?
]: Lithium-hydrogen-fluorine tripropellant.  Near-nuclear Isp.  But practical
]: it's not.
]
]This leads to a question that I keep having in response to the recent
]discussion of monoatomic fuels.  I was under the impression, apparently
]mistaken, that LH + LOX yielded the highest possible Isp achievable from
]chemical fuels, namely about 460s.

No, you can beat that by a little even with (relatively) straightforward
chemical fuels, like using liquid ozone or liquid fluorine as the oxidizer.
Not by a lot, but some.  Also, the exact Isp, and even to some extent the
relative ranking of propellant combinations, depends *heavily* on the test
conditions chosen.

]Now, I've seen mention of monoatomic fuels, which I don't yet understand,
]but are apparently quite efficient, if we can make them work.  That's fine;
]it sounds like there's something more than simple combustion going on here,
]and I'll get around to learning about it one of these days.

The basic idea is just extremely energetic chemical reactions using fuels
which are not stable, e.g. atomic hydrogen (which recombines, H+H -> H2,
yielding very large amounts of energy and an exhaust gas with very good
energy-conversion properties).  The trick is stabilizing the fuels well
enough to put them in a tank.  (Ozone has the same problem, but atomic
hydrogen is lots worse.)

]But what really throws me for a loop is Henry's comment, which seems to
]indicate that higher Isps are indeed achievable from the proper combination
]of reactants.

Indeed so.  Li+F2+H2 has been *measured* at 542s, although that is with a
long high-expansion nozzle for use in a vacuum.  The Li and F2 burn to
LiF, yielding a lot of energy with fairly small mass, and the H2 is just
along to improve energy conversion.  Sort of a poor man's nuclear rocket,
with hydrogen heated by chemical combustion rather than nuclear reaction.

]Given that high Isp combined with the high thrust of a
]chemical rocket would be a huge advantage, there must be some pretty
]serious offsetting disadvantages, or else I've misunderstood something.

Lots.  The hydrogen is inordinately bulky and has to be kept very cold,
the lithium has to be kept hot, the lithium and the fluorine are both
ferociously corrosive in different ways, liquid lithium has inordinately
high surface tension which makes injector design difficult, the lithium
ignites on contact with air, the fluorine ignites on contact with almost
anything else, the fluorine is highly poisonous, both lithium and fluorine
are quite expensive and in limited supply, and generally it's just an
obnoxious combination.  Nobody has needed absolute maximum Isp quite badly
enough to want to deal with it.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

From: "Jeff Greason" <jgreason@hughes.net>
Newsgroups: sci.space.tech
Subject: Re: Stuff
Date: Fri, 10 Sep 1999 20:00:00 PST

Henry Spencer <henry@spsystems.net> wrote in message
news:FHExK5.n4u@spsystems.net...
] Lithium-hydrogen-fluorine tripropellant.  Near-nuclear Isp.  But practical
] it's not.

Unfortunately true.  Actually, it was running numbers on this propellant
combination that finally convinced me that the then-conventional
wisdom "Isp uber alles!" was wrong.  I competed some notional
vehicles using Li-H-F propellant against H-O and CHx-O and found
that while H-O and CHx-O were competitive (one or the other ahead
depending on a *lot* of other vehicle design choices), Li-H-F was
a clear *loser*, in spite of having a fantastic Isp.  The density is just
too low, the propellant cost too high.

But I still look at it wistfully (along with other halogen oxidizers) whenever
I'm facing the usual performance trades.  But given the price of
fluorine compounds and the price of environmental regulation for all
the HF you generate, I always wind up doodling for a hour or two
and then forcing myself to drop it and think about something practical.

----------------------------------------------------------------
"Limited funds are a blessing, not         Jeff Greason
a curse.  Nothing encourages creative      ex-Rotary Rocket
thinking in quite the same way." --L. Yau  Propulsion Manager
   (Hughes is my ISP, not my employer)     <jgreason@hughes.net>