From: ahahma@utu.fi (Arno Hahma)
Newsgroups: rec.pyrotechnics
Subject: Re: AN propellants (was: HMTD #2!!)
Message-ID: <CJGLLu.H91@utu.fi>
Date: 11 Jan 94 09:41:53 GMT

In article <2gsu4k$act@bunyip.cc.uq.oz.au>,
Mark Fernee <fernee@dingo.cc.uq.oz.au> wrote:

>efficiency (low thrust/weight ratio). Also one of the major disadvantages
>results from the various crystaline phase changes that AN undergoes at about

The crystalline phase changes can be controlled. If AN is mixed with
copper(II)oxide or better, nickel(II)oxide, the phase transfer
temperature is shifted much higher (something like +70 oC). At the same
time, the additives act as burn catalysts and enchance the burn rate of
the propellant. Usually, dichromates are used as catalysts, especially
ammonium dichromate, as it is soluble into ammonium nitrate.

>don't think this will matter in a small rocket and I am investigating the
>use of double based propellents to increase the burn rate/Isp. Not that

Double base propellant would make a nice binder as well.

Regarding the addition of aluminum into AN propellants, there is a good
reason why not to do so. It is of no use, it is only disadvantageous.

The Isp keeps nearly the same, no matter how much aluminum is mixed
in.  If the binder content is kept constant, the Isp will be
_decreased_ from the Al addition. I calculated a few examples, so you
can see the temperature is the only parameter that is strongly
affected. I would at least prefer to have the temperature as low as
possible because of constructional reasons and trade a little Isp
instead.

Mixture			Isp/(m/s)		T/K

AN/HTPB
93/7			2449			2287

AN/HTPB/Al
88/6/6			2422			2625

AN/HTPB/Al
83,2/5,6/11,2		2563			2886

AN/HTPB/Al
78,7/5,3/16,8		2689			3117


The pressure ratio in the Isp calculation was 70 bar to 1 bar.

Note, that it would be very difficult to make the above propellants, as
the binder content is very low. If the amount of the binder is
increased, the Isp will drop sharply. I simply kept the amounts of the
AN and HTPB constant and added aluminum - that was the easiest to do.
The first mixture is close to the maximum Isp obtainable from AN/HTPB.

>Mark.

ArNO
    2

From: ahahma@utu.fi (Arno Hahma)
Newsgroups: rec.pyrotechnics
Subject: Re: AN propellants (was: HMTD #2!!)
Message-ID: <CJIAFD.JrE@utu.fi>
Date: 12 Jan 94 07:35:37 GMT

In article <940111.42953.LC2B@delphi.com>,  <LC2B@delphi.com> wrote:

>I haven't done the calculations, but I wonder if the calculations took into
>account the higher oxygen availability at high temperatures from H2O

Sure - I used a computer model with 18 possible combustion products and
the BKW (Becker-Kistiakowsky-Wilson) equation of state.  The model
calculates an equilibrium composition using the given reaction
products.

With increasing content of Al you also get an increasing content of
hydrogen and decreasing content of H2O, just as you expect.  Aluminum
is burned nearly completely into liquid Al2O3 and the rest of the
oxygen is captured by carbon to CO2 and CO. What is left after that, is
consumed to H2O. Other products like HCOOH, Al2O, CH4, H, O2, O etc.
play only an insignificant role.

>decomposition. Even if Isp were about the same, a higher binder proportion
>might be possible, and with AN, this is an advantage in itself. Also, Al

A higher binder proportion will make the Isp smaller in the aluminized
fuel than in the non-aluminized one. That is, if you keep the binder
content constant and add aluminum, the Isp will decrease.

>is dense compared with AN, and this is another advantage. The big

That is true. The volume impulse might still increase, although the Isp
does not.

>problem I'd worry about is the possibility of detonation . When
>I tried Al/AN compositions years ago, I couldn't get them to burn in

True - Al tends to increase the pressure exponent. Unfortunately, AN is
able to detonate at its crystal density easier than even ammonium
perchlorate is. Still, AN is not too sensitive but the risk exists. A
higher combustion temperature will make such a detonation easier to
start.

>be surprised if these explosions were actual detonations, as windows
>shattered in the immediate area :(

I'd guess you had real a read detonation, especially, if the fuel was
not degassed well before casting the motor.

>Best regards,
>-Larry C.

ArNO
    2

From: ahahma@utu.fi (Arno Hahma)
Newsgroups: rec.pyrotechnics
Subject: Re: HTMD #2!!!!
Message-ID: <CJGKE8.GDw@utu.fi>
Date: 11 Jan 94 09:15:44 GMT

In article <9401110045591.rwenzlaff.DLITE@delphi.com>,
Robert Wenzlaff <rwenzlaff@delphi.com> wrote:

>Exxon sells cured butyl rubber in 75 pound bails.  Are there any polymer
>people out here who could tell me if it comes as uncured liquid?  Or is it
>strictly used as a thermoplastic?  And if it comes as a liquid, how much

Butyl rubber (polyisobutylene) comes only as cured rubber, that is
cross linked using sulphur compounds or peroxides. To mix a rubber with
80+% of a filler you will need some really heavy machinery: hot rollers
that can knead the rubber effectively. Most of the heat comes from the
mechanical action i.e. friction between the rollers and the rubber,
btw. The driving motors tend to be _large_. Car tire rubber is mixed
with such machines - not something one is able to do at home.

Besides, it is not too safe to knead a rubber mixture loaded with an
oxidizer at high temperature. You might ruin the machine, although the
AN propellants do burn with a low temperature flame ;).

There are also liquid or semiliguid polyisobutylenes, but they are not
suitable for rocket fuel binders. The molecular weight is too low. If
you cross link such a polymer enough that it becomes mechanically
stable it also becomes glass brittle.

If you wish to see what butyl rubber looks like, find a soft,
transparent hot glue stick.  They are made of non-crosslinked butyl
rubber.

>RWenzlaff@Delphi.com (Durn signature's broken)

ArNO
    2