Newsgroups: rec.pyrotechnics
From: ahahma@utu.fi (Arno Hahma)
Subject: Re: Composite Rocket Fuels
Organization: University of Turku
Message-ID: <Cs6Jqt.8Lw@utu.fi>
Date: Wed, 29 Jun 1994 22:28:04 GMT

In article <1994Jun29.185525.10297@hpcvaac.cv.hp.com>,
bill nelson <billn@hpcvaac.cv.hp.com> wrote:

>spiegl@lobster.cig.mot.com (Mark Spiegl) writes:

>: I am curious how these issues are handled professionally. Is the chemistry 
>: tweaked with various processing aids to create an easily castable brew? Do 
>: professional shops have sufficiently sophisticated mechanical capabilities 
>: to extrude and degas a very thick batter? Other? 

>If the particle size of the AP is fairly uniform, this can be a real problem.
>It helps a lot if the mesh is quite varied - the mix flows much better.

There is more to consider than just the rheology. The composition of
the binder can be very significant and so are the additives used. AP
propellants invariably contain surface active agents to aid flowing.
Still, the propellant mass is hardly free flowing but has to be forced
into the molds with either under- or overpressure. This, of course,
depends on the composition.

If enough binder is used, then any propellant mixture can be made pot
castable - with a tradeoff of Isp. Usually, one wants to maximize the
Isp with acceptable mechanical properties.

As a note about using epoxies with AP, it is not necessarily a good
idea, if the epoxy resin is amine cured. The polyamine is basic and
liberates ammonia from the AP. Part of the ammonia gas reacts with the
epoxy and cures it to a glass-like material rendering the propellant
grain brittle, at least considerably more brittle than
expected. 

High viscosity is another problem, that will be encountered.  The basic
polyamine forms a perchlorate salt and thus a chemical bond to the AP
crystals. As the amine is polyfunctional, it kind of crosslinks the AP
grains together and the mass will be very sticky and highly viscous.
The mass also has a very high dynamic viscosity and becomes tixotropic,
i.e. impossible to pot cast and very difficult to degass. In addition,
the ammonia gas will expand the mass and make it even more difficult to
degass (more gas is developing constantly).

If epoxies are used, acid or alcohol functional curatives are a much
better choice. In addition, such crosslinkers enable elastic properties
for the resulting resin, basic curatives result resins of low
elongation of break (usually). As a drawback, acid crosslinkers react
very slowly and it is usually necessary to bake the
propellant at 60..100 oC for even days until the resin can be
considered fully cured.

>The professionals use more complex formulations, and probably vacuum degass
>the grain.

The grain is _always_ degassed, if not because of removing voids then
because of strength. Degassing is usually done simultaneously with the
mixing, i.e. the propellant is mixed under vacuum. Another method is to
cast under vacuum filling the mold with a thin stream of propellant,
whereby the stream is rapidly and effectively degassed.  After letting
atmospheric pressure to the full mold, any voids left collapse.

>Bill

ArNO
    2

Newsgroups: rec.pyrotechnics
From: ahahma@utu.fi (Arno Hahma)
Subject: Re: Composite Rocket Fuels
Organization: University of Turku
Message-ID: <CsK1so.J1u@utu.fi>
Date: Thu, 7 Jul 1994 05:26:48 GMT

In article <spiegl.773486746@lobster>,
Mark Spiegl <spiegl@lobster.cig.mot.com> wrote:

>This is most interesting. Are you saying that the grain is returned to 
>atmospheric pressure before the mold fully cures? 

Yes, that is what I meant. Fill the mold under vacuum with a thin
stream of propellant, keep it evacuated for some time (to remove any
large bubbles left) and then release the vacuum and let the propellant
cure.

>I would guess that after 
>the grain hardens, voids and air bubbles cannot be removed. 

You are right, they'd probably stay there and only get filled with air
slowly. Or at least the grain would shrink, as the atmospheric pressure
is squeezing it.

>for quite some time. Ie, bubbles continue to form after degassing and 
>during cure. Do these bubbles dissipate from the slurry simply by curing 

Small amounts of ammonia do no harm, because ammonia gas reacts with
the epoxy resin. Therefore, small bubbles disappear by themselves,
provided the bubble is pure ammonia and no air. One only has to get the
most of the gas off as the curing reaction is slow and its ability to
remove ammonia is limited.

>with the chemistry of this beast, but I guess that it probably behaves as 
>a surfactant. I do know that amino-zircoaluminate sometimes interferes 

The amino-zircoaluminate is probably a combined surfactant and curing
catalyst for CTPB or other epoxy resins.

>propellant viscosity to processable levels. Maybe careful consideration of 
>the order of mixing would yield some benefit. 

Not really. Have you ever tried mixing amine cured propellants with
ammonium perchlorate? Besides, the solids loading is already so high,
you have to add all of the liquids at once, not one by one, unless you
have a hydraulically operated, heavy duty planetary mixer or
equivalent.

>could be easily fabricated in an amateur lab. The "batter" would be poured 
>into the upper chamber, and then vacuum extruded through small jet into 
>the lower chamber, where curing occurs. Opinions? 

It works. Actually, you only need one chamber with a hole on the top to
let the propellant in. Try using a vacuum desiccator with a
ground glass joint on the lid and make an adapter, that allows pouring
in the propellant while pumping the vacuum.

>   |U|     Mark C. Spiegl

ArNO
    2

Newsgroups: rec.pyrotechnics
From: ahahma@utu.fi (Arno Hahma)
Subject: Re: Composite Rocket Fuels
Organization: University of Turku
Message-ID: <CsM5pJ.J2I@utu.fi>
Date: Fri, 8 Jul 1994 08:46:31 GMT

In article <2vifjp$2lg@azazel.sdsc.edu>, Bob Mackey <bob@ariel.sdsc.edu> wrote:

>I suspect that choosing the appropriate viscosity and reactivity
>resin will greatly improve the handling properties of the heavily
>loaded mixtures.

Certainly. You want to have the viscosity about in the range 2000
.. 20000 cP. Too low a viscosity will cause sedimentation,
too high will make mixing difficult. Practically all epoxy resins
(liquid ones) fall in this range.

>be much faster with a low viscosity resin.  In addition, the selection
>of an elastomeric resin will yield a shock- and crack-resistant
>propellant mixture.

The problem is, that elastomeric resins have a high molecular weight,
thus a high viscosity. The higher the prepolymer MW is, the better
elastic and mechanical properties you are able to get. With epoxy
resins, the MW is usually under 1000 g/mol for room temperature liquid
resins. That means, you can hardly get rubber elastic resins and even
if you can, the glass transition temperature will be high (often close
to room temperature). Below that temperature, the resin will be brittle
like glass. Urethanes are a bit better, you can have up to 1500 g/mol
polyol and still pot cast the propellant.

Of course, one can alter the viscosity by changing the mixing
temperature. That approach is one of the newer methods of making high
mechanical strenght propellants; they usually call such binders curable
thermoplastic elastomers. This kind of technology needs an extruder
process to work well - something that is a bit hard for home use...

Currently, about the only resin that can be made with a MW higher than
3000 g/mol and yet having viscosity lower than 10000 cP is
polybutadiene in various forms. Well, there are others, too, like
polybisazidomethyloxetane, but they are still at experimental stage.
Guess why nearly all solid fuel rockets are being made with HTPB or
CTPB ;).

>-bob mackey

ArNO
    2