Newsgroups: rec.pyrotechnics
From: ahahma@utu.fi (Arno Hahma)
Subject: Re: DB Propellants/Nozzleless rockets
Date: Mon, 27 Dec 1993 09:30:54 GMT

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

I'm sorry, if this article shows up for the second time. I posted this
again, as our news server did not show this here at all.

> propellent. I have yet to investigate the types of DB propellents
> available for pistol reloading etc., but I envisage forming a castable

For such a purpose, you will want to get ball powder, preferably non
graphitized.

> acetone/ethanol solvent. Such DB rockets reportedly can achieve

I would use ethanol/diethyl ether, as acetone tends to gel the NC too
fast. The gelled surface will block the solvent from penetrating
through the whole mass. It might also be useful to add plasticizers,
such as dibutyl phtalate to the solvent. Gunpowders are usually rather
hard and for a rocket you need a flexible grain.

Another problem you will face is the pressure dependency of the
combustion. Gun propellants are designed to have a pressure exponent as
close to unity as possible but not over. For a rocket, you will need a
pressure exponent of close to zero or even negative.

To do that, ballistic modifiers are needed. For smokeless powder, lead
chromate is rather efficient in decreasing the pressure exponent.
However, it will be effective only at relatively high pressures. If I
remember correctly, plateau burning characteristics are reached at
100..150 bar pressure.  Below that, the pressure exponent is strongly
positive and above it, positive again.

> 15 MPa. This seems to indicate metal construction and the use of a
> burst plate. I would like to conduct the launch from a tube using fold

Metal construction is a must anyway, as it is very hard to case bond
the NC-propellants. External burning grain is much easier to deal
with.  It is possible to use a case bonded structure, but then you'd
have to cast the grain in a mold and then cement it to the case
afterwards, after making sure the grain is not going to shrink any
more.

> out spring steel fins for stabilization.  I have heard of one such

Folding fins are nice, but laborious to make. Why not make fixed fins
and construct a lauch rail to shoot the rocket from?

> polyester binder and a HMX additive to increase the Is. This design was

Polyester/HMX is a gun propellant, hard to use in rockets. Besides, an
Isp of 2000 Ns/kg (m/s) is quite enough for "home use" ;). Getting the
Isp higher than that requires aluminum or other high energy additives.
Then you will get great difficulties with your nozzles, as the
temperatures easily exceed 3500 K.

I recently constructed small rockets (100 grams of propellant) to lift
fireworks with a fuel Isp 2200 Ns/kg. The motors can still be faster
than your eye even with that little Isp. If they are launched with a
light nose cone and small fins only, they will reach speeds in excess
of Mach 5. They'll be out of sight faster than you can turn your head
after and then you only hear a very loud, ripping noise reflecting from
the surroundings. You will have to know, where the rocket is going to
fly, if you want to track it. No more Isp is necessary, in fact, I will
have to downgrade the motor thrust and chamber pressure for fireworks
use.

That reminded me, I think Larry (L. Curcio) asked about nozzleless
rockets. For such a rocket, any propellant with a low burn pressure
limit will work.

However, such propellants are pretty hard to make, at least AP based
propellants tend to have a pressure threshold of 5..10 bar at least. It
means you will have to make the fuel burn so fast it will hold the
pressure above the minimum burn pressure even without a nozzle. The
grain configuration (length, mostly) also affects this.

A very long grain will still not be enough alone, fuel modifications
are necessary in any case. That is because the pressure will not be the
same throughout all the core area. A strong pressure gradient will
form, the low pressure at the exit end of the grain and high at the
other end. If part of the grain is above the minimum pressure and part
of it is not, the grain will start to burn in a really strange manner.
It may chuff but still generate continuous thrust + the oscillating
thrust, it may start to whistle or it may even crack or blow up due to
the stresses and pressure waves generated. Another and better
possibility is to make a fuel that burns are lower pressures.

AP of smaller than 5 micron particle size will make fast enough fuel
and using special additives should not be necessary. Suitable additives
are magnesium, zirconium, boron and silicon powders, they will help the
fuel burn at lower pressures. Using both a fine grained oxidizer and
fast burning additives and binders will make the effort most likely to
succeed. A suitable binder is silicone rubber, it will make extremely
fast propellants with a low pressure exponent.

Potassium perchlorate can also be used, if the binder has a
decomposition temperature close to that of KP itself and it decomposes
suddenly (does not char and smolder). Such fuels burn at atmospheric
pressures and make nice nozzleless rockets. You can even mix color
producing agents into them and make blue, violet, red and pale green
rockets. That's nice in fireworks, but hardly of use in rocketry.

I managed to make a nozzleless rocket once by accident. I cast a
propellant grain into a mold and it got a little stuck. As a result,
the grain cracked when I tried to release it and I could not use it in
a rocket motor. So, I took a hacksaw, sawed the cracked part (about 1/3
of the grain) off and inserted a fuse into the grain core (the grain
has a 5-pointed clover leaf configuration). I thought it would chuff
like h*** and make nice fireworks.

To prevent the grain from jumping all around, I taped it into a meter
long stick and lit the fuse. But the grain did not chuff at all, it
took off like a rocket and flew several kilometers with a spiral
trajectory before it burned out. The burn time was long, maybe ten
seconds or so and the efficiency of the "rocket" was probably lousy.
Still, it worked ;). 

> Merry Christmas -- Have a good one.

You too!
 
> Mark.

ArNO
    2