[BProckets] Re: Help with NC chemistry

[David Sleeter]

Flint said:

> I have tested all the binders that I have (except for
> guar gum...Here is my problem -
> I tried using NC at a concentration of 1ppb in 100cc
> acetone (ping pong balls = ppb). Adding 3ml of NC
> laqour to 10gr of BP does the trick and
> casting/ramming is easy.
> It's even easier to work with, and the flight is WAY
> WAY better!!! A VERY rapid liftoff and flight. Very
> impressive. Far superior to the rest of the binders
> (no surprise though).
>  BUT -
> Out of 3 motors 2 will fly like a bat from hell, but
> the last one will cato (crossette like!).
> I'm assuming the problem lies in cavities left by the
> evaporating acetone, even tough I'm ramming the stuff.
>
Dave Sleeter says:

In the mid-1990s, while doing the propellant development for my book,
"Amateur Rocket Motor Construction", I too experimented with numerous
binders, and among them was nitrocellulose. For my own work I used
single-based rifle powder, because it's basically pure nitrocellulose, you
can buy it at any gun shop, and it's compartively cheap. As Flint has tried,
I dissolved it with acetone. At normal atmospheric pressure the propellant
grains made with NC burned beautifully, but at elevated pressures, just like
Flint's nozzless motors, they exploded.

To get accurate data on my propellants' performance I built a device that
worked like a Crawford bomb, a stainless steel chamber where I could test
the burn rate of sample propellant grains at controlled pressures up to 200
psi in an atmosphere of pure nitrogen. To read and set the pressure I used
an Ashcroft test gauge with a stainless steel Bourdon tube.

I too thought that NC would make a GREAT binder because it was readily
available and easy to work with. My propellant test-grains were essentially
like Flint's nozzleless motors, 5/16" dia. grains packed into small lengths
of convolute cardboard tube.

I remember how disappointed I was to discover that propellant grains that
burned beautifully at atmospheric pressure would explode with just a modest
increase in pressure. Though I can't locate my notes on the subject, I
remember that the critical pressure was amazingly low, something like 25-35
psi, and FAR below the operating pressure of a good black powder motor.

NUMEROUS experiments in which I varied the amount of NC would not solve the
problem, so in frustration I called a professional rocket engineer-friend,
and he suggested the following. Black powder is a "low explosive", and NC is
a "high explosive". At atmospheric pressure, the burn rate of NC is SLOWER
than the burn rate of BP, so at atmospheric pressure a BP grain made with an
NC binder works fine. But as the operating pressure is increased, the burn
rate of the NC RAPIDLY overtakes the burn rate of the BP. My
pressure-chamber experiments seemed to indicate that this was happening at a
pressure oa 25 to 35 psi (as read on the Ashcroft gauge).

If the burning surface of the NC-component of the grain receeds faster than
the BP it is supposed to bind, it leaves behind a zone of POWDERED BP that
burns explosively and causes the motor or the grain to CATO. In my own
experimental test-burns, at anything over 25-35 psi, my test-grains didn't
just fall apart, they exploded like firecrackers.

Regarding Flint's experimental motors, my guess is that the ones that SEEM
successful and go "like a bat from hell" do so, because their operating
pressure is staying right at or just below the critical point, and the
grains are just beginning to disintegrate. The motors that CATO are the ones
in which the operating pressure has exceeded the critical point, causing the
propellant grain to explode.

I struggled with BP-NC mixtures for several months before I realized that,
barring some more elaborate (and impractical) chemistry to slow the burn
rate of the NC, NC could NOT be used as a binder in BP rocket motors. Hope
this helps,

David Sleeter

As my