From: ahahma@polaris.utu.fi (Arno Hahma)

>I have never made a rocket using composite propellants - I assume that 
>that is the type of fuel you are describing. I didn't realize that they
>were so efficient - or that the amateur could easily make a rocket 
>nozzle that would help produce such efficiency.

Even black powder can do it, if you just can make the rocket burn fast
enough, i.e. to generate enough force to lift the payload. The
specific impulse of BP is about roughly 500 Ns/kg. So, if you use 2 kg of it
you can get 1000 Ns of impulse. That is 100 m/s for a 10 kg load and
should be enough to lift it up to 300 meters.  With
composites, say, with 1500 Ns/kg and 1 kg the velocity would be 150
m/s, enough for 500+ m. Such composites can be easily made without
aluminum. With aluminum you can get up to 2600 Ns/kg. 260 m/s initial
velocity is more than enough for 1000 meters altitude. 

>>really awesome equipment.  we make them out of PVC plastic pipe with wood

>I don't like using such things as PVC for rocket casings. I will stick with
>wound paper tubes. Call me old fashioned, but I consider them to be far
>safer - another advantage is that they are bio-degradable, which the plastic
>is not.

Paper is about the best material for black powder motors, since BP
adheres paper well. Even more important, its mechanical properties are
suitable for a brittle propellant. For BP motors the casing has to
carry a static stress well. For composites, it has to withstand a high
dynamic load (dynamic over the burning time). Plastics do not
withstand a static load well, so they are not good for BP rockets.

As you ram the powder in, the paper tube expands a little. Thus, the
powder stays under (radial) pressure in the tube, since the tube is under
stress. Plastic tubes also stretch, but plastics will undergo a
plastic deformation during time (hence their names) and not tend to
return to their original measures, at least much less than paper. The BP
is not under pressure and may crack, as the rocket is fired.  For BP
you need a rigid, but still a little elastic material for the casing.
Paper fills these requirements quite well, at least there are hardly
other, as cheap and fragment free alternatives.

As a drawback, the tensile strength of paper is not very high and as a
result you can not use very high pressures with BP. If the pressure
exceeds the static strain generated during loading, the BP charge will
crack (since the tube expands a little at this point and BP is
brittle) and the motor explodes. This is also, why you have to keep
the paper BP motors dry - if they get even slightly moistened, the
strain in the casing will be released and the motor is likely to
explode. For the same reason, loading the powder moistened too much is
only bad.  Diethylene or polyethylene glycol (1 %) should be used
instead of water - these will not disturb the paper and will lubricate
the powder during pressing. They also dissolve the saltpeter somewhat,
not as much as water, though.

With composites the situation is somewhat different. They are strong
and can hold part of the pressure themselves. Also, most of them (or
all of the case bonded ones) are elastic and permit some deformation
without cracking. Thus, they can transfer the stress evenly to the
casing. In this case only the (dynamic) tensile strength of the casing
determines (provided the casing is stronger than the fuel), how much
pressure the rocket can hold. In this case, a plastic tube works well.

With brittle fuels, such as BP, I wouldn't try to load in plastic
casings, unless the casing was of something very hard and tough
material, like glass reinforced phenolic resin. Still, it would be
difficult to make the motors work because of a poor bonding with the
case. Such a casing may also be dangerous to use, so that is not a
good idea either.

>I have made many rockets - I wouldn't call it really safe, but at least with
>care you can survive the experience.

Well, as with all pyrotechnics, nothing is absolutely safe and the
relative safety increases only with knowledge and especially with
experience.

>Why do you have to pound the propellant in with so much force? It is true
>that BP is hard to ignite by percussion, but if I was going to use that

The more force while loading the higher the burn pressure can be and
the more you get impulse of the rocket. Of course, if you ram so hard
the tube ruptures, deforms or the layers of paper break, the maximum
pressure will get lower again. The optimal loading pressure is really
critical for (paper tube) BP motors. Automated presses have to be very
carefully calibrated to make functioning rockets with a high
reliability. A fireworks factory here had great problems with this, as
they went over to automated rocket production. Before the workers
simply rammed the rockets with hammers. The pressed motors kept on
failing, until they used a high precision force sensor to get a
constant loading pressure.

>>burns, the surface area of the powder available becomes greater.  We've made
>>dozens of these things, with a rather high success rate.  We're still at the
>>experimental stages, however.  <grin...boom,splat!>

I think your problems could be easier to solve with paper tubes
(because of the mentioned reasons). Plasticized PVC tends to deform
permanently under static load. I wonder how your PVC-rockets would
work after being stored for a few days or weeks.

>>The really nice thing about them is that they're cheap.  A rocket engine
>>the size of and Estes C class engine costs about 10-20 cents to make.

An 80 Ns composite motor (an E-class motor or am I correct?) costs
as much and is smaller physically.

>if I were making engines of that size, I would use plastic shotgun hulls.

They are made of polyethylene or something like that. Polyolefines
have even stronger tendency than PVC to deform under static load. So,
BP with plastic hulls does not work. With paper hulls, it does.

>Bill

ArNO
    2
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Black powder for rockets is made commercially, at least by Wano in
Germany. It is quite different from ordinary gunpowder, the proportions
are closer to 60/30/10 (saltpeter/charcoal/sulphur) rather than the
usual 75/15/10. In addition to that, the powder contains 0,5 to 1,5
additional per cent polyethylene glycol as a lubricant and binder. The
rocket powders are not polished nor graphitised and are fine grained
(like 4F). Such a powder can be pressed into a motor in single
operation and forms a very hard powder grain.

If you are able to open one model rocket motor, you can check if it has
been pressed from granulated powder. In that case, the powder is very
likely to be commercially made outside the rocket factory and the
propellant grain is not one homogeneous solid.  The grain interphases
can easily be seen, the propellant looks like it had been built from
tiny tiles.

>Ed sometimes talks about "Slow black powder" in his letters,
>hypothetically, but ESTES uses one grade. 

That "slow powder" is probably the rocket formulation, i.e. 60-30-10.


ArNO
    2