PVC Plumbing Pipe as Fuel

21.5mm diameter pipe from B&Q is an almost perfect fit inside the Scaerotech casing. (It's very cheap, about 5p for each length, and almost no assembly required). There is enough space around the outside of the pipe to wrap a couple of layers of paper as a precaution against the pipe melting and sticking to the tube. The wall is quite thin (only about1.5mm), but the calculator suggests that less than 0.5mm will be burned.

The video shows what happened during the test.

The motor burned quite well in the latter part of the burn. I'm not sure what it was that was burning in the first part of the experiment, but it quite possibly was not the PVC pipe. There is no logging (still) on the load-cell, but this time I managed to get the camera pointing at the scales properly. If the reading on the scales is accurate, the motor thrust is probably less than 13N. (That's a bit disappointing compared with the prediction of 20N or more).

PVC generates much more smoke than paper (which burns quite cleanly). On removing the fuel grain I found that I might have had a lucky escape with my paper liner. The pipe was completely obliterated over about half the circumference, but the paper had held out. The disintegration of the fuel left the o-rings rather under-compressed when I came to disassemble the motor, but, I think, during the firing the internal pressure in the combustion chamber will have prevented them from leaking.

I had thickened the forward end of the fuel grain (the end it is standing on) by gluing a second piece of pipe split and reclosed inside the main grain. That section of the grain is the only part which is intact.. I think in future firings I will line it all the way down. With a double thickness fuel grain, the cost of the fuel rises to about 10p per launch.

Nozzle Degradation

The other thing I hadn't noticed until this test was that my nozzle has been degrading. It is possible that almost all the change happened in this test; I'm not sure. It's something I'll have to keep a closer eye on in future tests. A “proper” micro-hybrid kit comes with a machined graphite nozzle. I've been reusing the polymeric nozzle that comes with the Aerotech reload kits. The nozzle on the right-hand side of the picture has been used about four and a half times, once with the original Aerotech motor, twice with a paper grain and once with the PVC. (It accumulated the extra half use in a MkI motor with a paper grain in what was a pretty insipid test). The throat is clearly quite a lot bigger now.

It might be that the opening of the nozzle hindered the ignition and caused the false start on this firing. I'll be trying another PVC motor soon with a new nozzle.

Second Attempt

I made three changes for the second PVC firing: I exchanged the worn nozzle for a new one; I made the PVC double thickness over the whole length of the fuel grain; I changed the ignition over to plastic fuze (because I have run out of copperheads for now). Quite which, if any, of those changes was responsible, but the ignition of the PVC was a lot more immediate this time around, and the full thrust was there straight away.

The indicated peak thrust on the scales was the same as the first shot. I've tried to calibrate the load-cell with the kitchen scales, and that reading corresponds to about 15N of thrust. I think the discrepancy between the measured and calculated thrust (above) is down to the actual burn time being a bit longer than the original calculation. From the video, it seems there is good thrust over a period of about 1s (perhaps a fraction longer), and so the total impulse is probably about 15Ns (almost exactly what the calculator says).

The inner layer of fuel ended up in pretty much the same state as the grain in the original test. The nozzle showed significant opening from only this single test; I think I will have to make a proper one from graphite after all.

For my next trick... more fuel Igor.