Multi-Port Nylon Fuel Grain

I managed to get a copy of “Space Propulsion Analysis and Design” from the library and reading it I have realised a few things. They all revolve around the fact that I am running this engine way too lean (not using enough of my fuel). In the calculator I had previously found, specific impulse was a fixed number for a given fuel type, and whatever you did to port geometry to burn more or less of the fuel in the designed engine, it made no difference to the thrust or total impulse calculated. The book (and intuition) tells me this is not right. But to find out the optimum oxidiser-fuel mass ratio for nitrous oxide and nylon, I needed to use another piece of software from here that knows all about the chemistry.

It turns out the optimum ratio of oxidiser to fuel for most nitrous oxide / polymer propellants is about four. [In the previous experiments with PVC fuel, I had probably been getting a ratio of something like 20 – way too high]. According to PROPEP, I might have been missing out on up to 20% of my impulse. The other thing that running lean does for you is to make the exhaust gases very oxygen rich, which might well explain why I started suffering heavy nozzle erosion (especially during the PVC tests).

There's no way (without using an much longer fuel grain) that I could get a design based on a cylinder with a hole in it which could add the 2g of fuel to go with the 8g of nitrous oxide in the cream whipping cannister. So, I'm trying a multi-port design. I've taken the same PVC tube and created a nest of 18 smaller nylon tubes inside. The nylon is 4mm pneumatic hose.

At the top and bottom of the cluster I have inserted an internal sleeve of PVC as before to help secure the outer ring of nylon tubes. Each nylon tube is about 30mm long (calculated to give the right surface area to burn 2g of nylon with 8g of nitrous oxide), which leaves a 10mm space top and bottom of the PVC. The space at the bottom will help make sure the fuel is completely burned before being exhausted (and extract a little bit more impulse). The space at the top will help the nitrous oxide spread more uniformly into the individual tubes and the spaces between them. However, I'm not completely convinced there is enough length to spread out properly, so I have extended to central tube above the height of the rest of the stack, and given it an “umbrella”, to try to deflect what is probably quite a concentrated jet coming out of the injector.

The photo is a little blurry, so in case you can't quite make out what it is, the “umbrella” is made out of a drawing pin. I hope I will be able to tell from the pattern of erosion in the nylon tubes whether it is too big.

Back to the Drawing Board

My first explosion! I did have concerns in advance over whether the cluster of tubes would hold together, but I had thought that if tubes did loosen they would stay out of the nozzle. (The slots in the drawing pin were there in case it made it to the bottom, so as to leave gaps for the gas to escape). I almost added a mesh underneath the nylon, but couldn't find any suitable material. I think though, that as well as loosening, the nylon melted, and the molten fragments blocked the nozzle causing the tube to blow apart. The fuze may have contributed to the problem. You can see in the video that the fuze does not get expelled from the nozzle for some time. It looks like the nozzle blocks twice, the first time clears quickly and takes the fuze with it. The second blockage caused the explosion. There was nothing actually in the nozzle at the end of the day.

There's quite a dint in the plank on the floor beneath the test and the test stand has taken a bit of a beating, but the tubes and the joiner are surprisingly ok. I'll check the thread out a bit more carefully under the microscope before reusing them. I'll need a new injector for the nitrous oxide; the one I used has lost its piercer, and the hole has blown out rather larger than it was before.

I think the next grain will need to be created so as to have each part separately supported so nothing can fall out.