Saturday, 14 April 2012

Today Scott, Buren and I tested the new 100N engine with %90 peroxide and silver mesh catalyst. The test went much better than I was expecting for a first try with complete decomposition and clear exhaust after a few warm up pulses. We slowly increased the length of the pulses, to one run of about 6 seconds. The runs were really smooth and we couldn't hear any roughness. Towards the end the runs did sound a bit funny But I think this is just the peroxide running out. We didn't bother measuring pressure or thrust, we just wanted to see if we could get complete decomposition. Here is a video of the test. Unfortunately I didn't have my video camera and had to settle for my phone zip-tied to a pole. As such the camera angle is a little strange.

We only need about 100 x 21mm diameter 20 mesh discs for our flow rate (3.6L/min), however we used about 144, 72 of which were 40 mesh. The 40 mesh were on top because they had more resistance. We had 3 anti channel discs in the pack, although I think these probably did more harm than good as they were too thick (radailly) and would have meant the peroxide wouldn't have perminated though the parts immediately before and after each anti channelling disc and effectively lowered the pack area. I think we can probably do without the anti channelling discs as the chamber is so small. 

We grossly overestimated the space the discs would take up and they when were compressed half the chamber was empty. I machined up a spacer but since the flow tray was part of the injector it made the tray useless as the flow would have spread out. The top few layers of 40 mesh were slightly melted/mangled which is, I believe because if the high speed flow hitting it as the tray is not doing its job. A few layers of stainless mesh should fix this, although we should shorten the chamber. If we went for a plated catalyst this would probably have stripped quite a few layers. The pack came out in one piece and seems to be slightly melted together. It could be just compressed together but seems to be fused. I didn't want to try breaking it apart and risk damaging it as its the only catalyst we have at the moment. I am getting quotes for chinese suppliers of silver mesh as however convient the 60mm jewellers mesh squares are buying in larger sheets is much better value. Here are some pictures of the pack after the test. I was surprised with how easily it came out. The brass was quite discoloured and clearly got really hot.

There was a small issue with the lower nitrile gasket melting and burning. Towards the end of the video you can see some smoke which doesn't seem to be coming from the nozzle; I think this is the gasket burning. We will probably want to rethink our gasket design for the next engine.  The left one is the lower gasket which does between the chamber and flow plate.

Here is a photo of the engine as it looked when we opened it.

The test itself went really smoothly. Following some advice we received at out last test we created a written procedure for filling and testing and followed it. The vent valve was unfortunately too small to work effectively but because the pressure drop over the new catalyst was much lower than the granular one venting was quicker anyway. I might just remove it as it is no longer necessary. 

Unfortunately the national instruments DAQ doesn't seem to work over the usb lan link. In addition it is actually worse for PWM'ing  than the arduino as it doesn't have any hardware timers. I didn't fancy going back to serial text based commands so after some investigation I discovered a way to change the frequency of the Arduino PWM command in  lab view. It was quite simple actually all I need to do was change the prescaler of the hardware timer on the arduino. Luckily it is easy to add your own code to the NI firmware on the arduino. Unfortunately 30Hz is about as low as it can do, at least with this method.

The next step is a light weight motor. We are Meting on monday to discuss possible designs and I would like to have a prototype ready by next weekend. I would also like to start work on a mock up of the vehicle so we can start work on integrating the tank and motors as well as electronics. I really need to start working on the simulator. I have been playing around with the aerospace toolbox in simulink which is exactly what I need but I am having trouble understanding some of the example simulations that come with it. 

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