Tuesday, 24 July 2012

Test Results and general discussion

Tonight we tested some modifications to the catalyst pack in one of the flight engines. The modifications included a new bottom retaining plate/disc with a higher open percent, top disc with fewer holes and anti channelling rings distributed through the pack every 3 sets of 5 silver and 1 stainless mesh. I hadn't bothered with the rings before as I was only running the packs for short durations and the silver mesh fit tight in the chamber so I didn't think channelling would be an issue. We put stainless mesh on each side of the rings to stop them sticking to the silver. I am not sure exactly sure what the percent open area of the new bottom spreader is as Buren went to town and drilled as manny holes as was possible with some being different sizes. My guess would be at least %50. From our highly scientific  "blow test" (we blow through the assembled engine) the new pack seemed to have a lower pressure drop. Unfortunately I didn't get a lot of photos from tonight as we were rushing to get everything done. We also installed a jet piece with 3 1.5mm holes.

Buren came up with a really good system for pre-assembling the new pack which will save time for the next 2 we have to make. He made a jig which has a hole in it the size of the chamber which the pack is assembeled in and compressed. The jig has a smaller hole in its other end which is used to push the pack out. The pack is transferred directly into the engine and engine is then placed on the press where it is compressed closed and the bolts engine bolts tightened. We found it much easier to tighten the bolts on the press and it also means we don't have to worry about the gasket moving.

The test went quite well but produced some unusual results. We broke the the new pack in with 2L of %85. It was only towards the very end of the run (about after 1.7L) that the engine started producing thrust an there was allot of un-catalyised peroxide that came out. Here is the thrust plot of the break in run with flow rate normalised between 0 and 6 for the purpose of illustration.

Although it looks like it made thrust early on the engine would build up pressure then flooded and spray out un-catalyised peroxide. This was really starting as I would have thought that as the pressure was increasing peroxide flow would decrease but it seemed to be the opposite, as soon as it seemed to get going it would get floded.

The second test went well and we finally got a smooth thrust plot:

 I didn't record chamber pressure as the flight engines don't have pressure ports. I improved the system by which the engine is mounted in the test stand so it is now fully restrained and can't move around producing which would have affected thrust readings. I try to get a photo of it next time I am at the workshop. I had previously thought that this was the some cause of rough thrust as the pressure plots didn't show occultations lining up with the thrust variations but now I am not sure it was entirely responsbale. In the above plot you can see that for the low flow rates the thrust is really rough but for the high ones it is quite smooth. The exception is at the end when the thrust gets rough briefly but I am fairly sure this is because the peroxide ran out and gas was coming out with it. Also the end roughness doesn't look the same as that initally. In this test I used a large "super pro shot" solenoid  (nitrous oxide accessories have silly names to appeal to their target market). This had a much higher flow rate than the small ones we are using on the vehicle which I should have used but we don't have any spare. I have noticed that at low flow rates the water coming out seems to pulsate and the solenoid makes a much louder pulsing noise so I think that the pulsating flow meant that the chamber never reached a steady state pressure. 

When we are breaking in the other 2 engines I will use the flight solenoid and try to figure out at what point the thrust becomes rough. If I can't get a good throttle range above the rough point I will have to add a jet to the solenoid to meter the flow or make the jet on the injector smaller. We probably should have made these smaller originally but Buren wanted to put 3 in as he though it would spread the flow out more evenly.

Sam also came along tonight and helped out which made setting and packing up much easier.  I am going to try to push the group side of things in the future as although it takes longer to get people up to speed which can be frustrating at times the long term gains rewards ave been well worth the investment. Overall  have also found working in a group to be much more satisfying than working alone. Sam has a  2nd year electrical engineering friend who might be interested in helping who would extremely useful with the electrical side of things. Now if I could only find a control systems engineer! I have always dreaded being a leader because I thought it would somehow give me less time to work on in-depth things myself but I have been pleasantly surprised to find that it it much more challenging and rewarding than any technical problem.

 Sam also finished the remote quick disconnect which despite looking a little dogie with the zip ties playing a structural role works every time. We decided to use a solenoid instead of a pneumatic ram. I didn't get a photo of it but will do so.

I also finally claimed victory in the troublesome battle for the pressure-gauge-in-bottle-jack! In the end I just put the jack in the lathe and settled a hole in the bottom on the jack. I did want one in the side so it could still rest on its base. This meant the jack had to be raised in the press to give the fitting room. The springs made this difficult as they are quite stiff and the press resisted  but eventually we got some spacers in. The press then fought back by bending the main plate which transfers force to the press-rod because the force was now off to each side and not directly on the rod. This caused the catalyst punch to fly out narrowly avoiding my head. Buren and I responded by reinforcing the plate.  In the end the victory was short lived as the high pressure gauge I used (reads between 0-10000 PSI) only really starts moving after the pressure is about about 500PSI which is useless for catalyst pack compression.

The rest of the photos are self-explanatory but this one is me trying to get the now longer jack back in the press.

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