On Tuesday we tried a 30 second fireing of the 100N engine measuring thrust and chamber pressure. We diluted the peroxide to %85 so we wouldent have any issues with the silver melting over the long run. A few seconds into the test one of the nitrile gaskets broke which resulted in most of the peroxide leaking out and the engine pulsing wildley.
Despite the engine only running propely for a few seconds we good some good data. I still havent calbrated the load cell propelly but it looks like it produced about 60N. I am not sure if this is because the engine hadent warmed up yet, or because the throat needs to be enlarged more. Engine pressure was 120PSI at its peak which is alright but I would like it at 200PSI.
So we really need a new design of seal. I have been looking at Armidillo’s engines and it seems that they use copper gaskets. I suppose it makes sense that a somewhat flamamable rubber seal (nitrile) which is heated to 800 degrees and exposed to peroxide would fail. The odd thing is that the gasket worked fine for the last test which was much longer than this one, although it did look slightly burnt/warped. Looking at the video it seems that the gasket actually popped out rather than burning through. Also when we took the egnine apart it wasent actually broken. My guess is that the engine bolts wernt as tight as it was for the first test.
In other news I found a great new tank which will make for a perfect flight tank on the hovering rocket as it is holds nearly the same volume as the heavy nitrous one (10L) and only weights 2Kg! Its designed for compressed air in automotive applications and is only rated to 200PSI but I figured it would have a generous safety factor so I got one to test. It also has holes on its top and bottom which means we could do without vacuum filling which is another bonus.
We hydro tested the tank and found it to burst at 950PSI. My previous method of hydro testing is hydraulic pump, however it runs on oil and makes a mess so I tried another method of using air from our high pressure cylinders and a regulator. I figured that because the tank was filled with water the failure wouldent be particularly catastrophic, however I think that some of the nitrogen dissolved in the water because when the vessel ruptured it made quite a loud sound. It was in the peroxide bunker (where we used to concentrate the peroxide) so I dident do any damage but it gave everyone quite a fright. The resulator method was good because I could slowly increase the pressure as opposed to the jack which required allot of force and usually vessels fail when applying force and when you are not looking at the gauge.
I still want to test one more tank to failure to ensure the failure mode is the same before we incorporate it into the vehicle. The vessel seems to have failed at one of the leg welds which is a bit unexpected as we all thought it would fail at the cap welds, but makes sense as the stress in the cylindrical section is double that of the hemispheres and the leg weld would have weakened a small part of it. The crack seems to have travelled down the tank to the other leg which would have been the next point of stress concentration. When you look inside you can see discolouration from the leg weld. I will film the next test with my high speed camera which should give us a better idea of what happend.
Assuming the next vessel fails at around 950PSI I will feel comfortable using the tank at 500PSI which is what we were going to use the nitrous tank at. The tank has 5 fittings in it, 4 on one end and one on the other. The 4 fitting end will probably go on the top of the vehicle. I am slightly worried about what would happen if the vehicle fell from a decent height and the tank took the impact causing it to rupture. I think we will meed to find a larger location to test at. I was actually thinking that we could pressurise the vehicle remotely so we dont have to be anywhere near it.