Tuesday, 24 April 2012

Simulator, New engine machining

Today I started machining up the new engine out of 304 stainless. I have never machined stainless before and for some reason I thought that it was going to be really difficult but as it turns out it wasn't too bad. I prefer aluminium or brass where possible but stainless does have some advantages;  unlike brass it doesn't break up into millions of tiny fragments which give you splinters. I have heard that 316 stainless is worse.

The stainless stock I has was 60mm diameter and as the actual chamber is only 26mm in diameter there was quite a bit to take off. The reason I got such a big piece was that we weren't sure how big to make the flange. As it turns out the flange doesn't need to be very big if we have a radial o-ring instead of a gasket/face seal. This design also saves weight. I was tempted to go with a design that diffident have any flange, only radial bolts to secure the thing, but because it is already so small a flange is really the only way we are going to be able to mount the engine to the vehicle. We are going back to top injection because on the vehicle we can just have a hole on the mounting bracket for the line to go through.

One issue I had was trying to cut the divergence for the nozzle. For brass and aluminium I use a special tool I made which has the divergence profile in it. It is made from soft steel and was not able to cut the stainless. What I really need is a tapered reamer however I have not been able to find one with the right angle. The other option is to get a big piece of tool steel and remake the tool.

Because I wasn't able to cut the divergence I thought it was pointless continuing on the new thruster as machining the 60mm stock to 26m is allot of work. I am looking for a tapered reamer which might take a while to get here. In the mean time I can get some smaller stock.

We still need to test a long duration firing so we can start work on the vehicle. Because we wont have the stainless thruster for a while we will need to make do with the brass one. I modified the test stand so it could be used with the load cell and drilled a hole at the bottom of the chamber so we could insert a pressure tap and measure chamber pressure. Hopefully this weekend we can do a 30 second firing and get good pressure and thrust data. The throat is currently 6mm which is a bit small for 100N so we might not get as much thrust as I wanted. The peroxide propulsion spreadsheet says 8mm but that's with a much lower feed pressure so we will see.

Simulator progress is going well. I have added in joystick functionality so I can command roll with it. I dident realise it before but the way the control is now the roll pitch PID's each only controll one thruster. This means that it is able to stabilise only using two thrusters but the system response is coupled so you cant have pure roll and pitch. This also seems to also induce a yaw.  One way to fix this would be for the output of the pitch and roll to be a torque which I could then convert to a thruster input.

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