Sunday, 4 November 2012

Thruster roughness frequency

I always figured that the thrust roughness was random but I thought I would do a FFT to see if it actually was.

As it turns out there is allot of thrust variation at 31Hz which is the frequency the solenoid pulses at.  I really wanted to believe that the roughness was just some artifact of the setup or issue with the engine but its clearly not. Highly Sensicial! Makes me wonder how much the roughness can actually be minimised.....

Thursday, 1 November 2012

Gettin scientific wit it.

We tested engine 2 (naming system for the engines on the vehicle) last night. There were a few issues with the new test stand but we got some good data.

We did 4 tests ramping the duty cycle from 0-254 and holding at each duty setting for varying times.  The title of the plots is Time_Dwell where time is the total test time in milli seconds and dwell is the time spent on each duty cycle. So for example 4000_500 has 8 thrust settings and spent 500ms on each setting. The first test was only for 3 seconds but it seemed a bit short so I extended it to 4 seconds.

The red is a 10 point moving average of the thrust which gives a general idea of what the thrust is doing. I was actually surprised at how well the smoothed thrust matched the input.

I probably wont upload video for future static tests as it takes time to edit and is not particularly exciting. If anyone wants to see the video let me know and I might just upload it unedited.

On the 4th test (4000_50) the peroxide ran out towards the end.

Apart from two small leaks another problem was that the load cell would not register any thrust bellow 1.8Kg of force, which is why I didn't bother converting the raw thrust readings to load for the above plots. This is probably why the thrust only starts at around 50. The other thing is that because the range of the load cell is 0-100Kg and we are measuring 7Kg at max, we are not getting particulary good resolution. I am not sure how much this contributes to the roughness however. I have ordered a 0-20Kg load cell which should give better resolution.

The other problem is that I couldn't manage to get the arduino to record data faster than about 100Hz. Some cycles were faster than others I slowed it down to 66Hz so it was recording at a consistent rate. It should be allot faster than that so either my programming is horribly inefficient or sending data over serial is a bottleneck. I will give recording to SD a go or otherwise get a faster Arduino (have been waiting for an excuse to get a Due...). Programming outside the arduino environment is another option.

Anywhoo, now we have a consistent test to see how well the thrusters are working we can start tweaking them. Next step is to test engines 1 and 3 to see how they compare then we can start making modifications. The first thing I want to test is what changing the injector size does. At the moment we have 3 1.5mm holes which I suspect has too low of a pressure drop.