Tuesday, 27 September 2011

Cancelled Test

We had been planing to conduct another test last weekend but unfortunately  it had to be cancelled the night before.

The source of the problem was that the new H-Bridge motor controller dent have the power to drive the valve motors. I hadn't tried it before then because I thought it would be a trivial task and a waste of time, but I learnt yet again that few things are simple. When buying the motor controller I noted at the motor's specs and noted that it retired 3A while moving. The motor controller could provide this, but I didn't take into consideration that it required allot more current to get moving. So If I hand started the motor it would work, but it couldn't start on its own.

I had been working really really hard last week to get everything finished and it is quite disappointing to have to cancel the test. We probably won't get another shot to test before the Christmas break, because I have exams coming up which I really need to concentrate on.

In the last 6 months I have learnt a really important lesson - things are usually harder than you think they are going to be - The entire reason I used actuated valves was so that I can vary the amount of fuel or oxidizer going into the engine. I think that to keep things simple I will replace the actuated valves with solenoids. Once I get the engine going they can be switched back.

Another example of how things were not as simple as I thought they were going to be is the relay back. Apparently (didn't  say it on their ebay page)  they wired active high. This means they are closed when they are open and open when they are off. If I used them like that it would mean that if the relays lost power everything would open (solenoids etc.) at the same time as the igniters ignited. Not the ideal situation. Ariel suggested I modify them, by removing a resistor and attaching the signal line to one side of the removed resistor which worked quite well.

So I guess I really need to make less assumptions and check everything!

On the valve front I would like to continue development of actuated valves as they will be extremely useful, but the current implementation is a bit haphazard. It will work for just opening/closing but I am not sure I will ever get the accuracy I want out of them without some major modifications. In the past I had thought that all the play had to do with the method used to attach the motor shaft to bearing plate. I have now realized that there is also quite allot of play in the reduction gearbox itself.

Originally I had considered using a large RC servo to actuate valves, but wasn't sure if they would have enough torque. After talking to someone the other day who told me about a 1/4 scale servo he had that nearly took his finger off and i thought... thats exactly what I need! (not the finger bit). I bought  cheap 1/4 scale servo, which I will test soon (once I get a torque wrench). It reckon it has 3.2Nm of torque, which might be enough to open a valve. If not there are two options. Get a better servo (the one I got was relatively inexpensive), or get a valve with less resistance. The valve I am using at the moment is good because its cheap and rated to high pressures but it has a few downsides. The torque required seems to vary greatly depending on the pretension on the ball and the pressure, so i might be able to reduce the torque required at a particular pressure. Alternatively I could get a valve with bearings. I would like to research the different types of valve available.

On another note I now have a engineering job which is keeping me busy. In someways having a job is good because I now have more money to put into rocketry, but the downside is I have less time to use the stuff I buy. I have so far avoided buying stuff I don't need and turning into one of those old men... but it is a struggle! At some stage I would like to purchase a CNC mill, but I really don't have any need for it at the moment (not to mention any place to put it). I have been considering moving into a dedicated workshop space for a while now and now that I have a income I am able to. Ariel and I are looking at something about 100m^2 which should provide plenty of room. We have inspected a few places in the inner suburbs, but they are all a bit pricey, so we will likely go a bit out of the city. Over the holidays I will be working full time so I just hope that I will have enough time to work on rocket stuff. I would also like to start building up a group of rocket enthusiasts to work on projects. There have been manny discussions about the best way to organize such a group.

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