Saturday, 23 July 2011

Finished valves, nearly ready for a test

Today I finished up the last of the electrical work to get the valves connected. After that I played around with the timing to open and close them, and also trying to get them to reliably go to specific positions. Unfortunately the two valves are not the same (one is geared almost double) so they require different times to actuate. I made the key to couple the plate to the motor shaft out of soft aluminium so that if it tuned too far it wouldn't rip itself apart (the motor and gearbox has about 150Nm of torque). I broke a few keys at the start but didn't break any after I got better at the timing. I was worried that the minium time required for the relays that control the motors would be too big to get fine control of the valve position but it seems as if I can get at least a 100ms pulse. I am having this wired problem where the motor would rotate more in one direction that in the reverse, even though it was being pulsed for the same period of time, which I am yet to solve. There is a small amount of play in the position, which i think i should be able to at least improve by using a Steel key. Also something to improve would be putting limit switches on the motor, as i am worried about a key breaking during a test and having limited control. I will do a few dry runs and if its an issue I will have to add them. I am not really confident in the valves ability to hold its position after a few movements, after about 4 movements I would not want to rely on it being where it should be. For the first test I really only need three positions, closed, 1/10th for startup and full open so the accuracy won't be that big of an issue.

Also I have started thinking about the startup sequence, which felt odd at first because for the most part there has been little theory and a lot of time consuming fabrication. I plan on using a pyrotechnic igniter because its simple, reliable and I believe will limit the possibility of unburnt propellant collecting and causing a hard start. I have two ideas for igniters, a thermite based igniter and one based on a cylindrical strand of solid propellant. The thermite based one has the advantage that it is more energetic and will throw sparks increasing the chance of ignition. The solid propellant one although not as energetic and having a smaller burning area will increase the chamber pressure to (if sized correctly) operating pressure. I am worried that because of the large initial pressure differential and the inability of positing the valves with precision that the starting propellant flow will be too large. Pressurising the chamber will reduce the chance of a large volume of propellant entering the chamber causing a hard start. The igniter will need to burn for at least 5 seconds, but the longer the better. The other advantage of thermite is that its burn rate not really sensitive to pressure, whereas too much solid propellant could in itself cause a  hard start.

After the igniter has started burning and visual confirmation of the burning has been acknowleged I will open the fuel and oxidiser valves to 1/10th or thereabouts. Form what i have read its better to have a fuel rich flame so the fuel valve will be open more than the oxidiser one. Once I have visual confirmation of the propellant burning I will open the valves to full. Once the chamber pressure is right it shouldn't really matter how open the valves are. I am still not really sure I understand why the chamber pressure settles to a particular value, dependant on the feed pressure (as the flow through the injectors isn't choked) . Thats something I would like to understand better. Whenever I try to think about it I go in circles because the chamber pressure is dependant of the mass flow which is dependant on the chamber pressure. I understand that with a fixed exit area the pressure will reach an equilibrium but how would you possibly calculate that? More reading required.....

Anyway so I am pretty much ready to go for a first test. All I am waiting for now is a location. Next I will run some full duration tests with 750PSI and make a few igniters to test. Good igniters will be a evolutionary process as i can't really test them properly without a firing. I will try firing a igniter while flow testing with water to see how the flow rate changes but the water will probably mess with the igniter. One other think I need to do is drill a hole in the chamber for the pressure transducer. If i have enough time before a location is found I will start working on having electronic sensors which are transmitted via telemetry (which i will need to do before a second test anyway), otherwise a pressure gauge and a camera will have so do. Sensor data (in particular pressure) would be very useful for knowing whats going on in the chamber but its not necessary. For the first test all i really want to so is a few 5 second runs. Also here is a few pictures of various parts. Since taking the batteries out of the relay box I remounted everything and its a lot neater now. The cooling jacket is on the chamber (only one of the two barbs in currently on). You can see little bits of oring coming out, but it holds pressure and doesn't need to so I am not really worried. See if you can guess what the last picture is of.

1 comment:

  1. Beautiful work John

    I'd love to know more about it
    Also we have a great rocket range to test such thing

    Contact me – Ashley(at)