20061111
Ball Valves
Update 2006-11-11
Motor Description
The motor is a bi-propellant liquid motor with nitrous oxide as the oxidiser and petrol as the fuel. The fuel tank is inside the nos tank and is pressurized with nos. The fuel tank is filled before assembly. The nos tank is filled through a non-return valve. There are two ball valves to start the flow of nos and petrol on ignition. The ball valves are kept closed with cotton string that gets burned through with nichrome wire. The ball valves are pulled open with springs once the cotton burns through.
The combustion chamber is a stainless steel tube (60mm), and the nozzle is an aluminium nozzle with a graphite insert. The test motor is an L-class motor. The motor uses a solid fuel starter grain to ignite the nos and petrol mixture.
Test Fire 2006-10-23
Once the nos tank was full, I power up the launch control box. 10-9-8-7, I press the button to start the starter grain, 5-4-3-2-1, I press the button to burn through the cotton and open the ball valves. A big cloud off black smoke forms and then the motor burst into life. It roars for about 5 seconds and then the combustion chamber burns through. It blows a cutting torch flame horizontally out the combustion chamber for a few seconds and then whistles to a stop. My hands are shaking from the adrenalin rush - It works!
(see the video)
The ball valve concept worked perfectly. One of the o-rings on the injector was damaged so we thought the combustion chamber burned through because Nos was leaking past the o-ring down the combustion chamber which caused a local hot spot (see next firing below).
Test Fire 2006-11-11
We tested the motor again with the same configuration as the previous test (making sure not to damage the o-rings). Same result. This time the motor didn't burn through but it would have in another half a second.
Somehow there is turbulence or swirling that focuses the burning on the wall of the combustion chamber. Unfortunately we don't have a video of this test.
The good thing is that the motor behaviour is repeatable. It is a stable design. Even when the combustion chamber fails, nothing dramatic happens.
In an attempt to smooth out the propellant spray a bit, I beveled the injector to look like this:
We will test this injector change soon.