Monotube
Our goal is to build a bi-propellant rocket motor that is scalable and cheap to make. We chose nitrous oxide as the oxidizer and petrol as the fuel because it is easy to get hold of and not too expensive.
I made a spreadsheet to help me with the injector sizing and nozzle sizing. For testing we chose a low thrust of 550N. The basic specifications are shown below:
Our first design looked something like this:
The fuel tank AND the combustion chamber is inside the nitrous oxide tank. Not shown in the CAD drawing is a siphon to feed the nos to the injector. There is a piston in the fuel tank to keep the nos and petrol separate. The nozzle has a graphite insert in the throat.
The injectors were simple tubes with slots cut into them. Here is a photo of one of the injectors:
The valve setup was a simple UC valve as used in hybrid motors. Below is a photo. The tube on the left is the nos feed that will be burned through with a solid starter grain to initiate the nos flow and ignition. The tube on the right blocks off the fuel injector and also gets burned through on starter grain ignition.
The first test went well, but the nos injector came loose and increased the nos flow rate significantly. The combustion mixing was not very good.
And the aluminium combustion chamber didn’t hold up to the pressure in the nos tank. You can see the siphon that feeds the nos to the injectors in this photo. So the combustion chamber was changed to a steel tube.
Here is a photo of the motor at the moment the aluminium combustion chamber failed. It made this bright white flash and then the motor died. No hassles.
So we redesigned the injector: fan sprays that impinge the nos and petrol. And it worked well – too well. The injectors melted:
If you look at the video, you will also see that it burned for 4 seconds at full thrust and then suddenly the thrust drops. We suspect that the nos vaporised and the mass flow rate dropped to a quarter of the design. That is why the burn was so long.
So back to the drawing board…
We redesigned the injectors: it is now a cup with four nos holes spraying towards the centre of the cup. The single fuel hole in the cup sprays down the centre of the cup along the axis of the motor. So we have four nos streams impinging on each other and the fuel stream spraying through the centre of this. This should mix well.
The siphon has been replaced with a sleeve to feed the nos to the injector cup.
An epoxy pyro grain is cast into the cup to act as a pyro valve. The nos is now filled from the outside of the tank through a non return valve.
Does this injector configuration work?
Don’t know. The starter grain was too big and burned for too long and heated up the nos in the motor. When the motor lit, it was just too much for the nos tank and it failed.
Lesson learned: don’t put the combustion chamber inside the nos tank. If the starter grain is too big or burns too long, the nos will over-pressure.
So here is the third design. The combustion chamber is on the outside, but the fuel tank is still inside the nos tank and will be pressured by the nos. We will try to use a ball valve to initiate flow. We want the ball valve to open slowly when we start the motor.
So the idea is to stick an igniter grain up the nozzle, start the grain and then slowly open the ball valve.
I’ve starting machining this design and hope to test soon. Here are the design specifications that we are looking at for this motor: