I'm back to thinking about hybrids again. I've been doing some work with composites and the more common high performance oxidizers such as AN, AP and KP. That work with the composites reminded me of why a hybrid appealed to me in the first place. The oxidizer is fairly inexpensive and available at many car parts stores. There are no transportation or storage issues to speak of. The propellant has a good specific impulse and is easily scalable.
I pulled out my first two hybrid engines and looked them over. I decided if I was going to work on a hybrid it may as well be an engine that can fly in a rocket. So I decided to build a new engine, one that would fit in an air frame without a lot of modifications. That would mean a mono tube design. I have a fair amount of 2.25" OD .065" wall 6061 T-6 aluminum tubing, so that would form the basis of the engine. N20 at room temperature is about 650 psi, push the temp in the upper 80's and it's in the 900 psi range. To keep a good safety margin I'll use 900 psi as my N2O pressure. At 900 psi the 2.25" aluminum has a pressure safety margin of 2.88x. While that's pretty good, it's still not something I want to hold in my hand and fill. So a remote, on the pad fill system will be needed.
I considered many options. I narrowed the options to two. 1) Build a proprietary disconnect system. 2) Use a more standard nylon tube through the nozzle/burn it off at ignition design. It's called a U/C valve. Named for Urbanski and Colburn, the wise gentlemen who came up with it. I must admit, for a small to mid sized engine it's a great design in it's simplicity.
I opted for the second design, I just like it's simplicity. For larger engines, I will move to a heavier tank that I'm more comfortable standing next to and filling.
The next consideration was how to separate the combustion chamber from the oxidizer tank. I considered a floating bulkhead, but that means the force from the tank pressure is exerted on the fuel grain. If I use R45 it's fairly soft and would compress. If I cast into a 2" thin walled PVC tube, the PVC would give me a load support of 3508 pounds, while the force of the N2O pressure would be 3175 pounds. That's too close for comfort. So I decided to use retaining pins to hold the injector bulkhead in place.
This is a basic diagram of the middle bulkhead between the N2O and the combustion chamber. The green area in the combustion chamber will be an insulating layer composed of a PVC casting tube with a layer of EPDM rubber around it.
My initial plans call for an overall engine length of about 32 inches. 17 inches of N2O should be about 59 cubic inches and hold about 1.6 pounds of N2O. The remainder being combustion chamber length and bulkheads.
For a starter grain, I think I'll use the KP/epoxy/Fe2O3 composite formula I was working on. It burns hot but not too hot, and has nice burn properties, very clean and stable.
I ordered some nylon tube and some compression fittings from McMaster-Carr. I'll deal with forming the injector orifice once the fittings arrive. I'm planning on a .15" orifice, and a throat diameter of about .5". According to my calculations that should yield a thrust of about 70 pounds and a chamber pressure in the under 350 psi range. Burn time should be around 5 seconds. Of course, in reality I know the initial thrust will be higher and the engine tapers off in thrust as the burn proceeds.
Here is an image grabbed form the McMaster-Carr web site showing the fitting I'll be using as the injector and the hose fill connection.
While the fitting has a through port opening of .19", the internal diameter of the hose is .15". I'm going on the assumption the 1/2" length of nylon hose that is inside the fitting will not burn out as the N2O should keep the injector cool.
N2O Tank: 17"
N2O Tank Volume: 59.41 cu in
Combustion Chamber Length: 11.25"
Fuel Grain Length: 10"
Fuel Grain Port: 1" diameter Cylindrical
Fuel: Cast Polyester Resin
Fuel Grain Weight: 442.6 grams
PVC Casting Tube Weight: 98.4 grams
Starter Grain: KP/Epoxy/Fe2O3, 1/2" thick, 1.88" diameter, 29.1 grams.
Here is pretty much all the major components of the motor.
I decided not to try to make the injector anything special. It will simply be the opening left when the nylon fill hose is burned off by the starter grain. While I was ordering from McMaster-Carr I ordered more O-ring cord. 100' for $14 and that should make nearly 200 o-rings for the 2.25" casing. I'll need a few too, as the engine will require six o-rings with each use.
Here is an end view. Yes this nozzle cone is cracked, It's not the nozzle I was going to use anyway. And don't ask about the Swiss cheese looking nozzle end of the casing! It was supposed to have only one set of retaining holes but my clamp moved and misaligned the series of holes so I had to drill a new set. From this view you can see the polyester resin in the PVC tube, and you can also see the notch in the injector bulkhead where the PVC engages it.