I was more than a little discouraged when the ATF informed me sugar propellants were now being classified as low
explosives and required a low explosives users permit (LEUP). Now another government agency (CPSC) is about to
limit the annual sales of oxidizers and fuels to non low explosives manufacturer permit (LEMP) holders. While I
likely will pursue a LEUP or LEMP regardless, I'm going to wait until this summer to let the cards fall first.
In the mean time I decided to start another hybrid project.
This project will be geared more towards learning than flying, I have limited experience with hybrids and I'd like to get some firm data from a modest "M" class or thereabout engine. My initial design is working with a 2.5" diameter combustion chamber 24" long. The plan is to use 2" PVC as a thermal liner, with a cast HTPB/aluminum grain inside the PVC pipe. In the event the HTPB burns through, the PVC thermal liner will sever as a secondary fuel grain. Given the high initial flow rate, then declining flow rate of N2O; the higher regression rate of R45, followed by the lower regression rate of the PVC; this should give me ballpark oxidizer to fuel ratios during the entire burn.
I'm going to try to get as much data from each test as possible. That will include oxidizer pressure, chamber pressure and thrust. So I have a fair amount of work ahead of me setting up the data acquisition hardware, but I do have plenty of load cells and pressure transducers, so it should be just a matter of wiring everything.
For these static tests, I'm going to use a standard 1/4 turn 3/8" ball valve to initiate N2O flow. I may build a pneumatic/pyro piston to "turn on" the valve, or I may simply use some sort of spring and cut wire.
To the right is the 2.5" diameter combustion chamber, it is .065" walled stainless steel. The 2" PVC pipe was a tight fit in the combustion chamber so I chucked it up in the lathe and made a quick pass to make it a little easier to get in and out of the combustion chamber.To the left is the bulkhead/injector coupled to a stainless steel 3/8" ball valve. The bulkhead has an offset to allow the lip inside the PVC for better thermal protection. All of the fittings and valve are high pressure rated stainless or regular steel.
Here is the injector/bulkhead in the combustion chamber. I still need to cut some snap ring grooves in the chamber.
Here you can see the new nozzle and the casing with snap rings grooves cut.
I've been a little indecisive lately as to what to use for a nitrous tank. For static tests, it would be nice to know exactly how much nitrous was used in a given test. About the only effective way to do that is to weigh the tank before and after a test. A flight weight tank wouldn't be safe to handle once it was filled, so I decided to use a DOT rated tank on static tests. It should also be easier for me to maintain a reasonable tank/nitrous temperature in cold weather. I can set up for the test, then attach the nitrous tank at the last minute. I can also wrap the tank with insulation to further maintain its temperature.
The good new is I have a good supply of cylinders to hold N2O, I have 3) 20 pound tanks and 1) 15 pound tank in aluminum, and a number of steel tanks in the 20 pound range. What does concern me is the valves on the tanks. I really don't think they will flow the amount of N2O I'll need on larger motors. So I'm going to make a valve insert to go into the tank with a 1/2" pipe thread output. So I ordered a full port 1/2" valve for the tanks, along with a 1/2" PTFE/stainless braid hose to go from the tank to the engine. While this may be overkill for the engine size I'll be testing now, it should be large enough to flow N2O to future, larger engines.
Forward bulkhead/injector
Thoughts on the injector system:
I had really intended to make a more elaborate injection system with impinging streams of N2O. But that would require a lot of machining, and would be difficult to change the injector sizes without replacing a complete, machined surface. The thing about N2O though, is that the liquid pretty much instantly changes to a gas once it leaves the injector and enters the combustion chamber. So I'm not entirely convinced an impinging stream injector is really needed. I'm leaning more towards a long combustion chamber improving performance more than anything. So I opted for a simple 3/8" plug injector system. I can drill out multiple holes for a mini shower head style injector, and quickly change it out to a different size or configuration.
I'm not entirely certain what the flow rate of N2O will be with this design. I do have some limited data from my early tests, which, if nothing else showed me that the formula for predicting flow rate and injector coefficient were way off. One rule of thumb for hybrids is the ratio of throat area to injector area should be between 10 to 1 and 20 to 1. With more stable burns in the higher 20 to 1 ratio, and higher chamber pressure and better performance from the lower ratios.
Here is the current state of the engine.
In the picture you can see a pressure cylinder for nitrous oxide I'll use for static tests. It's a little big, but I don't have to fill it all the way, likely I'll start out with about 5 pounds. The original tank valve was bored out, so now I have a minimum of .443" inside diameter through the valve. I used the old output nipple and threaded it for a standard 1/4" pipe thread to connect to my pressure transducer. This will allow me to record N2O pressure along side the engine thrust. I'll have to wait with recording chamber pressure until I move on to a larger engine, as there just wasn't enough room for a port in the bulkhead.
The valve threads into a 1/2" full port ball valve. This will be the valve to open and close the tank now that the original valve has been bored out. The 1/2" valve connects to a 1/2" PTFE/stainless braid hose via a threaded coupler. The 1/2" hose is reduced above the engine to 3/8" pipe, through the 3/8" full port ball valve and to the bulkhead injector. I know there's a lot going on for such a simple task, but I wanted this to be expandable to larger engines, and I really felt I needed the large diameter plumbing to provide an adequate N2O supply.
Here I'm doing a pressure test of the fill line with nitrogen.
I picked a few adapters and did a pressure test of the plumbing with compressed nitrogen, all went well with no leaks up to 500 psi. I ran the fill line up to 1,000 psi, but only had enough nitrogen to get 500 psi in the N2O tank. I talked to our gas salesman the other day about my K bottle of nitrous, he's going to try to get me a bottle with a dip tube so I don't have to invert the big bottle to get at the liquid N2O. I also decided to trade in my small N2O bottle for the K bottle of nitrous so I don't have to pay rent. It's only $65 more to go from the small nitrogen bottle to a large nitrous tank.
The stack of fittings on the small nitrous bottle does create a bit of a hazard if the bottle were to fall and bang against the fittings. While all the components are rated well above the pressure of the N2O, they could easily break if impacted. So I think I'll lug the big K bottle of nitrous to my test site, and fill the small tank on site.
With a static test now in sight, I decided it was time to cast a fuel grain. For the first test I wanted to use 80% R45M and 20% powdered aluminum. So I set up a casting rig, mixed up the R45/Al and cast the grain. Much to my dismay, the R45 never reached a full cure. I'd used Mondur MR rather than Isonate 143L because I expected a slightly faster cure. But I think perhaps there is a shelf life on the Mondur, and it lost it's effectiveness since the last time I used it.
I had used the majority of my R45, so if I wanted another fuel grain, it was going to have to be something else. So I decided to give epoxy/aluminum a try.
Here is the casting rig prior to casting.
The casting rig is the 2" PVC pipe turned down to fit inside the combustion chamber case, with a 1" PVC core giving me an actual core diameter of about 1.32". I'm using 1.25" PVC end caps with a hole bored out to form the centering/core holders. I drilled a dozen 1/4" holes in the 2" PVC to key the epoxy into the PVC for a better grip. Prior to casting the entire core and base were painted with melted paraffin. The idea is to heat the assembly to about 175 F. after the epoxy cures, melting the wax and allowing the core mandrel to be easily removed. This actually worked very well the first time with the partially cured R45, so I'm hoping it works as well with the epoxy.
Fuel grain cast and curing. I sealed the key holes in the PVC with foil tape.
The fuel grain has been decored and looks fine. But I found one interesting thing, some of the wax must have combined with the epoxy, because at the very top of the grain there is about 1" of soft, putty like epoxy, the farther down into the grain it goes the harder it gets. My guess is the self heating of the epoxy softened the wax. I wouldn't have thought the two would have combined, but they must have. The good thing is that I overfilled the tube, so the soft area is excess that will be trimmed off anyway.
My first big bottle of nitrous oxide!
My nitrous arrived today, it was just what I needed, dip tube installed as I had requested with a CGA 660 valve. It's labeled as 64 pounds of N2O and I paid $72 for it. A good deal any way you look at it. I traded in my 120 cubic foot nitrogen tank and had to kick in an extra $40, but no tank rent to worry about now. I also asked the supplier about my helium needs for the high altitude balloon project. He said no problem as they kept a good supply on hand.
The HR 4 engine has been tested in Static Test 138
Results of the first test were encouraging. I'll have to work on a better configuration for my load cell so I get better data, but intuition tells me the Isp was very good. One thing that surprised me was the amount of fuel consumed, 2.2 pounds to 5 pounds of N2O. In other words, virtually all the epoxy/aluminum and a good portion of the PVC as well. It would seem the EP/Al fuel has a pretty good regression rate. Using a large N2O tank resulted in a long gaseous blow down period after the liquid phase ran out. While the late burn didn't add much thrust, I'm assuming a fair amount of the PVC continued to burn. So my Oxidizer to fuel ratio was likely in the 65/35 range, a little fuel rich which cost me about 6 seconds theoretical Isp.
Here's a picture of the upper bulkhead and injector after the first test.
This first test used 3 injectors .125" diameter, with a nozzle throat diameter of .75", yielding about a 12 to 1 throat to injector area ratio. I think for the next test I'll try to reproduce this test only using R45 rather than epoxy.