My hybrid rocket project now moves on. My first engine was modestly successful, probably better than anticipated. It actually worked! With a little more knowledge gained, I felt the original design lacked the flexibility I needed for more complete testing. Thus the HR-2. While not really changing the design much, I am adding a ball valve to initiate the flow of nitrous oxide, rather than using a pyro valve. I will continue to use the same oxidizer tank, and the combustion chamber will use the same nozzle. I have made a new injector plate/bulkhead to mate with the ball valve.
Here is an image of the new bulkhead. It is tapped on both ends for 1/8" NPT. One end will have the injector screwed into it, and the other will connect to the 1/4 turn ball valve. The bulkhead is .33" thick, and has a slot cut for an o-ring.
Above is the bulkhead installed in a new chamber casing. As you can see, the bulkhead is retained by 8) 10-32x3/8" steel screws. Originally, I was going to make the bulkhead thicker, and cut a groove for the screws to set in. After giving it some thought, I decided there was no good reason to make the bulkhead that thick. Chamber pressure will hold the bulkhead against the screws, there should not be any significant force pushing in on the bulkhead. The chamber casing is .065" steel, and was drilled and tapped to hold the 8 retaining screws.
Here are the components of the combustion chamber. The chamber is 1.5" EMT, actual ID of 1.61" and is 11.5" in overall length. That allows room for the nozzle and bulkhead, a 1" pyro ignition grain and 8" R45 fuel grain. The nozzle uses the same retention method as the bulkhead. The nozzle is the original nozzle from my first hybrid. The nozzle has been fired four times, and has suffered no throat erosion. There was only one small spot that suffered slight heat damage, it was in the aft area of the throat just moving into the divergent cone area. The heat damaged area was polished smooth in the lathe.
Update: October 23, 2003. I purchased the ball valve and associated fittings today. All I need now is a spring to open the valve, that should be an easy find at the local hardware store.
Above is the assembled valve system, connected to the bulkhead. The spring shown here is a little too long, so I will have to find a more suitable spring. The valve is shown here in the open position. I shortened the valve handle by about 3/8", just to keep it inside the oxidizer tank diameter. The valve is rated for 600 psi, just under the working pressure of N2O. With fairly large safety factors built into the valve, there shouldn't be any problems.
Here is the valve in the closed position. I will hold the valve closed with a small nylon or poly cord. After the pyrogen starter grain is lit, I will use a resistance wire to cut the cord. I leak tested the assembly with air at 650 psi.
Update: October 24, 2003
I purchased a small spring today, it looks like it should work fine. With the spring in hand, it now looks like the HR-2 is about ready for testing. I decided to get a fuel grain cast.
I decided to make casting R45 grains a little easier. I made a bottom plug for an 8" length of 1.5" EMT, I then turned out a hole to receive the 1" dowel I use as a coring tool. I put a small square of wax paper on top of the plug to keep the R45 from sticking to the plug. The 1" dowel also has wax paper wrapped around it, then taped on the exposed edge to keep it from unwrapping. The wax paper covered dowel is then pushed through the wax paper on the bottom, into the hole. I also used a cardboard casting tube this time, as I was not casting directly into the combustion chamber. Hopefully the cardboard tube will be easy to extract from the EMT.
The R45 has cured, so I removed it from the casting support and checked the fit in the engine. I had some trouble removing the cured grain from the support tube. Some of the R45 had bled past the cardboard casting tube and cured onto the steel support tube. I used an extacto knife to cut the grain free. On the top you can see the combustion chamber with the grain partially inserted. To the right, is the starter grain. The starter grain is 75% KnO3, 23% West Systems epoxy and 2% Fe2O3. The starter grain is 1" in length, the R45 fuel grain is 8" long. Below you can see the oxidizer tank with the valve system attached and at the far right is the quad injector originally used on my first hybrid.
Here you can see how the started grain sits around the injector.
Above is a picture of the assembled HR-2 engine. The white arrow indicates a small hole drilled into the uppermost edge of the casing. The hole is for the nylon cord that will hold the valve closed until cut by a resistance wire. As an afterthought, I wonder if the starter grain would heat the casing up enough to melt the nylon cord? I don't think I'll chance it, I'll use the resistance wire. The black line below the arrow indicates where the nylon cord will go.
Update October 27, 2003:
I decided to do a test of the valve today, using a resistance wire to cut the cord holding the oxidizer valve closed. I really didn't anticipate much of a problem, it all seemed pretty straight forward. I filled the oxidizer tank with about 400 psi air. Then tied the valve closed with some nylon cord. On my first attempt I used a single strand of the steel picture hanging wire I use on my igniters. The wire burned through before the cord was cut. So for the next try I used three strands of the wire.
Here is a capture from the video I shot of the test. I just used jumper wires to connect the cutting wire to the battery, and a momentary switch to apply power. A 12 volt garden tractor battery was used as the power source. It's hard to see in the picture, but the wire strands were wrapped around both sides of the cord with 2 turns.
Here is the moment power was applied to the wire, as you can see it burned very well.
Click Here for a short video of the test.
If you watched the video, you will notice very rapid opening of the valve. The cord burned through almost instantly at the end of the count down. When I go to fire the motor, I will do a normal count down then at T-0 I will push the ignition button for the starter grain. Once the starter grain is burning well, I will do a 3 second count down to the release of the oxidizer valve. The good thing is, I will have the ability to abort a launch even after the starter grain is ignited, if there should be a reason to do so. Of course, the down side is I will need two sets of wires going to the rocket, and two start switches from my controller. A standard three wire extension cord will still work fine, I'll use the ground plug/wire for the negative side, and one each of the neutral and hot wires for the individual positive circuits.
I have completed a new launch controller. Click here for details.
I decided to do a test fire of the new engine today. I set up two video cameras, one zoomed in on the test stand
pressure gauge, the other a wider view of the engine and exhaust. I set up my new launch controller with two 100'
extension cords, one circuit running to the pyro ignition grain, the other connecting to the oxidizer flow cut
line. I did a test several days earlier, to see how well my igniters. would light the pyro starter grain. As I
expected, the igniters were somewhat unreliable. It would take 2 or 3 igniters to light the grain. I intended to
make some new igniters using the epoxy pyrogen, but didn't get to it yet. So I knew it may take an igniter or two
to get it lit.
I filled the nitrous flight tank with about 1.4 lbs. of N2O. I used my car battery for a power supply for the launch controller. For added safety I perform my test fires from behind my car, looking through both side windows at the test. The video cameras are in the open, one on a tripod and the other on the roof of the car.
Everything set, I turned the power key to on, then main power on, I tested continuity on both circuits, all was a go. The first igniter burned, but did not ignite the pyro grain. The second igniter also failed to light the pyro grain. On the third attempt, the pyro grain ignited. I waited 3 or 4 seconds, to let the pyro grain get burning well, then pressed the oxidizer cut line switch. I held the switch for 4 or 5 seconds, I could see the cut wires glowing a bright white, but nothing happened. The pyro starter grain burned itself out after about 10 seconds.
I waited several minutes before approaching the engine. It was soon apparent the cut line was cut, but the spring had failed to open the valve. I removed the spring, and tried turning the valve by hand. There was a little catch in the valve when I tried to open it. Once it moved the first time, it moved smoothly thereafter. I'm not sure what caused the initial catch, maybe just sitting unused for a week, maybe the cold weather, it was about 40 F.
There was no sense in leaving the N2O in the flight tank, so I opened the valve fully to vent the nitrous.
Above is a picture of me venting the flight tank. You can see one camera on the tripod to the left.
Here are some numbers:
While I wasn't thrilled the engine didn't run. It did give me a good look at the oxidizer flow rate. Everything worked except the spring. I think I need to work the valve a couple of times before attempting a burn, and also I'll use a little heavier spring. The pyro grain needs to be replaced, but the fuel grain should be fine. I may get one more fill out of my nitrous fill tank, then it will need filling again.
Update: 11.12.2003
The weather has been unsuitable for another test attempt. So I decided to work on a better igniter for the hybrid starter grain. I decided to use the same epoxy pyrogen I had used on the previous hybrid engine, only to cast it in a straw with a resistance wire embedded in the pyrogen. I was a little nervous, in the past the epoxy pyrogen has shown a tendency to fracture the pyrogen without igniting it. Keeping the resistance wire close to or on the surface of the pyrogen solved that problem. I hoped the slender pyrogen in the straw would not fracture, but ignite.
I mixed up a 60 gram batch of pyrogen, KNO3 70%, Fe2O3 7%, Epoxy 23%. I cut the soda straws into about 1" lengths, and tapped the straw into the pyrogen to fill the straws. After each straw was filled I inserted the resistance wire and set them aside to cure. The next day I cut the soda straw away and was left with the finished igniter.
I tested 3 of the new igniters, each one in turn popped and blew a chunk of the pyrogen away without igniting. It seems the epoxy is just too brittle to ignite that way.
So I cut the wire off an igniter, and wrapped a resistance wire around the pyrogen with 3 wraps. I then resoldered the leads to the resistance wire. The first attempt at lighting the pyrogen was successful. Three more tests all worked as well. To keep the resistance wire in place, I just applied one wrap of tape around the pyrogen.
With the igniter problem solved, I needed to change the spring on the flow valve. A quick trip to the hardware store yielded a slightly shorter, stronger spring. I think this spring will open the valve easily. Next, another test fire of the engine.
Not a great picture, but you can see the load cell now attached to the angle iron test rig. I need to reattach the guide tube before use.
The next job I did was to test the new, heavier spring to open the N20 flow valve. I did several tests with air pressure last fall, and another test this Spring with full N20 pressure. In all tests the new spring opened the valve quickly and cleanly.
Now it was time to make some more fuel grains. I had one R45 grain from last fall, but I wanted to test some other fuel options as well.
From left to right; a left over R45 grain from last year cast in a cardboard tube, a new R45 grain cast in wax paper only, a polystyrene grain cast in wax paper and lastly a paper/wax grain. All of the grains still need to be trimmed to length.
The R45 grains I have used before, so I'm not too worried about their performance. The polystyrene grain I hope will work well. It's Bondo polyester resin, very inexpensive compared to R45. Theoretical performance is as good as R45, but I'm worried the regression rate may be low enough that the engine runs lean, causing too high combustion temperature. The paper/wax grain should be interesting, the combustion temp of wax is low, and the temp of paper is high. So we'll see what happens there. I think regression should be very high, so I think the combustion temp will stay fairly low. What I am worried about, I'm really concerned the wax will start melting and the whole grain will sag into the nozzle. If you look at the top of the grain you will notice I applied a layer of aluminum foil tape. I think I will cover the entire outside surface of the grain in the tape, hoping it will prevent some heat and melting of the outer grain surface.
I'll do a test of the R45 first, gathering thrust data and use it as a bench mark for future fuel tests. I have a feeling at some point in these tests I will melt the nozzle. So I need to start looking for some graphite to use as a nozzle insert. The reason I went with R45 in the first place was its low combustion temperature, but I don't like the expense of the R45, and the fact I have to mail order it. I guess I could live with a graphite nozzle and some casing insulation if I had to.
Next, I plan on testing within the next couple of days.