There you have it, the page title tells the story. I've decided to step into the world of bipropellant liquid engines. This is a big step and will be long process no doubt. I'll continue to fly the amateur solid propellant rockets I have, but I doubt you'll see any new, big solid or hybrid rockets in the foreseeable future. I suppose the real reason I feel ready for this step is the fact I now have a Bridgeport milling machine. A mill is really essential for the type of work that will need to be done. But, I'm still going to need more equipment. My short list of needed tools includes a rotary table for the mill, a welding/cutting/brazing torch and a bottle of nitrogen gas.
My initial plans are to use N2O and ethanol. The N2O is of course storable at room temperatures and self pressurizing. The ethanol will use a pressurized blow down system using gaseous nitrogen as the pressurant. I know a lot of people are going to ask, "Why not use LOX?". It is cheaper and slightly higher performance, but it doesn't store well, it IS more dangerous than N2O, it requires more expensive hardware/plumbing and would require expensive helium to pressurize. I won't say I'll never use LOX, just not at this time. The fact that I already have the infrastructure in place to handle N2O also played a roll in my decision to use N2O.
I'm looking at several possible combustion chamber configurations. A simple ablative thermally protected chamber with a graphite nozzle would be the easiest, and may well be my starting configuration. Another good option would be a steel or stainless steel combustion chamber/nozzle using film cooling. A regeneratively cooled chamber is also an option of course, but due the complexity of the design, I don't plan on starting there.
My biggest early decision is how large to make the first engine. I don't really want to start with something too small, as designs don't always, or rarely, scale up well. I also want to be able to fly a biprop in a smallish rocket. That would be a huge accomplishment in itself... But I don't want to spend thousands of dollars in expendables in the process. So I'm leaning towards something in the 500 lbf range. The early designs will need to be trimmed down for flights, but there's a lot of data that needs to be collected before I even start thinking about flying a biprop.
I'm planning to start with about a 3" ID combustion chamber with an L* in the 40 to 60 range. I'll try a triplet injector scheme with dual N2O injectors impinging on a single fuel stream. The ability of N2O to vaporize at atmospheric pressure should help to atomize the fuel.
I spent the last couple of days ordering some odds and ends I need to get started on this project. First I ordered a clamping kit, some end mills and a dial indicator for the milling machine. Next was some steel tubing for the combustion chamber and aluminum stock for the injector plates. I've really been he-hawing about what to do for a rotary table. I looked at my injector designs, thought maybe I could do some of the work on my lathe and finish up on the mill, but in the end, I decided a rotary table was the only way to go. I need a good sized table, at least 8 inches, so I looked at tables on eBay, from Grizzly and from Harbor Freight. I did bid on one table from eBay, but wasn't willing too bid to much on something that was going to cost me so much to ship. Grizzly Tools has a couple of nice 10 inch tables, but they would run over $400 with shipping... I might regret this, but I went ahead and ordered an 8" table from Harbor Freight. The table was only $200 and shipping was under $14, I suppose if it turns out to be junk I can always sell it and get most of my money back out of it. Tools from Harbor Freight can be a mixed bag, some really good deals on pretty good equipment - to some tools that are out of spec and wear out prematurely. I'm hoping this table can at least get me by for a year or two until I can find a good deal on a larger, better quality unit.
As you may have gathered from what I ordered, I'm going to make the combustion chamber out of DOM steel tubing and go with a graphite nozzle. I'm going to try film cooling and see what kind of results I get. I fully expect to burn through a few chambers in the testing process... I think I have enough valves and plumbing from the hybrid work to get me started. I will have to make some sort of dual valve actuator, depending on how testing goes, I may needed to incorporate some sort of delay on one of the valves in the start up sequence.
26 October, 2009:
I've been kicking around several injector head designs and finally decided to start with the easiest to make design first, since I'm learning as I go with the milling machine that seemed the best prospect.
Here's the start of the inside half of the injector head. I've just finished facing the top surface with a fly cutter.
I faced both surfaces of the injector head with a fly cutter. I bought a set of three fly cutters from Grizzly Tool along with a few other tools, end mills and a new boring bar. I wondered how well these inexpensive fly cutters would work, and for aluminum at least they work great. The fly cutter produced a semi mirror surface. I'm running the mill quill speed at 1060 rpm and just taking a couple hundredths off with a pretty slow table speed. It's hard to see in the picture, but I've also taken a pass off the outside perimeter with an end mill, I need to make a step in the side so the head drops into the combustion chamber. Later I'll mill an o-ring gland above the step.
I really need a better camera for this sort of thing, but here the main passages are milled out.
You may have noticed the little clamps I made to hold the disk in place. It takes me a few minutes to get it set up with a dial indicator but I've had good luck with the setup so far. The four 1/4" bolts are threaded into the brackets so I can fine tune the adjustment. The outer milled ring is the fuel passage, the inner ring is the N2O passage. The four triangle looking things in the center are going to be threaded for bolts, a pattern of bolts will also run outside the fuel passage. So far I'd have to say I'm pleased with the rotary table. It's an 8" from Harbor Freight and was very reasonable at $199, it runs very true and seems nice and stable. I put the dividing plate back on it in anticipation of drilling the injector holes...
Here's my new project consultant, Casey. Of all things he likes to stand next to me when I'm on the lathe or mill and try to catch the chips as they fly off!
I need to order some more material and tooling... I guess I expected that, but even purchasing low end tooling this is quickly getting expensive. To try to save a few bucks I made a set of parallels, a lot of work all things considered. I cut some steel flat stock, rough filed the edges and then proceeded to hand stone them to within a thousandth or so. More work than it was worth, I'll be ordering some parallel sets in the near future!
3 November, 2009:
I didn't get as far as I expected last week, I was going to start drilling injector holes when I took a look at the drill chuck that came with the milling machine, the jaws were trashed and would never hold a small drill bit. So I ordered a new drill chuck and R8 arbor, some drill bits and a gasket punch set. They arrived yesterday so I was able to get back at it...
Here's the new drill chuck working on the inside set of injector holes
I had some problems getting the very small .065" fuel holes drilled. The holes are drilled at an 18 degree half angle to form unlike impingement points, as I assumed, I had to spot drill first to get the drill bit started accurately. Then I proceeded to break off my first drill bit in the hole. So I chucked up a new bit on the second hole, worked more slowly and broke it too. Then I moved from cobalt to HSS bits, and just lightly pecked at the holes, cleaning them out after each peck. I managed to get a few holes drilled, then broke another bit. More holes drilled successfully, then on the last hole I suppose I got in a hurry and broke off another bit. So in all, I got 8 of 12 fuel holes drilled.
The N2O holes are a couple of sizes bigger, after spot drilling I got all 12 holes drilled with no problems. I'm not sure what I'll end up doing with this injector plate. I'm going to go ahead and finish it just to get the practice in, but I may try re-drilling the holes alongside the bad holes and use it the way it is, I'll decide that after I get it done and do a flow test on it. That is, providing I don't screw up something else before I finish it...
Here's the injector plate drilled out, although it's hard to actually see the holes. At least they all came out in the center of the flow channels. You can see the drawing behind the plate with the hole angles on it.
Here's the upper plate of the injector. Here I've just finished facing the plate with a fly cutter.
Notice the little arrow shaped tabs holding the part in the rotary chuck. I bought some 1/2" flat plate and cut the diameter with an end mill on the rotary table. It's cheaper and probably easier to make bulkheads on the mill than it is on the lathe, but the real benefit is that I can make thinner bulkheads on the mill. On the lathe I need at least 1" thickness to get it chucked in the lathe and still be able to turn it down. At some point I'll cut off the tabs too, but for now they're a handy way to clamp the part to a table.
Here's the top plate sitting on top of the injector plate. I've drilled 12 holes on the periphery and 4 holes in the center for bolting the plates together. The very center hole with the transfer punch in it is for centering the 2 plates. When done, the center hole will be drilled out larger and tapped for the N2O line.
It's a good thing I'm not 19 or 20 years old anymore. I didn't have much patience back then and one thing I've found with the mill is you need a lot of patience. It takes as much or more time setting up the part and the mill as it does to do the machining. I'm sure things will speed up as I get more experience...
4 November, 2009:
Here the 2 plates that form the injector are pretty much done. All 16 bolt holes have been drilled and tapped as well as the fuel and N2O inlet holes. I made a gasket out of EPDM rubber and used my new gasket punch set to cut the holes, it worked very well and was a good $17 investment! I have no idea how well the gasket will seal under pressure, but EPDM is compatible with both N2O and ethanol.
Here's the injector plates bolted together and a couple of 1/4" pipe fittings threaded into the injector top plate. Next time I place an order for hardware I'll replace the flat head bolts with some hex head bolts. I want to be able to torque the bolts with a torque wrench for consistence. Next I'll run some water through the injector and see how it performs.
5 November, 2009:
Here's a water flow test of just the fuel injectors. This isn't what the stream will look like when the N2O injector holes are flowing, but it does show a pretty good pattern indicating my holes were drilled properly.
Now I'm flowing water through both fuel and N2O injector holes. Remember there are 4 fuel holes that didn't get drilled, so the "odd" looking flow pattern is from those lack of holes.
Here's a close up of the impingement points. Very nice if you don't mind my saying... This was exactly what I was hoping for.
Since this injector seems to be working the way it was intended, I'll try drilling the bad fuel holes just outside the original holes. While this will move the impingement point some, I think it will still work.
21 January, 2010:
It's been a while since I've worked on anything in the shop. Very cold weather and a series of winter storms have made working in the shop problematic, not to mention any outside work out of the question.
Here's the aftermath of the most recent storm, the wind is still blowing but at least the skies have cleared.
Same day looking north, these drifts are about 7 to 10' high.
And my living room window, now completely covered with snow. The snow reached the roof about 12' high in this location.
Back to the rocket engine...
Here I'm doing a test cut of an o-ring gland.
I finally turned the shop heat on for a couple of hours and did a test cut of an o-ring gland in an upper injector bulkhead. I'm using a Woodruff key cutter to make the slot. It seemed to work very well and I'll cut the actual part next.