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Amateur Rocket Launch Assist (ARLA)

Newsletters and Random Thoughts

This page will include progress reports, notes, random brain farts, and other ramblings.


August 17, 2001

Requirements Changes

I've been talking with friends and decided to reduce the requirements for my first launch vehicle. The cost of flying and recovering an instrumented vehicle is likely to cost in the neighborhood of $500 plus months (at my current pace) of additional work. With that in mind I decided that all I really need to know is that it works. Therefore, I am working toward a minimal vehicle with no parachute, no instrumentation, and no recovery markers. Just fuel tank, igniter, flameholder, and body. If it falls back to Earth within a few seconds I'll know it failed. If I see smoke (there should be unburned fuel till about Mach 1) then I'll know it worked, to some extent.

I've looked at a couple of options for the launch tube, one cold and one hot. I'm looking around for a used air compressor tank in the 100-200 gal category. There should be more than enough air for a launch, though the G-loads will not be consistent throughout the launch. Considering the simplicity of the first launch vehicle this should not be a problem. The alternative is to add heat via a propane flame in the air stream. This would reduce the amount of air required and, for light loads, allow a supersonic exit. For now I'll stick with cold air.

Fuel Tankage

This has been another thought marathon and a little help from my friends. Options seem to be propane and pressurized hydrocarbon (mineral spirits or naptha). The propane is self pressurizing and highly volatile. The naptha requires pressurizing but is denser and closer to what I had originally wanted to do.

Either way I have starch cans that hold one lbm (my design point for first flight) and I've already demonstrated that they can hold the pressure of propane. And they are light weight. These cans fit easily inside a 3 inch aluminum tube. I intend to order up some fiberglass tubing from the rocket hobby stores, along with a nose cone just to see what that looks like.

Fuel Bladder

After months of thought a friend said that he plans to use a bicycle inner tube. Now why couldn't I think of that? Within days I noticed that Wal Mart uses long narrow bags to put fish in that they sell. So I took one home and, sure enough, it holds up well to naptha, even after a couple of days of soaking.

Today I realized that I may not even need a bladder. If I can find a lightweight liquid that will float on top of the naptha but not allow the pressurant to pass through (the naptha tends to absorb it) then all I need to do is add a little of that to cap the fuel.

Pressurant

A friend who is also working on a ramjet told me about a report on fuel pressurization. In some old government/industry tests they found that high pressure is needed to prevent the pressure waves from the engine backing up into the fuel system. Preferably about 300 psi. Butane is only about 20-40 psi. So, unless I vent the fuel into the air in front of the inlet (an actual possibility) I need to have a pressure of at least 100 psi). So I'll be looking at propane and some of the refrigerants.

I believe I can build a simple pressure tank using a torus (or partial) of metal tubing. This torus would lay flat on top of fuel tank. It would have two small tubes attached near the top of the torus. With these I could fill in one and vent from the other. Once filled with liquid (propane or a refrigerant) one tube would be close off. The other would have a vinyl tube running to the top of the fuel tank. This tube would be pinched closed by a couple sticks of wood or plastic. The sticks would be held tight with thin wire. The wire would be connected to heavier wires running to the igniter switch. When the switch is turned on the thin wire burns through, the sticks fall away, and the fuel flows freely. A cheap, lightweight, valve.

Launch Tube

I bought a 20 ft piece of 6 inch, Schedule 40, PVC. I can't remember for sure what I paid but I think it was about $55 and it weighs 65 lbm. 80 ft would weigh about 260 lbm and be a bit difficult to raise to the vertical. The couplings were almost $10/each. I also found a local source of 6 inch, Schedule 20, PVC but it runs about $4/ft. The weight should be about 60 percent of S-40 or about 160 lbm. Six inch aluminum irrigation pipe would be about half this weight but cost about $6/ft new and I'm not sure what kind of couplings I could use. So, I'm still thinking about this.

Launch Site

Non-powered launches I can do from my back yard (I have a 2.5 mile range with only a town well in the path). Powered launches will have to be conducted from a more controlled site. I've sent off my application to the Rocket Research Society and hope to be able to use the Mojave Test Site about 40 miles away.

Ignition

I'm sure I can do this with amateur rocketry materials but I also want to look at obtaining some magnesium.


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This Page created August 17, 2001

Last Updated August 17, 2001