news7.htm Tri-Mode ARLA

Amateur Rocket Launch Assist (ARLA)

Newsletters and Random Thoughts

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


February 18, 2001

New Direction

Family matters are taking even more time than ever. I've had four interuptions just writing this. These matters have caused me to make several changes, one of which is to reduce the scope of my ramjet studies. I've decided to focus on combustion chambers and flame holders. Trying to develop a model airplane to fly subsonic ramjets or a launch tube to do supersonic ramjets (which would also require instrumentation) is far too agressive. On the other hand, combustion chambers I can do in my yard.

My temporary shop is a gutted out 14x40 trailer home. This is unsafe for operational testing inside, so I go outside with all the weather and related problems that poses. I've decided to have a real shop built, rather than do it myself, and have started laying out the floorplan. I hope to have this completed by May.

Blower

I built an adaptor and connected the blower to my tractor but ran into two problems. First, the tractor PTO runs CCW (the wrong way). Second, the PTO runs at 560 RPM (rather than 1650 like I thought). This left me to decide how to change these and how much to spend. In the end I decided to try a different approach.

I now believe that the blower was designed for a 5 hp motor running at 1750 RPM (the Westinghouse people I talked to didn't have the old records and refused to speculate on the proper motor requirements). I found a 5 hp gas engine for cheap (because it doesn't run very well). It spun the blower and moved air but I had trouble with the belt and engine, especially at high speed. More recently I bought a 5 hp, 240 V, 3450 RPM motor at Harbor Freight for $80. Using a 5 inch pully against the blower's 10 inch I had the design speed for a few seconds before a wire came loose and I had to quit for the day (week). It also had a little problem with the belt and the air speed was less than desired. I hope to get back to it this weekend (if the rain stops).

Enhancements in the works include overspeeding the blower by about 20 percent. This means a 6 inch pulley on the motor and adding the power of the gas engine (I mounted them so they can both run at the same time). I'll also neck down the exhaust as far as I can without stalling the propeller blades. I'm hoping for at least 200 fps air.

The backup plan is to build (or buy) an axial blower to give myself the 200 fps air needed for combustion chamber work. I have a good source of old blowers but documentation is non-existent and the salvage yard owner isn't familiar with the safe operation of these. I may have to resort to engineering analysis, of which I've done little since college in the 70's.

Flame Holder Thoughts

I took apart an old gas furnace and it has an interesting flame holder idea that might work for a ramjet. I've also come up with a couple of other ideas that could be very simple and inexpensive, but testing will have to wait for now.

March 14, 2001

Small Flame Holders

I've put considerable thought into flame holders. One of the problems with ramjets is throttleability. If the fuel/air (F/A) ratio gets too high or low the flame blows out and there's usually no way designed in to relight it. This is where the two-part combustion chamber comes in. If about 10 percent of the air is drawn off and combusted near stoichiometric then it should a) not blow out and b) provide enough heat to vaporize and ignite the fuel for the other 90 percent of the fuel. Therefore, the ramjet engine should be throttleable from extremely lean (10 percent stoichiometric) to very rich.

For amateur ramjets this means relatively small pilot cans. Such cans should be manufacturable from food or other cans obtainable at the grocery stores (50 cents each at the local 99-cent store). Many of these are very close to 2.6 inches diameter, the same as many off-the-shelf amateur rockets.

The graphic above describes such a pilot can.

There are several possible ways to inject primary fuel into main combustion chamber. One such is shown in the graphic above (right). The primary fuel is injected into the pilot can exhaust stream where it is heated, ignited, and then directed onto a hemispherical flame holder (possibly the bottom of a soup ladle). The already heated fuel droplets are further heated by the hot flame holder and vapor/droplets are scattered into the airstream of the main combustion chamber. Additional flame holding is accomplished behind the hemispherical flame holder.

Solid Fueled Pilot

One of those late-night thoughts came to me. For short flights a liquid-fueled ramjet could use a small, solid-fueled, slow-burning, rocket motor (or flare) for the pilot light.

Combustion Chamber Test Cells

A couple of thoughts came to me on how to improve testing of ramjets and many variations on pilot flame holders.

In the meantime I'll just use my large shop-vac for preliminary testing.

Speaking of a Shop

The first estimate on the shop I wanted was about $50K. I got a line on a steel building, 30x100 which would cost about $30K, set up on a full concrete pad. I heard about a guy who found a used 40x60 steel building for $1K. Installation would still cost about $18K. Further reductions could be made by having only the footings laid first and fill in with concrete later. I'm pursuing financing options on the 30x100 but keeping my eye open for better deals. A shop would definitely be a help.

The Big Blower

I re-connected up the 5 hp electric motor and, while it turned, it also smoked within 20 seconds. It still works but it took an hour to cool it off. I'm still scratching my head on what the design speed and horsepower really are.

I re-read my Kamps and he shows the math behind radial and axial compressors. He says that axials do not produce much compression with one stage but move a lot of air. I need to get back to the salvage yard to see what he has in the way of axial compressors. I thought about making one but two things bothered me, strength of materials and balance. After looking at the Schreckling design, which uses a plywood compressor, I decided that materials were not a serious problem. Balance, though could be.


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This Page created February 18, 2001

Last Updated March 17, 2001