news4.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.


April 8, 2000

Servo

I partially disassembled an FPS3003 servo. At the top there is a neat gear drive with a very high drive ratio. With a little minor surgery it can be made to turn without restriction. In the middle is the motor. At the bottom is a small circuit board which converts the controller signal into positioning information.

This looks like an excellent drive for the pilot fuel controller.

Sheet Metal

I still have not contacted the sheet metal companies. My available cash is too low for purchasing additional sheet metal stocks at this time.

Combustion Testing

See Test Report TR-2 for the details.


April 11, 2000

Bibliographic Additions

Two interesting papers to read which have flame holders similar to mine:

Upcoming Testing

This weekend I hope to test the second fuel distribution system. If it works well then it will be time to build a full-up prototype engine.

Air Supply

I recently bought a 1947 Ford 9N tractor for leveling the ground around the house. The bad news is that I have to work on the house rather than the project. The good news is that it has a Power Take-Off (PTO) that I can use to drive a large blower. The engine can put out up to 20 hp all day long. Shaft speed is about 500 rpm. I just need to get back to the junk dealer and buy one of his old blowers.


April 13, 2000

Butane Pressurization

Today I checked out the possibility of using butane as the pressurant for the fuel tank. At about 68 degrees f the vapor pressure was 25 psi. This is more than sufficient and yet not too high for a lightweight aluminum tank. It will be a little higher on hot days but the tank can be chilled if necessary. Maybe someone can find or design a very small pressure regulator. Another way is to use a splitter valve so that excess pressure bleeds off. For the short flights expected this should not be much of a problem.

Here are two butane containers of interest. The top one is a "Pencil Torch" and costs $3-5 at the hardware stores. The refill cylinder costs $1.78. Both weigh approximately 2.5 oz. The smaller one takes up less space, is refuelable, and comes with its own valve. If a splitter valve is to be used then it might be best to use the larger tank to ensure adequate pressure throughout the flight. All things considered $1.78/flight is minor.

Fuel Tank

Building a lightweight fuel tank that can hold 5 lbm of fuel (nearly one gallon) should be interesting. I've been considering different designs but haven't settled on one yet. The hardware store sells rolls of aluminum sheet for flashing that is probably close to the right thickness and size. The tank will probably need baffles that can double as stiffeners. Flap valves at the bottom of the baffles could help move the fuel to the center. I'll have to work up a drawing for this.


April 23, 2000

Fuel Tank Pressurant

After giving it more thought I think that it might be better to pressurize the fuel tank directly. That is, inject the liquid pressurant directly into the fuel tank. This should reduce the weight and complexity. The fuel would act as the heat sink to keep the temperature, and therefore pressure, constant.

I picked up a can of "Dust Off" from the computer store. It stores a refrigerant in a liquid state and directs the boil-off gas to blow dust from a computer monitor or other device. I haven't checked the pressure but I think that there are several different such refrigerants on the market. By testing each and selecting them based on the temperature of the day I can keep my fuel tank at a fairly constant pressure.

Mineral Spirits Variability

I checked the MSDS on Mineral Spirits and found that there is a wide variance in properties, as sold. This includes vapor pressure, density, and etc. Some manufacturers sell something close to kerosene, some closer to gasoline, some use a blend of kerosene, naptha, and other additives. Mineral spirits is also sometimes known as Stoddard's Solvent. When I looked through the MSDS on that I found an even wider range of properties. This is not good but I bought six gallons yesterday which should supply me for a while.

Additional Testing

I tested the latest combustion chamber using a second fuel line for the afterburner. I also canted the afterburner holes to add a small amount of swirl. It added the swirl but seems to introduced some turbulence in the pilot can. This could be more from the rough nature of the afterburner holes and the crude way I bent them but I didn't like it anyway.

I checked the stability of the pilot flame by waving my hand between the air source (shop vac) and the engine inlet. While the flame fluttered wildly it never went out. This means that it should be robust even under turbulent and angle of attack variations.

The afterburner fuel line did not perform as desired. To get the fuel to burn in the afterburner there definitely had to be a fuel source (hole in the fuel line) inside the pilot can to carry the flame downstream. The holes (1/64th inch) that I drilled seemed to be a good size but droplet size was too large and too poorly distributed. This resulted in droplets being ejected out the back end at high fuel flows.

In order to get maximum thrust I need to be able to combust all the air. As built and tested I probably wasn't getting more than about 75% of the air. This needs to be addressed but I'm still thinking on it.

Near Flight Version

I've started building the next version combustion chamber which I'm calling the near flight version. This one will have a longer pilot can with three rows of cuts instead of two. The afterburner fuel line will be placed with the outlet between the last two rows of cuts to help heat and vaporize it. I'm hoping that this will give me the afterburner characteristics I'm looking for.

This engine will also have a lighter weight mounting ring. The last one was very heavy. It will also have inlet guide vanes that will keep the pilot can centered and provide a mount for the inlet diffuser cone. This will look something like a supersonic inlet but, hopefully, act like a subsonic inlet.

I've realized that I need to do angle of attack testing on the engine (duh!!). This means tipping the engine up and down as well as taking this into consideration in my design work.

Scate Boarding

I've been wracking my brain as to how to test the engine before going through the long and expensive effort of building an airplane. My latest thoughts are to build a scate board type of car that can go on the street and be remote controlled. The winds lately have been over 60 mph at times and we'll probably see many more of these before spring is over. I'm guessing that inline roller scate wheels, lightly loaded and sprung, will support the weight and give me controllability. With an electric motor, similar to those on RC race cars, I should be able to get up to 60 mph ground speed. This will give me 120 mph airspeed on the windier days. Yes, I know about boundary layers and such but if I can get at least 80 mph airspeed I'll be a lot farther along than I am now. At that speed I should be able to see some thrust even though it won't be enough to sustain flight speed, let alone accelerate. I should also be able to see any combustion, or other, problems that I have not accounted for.


May 9, 2000

Near-Flight-Weight Engine

The near-flight-weight engine is nearing completion. The flame holder and body together weigh about 1.6 lbm. I now need to finish the fuel lines and it will be ready for testing. I've decided to move the fuel line in the pilot can a little so that I can place a piston up the tailpipe all the way to the top of the pilot can. For vertical launches this will allow transferring all the force directly to the launch stack rather than through the relatively flimsy combustion chamber. This piston can also hold the igniter for either horizontal launch or vertical launch.

Cheap Igniter

A thought came to me that I could build an igniter using a cheap cigarette lighter. All I need is a way to spin up the striker with the butane turned all the way on I should get a real quick start on the engine.

Source of Sheet Metal

About two weeks ago I ordered some 0.010 stainless 302/304 from Thin Metal Sales in Chino, CA. Unfortunately, I sent them the wrong area code (it changed a while back) and they couldn't get hold of me to say they don't take credit cards. Yesterday I called them and we worked out a deal. I'm sending them a check and they're sending me a 24 in wide, 10 ft long sheet for $125. Kind of pricey but the cost/lbm goes down with larger orders. I'm looking forward to seeing how difficult it is to work with this stuff. I have a 3 ft x 3 ft sheet of 0.016 which is a little too heavy and stiff for model airplanes but might be good for rockets.

Model Airplanes

I've been talking with a couple of people about an airplane for flight testing. Both are still in the early discussion stage. I figure my material costs will be in the $5-10 dollar bracket for a flight-worthy engine. Tanks and such are additional but shouldn't be much more. Labor should be down to the 10-15 hour range soon and after 4 or 5 more I should be able to build one in about 6-8 hours. All these months of wracking my brain for ways to make these simple and cheap are paying off.


ARLA Back Pages
ARLA Home Page
Alternative Accelerators Home Page


This Page created April 8, 2000

Last Updated Sept 4, 2000