Test Report 2, April 8, 2000

Tri-Mode Launch System
Test Report 2
April 8, 2000

Summary

It's hard to believe that it's been 14 months since my last test report. On the one hand it's disheartening that progress has been so slow. On the other hand I had to follow my old notes to build the new combustion chamber and it came out great the first time. This means that the path is clear for others to follow.

This test successfully demonstrates a) mineral spirits as a fuel b) fuel preheating, and c) low pressure fuel feed. It concludes that "mineral spirits" is the preferred fuel at this time. It makes recommendations for future designs and testing.

Test Cell


This shows my extravagant test cell :-). Not shown are the fire extinguisher just inside the shop and the water hose on the ground. Considering the low fuel pressure, general lack of combustible material around (some loose hay and grass, otherwise desert), and wiring placed for easy disconnect, I considered this adequate safety.

Test Plan

The original test plan was to test:

In the end the copper and brass fuel lines and kerosene tests were dropped. The copper and brass were dropped because the temperature was estimated to be too high. The mineral spirits ignited and burned so well, compared to previous kerosene tests, that the kerosene was dropped. A total burn time of about 20 minutes was accomplished without undue incident.

Flame Holder - Construction

This picture shows the combustion chamber and its principle components. The fuel is mineral spirits, the pilot can is a 2.25 inch diameter furniture polish can, and the cone is sheet metal from a mineral spirits can. The ring at the bottom of the flame holder is a mounting ring made for test purposes. This combustion chamber design bears some similarities to the one described in NACA RM E51G26. Major differences are the air entry to the pilot and the fuel lines.

This graphic shows the general layout of the flame holder including the dimensions. The pilot slits were 3/4 inch long, cut with a Dremel, and opened up with a screwdriver.

This picture shows the mount and the afterburner holes. These holes were started with a drill then burned with oxy-acetylene and dressed up a little with a grinder. When I tried to drill the holes after fabricating the cone the drill tended to tear the metal rather than cutting it. In production the holes would be drilled before rolling using a fixture to keep them clean. Testing indicated that they should also be canted to swirl the flow even in the afterburner.

The afterburner cone was made from tinplate from a gallon can. This is about 0.008 inch thick. It was rolled into a cone, roughly truncated, spot welded, truncated, then brazed. The brazing was not needed but I wanted to see if the flame would melt it. It didn't, which means that future flame holders for model airplanes can be brazed rather than spot welded.

The cone was then brazed to the pilot can. The pilot can inlet slits were cut with a Dremel and shaped with a screwdriver. The afterburner holes were then made. The mounting ring was from 1 inch strap purchased at the local hardware store, cut, bent, and gas welded into a ring. This ring simplified some of the construction by adding stiffness. In an operational engine this will probably not be needed or a lighter weight one used.

Flame Holder - Operation

The total area of the combustion chamber (4 inch pipe) is 12.5 inches. The pilot can (2.25 inches dia) occludes 4 inches for an air path of 8.5 square inches. When incorporated into a model airplane there will be a styrofoam inlet cone that will occlude another 2 square inches giving a .5 inlet convergence ratio.

Fuel was provided by a 6 psi auto electric fuel pump connected to a battery. Fuel regulation was by a small brass valve normally used for water. While acceptable for test purposes this valve did not provide smooth, predictable fuel flow variation. Fuel was introduced into the flame holder without pressure or spray. The first test used a small hole in the end of the steel (auto brake line) tube. This resulted in too low a fuel flow and pulsing because of the gasoline boiling in the tube. After that I cut the end off the steel tube and ran it open at the end. This allowed the vaporized fuel to exit freely. It reduced the pulsing with the gasoline but did not eliminate it.

Test Results

Test Results - Fuel Preheating

Moving the fuel line around in the flame had a noticable effect when the fuel was gasoline but none with the mineral spirits. There was no attempt to move the fuel line completely outside the flame so conclusions about the effects of preheating on mineral spirits could not be substantiated. But, it worked well so I'll stick with it. Future testing with the afterburner fuel line may show some impact.

Test Results - Gasoline

Testing with the gasoline was unsatisfactory. It surged with visible fluctuations in the fuel line (clear tubing section with bubbles). It left soot and coked easily. It was, however, easy to start.

Test Results - Mineral Spirits


This picture shows the nice flame pattern with a moderate amount of fuel. The following series shows the range of fuel flows allowed by this design.

1. 2. 3. 4. 5.

I had no flow meter but I estimate the throttling to be about 20 to 1. In operation the pilot should not need much variation. For amateur airplanes the fuel flow can remain constant while the airflow varies with speed (approximately 120 to 300 mph). For amateur rocketry I hope to have a fuel controller maintain nearly stoichiometric flow in the pilot. In both cases most of the throttling will be done in the afterburner (conical) section.

The flame-holding was exceptionally smooth and satisfying. An excellent fuel and a design better than expected or hoped.

Test Results - Kerosene

Based on the success of the mineral spirits and past experience with kerosene I chose to skip the kerosene testing. In previous testing kerosene was difficult to light and slow to burn allowing droplets to blow out the tail pipe (ref TR-1).

Test Results - Tubing

The steel tubing (1/4 inch brake line, $2.50/5 ft) worked well. The weight felt reasonable. There was no indication of excess heating, though the gasoline did leave soot and coke on it.

Based on the observed flame I chose not to try the copper or brass even though these would have been lighter and had better heat transfer characteristics.

Test Results - Flame Holder

The pilot can showed little paint blistering and no melting. The afterburner showed no signs of heating, other than soot on the inside. The brazing showed no signs of discoloration or melting, even at the pilot/afterburner interface.

For model airplanes it might be best to leave the bottom of the pilot can without holes so that a) unburned fuel will pool helping keep the flame going and b) provide a deck to set the igniter on (assuming something like black powder or road flare is used). A small lip at the aft end could help for both.

I believe that the afterburner holes should be canted to swirl the air in that section. As built and tested the swirl appeared to end at the aft end of the pilot can.

Conclusions and Recommendations

  1. Spot welding will not be needed for amateur airplanes. Brazing will do.
  2. Simple materials, tools, and techniques should suffice, at least for model airplanes and low supersonic rockets.
  3. Mineral spirits appears to be the fuel of choice.
  4. This design allows deep throttling and could be used without the afterburner fuel flow, though not recommended.
  5. Minor changes in the pilot can could improve model airplane operation.
  6. Low-cost auto brake line makes a good fuel line.

Future Testing



Page First Created: April 8, 2000
Page Last Updated: April 10, 2000