up-jan8b.htm
Attended the IMS conference in Pasadena, bought some materials, started building the Small-Pogo model, and philosophized on ejector ramjets.
The IMS was crowded but there were many displays by Futaba and etc. I spoke with three of the model airplane radio manufacturers about distance. They were all uncomfortable about distances of 10,000 ft. One attendee said his buddy was able to maintain control to 4 miles in the open desert using a Futaba. At which time the Futaba rep agreed that it might work.
At the IMS I bought some balsa and spruce for constructing the Small-Pogo model. I also bought three 12 inch props which defines the approximate diameter of the cowling (for which I bought some sheet spruce. I bought two Cox .049 engines to play with along with two .049 tank/reed valve assemblies for possible use in miniature pulsejets. The reed valve assemblies are extremely simple.
One vendor was selling the Sophia Turbine J450 Turbo-Jet engine. The one on display (and used in two speedboat models on display) had a shorter tailcone and looked cleaner than the pictures in their catalog. I was allowed to pick it up and look at it. The compressor is a complex shape and looks made for this engine. The turbine blade also looked production manufactured. This is not something put together in someone's garage. The price for the engine alone was $2,500 US. If you wanted the simple fuel controller it was $3,300. If you wanted the full engine controller it was $3,900.
One swap-meet vendor had two pulsejets (about 3" dia and 4" dia). He was selling them for $75 and $150 as I recall. The big one goes for about $260 out of the catalogs.
After the show I stopped by the local hobby shop and bought a round 12 oz fuel tank. This defines the approximate size of the body tubes. The flat pieces of balsa I bought will work just fine for the internal vanes. I also bought a model rocket to go on top of the Small-Pogo model, more for reference than anything. I've all but completed the assembly of the rocket.
The Small-Pogo model is looking like it will be very light and powerful. I haven't cut any wood for the model but I've been laying the pieces out and getting them ready. I have to pick up just a couple more pieces from the hobby shop before I begin assembly, which should be tomorrow. The design will be heavily influenced by the need for quick and easy operations and maintenance.
Ejector Ramjets: I've reviewed the Gluhareff design (which I previously mislabeled as a pressure jet), the Marquardt ejector ramjet, descriptions of the Hybrijet, and other liquid fuel to gas generators and feel that there may a way to combine these into a cheap and simple ramjet with thrust at zero speed. The Marquardt engine uses small rocket engines as the gas generators. The Gluhareff uses a coil of tubing inside the combustion chamber to vaporize and superheat the propane. The Schreckling turbine design uses a coil of tubing in the combustion chamber to vaporize the kerosene fuel. Coleman camp stoves have a tube going across the primary burner to vaporize the white gas that they use.
With these as models, it should be possible to build a kerosene gas generator, either inside the combustion chamber or separate, to obtain thrust from the ramjet at zero speed. The Gluhareff G8-2-130R ad claims 130 lbf static with a 9 inch diameter combustion chamber. The Hybrijet ad claims 16 lbf static and 25 lbf at 200 mph. If you mount a spray nozzle in front of the ramjet inlet such that the spay cone is just the diameter of the inlet at zero speed it should entrain enough air to work like the Gluhareff and Hybrijet. As speed picks up additional fuel can be injected in the normal ramjet mode to increase speed and performance. This should work well with models and possibly with larger vehicles. A variation would be using superheated steam in the injector to entrain the air. This would eliminate the hazard of having a high pressure combustible gas and any coking that might occur in the gas generator. At hypersonic speeds it could also be used to keep the combustion chamber below meltdown and add to the exhaust mass.
Some enterprising startup company could take some of these ideas and develop a line of inexpensive jet engines for modelers and homebuilt aircraft people. The investment costs would be low and the return on investment could be very large.
Tried building up the lower body tube with layers of paper and spray glue. Terrible failure. Tonight I used layers of 1/16 " balsa that I bought at the IMS show. Works great. The second layer is drying and will do the third and final layer tomorrow.
After the lower body tube is completed I will install the central landing strut, engine mount and vanes. After that comes the engine shroud and flight controls. I've already picked up the hinges for these. The radio, receiver, and servos should be here in about 10 days. I hope to begin tethered flights in about three weeks.
Completed laminating the balsa body tube. Cut a 4" piece for the lower body tube. At one end will be a piece of plywood for mounting the engine. At the other end will be balsa sticks extending down about 10 inches and terminated by plywood. This will form a cage for the fuel tank and avionics. At the very bottom will be the landing strut. for flight a plastic sheath will be slipped over the cage to protect the avionics. There will be eight vanes radiating from the lower body tube. These will support the engine cowling. By tomorrow I hope to have the plywood ends and cage fully constructed. Possibly the vanes mounted and landing strut built.