news1.htm
This page will include progress reports, notes, random brain farts, and other ramblings.
For months I've been wracking my brain trying to come up with a way to build ramjet inlets and exhaust nozzles for really cheap. That is, without expensive welders and such. Last night I hit on an idea that should work well for at least subsonic ramjets. I call it the "pinched tube design."
Picture in your mind (I'll add a picture later) a thin-wall steel tube about 4 inches in diameter and 24 inches long. The cross sectional area through the tube is about 12.6 sq inches. Typical inlets and exhaust nozzles have half the cross sectional area of the combustion chamber. Now imagine squeezing or flattening each end so that they create an oval with about 5 inches by 1.3 inches. This will have about 6 inches of cross sectional area. The pinched slope should be gradual and extend back about 8 inches. You now have a low-cost ramjet case.
The ramjet could then be made in three pieces. Cut the tube about 8 inches from the exhaust end (where the tube is still round). A conical-shaped can-type flame holder can be built, tested, and attached to circular ring of metal or ceramic about 1 inch long. This assembly is then inserted in-between the two case halves and the halves attached by small screws.
This should work for any size ramjet. The pinch can be two-lobed (oval), three-lobed, and etc. If the lobes are pinched flat then they can be brazed and the excess cut off, either inside or outside, giving a rough simulation of a conical shape..
I was looking for some commercial product that had a nice bullet shape that would work for a subsonic ramjet inlet. This is when I came across the 2-liter soda bottles. They are about 4 inches in diameter (a good size for model airplanes), are easily cut, and are see thru. If a conical-shaped can-type flame holder is used very little of the heat should be felt upstream of the flame holder. This would allow watching the flame stability during testing and flight.
Since then I've seen many other commercial products with nice shapes.
In talking with an engine analyst I learned that it's common to preheat kerosene before it's combusted in ramjets. This is because kerosene is endothermic. That is, it takes heat (quite a bit) to break the molecules down into carbon and hydrogen so it can burn. The preheat temperature is in the 400-500 degree f range.
Some of the newer leaf blowers advertise velocities up to 240 mph. These are probably with the absolute minimum airflow. Part of the problem is the power requirement. My shop-vac advertises peak power of 4.6 horsepower and moves a lot of air. I've seen some that claim over 6 horsepower. The peak velocity may be a little less but the volume should be massively greater. And these are easy to obtain for not much more money.
Another idea is to use a model airplane engine and propeller to create a wind for a wind tunnel. In terms of horsepower the weed-whacker engines, such as the Ryobi 31cc, have on the order of 4-5 horsepower. Kits to convert these for model airplanes can be purchased for as low as $35 and they swing 18-22 inch blades.
I've also considered building a large centrifugal impeller to get high volume, high speed air. The problem was the power to run such a beast, 10-100 horsepower. The tip speed should be at least 400 mph. This speed is sufficient for a low-efficiency ramjet. I just need to build the equivelant of a ramjet-tipped propellor or wheel to drive it.
During WW-II Focke-Wulf built a powered wing design called "Triebflugel" or "Triebflugeljager" (rotating wing fighter) using three Lorin ramjet engines on the ends of wing-type rotor blades. This VTOL plane looks wierd but may have actually been flown. An earlier model was built by Erich von Holst, who called it the Libelle (Dragonfly) or Schwingenflugzeug (Ornithopter, flapping wing aircraft), and demonstrated it in 1940. Holst published his findings in the 1942 Jahrbuch der Luftfahrtforshung (Aeronautical Research Annual). I'm hoping to get a copy of the paper.
Tip-jet powered model airplanes and helicopters would make interesting applications for amateur ramjets. Ramjet powered model airplanes might be more interesting but at any appreciable speed they would be very hard to keep under control, considering the range of model airplane transmitters.
I have the address for obtaining Dynajet Pulsejets.
Dynajet (Citizen-Ship) 17335 U.S. 31 N. Westfield, IA 46074 (317) 896-2561 American Hobby Center (retail outlet) (800) 242-1931
I actually got back to work in the shop. I modified a propane cylinder to allow high volume gas or liquid to flow. This now flows into a series of valves and taps to allow me to fill and use the cylinder as a fuel tank for an operational ramjet. It's still a little heavy but it's a start.
Now I have to test everything for pressure before I actually begin using it. I hope to be able to use my 12 volt auto air compressor which is supposed to reach 200 psi. Propane at room temperature is about 164 psi.
I've also been looking at model airplane designs which would allow me to test a subsonic engine. I found an interesting site about a pulsejet-powered delta-shaped model airplane. They claim a top speed of 445 km/hr which is faster than I thought could be controlled. But, if they can then I can.>
I've been downloading and reading some of the old NACA ramjet reports (NACA Technical Report Server) and have run into some real nuggets.
I found many others that I want to download. One talks about operation from Mach 0.8 to 2. This is the transonic ramjet data I've been looking for. The NACA site is a gold mine of information.
I've been reviewing so many of these that I've moved the descriptions to the bibliography page. One in particular, though, needs to be mentioned. I've been upstaged by 47 years. NACA report L53H28 talks about a twin-engine ramjet powered missile that launches at Mach 1.74 and accelerates almost straight up to Mach 2.61. This is a 242 lbm missile (with 22 lbm fuel). The throttle stuck and it flew until it ran out of full. The acceleration is about 1.5g vertical (with gravity 2.5g). The 6.6 inch diameter engines produce about 500 lbf thrust each. Doing better should be fairly easy.
I tested a thin tin-can against a propane flame and the can won. The can was a WD-40 can (empty), which is about 0.01 inch thick. I pointed a propane torch (nearly stoichiometric) at it for about five minutes. While red hot I tapped it with a screwdriver and it still rang so it still had strength to spare. Afterward I checked it for erosion and found none. This means that these types of cans (and materials) should do just fine as combustion chambers. I think they're a stainless steel alloy.
I also tested an aluminum soda can. It didn't last 2 seconds.
The propane can and valve assembly has been built and tested at propane pressure.
For a model airplane engine I have the following ideas. The combustion chamber will be a 4 inch diameter tin can about 18 inches long. The nozzle will be of the pinched type (my invention). The inlet will be just the open end of the can but with an inlet cone. This cone will not act as a supersonic diffuser but will reduce the inlet cross sectional area to about half that of the combustion chamber. The fuel line and star-type flame holder will be attached to the inlet cone. This will make it easy to install and remove for inspection/maintenance.
The RJSC is public domain code. It's windows based and very easy to use. Hopefully you can download it here. Otherwise just go call up the site http://www.alt-accel.com/rjsc.zip.
Another ejector ramjet analysis program that might be available is owned by John Olds of Georgia Tech. I've heard that this is limited and quirky but useable.
I pressure tested the fuel tank (propane) and manifold. I gave it several minutes at 200 psi (mostly water). At one point it went to 220 psi by accident.
Filled the propane bottle with liquid and flowed liquid out. I'm going to need a filter to clean up dust and other junk in the line before it plugs up the engine.
This report describes a missile that accelerates almost vertically at up to 4.13 g and coasts to 159,000 ft. Now all I have to do is duplicate their work, on a smaller scale.
L50L27 (104). Dettwyler, H Rudolph; Bond, Aleck C, Flight performance of a twin-engine supersonic ram jet from 2,300 to 67,200 feet altitude, NACA RM-L50L27, February 19, 1951, pp. 26.
While reading about all the different gutter-type flame holders it occurred to me that it might be possible to build one that spirals down through the combustion chamber. This would block little of the CC at any one point and allow the flame to follow the stoichiometric mixing point, wherever it may wander.
This Page created November 9, 1999
Last Updated Sept 4, 2000