rj-bas2.htm Tri-Mode ARLA

Amateur Rocket Launch Assist (ARLA)

Airbreathing Mode - Ramjet

Ramjet Basics - Part 2, Additional Ramjet Notes

Ramjet Fuels

Fuels are selected for a variety of reasons. The principle one is the amount of energy it provides. The measure of this, for the rocket community, is Specific Impulse (Isp). This is measured in seconds and says how many pounds of force (lbf) are produced per pound of fuel mass (lbm) when burned in one second. For comparison, liquid oxygen/kerosene rockets typically get only 350 seconds of Isp. A ramjet typically gets 1,200-1,800 seconds, though theory says they can get 2,400 seconds. For this reason the ramjet gets 4-5 times as much thrust per pound of fuel. For amateur rocketeers the cost of the fuel is small but this higher Isp means that the stage can be 8-10 times smaller.

The traditional jet engine fuel is kerosene. Other fuels that have been used are diesel, alcohol, propane, butane, and hydrogen. Each fuel has it's benefits and drawbacks. For example, alcohol has a lower energy density but, because of this, the engine will run a little cooler.

Ramjets can operate on a wide range of fuels including solid plastics, liquid hydrocarbons, and hydrogen. The solid fueled ramjets were not researched for this study. The German Lippisch P13a experimental fighter used coal as its fuel. Each fuel has certain characteristics that may be preferable, depending on the particular design of the ramjet.

Kerosene is the traditional ramjet fuel. It is widely available for about $0.45/lb in bulk and is much safer than gasoline. Diesel fuel is very similar to kerosene. Gasoline is slightly less dense than kerosene and has about the same density Isp. It is easier to ignite but can be very explosive. Alcohol has a lower density Isp and a lower flame temperature. For this reason it would be useful if lowering the flame temperature were needed.

Hydrogen provides almost twice the Isp of kerosene but is about a fourth as dense. Because of this the fuel tank will be about twice as large for hydrogen as for kerosene but the fuel will weigh less. The flame temperature for stoichiometric hydrogen operation will be much higher than for kerosene and so will cause problems with combustion chamber and nozzle design. Hydrogen is considered by some, but not all, to be essential above Mach 7.

Fuel/Air Mixture Ratio

Ramjets operate most efficiently with a stoichiometric fuel/air ratio (about 1/17) but for a variety of reasons have been operated otherwise. To limit velocity, pressures, and temperatures they are often operated very lean. They can also be operated very rich, especially for very high velocity operations. An excess of fuel allows for full combustion of the available air plus some cooling and increased mass flow. This has the benefit of reducing thermal stresses in the hot section of the engine but requires more fuel. This cooling can also be achieved by water or alcohol injection as is done in the Mig 25 and the older B-52s.

When hydrocarbon fuels, such as kerosene, are used above about Mach 5 the exhaust temperature is high enough to begin dissociating some of the exhaust products. While this reduces the Isp it does not become a major factor until about Mach 7. Because the CO2 dissociates easier than the H2O the net effect of operating very fuel rich at high speeds is that the ramjet begins to act like the fuel is hydrogen with some excess carbon thrown in. An interesting effect is that the very hot carbon particles would probably burn after they exit the exhaust nozzle and cause a comet-like tail of flame behind the vehicle.

Construction Materials

Up to about Mach 3 sheet aluminum (for the cool sections) and stainless steel (for the hot sections) purchased from the local hardware store should be sufficient. Above Mach 3 materials like stainless steel, titanium and inconel (available from mail order industrial supply houses, such as McMaster-Carr) or techniques like excess fuel, water injection, switching to alcohol, or cooler designs may be needed. Titanium is likely to be difficult to work. Above Mach 7 the builder may need to go to carbon-carbon composites which are becoming readily available for the model airplane and rocket builders and through industrial supply houses.

Variations

This ramjet, shown behind the rocket, has all the basic elements of a ramjet but with an inlet that gathers air flowing past the rocket upper stage. The upper stage provides bodyside compression in place of the diffuser.

Ramjet Efficiency

High efficiency has its value but at high cost. There's an old saying, "The last 20 percent of performance costs 80 percent of the budget." This is especially true in air vehicles. Acceptable efficiency can be obtained from approximate shapes such as cones and tubes rather than compound curves.

Structure

The structure can be anything that works. Improving efficiency is mostly a function of reducing drag and increasing thrust. Sharp changes in the speed or direction of the air flow cause drag so it's best to keep things as smooth as possible (without making them difficult to build). Round shapes and spherical volumes are the most efficient while thin, narrow, or complex shapes lose efficiency. The difference is often only a few percentage at each point but if the entire engine is rough and convoluted those percentages can add up to enough to keep the engine from producing net thrust.

Placement of the inlet along the shape of the air vehicle can add or detract from efficiency. Behind the nose of the air vehicle, under wings, and along different parts of the body there are places where the air is already compressed and more efficiently used. The nose cones an airplanes and rockets act as supersonic diffusers. Both the F-16 and the ASALM used chin mounted subsonic inlets behind the nose cone while flying at supersonic speeds. This is often referred to as bodyside compression.

Intake

The efficiency of the engine is highly dependent on the inlet efficiency and the compression ratio.

Supersonic Diffuser

The supersonic diffuser is most efficient when it is optimized for one air speed. At this speed the air goes through a number of small shocks with the final normal shock fitting perfectly at the inlet. In the case of the ARLA, the ramjet is used as an accelerator and therefore operates at it's design point for only a brief moment. In reality, very few inlets are operated at their design point. In fact, most, like aircraft, almost never operate there.

Where the inlet is blended into the body (i.e. ARLA) the optimum design speed is not so easy to guess at. However, for the ARLA a starting point can be the assumption that the air speed along the side of the rocket after a couple of feet is about 20 percent slower than the vehicle itself. Therefore, at Mach 5, the airspeed near the inlet to the ramjet will be about Mach 4. If anyone has a better assumption then please let me know, along with the basis for the assumption.

Subsonic Diffuser

The efficiency of the subsonic diffuser is a function of how smoothly the air expands and compresses. If the walls of the diffuser are smooth and shaped similar to that of a wing then it should be quite efficient. For supersonic ramjets the back end of the shock cone tends to taper down to a point allowing the air to, again, smoothly flow along it's surface and expand.

Combustion

Fuel Injector

The highest efficiency for the fuel injector is when the fuel is mixed at the precise fuel/air ratio desired, is completely vaporized, and perfectly mixed. Good enough depends on the length of the combustion chamber and how well the flame holder works. Fuel injectors should also be designed to not introduce significant drag.

Flame Holder

The flame holder should provide as many flame sources as possible to spread the flame quickly and ignite all the fuel/air mixture.

All flame holders have some drag due to the fact that they create turbulence in the air flow. For gutter-type flame holders they should be as small as possible and still hold the flame. They should also have a sharp V shape. Can-type flame holders, on the other hand, should be fairly large and have as many holes as possible and still hold the flame. Creating a swirl helps the combustion process and usually does not create drag.

Igniter

Igniters should start the flame then go away so that they don't interfere with combustion or induce any drag. In the case of spark igniters that are part of the engine this can't be done but they can be made small and be placed behind a flame holder where an eddy is expected.

Combustion Chamber

For efficiency, the combustion chamber should have smooth surfaces and be as large as possible. In addition, combustion chambers can lose a considerable amount of heat out the sides reducing the exhaust velocity and efficiency. The use of can-type flame holders helps reduce this loss.

Exhaust

Nozzle Throat

Like the diffuser, the nozzle throat should allow the air to smoothly accelerate and exit the combustion chamber. Long round entrances to the throat are most efficient with a gentle curve through the throat. In practice this might make the engine too long and heavy. Most important is probably a smooth, curved surface right at the throat.

Nozzle Exit

The nozzle exit should allow smooth expansion and be sized such that the exhaust gas pressure equals that of the outside air. A bell curve, like a rocket nozzle would also be optimal. In practice, they are typically the diameter of the combustion chamber and have a conical shape.



This Page Last Updated 18 Nov 98