rj-basic.htm Tri-Mode ARLA

Amateur Rocket Launch Assist (ARLA)

Airbreathing Mode - Ramjet

Ramjet Basics - Overview

Ramjets are, by far, the simplest air breathing engines available. Whereas piston engines and turbine engines use complex mechanical components to compress the air then extract the energy for useful work the ramjet compresses the air and expands the exhaust simply by means of it's shape and it's movement through the air. It requires no moving parts. See also www.allstar.fiu.edu/aerojava/flight67.htm.

Ramjets have been operated at speeds as low as 250 mph (ref Casamasa) on an F-80. Above about 400 mph they are more efficient than similarly fueled rockets (i.e. kerosene vs LOx/RP1).

Two gross estimating methods of thrust for ramjet were found. The first is 20 lbf/sq in combustion chamber cross section. The second is 90 lbf/sq in inlet area (ref Jaumotte). The first is probably conservative for a lean burning engine and the second is more of a maximum.

Like other internal combustion engines, it has the same Intake, Compression, Combustion, and Exhaust functions. The intake and compression are both accomplished by the intake features.

Structure

In some cases the construction is straight forward and the components are easily visualized, as is the case with the Bomarc ramjet. But, because the function is the important factor, a portion of the construction may perform the function of more than one component. Further, many components are the same whether they are on aircraft or missiles. For example, an inlet shape that works on a turbojet engine will also work on a ramjet, provided it's the right size for the application.

Intake

Air enters the inlet at about the speed of the vehicle. Because the inlet is narrowed at the front the air splits with some of it going around the outside and some of it going in. The air that goes outside flows smoothly around the engine causing very little drag. The air that goes inside (intake) finds a widened area called the diffuser and spreads out. When it does it slows down and increases in pressure (compresses).

Combustion

After the air enters the engine, the fuel is added through an injector to provide a combustible mixture. This fuel/air mixture is ignited to raise it's temperature so that it exits out the nozzle at a much higher velocity than it came in with. The air entering the combustion chamber is moving at about 200 mph. To ensure that the flame doesn't blow out under these conditions a flame holder is placed in the air path.

There are several variations on this, such as different types of flame holders and different locations for injecting the fuel.

The combustion chamber is an open area where the fuel/air mixture can burn completely before being exhausted. The combustion chamber includes the flame holder.

Exhaust

As the fuel/air mixture burns to become exhaust products it heats up and accelerates towards the exhaust nozzle. The nozzle throat provides a constriction to help maintain pressure inside the combustion chamber. The hot exhaust products pass through the throat and expand in the nozzle exit to approximately atmospheric pressure.

Thrust

The ramjet net thrust is the difference between two opposing forces, drag and thrust.

These may sound daunting but need not be.


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Ramjet Basics - Component Details

There are two basic types of ramjets, subsonic and supersonic. A third, which is theorized but not really demonstrated, is the supersonic-combustion ramjet or scramjet. Since this is unproven technology it will not be considered here.

The following description of the ramjet and it's components is based on the classical subsonic ramjet which typifies all ramjets in general. The differences between these and supersonic ramjets will be covered afterward.

Classical Subsonic Ramjet


Structure

The classical subsonic ramjet is basically a tube with a narrowed inlet and exhaust. Inside will be the fuel injector and a flame holder. Nothing else needs to be part of the engine. The fuel, fuel pump (or pressurized fuel tank), and igniter can be external to the engine.

Intake

Subsonic Inlet

The classical subsonic ramjet has an annular inlet which directly faces the air stream. The inlet is about half the area of the combustion chamber and smoothly shaped to reduce drag on the air flowing around it.

Diffuser

Usually right behind the inlet is the subsonic diffuser which slows the air and thereby compresses it. On some ramjets, such as the ASALM and FastHawk, the air enters the inlet near the front of the vehicle and travels along a duct before entering the diffuser. On some ramjets the expansion may be accompanied with a dramatic change in shape.

Combustion

Fuel Injector

The fuel injector adds fuel to the air to provide a combustible fuel/air mixture. This is best accomplished when the fuel is vaporized or at least atomized into very small droplets and thoroughly mixed with the air. The fuel is typically sprayed in from nozzles, often through a spray bars. For can-type combustion chambers the fuel is often sprayed directly into the combustion chamber. Some microjet engines run the fuel through tubes in the combustion chamber to heat and vaporize it first. It may also be advantageous to inject fuel in more than one location, such as near the inlet plus at the flame holder.

In some cases it is desireable to have an uneven mixture of fuel and air. If the engine is running at it's highest temperature (stoichiometric) then a complete mix is best. But, if it's running lean or rich to affect thrust, range, or temperature, then a flame may be difficult to maintain. In this case a turbulent, uneven mixture will have some regions that are rich, some that are lean, and more importantly some that are stoichiometric. It is these regions that will sustain the flame and ignite the other regions.

Flame Holder

The purpose of the flame holder is to provide an environment where the fuel/air mixture will burn without blowing out. The fuel/air mixture is typically traveling through the engine at about 200 mph (about 300 fps) making it difficult to keep the flame lit. The classical flame holder is the gutter type. This is just a V or U shaped piece of metal with the open side facing downwind. Behind (and inside) the gutter the fuel/air mixture is highly turbulent with small pockets of slow moving eddies. It is in these eddies that the flame is actually held, spreading to the rest of the fuel/air mixture. Designing a low-drag, efficient flame holder is somewhat of an art but it can usually be done on a bench with a simple air source.

Another common flame holder is the can-type. As the name implies, it is shaped like a can (though it may be conical, spherical, or other). Fuel is typically sprayed in at the front end, the air enters through holes along the sides and mixes with the fuel, and the combustion products leave out the back. The fuel/air velocity is low in the front end of the flame holder allowing the flame to hold without being blown out. Often the holes are canted to swirl the fuel/air mixture which helps maintain the flame.

There are many other types of flame holders available as well as innumerable variations on each. Entire books have been written on the subject. Also, the homebuilt microjet web sites are good sources of information.

Igniter

The function of the igniter is ignite the fuel/air mixture (i.e. get the flame started). The classical way is a short burning flare in the flame holder. It can also be done with a spark, addition of special chemicals (such as phosphorous), or a small torch. It is also possible to add a continuous spark so that the flame can be restarted if it goes out, but that is not typical.

Combustion Chamber

The combustion chamber provides an area for the complete combustion of the fuel/air mixture. It is typically the widest part of the ramjet engine (in terms of cross sectional area).

Exhaust

The purpose of the exhaust nozzle is to maintain pressure in the combustion chamber by restricting the outflow of the gasses. The classical nozzle is circular but may be more complex. It may be oval, square, rectangular, or more complex. Highly complex nozzle exits may have moveable surfaces to improve efficiency, but these are very expensive and only rarely used.

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Supersonic Ramjets


Classical Supersonic Ramjet Engine

The supersonic ramjet differs from the subsonic ramjet by the addition of the inlet shock cone (supersonic diffuser) and a converging-diverging exhaust nozzle. The diffuser helps slow the incoming air more efficiently at supersonic speeds than the subsonic inlet does. It does so by introducing one or more small shock waves in the air before it reaches the lip of the inlet where a normal (engineering term) shock slows it to subsonic speed. The exhaust nozzle provides the flow restriction to maintain the combustion chamber pressure, to accelerate the exhaust to supersonic speed, and to provide some matching of exhaust gas pressure with the outside air. Everything between is the same as the subsonic ramjet, though some dimensions may be varied to optimize the engine for higher speeds and altitudes.

Subsonic ramjets have been operated to about Mach 1.7 but the drag goes way up limiting their speed. Supersonic ramjets can be operated at subsonic speeds but inlet cone effectively reduces the inlet area considerably allowing for much lower thrust. One method being considered to overcome this restriction is the use of an aerospike rather than an inlet cone. At subsonic speeds the aerospike offers almost no restriction to airflow while at supersonic speeds it would have almost the same effect as the cone.

As with subsonic inlets, supersonic inlets can be almost any shape. The inlets on the Mach 2.5 F-15 are adjustable to improve operation at both sub- and supersonic speeds. At supersonic speeds the top of the rectangular inlet is tilted forward so that the lip forms a wedge which helps shock the air down to subsonic speed before entering the inlet.



This Page Last Updated 18 Aug 99