rj-const.htm Tri-Mode ARLA

Amateur Rocket Launch Assist (ARLA)

Garage Level Ramjet Construction


Introduction

A review of the available literature shows that there is no need for a specific shape and that function is the only real requirement. Further, most of the detailed design work of operational ramjets was done to increase the efficiency and range without increasing the vehicle size. Very simple designs should suffice for the 25-30 seconds that the amateur ramjet will be operating .

The following examples are based on the assumption that these will be tube launched and will function like expendable rocket stages.


Ramjet Powered Rocket

This figure shows the ramjet mounted as separate stage of a launch vehicle.


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Example 1, Simple Amateur Ramjet Design

This example was created to be as simple to build as possible. It has not yet been built but is shown here to demonstrate what could be built and how. The sketch is not necessarily to scale.

This example shows a ramjet made of two stock tubes, two cone sections, six alignment vanes, and a fuel spray nozzle. The igniter and fuel plug would be attached to the sabot. The combustion chamber would be created using holes in the central tube. The dimensions are:

Fuel Consumption and Fuel Tankage

Components

Structure

The principle structure for this example is two metal tubes, a conical section, and some sheet metal braces. Additional parts are needed for the fuel system and igniter.

Intake

Supersonic Diffuser

This inlet is similar to that of many aircraft and some airbreathing missiles. That is, it is fixed and located well behind the nose of the vehicle. The rocket acts as a very long supersonic diffuser and also provides bodyside compression.

Subsonic Diffuser

Subsonic diffusion is achieved when the air expands and slows after it enters the inlet. In this example the additional cross sectional area is provided by the combustion chamber. The many holes provide a quasi-smooth area expansion.

Combustion

Fuel Tank

The central tube of the ramjet may be built to take a standard fuel cylinder, such as propane or butane.


Or it may be blocked off to form a fuel tank for a liquid such as kerosene.


Fuel Injector

One way to inject the fuel would be to use pitot air passing through a venturi to draw fuel into the combustion chamber. This would also help atomize the fuel before injection. It would have the additional benefit of acting like an automatic mixture control since air flow is directly ralated to q.


Otherwise, the fuel tank will probably need to be pressurized to spray the fuel into the combustion chamber. At maximum q the pressure inside the combustion chamber will be about 260 psi. Garden sprinkler nozzles are usually designed for about 40 psi. This means that the pressurization will need to be above 300 psi to cover the differential. The fuel flow for 300 lbf thrust will be about 2 gal/min and will run for 30 seconds. It may be possible to use a pellet gun CO2 cartridge. Other means of pressurization could be from refrigerant cans, like those used to spray dust off camera lenses.


If a spray nozzle is not used then a splash plate might be used to break up the spray.


The central tube could be blocked off and filled with kerosene then pressurized. A similar nozzle arrangement to that of the butane concept could be made.


It may also be helpful to spray the some or all of the fuel into the air just after it enters the inlet so that it is better mixed when it enters the flame holder.


Another option would be to pump kerosene through copper tubes looped through the inside of the combustion chamber. This will heat and vaporize the fuel for better mixing. It is commonly done with microjet engines.

Fuel Control

Some form of fuel control will be needed.

Flame Holder

As the air passes from the inlet through each hole in the can-type combustion chamber there will be local variations in speed causing turbulence and low-speed eddies. It is in these eddies that flame holding is achieved. The air flowing around and through the flame holder keeps the metal from overheating. In this example the central tube is drilled with different sized holes. Design and construction of flame holders is an art requiring trial and error. The best way to proceed is to look at a wide variety of different flame holders that have been built (the micro-jet web community is a great place to start), build one, and test it. A make-shift direct-connect wind tunnel can be built from a high speed leaf blower. These often advertise velocities of 240 mph where the typical air velocity in a ramjet combustion chamber is 200 mph.

If the holes are cut then they could be half-moon shaped and arranged vertically so that the tabs can be bent to deflect the air sideways. This will swirl the air helping mix the fuel/air and also helping to keep the flame holder wall cool. If the holes are drilled then they can be bent to swirl the air by inserting a rod into the holes and bending the edges radially.

Homemade Flame Holder

This picture shows a flame holder made from a Folgers Coffee can that Larry ? in Oregon built. It worked well in a homemade jet engine. Note the larger hole in the middle. This is where the builder attached an automotive spark plug as the igniter.

See http://pfranc.com/projects/turbine/duh.htm for more details.

Microjet Engine Flameholder/1

This photo shows part of the combustion chamber/flameholder built for my friend Alan's microjet engine. He cut out the stainless steel with a Dremel and drilled the holes with a drill press. Together we shaped the cone with a hammer over a steel rod then welded it with a Lincoln Weldpak-100, MIG gas kit, and argon. Weld cleanup was done with a Makita 4" grinder and a Dremel. While not inexpensive, these are all garage-level tools.

Both of the above are considered "can-type" flame holders.

Igniter

A simple, low-cost igniter is steel wool and a 9V battery. Extra fine steel wool burns well in still air and burns like a blow torch in a strong wind. It is easily ignited by as little as 3 volts.

The steel wool and two thin insulated wires (with the ends stripped) would be packed into the flame-holder such that the wires short through the steel wool near the fuel injector. For high g launches it may be necessary to insert cross wires inside the flame holder to keep the steel wool from falling out. Under high airflow conditions it may also be necessary to mix both extra fine steel wool (for easy igniting) with heavier steel wool (slower burning).

The battery would be attached to the launch tube piston or sabot with a pressure switch so that ignition begins at launch. The short time in the launch tube should give the steel wool enough time to fully ignite before it hits the air and the battery drops away. An alternative would be igniting the steel wool from an external voltage source just before launch. This would give the steel wool more time to ignite and reduce the weight of the sabot.

Another alternative is to obtain an igniter from an electronic barbeque lighter. These cost about $30 and the igniters weigh about an ounce. They run on a single AAA battery and provide many minutes of continuous sparking. Thin, rigid coaxial cable can carry the spark from the igniter located anywhere on the vehicle to the combustion chamber.

Combustion Chamber

In this example the combustion chamber begins with the first hole in the flame holder and ends at the nozzle throat. The combustion chamber needs to be long enough to allow complete combustion.

Exhaust

The nozzles depicted and discussed have all been made from sheet metal. However, they could be made of graphite or even wood. The Russians and the Chinese have both used oak for ablative shields. The oak chars to a fibrous carbon which then resists further heating.

Nozzle Throat

In this example the nozzle throat is the narrowest area between the combustion chamber and the exhaust nozzle.

Exhaust Nozzle

The exhaust nozzle shown is a double conical section. It could be made from any stainless steel part that has the right shape and wall thickness. A long funnel could work very well. If none of these can be found then it can be made from stock sheet available from many scrap yards and supply houses.

The best taper will depend on what's available or easy to make. Additional length increases efficiency (to a limit) but adds weight. The metal thickness should be about 0.030 inches but this will vary depending on the exhaust temperature, the grade of steel used, and operating time. For vertical launches the flight time should not exceed 40 seconds.


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Example 2, Advanced Amateur Ramjet Design


This design is similar to the first but has more metal work. It should be more efficient, especially at the higher speeds. The throat diameter is not to scale and the drawing needs to be updated to clean it up.

This design uses

Most operational ramjets have inlets with the cross sectional area about 1/2 that of the combustion chamber. For those with supersonic diffusers, the inlet area includes the supersonic diffuser. In this example the rocket provides some diffusion and the widened fuel tank/body provides additional diffusion. Because of these it is uncertain just what would constitute a 1/2 ratio. However, if the area between the inlet lip and the body is between 1/4 and 1/2 that of the combustion chamber then the engine should work well. Some experimentation and wind tunnel testing should help with the design work. There are also several software models available which could aid in the design.

Bomarc-Style Amateur Ramjet

Conclusion

Safety and legal constraints cannot be emphasized enough. The energy levels that can be achieved from amateur ramjets equal those of the biggest and best amateur rockets. Together they can endanger not only people on the ground but also aircraft at all altitudes (including the SR-71) and even the Space Shuttle itself.

There is nothing magic about ramjets. They have been flown for over 60 years. Reading up on inlets, flame holders, and exhaust nozzles will help understand their approximate design characteristics. Garage level experimentation will help understand the underlying physical concepts and relationships. With these accomplished almost any well thought-out design should work reasonably well.



This Page Last Updated 22 Dec 98