arla-tes.htm
Safety is the first, middle, and last things to consider. The equipment discussed in this paper is very high energy, on the order of the most powerful amateur rockets. It can kill people, take down aircraft, and destroy houses and vehicles.
If you use PVC then take the extra cautions related to it. PVC is not recommended for gasses, even though many people do it. The manufacturers, distributers, sellers, and the professional societies (i.e. ASME, ASE) all say don't use it for gas. Unlike metal pipes, PVC shatters instead of tears, and sends schrapnel in all directions at high speed. This stuff can maim and kill.
In testing you should take a go-slow attitude. Build up in the energies slowly with many tests. Inspect everything between tests to ensure that nothing is wearing excessively, cracking, or otherwise becoming unsafe.
Keep a large safety margin in all your designs. Use automatic controls that you can operate from a distance. Treat this equipment as you would a high powered amateur rocket you built in your garage.
Legality - PermitsIn all your planning, testing, and operations keep in mind that this equipment probably requires a multiple of federal, state, and local permits from a host of agencies. These include, but are not limited to, the FAA, BATF, police, fire marshall.
You may want to team up with NASA or the Navy to use their ranges. If you can claim a military application then they may help pay for the development.
Once you get past the concept stage into detailed design and testing then the results of your work could well become export restricted. This means that you cannot legally divulge those details to any non-US citizen, take any of the equipment out of the country, or post those details on the internet. Yes, this is serious stuff, but most of the amateur rocketeers who build their own equipment already know this.
Legality - Use of Private or Public LandsLand use may also require permits. One of my testing concepts for the ramjet is to fly it horizontally to measure performance. This means that I have to obtain permission to use someone's land to do this. Many amateur rocketeers here in the Western US go to the dry lake beds to operate. Black Rock is famous for this. At 2,000 fps muzzle velocity it takes only 2.6 seconds to go a mile.
The first tests should be for basic function. Check each component as you go along for function. As you begin assembling parts continue to check function. Parts integration is where you're likely to find most of your problems.
Brass-Board TestingBrass-board refers to building low-cost prototypes that are not suitable for flight but work well enough to help in the design and testing effort. For example, a brass-board ramjet may be made of heavy gauge mild steel for verifying ramjet ignition and combustion in a home made wind tunnel. This keeps the cost down, makes construction easier (than high grade stainless sheet), and allows the part to be tested many times without repair.
Brass-Board exhaust nozzles can be made from concrete or pumice until the correct shape can be found. These will not be flight weight but will be easy to make and should hold up well under testing. Fine pore pumice can be obtained from restaurant supply houses such as Smart & Final.
Ground TestingComponents and assemblies should be tested as much as possible on the ground, either in actual operation or in simulations. Once the projectile starts moving down the tube it's hard to stop things and make adjustments.
Direct-Connect Ramjet TestingInitial testing of the combustion chamber can be done direct-connect. The air for this can come from a large industrial blower or a good garden leaf blower. Some of the newer leaf blowers claim air speeds of 240 mph and high volume, though maybe not at the same time. The air flow through a ramjet is typically about 200 mph, making the leaf blowers a good starting point. To be tested are fuel flow, ignition, complete combustion, and life expectancy.
The launch tube should be designed for a healthy safety margin. Cranes and other equipment used around people usually have a 8:1 or 10:1 safety margin. Aircraft, which have to be light weight to fly, have about a 1.3:1, but that's on ultimate load (i.e. the worst storm the airplane is expected to have to fly in). Most of the time they operate at 4:1 or better.
PVC does not allow a healthy safety margin. The work around is to use all automatic controls and stand back a hundred feet or more. If you must use PVC then include automatic pressure relief valves so that you can de-pressurize everything before you approach it.
Pressure test your equipment (from a safe distance) to about 90 percent of the design strength (well above your working pressure). If it can withstand this then it should hold up to normal operations.
Valves and Gas FlowThe same goes for valves as for the tube itself.
Check your gas flow in a build-up manner. Verify that the gas flows as it's supposed to and that no harmonic vibrations are set up.
Tube WearCheck for tube wear after each launch test.
Measuring Exit VelocityThe sporting gun community has a number of devices for measuring the velocity of bullets. At least one, the "Shooting Chrony," (available on the web) has been used to measure the speed of potato gun projectiles.
Structural testing needs to be conducted to ensure that the ramjet will not crush during the high g launch with the weight of the rocket on top. This can be done initially by assuming that the weight of the ramjet engine components is negligible and all the force will be through the interstage connection. For this you build a short launch tube section with the muzzle end capped off (though allowing for gasses to escape). Next insert the ramjet stage and piston or sabot. Last, close the breech end and pressurize the tube to your launch pressure. If it doesn't crush under this load then you should be safe with a small margin (the weight of the ramjet components).
Fuel InjectionThis could be done in a variety of ways. If the fuel is injected from a nozzle then the nozzle can be tested separately using the pressure expected during flight. Look for a fine droplet size and good dispersion. Higher pressures and smaller nozzle sizes tend to give better performance. The actual flame can be verified in a brass board flame holder. This, with a strong air source, should verify basic operation.
IgnitionBasic function of the ignitor can be verified on a bench, especially if it's a spark generator. Flares should be tested in a safe place where they won't set anything on fire. Homemade flares should be used only with great caution, they may explode.
Once basic function is verified then it will need to be operated in the real environment. This can be done either in the actual engine or, preferably, in a brass board engine. Because the engine will go from essentially zero internal air flow to 200 mph air flow in a fraction of a second this transient should be tested. If you use a blower this should have a valve that can be operated rapidly during the ignition procedure. Otherwise, the air can be obtained from the gas storage tank.
CombustionIf you are getting flame well behind the exhaust nozzle then you are getting a) excessive fuel, b) poor fuel mixing, and/or c) too short a combustion chamber. A longer combustion chamber makes up for poor fuel mixing but both should be considered.
If you are getting no flame out of the combustion chamber then you may be running lean. The hottest flame with no afterburning is the most efficient. For maximum thrust you will want to err on the side of a rich flame. For efficiency and moderate exhaust temperatures you'll want to err on the side of lean.
If your flame tends to blow out then you may need to change the flame holder design by adding/subtracting holes, changing hole size, and/or changing hole arrangement. It may also be that you are running too rich or too lean.
Wind TunnelThere are a lot of advantages to having a supersonic wind tunnel to test in, especially for improving performance. However, few of the people reading this will be able to afford the use of wind tunnels.
Pistons and sabots take the full force of the gas pressure on one side and almost the same force on the other side from the launch vehicle. These can be tested outside the launch tube by placing the pressure side on a smooth flat surface and piling weights on the other side, making sure that footprint is the same as that of the vehicle. A better test would be to use the actual air pressure and test it against an immovable wall (remember, there will be hundreds to thousands of pounds of force being applied).
The proof is in flight testing. This should be done conservatively, safely, and legally. Start with no fuel, an inert rocket, and a parachute. Also start with low launch tube pressures. This means many trial launches but will allow you find design problems before they become dangerous. For example, the g loads may partially pull your igniter wires out. You won't see that if you fire it every time. You'll want to recover it intact through a range of conditions to verify that there are no structural or other problems showing up.
Launch one with the igniter but no fuel. From this you can verify how well the igniter is working under real conditions.
Launch a series with increasing fuel loads. At some point they will go so high and fast you will no longer be able to recover them. You'll want as much data as possible before that happens.
In order to determine performance you will want some kind of instrumentation to determine altitude and, hopefully, velocity. Amateur rocketeers have such equipment available to them.
Measuring PerformanceInitially performance can be tested with horizontal flight into a cliff or out to sea.
For measuring ramjet operation a set of markers can be laid out between the muzzle and the cliff. A camcorder well off to one side can record the vehicle's travel in frames. Analysis of the position of the vehicle relative to the markers (photo theodolite) will give velocity and acceleration. By firing the first vehicle without the ramjet operating you get a deceleration curve. The next time, if the ramjet is providing thrust you should see an acceleration curve. From that curve you can calculate the net thrust from the engine.
Improving PerformanceHorizontal testing is adequate for improving performance at one altitude only. To verify and improve performance for the entire flight vertical measurements need to be made.
This Page Last Updated 9 Mar 99