CREATIVE SCIENCE PROJECT #2
PULSEJETS AND RAMJETS.
Disclaimer: If you fire one of these up within half a mile of any other form of habitation, you will not just annoy any neighbours, but stand a good chance of waking the dead. You have been warned. They are outrageously loud.
Jet engines are very loud. Rocket engines are deafeningly loud. Pulsejets of the same thrust levels of similar rocket engines redefine loud. They are painfully loud. Even very small pulsejets. Certainly, if you have noisy neighbours who won't shut up, get yourself a pulsejet and fire it up. If the noise doesn't permanently shut them up, the flames from the exhaust probably will.
As for ramjets, well, given that they need to be travelling at over 400 mph just to get started, I can't say I've heard one in operation...
...yet.
Bigger rocket engines do seem to be louder than pulsejets above a certain size (or at least that's my experience of them when I've been in the vicinity of them being tested), but because the options for locations for firing them up are generally more restricted than for pulsejets, you don't get the opportunity to entertain yourself with them.
In amateur rocketry, it is not unknown for people to make the connection between pulsejets, ramjets and rockets. Whether it is the sheer power of coupling such exotic forms of propulsion together, or a requirement for lots of noise, I don't know, but the thought of combining them does cross rocketeers minds from time to time. As a consequence, the information presented here is from a rocketry perspective.
This is a rocket motor - it is very loud.
PULSEJETS
Pulsejets are little more than a tube which acts as a combustion chamber, with a valve at the front of the tube. Pressurised air passes throught the tube, and is mixed with a vapourised fuel (such as petrol or kerosene). This mixture explodes, closing the front valve, and forcing the combustion products out of the rear of the tube to provide thrust.
Pulsejets have been around since the 1940's, and were used as the propulsion system of the German Fiesler Fi-103, better known as the V-1 Flying Bomb. The V-1 used an Argus Pulsejet which produced about 300 kilograms of thrust, and had a pulse frequency of around 47 Hz. In order to get sufficient air through the Argus Pulsejet, the V-1 had to be accelerated up a ramp by means of a steam catapult, powered by T-Stoff (Hydrogen Peroxide) and Z-Stoff (Calcium or Sodium aqueous solution used as a catalyst for the Hydrogen Peroxide). The Argus Pulsejet was also used in other German aircraft such as the Messerschmitt Me-328.
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| A pulsejet in action - this is far louder than the rocket motor firing above (believe me, I was there when both of these were fired, and my hearing was far less impaired by the rocket) |
Yes, pulsejets really do run very hot |
I can't take credit for this pulsejet, it's the pulsejet of Sally, a fellow crazy in MARS, the rocketry group that I am rather heavily involved in. This is her "small" pulsejet. If this one is mildly concerning, the large one she is working on is a bit more worrying. Note, the reason it is glowing is because it does not have a constant stream of air flowing past it to cool it, as would be the case if it were used on an aircraft, consequently, a pulsejet heats up very quickly when static tested, and cannot be operated for too long, otherwise it will literally melt!
Pulsejet points of note for rocketeers
Pulsejets are incredibly temperamental and fiddly to get started. I've heard quite a few people in the rocketry community express an interest in using pulsejets in conjunction with rockets (to sustain longer duration powered flights) - think again - pulsejets are hard enough to get started on the ground with plenty of extra support equipment, let alone clustered on a rocket or a rocket plane. You could start the pulsejet first on the ground then airlight rocket motors, but that rather defeats the object. Getting pulsejets, or ramjets to light whilst in flight is difficult.
RAMJETS
Ramjets and Pulsejets are often confused, but they are actually quite different types of propulsion. Whereas a pulsejet is reliant on a valve system to mix and ignite the air/fuel mix before combustion and subsequent ejection - all as a series of pulses, a ramjet is a very simple device which literally consists of an intake, combustion and nozzle system in which the cycle pressure rise is achieved purely by ram compression. Consequently a separate propulsion system is needed to accelerate the vehicle to speeds at which the ramjet can takeover. A ramjet is literally just a tube into which fuel is continually fed and ignited. The ramjet does not use a pulsed mode of combustion, but rather continuous combustion.
Ramjet points of note for rocketeers
Ramjets should in theory be more amenable to being used in the amateur community if combined with a rocket to get them up to speed. There's very little else that a clued up amateur could use to accelerate a ramjet up to starting speed other than a rocket. The drawback to rockets however, is that they consume propellant at a rate that makes ramjets and pulsejets look like they are fuel efficient in comparison. The result of this, is that when a rocket accelerated ramjet hits around 400 mph, there would be little time to ignite the ramjet before the rocket would run out of propellant.
Most amateur and High Power rockets can manage no more than around 10 seconds of thrust, so they need to get a ramjet up to 400 mph well before they run out of propellant - this is really not as easy as it may sound, since a rocket needs to be lightweight to get up to an appreciable speed, and dragging a big metal pipe which constitutes a ramjet, would not be light compared with the rocket it would need to be paired with. I have worked on and launched amateur rockets that have reached speeds up to Mach 2.6, but they involve phenomenal acceleration (0 to Mach 2.6 in 2 1/2 seconds), and this is really not going to help with ramjet ignition, since any flame in the ramjet would likely be extinguished.
RAMROCKETS
Ramrockets, ducted rockets, ejector ramjets or air augmented rockets, provide an effective way for the amateur to develop a propulsion system similar to a ramjet, without the worry of how achieve ignition. The drawback is that these types of propulsion system will only operate as long as the rocket motor is operating, however, it can provide an appreciable thrust increase over a conventional rocket when used at subsonic velocities, as well as an improvment in specific impulse (a measure of the efficiency of the rocket motor).
The key to building a successful ramrocket is careful design of the duct. This is easier said than done, since there is little to be found in the way of literature on these types of propulsion systems. What little is available, seems to refer to military propulsion systems, which probably explains why there is so little information available.
In this type of propulsion system, the rocket motor provides the main propulsive force at low velocities, with air being drawn into the duct and compressed, and heated by the rocket exhaust.
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