Build your own cruise missile

Meant to mention this earlier.
Doesn’t the latest Harrier or AV8B have a fully composite wing?

exactly…

two methods to start it when stationary…one as suggested is to use a puff of compressed air, another more exotic technique is to design it so a controlled explosion is initiated in the engine which then is strong enough to start the resonance processes. As to throttling it…yes you should be able to throttle it if you vary the fuel injected. Most likely the simple model versions don’t incorporate a FADEC like most model jets does now. As to composites Mixtec, i’m pretty sure you can make an ENTIRE plane out of it and fly at 500mph if you want (the design method should be different than how a metal wing is constructed to take advantage of the properties of composites).

Garry
Can’t be certain but I’m fairly sure that design can be started when stationary. As the exhaust pulse leaves the tailpipe, it creates a partial vacuum which opens the intake again. I’m prepared to be proved wrong but I believe that’s one of the fundamental principles of a pulse jet.
Certainly the model ones are started with compressed air on the ground.
I’ve seen films of V1s being launched several times. Can’t remember clearly but I think the engine’s running before the catapult fires.

The pictures and drawings I have seen of the V-1 show the blind design as slightly different from those used as curtains.
Think of vertical tubes forming a grill over the front of the intake, then on each rod that makes up that grill put two flat pieces of metal sticking out either side but hinged on the rod so with just one set of vanes you have a single rod with a flat strip sticking out either side… with the airflow able to push it back… closing to form a v shape… the design prevents the two strips actualy touching and going flat.

With all of the strips fitted, they have no actuators to move them, and the explosion is what pushes them forward to seal the intake… the blast providing an impulse forward, the forward movement forces the, now flat strips back to let air in and fuel is added and ignited which blows the strips flat again to seal the inlet.

This design does require forward motion before it can be started…. for obvious reasons.

It is also the simplest and cheapest design.

It is not great for acceleration, but can generate enough force to get a Buzz bomb to a speed that makes interception difficult… (for prop driven aircraft, and very difficult for ground based air defences)

Seen the model pulse jets, very impressive. I’m not sure they’re actually illegal in the UK but they’re certainly frowned upon. Probably can’t get insurance on them.
There’s a Dutch team that do displays over here.
One major drawback to them is they have only 2 throttle settings, flat-out and stopped.
The Dutch pilot was amazing. The ‘plane was/is a delta with the engine stuck on top. Flies around at 2 – 300 mph for as long as the tank lasts and then glides down to land at his feet. I’ve seen them several times, probably 7 or 8 flights in total and I’ve never seen him even have to take one step to pick the plane up.
I’ve got, somewhere, a set of drawings for a model pulse jet, published in Aeromodeller in the 50s but I could never work out how to make the petal-shaped spring steel inlet valve.

Keep in mind that a cruise missle is an aircraft not a rocket. Its always amazed me they have never used cruisemissles for recon or AA as these are UAVs that are capable of these missions. All composites airframes are not practical for such aircraft
because though composites are very strong, they do not have the inherent rigidity of metals. Dont confuse flexablity with elasticity. This is why you dont see any 100% composite fighters. The wings especially can not be composte because they provide lift and are load bearing whereas a rocket is more of a projectile where the control surfaces of merely provide control and stabiliy (in a tight turn techinally the fins of a rocket are providing lift, but not efficiently as their tiny area only work at high speed (with thrust or momentum). A slight flexing in a 500 mph 1000 lb fuselage is not tolerable.

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there are many types of pulsed jets…the shutter valve type used in the the V1 is sometimes called a Helmholtz type pulse jet, but some times also called by another name (sorry, forgot). Unlike a ramjet, you CAN takeoff using a pulse jet by itself because it’s efficiency isn’t really based on the ram pressure. Theoretically, it has a very high combustion efficiency…probably the highest of all engines, but unfortunately it also have a very high loss of heat at the tailpipes…which makes the overall efficiency somewhat low (of course depends on your design). Hence, this is why most household central heating uses a type of vertical pulsed jet combustor called a Rijke tube which can actually burn some rather nasty and cheap fuels due to it’s rather stable combustion method and transfer the heat via a heat exchanger very well with such a short exchange surface length. Many modellers also use pulsejets around 2-3ft long producing under 10lbf of thrusts. Never seen on in real life, but seen many pictures and designs. It uses a single spark plug in the centerline (notice you only need it during ignition) and a spinning inlet valve disk where the rotation is provided by the pulsation. It’s kinda like a timing valve that opens the inlet. I believe this is more efficient than the Venetian blind type used in the V1…at least for this scale. The biggest problem in pulsejets for operational usage is sound and heat signature if you already guessed it.

Actually pulse jets are rather simple and relatively efficient. They certainly used them on so called Buzz bombs (they got their name from the noise the pulsejet engines made), but they are also used on some types of RPV (UAV).
Like a ramjet, they are not really powerful enough for a conventional roling takeoff and more often have some form of rocket booster to get it airborne and up to speed.
To give you an idea of how simple they are… think of a hollow tube. Put fuel injectors around the inside of it and put what look like venitian blinds (ie those flat strips of material people use instead of curtains) at the front.
In operation moving forward the blinds open to let air in. The fuel is pumped in and ignited… and the blinds are shut… the explosion can only go one way… out the back… this generates a forward impulse, the blinds are opened and o2 rich air fills the tube again… more fuel and ignite… you can’t get much simpler than that…

The only operational use that Im aware of is by germany in WW2 for the V-1 buzzbomb. The reasons the germans were forced to use this concept is due to the extreme lack of stratigac metals, which
is also why the Me-262 had such pathetacly short engine hours (16 as I recall). You have to realize the concept of a pulse jet is like a diaphram or piston that moves back and forth at an extemly high rate, pressing in air in one side, than releasing it out the back at one time. Try too imagine the pistons in your car using compression stroke to create thrust, were talking very low efficiency here.

What about the pulse-jet engine? Why haven’t I heard about this type of engine before? If it is so easy to build, then why don’t countries which require jet engines but don’t have the technology to build turbojets use this type of engine? Are there any aircraft which use pulse jets?

that’s what i’ve been talking about…

yes a person with multidisciniplanry engineering background, ie some Aero, some Computing, some EE experience with good hand on knowledge CAN built it. Although i doubt it for $5k if you want to fly at 500mph. Most RC aircraft flying with widely available “large” jets (say greater than 40-50lbf of thrust) will need to pay about $5k for the engine alone….altough it isn’t all that tough to build one IF you have a machine shop at hand with some CNC capabilities. The Airframe should have NO FOAM, but simply Glass Composite or Graphite “hollow” structures. Simply use FOAM as mold and melt it alway with solvents (ie Acetone) when done with the airframe. The navigation isn’t all that hard if you have GPS ( i believe a GPS chip type receiver with antenna, not those handhelds, just chips is ~$100 or less). On top of that add some missile controls grade (not inertial grade and easily and widely available) gyros and accels and you got yourself some short time navigation redundancies…maybe add a magnetometer just in case. The missile can be made to fly at low altitude with some polaroid sonar ranging sensor (at most a couple of hundred dollars for 3-5 sensors and controls electronics) or if you’re willing to spend more, use mininature radar ranging sensors for terrain following. Now, let’s add a video camera with satellite phone access and you got yourself some low frame rate video image. On top of that, imaing IR sensors may be expensive, but if you simply buy single element IR sensor and add some linear stage mechanism to it (not different from a inject printer)…you got yourself a IR line scan for a “Synthetic Apearture IR” for terminal homing capabilities. Scarry I’ll say, fortunately not many “single” person can do all of this (but enough people do know) but if a group of people do this simply means a higher chance of being found out.

Yes it can be done, but this isnt the guy to do it. The fact he mentions:

“An airframe, built using the type of foam-composite construction found in a number of home-built light aircraft would cost between $1,000 and $5,000 depending on the size of the craft. “

indicates he doesnt realize the engineering requirements of a small, heavy 500 mph aircraft. Thats not to say that cutting edge techonolgy is required, but something on the level of the German V-1 bomb. Programing software to build the “autopilot” that takes input from the GPS, gyros, etc, isnt something the amateur can sit down and do either.