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Can the Guy Next Door Build His Own
   Cruise Missile? YES!  [and why that may not be
   the disaster you're expecting!]  - Part II -

   by Larry Cottrill
. . . featuring a jetZILLA EXCLUSIVE:
Rebuttal to Part II

   by Bruce Simpson

jetZILLA  JUNE 30, 2003  S P E C I A L   E D I T I O N  -

jetZILLA Online Magazine of Amateur Jet Propulsion Development
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Issue 2003-0630-0105-00                       June 30, 2003
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I N   T H I S   I S S U E . . .
   Can the Guy Next Door Build His Own Cruise Missile? 
    Yes!  [and why that may not be the disaster 
    you're expecting!] - Part II -
   by Larry Cottrill

   Rebuttal to 'Can the Guy Next Door  
    Build His Own Cruise Missile? - Part II -'
   by Bruce Simpson

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   > Can the Guy Next Door Build His Own Cruise Missile?
     - Part III -

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Can the Guy Next Door Build 
  His Own Cruise Missile?  YES!
    [and why that may not be the disaster you're expecting!]
  - Part II [June 26, 2003] -
by Larry Cottrill, Editor, jetZILLA online magazine

Note from the Author:
All opinions expressed in this series of articles are my own. 
Mr Bruce Simpson was not consulted or interviewed in the
preparation of this article. He may, of course, respond publicly 
or privately at any time, and is especially welcome to point out 
anything he perceives to be in error in my description of this 
project. This article is not meant to either endorse or 
disparage Mr Bruce Simpson or his ‘D-I-Y Cruise Missile’ project,
and is solely intended to give the public needed information, 
along with another viewpoint, on this topic.

Recap of PART I

A few months ago, New Zealand pulsejet experimenter Bruce Simpson
set  up a page on his popular Website purporting to show the
public how easy it would be for a normally skilled person with
malicious intent to build, from scratch, a small jet-powered 
cruise missile at low cost and without arousing suspicion when 
procuring the materials needed. While assuring the world that he 
would not be revealing anything of a critical nature that would 
be of use to a domestic terrorist in executing such a plan, the 
project nonetheless quickly generated a high level of public 
anxiety and a great deal of controversy. There seems to be little
doubt that Mr Simpson CAN accomplish his plan, given his high 
level of technical skill in several critical areas. The question 
most people are concerned with is whether the project should be 
done. The question I would pose, however, is this: Even if Bruce 
Simpson can succeed in this, does it really matter, in terms of 
an actual terrorist threat of this kind that might materialize?

The purpose of this series is to try to come to a reasonable 
answer to that question.

To recap what a terrorist would need to be able to do to bring 
such a scheme to fruition - after the necessary planning and 
technical design work, the plan would involve at least the 
following tasks:
- Build a large, fast, rugged model airplane of sufficient 
   size to carry the deadly payload [and necessary fuel and 
   equipment] successfully to the desired target site
- Build a working jet engine out of new and/or scrap metal 
   [mostly some kind of sheet metal, assembled by welding]
- Build a guidance system [could be radio controlled, pre-
   programmed dead reckoning with gyro guidance, GPS guidance,
   or some combined system] 
- Build a launching device to get the model up to speed 
   quickly when finally needed [this could supposedly consist
   of a fast car, van or small truck]
- Perform extensive testing, without detection
- At the chosen moment in the wee hours, make a fast run down
   the road and launch your device at the intended target; 
   then beat it back home and wait for the media coverage to 

Bruce’s “construction diary” site page lays out his picture of 
this process in better detail. [The link to this page on Bruce’s
site is listed at the bottom of this article.] 

Bruce’s vision of how to accomplish the project is basically a 
process of gluing together a great deal of easily available 
‘over-the-counter’ technology, rather than coming up with a lot 
of new and exotic stuff. This is the main reason that there is 
little serious doubt that such a scheme is possible. Where I come
up with doubts about the viability of this threat has nothing to
do with what is possible, but rather with what is feasible. In 
the area of pulsejet-powered missiles from the neighbor’s garage,
I will try to show that there is a great gulf between the two.

In Part I of this series, I made the following claims, all of 
which are meant to cast some doubt on successful completion and 
deployment by your neighborhood domestic terrorist:
1. Just because you can design a pulsejet doesn’t mean you can 
build it
2. Just because you can design an airframe doesn’t mean you 
can build it and get it flyable
3. Just because you’ve built the engine doesn’t mean you can 
run it
4. Just because you can get it running doesn’t mean it will 
perform well enough to deliver the payload
5. Just because you can get it running right doesn’t mean you can
launch it
6. Just because you can launch it doesn’t mean you can pilot it
7. Just because you can pilot it doesn’t mean a robotic system 
can pilot it to the intended target point

Now it’s time to flesh out these claims so they can be heated in 
the crucible of public opinion. You can let me know how much of 
this looks real and how much you think is "fool's gold".

Just because you can design a pulsejet doesn’t mean you can 
build it

When you study the possibilities for available engine designs to 
use, you encounter a staggering variety, each claiming to be 
successful, as constructed by the original designer. Basically, 
there are two categories of designs to choose from – valved and 
valveless pulsejets. This division is more than just technical; 
it has a lot to do with construction methodology and run-time 

There are few valved designs that work out well for amateur 
builders with limited tooling. The best valved engines ever 
created and successfully deployed are probably the very large 
German Argus engine [used on the famous WWII “buzz bombs”] and 
the somewhat more modern, but vastly smaller, DynaJet. While very
simple in mechanical principle, both involve very fine machining 
of the front-end ‘valve plate’ to support the thin metal reeds 
that enable them to work. Any lack of good fit or poor seating 
between the valve plate and the reed valves results in degraded 
performance and increased valve wear and breakage [and the 
breaking of a valve can shut down the engine instantly!]. 
Technical improvements were made to the Argus design after the 
war, but these did nothing to simplify manufacture of the 
critical parts. Here’s an original Argus engine, partially cut 
away to show interior details [photos by Dmitry Petrov]:

Dmitry Petrov with Argus pulsejet - photo by Dmitry Petrov VIEW LARGE
THE ORIGINAL Mr Dmitry Petrov of Russia stands beside a captured Argus engine, a relic of WWII German war technology. The Argus is probably the largest pulsejet engine ever produced, and is certainly the first pulsejet design ever put into mass production. This machine, designed by Paul Schmidt, is the engine that propelled the infamous V-1 "buzz bomb" across the English Channel to terrorize London and a few other target areas. Static thrust of the Argus was approximately 550 pounds [275 force kg]. The noise of this engine was unbelievable - and unmistakeable. This one has been heavily cut away at the front end [at the bottom in this shot], to show the precisely machined valve plate assembly [see detail shot, below]. Photo provided by, and property of, Dmitry Petrov
Argus pulsejet front end showing valve grid - photo by Dmitry Petrov VIEW LARGE
CLOSE UP AND PERSONAL This closeup shot of the cut away front end of the Argus shows the back side of the intricate valve plate, which in the Argus is a 'grid' made from a precision casting that is machined to support both the thin spring steel valves and the fuel injectors. Surprisingly, the reed valves themselves are so small and numerous that they can't really be seen in the photo, but three of the injectors are visible protruding from the rear face of the grid. The steel tape Dmitry is using in these pictures is metric, of course - each major division [between red marks] is 1/10 of a metre, or just a hair under 4 inches. Photo provided by, and property of, Dmitry Petrov Valveless designs generally eliminate altogether the need for fine quality machining. It is often said that these are the simplest jet engines ever designed. What’s needed here are skills in welding, sheet-metal forming and blacksmithing, but these are needed [though to a lesser extent] in building a valved engine, too. Basically, you’ve got to be able to put together a sheet metal weldment of extremely light weight with sufficient skill to ensure no weakness or leakage anywhere in the structure. With some designs, such as the Lockwood [often called the ‘Lockwood-Hiller’] type, just forming the tubular parts is a major test of skill. Mention of the Lockwood brings up the difficulty of scaling, since the Lockwood is one of the few designs that has been successfully worked out in medium-size versions that could be used without re-scaling from something significantly larger or smaller. Most published plans for pulsejet engines are for very small engines of insufficient thrust for a terror weapon that could really deliver something meaningful. Pulsejets are notoriously hard to re-scale and get a successful engine, although it is true that ‘scaling up’ [going larger] is somewhat easier to handle than ‘scaling down’. Re-scaled engines often have poor power for their size, or don’t start or run reliably. These problems can be overcome, but not very easily. You either need the ability to take a methematical approach to solving the fluid dynamics problems involved in re-scaling, or work the problems out experimentally through a lot of difficult tearing apart, re-building and noisy test running [very hard to do without detection, in most places]. Yes, there are plenty of amateur experimenters who possess these skills, and the tools to go with them. However, this is just the first phase of our domestic terrorist's cruise missile project. A lot of other things have to fall into place beyond this to move the project forward. Just because you can design an airframe doesn’t mean you can build it and get it flyable At first thought, the airframe might seem to be the easy part of the project - after all, we just need a big model airplane that's capable of fairly high-speed operation [one figure Bruce has mentioned is about 400 miles/hour, but this may be optimistic]. Again, the design and construction of a moderately high speed airframe is found to be not quite as easy as 'scaling up' from some design that is known to work as a 'speed trial' model plane. With sport models, weight is usually the main problem - you want to get a reasonably low wing loading [on the order of a few ounces of aircraft weight per square foot of wing area] and a moderate power loading [perhaps 1/2 HP per square foot] to go along with it. A typical model might be a scale model of a WWII fighter, for example. When it comes to designing record-breaking speed models, however, weight is no longer the issue - drag at the anticipated airspeed determines the design, almost exclusively. That's why no one goes the route of scaling down a full-size record racer, because you'll never get the desired performance. Instead, the design becomes a kind of 'minimalist sculpture' model -- getting the most power into the least surface area possible, while packing enough 'beef' into the structure to handle the considerable stresses of high flight speeds.
Eniks target drone on tundra - photo from Russian sales brochure
DRONING ON A Russian export that isn't caviar - an Eniks target drone, from the 1990s. A target drone is a small plane designed to be a moving target for fighter pilots, air defense rocket launch teams, etc. They need to be cheap and fast, easy to see and hard to hit [good evasive maneuvering is an important part of the training process, especially when training fighter pilots]. This one is powered by a valveless engine that appears to be based on the Lockwood design. I have no technical data, except that the engine is supposed to supply about 34 lb [17 kg] thrust, which makes this a very small, light craft [typical of target drones]; I would guess that a couple of guys could heft this onto the launcher [shown in a separate photo, below]. Note the external mounting of the engine above the fuselage, and the aluminum reflector to prevent heat-blistered paint. Note also the relatively small wing and tail surfaces - minimal area for low drag at full operational speed. A homebuilt missile might be similar in overall configuration to this, but somewhat larger and almost certainly less colorful. This photo is from an Eniks company marketing brochure. It is easy to underestimate drag stresses when you first think about high-speed flight. Aerodynamic drag is basically a GEOMETRIC function of speed - to double the speed of a model, you must overcome FOUR TIMES the drag; to triple the speed, you must deal with NINE TIMES the drag; and so on. This is not just a matter of power, but also structural strength. The wing of a slow sport or stunt plane doesn't even need any special design for the drag forces, but for a speed ship, the drag force on the wing is one of the highest stresses in the entire plane. For this very reason, both prop-driven and jet-propelled U-control speed models are often simply carved out of solid pine or basswood [an excellent illustration of the fact that weight is not considered much of an issue]. But that's only a practical construction scheme for the smallest models that can get by with minimal control complexity. Construction of larger models would need to involve advanced 'stressed skin' design concepts, and would probably need to be built using some form of 'composite' construction with 'fibreglas' or carbon-fiber reinforcing. This type of construction requires a lot of experience to be effective - it's not easy to get high strength where it's most needed in the root section of a small wing, and the slightest structural flaw can bring catastrophic failure in a sudden maneuver. Another issue is the moving of the control surfaces. Every R/C modeler knows how critical smoothness and reliability are, but in the case of the missile, the speed and weight throw an extra wrench into the works. In the case of small, slow models, there is almost no significant aerodynamic force on the control surfaces as they move further into the airstream; not so, when you have a couple of hundred pounds of plane moving at 400 MPH. The forces involved will be significant, and standard hobby servos aren't going to deliver the power needed. Industrial units are expensive [though possibly not enough to blow the budget], and take a lot of electrical power to operate [which means that large batteries will need to be carried on board]. Homebuilding small servos out of easily obtainable DC motors is not out of the question, but demands yet one more skillset for our terrorist. 'Aerodynamic balancing' of the controls can reduce power demands to some degree, but this isn't feasible for ailerons and wing flaps [if these controls turn out to be needed]. Also, control hinges and linkages need to work smoothly under large stresses, requiring critical design and accurate alignment. Again, it's very doubtful that a beginner could get the job done right. There are a lot of people who do have the experience to accomplish the tasks needed, but this has to be on top of building that perfect engine. And, the plane being built will be moderately large and needs to be carefully concealed during all this construction and finishing. But wait - there's more! Just because you’ve built the engine doesn’t mean you can run it One more important thing that hobbyists have discovered about pulsejets: No matter how good a builder you think you are, you can’t get by without a lot of engine testing. Even with the simplest pulsejet designs, there’s still plenty that can go wrong. Even building from a proven design of the right size, you still need to test, and you need to get your starting technique perfected. You’re not going to do that on your first test run. So, even if you have a “perfect” design and top-drawer quality construction, you’re not going to get by without a considerable amount of noisy testing, so you know what you’re doing at launch time. Thrust measurement requires acquiring or building even more equipment [at considerable expense, for a medium size engine such as we're talking about here]. Once you start measuring thrust, you'll find that it's not quite up to expectations. There is considerable 'tuning' to pulsejets to get them really working right, even if your construction skills are very high. Almost any pulsejet hobbyist will be glad to tell you how easily an eagerly anticipated "first run" turned into a research project that stretched out over weeks of spare time. Getting a pulsejet engine right takes time, effort, possibly considerable extra out-of-pocket expense, and a lot of very noisy testing. Unless you're working with an exactly full- sized model built from a carefully dimensioned plan of a totally proven design, there's just no way around it. Our domestic terrorist is going to need a test site pretty far off the beaten path to get through this phase without arousing suspicion. Just because you can get it running doesn’t mean it will perform well enough to deliver the payload Getting a good static run out of your engine and getting it to carry you all the way from point A to point B are two somewhat different things. Some pulsejet designs get weaker in thrust and/or less fuel efficient when you introduce 'ram air' into the equation. The Argus design, on the other hand, has been shown to have actually performed better once it got moving, due to the constricted opening of the front end, which creates a 'diffuser' section [something like a ramjet front end] right in front of the valve grid. All jets are 'gas hogs' and pulsejets are poorer in fuel efficiency than most other types, because of the absence of compression during most of the combustion cycle. So, one of the big problems for our missile designer is the need to carry a heavy load of fuel [or on the other hand, to accept very limited range]. The takeoff phase of the flight, where the machine is struggling to gain speed, is particularly wasteful. [One way to get around this is discussed later, when we talk about how to launch the missile.] Reliability of the engine is also an issue in getting your deadly payload to the chosen target. All you need to stop a running pulsejet is to make it miss one explosion, because the energy of each explosion is used to pull in the next charge of combustion air. This makes ultra-reliable fuel feed essential. With liquid fuels, sloshing in the tank due to sharp maneuvers [or even rough air along the route] could create enough of a bubble in the fuel line to stop the engine cold. An explosion behind the tailpipe [say from a small rocket in pursuit] can halt engine operation, also. I'd speculate that the same thing could be accomplished at the front end, though I'm by no means certain of this. Most fuel feed problems could be avoided with the use of a pressurized gas or liquified gas fuel, such as propane, but it would still require careful design and construction, including an appropriate pressure reducing valve, at the cost of some additional weight. In general, pressurized or liquified gas fuel systems are thought to be bulkier and heavier than the energy equivalent in combustible liquid fuel setups. In this case however, that may not be true, since we'd almost certainly need an electric pump to deliver liquid fuel [with a sizeable and heavy battery to power it, of course], and a pressure regulator to make sure the engine will keep running steadily. There are only a couple of practical ways to test how your engine will run in the air - test it in a wind tunnel [way too expensive and complex to add to the project], or flight test it [risky and hard to do while avoiding detection]. Of course, our backyard terrorist might elect to just get it running the best he can on the ground, and 'hope for the best' in flight; that would be a poor choice, however, considering the likelihood of failure or unacceptable performance. Just because you can get it running right doesn’t mean you can launch it Contrary to popular belief, the most critical phase of any airplane flight is not the landing, but the takeoff. This is precisely where the vast majority of accidents happen, especially in the case of light aircraft with non-professional pilots. It's usually the best opportunity for engine failure, and the best chance for stalling the airplane without sufficient room to recover - and you can stall a plane with your engine delivering full power! It's an interesting combination of dangers: you're low and slow, but need all the power you can get to keep going - and, you have no room to maneuver if you 'lose it' for even a split second. And, all it takes to 'lose it' is a little too much back pressure on the stick or tightening a turn a bit too much in the climb! But what you have going for you in a light plane at takeoff [that we know our terrorist won't have in his missile at launch] is a trained, thinking human being inside who can see, hear and feel what the airplane is doing, and react properly. All three senses may be involved in making a life-and-death decision, with only a moment to react to what's happening. With luck, a human pilot can save the situation, because the technique is practiced until it becomes 'second nature' to react quickly and correctly [the key word here being "correctly"]. At this point, someone should ask, "So, how is it that the German V-1 crews were able to successfully launch several hundred such missiles per day?" The secret is getting your missile up to MUCH HIGHER THAN STALL SPEED at the launch -- and here is the one area where weight really DOES work against you. The Argus engine, like all well-developed pulsejets, had a quite acceptable thrust-to-weight ratio; however, the V-1 missile as a whole must have weighed at least a couple of metric tons [roughly 4000 lb], so the 550 lb thrust developed amounted to a fully loaded aircraft T/W ratio of 1:7 or less - pathetic, even by WWII standards. To allow the thing to take off under its own power would have taken half a mile or more, with an unbelievable waste of fuel in the first minutes. So, a launch scheme was designed that was far superior to a normal takeoff run: booster rocket launch. The V-1 was positioned for launch at the end of a long 'launch rail'. Under the surface of the rail, linked to the V-1 by a simple lug that extended up into it, a solid-fuel rocket [the so-called 'launch piston'] was triggered as soon as the Argus was running full bore. Thus, the Argus and the booster 'piston' both contributed to the launch thrust [much like the US Space Shuttle and its solid boosters today]. The result was the V-1 getting off the end of the rail with MUCH higher speed than the minimum that would keep it in the air, virtually assuring a successful beginning to its flight. The Eniks drone shown earlier uses a clever portable launch rail, with a [presumably hydraulic] cylinder and cable catapult arrangement taking the place of the V-1's booster rocket system:
Eniks target drone on portable launcher - photo from Russian sales brochure
OUT TO LAUNCH Another shot from the Eniks sales brochure - here, the Eniks drone is set up on its portable launch rail, ready to go. This is an extremely short rail, mostly due to the small size and weight of the drone; it also appears that there is a 'boost' catapult built into this launcher, in the form of a hydraulic cylinder and a cable system that acts as a velocity multiplier [looks like it might apply four times the cylinder's speed to the drone at the first part of its forward slide up the rail]. The V-1, being much larger, had permanent launch rails built, with rocket boosters to get the craft moving. Without such an added boost, the takeoff run would be hopelessly long, with very high probability of crashes after takeoff. The launcher shown here seems to be about the size of a large boat trailer, and would probably cost far more to make than Bruce Simpson's entire project budget! Now, this is a wonderful method of getting our missile into the air, except for one little thing: a launch rail with booster could never be built for anything less than the cost of the missile! For the V-1 or the Eniks drone, this makes sense because multiple missiles will be launched from a single rail. But unless our homegrown terrorist is independently wealthy [and can do all this in an area as secluded as the Australian 'Outback'], this method is out of the question. I agree with Bruce that the only practical alternative is a launch from a fast land vehicle. I do NOT agree, however, that this is very likely to work as anticipated. Such a launch can be nothing better than a minimally assisted takeoff. Up to this point, I would have every confidence in Bruce's skills to do whatever is needed to make the project work. Right here, though, I think he's up against something fundamentally different. Everything up to now has been build-and-test, go-at- your-own-pace stuff. But at this point, we're talking about a plane designed to cruise at 400 MPH lifting off at some humanly attainable highway speed [granted this could be 100 MPH or more] and controlling itself perfectly at the lowest speed it can fly, for at least a second or two [long enough to gain some speed so that it can safely pull up into a climbing attitude]. Everything about this couple of seconds is critical -- the slightest excess in nose-up attitude will stall it; all the control surfaces are at minimum effectiveness, so responses will be sluggish; the height above the roadway will provide mere feet in which to gain any speed needed to recover. This exact scenario has cost untold lives in normal category light aircraft, and this situation for a truly high performance craft is far worse than for any single- engine plane that gets off the ground at 50 or 60 MPH! Is it possible to create control software that will handle the takeoff so well that the potential problem never develops? Well, of course, you can argue that anything is possible. What I know is, IF the slightest thing goes wrong in those critical first seconds, you've lost it, and you won't have the space and time needed to get it back. If you're lucky when that happens, your missile will be out in the clear in front of your truck before it splats; but, since we know it's struggling along at minimum speed, there's about as much chance of this as of winning the Irish Sweepstakes. As an old pilot who remembers what these stalls are like [and how easy it is to get into them], I'd conservatively give this launch method about a 2 percent chance of success. [And, I'll hope that Bruce has his fire retardant suit and helmet on and in good condition, when he's speeding down the highway on his launching run in that pickup!] [There is a link to an excellent V-1 history site, with several good V-1 launch photos, at the bottom of this article.] Just because you can launch it doesn’t mean you can pilot it Ask any R/C plane builder how often a new plane handles perfectly the first time you take it out after building: never. A new plane, even one kit-built and supposedly identical to a thousand others, always has new quirks of its own. Usually, these are minor issues of getting your plane 'in trim' [hard to define, but one way to say it is, the ship will fly itself straight and level without pilot intervention]. Sometimes, though, major problems will be uncovered that need correction, and on certain rare occasions, catastrophe strikes. The point is, you have to fly the plane under the remote control of an experienced human pilot to have any chance of knowing whether all the systems work as planned. No modeler can ever assume that everything will be perfect the first time out. For our backyard terrorist, this means test flying a large, fast, noisy aircraft without giving away the game. Oh, and one more thing: He'll need to be, or have the help of, an experienced high-performance R/C pilot to do this testing successfully [i.e. with the airplane still in one piece afterward]. Definitely not for novice flyers. The fact that good control will need to be established immediately after a vehicle- assisted launch at marginal flight speed only adds to the chance of catastrophic failure. It is entirely likely that multiple such test flights will be needed to know that everything works reliably, all achieved without either public awareness or private disaster. Just because you can pilot it doesn’t mean a robotic system can pilot it to the intended target point From a purely technical standpoint, this may be the most controversial part of the entire plan. I have no idea what Bruce's software development skills are, but bringing this kind of guidance and control system together would be a major challenge for even an expert developer. There is a serious question as to whether any over-the-counter GPS systems really have the necessary resolution for such accurate targeting, and apparently, device position is only updated once per second - good enough for a hike through a national forest or even for a sailing trip around the world, but pretty difficult to handle properly in a device moving at almost 600 ft [about 180 m] per second! Guidance to a target is not just a matter of monitoring where you are at a given moment. You need to process that information, along with some number of previous positions [so you know your actual direction!] and work this into a course correction. The closer you get to the target point, the more rapid and accurate such corrections will need to be [although you hope these will be smaller and smaller corrections as you go]. Guidance to a precise target point is not simple; even the military can't guarantee 100 percent accuracy or reliability, and they've spent millions trying to perfect precision weapons guidance technique. This is just one more example of something that you'll never have working properly on your first attempt at it. Admittedly, your initial testing could be done with small models [even prop-driven ones] to wring out the most basic problems. Eventually, though, you have to tie guidance in with the actual control systems in your missile and make sure it all works as planned. Remember also that this is a three-dimensional guidance problem, not just making sure you follow a path on a map. You want to take advantage of 'tree top level' operation, but that means knowing the terrain along the route and being able to program in the altitudes you need everywhere along the path. At the end of your flight, you want a smooth, fast descent into the target point [for most kinds of potential targets, anyway]. Man-made obstacles are a considerable hazard, and require constant updating of stored data; in the rural Midwestern US, where I live, a new cell phone tower 50 or 60 feet high seems to spring up out of nowhere every two or three weeks! This is a much too advanced topic for me to begin to appreciate fully, but seems like a major problem area in the overall plan. And again, we encounter the same recurring problem: there has to be some practical testing in the final stages, without arousing public interest. And, I'm not sure that this aspect of the project will really ever be adequately proven [even by completing a successful test!] using a path laid out in some sort of 'safe' test flight range, since most meaningful targets for a real terrorist wouldn't exist completely isolated, out in the middle of nowhere. (breathing a sigh of relief) - Conclusion to Part II – I hope to have shown some of the problems our would-be pulsejet missile terrorist will have to deal with in getting his evil goal accomplished. The more I think about this, the less likely it seems to me to come to reality as a one-man job; there's just an awful lot to this, and I've just covered the basics - people working in these technical specialties could point out a lot more pitfalls than this [and correct a lot of things I've said in blissful ignorance], I'm sure. This doesn't necessarily mean that a man with the technical skills of Bruce Simpson can't bring the project off; BUT, it does cast doubt on the reality of the threat as perceived by the general public. I’ll try to zero in better on the real nature of the threat in Part III. And in Part IV, I’ll wrap up by talking about how I think the hobby pulsejet community can relate in a meaningful, educational and non-threatening way to the rest of the world. Bruce Simpson’s ‘Construction Diary’ page is at: An excellent site on the German V-1 missile program, with an impressive gallery of historic photographs, can be found at: -- End of Article Part II –- ____________________________________________________________ Larry Cottrill is Editor of jetZILLA online magazine and Director of Product Development and CEO of Cottrill Cyclodyne Corporation of Mingo, Iowa, USA - striving to create the world's smallest, safest and most practical hobby jet engines. You may contact me concerning this article at: This article first appeared in jetZILLA ezine, Special Edition, June 30, 2003. Permission is hereby granted by the author and publisher to freely distribute this article, so long as you agree to use the article in its entirety, without alterations or additions, including this resource box. Do you write articles? Click Here To Learn How You Can MASSIVELY BOOST Your Exposure for FREE! The author is a member of the free Writer's Viral Syndicator and sponsored by ad-CLiX Traffic Exchange Network ____________________________________________________________ Don’t miss the remaining articles in this series! Simply subscribe [at no cost] to jetZILLA online magazine – just send a BLANK email: Be sure to do this from the system where you want to receive each email edition of jetZILLA. You should receive a validation email immediately.


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Rebuttal to 'Can the Guy Next Door Build 
   His Own Cruise Missile?  YES! [Part II]'
by Bruce Simpson

    Bruce Simpson with his PJ100 engine prototype - photo used by permission
  Photo by Bruce Simpson - used by permission
Bruce Simpson is an experimenter who lives and works in New Zealand. He has been heavily involved in pulsejet design and experimentation for about the last three years. _____________________________________________________ Note from the Editor: Once I had finished writing Part II, I again notified Bruce Simpson and requested his comments. He promptly offered this excellent rebuttal to many of the specific points I argued, for publication. Note that one error I [and some others] have made is the assumption that Bruce is trying to prove that this could be done by a 'lone wolf' working in isolation. Looking back at Bruce's site, I see that this is not specifically claimed as an objective. This inevitably modifies some of my arguments, of course. And, not being an active R/C hobbyist myself, I was blissfully unaware of the FMA Co-Pilot device he describes, which apparently makes in-flight trim a 'shoo-in'. Just another example of the mind-boggling variety of low-cost, easy-to-use technology now available, right off the shelf. Anyway, here it is from the man himself - you be the judge. - Larry Cottrill, Editor _____________________________________________________ To the Editor - Larry, Firstly, you make the assumption that an LCCM would be built and deployed by a solo terrorist -- but this is far from reality. Recent history has shown that terror attacks have almost always been perpetrated by groups of terrorists (sometimes, as in the 9/11 attacks, quite large ones). Secondly, you point out that a lot of different skills are required, and that is true. However, terrorists have shown themselves capable of recruiting or learning quite a diverse range of skills when necessary. For example -- how many people could successfully pilot a wide-body passenger jet and successfully hit an object as narrow as the WTC towers or as low to the ground as the Pentagon? Remember that many terror groups are well funded and have plenty of time at their disposal -- and the world is filled with information and places of learning. You talk about the difficulties in designing, scaling, building and testing a pulsejet. Yes, it's more difficult than many people imagine but in the case of a well funded/organized terror group, much of this design and testing work could be done in a friendly country. I'm sure there are plenty of places in Syria or Lybia where you could do this without attracting any unwanted attention. Once the designs were completed, tested, tuned and debugged, it would be a simple matter to transfer the plans to the target country via encrypted email, the postal service or other means. The builder would then simply need to construct the engine to the supplied plans with a modest degree of accuracy. Once correctly designed, a pulsejet engine can be built with a high level of confidence that it will work -- I don't recall the Germans having to "tune" or play with the individual Argus engines before they fired up each V1. As for the amount of tooling and specialist skills required, I think you underestimate the number of home-workshops equipped with mills, TIG welders and the other tools needed to build good pulsejet engines. The level of skill required isn't that great either. Rolling and welding sheetmetal is an easily acquired skill (if I can do it, anyone can), and there are plenty of night-classes that will teach basic machine skills to the level needed for such an undertaking. As far as the airframe design is concerned, you are right that scaling up a traditional model airplane design would not work. These craft are generally quite small and designed to fly at speeds which are generally less than 1/5th that of the LCCM. However, basic aerodynamics is not rocket science and there is plenty of reference material and prior art around that allow anyone with modest math skills to design a craft with a form suitable for carrying the intended payload at the desired speed. A particularly lazy terrorist could simply copy, and scale down slightly, the V1 airframe if they wanted to. In regards to construction, you make composite technology sound complex and difficult -- something it is not. There are thousands of home-built light aircraft and model airplanes being flown every weekend which have been built from composite materials. Many of these were designed by their owners who have neither degrees in aerodynamics nor materials engineering. Certainly there are some basic rules and skills that must be used when working with composites, but these are easily acquired. In fact, in many ways composite construction is a lot simpler and more reliable than the traditional techniques of ribs, spars, bulkheads, longerons, stringers, and a tensioned skin -- it's certainly a lot quicker. You talk also about the complex issue of flight control and aircraft trim. Yes, these are the most difficult problems to be addressed but (un)fortunately, modern technology comes to the rescue again. Originally I had contemplated using a complex system of gyros and accelerometers to cope with short-term course deviation and fundamental stability. However, a little device sold as the FMA Co-Pilot tackles the stability and trim problem in a very elegant manner. The manufacturer's product sheet can be found here: and the fact that it will even stabilize a craft as inherently difficult to fly as a model helicopter is powerful testament to its capabilities. Independent reviews can be found at: This device significantly simplifies the guidance system by taking care of trim and stability, allowing the targeting system to concentrate its efforts solely on heading and altitude. It also significantly reduces the effect of a badly trimmed craft, since the Co-Pilot will automatically apply the required roll and pitch corrections to ensure straight and level flight right out of the launch. And speaking of launch ... As you point out, and as I've already acknowledged on my website, the biggest challenge is that of the launch. At the moment of launch the craft is at its heaviest and flying at its slowest. This has mandated the design of a craft with very low induced drag at low airspeeds and a very low stall speed. These characteristics are not always compatible with low form drag and a high top speed so compromises have had to be made. Those compromises and how they've been designed-around will be documented on the website, but it's not an impossible feat. The Concorde has a minimum/maximum speed range of over 7:1, the LCCM only needs a range of 4.5:1. Compared to the V1, the LCCM has the advantage of a launch- time power to weight ratio of 0.7, versus the V1's 0.25, and a much lower wing-loading. The launcher for the LCCM consists of a 3m [approx. 10 ft] long catapult-equipped launch ramp mounted atop a pickup. If the pickup can reach 80mph and the catapult can add an additional 30mph then the craft will be released at well above its designed minimum flying speed and in a good aerodynamic position to accelerate quickly to its optimum cruising speed. In respect to engine reliability, fuel feeds, etc -- I think the fact that so many V1's made it across the English Channel is evidence that this is not the problem you feel it might be. Sure, a percentage of LCCMs launched will fail for whatever reasons: bad launch, faulty guidance, engine failure, etc -- but the same has been proven true of the US$1M+ Raytheon Tommahawks -- as witnessed in both Gulf Wars and the attacks on Afghanistan. So in summary: Don't make the mistake of assuming terrorists operate alone -- history has shown this to be the wrong conclusion. Don't forget that R&D can be done in a "friendly nation" and the results of that work (in the form of plans) can be delivered electronically to any point on the globe. Be aware that much of the "off the shelf" technology is actually extremely capable and effective. It's conversion for use in an LCCM, while not trivial, is not that difficult either. Don't underestimate the resourcefulness of those who are driven by fanaticism or ideology. They can often do the impossible, especially if it's easier than you might think. As I hope my response has shown, none of the obstacles you've described are insurmountable, or even particularly difficult to overcome. I would not have embarked on this project unless I (and a surprising number of serving and ex-military respondents) didn't think the risk of a terrorist LCCM wasn't very real and viable. Fortunately, thanks to all the sweat and hard work I'm expending right now, we'll have a chance to find out who's right sometime in the pretty near future. - Bruce Simpson June 27, 2003

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  Larry Cottrill with Reynstodyne(TM) Shark(TM) engine prototype after early test firing - Photo Copyright 2003 Cottrill Cyclodyne Corporation
  Larry Cottrill with ReynstodyneTM SharkTM engine prototype after early test firing
Photo Copyright 2003 Cottrill Cyclodyne Corporation
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