A DIY Cruise Missile
Last Updated: 13 May 2003

Phase 2: Airframe Design

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Designing a suitable airframe for the LCCM project has necessitated a sea of compromises.

Unlike an expensive Raytheon Tomahawk, the DIY LCCM won't have a solid-rocket booster to launch it into the air and get it rapidly up to the optimum flying speed of several hundred miles per hour. Instead, it will be catapulted from the back of a speeding pickup truck so will have to start flying at a mere 70mph or so.

What's worse, at launch, the craft will be carrying a full load of fuel, adding significantly to the amount of lift needed to keep it in the air.

Of course it would be nice if we could just use our own solid-rocket booster but that would significantly complicate the whole project. The design of reliable, effective solid rocket boosters is not a trivial affair and often requires the use of materials that aren't readily available "off the shelf".

So, we're stuck with a relatively low-speed launch mechanism and this makes the task of designing a suitable airframe a whole lot more difficult. That's because, due to the laws of aerodynamics, there are some significant differences in design between a craft designed to fly efficiently at 70mph and one designed to do so at 400mph.

F104A Starfighter Aircraft intended for high-speed flight are designed to create the absolute minimum disturbance (drag) as they slice through the air. This design objective generally manifests itself as a long slender body and relatively small broad yet thin tapered wings that have just enough area to provide the lift needed to keep the craft airborne at the optimum flying speed.

By comparison, an aircraft designed to fly most efficiently at low speeds tends to have larger, thicker and more cambered wings for any given weight in order to generate plenty of lift at those lower speeds.

Just compare a high-speed fighter plane such as the rather extreme F-104A Starfighter to something like a modern microlight designed for much slower flight. Note the remarkable similarities between the F-104A and the world's first cruise missile, the V-1 Flying Bomb.

The problem with big, thick, cambered wings is that although they work well at low speeds, they generate so much drag and excess lift as velocity increases, that they make it very difficult to obtain the high speeds we're seeking with our LCCM.

As you can see - this isn't going to be simple and no matter how much thought is applied, our LCCM is going to be a set of aerodynamic compromises.

If you're unfamiliar with the basics of aerodynamics then this may be a bit of a learning exercise.

You'll see why when we analyze the goals:

  1. a very low drag to enable maximum flight speed and range
  2. a comparatively low stall-speed to allow the use of a simple launcher
  3. a good level of inherent stability across the entire flight envelope
  4. a form that lends itself to simple construction techniques
  5. a form that allows the use of a pulsejet engine which radiates significant heat

Let's take an overview-type look at the factors that will affect our design choices. Note that in order to make this material more understandable by those without much prior knowledge, I've (over)simplified things a little, used analogies, and avoided the use of complex math wherever possible.

1. Drag
Drag is one of the four key forces that act on any aircraft in flight (the others are gravity, lift and thrust). Read more....

2. Low Stall Speed
Since the DIY LCCM won't have a rocket-booster or be dropped from an airplane already moving at several hundred miles per hour, it will have to tollerate being launched at less than one quarter it's designed flying speed. More to be uploaded shortly

3. Inherent Stability
Without some degree of inherent stability, our LCCM would be solely reliant on the guidance system to correct every little change in attitude or direction that might be caused by turbulence or other factors. To simplify the guidance system, the craft must be designed so that the aerodynamic layout provides a basic level of inbuilt stability. More to be uploaded shortly

4. Simplicity Of Construction
In accordance with the projects "DIY" title, the construction techniques used for the airframe must be relatively simple and not reliant on complex or expensive tools and materials. This goal is met by using modern composite materials based on foam and fiberglass. More to be uploaded shortly

4. Form Considerations
Bearing in mind all the other design factors previously discussed, the airframe must be designed to take into account several other factors -- not the least of which is the very high operating temperature of a pulsejet engine. More to be uploaded shortly

Final Decisions
Accepting that the LCCM's aerodynamic design will be a mix of compromises, the following choices were made which, I believe, are the most sensible balance of factors:

  1. While attention will be paid to the requirements of low-speed, low-drag flight, more emphasis will be given to the dictates of the high-speed cruise-mode. As you will see when the issue of the engine design is covered, a simple but effective method of temporarily doubling the engine's thrust at low speeds will be used to help overcome the high drag intrinsic to the slow-speed flight of such a craft.

  2. Obvious enhancements that represent very little compromise (such as wingtip fins) will be used to reduce drag.

  3. Wing-loading will be kept as low as possible, contingent on maintaining structural integrity and minimal form-drag.

  4. A low-drag laminar flow airfoil section will be used.
CAD drawings of the resulting airframe design will be posted in the next few days, along with the calculations used to determine key aspects of the design.

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