How to build an all Carbon composite airframe on an aluminum mandrel.
Rocket Airframe Design Background
A rocket airframe may experience various loads and stresses, but one of the most critical is column buckling. Column buckling is an issue whenever a relatively thin column experiences a vertical compression load. There is a tendency for a thin column to bow sideways which causes the bending stress to increase rapidly. This causes an overloaded column to fail suddenly and completely. In rockets, the airframe behaves as a column, compressed between the engine thrust and the dynamic air load on the nose. If the load exceeds the critical buckling stress, the rocket may appear to explode.
In an rocket airframe, the main variables to resist buckling are: reducing the airframe length, increasing the airframe diameter, increasing the wall thickness, and increasing the modulus (stiffness) of the airframe material. (There is one other method, used on many liquid fueled rockets, is to pressurize the airframe.) Short and fat airframes resist buckling but impair performance. Increasing the wall thickness adds considerable weight with only modest improvement in buckling resistance. The best method for improving buckling resistance is to select an airframe material with high specific modulus, that is, stiffness per pound. Paper, phenolic paper, and PVC all have low specific stiffness. Glass fiber laminates have a specific stiffness 5 to 10 times higher than PVC, depending on fiber orientation. Carbon fiber laminates have a specific stiffness 40 to 80 higher that PVC, again depending of fiber orientation.
Higher material strength is important is helpful in preventing damage from handling and recovery, but it is material modulus that is important to prevent airframe failure during launch.
Composite Airframe Construction Alternatives
Wrapping a paper or phenolic paper base tube with glass and epoxy or carbon fiber and epoxy is a common method for improving the strength and modulus of an airframe. The disadvantage of this approach is that it is structurally inefficient. The paper or phenolic paper mandrel shares almost no load; it is mostly parasitic weight. However, if a paper base tube is used, it may be soaked with water and removed, leaving a composite shell. But a paper base tube may cause trouble holding straightness and circularity in larger sizes, without additional support.
Commercial filament wound tubes and sock type reinforcements are commonly used, but these method share the common drawback of sub-optimal fiber orientation. For maximum bending performance, at least half of the fibers should be running lengthwise.