Why are ProtoStar vanes made of stainless steel (and not aluminum, or carbon fiber)?
Aluminum has a superior strength-to-weight ratio compared to steels, but for a spider a high strength-to-volume ratio is desired. In order to keep the vanes thin, it’s necessary to fit as much strength into as small of a volume as possible. A spider made of aluminum would need to be about 2.5 times as thick as a steel vane to have equivalent rigidity. (The modulus of elasticity of steel is three times aluminum’s.) A similar thing is true for carbon fiber materials, but to a lesser degree. The modulus of common carbon fiber materials is about 24-28 Mpsi, whereas steels are around 30 Mpsi. This means the vane would have to be slightly thicker to have the equivalent rigidity of a steel vane. There is also a problem processing carbon composites that are as thin as our vanes (~ 0.5 mm). This reason alone usually forces a carbon composite vane to be 1.0 to 1.5 mm in thickness.
