I read about the Composite Crew Module and it looks like they used aluminum honeycomb between two layers of composite. Why didn't they just make the whole thing out of composite? Why is carbon composite not used more for the construction of space station modules and spacecrafts? https://ntrs.nasa.gov/api/citations/20120009267/downloads/20120009267.pdf

  • $\begingroup$ The aluminum honeycomb is used for over 100 years now. The method to build them may be used with bendable aluminum foil but not with a brittle composite of fibers and epoxy. $\endgroup$
    – Uwe
    Commented Dec 29, 2023 at 18:41
  • $\begingroup$ Your title does not contain a question, but there are two different questions in the body. Please edit to clarify the question. If there is more than one, please post separate questions. $\endgroup$
    – Woody
    Commented Dec 29, 2023 at 22:59
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    $\begingroup$ What do you mean "the whole thing out of composite"? Sandwiching a honeycomb between carbon layers is classic composite construction. Did you mean to ask why did they not use a solid slab of fabric and resin? Or do you mean why did they not also make the honeycomb out of fabric and resin? $\endgroup$
    – DKNguyen
    Commented Dec 29, 2023 at 23:36
  • $\begingroup$ @DKNguyen Yes, I meant why isn't it all one layer of composite. $\endgroup$
    – Saturn V
    Commented Dec 30, 2023 at 7:34
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    $\begingroup$ @SaturnV Because it would be hella heavy. About the same as if it were a solid sheet of glass. Composites sandwiches are used because they distribute the material for best strength-to-weight. A solid sheet is a lot of dead weight that isn't contributing to strength or stiffness. $\endgroup$
    – DKNguyen
    Commented Dec 30, 2023 at 17:21

1 Answer 1



Why didn't they just make the whole thing out of composite?

Answer: to save structural mass. Cored construction (with honeycomb or other materials) can minimize mass, especially compared with solid construction.

Stress is concentrated on the surface of a structure subjected to bending. One side experiences compression, the other side experiences tension and there is a neutral axis which experiences neither.

These surface forces are inversely proportional to thickness of the structure, so a thicker beam experiences less stress for a given load. But the penalty (for a solid beam) is increased mass along the neutral axis; mass which is not carrying load and therefor not contributing to strength.

One solution is a hollow beam, like a pipe or channel. This is why space frames (like bicycle frames) are constructed of hollow tubes.

enter image description here

However, there are also shear stresses in the beam at right angles to the axis.

enter image description here


These shear forces are responsible for failure by buckling. The shear forces are very small compared to the tension and compression forces in the wall of the tube, so lightweight materials such as honeycomb or foam are adequate to prevent buckling. The honeycomb core is ideally shaped to take these shear forces.

enter image description here


No material is “best” for all design applications. This includes Carbon Fiber Reinforced Plastic (CFRP). All materials have their strengths and weaknesses (pun intended). The “super power” of CFRP is high rigidity for its weight (specific modulus)

The stiffness-to-weight ratio of CFRP is 60% higher than steel or 6061 Aluminum. https://dragonplate.com/what-is-carbon-fiber

However, the compressive strength of CFRP is significantly lower than steel, as demonstrated by a recent submarine accident. Also, it does not yield before fracture so it tends to fail catastrophically, like glass.

CFRP also has a limited temperature tolerance (determined by the matrix material) compared to most metals.

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    $\begingroup$ highly flexible and durable alpine skis have been using this tech (composite/honeycomb/composite) for a long time (60 years?) It is well known how to tailor a custom combination of materials and adhesives for just about any purpose. $\endgroup$
    – BradV
    Commented Dec 29, 2023 at 19:29
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    $\begingroup$ I think this answer is a little too much "why honeycomb" and not enough "why not pure composite" but it is good and I have upvoted $\endgroup$
    – Erin Anne
    Commented Dec 29, 2023 at 21:16
  • $\begingroup$ @ErinAnne ... I agree with you. Published materials data on CFRP concentrate mostly on its strengths (tensile, that is). Its hard to find good data in CFRP's weakness (compressive strength) because it is so dependent on the matrix, as opposed to the CF. $\endgroup$
    – Woody
    Commented Dec 29, 2023 at 21:34
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    $\begingroup$ @ErinAnne ... as well, there is very little published information on non-honeycomb core materials, I'm currently building extruded polystyrene foam core CFRP motorcycle components. It is truly spooky how light and rigid the structures are. Unlike honeycomb, which is planar, the foam core can be a complex curve which really takes advantage of strength-by-shape of the finished structure. $\endgroup$
    – Woody
    Commented Dec 29, 2023 at 21:41
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    $\begingroup$ @Woody ah I thought honeycomb was instead easily "drapable?" along unruled or complex curvatures, in particular aluminium honeycombs due to their very plastic behaviour $\endgroup$
    – user19132
    Commented Dec 30, 2023 at 19:13

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