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Extruded Rocket Structure


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Service pack 1 for the wound graphite stage over a P120.

Balsa doesn't insulate the tank heads so foam must do it, 10mm thick or less at oxygen (+16kg) and 15mm at hydrogen (+24kg).

The core between the tanks must be removed once the cylinder is wound. It can be molten, taken down... but a hole is necessary. I imagine the graphite fibres can by-pass the hole(s) when they're laid down and get a local reinforcement. Ask the experts.

I addressed only the azimutal contraction of the skins at cold. The axial one creates in the core a shear stress that decreases exponentially from the temperature transition over a distance (e*s*E/G)1/2 (thicknesses and moduli) and is small, about 300kPa if G=1GPa.

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  • 5 years later...

Trusses often consist of light tubes with welded stronger ends for assembling at the nodes. Usually, the ends are welded with the tubes by friction, where the tube rotates and the end is pressed strongly until they melt together. This leaves the full tube strength or nearly, even with materials unsuited to other processes like TIG. But the relative orientation of the ends is undefined.

If the assembly method at the nodes accepts any orientation, fine.

As a second possibility, the ends can have an axial symmetry when they're welded with the tubes, and be milled afterwards with the adequate relative orientation and length.

I propose a third possibility: build a special machine that welds by friction both ends simultaneously. The machine rotates the tube, holds both ends with the desired relative orientation, and presses both ends against the tube until the desired length is achieved.

The machine must hold both tube ends firmly to avoid torsion in the tube. It could use slotted cones like the ones that hold milling cutters.

As I didn't check if this is already done. The uses exceed the rocket structures I proposed.

Marc Schaefer, aka Enthalpy

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  • 4 weeks later...

Friction welding, as in the last lessage, saves raw material and keeps the full tube strength. Big series, as in architecture, justify the special machine. Smaller amounts, as for a launcher or a satellite, can more classically use the big outer or inner surface of a tube to exploit the full section's strength.


Top left on the sketch, smooth tube heads fit in or on the tube ends over enough length. Or the tubes can fit at the truss nodes directly, without tube heads. Low-temp filler exists to braze aluminium. A nickel layer on aluminium lets solder wet and adhere. Glue could be considered too.

Bottom left on the sketch, left and right threads too let adjust the distance and orientation of the heads or nodes. The rotation can be stopped by brazing, soldering or glueing, or with a lock-nut, which a metal wire can hold to the nut.

Right on the sketch, the tubes connect the nodes without heads, with threads as displayed or without. An isostatic truss and short overlaps help insert new tubes.

These processes assemble the hardest alloys, without the possible limits of friction welding. If wall buckling limits the compression strength, stiffening rings can help with little added mass and effort. This needs experiment, as theories fail. A bigger effort lets mill integral stiffeners in the wall, like an isogrid, or cut a truss from the wall, which a laser or water jet achieve too.

Marc Schaefer, aka Enthalpy

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