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Ground testing has shown that the cup does not buckle under normal piston operating loads, even those as high as 3500 lbs (an order of magnitude greater than normal operating loads) as we predict we saw in Ground Test November 10, 2017.

v2.0: Hermes Dimensions

In addition to a change in outer diameter to accommodate the switch from a 6" ID to 6" OD rocket body, v2.0 of the Hermes cup involves making mass saving cuts.

To quickly determine structural integrity, we can perform a buckling calculation, assuming long vertical cuts are made (or a similar, equivalent cut):

Image Added

Given some amount of aluminum from the cup on both ends of the column, this scenario is best approximated as a double-wall-mount buckling problem. Only first order buckling is possible, because the mission package tube constrains the cup in one direction. Thus the critical buckling load is:

Mathinline
bodyP_{cr} = \frac{4 \pi ^2 E I}{L^2}

From Matweb, the Young's Modulus of aluminum is 68.9 GPa = 1.00e+7 psi. The area moment of inertia for a rectangle under buckling is

Mathinline
body\frac{bh^3}{12}
. Given a 0.1" cup thickness, b, we can solve for the minimum width of each column, b. We apply a 2x factor of safety on the piston load (360 lbs) and generalize the number of columns as nc. An additional factor of safety is applied when we set L = 14", the total height of the cup:

Mathinline
body\frac{720 lb}{n_{c}} = \frac{4*\pi ^2 *10^7 psi * b *(0.1 in)^3}{12*(14in)^2}

Mathinline
bodyb = \frac{4.3 in}{n_{c}}

So, for example, if 8 evenly-spaced columns are used, the minimum necessary width of each column is 0.54 inches.

Resources

NASA, Buckling of Thin-Walled Circular Cylinders