The Hermes Pseudo Forward Closure goes at the top of the extension tube to prevent the drogue falling down, and to attach the webbing via a u-bolt. It must be able to withstand the "snatch" force of the booster separating from the mission package, as well as potentially any loads transferred from the mission package to the booster section during drogue or main parachute opening.

The Hermes Pseudo Forward Closure is designed to bring a smile to the face of tired rocketeers as they prep for flight. Eight bolt holes affix the closure to the extension tube. Moderate mass saving cuts decrease the weight.


Simulations

I ran two preliminary simulations on the forward closure using the eight bolt holes as fixed geometries and sharing total upward loads between the two "eyes."

First, I ran a 2250lb simulation, as this is the breaking strength of the fin can to mission package webbing as of 4/16/2018. Results showed that we would exceed the yield strength, but that total deformation would be small:

 

Then I ran a 1000lb simulation. This is still an over estimate, as preliminary analysis showed that the mission package to fin can webbing would see a resulting force of < 200 lbs. Results still showed that we would exceed the yield strength, but again, total deformation would be small:

There are a few reasons why I'm not actually worried about this, even though all results show that we will exceed yield strength:

  1. Stress is clearly concentrated around the bolt holes. Solidworks simulations historically exaggerate stress in these areas.
  2. As I previously mentioned, my chosen forces are overestimates.
  3. When the U-bolt is attached, it isn't going to be threaded into the smiley face's "eyes," which is what my simulation currently implies. Instead, it will be affixed a long, bar on the other side and then nuts. This bar will distribute the load better than my simulations show.

Bolt Strength:

Using Size 4-40 bolts

There are 8 bolt holes. Assuming that they are size 4-40, the distance from the bolt hole center to the edge is > 1.5 times that of the bolt diameter (~1.67).

Bolt shear is estimated as 0.6*60,000psi = 36,000psi per bolt. Then, estimating bolt area using the minor diameter yields pi*(0.0805/2)^2 = 0.00509 in^2. Then, estimated shear force is ~183 lbs per bolt. Given that there are 8 bolt holes, this gives an estimated shear force of 1465.8 pounds.

Using Size 6-32 bolts

Assuming that they are size 6-32, the distance from the bolt hole center to the edge is < 1.5 times that of the bolt diameter (~1.36).*

The new minor diameter yields an area of pi*(0.0989/2)^2 = 0.00768 in^2. Then, estimated shear force is ~277 lbs per bolt. Given that there are 8 bolt holes, this gives an estimated shear force of 2212.5 pounds, which is good, given that the webbing will break at around this force, anyways.

*Note that if we increase thickness of the closure from 0.375" to 0.5", this number increases to 1.8.

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