December 2, 2017 Hermes Ground Test

This was a successful test that used 0.2 grams of black powder in the piston and 3x 6-32 shear pins. Webbing was used to connect the sections and the rocket contained a drogue and main parachute. Webbing lengths were not representative but were merely a mock-up of the planned webbing lengths (excluding a load path for the tender descender, which was not included in this test). This test did not implement any form of flight avionics. Following the test, we also performed a tug-test on the parachute, the results of which are summarized below.

Test Case	Approximate Maximum Force (lbs)
Fish scale attached to drogue shroud-line quicklink; pulled main parachute out of cup.	4.52
Held the top of the deployment bag in place; attached the fish-scale to the main parachute's shroud-line quicklink; pulled main chute out of deployment bag.	9.02

Black Powder Calculations

This ground test used 0.2 grams of black powder, which was calculated in the following manner:

Step 1: Determine volume in the piston prior to rod extension

For this test, we assumed that the Piston Cup would be seated perfectly on the coupler. Given known integration challenges, this is not a perfect assumption. We also assumed that, in order to do this, we would not have to extend the rod of the piston to have an initial displacement. This is not a correct assumption, as the rod certainly requires an initial displacement. We will be working to qualify the impact of this initial displacement and will update this page as necessary.

Flight Piston Dimensions: An estimate for piston volume prior to rod extension is 0.5599π [1.08 in for depth and (1.44/2)²π = 0.5184π in² for area]

Step 2: Determine necessary force to break shear pins

This test used 3x 6-32 shear pins, which take approximately 60 pounds each to break according to a rocketmaterials.org experiment. This totals to 180 lb necessary to separate the rocket. A 2x factor of safety was applied for a desired 360 lb.

Step 3: Determine PSI inside the piston necessary to achieve required force

360 lb-f = x * 0.5184π in²

x = 221 psi

Step 4: Determine PSI inside the piston necessary to achieve required force

Using this black powder calculator and the previous quantities, we determined that 0.2 grams of black powder would be necessary to achieve 360 lb-f.

Step : Evaluate Assumptions

Additional assumptions (besides those already mentioned are):

Admittedly this math is sensitive to the volume, which we only estimated.
The BP calculator assumes an ideal gas, which isn't true.

Media

Video of test: Ground Test 12-2.MOV

During the previous ground test, the large amount of force applied (estimated at 3500 pounds), damaged the fin can coupler's bolt holes, pushing the coupler further into the fin can. This made mating the fin can with the mission package difficult during integration.