Introduction
The Hermes II soft goods have to be stronger than the Hermes I soft goods because the rocket is heavier so the loads are higher. The drogue system was relatively unaffected, while the main system had to be completely redesigned. Design work was done on the Stage One Parameters Sheet and on this page.
The scope of this page is limited to the parachutes, lines, and risers. Deployment bags, links, and swivels are on separate pages
Assumptions and Design Goals
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Drogue
Parachute
Lines
Lines Confluence (swivel, ql, etc.)
Riser
Y-Bridle
TD loops
The Hermes II drogue system is designed for deployment at apogee +/- 10 seconds. It is designed to operate at a temperature of 300 F because it could contact the side of the motor case. The current design temperature is arbitrary; it will be updated after a static fire. The deployment conditions are given by sims. Two edge cases were considered (A) a light, powerful rocket launched pointing straight up and (B) a heavy, weak rocket launched at an angle of 10 degrees. Case A gives the maximum mach number at deployment, which is .8. We begin to worry about trans and supersonic effects at or below this number. Case B give the maximum dynamic pressure at deployment. The Hermes II drogue deployment force is lower than the Hermes I deployment force because the rocket is going higher. At higher altitudes the air is thinner, so at the same speed the dynamic pressure is lower.
- Make inventory tags.
- Measure existing drogue lines. Put length in inventory and in parameters. Put other details here. Put a diagram of the markings on the line here.
- Cut one 500 lb line. Have everyone practice tying double figure 8. Attach line to drogue.
- Create a deprecated Hermes soft goods box. Put the old line in the box.
- Sign off on the drogue lines on the parameters sheet.
- Measure existing drogue riser. Put length in parameters. Put other details here. Put a diagram of the markings on the line here.
- Cut one 2500 lb line. Demonstrate finger trapping and sewing. Write procedure.
- Determine the material required for the TD loops. Write procedure.
- Have everyone manufacture 2 TD loops. Store in the orange box.
DESIGN TODO
Main
The Hermes II main system is designed for deployment at 2000 ft above the launch site under a range of reasonable masses. It is designed to operate at a temperature of 150 F because it the inside of the rocket could reach that sitting on the pad.
Thoughts:
There are four inputs to the design process: parachute parameters, rocket and environmental parameters, the force correction factor, and design and safety factors.
The parachute parameters are derived from past flights and from the scientific literature. There are difficult to measure and our conditions do not quite match those in the literature. The error in the measurement Cd stems from only having a single axis velocity measurement, the drag of the rocket body, and from difficulty in measuring air density. These are confounded by data loss, poor recording of ambient conditions at launch, and that Cd is generally recorded without context (data sources, parachute diameters, and ambient conditions are not listed.
The rocket and environmental parameters encompass the estimated mass and the predicted air density.
The force correction factor (Cx) is coefficient in the deployment force calculations that is interpreted by the designer from tables in the literature.
The design factors encompass the joint efficiencies, abraision losses, etc. and the safety factors. and My intuition tells me that the design is too conservative. I feel this way because I am biased from anchored on the original design and the swivel suggests that the measured force was roughly 50% of the predicted force.
Fix safety factors, then determine Cx. We need to do it in that order so that we are not fi
I feel that the main design is too conservative, but I don't want to be rash and under design. Some issues with the current design are the drogue Cd, temperatures, mass range, safety factors, Cx, abrasion, and UV performance.
Drogue Cd
Should I use the measured value at the nominal and then use the standard deviation of the wind tunnel test off of that? Should I use any variation at all? I'm unsure that the measured value is correct. I should contact MJ confirm the Cd. I should include a section in the post-flight procedures to measure the drogue Cd.
Temperatures
What is are reasonable temperatures to design the main, drogue, and motor section components at?
Mass Range
The mass budget gives and uncertainty of 13 pounds. First, can it be trusted? Second, will it go up or only down? Do we expect Hermes III to be lighter or heavier? How should uncertainty be distributed between the sections of the rocket?
Safety Factors
Should there be separate asymmetry, dynamic, and safety factors? Should the lines factor be lower than the other factors? What is an appropriate safety factor? Who can we contact to learn about this?
Cx
I used the Cx table in Potvin, 2009. Is this the right table? What do other tables say? Can we ask Potvin or other experts?
Abrasion
Is a abrasion factor necessary? Should it only be included on some materials or on flown goods?
UV
We use a variety of UV sensitive materials: Kevlar, Technora, Polypropylene, and Polyethylene. How well have these been stored? Are there methods that we can use to test them? Do we need to test them? The charts for UV exposure look terrifying, are they useful?
Motorsection