The Hermes Recovery deployment sequence is "lines first," which has several advantages, noted by Wolf in his Parachute Seminar.
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We will conduct research into the deployment sequence to mitigate possible failures, ensure a stable descent, and help inform our camera choices.
Chute Inflation
To inform our choice of camera, we need to gather information on chute deployment and inflation rates.
Drogue Parachute Inflation
The rate of drogue parachute inflation will depend on the airstream conditions and the parachute dimensions and its materials. To begin analysis, we examined the NASA TM X-1786 "Wind-Tunnel Investigation of Inflation of Disk-Gap-Band and Modified Ringsail Parachutes at Dynamic Pressures Between 0.24 and 7.07 Pounds Per Square Foot."
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Using these estimates, we generate the following plot of fill time as a function of velocity. To select a representative range of velocities, we examined the range of possible main-deployment conditions (using the chart featured in the Hermes Disk Gap Band Design page as a basis for our analysis). This analysis also made use of 1976 COESA Standard Atmospheric model, as calculated using the MATLAB function atmoscoesa.
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Then, we can generate the following graph of inflation times based on velocity:
Timing of Deployment
We currently do not know how to qualify exactly how long after apogee the drogue parachute will begin inflation. This may depend (to varying degrees) on the following things:
- Avionics system apogee detection response time
- Webbing lengths (example: if the drogue parachute is loosely held in the booster section, full line extension will draw it out)
- Deployment event energy (example: after a non-energetic deployment event, such as we saw in ground tests 1 and 2, the bodies may need more time to achieve sufficient distance from one another to pull the drogue parachute out)
The time after apogee that inflation occurs depends for two reasons. First, it will impact the inflation time, as seen the graph above. It is important to note that in this graph, the airstream velocity will be increasing during inflation (as the drag force may not yet be sufficient to fully counteract gravity), so you cannot simply calculate the inflation time by determining the airspeed at the moment inflation begins. It will be faster.
The second reason why timing matters is "the v2 law" which was found by skydiving research conducted by Potvin and Peek. This states that "the maximum deceleration sustained... is proportional to the square of the jumper's speed prior to slider descent." In simple terms, the faster Hermes is going prior to parachute inflation, the worse the deployment forces will be.
Stages of Parachute Inflation
- Line Snatch: occurs upon line extension right at the start of parachute inflation; forces during inflation outweigh the snatch force (Potvin and Peek)
- Note that this source relates to skydiving applications which include sliders that affect the deployment sequence. Thus, things may be different for rocketry.
Main Parachute Release
Our main parachute will be released at an altitude of approximately 2,000 ft. We will use two Tender Descenders for redundancy. There are two configuration options, series and parallel:
Parallel Configuration
There are many advantages to a parallel configuration. Notably:
- Saves a lot of space vertically in the cup, which in turn will reduce the likelihood that the main parachute's deployment bag is in the airstream prior to descent.
- Reduces mass (fewer quicklinks)
One disadvantage is that you have to be sure that the Kevlar line (or another type of line if you so choose) will not abrade.
Series Configuration
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Terminology
geometric porosity: the percent of the nominal canopy surface area that is removed due to vents and gaps
Resources
Potvin and Peek, Parachute Opening Shock Basics
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