Webbing lengths must be selected carefully to satisfy several constraints such that parachutes enter the airstream (and do not remain confined within the vehicle). For Project Hermes, the webbing constraints and resulting calculations are summarized below:
For the Hermes Pathfinder vehicle, the relevant dimensions are:
| Variable | Length (in inches) | Description |
|---|---|---|
| hmp | 51.3 | Total height of mission package |
| lmp | 17.8 | Distance from diaphragm to end of mission package |
| lfc | 4.75 | Depth of booster coupling section |
| hfc | 40.5 | Total height of booster including coupling section |
| RB | 27 | Height of deployment bag, includes the flap length |
| DM | 114 (9.5 ft) | Diameter of the main parachute |
| RM | 80.6 | Height of the main parachute (found using 0.707 aspect ratio): RM = DM*0.707 |
| SM | 126 | Length of shroud lines on the main parachute |
| DD | 34.4 (2.87 ft) | Diameter of the drogue parachute |
| RD | 24.3 | Height of the drogue parachute (found using 0.707 aspect ratio): RD = DD*0.707 |
| SD | 48 | Length of shroud lines on the drogue parachute |
Using these variables, a set of constraints was generated to ensure successful Recovery system deployment. The constraints are summarized in the table below:
| Constraint | Description |
|---|---|
| WA,H > WA,E,F,G + RD + SD + lfc | When lines between the mission package and fin can are fully extended, the drogue parachute must be within the airstream. |
| WA,E,F = lmp | Placing the junction between the tender descender (TD) and the main chute load paths at the edge of the mission package reduces the likelihood that the main parachute will enter the airstream prior to the designated release altitude of 2,000 ft. |
| WC,D > RB | The line connecting the top of the main chute canopy to the inside of the deployment bag must be long enough such that the main parachute is not stuck inside the deployment bag upon full line extension. |
| WA,B > lmp + ~4DM | As a temporary value, the main parachute must be at least 5 diameters away from the mission package when inflated. This constraint also ensures that it is outside of the mission package, assuming full line extension. Further analysis will be performed to refine this constraint. |
| WA,E,F,G - lmp > 5DD | This constraint reduces the impact of forebody wake on the drogue parachute during drogue descent. |
| WA,H > lmp + hmp + lfc | Assuming full line extension, the mission package and booster section should not hit one another during descent. This neglects the effect of drag on these sections and their respective masses. |
| WA,E,F < WA,B + SM + RM + WC,D,F | Prior to firing the tender descender, the load should go through the shorter (TD) path. |