Primary Failure Mode


As with all Recovery operations, the tangling of various lines (webbing, shroud lines, etc.) is a potential cause of failure. To date, the primary tangling-based failure mode that has been identified for Hermes' Recovery system is shown to the left. In this scenario, prior to main parachute deployment, the webbing from the fin can wraps itself around and tangles with the webbing connecting the drogue parachute to the mission package. Three potential consequences of this failure mode are depicted and described below.

Note: these images are not to scale. The lengths of lines and components will be very different for the flight vehicle (learn more about our webbing lengths here). Furthermore, during descent under drogue, the relative forces on each falling body may be similar enough that all parts (mission package, drogue, and fin can) are tumbling together in a more chaotic arrangement than what is featured here.

 

Consequence #1: Load Path Failure

The first potential consequence is that the tangling of lines will be tight enough such that the load path from the drogue parachute travels down the fin can webbing. Notably this would occur if two conditions were satisfied:

  1. The fin can path length from the knot to the mission package is shorter than the tender descender path length.
  2. The knot is tight enough such that load from the drogue could be offloaded to the fin can webbing path rather than the tender descender webbing path.

This failure mode renders the tender descender useless since, after firing, the drag of the drogue will not pull the deployment bag off of the main parachute. This failure mode is less likely than the following two modes because two conditions must be satisfied.

Consequence #2: Main Escape Failure

In the second potential failure mode, the tender descender fires and load from the drogue (appropriately) pulls the deployment bag off of the main parachute. In this failure mode however, the knot prevents the deployment bag and main parachute from fully exiting the vehicle. This is perhaps the most concerning failure mode because it is more likely than Failure Mode #1, and unlike Failure Mode #2, the main parachute cannot achieve even partial inflation.

Consequence #3: Main Performance Failure

In the final failure mode, the tangle occurs far enough along the length of fin can webbing that the main parachute is able to escape the vehicle and inflate. Unfortunately, the weight of the fin can pulling on the knot above the main parachute will likely hinder its performance and inflation.

The behavior of this failure mode is dependent on the following factors:

  1. Relative forces (weight and drag) on the main parachute, mission package, and fin can: this determines how much the fin can will hinder the performance of the main parachute. For example, let's examine the limiting case in which the fin can has the same sum of forces (drag and weight combined) as the main parachute/mission package assembly. In this scenario, the fin can will not affect the performance of the main parachute because they will be falling at the same rate at all times.
  2. Position of knot along the piece of fin can webbing: if the knot does not occur far enough along the piece of fin can webbing (relative to the mission package), the main parachute may fully enter the airstream, but airflow could be very poor in the immediate wake of the mission package. This is, of course, highly dependent on the dynamics of the falling mission package, as it will not necessarily be most stable in the illustrated position.
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