Last month, on June 8, 2015, NASA ran the second Low-Density Supersonic Decelerator (LDSD) flight test off the island of Kauai, Hawaii. The inflatable decelerator portion of the test was successful, but the supersonic parachute shredded as it opened, similarly as it did during the first flight test.
Do they know specifically why it shredded when it opened, and what are they doing to improve it for the next year's third LDSD flight test?
The third test of the Low-Density Supersonic Decelerator (LDSD) never occurred, the program was cut as part of a budget shortfall in 2016. However the paper "Reconstructed Parachute System Performance During the Second LDSD Supersonic Flight Dynamics Test" provides an in-depth analysis of the parachute system during the test including some great graphics like this:
Here is the relevant section of the paper discussing the cause of the parachute failure:
D. Assessment of Parachute Failure
After reviewing the data on the failure of the SSRS on SFDT-2, the LDSD project convened a meeting of experts in the parachute community to conduct a joint assessment of the parachute failure mechanism and its causes. The panel of experts and LDSD project engineers emerged with four leading hypotheses for the parachute failure in SFTD-2:
An asymmetric, uncontrolled deployment caused damage to the canopy prior to inflation. This damage was aggravated by inflation loads and propagated throughout the canopy, leading to failure.
The effects of fabric inertia and fluid inertia caused transient loading on the canopy during inflation beyond the levels predicted by pre-test analyses and beyond the capabilities of the canopy materials.
Transient loading due to pressure waves during the supersonic inflation resulted in regions of higher stress on the canopy. The stress in these regions exceeded the capabilities of the material and the margins allowed for in the design analysis, and led to failure.
The strength of the skeletal elements under high-rate, dynamic loading conditions was much lower than the results of static material testing suggested. During inflation, the high onset rate of the loads on the circumferential skeletal elements led to failure.
As a result, the project has undertaken the following steps to determine the cause of the failure and to to improve the analysis of loads and margins on future designs:
Carrying out dynamic, off-axis, strength testing of the circumferential elements in a canopy, especially near the region of the canopy where the SFDT-2 failure occurred. These dynamic strength values can then be used to re-compute the margins on the SFDT-2 canopy elements.
Investigating the dynamic loads on the canopy during inflation due to fabric and fluid inertia using analytical tools.
Investigating the dynamic loads on the canopy during inflation due to traveling pressure waves during inflation using final element analysis software (LS-DYNA)
A much more recent paper "Overview of the Mars 2020 Parachute Risk Reduction Activity" includes more information drawing conclusions from the LDSD test:
In summary, LDSD had the following implications on heritage methods used to verify that a parachute could survive asupersonic inflation at, or below, its flight limit load:
It indicated that peak stress in the canopy does not necessarily correlate with peak load and indicates that drag force generated by the canopy may not be well correlated with canopy stress at all.
It indicated that significantly higher stresses can be generated in the canopy than those predicted by quasi-static analyses of a full open canopy, even if the quasi-static load is amplified significantly to attempt to compensate for the additional dynamics and asymmetries that can occur during a supersonic inflation.
It indicated that a subsonic overload test of the full open canopy does not provide sufficient evidence that the canopy will survive a supersonic inflation at, or below, the flight limit load.
