Spacecraft Landing Risk Characterization

INTRODUCTION
United States On-orbit Segment (USOS) astronauts have been returning to Earth from the International Space Station (ISS) aboard the Russian Soyuz vehicle since 2003. The energetics associated with Soyuz landings are comparable to our next generation of US space vehicles (Orion, Crew Dragon, and Starliner). Soyuz also lands crew in a similar state of spaceflight deconditioning that is anticipated using the future vehicles. A number of injuries have now been observed during these Soyuz landings. Currently it is unknown how Soyuz landing accelerations and the numbers and types of associated injuries relate to the new occupant protection requirements levied upon future space vehicles. Understanding this relationship will allow better quantification of the risk of injury for future spacecraft designs. Within the last several years, NASA and USOS crewmembers have been flying on the SpaceX Crew Dragon vehicle as part of the Commercial Crew Program (CCP). This study has been expanded to include USOS crewmembers flying on CCP missions and private astronaut missions (PAM).

METHODS
An estimate of injury occurrence during Soyuz landings has been determined using data contained in NASA flight medicine databases and supplemented with data collected from crewmembers and flight surgeons. Thus far, there have been 84 total Soyuz data collections (61 NASA, 22 USOS, and 1 spaceflight participant). For Dragon, data is primarily collected through crewmember debriefs, with a total of 46 data collections (24 NASA, 6 USOS, and 16 PAM). Injuries are classified either as contusions/abrasions, Class I (minor, no medical follow-up), Class II (moderate, medical follow-up necessary), Class III (severe), or Class IV (life threatening). Current NASA occupant protection standards are primarily based on the Multi-axial Dynamic Response Index (MDRI), which is a simple lumped-parameter spring, mass, damper model tuned to human impact responses in three orthogonal axes.

RESULTS
Using what is known about Soyuz landing dynamics from airborne and drop test data, the MDRI predicts essentially no Class IV or III injuries, < 1% Class II injuries, and < 5% Class I injuries. Approximately 30% of all USOS crewmembers for whom data has been gathered have suffered some sort of injury related to landing in the Soyuz vehicle. Most of these injuries fall into the contusion/abrasion category (20%) (n.b. This is the percent for whom contusion/abrasion was the worst injury. Those with class I or II injuries typically also received contusions/abrasions). Class I injuries have been sustained by 5% of all crew, and Class II injuries have been observed in 4%. No Class III or IV injuries have been sustained to date. Forty-six injuries have been documented, with some crewmembers sustaining more than one injury in a single landing. For Dragon, about 11% of crewmembers report some sort of injury, with the majority being contusion/abrasion injuries (9%), and only a single Class I injury (2%).

CONCLUSION
Soyuz landing dynamics are the most comparable analog environment for the anticipated loads associated with Orion and commercial crew vehicle landings. The MDRI under predicts the percentage of Class II injuries experienced during Soyuz landings, but it is better at predicting Class I injury occurrences. Thus far the Dragon injury rate is significantly lower than Soyuz. One original concern from the Soyuz findings is that because all vehicles certified by NASA use the same tools that are unpredicting Soyuz injuries, future vehicles are potentially at risk to injury under prediction as well. Despite Dragon showing lower injury rates than Soyuz, it is important to continue to evaluate the injury rates for all current and future vehicles, with addition data working toward improving the predictive tools for spaceflight use. Spaceflight deconditioning may account for some of the under prediction, and work is ongoing to assess the relationship between landing injuries and impact tolerance changes due to spaceflight deconditioning. Future work includes expanding this study to involve private space missions on new vehicles to better characterize the effects of time in flight, suit and seat design, and vehicle dynamics.