Assessing the Risk of Crew Injury Due to Dynamic Loads During Spaceflight

Spaceflight requires tremendous amounts of energy to achieve Earth orbit and to attain escape velocity for
interplanetary missions. Although the majority of the energy is managed in such a way as to limit the accelerations
on the crew, several mission phases may result in crew exposure to dynamic loads. In the automotive industry, risk
of serious injury can be tolerated because the probability of a crash is remote each time a person enters a vehicle,
resulting in a low total risk of injury. For spaceflight, the level of acceptable injury risk must be lower to achieve a
low total risk of injury because the dynamic loads are expected on each flight. To mitigate the risk of injury due to
dynamic loads, the NASA Human Research Program has developed a research plan to inform the knowledge gaps
and develop relevant tools for assessing injury risk.
The risk of injury due to dynamic loads can be further subdivided into extrinsic and intrinsic risk factors. Extrinsic
risk factors include the vehicle dynamic profile, seat and restraint design, and spacesuit design. Human tolerance to
loads varies considerably depending on the direction, amplitude, and rise-time of acceleration therefore the
orientation of the body to the dynamic vector is critical to determining crew risk of injury. Although a particular
vehicle dynamic profile may be safe for a particular design, the seat, restraint, and suit designs can affect the risk of
injury due to localized effects. In addition, characteristics intrinsic to the crewmember may also contribute to the
risk of injury, such as crewmember sex, age, anthropometry, and deconditioning due to spaceflight, and each
astronaut may have a different risk profile because of these factors. The purpose of the research plan is to address
any knowledge gaps in the risk factors to mitigate injury risk.
Methods for assessing injury risk have been well documented in other analogous industries and include human
volunteer testing, human exposure to dynamic environments, post-mortem human subject (PMHS) testing, animal
testing, anthropomorphic test devices (ATD), dynamic models of the human, numerical models of ATDs, and
numerical models of the human. Each has inherent strengths and limitations. For example, human volunteer testing
is advantageous because a population can be selected that is similar to the astronaut corps; however, because of the
inherent ethical limitations, only sub-injurious conditions can be tested. PMHSs can be tested in a variety of
conditions including injurious levels, but the responses are not completely analogous to living human subjects. In
addition, it is exceedingly difficult to select a PMHS population that is similar to the astronaut corps. ATDs are
currently widely used in the automotive industry and military because they are highly repeatable and durable.
Unfortunately, because they are mechanical models of the human body, the biofidelity of the responses are limited
to dynamic conditions used to validate the ATD. Numerical models of the ATD, in addition to the strengths and
limitations for ATDs, are easy to use for a variety of designs before a design is fabricated, but also have additional limitations for ATDs, are easy to use for a variety of designs before a design is fabricated, but also have additional
uncertainty. Dynamic models are simple and easy to use, but do not account for localized effects of the seat and suit.
Finally, numerical models of the human have the potential to have the most advantages; however, the current models
are not validated for the conditions expected during spaceflight. To properly assess spaceflight conditions with
numerical human models, human data would be needed to optimize the model responses for those conditions.
Using the appropriate assessment method with the knowledge gained for each risk factor, an appropriate approach
for mitigating the risk of injury due to dynamic loads can be developed ensuring crew safety in future NASA
vehicles.