NASA - National Aeronautics and Space Administration

Problem
The Orion Project requested alternate seat attenuation designs to be developed and analyzed for the Orion Vehicle with primary emphasis to provide improved crew survivability for nominal and Contingency Land Landing (CLL). An emphasis was placed on improving robustness and maximizing crew protection from acceleration forces given project directed requirements and goals. Due to the team’s in-depth knowledge of the problem and work with isolation systems, the NESC was later asked to evaluate design options in the crew seat area for the Ares Thrust Oscillation problem and its effect on landing loads.

NESC Contributions
The assessment team consisted of designers and analysts from multiple NASA centers including GSFC, JSC, JPL and LaRC, contractors, academia, NASCAR seat design experts and engineers from the Apollo era with design and test experience on its landing systems. Prior to developing alternate concepts, assessment members had the opportunity to evaluate the seat layouts in the Apollo 17 capsule, and in mockups of the Soyuz and Orion vehicles at JSC (see Figure 1).

Figure 1. NESC Team Members (left) performing a fit check in an Orion seat mock-up and (right) inspecting the Apollo Capsule at JSC for lessons learned related to landing attenuation.

The Orion landing approach orients the capsule such that the reclined crew impacts the earth surface (water or land) “feet first.” The result is that a major portion of the crew impact force vector is along the axis of the crew’s spine (see Figure 2 for orientation) and maximum energy absorbing stroke would be required in this direction. Alternate designs for CLL considered a variety of concepts with different approaches to improve the interface such as 1) between the attenuating struts and the pallet, 2) between the pallet and the seat, and 3) between the seat and the crew.

Figure 2. NESC Cockpit Concept Layout to achieve additional Z-Axis stroke.

Analysis performed by another NESC team assessing Orion Occupant injuries found that improving the lateral restraint of the crew as well as holding the crew tighter in a conformed seat reduced injury risk. A NESC seat design expert developed and tested mockups of improved harnessing techniques to achieve these results (see Figure 3).

Figure 3. Harness tension mechanism mock-up (lap strap – left, shoulder strap – right). Doubles the possible tension in the harness strap from heritage systems to better protect the crewmembers and can be self-operated.

Based on the stakeholder review of the alternate concepts, the team focused detailed design and analysis effort on investigating the effectiveness of incorporating an isolation system between the seat pallet and the Orion pressure vessel structure. Two concepts for a new pallet interface emerged. The first tilted the crew pallet during landing to provide a greater stroking distance to absorb more energy in the spine axis direction. The second concept focused on providing isolation at the strut-pallet interface.

The NESC team developed simplified dynamic response models and utilized the Orion baseline models to examine a range of pallet isolation properties for crew landing attenuation. The analytical tool development and the pallet isolation analysis performed by the NESC led to an additional request from the Orion Project to examine mitigating the effects of Thrust Oscillation (TO) from the Ares stage 1 solid rocket booster on the crew using isolation concepts. Coupled loads models for launch and landing models were used to examine the optimal TO isolation frequency that would minimize crew loads during all phases of the Orion flight. In addition, hardware design concepts for implementation of this feature have been generated.

Figure 4. LSDyna model results from isolation study for landing attenuation.

In addition to the NASA and industry teams working this problem, the NESC is funding Massachusetts Institute of Technology (MIT), Pennsylvania State University (PSU), and the University of Maryland to investigate new innovative ideas for addressing the landing attenuation problem. The University of Maryland provided a smart material concept that could offer both isolation and attenuation using shape memory alloys. Their evaluation results were provided to the NESC team with some improvement from standard spring, flexure isolation systems.

Graduate and undergraduate students at MIT and PSU teamed together to evaluate a concept that was the result of a one week “Innovative Engineering,” NESC Academy class. They designed and built a test article for a crewmember personal airbag system. The advantage of such a system is its low weight and potential for low risk of impact injury due to its conformity and attenuation stroke. It could also provide additional on-orbit free volume by easily deflating and stowing (see Figure 5). Testing of this system is ongoing (see Figure 6).

Figure 5. PSU & MIT Personal Airbag Project

Figure 6. Personal Airbag Test Article

Results
The NESC improved seat harness test results were briefed to the Orion project, and hardware developed has been delivered to them for further evaluation. Specific recommendations for the baseline seat system, such as the shoulder harness attach point, were also forwarded to the Project.

Modeling of isolation versus the baseline design showed improvement in the impact acceleration forces experienced by the seat occupant in all three axes for most load cases by reducing the dynamic amplification portion of the load (see Figure 4 for crew response in the axis along the crew spine). Other load cases did not show significant impact acceleration improvement with isolation, yet increased stroke in some instances. The NESC team is currently evaluating active closed loop control systems to improve the strut performance over the baseline for all landing cases.

Results from the TO study confirmed the optimal crew isolation frequency of 4.5Hz and revealed a potential problem with a rocking mode of the crew seat should isolation be implemented at that location. The NESC team also examined pallet isolation and found that approach to be more appealing from a load mitigation perspective. Isolating in series with the pallet struts has a lower effect on crew landing loads than seat isolation and a higher probability of successful implementation. The NESC team continues to evaluate updated load cases and refine the hardware design concept.