Turbolift

Future crewed space exploration missions into deep space will require enhanced countermeasure technologies to ensure astronaut health. One such hazard is extended exposure to reduced gravity levels (i.e., microgravity, lunar gravity, or Martian gravity). Reduced gravity negatively impacts many physiological systems, leading to hydrostatic intolerance, musculoskeletal atrophy, sensorimotor impairment, bone demineralization, cardiovascular deconditioning, and visual alterations1. Various countermeasures have been employed for mitigating these effects, such as exercise, pharmaceuticals, diet, and fluid loading. However, these approaches treat individual symptoms, such that each physiological system is addressed with typically one countermeasure. An alternative to this approach is artificial gravity (AG), which promises to be a holistic, comprehensive countermeasure2. The traditional approach to creating AG is through centrifugation. However, centrifugation is not a pure form of AG and typically includes the drawbacks of Coriolis forces, gravity gradients, and vestibular cross-coupled illusions. As an alternative, we have proposed a Linear Sled Hybrid (LSH) AG system to mitigate astronauts' physiological deconditioning. This system functions by applying pure linear acceleration to produce footward loading. There is a half rotation (180°) to reorient the rider between acceleration and deceleration phases, such that the loading remains footward, as when standing on Earth. The rotation also provides some footward acceleration to the lower body through centripetal acceleration; hence the hybrid aspect of the design (Figure 1). At the end of the deceleration, the rider than accelerates back in the opposite direction and the sequence repeats. This proposed system could be integrated with future crewed space vehicles in a variety of manners. One approach that we have explored is for it to be added to the outside of the vehicle as a subsystem. We propose a pressurized pod to enclose the rider, which performs the sequence of motions in Figure 1. The system could utilize both sides of the track and have two pods, such that two astronauts could ride on the system at a time. The LSH AG system could broadly prove beneficial for any long-duration space exploration mission. As previously mentioned, extended duration exposure to microgravity impairs astronauts' ability to function and negativity impacts their health. Many of these deleterious effects are expected to grow with even longer duration missions than current 6-month International Space Station (ISS) stays. Furthermore, longer exposures to microgravity may uncover additional physiological concerns and interactions that have not yet been identified. For planetary landing missions to the moon or Mars, it is currently unknown whether these reduced gravity environments (0.16 and 0.38 G, respectively) will be sufficient to help mitigate or slow astronaut deconditioning. Thus, the LSH AG system may be critical to enabling crewed long-duration lunar stays, cis-lunar exploration, Mars orbital missions, exploration of Martian moons, Martian landings, or any further destination in our solar system (e.g., Europa). In the foreseeable future, we envision the LSH AG system to be directly applicable to crewed missions to Mars, which will require 1+ year of microgravity exposure, in addition to any time spent on the surface (potentially ~2 years). There are three aspects to be considered regarding the feasibility of this system; human health benefits, human tolerability during LSH operation, and the associated cost of engineering and designing the system. Regarding the human health benefits, while AG has not been validated as a countermeasure for astronauts in space, presumably replicating 1 G would be beneficial in maintaining human health as it is here on Earth. We consider a range of different motion sequences that might prove optimal in maintaining astronaut health during long-duration exposure to microgravity. We investigated the human tolerability of the LSH motions via simulation of the well-validated observer computational model of orientation perception. The motion sequence of the LSH system was found to be well-perceived with no vestibular cross-coupled illusion occurring, even if the simulated rider tilts his/her head3. Human studies have been pilot tested, assessing the potential concern of motion sickness and physical discomfort during the 180° rotation phase, which have been successful. A tolerable LSH AG system may allow for a comprehensive countermeasure for spaceflight-induced physiological deconditioning. Mass, power, and volume, and the associated cost of the LSH system were the main design drivers for defining this concept. Total added mass was the sum of masses of the pod, actuators, and structure of rail on which the pod travels. As a preliminary estimate of the mass required for such a system, we considered the mass of the pressurized pod and performed calculations regarding the required track length. We assumed that life support for the pressurized pod would be provided from the crewed vehicle, but that the pod would need to be capable of sustaining one astronaut for a maximum of 2.5 hours at a time. This timeframe was motivated from studies demonstrating centrifuge AG of 1 hour per day to mitigate physiological deconditioning that otherwise occurs during the ground-based space flight analog of bed rest. This also compares well with the ~2 hours per day of exercise each crewmember performs on the ISS4. Accounting for some buffer time for entry/exit and contingencies, we assumed the pressurized pod would provide Environmental Control and Life Support System (ECLSS) for this time5. Using mean ECLSS requirements, an average rider, and associated systems, we estimated the required mass of the pressurized pod. The power inside of the pod was dependent on the electronics used inside such as a fan for ventilation, cabin lights, and heat removal from inside the pod. The mass of the structure was a function of the length and material used for the railing. The duration of each phase of linear acceleration/deceleration and half rotation dictated the length required. We included a margin of safety at both ends of the track to allow for a tolerable emergency stop. We explored a range of motion profiles, and present two cases studies that yielded the maximum and minimum track length in Table 1, where Ta/d is time spent during acceleration or deceleration, TR is defined as the time during rotation phase, TT is a transient time between rotation and acceleration or deceleration phase. The mechanism of actuation of the LSH is dependent upon the profile of the motion. After determining the motion profile for the LSH, the theoretical power/energy requirements for both linear acceleration and rotation phase were computed for the structure of the LSH, the values are presented in Table 2 for the max and minimum of length, mass, and power/energy required for our design parameters. Note that some of the LSH system configurations yield a very short track length. As shown in Figure 2, the LSH is attached outside of the crewed vehicle. Therefore, it would not impact the existing internal habitable volume of the vehicle. The pod design adds a small habitable volume of ~1.5 m3. This volume was designed to keep the astronaut alive for the duration of intended use (<2.5 hours). Based upon our preliminary analysis, the LSH system appears to be a feasible approach to creating AG, which is likely to be beneficial to protecting against astronaut physiological deconditioning on a gateway spacecraft in cis-lunar space or even further away from Earth. Specifically, we found the motion sequence is likely to not be disorienting for the rider and provided preliminary engineering analysis of the track length, and pod in terms of mass, power, volume, and monetary cost. Future work should further refine estimates for the LSH system's mass, power, and volume, as well as provide a cost analysis. Human testing can further verify the system, particularly the 180 degree rotation, is tolerable in terms of motion sickness and physical comfort. It can also help inform the required length of the rotation phase. Finally, future work should aim to demonstrate the system indeed mitigates physiological deconditioning that otherwise occurs in microgravity. However even at this point there is strong reason to believe replicating gravity through the LSH AG system will be beneficial for astronaut health.