Four different system CG locations were tested during IST-3. The perfect CG was chosen because it represents the ideal condition of adding significant mass to the subject in perfect proportion so that it does not alter the subject’s own CG. The Backback CG was chosen because it represented a potential option based on current suit and Portable Life Support System (PLSS) modeling. The CTSD CG was the CG of the suit. The POGO CG was determined to be the system CG as tested during previous suit tests, including the EWT, IST-1, and IST-2.

Three different offload profiles were used during this study. The first profile involved no offload, with the subjects performing all tasks in 1-g with no CG rig and unconnected to the POGO (1-g). The second profile had the subject don the CG rig at the Perfect CG, but with the weight of the CG rig offloaded so that the subject’s total gravity adjusted weight (TGAW) was the same as the first profile, his 1-g body weight (1-g [Perfect]). TGAW, which is defined as the weight remaining on the ground, is the product of the total system mass and the offload level. The third profile had the subject don the CG rig before the total system mass (combined mass of the subject, gimbal, and CG rig) was offloaded to represent lunar gravity (1/6-g) (Perfect, Backpack, CTSD, POGO).

RESULTS:
CG locations for the system CG in relation to the subject and the gimbal axes of rotation were modeled before the test to provide the configuration of the CG rig for each condition, but these locations were based on the standard subject model. To determine how much variation occurred because of varied subject anthropometry, further analysis was performed using each individual subject’s height and weight to modify the standard subject model on completion of the test sessions. Subjects were lined up in a consistent orientation to the gimbal support structure before the CG of the system and the relation of the system CG to the gimbal axes of rotation were recalculated. This recalculation proved valuable for another reason: Three of four CGs were well tolerated, but the CTSD CG was consistently the most difficult for all subjects to cope with. When performing tasks in this CG, many subjects described the feeling as similar to a “horse-collar” tackle in football, with subjects feeling as though they were being pulled down and backwards from the base of the neck. Because these subjects did not experience this with any of the other conditions, the investigative team believed there was an error in the initial model calculations.