Neutral Buoyancy Simulator

The Neutral Buoyancy Simulator was a neutral buoyancy pool located at NASA's George C. Marshall Space Flight Center (MSFC). The simulator was used from 1968 through its decommissioning in 1997. Marshall recognized the need for underwater simulations of extra-vehicular activities (EVAs) and developed three successively larger tanks for the purpose. The Neutral Buoyancy Simulator contributed significantly to the American crewed space program. Astronauts involved into Skylab, the Space Shuttle, Hubble Space Telescope, and the International Space Station missions trained in the facility. Until Johnson Space Center constructed the Weightless Environment Test Facility in the mid-1970s, MSFC had the only NASA-owned test facility that allowed engineers and astronauts to become familiar with the dynamics of body motion under weightless conditions. The water within the simulator was temperature controlled, continuously recirculated and filtered. Special systems were integrated into the tank for underwater audio and video, pressure-suit environmental control and emergency rescue and treatment. Life support was simultaneously provided by these systems for up to four pressure-suited subjects. Additional systems included data acquisition and recording, underwater lighting, special underwater pneumatic and electrical power operations of motor, valves, controls, and indicators that required for high fidelity and functional engineering mockups and trainers. Demolition of the Neutral Buoyancy Simulator began in mid-December 2025.

Principles of operation Neutral buoyancy simulates the weightless environment of space. First equipment is lowered into the pool using an overhead crane. Suited astronauts then get in the tank and support divers add weight to the astronauts so that they experience no buoyant force and no rotational moment about their center of mass. One downside of using neutral buoyancy to simulate microgravity is the significant amount of drag presented by water. Generally, drag effects are minimized by doing tasks slowly in the water. Another downside of neutral buoyancy simulation is that astronauts are not weightless within their suits, thus, precise suit sizing is critical.

Origins and first tank

NASA has flown zero-g flights on various aircraft for many years. In 1959, Project Mercury astronauts trained in a C-131 Samaritan aircraft, which was dubbed the "Vomit Comet". Airplane weightlessness is limited to 25 seconds at a time, which hampers efforts to practice EVAs which might last hours. Prior to May 1960, NASA recognized the possibility of underwater neutral buoyancy simulations and began testing its efficacy. NASA engaged Environmental Research Associates of Baltimore to try neutral buoyancy simulations first in a pool near Langley Research Center. Visitors and other issues disturbed those efforts, and they moved the operation to McDonogh School where Scott Carpenter was the first astronaut to participate suited. Then, after difficult EVAs through Gemini 11 in mid-September 1966, the Manned Spacecraft Center fully understood the importance of testing procedures underwater and sent the Gemini 12 crew to train at McDonogh. Meanwhile, MSFC was looking ahead to the Apollo Applications Program which would involve EVAs to convert a mostly-empty S-IVB rocket stage into a space station, and the people designing the hardware needed a thorough understanding of the challenges of weightlessness. Charlie Cooper at MSFC theorized that neutral buoyancy exercises could help with EVA planning while he was reviewing film of the Gemini 4 EVA. He and Charles D. Stocks pursued the idea with a couple of NASA scuba suits and an 8 feet (2.4 m) diameter, 8 feet (2.4 m) deep pool which had been previously used for forming metal parts explosively. November 1965 tests included removal of the ST-124 and a J-2 engine propellant utilization valve - early steps in a nascent Skylab mission, then called S-IVB orbital workshop.

Second tank After the utility of the technique had been demonstrated, in January 1966, workers repurposed a larger explosive forming tank for neutral buoyancy testing. The larger pool was 25 feet (7.6 m) diameter, 15 feet (4.6 m) deep. A 10 feet (3.0 m) tall 35 feet (11 m) diameter damaged Saturn V corrugated section with a conical roof atop enclosed the now-heated facility with adequate lighting and more room for larger hardware than the initial tank. The budget for the second tank was especially tight. It included a swimming pool filter from Sears and a tap into an adjacent steam line for water heat. Algae engaged chemists for nearly a year to find an appropriate balance of chemicals for the unique arrangement. Initial tests again focused on S-IVB workshop: airlock ingress and egress, and S-IVB hatch cover removal.

The second tank saw astronaut training in pressure suits beginning with the Navy Mark IV. The MSFC Manufacturing Engineering (ME) lab developed a constant pressure valve which "made it possible to maintain neutral buoyancy at any depth." Divers first tried the Mark IV in the tank in July 1966, and Alan Bean spent two hours "executing typical astronaut activities" in the tank on September 6, 1966. Bean was "quite enthusiastic and outspoken about neutral buoyancy as one of the mandatory methods of simulation for all the S-IVB Workshop experiments," according to Manufacturing Engineering director W. R. Kuers. Stocks, the controller for that exercise, reported Bean's dive began with a leaky glove followed by his Mark IV suit tearing under the arm and requiring diver-assisted evacuation. Bean proceeded with exercises in regular scuba gear.

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