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Astronaut Scott Carpenter underwater in the pool at The McDonogh School in 1966 removing bolts in a simulated airlock to create access to the dome of the spent SIVB stage.

A personal history of underwater neutral buoyancy simulation

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David J. Shayler’s book Walking in Space gives an interesting historical account of space walking, including many examples of science fiction writers and scientists forecasting such extravehicular activities (EVA). Apparently it was not until spaceflight was a reality that any attention was turned toward training for EVA.

ERA was a small research firm that became a pioneer in neutral buoyancy simulation of EVA almost by accident.

Because military and civilian pilots selected as astronauts were familiar with aircraft maneuvers such as loops and dives that increased or decreased their perceived gravity loads, they were naturally comfortable with airborne weightlessness for EVA training. In 1958, after proving the concept using jet trainer aircraft, the US Air Force started flying a C-131 to get up to 12 to 15 seconds of weightlessness in its spacious interior, but only for about three parabolas before it needed to climb back up to a safe altitude for the next three parabolas. By 1962, the Air Force had transitioned to the KC-135, which could provide 30 or 40 maneuvers continuous for up to 30 seconds each.

Meanwhile, NASA and contractor engineers developed and tested various harnesses and frictionless surfaces as training models on the ground, without the expense and limitations of airplane parabolic maneuvers. Neutral buoyancy simulation under water was an obvious modality, and had been studied using SCUBA-equipped subjects.

I was fortunate to be present at the beginning of underwater simulation of EVA, as one of the founders and the senior partner in Environmental Research Associates (ERA). My good friend Harry Loats was the other founder and Chief Scientist. ERA was a small research firm that became a pioneer in neutral buoyancy simulation of EVA almost by accident.

The EVA connection came about in part because ERA had interested NASA Langley Research Center in a study of seals for airlocks as applied to a space station concept under study at Langley in 1963–1964. The study was a review of seal materials and flange sizes and configurations. One of its conclusions was that there was very little information available on airlock usage as applied to lightweight structures such as an early space station. A small addition to the contract allowed ERA to develop a conceptual airlock with various door and seal designs to be tested in the vacuum chambers at Langley. The technical contract monitor, Langley engineer Otto F. Trout, procured some hardware for testing and had the Langley model shop build a full size (4 feet (1.2 meters) diameter by 6 feet (1.8 meters) length) cylindrical plastic model of the airlock to illustrate the scope of his planned test program to Langley management. This plastic airlock model was only intended for demonstrations, not for use in a vacuum chamber.

At this same time, ERA had also proposed another EVA-related study to Langley concerning the use of tethers as a safety measure. The study “Retrieval of a Tethered Astronaut” was funded by the Gemini Project Office with Langley engineer Gary P. Beasley as technical monitor, and was awarded to and performed by the Marquardt Corp. of Van Nuys, California, with ERA as a consultant. The scenario to be studied was to have the EVA astronaut secured to the Gemini spacecraft by a 5,000-foot (1,500-meter) lightweight extendable tether that would be “reeled in” to return the astronaut to the Gemini vehicle. In those days before rendezvous was even understood, much less perfected, the notion of an astronaut venturing so far from his spacecraft to accomplish some task, perhaps on another vehicle, was not outlandish: remember the EVA repair scene in the movie 2001: A Space Odyssey? The Langley study concerned orbital mechanics and tether dynamics. The tether was not to be used to assist the astronaut in reentering the spacecraft. It was obvious even then that reentering the Gemini vehicle, especially if the astronaut was incapacitated, would be very difficult because of the inflated space suit.

With this background, it is not surprising that Otto Trout’s plastic airlock model provoked interest in the question of whether an astronaut could, in fact, operate such an airlock in weightlessness. It was clear to us from the very start that neutral buoyancy under water could simulate weightlessness. A space suit, or pressure suit, was a requirement for EVA so it would be an important component of our EVA studies. The problem was that space suits were not readily available.

In the early 1960s, pressure suit manufacturers were concerned with mobility in a high-altitude jet fighter’s cockpit but, apparently, not free-floating outside a spacecraft in weightlessness. The Goodrich Mark IV US Navy full pressure suit was adequate for operating the controls in a confined cockpit, but the Arrowhead version of the Mark IV had better mobility because it used a bellows concept at the body’s major joints. This reduced the effort required to bend a joint and maintain the flexed position. It did not make it easy to bend joints, but it made it easier than the other suits of the day.

Loats and I had worked on Naval Aviation contracts with a previous employer and were aware of the Navy High Altitude pressure suit operation at nearby Norfolk Naval Base. It turned out that the pressure suit school not only trained pilots in the use of pressure suits but also had a small pool for training in procedures for ditching at sea. Navy pilots had to practice egressing a ditched plane while wearing an unpressurized full pressure suit; the Arrowhead suits used for the training were out of service for flying but still adequate for training. It did not escape our notice that there might be some similarities between underwater suited egress training and underwater suited simulation of EVA.

The school, under the direction of Lt. Charles C. Cole, agreed to let ERA borrow a suit but not without appropriate training, so Loats and I became suit-qualified, including time in the altitude chamber at a simulated altitude of 70,000 feet (21,300 meters) and a dunk in the water. For our efforts, in addition to the use of the suits, we were awarded membership in “SPACE,” the Navy’s Society of Pioneering Astronauts and Celestial Explorers.

The school was not confident that the pressurized Arrowhead suit could be made neutrally buoyant without excessive weights, but they supported ERA by providing and maintaining several Arrowhead suits and associated hardware. This was not a contracted effort. It was a gentlemen’s agreement to expand knowledge relative to the space program.

It quickly became obvious that neutral buoyancy was useful for evaluating general EVA problems.

In June 1964 Otto Trout requested and was granted use of the swimming pool at the officers club at Langley Air Force Base for a single three-hour period. The plastic airlock was immersed and I was suited up and weighted to approximate neutral buoyancy. No attention was given to degrees of freedom or rotational stability, only to total neutral buoyancy, so the weighting included lead shoes, body-worn weights, and handheld dumbbells. The test was inconclusive: I managed to pass through the plastic airlock but was not able to demonstrate the six degrees of freedom of weightlessness. It became obvious that such a program would require much more time and planning. After this single exercise, it was decided that ERA would take possession of Langley’s plastic airlock model and conduct a program to evaluate the simulation procedures, complete with detailed underwater photography to document the exercises.

The very next day, I approached the McDonogh School, a local boys’ military school in Owings Mills, Maryland, near the offices of ERA in Randallstown. McDonogh had an indoor pool for year-round use by their swim team and by the Red Cross for swim instruction. I knew that the pool’s water quality would be adequate for photography because I had installed the filtration system in a previous business. Robert Lamborn, McDonogh Headmaster, agreed to ERA’s use of the pool, for the same hourly rate the Red Cross was paying, on off hours as long as no equipment remained in the pool building after its use and the program did not impact other pool users. It was intended to be a short-term program so no formal lease agreement was necessary, just another gentleman’s agreement that ERA would keep track of its usage and pay monthly.

We conducted several simulations to evaluate the Arrowhead full pressure suit in a neutral buoyancy mode using bags of welding shot tied on to the suit at various locations. On July 19, 1964, less than a month after the test in the Langley pool, ERA made the first formal neutral buoyancy test of airlock ingress and egress. Trout had acquired a Milliken 16 mm gun camera from the Air Force, which exposed 400 feet (120 meters) of 16mm film in 12 minutes, and ERA designed and built a Plexiglas housing for it for underwater use. I was in the suit, Loats was the photographer safety diver, and Trout was the observer safety diver.

A presentation using the film was made to Paul Hill, Deputy Chief of the Structures and Mechanics Division at Langley, along with a proposal for a series of simulations to evaluate airlock features and sizes. Hill had supported the original seal study, and was impressed and supported the proposal, which became Contract NAS1-4059. ERA hired several young SCUBA-experienced personnel and conducted simulations.

Because zero-g in parabolic flight was accepted as the standard simulation of weightlessness, neutral buoyancy simulation would have to be compared with zero-g, and with one gravity demonstrations as a baseline, to gain acceptance. Langley, however, had no zero-g flight capability. Loats and I were familiar with the Air Force at Wright-Patterson AFB from previous employment and were able to make contact with John C. Simmons, who was conducting studies of human motion control in weightlessness using a C-131 aircraft. Simmons, who had co-authored “Self Maneuvering for the Orbital Worker” for the Aerospace Medical Division in December 1960, was impressed with the neutral buoyancy film and arranged to have me, as a former Army Air Corps flight engineer, qualified as a test conductor for the USAF zero-g flights. He then provided use of the C-131 for a test of airlock ingress and egress using the Langley plastic airlock. Again, there was no formal contract for performance: it was another gentlemen’s agreement intended to further the understanding of weightlessness simulation.

ERA filmed the simulations at one gravity, in the zero-g aircraft, and in neutral buoyancy. In September 1965, a report was generated and was published in January 1966 as NASA Technical Note TN-D 3054, “A Water Immersion Technique for the Study of Mobility of a Pressure Suited Subject Under Balanced Gravity Conditions.” A film supplement was also produced showing one gravity simulation, zero-g flight simulation and neutral buoyancy simulation. The report and the film are available at Wikipedia under “Neutral Buoyancy Simulation as a Training Aid.”

It quickly became obvious that neutral buoyancy was useful for evaluating general EVA problems and a contract extension was negotiated at Langley to broaden the scope of the simulation. Additional contract extensions were negotiated to continue the program until July 1966 and presentations were made at the NASA facilities and at symposiums. However, in the meantime one Soviet and two American EVAs were completed. Although the second American experience was not successful (and the Soviet EVA had serious problems that were not publicized for decades), there was a sense among NASA mission planners that EVA was a done deal. Astronaut training was expected to be conducted in zero-G on the KC-135 and on frictionless air-bearing platforms. The Langley/ERA neutral buoyancy program was considered interesting but not necessary and the NASA Manned Spacecraft Center (MSC) in Houston was not interested in further work in this area. In fact, underwater training was considered “beneath the dignity of an astronaut,” according to Kenneth Kleinknecht, then the Deputy Manager of the Gemini Program. However, the problematic American EVA on Gemini 9 raised questions of EVA training and the Gemini Program Office took interest in neutral buoyancy, encouraged by Robert Gilruth, the MSC Director.

ERA had completed the work required on contract NAS1-4059 but had not yet produced the final report. The McDonogh pool was available full time for a few weeks during the summer break, so equipment could be left in the pool building. Loats and I took the initiative to conduct demonstrations of neutral buoyancy relative to space station development and to invite the various NASA centers. Langley, Marshall, MSC and the Air Force sent representatives to the demonstration, which was the removal of the S-IVB dome cover from inside the airlock planned for the “Wet Workshop” of the Apollo Applications Program, the predecessor of Skylab. We thought it looked like an easy part of the mission-critical process of converting a used rocket stage into a habitable space station. ERA built the mockup using dimensions identified in a trade magazine in the spring of 1966.Various tools were used for bolt removal, including the Air Force’s new “Torqueless Wrench”, which was developed by Black & Decker and the Martin Company to minimize the force required to remove bolts. The demonstration clearly showed the difficulty of using tools and providing forces in confined spaces while wearing a full pressure suit. In particular, the pressurized glove made it difficult to squeeze the one-finger trigger on the wrench with the wrist flexed to access the bolts behind an obstruction.

Don Jacobs of the Gemini Program Office at NASA MSC was in attendance. He asked ERA to do an evaluation of the Gemini 10 EVA task and provided a small section mockup of the Gemini adapter for the evaluation. The task involved attaching an umbilical fitting from a maneuvering gun to a nitrogen valve. The simulation showed that the task was doable but would require three hands to complete without incident. Mike Collins’ flight experience confirmed the simulation. Shortly after the Gemini 10 flight, astronaut Scott Carpenter visited unannounced, donned SCUBA gear and observed one of the ERA divers in a Gemini suit attempting the dome cover removal task. The diver worked for a half hour and removed three of the 72 bolts. Carpenter said, “I can see that he is having trouble, but I can’t see why.” So, we put him into the Gemini suit and he attempted the dome cover removal task. After a half hour, he had removed one of the 72 bolts.

Not everyone was in favor of including neutral buoyancy training into the busy work schedule—Chief Astronaut Alan Shepard would only support it on direct written orders from Dr. Gilruth.

Impressed by the difficulty of the task and the usefulness of underwater simulation, he reported back to MSC that our work at McDonogh had value. The Gemini Office then provided funding to Langley for an extension of Contract NAS1-4059 and had a partial simulator of the Gemini adapter section flown in from the St. Louis McDonnell plant. It included mockups of some of the EVA task of Gemini 9 and tasks planned for Gemini 11 and 12. ERA personnel George Hay and Bruce Tharp evaluated the Gemini 9 task. They exposed some of the difficulty and suggested modifications. This was observed by Gemini 9 Astronaut Gene Cernan, who was then suited and re-enacted the portion of the Gemini 9 EVA involving activation of the Air Force’s Astronaut Maneuvering Unit (AMU) preparation for its independent test flight. He reported a similarity between neutral buoyancy and his space experience and said the simulation had value. In fact, his relief was palpable, because it had not been universally accepted that the AMU failure on his EVA had not been his fault.

The upcoming Gemini 11 EVA task was then simulated using ERA test subject Bruce Tharp who, by that time, had many hours (probably 50) of experience pressure suited in neutral buoyancy. The task was considered difficult but doable and was filmed in 16 mm black and white and rushed to MSC. There is no record of review by Astronaut Dick Gordon who later attempted the Gemini XI EVA but had difficulty resulting in an aborted EVA.

There was no time available in Gordon’s Gemini 11 EVA schedule to consider neutral buoyancy training but Charles Matthews, Gemini Program Manager, petitioned Dr. Gilruth to have the Gemini 12 Astronaut Buzz Aldrin include neutral buoyancy EVA training at the ERA/McDonogh facility. As has been reported, not everyone was in favor of including neutral buoyancy training into the busy work schedule—Chief Astronaut Alan Shepard would only support it on direct written orders from Dr. Gilruth—but Matthews prevailed and Aldrin became the first astronaut to prepare for EVA in neutral buoyancy. One of his early visits to McDonogh coincided with the launch of Gemini 11. There was a break in our preparations so that a TV set could be brought in and Aldrin could watch the launch while he was suited and standing in the four-foot-deep area. He then proceeded with an evaluation of the original Gemini 12 EVA plan, which included activation of the AMU. Although he was confident that the task was doable and some of the restraints and hardware had already been redesigned and upgraded from the Gemini IX EVA, that plan for the AMU was dropped from Gemini 12 by the EVA Review Board. While the Board was being briefed on the AMU at its first meeting, Aldrin was practicing with it under water in a swimming pool at McDonogh Maryland.

The final version of the Gemini 12 EVA hardware was then incorporated into the simulator and Aldrin returned for additional training sessions. Some of these sessions included astronauts Gene Cernan and Gemini 12 Commander Jim Lovell (unsuited) in radio contact with Aldrin underwater. Observing NASA officials from Headquarters included George Mueller, Bill Schneider, and several others. Aldrin’s Gemini 12 flight EVA was considered totally successful.

During this period major aerospace firms began studying neutral buoyancy simulation, and Ellington AFB near MSC had a pool that was being used to practice egress from the Gemini capsule after splashdown. Mr. Matthew’s decision to request the inclusion of neutral buoyancy training for Gemini 12 was an engineering decision supported by human factors study. The decision to use the ERA/McDonogh facility was probably the result of consultations with astronauts Scott Carpenter and Gene Cernan. It must have been a difficult decision. ERA was a very small firm with fewer than a dozen employees and was using a rented pool on a part-time basis. However, we had two years experience in neutral buoyancy simulation and had produced the comprehensive report mentioned earlier. Scott Carpenter had visited other aerospace firms before visiting ERA and was a recognized underwater expert within NASA for SEALAB, the Navy program for long-term underwater habitation studies. Gene Cernan first visited ERA after his Gemini 9 experience and did so at the request of Dr. Gilruth.

At the conclusion of the Gemini program there were still mixed feelings about neutral buoyancy simulation. However, underwater training was judged to be sufficiently important to success in EVA that water tanks for astronaut training and procedures development quickly appeared at NASA and contractor facilities nonetheless.

The training sessions and equipment required many concessions from McDonogh Headmaster Lamborn. The Gemini simulator equipment was assembled in the pool building and had to be left on the deck when not in use. There was also a full schedule of other pool users such as the school’s swim team so all simulation equipment had to be removed from the pool to the deck at the end of each session. Some sessions were conducted overnight. An overhead handling structure was installed by ERA and the breathing air supply had to be completely revised using a series of about forty “K” bottles also stored on the deck in the pool building. The water temperature was 70°F and ERA restricted the safety divers to 30-minute shifts to avoid hypothermia. Additional divers were hired and trained in suit operation and maintenance. At one point, in order to submerge the entire mockup, the water level was raised 14 inches by stopping up the pool scuppers. Three more Milliken cameras were submerged and ganged for continuous filming, each providing its 12 minutes of coverage before it was hoisted out of the water for film changing. Finally, in order to make a color film of the EVA, a 28-volt generator was brought in from Andrews AFB and a group of landing lights was submerged. What had started as a couple of morning hours a couple of days a week for four or five people became 15 ERA people and 15 visitors working four or five hours at a time. The McDonogh School managed to survive, and ERA established the basis for future EVA training without ever competing for a contract.

At the conclusion of the Gemini program there were still mixed feelings about neutral buoyancy simulation. About a third of the people involved said “I always knew that it was the right way to go.” Another third felt that it was useful but that restraints and handholds were the important criteria. The final third felt that neutral buoyancy had nothing to do with the success or failure of EVA, and that it was entirely the result of the restraints and handholds.

However, underwater training was judged to be sufficiently important to success in EVA that water tanks for astronaut training and procedures development quickly appeared at NASA and contractor facilities nonetheless, and eventually in other countries as well. It was central to the EVA repairs of the Skylab space station in 1973, and the Hubble Space Telescope in 1993 demonstrated that neutral buoyancy was critical for evaluating EVA plans and for training astronauts.

Today, astronauts bound for the International Space Station train in the Neutral Buoyancy Laboratory (NBL) in Houston to develop the whole set of required skills instead of rehearsing complete EVAs, allowing NASA and the partners to plans EVAs as tasks arise.

In early December 2009, John Charles arranged for me to visit the NBL. I just wanted to see the operation: how it was conducted and, frankly, to see what differences there were from our ERA experiences in the McDonogh pool. (In August 2012, I invited three of the ERA divers to meet John at my home for a small Eastern Shore crab feast. The discussions that day refreshed some of the memories in this article.)

The overall operation, especially the current video capability underwater from several angles in real time, makes our efforts with 16mm cameras seem prehistoric. Now you can see what is happening as it happens without having to go underwater, or peer down from a diving board, as George Mueller did, when as NASA associate administrator for manned spaceflight he watched Aldrin rehearse back in 1966.

It is an impressive place and an impressive bunch of people operate it. I shook hands with Astronauts Chris Cassidy and Akihiko Hoshide just before they went on a several-hour simulated EVA at the submerged ISS mockup, and talked with the technicians and the divers and was particularly pleased that we had rapport.

Before I left, NBL Director Ron Lee gave me an NBL challenge coin. It is serial #114. I will always treasure it as a marker on the long road connecting their work today with our early efforts almost half a century ago.


Oral History Transcript, Kenneth S. Kleinknecht, Interviewed by Carol Butler, Littleton, Colorado – 10 September 1998, p. 12-18 (accessed June 25, 2007).

Campbell, M.R., M.D., Classics in Space Medicine, Aviation, Space, and Environmental Medicine, Vol. 80, No. 12, December 2009, p. 1077 (accessed Dec. 28, 2012).

Hacker, Barton C., and James M. Grimwood. On the Shoulders of Titans: A History of Project Gemini. Washington, D.C.: National Aerosnautics and Space Administration, 1977. (accessed Jan. 21, 2013.) See pages 356, 370 and 372.

Shayler, David. Walking in Space. Chichester, UK: Praxis Publishing Ltd., 2004. Print. P. 121.