At the height of the Cold War, if you wanted a peek behind the Iron Curtain, it had to be a birds’s eye view from 63,000 feet — above the reach of Soviet SAM batteries. And to fly that high, America’s elite SR-71 pilots had to wear the most advanced flight suits this side of the Apollo program.
In SR-71, USAF Col. Richard H. Graham (ret) recounts the rigorous testing both suit and wearer went through before they ever got off the ground.
Physiological Support Division and the Pressure Suit
Space begins at about 200km above the earth, but as a physiological environment it begins at around 63,000 feet (19km), where the atmospheric pressure becomes so low that fluids boil at the body temperature of 37C. The main function of the pressure suit was to save your life at the extreme altitudes, temperatures and speed at which the SR-71 flew. Above 45,000 feet (14km) any crew member’s effective performance time (EPT) is between nine to 12 seconds without oxygen. EPT is the amount of time an individual is able to perform useful flying duties in the absence of oxygen before going unconscious. Without a pressure suit at 80,000 feet (24km), and the loss of all cabin pressure or an ejection scenario, you could not survive.
The organisation at Beale that maintained the pressure suits was the Physiological Support Division (PSD). The facility was located on the flight line and was the US Air Force’s entire repository for all pressure suit operations. Consequently, PSD had a high level of experienced personnel, who had the technical expertise and capability to do a complete overhaul on pressure suits. To SR-71 crew members, they were highly qualified “technicians” in every sense of the word.
Although not considered a military uniform, the David Clark Company Model 1030 pressure suit was the most prized uniform for aspiring Habus. It meant you were one step closer to flying the aircraft! Before donning the pressure suit each crew member was given a locker in the PSD changing room to store personal effects and their military uniform before flying. Each crew member was issued four pairs of cotton, long underwear (very sexy!), and thick white socks to be worn under the pressure suit. The underwear provided a layer of warmth and reduced skin irritation from the inner liner of the pressure suit.
The Model 1030 pressure suit came in twelve basic sizes. From there everyone was individually fitted with adjustments by arm length, glove size and foot size. Although the pressure suit was a six-layer outfit, three layers were significant. The outermost layer (exterior) was made of fire-retardant material called Nomex. It contained zippered pockets on the upper and lower legs, Velcro patches on the upper legs to secure checklists, and, most importantly, the parachute harness connections.
The inner layer, called the bladder, was made of rubber compound and became inflated, much like a balloon, when air pressure was added to the suit. The rubber layer was irritating to bare skin, thus the need for a comfort liner made from lightweight Dacron material. Also located inside the bladder layer was a network of tubes to direct cooling air throughout the suit, particularly to the extremities.
Between the outer layer and the bladder of the suit was a tightly woven mesh netting designed to keep the bladder from inflating too much. The netting was woven in a special manner so that increased pressure within the inner lining caused the webbing to grow more rigid and contain the expansion.
The PSD facility also housed an altitude chamber capable of evacuating the air to an altitude of 85,000 feet (26km). The purpose of the small chamber was to expose new crew members to the mating of the pressure suit to the ejection seat, build their confidence in the suit, teach them how to operate and control the suit’s temperature and pressure, and, finally, to experience a loss of all cabin pressure at 85,000 feet (26km). This was called a rapid decompression, since it happened in an instant.
Crews put their lives into the hands of the PSD technicians every time they flew. Wearing the pressure suit made it impossible to strap oneself into the SR-71’s ejection seat making all the necessary connections. Consequently, crews were taught to extend their arms out each side of the cockpit and sit there patiently while PSD technicians, on both sides of the cockpit, mated the pressure suit to the aircraft and its ejection seat. They were extremely professional and safety conscious at their job, alert at all times to the danger of being complacent.
Once the crew member was strapped in the ejection seat inside the altitude chamber, the technicians departed and closed the thick door leading out of the chamber. After the chamber was fully sealed, they began evacuating the air pressure inside. As the chamber reached around 25,000 feet (8km), the technicians asked the crew member over the interphone if he was experiencing any sinus problems. If not, the steady climb to 85,000 feet (26km) continued. The first thing you noticed on the climb happened at 63,000 feet (19km). A flask of warm water (37C) placed inside the chamber slowly began to boil. By the time the chamber reached 70,000 feet (21km), it was boiling rapidly, and at 80,000 feet (24km) the water was all gone — evaporated! The water was placed in the chamber to give you an appreciation of what could theoretically happen to your blood or other fluids without a pressure suit on.
To prepare the chamber for the rapid decompression, the greatly reduced air pressure at 85,000 feet (26km) was captured and stored in the larger altitude chamber, connected to the small chamber by huge air valves. Once the thin air was captured, they brought the small chamber back down to 26,000 feet (8km). This was the normal cabin pressure inside the SR-71’s cockpits, flying at 85,000 feet (26km). Since the RD happened so rapidly, the supervisor briefed you on what to expect before throwing the decompression switch: “There will be a loud, explosive ‘BANG’ accompanied by immediate fogging, and then rapid clearing inside the chamber. Simultaneously, the pressure suit will fully inflate and become very rigid.”
When he moved the switch, instantly the cabin pressure would be at 85,000 feet. Just as advertised came the bang, fogging and clearing and full suit inflation. To practice various cockpit chores with a fully inflated suit, they kept the altitude at 85,000 feet (26km). It is surprising just how difficult it was to turn your head inside the inflated pressure suit. The most important task was to be able to reach for the ejection seat handle located between your legs and pull it upward to initiate the ejection sequence. Hopefully, crews would never have to experience an RD for real, but for now they had an ideal opportunity to experience one in a controlled environment.