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U-2 pressure suit
The pressure suits for U-2 pilots undergo thorough inspection and testing prior to each mission. The U-2 is operated by the 9th Reconnaissance Wing based at Beale AFB, California. (credit: USAF)

The stratosphere and suborbit: shirtsleeves or pressure suits?


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On October 31, 2014, Virgin Galactic experienced the aerospace equivalent of a black swan event. The phrase was coined by author Nassim Taleb, who wrote a book about unanticipated financial events that have sweeping and often dramatic global consequences. It is a metaphor drawn from the common belief that all swans are white, until one encounters a rare black swan that upends that belief.

Ever since the historic suborbital flight of SpaceShipOne in 2004, the pilots and co-pilots of these cutting-edge spacecraft have never worn pressure suits. With a pressurized cabin, it was thought a pressure suit was unnecessary for trips into the stratosphere.

There was considerable speculation in the media over the cause of the SpaceShipTwo accident before the facts could be collected. This event became the first commercial human spacecraft accident investigation led by the National Transportation Safety Board (NTSB). The NTSB issued several preliminary statements in the week following the crash. A full investigation report, though, will not be released for many months.

It was soon established the feathering system, designed to deploy to slow the aerodynamic reentry from suborbit, deployed prematurely. The spacecraft was at an altitude of about 15,000 meters and traveling at roughly Mach 1 when this occurred. The extreme aerodynamic forces induced structural failure and explosive decompression of the crew cabin.

Pilot Peter Siebold told the NTSB that the spacecraft broke apart around him. Both pilots were wearing conventional flight helmets with oxygen masks, but not pressurized flight suits. Siebold was immediate exposed to a temperature of –57°C, an atmospheric pressure of just 15 percent of sea level, and had less than ten seconds before experiencing hypoxia—oxygen starvation. While he was still conscious and physically able, he unbuckled his seatbelt and fell clear of the still disintegrating spacecraft. His co-pilot, Michael Alsbury, did not survive the disaster.

“I can’t imagine the forces he must have experienced being thrown out of the aircraft,” Jeff Sventek, executive director of the Aerospace Medical Association told Bloomberg News. He described Siebold’s survival in such brutal conditions as “extremely remarkable.”

It is believed Siebold was unconscious as he fell and that a sensor automatically deployed his parachute a lower altitude. He regained consciousness and was alert when he landed, but sustained injuries.

Ever since the historic suborbital flight of SpaceShipOne in 2004, the pilots and co-pilots of these cutting-edge spacecraft have never worn pressure suits. They have worn a casual, one-piece Air Force flight uniform of comfortable fabric, with the requisite zippered pockets from shoulder to calf that practically shouted “pilot.” With a pressurized cabin, it was thought a pressure suit was unnecessary for trips into the stratosphere.

U-2, X-15, and Mercury astronaut flight suits

The stratosphere begins at an altitude of roughly 10 kilometers and extends to 50 kilometers. For over half a century, this has been the flight domain of the U-2 reconnaissance aircraft. It is a little known fact that this able aircraft is still operated today by the 9th Reconnaissance Wing at Beale Air Force Base, California, with an active force of 22 aircraft. The U-2 is employed for all-weather surveillance, reconnaissance, and signal intelligence anywhere in the world in support of the United States and allies.

The U-2 has an operational ceiling above 21,000 meters. The pilots of this aircraft have always worn pressure suits and helmets manufactured by David Clark Company of Worcester, Massachusetts. This company is America’s premier flight suit manufacturer for pilots and astronauts. David Clark also made the suit for Felix Baumgartner’s free-fall jump from over 39 kilometers.

The U-2 flight suit is built up from four separate layers. The suits undergo thorough inspection by technicians prior to the pilot suiting up, a task which typically requires the assistance to two technicians. These suits provide not only oxygen but proper temperature and dehumidification. They are completely capable of protecting the pilot in the event of a bailout at 21,000 meters.

Neither the Air Force nor NASA considered attempting human flight in these environments without a pressurized flight suit. Virgin Galactic has consistently maintained that pressure suits will be unnecessary on their suborbital flights.

The X-15 was the US Air Force and NASA’s hypersonic research vehicle between 1959 and 1968. (see “Review: The X-15 Rocketplane”, The Space Review, July 22, 2013). It was built by North American Aviation and was powered by a Reaction Motors rocket engine. It was carried aloft by a modified B-52 bomber, and released at an altitude of around 13,500 meters.

The X-15 pilots executed a very specific profile for each flight, but in every case the cockpit became pressurized at about 10,700 meters and the pressure suit supplied oxygen for the pilot. Thirteen test flights exceeded 82 kilometers. Two flights by pilot Joseph A. Walker in 1963 exceeded 100 kilometers, which the Fédération Aéronautique Internationale considers to be the threshold of space. The X-15 operated at speeds in excess of 5,500 kilometers per hour. William Knight’s flight in October 1967 achieved a speed of more than 7,200 kilometers per hour.

The X-15 suit was multilayered with a distinctive silver outer layer. The X-15 had an ejection seat but this was never used during the program, not even during the fatal flight of Michael Adams in November 1967. This accident is fully described in the book cited above.

Mercury astronauts wore a pressure suit that was similar in appearance to the X-15 suit but constructed differently. The first two Mercury flights were suborbital. A Redstone rocket launched Alan Shepard on May 5, 1961, and his capsule reached an altitude of 187 kilometers. That July, Gus Grissom flew a second suborbital flight, also lasting 15 minutes.

Armstrong and Shepard
X-15 pilot Neil Armstrong (left) and Mercury astronaut Alan Shepard wore similar pressure suits for their flights. They both went on to command Apollo 11 and Apollo 14, respectively. (credit: NASA)

The suborbital flights of SpaceShipTwo were significantly higher than those of the U-2 aircraft, about the same as the X-15, but significantly lower than the Mercury suborbital flights. Neither the Air Force nor NASA considered attempting human flight in these environments without a pressurized flight suit. Virgin Galactic has consistently maintained that pressure suits will be unnecessary on their suborbital flights.

“We have always been clear that a shirt-sleeve environment was part of the baseline design,” said Virgin’s Stephen Attenborough in a 2013 magazine article. “However, safety remains the priority, and should any new factors emerge that mean we should change that or any other element, then of course we will do so.”

Just two weeks before the crash of SpaceShipTwo, Virgin Galactic’s president and CEO George Whitesides told the British newspaper The Telegraph, “We think the safest thing is to not have people in pressure suits but to have them in flight suits and then in a cabin which protects them and allows them the freedom of microgravity because these people will be able to get out of their seats and float around the cabin.”

Virgin Galactic has a significant, real-world precedent to point to in support of this view.

Counterpoint: the Concorde supersonic transport

The supersonic commercial passenger jet Concorde was flown by British Airways and Air France from 1976 to 2003. It had an operational ceiling of 18,300 meters, somewhat lower than the U-2, and flew at Mach 2. It used conventional cabin pressure equivalent to roughly 2,000 meters above sea level. The passenger windows were smaller than those on a conventional passenger jet, but the loss of a single window would still have resulted in complete loss of cabin pressure. Professional aerospace journals state that at that altitude, passengers would only have a period of useful consciousness of no more than fifteen seconds, emergency oxygen masks notwithstanding.

In this situation, the Concorde pilots would have initiated a rapid descent, but this would have required more time than that for standard jets flying at half the altitude. However, in more than 25 years of operation, Concorde never experienced a rapid decompression. The passengers were permitted to move about the narrow cabin during most of the flight. Of course, pressure suits as worn by the U-2 and X-15 pilots are primarily used in the event of the need to eject. In the event of a decompression event on the Concorde, the passengers would have had to endure the predictable consequences of hypoxia.

A mere one-cm hole in the wall of the cabin would give passengers and crew less than 50 seconds before they were incapacitated by hypoxia. Even if the pilot and co-pilot remained conscious, it would take a suborbital vehicle many minutes to return to Earth and to an adequate atmospheric environment.

Ever since the International Space Station (ISS) has supported crews, the world has seen video of the men and women insides wearing shirtsleeves and shorts or pants. We saw similar images televised from the Space Shuttle. Going even further back, one can see astronauts aboard Skylab performing somersaults and other whimsical feats in nothing more than T-shirts and shorts. These images all reinforce the public perception that space travel is safe, belying the fact that a very hostile and dangerous environment exists just centimeters away.

There are very reliable safeguards and protective protocols for astronauts aboard the ISS that permit them the comfort of casual attire. They are too lengthy to list here, but crews know how to respond quickly and safely in an emergency.

There has always been another risk involved in flight at high altitudes, known as ebullism. This results from the combination of subfreezing temperatures and lowered boiling point of water at these altitudes. Body volume rapidly expands, moisture in the respiratory system freezes, and this causes paralysis in about thirty seconds. It is a horrific form of death.

Comparing commercial spaceplanes

How does Virgin Galactic’s take on crew and passenger protection compare to XCOR Aerospace’s Lynx Mark II and Sierra Nevada Corporation’s Dream Chaser?

The XCOR Lynx Mark II, the commercial variant of the current test vehicle under construction, will have a pressurized cabin, but both the pilot and the one spaceflight participant on board will wear pressure suits. The XCOR website states the following:

Lynx will have a pressurized cabin; however, pilot and participant will wear full pressure suits during flight for safety in case of an emergency. The pressure suit will be custom designed for XCOR by one of the leading manufacturers of pressure suits such as David Clark Company or Orbital Outfitters.

The Dream Chaser spacecraft is moving forward despite Sierra Nevada Corporation (SNC) losing a NASA competition provide commercial crew transport to and from the ISS. Dream Chaser is the product of a military contractor, so it comes as no surprise that the crew will wear pressure suits. This was the original intent when the company was competing for the commercial crew contract and it remains so today. SNC is also looking at a scaled-down version of Dream Chaser, about 75 percent the size of the original, which could be launched from the massive Stratolaunch carrier craft.

Sir Richard Branson is one of the most successful and colorful entrepreneurs in the world. The success of his many companies in the Virgin Group has made him a multi-billionaire. When he launched Virgin Atlantic Airways in 1984, he did not have to be concerned with the reliability or safety of the aircraft. Commercial jet transportation had a proven technological history spanning three decades. He only had to concern himself with superior customer service and routes.

When Branson announced Virgin Galactic, many in the financial community saw it merely as an extension of his existing transportation empire. However, Branson was now confronted with a completely new and unproven form of commercial transportation, one that would take years to develop and prove safe. He doesn’t only see the company performing suborbital flights from and back to the same site. He wants to eventually develop vehicles to perform intercontinental flights. After the crash of his spacecraft and death of one of its pilots, there are certainly some issues to reconsider.

Relative to commercial passenger jets, the cubic volume of the suborbital spacecraft currently under development is quite small: less than 35 cubic meters. A mere one-centimeter hole in the wall of the cabin would give passengers and crew less than 50 seconds before they were incapacitated by hypoxia. Even if the pilot and co-pilot remained conscious, it would take a suborbital vehicle many minutes to return to Earth and to an adequate atmospheric environment.

Michelle La Vone, writing about Peter Siebold’s harrowing survival for Space Safety Magazine in December, stated:

What we do know is that even if Siebold did not experience ebullism, future space tourists—in the event of a cabin depressurization or spacecraft breakup—could. That’s because the space industry has defined the ‘outer edges of space’ as 62 miles, or nearly 100 km, well past the Armstrong limit (the point at which water boils at 98.6 degrees Fahrenheit, the temperature of the human body). The possibility of ebullism (and other pressure-related elements) drives home the need for all passengers to don pressure spacesuits and oxygen masks, not T-shirts and shorts like some idealized visions of consumer space travel.

Branson wants to make national and international suborbital spaceflight a successful long-term business reality. Making it safer than it currently is will be a challenge, but not an impossible one. At the same time, Virgin Galactic doesn’t want to see their passengers tethered to their seats with oxygen hoses and restricted by bulky pressure suits. Richard Branson and those at Virgin Galactic are sure to be giving the best solution a great deal of thought.


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