Passenger Training and Certification

  • Erik Seedhouse
Part of the Springer Praxis Books book series (PRAXIS)


How much training will the new breed of commercial suborbital astronaut require? I think it is safe to presume he or she will require more training than the few days set aside by XCOR for its spaceflight participants and certainly much less training than that required by government trained astronauts preparing for increments on board the International Space Station (ISS). But where does the sweet spot lie? How long will it take to train a payload specialist to work efficiently and productively in an environment that will be extremely unforgiving of real-time snafus? And let’s not forget what the demands of that environment are. First there is the wow factor brought on by those jaw-dropping views (Figure 7.1) and then there is the cost of the time spent in microgravity: $150,000 divided by four minutes equals US$37,500 per minute, or close to US$625 per second. Don’t drop anything!! My suggestion to you if you plan to fly as a payload specialist is to gain as much experience in analog environments as possible. Become a scuba-diver, learn to fly an aircraft, and gain as much exposure to weightlessness on board parabolic flights as possible.


International Space Station Parabolic Flight Oxygen Mask Hypobaric Chamber Flight Director 
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How much training will the new breed of commercial suborbital astronaut require? I think it is safe to presume he or she will require more training than the few days set aside by XCOR for its spaceflight participants and certainly much less training than that required by government trained astronauts preparing for increments on board the International Space Station (ISS). But where does the sweet spot lie? How long will it take to train a payload specialist to work efficiently and productively in an environment that will be extremely unforgiving of real-time snafus? And let’s not forget what the demands of that environment are. First there is the wow factor brought on by those jaw-dropping views (Figure 7.1) and then there is the cost of the time spent in microgravity: $150,000 divided by four minutes equals US$37,500 per minute, or close to US$625 per second. Don’t drop anything!! My suggestion to you if you plan to fly as a payload specialist is to gain as much experience in analog environments as possible. Become a scuba-diver, learn to fly an aircraft, and gain as much exposure to weightlessness on board parabolic flights as possible. You may be thinking why a payload specialist flying in the right seat of the Lynx would need all this supplementary training given that the astronaut will be strapped into their seat for the duration of their flight, but spaceflight – whether you happen to be strapped in or floating freely – is a profoundly disorienting experience, so it makes sense to spend as much time exposed to similarly challenging environments. In terms of what phases of training will be required, payload specialists will need to complete XCOR’s vehicle familiarity training, which is included in the ticket price. That phase will take about three days. Beyond that, the payload specialist will need to complete training specific to operating their payload in addition to familiarization on board a parabolic flight. Total time? It depends on the complexity of the payload and/or science being flown, but training will take at least a week and perhaps as long as three.

The view from orbit. Credit: NASA

This chapter is all about teaching the average person in the street how to fly in space. Some will be buying a ticket on the Lynx for the thrill of rocketing into space, some will be on a mission to test a payload, and some will be scientists. But, no matter what their background, they will all need to be trained because once the Lynx starts flying we will be leaving behind the era of governments selecting astronauts based on intelligence and aptitude for spaceflight and we will be entering a period when astronauts select themselves based mostly on the thickness of their wallets. Way back when the Shuttle was flying, astronauts trained for at least a year for a two-week flight. Today, in the ISS era, astronauts typically train for four years for an increment lasting up to six months. Four years! With the advent of suborbital flights, we will have an extreme end of the astronaut training spectrum, with spaceflight participants requiring perhaps as little as three days of training. What will that training include? Well, below is a generic schedule of the sort of training XCOR will be delivering for its passengers.
  • Day One AM
    • Classroom:
      • Regions of the atmosphere

      • Altitude physiology and the hypobaric chamber

      • Unusual attitude flight profiles and Mach flight

  • Day One PM
    • Chamber:
      • Rapid and slow decompression in the hypobaric chamber

    • Classroom:
      • Acceleration physiology and the anti-G straining maneuver (AGSM)

      • Spacecraft safety and emergency egress

      • Lynx indoctrination: safety systems and mission architecture

  • Day Two AM
    • Classroom:
      • AGSM review, theory, and practical. Centrifuge manifest assignment

    • Centrifuge:
      • Gradual onset runs (GOR) # 1 and 2 (familiarization to 6 Gs)

      • Rapid onset runs (ROR) # 1 to 3 (ROR4 for 15, ROR5 for 15, and ROR6 for 15)

      • Debrief/review of G-videos

  • Day Two PM
    • Classroom:
      • Lynx life-support systems

      • Final frontier design (FFD) spacesuit indoctrination

      • Spacesuit donning and doffing (practical)

      • Spacesuit ingress and egress (simulator)

    • Chamber:
      • Armstrong line chamber run to 24,380 meters wearing spacesuit

  • Day Three AM
    • Classroom:
      • Flight briefing

      • Unusual attitude and high-G flight in Extra 300. 45 minutes


In 2008, Nassim Nicholas Taleb published The Black Swan: The Impact of the Highly Improbable and so the “black swan” event was born. A black swan event is one that is rare and difficult to predict, such as the 1987 stock market crash, the 11 September attacks, or the SpaceShipTwo accident (Figure 7.2). As I’m sure you’re aware, SpaceShipTwo crashed in October 2014 when its feathering system deployed prematurely when the vehicle was traveling at Mach 1 at an altitude of 15,000 meters. Extreme aerodynamic forces caused the vehicle to disintegrate and the cabin suffered an explosive decompression. While the pilot Peter Siebold and co-pilot Michael Alsbury were wearing flight helmets and were hooked up to oxygen masks, they were not wearing pressure suits. The consequences were dire. Immediately, Siebold and Alsbury were exposed to −57°C and an atmospheric pressure of 15% of sea level, which meant they had less than 15 seconds of useful consciousness. Fortunately, even though Siebold was unconscious, his parachute opened automatically and he survived. Alsbury wasn’t as fortunate. Ever since the X-Prize-winning flight of SpaceShipOne in October 2004, the Virgin Galactic pilots have worn one-piece flight suits because the thinking was that the pressurized cabin would be sufficient protection against the elements. Should they have worn pressure suits? After all, the U-2 pilots have been wearing them for decades, and this legendary reconnaissance aircraft routinely operates at altitudes similar to those encountered by SpaceShipTwo during its testing program. With an operational ceiling of 21,000 meters (the actual ceiling is classified), the U-2 is used for weather surveillance and signal intelligence among other activities. Its pilots, who are attached to the 9th Reconnaissance Wing at Beale Air Force Base in California, have always worn pressure suits, provided by the David Clark Company, which also happens to be the same company that made the pressure suit used by Felix Baumgartner for his free-fall jump from 39,000 meters. These state-of-the-art suits (Figure 7.3) provide the pilots with oxygen and also ensure comfort by regulating temperature and humidity. In the event that a flight goes pear-shaped, the suits are more than capable of protecting a pilot from bail-out, even if the bail-out altitude happens to be above 20,000 meters.

The SpaceShipTwo accident. Credit: NTSB


U-2 pilot. Credit: USAF

Another aircraft that required its crew to wear pressure suits was the X-15, the hypersonic research vehicle that flew between 1959 and 1968. Built by North American Aviation and carried aloft to 13,500 meters by a B-52, the X-15 (Figure 7.4) featured a cockpit that became pressurized at 10,700 meters and pilots wore a pressure suit that supplied oxygen. In addition to the 13 test flights that exceeded 82,000 meters, the X-15 was flown into space on two occasions, each time piloted by Joe Walker in 1963. In addition to Walker’s suborbital excursions, it’s worth remembering the two suborbital flights of the Mercury program: the first, piloted by Alan Shepard on 5 May 1961, reached an altitude of 187 kilometers, and the second, piloted by Gus Grissom, reached an altitude of 190 kilometers. Grissom and Shepard (Figure 7.5) wore pressure suits. And then there was the most seriously badass aircraft ever to take to the skies: the Blackbird, aka the SR-71. Back in the days of the Cold War, if the US wanted to know what was going on behind the Iron Curtain, it had to deploy an aircraft capable of flying very, very fast and very, very high. That aircraft was the SR-71 (Figure 7.6) and its elite pilots wore the most cutting-edge pressure suits available, which happened to be the David Clark Company Model 1030 back in those days. The organization responsible for fitting and testing the suits was the Physiological Support Division (PSD) at Beale Air Force Base facility that also housed an altitude chamber capable of testing to 26,000 meters. The 1030 suit comprised an inner rubber layer – the bladder – that was lined with tubes connected to cooling air, and between the bladder and the outer layer was a mesh material that prevented the bladder from over-inflating. As a confidence test, each SR-71 pilot was subjected to a rapid-decompression (RD) test to 26,000 meters. After struggling into the 1030 suit, they were strapped into the SR-71’s ejection seat (wearing the pressure suit meant the pilots couldn’t do this themselves) by PSD technicians inside the chamber. The chamber’s door was closed and the decompression test began. Once 7,620 meters had been reached, the pilot was asked whether he had any sinus issues. If everything was clear, the ascent continued onward and upward to 26,000 meters as the pilot kept a close eye on a glass of water inside the chamber. As the altitude approached 19,200 meters (the Armstrong Line), it slowly began to boil, which was a stark reminder of what would happen to the pilot’s body fluids if he wasn’t wearing a pressure suit. After the 26,000-meter ceiling was reached, the altitude was brought down to 8,000 meters, which was the SR-71’s cabin altitude. In an adjacent chamber, the altitude was brought up to 26,000 meters and the pilot was warned of the impending RD event. At the flick of a switch, the cabin altitude instantly rocketed up 26,000 meters accompanied by a loud bang and fogging. As a result of the rapid pressure change, the suits would become rigid to give pilots an idea of how difficult routine cockpit tasks – such as pulling the ejection handle, for example – would be following an RD.

X-15. Credit: NASA


Al Shepard. Credit: NASA


SR-71. Credit: USAF

“We have always been clear that a shirt-sleeve environment was part of the baseline design. 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.”

Virgin’s Stephen Attenborough in a 2013 magazine article

“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’s president and CEO George Whitesides, The Telegraph, two weeks before the SpaceShipTwo crash

Virgin Galactic’s position on pressure suits seems to be contradictory given the history of manned suborbital spaceflight, doesn’t it? After all, NASA and the USAF never for one moment considered flying their vehicles without their pilots wearing a pressure suit, so why did Virgin Galactic think it would be safe? Perhaps they based their mode of operations on the Concorde experience? Concorde was a supersonic passenger jet that was flown between 1976 and 2003 at an altitude of 18,000 meters. At this altitude, a puncture in the skin of the aircraft would have meant passengers would have had about 15 seconds of useful consciousness unless they managed to grab their oxygen masks. Another factor that may have swayed Virgin Galactic away from the requirement to wear pressure suits was the work attire of astronauts on board the ISS. Those working on the orbiting outpost wear nothing more than shirtsleeves and pants – an image that reinforces the perception that flying to space is safe. Of course, it is anything but, because astronauts are among the most highly trained humans on and off the planet, and there are myriad emergency protocols in place to protect crewmembers in the event of an off-nominal event. At this point, it is worth comparing the approaches of other vehicle providers to see what their perspective on pressure suits is. XCOR we know will require its pilot and passenger to wear a pressure suit, and those ferried on board Sierra Nevada’s Dream Chaser will also be wearing spacesuits, so why does Virgin Galactic insist on only flight suits for its crew? After all, if a commercial passenger jet were to suffer a puncture, the oxygen masks drop down and the pilot dives to lower, breathable altitude. This will not be the case in a suborbital flight because, once the vehicle is on its way to space, it will be several minutes before it reaches a survivable altitude, by which time the passengers could be … well, let’s not think about that.

“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.”

Michelle La Vone, Space Safety Magazine, December 2014

Of course, Branson wants to make spaceflight fun and that will prove difficult if his passengers are tethered with oxygen hoses and constrained by bulky pressure suits but, in light of the SpaceShipTwo accident, the pressure suit issue may be one worth reconsidering.

Altitude Physiology

One of the academic sections that will be delivered to prospective Lynx astronauts will be a lesson on altitude physiology. It’s an important module because knowledge of basic altitude physiology helps you understand why pressure suits are so very, very important. But first some background. To earn your bragging rights as an astronaut, you need to fly to 100 kilometers of altitude, which is the internationally recognized boundary of space – unless you happen to be a Virgin Galactic passenger, in which case the altitude is a little lower.1

But, before we talk about what happens to the body in space, we need to understand what happens on Earth. We’ll begin with the atmosphere, which is divided into layers (Figure 7.7). The lowest layer, which is the troposphere, is the one that we’re most concerned with during most day-to-day activities. This layer also happens to be the most complex because there are so many variables that can affect conditions in this section of the atmosphere. Beginning at sea level and extending to an altitude of 7,000 meters at the poles and up to 18,000 meters at the equator, the troposphere contains about 75% of Earth’s atmosphere. In this layer, temperature rises by six degrees Celsius for each 1,000-meter rise in altitude and, as you gain altitude, pressure also falls. At sea level, this pressure is 760 mmHg, but this number falls as altitude is gained, and this is when problems may be encountered.

Layers of the atmosphere. Credit: NASA

Continuing up through the troposphere, we eventually encounter the tropopause – a boundary layer characterized by stable temperatures. Next is the stratosphere – a layer which will be familiar to all of those who followed Felix Baumgartner’s Project Stratos. The stratosphere, which extends all the way up to 50 kilometers, contains the ozone layer and is defined by rising temperatures with increasing altitude – a characteristic that results in very stable atmospheric conditions. If you are a WorldView (Figure 7.8) passenger, then this is the layer you will be visiting in your gondola.

WorldView. Credit: WorldView

Stacked above the stratosphere is the mesosphere, which extends almost all the way to the top of our atmosphere at an altitude between 80 and 85 kilometers. In common with the trend in the troposphere, temperature falls with increasing altitude, but temperatures in the mesosphere are much colder than the lower layers and can be as low as −100°C: that’s cold enough to freeze water vapor into ice clouds, which are known as noctilucent clouds – the subject of Project PoSSUM. Separated from the mesosphere by the mesopause is the thermosphere, which is the outer layer of our atmosphere. This layer extends all the way to an altitude of 640 kilometers and is marked by temperatures rising to as high as 1,000°C. Now that sounds hot, but you have to remember that up at thermosphere altitudes, there are so few molecules that there would not be enough energy for us to feel that heat. Beyond the thermosphere is the exosphere, which gradually merges into deep space.

So that’s our atmosphere. But what happens as we fly through those layers? Well, the most important point to remember is that as altitude increases so does air pressure, which is why spacecraft cabins are pressurized. If you have flown commercial, you may have noticed a faint hissing sound once the doors have closed. That’s the sound of the aircraft being pressurized. When your aircraft takes off, the cabin pressure is slightly higher than the outside air pressure and, as the aircraft ascends, the pressure is adjusted to decrease the differential between the internal pressure and outside air pressure, which is why most commercial aircraft fly with a cabin pressure of around 1,600 meters. If the cabin pressure was a little higher (say 2,500 meters), people with respiratory problems would have difficulty breathing and, if the cabin pressure was a lot higher (say 4,000 meters), then all the passengers would experience low-pressure symptoms – headaches, dizziness, vomiting – and that would be bad for business. Reduce the altitude some more and eventually all the passengers would lose consciousness (Table 7.1).
Table 7.1

Time of useful consciousness

Altitude flight level

Altitude (feet)

Altitude (meters)

Time of useful consciousness




30 min or more




20–30 min




5–10 min




3–6 min




2.5–3 min




1–3 min




30–60 sec




15–20 sec




9–15 sec

500 and above



6–9 sec

The reason we humans lose consciousness at high altitudes is because our brain cells just weren’t designed to function properly at low pressure, hence the need for those bulky pressure suits we talked about earlier (see sidebar). Here’s some history. There are two primary types of pressure suit: the full pressure suit and the partial pressure suit, although this latter term is a little misleading because both suits fully protect the human inside the suit, it’s just that a different approach is used depending on the suit. The full pressure version (EVA) (Figure 7.9) encloses the human completely in an airtight pressurized suit, whereas the partial pressure version (Figure 7.10) relies on figure-hugging material and tubular air compartments running parallel to the arms to exert pressure on the wearer’s skin.

EVA. Credit: NASA


X-1. Credit: NASA

No Pressure Suit?

Those of you who have watched 2001 A Space Odyssey will no doubt remember the explosive decompression scene. In this scene, Bowman (Keir Dullea) is performing a spacewalk inside an escape pod and is prevented from re-entering the Discovery by the HAL-9000 computer. But Bowman blows the bolts on the pod and enters Discovery’s airlock – an act that causes him to be exposed to a vacuum for 14 seconds before he can re-pressurize the compartment. Fiction or fact? Actually, such an exercise would be survivable, although not advisable. Way back in the 1960s, the USAF subjected chimpanzees to explosive decompression which left the hapless test subjects exposed to vacuum for more than three minutes in some cases. All of the guinea pigs survived except one. If you were to be exposed to an explosive decompression event, then we know you would have up to 10 seconds to help yourself, so think quickly! After 12 or 13 seconds you would begin to experience impairment and, if you weren’t wearing a pressure suit, your body would begin to swell because the liquid in your soft tissues would begin to vaporize. Contrary to what you may have seen in Hollywood movies – the decompression event in Outland comes to mind – you would not explode because the skin is extraordinarily resilient. After that swelling sensation is noticed, your blood would stop circulating. At this point, you will have been exposed to a vacuum for around 60 seconds. Next, the vacuum would go to work on your lungs with catastrophic results because your pulmonary system is perhaps the most vulnerable to extreme low pressure. So, if you are exposed to a vacuum due to an explosive decompression, there is time to take action. But be quick!

Full pressure suits, which were descendants of undersea diving suits, were the first to be tested for high-altitude operations by scientists in the US and Great Britain in the 1930s, whereas the partial pressure suit was born out of a need for such a garment in the Second World War. Much of the research conducted on partial pressure suit design was carried out by Dr. James Henry at the Biophysics Branch of the Army Air Force’s Aero Medical Laboratory, which today is known as Wright-Patterson Air Force Base. Much of Henry’s work on the design on the partial pressure suit came from the world of G-suit protection for fighter pilots, which is why his mechanical pressure suit featured tubular assemblies (capstans – in the G-suit, these devices were inflated to maintain blood pressure). Henry’s partial pressure suit was capable of protecting a pilot at altitudes as high as 25,000 meters, which was a performance capability that was so impressive that the army’s high command recommended that further research be carried out in cooperation with the David Clark Company which was a prime contractor for the US Army’s G-suits at the time. During the post-war cooperative research, Dr. Henry and the David Clark Company developed the S-1 and S-2 suits, which, after a series of refinements, morphed into the MC-series suits in the 1950s and 1960s. But, by the 1960s, interest in the partial pressure suit was waning due to an emphasis on full pressure suits that were required for those crewmembers involved in projects such as the X-15, X-20 (DynaSOAR), the Manned Orbital Lab, and Project Mercury. These space and near-space projects demanded very high-altitude life-support requirements which could only be met by the full pressure suit and so the partial pressure suit died a natural death, with the exception of a space activity suit that was developed as part of a special NASA program in 1971.

Today’s full pressure suits are a layered system that act together to protect the occupant. At its simplest, the suit is a balloon filled with air and the balloon in this case is the bladder, which acts as a restraint layer that prevents the suit from rupturing. Other restraint layers may be included, such as anti-G layers that help counteract the effects of G during high-G turns or during launch and re-entry. The exterior layer, which is usually brightly colored, is often made out of fire-retardant material (Nomex). For many decades, the basic design of the full pressure suit has remained the same, but changes may be on the way thanks to Dr. Dava Newman, a MIT bio-astronautics engineer (and now Deputy Administrator of NASA) who developed the BioSuit (Figure 7.11).

Dava Newman wearing the revolutionary BioSuit. Image courtesy: Professor Dava Newman, MIT: Inventor, Science and Engineering; Guillermo Trotti, A.I.A., Trotti & Associates, Inc. (Cambridge, MA): Design; Dainese (Vicenza, Italy): Fabrication; Douglas Sonders

Classed as mechanical counter-pressure (MCP), the BioSuit appears more like a skinsuit than a standard full pressure suit, but it is still designed to do the same job. The principle behind the suit is applying the same pressurization as is applied by a standard full pressure suit but applying that pressure directly to the skin in a way that sidesteps the need for gas pressure. Newman and her team reckon this can be achieved by using the latest in active compression fabrics. If they are right, then future astronauts will be able to do away with the bulky restrictive balloon suits and look forward to a day when they can don a lightweight high-mobility suit instead. But what about the suit you will be wearing in the Lynx? Well, there is a good chance it will be provided by the innovative engineers at FFD, a spacesuit company based in Brooklyn. FFD came about as a result of a meeting between Ted Southern and Nik Moiseev in 2007. Nik had spent the best part of 20 plus years working in the Soviet Union designing cutting-edge spacesuits, whereas Ted was an artist who had a hand in designing Victoria Secret garments (the angel wings happen to be one of Ted’s signature items). As competitors in NASA’s Centennial Challenge to design an astronaut glove, Ted and Nik’s design was placed second, which was good enough for US$100,000 in grant money from the agency. FFD was born out of that grant money and the rest is history. Today FFD is at the cutting edge of spacesuit design (Figure 7.12) and the company is already working on their third-generation suit which has been tested by Project PoSSUM astronauts in Embry Riddle’s suborbital simulator.

Ted Southern. Credit: Final Frontier Design

High-Altitude Indoctrination (HAI) Training

To train you how to react to an RD, operators will require you to complete high-altitude indoctrination (HAI) training. This will begin with a ground school component which will cover the basic principles of altitude, which we have already covered in this chapter. You will then be introduced to the altitude chamber (Figure 7.13) where the next phase of training will begin (see sidebar). Here you will be assigned seats, given a brief overview of the flights and what you can expect at each flight level, and a safety briefing. You will be accompanied in the chamber by one or two observers who will talk to the physiological training officer or flight surgeon observing the flight from the console outside the chamber.

Hypobaric chamber. Hospital Corpsmen 2nd Class Kyle Carswell and Daniel Young monitor members of the 2009 class of NASA astronaut candidates for hypoxia in an altitude chamber. Credit: NASA/USN (File: US Navy 091006-N-9001B-017)

The MACC Suit Chamber

Capable of flying to 30,480 meters of altitude and supporting RD rates of sea level to 30,480 meters in less than five seconds, the MACC Suit Chamber comprises a two-meter-diameter cylinder that seats two fully suited astronauts. It’s the perfect environment for all manner of astronaut training, including simulating flight profiles, practicing emergency procedures, evaluating suit performance in low-pressure conditions, and building confidence while wearing a pressure suit during rapid or slow decompression.

When it is time for your flight, you will find the chamber crew ready and waiting with the chamber prepped and ready to go. You will be shown into the chamber, don your helmet and mask, hook up your mask, and check your communications and oxygen systems. The inside observers will then complete a physical check of everyone’s oxygen connection to make sure the connections are secure. This will be followed by a communication check beginning with the students and ending with the chamber crew. As chamber technicians ready the chamber for the flight, the inside observers will go through some of the material covered in the lectures by asking the students questions related to altitude physiology and safety procedures during the flight. With the final checks complete, the hatch will be closed and the flight director will give a thumbs-up, indicating that the flight is about to begin. Once the flight director receives a thumbs-up from each student, the chamber will begin its ascent to 1,500 meters at a rate of 1,500 per minute. At 1,500 meters, the chamber will level off and the inside observers will ask each student to complete an ear and sinus check followed by a confirmatory thumbs-up. If everyone has clear sinuses, the chamber will return to sea level and the inside observers will once again ask for a thumbs-up to make sure everyone is ready for the hypoxia demonstration flight to 7,620 meters. After leveling off at 7,620 meters, the students will be divided into two groups. The first group will drop their oxygen masks to experience hypoxia after being told to put on their oxygen masks as soon as they experience one clear-cut symptom of hypoxia. As a flight director, I have been in charge of dozens and dozens of chamber runs and it’s always interesting to see how people react. Pilots tend to be the worst when reporting symptoms because this group is a competitive lot and each pilot wants to have the bragging rights of having been without oxygen the longest, or at least for the six minutes permitted. While off oxygen, the students perform simple tasks such as drawing cats and dogs, subtraction and multiplication, and answering general-knowledge questions. These tasks are intended to demonstrate to the students just how insidious hypoxia can be. During the exercise, the students are confident that they are completing the tasks correctly, but the results – which are presented to the students following the chamber run – often tell a different story. To help the inside observers monitor symptoms, the group experiencing hypoxia wear pulse oximeters that display oxygen saturation. These devices don’t lie, although this doesn’t stop some trying to push through their hypoxia symptoms. I remember one fighter pilot who had been off oxygen for more than five minutes who insisted he had no hypoxia symptoms despite his pulse oximeter reading 61%! On another occasion we had a guy who completed the six minutes off oxygen with no clear hypoxia symptom. After the chamber run, we asked him some background questions which revealed he was a 40-a-day smoker: this guy’s body was in a permanently hypoxic state! Every once in a while, a student pushes the hypoxic limit and it is up to their buddy, sitting opposite wearing their oxygen mask, or the inside observer to step in and hook up the incapacitated student to their oxygen mask. Once everyone in the first team has experienced one clear-cut symptom of hypoxia and is hooked up to their masks, it is the turn of the second group to drop their masks and the exercise is repeated. After the flight, the students are quizzed about their hypoxia symptoms (Table 7.2). Some lose their color vision, some become dizzy, and others feel tingling in their extremities. One interesting characteristic of hypoxia is that symptoms can be very variable. Just because you experienced a particular symptom one day doesn’t mean you will experience the same symptom the next time, so it’s helpful to know the range of symptoms.
Table 7.2

Hypoxia symptoms


Indifferent: 90–98% oxygen saturation

Compensatory: 80–89% oxygen saturation

Disturbance: 70–79% oxygen saturation

Critical: 60–69% oxygen saturation

Altitude (1,000 feet)






Decrease in night vision


Poor judgment

Impaired coordination

Impaired efficiency

Impaired handwriting

Impaired speech

Decreased coordination

Impaired vision

Impaired cognitive function

Impaired judgment

Circulatory failure

CNS failure


Cardiovascular collapse


The day after their hypoxia demonstration, the students enter the chamber for a second flight to 13,000 meters. The purpose of this flight is to demonstrate positive pressure breathing (Figure 7.14) and also to provide another opportunity for students to practice clearing their ears during the descent. At 3,000 meters, students are told to drop their masks, although the inside observers keep theirs on until the chamber reaches ground level. Two chamber runs down, two to go. The third chamber run is the RD flight, which is intended to simulate an immediate loss of pressure as a result of a puncture in the skin of the Lynx and spacesuit.

Fighter pilot. Credit: USAF

Before describing what happens during the RD, it is helpful to understand the layout of the chamber. The chamber has two sections, one of which is the main chamber and the other the secondary chamber or lock. The chambers can operate independently thanks to a hatch and valve separating the two, which means that the main chamber can run at a different pressure than the lock. During the RD run, the main chamber is sealed and flown to 12,000 meters. As the main chamber is being flown, the inside observer and the group of students enter the lock, complete their hook-up and checks, and fly to 2,400 meters. As they ascend, the inside observer reminds the students what they can expect during the RD and, once the lock reaches 2,400 meters, the chamber engineer prepares to open the valve separating the two chambers. Once the inside observer has briefed the students to expect an RD, all the students can do is wait. But not for long. After a few seconds, the chamber engineer pushes the button opening the valve. A moment later, there is a loud bang that gets everyone’s attention, followed by fogging and a rush of wind as the air in the lock is rapidly evacuated. It’s always interesting to watch how students react to an RD. Even though everyone knows an RD is imminent, that bang is loud enough that everyone jumps as if someone has just stuck a wide-bore needle into them. Some sit there with a “deer in the headlights” look and have to be prompted to carry out their safety checks, but most recover from the shock and busy themselves with checking their connections as instructed before giving the inside observer the required thumbs-up. With the RD over, all that remains is the final and fourth flight to 24,000 meters (see sidebar) (Figure 7.15).

Pressure suit test. Credit: NASA

The MACC Chamber

The MACC chamber located onsite at Midland Space Port is capable of ascending to 30,480 meters and can accommodate up to 10 astronauts wearing their spacesuits. It’s a very spacious chamber that has enough room to fit an entire vehicle cabin, thereby providing astronauts with a high-fidelity and very realistic training platform. Seated in their mock-up cabin, astronauts can not only test their spacesuits, but simultaneously check interfaces within the cabin, assess mission performance, practice manual operations, learn how to react to emergencies, and simulate an entire spaceflight from launch to landing.

One of the most important objectives of all this chamber training is to expose suborbital astronauts to simulated altitude so they can learn about their limitations and dangers of working in what is a very, very dangerous environment. It also provides an ideal opportunity for this group of astronauts to familiarize themselves with the spacesuit they will be wearing on their flight, hence the flight to 24,000 meters. After donning the spacesuit with the assistance of two spacesuit technicians, the astronauts will ascend to 24,000 meters, keeping a close eye on that glass of water that was mentioned earlier while simultaneously listening to instructions from the flight director. As the chamber ascends to altitude, the astronaut will be told how the suit should feel as the suit inflates. As the chamber passes 19,000 meters, the astronaut will notice that the suede patches will begin to smoke and the flight director might mention that, if the astronaut wasn’t wearing a suit at this altitude, they would be dead. At 24,000 meters, it is time to perform some simple tasks such as pulling a pen from a pocket, checking D rings, finding the drink bottle, performing a simple press to test, and going through the emergency checklist. Once the astronaut has completed the tasks, he or she will be left at altitude for a few minutes so they can fully appreciate the potentially lethal situation they are in and also to build confidence in the suit. With the familiarization to 24,000 meters over, the chamber will descend to 7,620 meters and level off in preparation for the final chamber test: with the punch of a button on the chamber console, a loud bang, and a tremendous rush of air, the two chambers will equalize at around 21,000 meters. After their second RD in as many days, the astronauts will descend at 1,500 meters per minute to ground level, they will doff their suits, post-flight records will be written, and a briefing will be conducted. Then it will be time for centrifuge training.

Acceleration Physiology

In addition to spending a number of years as a flight director for the hypobaric chamber flights in the Canadian Forces, I was also lucky enough to work as Acceleration Training Officer, which meant strapping pilots into the centrifuge at Downsview (Figure 7.16). As with HAI, the practical element of acceleration training was preceded by a theoretical component on the subject of acceleration physiology. Let’s begin with the basics and start with the unit of measurement: Gs. If you are a fan of Formula 1, you will have no doubt heard the commentators talk about how many Gs the drivers are subjected to in the many high-speed corners that make up your standard Formula 1 track. In some of these corners, Lewis Hamilton, Sebastian Vettel, and Jenson Button may have to deal with lateral G-forces that exceed 5 Gs. When you consider that the average Formula 1 track has 16 or 17 corners, with perhaps half a dozen of those being high-G corners, and that your average Formula 1 race is more than 60 laps, you can begin to appreciate just how fit these guys are. But, if pulling 4 or 5 Gs repeatedly sounds like a workout, consider your average fighter pilot who may be subjected to more than 8 or 9 Gs. And these Gs may be sustained for several seconds. Whereas Fernando Alonso and his Formula 1 colleagues are subjected to Gs for a second or so, fighter pilots may have to deal with high-G loads for as long as five or six seconds. I remember a presentation given by the Flight Surgeon for the Blue Angels that included in cockpit footage of a turn in an F-22 that pegged the G-meter at 7 Gs for 22 seconds!

Author in the Downsview centrifuge. Credit: Author’s collection

At this point in the introduction, it is important to note the different types of G, which are linear, radial, and angular (Figure 7.17). Linear acceleration is the sort of acceleration that is experienced during take-off or by Formula drivers at the start of a race, while radial acceleration is the type of acceleration a pilot is subjected to during a sharp turn or when pushing into and pulling out of a dive. The third type of acceleration is angular, which occurs during a simultaneous change in speed and direction, which happens during a spin or a climbing turn. The G-forces induced by these types of acceleration are abbreviated as Gx, Gy, and Gz (Figure 7.18).

Unusual attitude. Credit: USAF


G-axes. Credit: NASA

Gx is the force that acts from chest to back and is experienced during take-off, Gy is the lateral force that is familiar to aerobatic pilots when they perform aileron turns, and Gz is the force that acts through the vertical axis of the body, from head to foot or from foot to head: if Gz is experienced from head to foot, it is termed positive Gz (+Gz) and, if acceleration is transmitted from foot to head, it is termed negative Gz (−Gz). As you can imagine, all these G-forces exert a significant strain on the body, particularly the cardiovascular system, which must keep blood flowing to the brain. While the cardiovascular system responds quickly to increased acceleration by increasing the heart rate, there is a point at which the physiological responses cannot keep pace with the Gs. When that happens, the cardiovascular system cannot pump sufficient blood to the brain and pilot performance is degraded, sometimes with fatal consequences. One of the first signs that things are going pear-shaped is loss of vision (LoV) because the eyes are particularly sensitive to low blood flow. As the Gs pile on, vision becomes more and more compromised and the pilot may suffer tunnel vision as his or her peripheral vision may become degraded. If the onset of Gs continues, the next sign may be gun-barrel vision which will be followed in short succession by grayout and blackout. At the blackout phase, the pilot is still conscious but cannot see anything – a sign that G-induced loss of consciousness (G-LOC) is imminent.

A pilot suffering from G-LOC may be unconscious for up to 15 seconds and it may take another 15 seconds for the pilot to recover their bearings and regain control of the aircraft, at which point it may be much too late. Fortunately, the symptoms of high-G exposure are fairly predictable, and pilots are trained to recognize these. For example, when G onset is gradual (0.1 Gs per second), visual symptoms normally precede G-LOC whereas, if the onset is rapid (1 G per second or greater), then G-LOC can be almost instantaneous. What does all this have to do with a suborbital astronaut flying on the Lynx? Well, under normal flight conditions, there is a low risk of either the pilot or his passenger suffering a G-LOC event but, if things go squirrely, then those Gs could pile on rapidly, and that is when all this acceleration training will be invaluable. So what can you do to increase your G-tolerance? Well, first of all, G-tolerance is degraded by alcohol, fatigue, and dehydration, so don’t drink, get plenty of rest, and drink plenty of water before your flight. Second, practice your AGSM (see sidebar) on a daily basis in the two to three weeks leading up to your flight.


The AGSM is a technique taught to all fighter pilots to increase their tolerance to the dreaded G. A perfectly executed AGSM will increase your tolerance by 2–3 Gs, so it’s worth becoming proficient. If you are interested in seeing yours truly performing the AGSM while being spun in the centrifuge, you can enter my name in Google and add the term “centrifuge training.” Enjoy!

The technique is all about timing and requires a deep inhalation of air while simultaneously tensing the big muscles in the legs, stomach, and buttocks. Following a count of three, the pilot exhales rapidly, inhales again, and repeats the exercise. In my role as Acceleration Training Officer, it was one of my responsibilities to ensure every pilot entering the centrifuge was capable of performing a proficient AGSM, which is why each pilot was required to demonstrate their technique to either myself or one of my instructors before entering the “fuge.”

If you are sitting down reading this book, your blood pressure will likely be around 120 mmHg systolic and around 75 mmHg diastolic. Systolic pressure, which is the highest pressure, is attained as the left ventricle contracts and ejects blood into the aorta, and diastolic is the minimum pressure that is measured just prior to the next beat of the heart. Together, your systolic and diastolic pressure is a function of your heart rate and the peripheral resistance as your blood makes its way around your circulatory system. The reason this is relevant to acceleration is because a significant percentage of that blood flow must be channeled to your brain and that blood must be pumped uphill (in the standing or seated position, your brain is above your heart), which results in a loss of pressure. By the time arterial blood makes its way to your brain, there is an arterial pressure drop of around 35 mmHg and, if that arterial pressure drops some more, then there is a concomitant fall in pressure in the brain. Now imagine you are being accelerated forwards during your suborbital flight and the acceleration, or +Gz, is four times the normal acceleration of gravity. This will lead to an acceleration-induced pressure drop of 4 × 35 mmHg, or 140 mmHg, at the brain. As the Lynx rockets upwards and acceleration continues, your blood will flow “downhill” to your extremities, especially your stomach and legs. Your blood will pool there because the venous return of your heart will be compromised by that acceleration, which means the amount of blood being pumped by the heart is reduced and arterial pressure is further reduced. From this point on, if no fail-safe mechanisms are implemented, those 4 Gs of acceleration will become fatal as blood flow to the brain eventually spins down to zero. Fortunately, your body is equipped with some fail-safe mechanisms. One of the first things the body does when faced with acceleration is to increase heart rate. This compensatory mechanism acts in tandem with the pressure receptors that can be found at strategic locations in the circulatory system. These pressure receptors – baroreceptors – keep the brain informed of blood pressure and send signals to the brain whenever blood pressure levels are too high or too low. These compensatory mechanisms work well, but they are limited, and it isn’t just the circulatory system that is affected. As you encounter those Gs during launch, your dense tissues will be driven downwards: for example, your liver will sink into your stomach and your heart will also sink into your chest, with the result that pressure will be exerted on your diaphragm. And as those Gs continue to pile on, your diaphragm will be displaced, which will make breathing increasingly difficult. As the G-meter scrolls past “3” you will feel a “dragging” sensation in your chest and stomach and as the Gs hit “4” you will be struggling for breath. Fortunately, thanks to your training in the centrifuge and by executing a proficient AGSM, you won’t have any trouble tolerating the Gs – hopefully. I say hopefully because it is not by any means certain that everyone will be medically cleared for this challenge to the cardiovascular system and here’s why. While the effects of +Gz and –Gz are well documented, there are certain effects of acceleration that don’t appear on an electrocardiogram (ECG). We know that heart rate increases and we know that vascular return is diminished under G and we know that all the muscle straining while performing the AGSM drives up systolic pressure. We also know that, if the AGSM isn’t performed in synchrony with the G-loading, flow resistance has a tendency to fluctuate and that can be bad news for the heart. Why not perform the ECG while exercising, you may ask? The problem is that a treadmill exercise protocol will not cause a drop in cardiac output so a stress ECG won’t tell you any more than a resting ECG will. In short, there is no way of assuring a potential suborbital astronaut that they can safely be exposed to sustained acceleration stress. If someone has a problem in the centrifuge, it is simply a case of punching a big red button on the control console, the fuge slowly grinds to a halt, and the rider can be escorted to the flight surgeon’s office for a medical review. Not so en route to 100,000 meters! In fact, if a cardiovascular abnormality was to manifest itself in the early phase of a suborbital flight, that flight could potentially be life-threatening. Now, you may be thinking that the aeromedical community would have a good handle on this since thousands and thousands of pilots have been tested in centrifuges over the years, and you would be right. In a study conducted at the USAF School of Aerospace Medicine that examined 1,180 centrifuge training sessions, a whopping 47% resulted in arrhythmias and more than 4%of these should have resulted in termination of the run. The subjects of this study were aeromedical course students who had been medically pre-screened and were a healthy group. But, despite being screened, there was a significant number who had potentially harmful responses to acceleration. Another study that examined 195 fighter pilots revealed a rate of 2.6% ventricular tachycardia, 1.5% paroxysmal supraventricular tachycardia, and 0.5% paroxysmal atrial fibrillation. Each of these conditions is a red flag that would prevent someone from performing centrifuge training. So what to do? Well, I suggest going beyond the minimum flight medical and insisting on a comprehensive cardiovascular examination, especially if you are over 40 because those compensatory mechanisms discussed earlier tend to become less and less effective with increasing age. Another precaution you can take is to be instrumented during your ride in the centrifuge, which is discussed here.

The few centrifuges that exist worldwide vary widely in their capabilities. Some, like the Star City behemoth (Figure 7.19), have an onset rate exceeding 12 Gs per second while others, such as the one I was in charge of at Downsview, struggled to spin up at its advertised 3 Gs per second (it actually never managed more than 2.8 Gs per second). But it isn’t just the onset rates that differ: some fuges, especially those focused on crew training, are fitted with closed-loop profiles and target tracking to make the whole acceleration experience as realistic as possible. Perhaps the gold standard in the centrifuge world is the Phoenix 4000 at NASTAR. Located in Southampton, Pennsylvania, this luxury fuge (Figure 7.20) is extremely versatile, which is one reason why it has been used by Virgin Galactic to train its spaceflight participants.

Star City centrifuge. Credit: Harald Illig


NASTAR’s centrifuge: the STS-400. “Flights” in the STS-400 are preceded by classroom lectures on the subjects of acceleration and the physical effects of spaceflight. Credit: NASTAR

In standard aircrew training, pilots are required to complete a series of GOR, ROR, and high sustained G (HSG) runs: a GOR is defined as an onset rate of 0.1 Gs per second, a ROR is defined as an onset rate of at least 3 Gs per second, and an HSG run is run in which a pilot is subjected to 7 Gs for 15 seconds wearing a G-suit or 5 Gs for 15 seconds without G protection. Those are some tough runs! Fortunately, the Lynx won’t be subjecting its passengers and crew to HSG but the acceleration forces will still be quite substantial, which is why centrifuge training will be required (see sidebar). The G-training course you will complete has one primary and three supplementary objectives. The most important objective is to increase your G-tolerance by improving the effectiveness of your AGSM. The secondary objectives include providing soon-to-be suborbital astronauts with a better understanding of the physiological stresses of increased G, increased confidence in their ability to tolerate high Gs, and a better appreciation of the hazards encountered in a high-G environment. A G course typically begins with some theory in the form of a couple of lectures introducing you to some basic acceleration physiology and an overview of the runs. This is followed by some one-on-one AGSM training with one of the instructors, and then it’s off to the fuge! The first run is usually a relaxed GOR that continues until peripheral light loss (PLL) is encountered; this is done to establish relaxed G-tolerance. The relaxed GOR is followed by a ROR specific to the vehicle, so you can expect a run that takes you up to 4 Gs.


Operated by TNO Netherlands Organisation for Applied Research, Desdemona (Figure 7.21) is a three-axis flight simulator mounted on the arm of a centrifuge. Its modular configuration means a cabin can be mounted on the arm which can then – thanks a fully gimbaled system – be rotated and/or spun in just about any axis. Perfect for training budding astronauts! This is probably why XCOR has been working with TNO to develop a simulated suborbital mission that is “flown” on Desdemona. The fuge mission begins with a high-G boost from Spaceport Spaceport Curaçao followed by the microgravity phase during which the pilot and passenger can gaze down on a simulated view of the Caribbean from 100 kilometers of altitude. After a few minutes of simulated weightlessness, the nose pitches down, the re-entry Gs begin piling on, and the Lynx glides to a graceful landing. Desdemona is an amazing tool. So amazing that some pilots have pronounced the Desdemona suborbital simulation more stressful than the real thing.


Desdemona. Credit: AMST

Space Motion Sickness

Nausea, vertigo, headaches, vomiting, and general discombobulation – these are all symptoms of space adaptation syndrome or space motion sickness (SMS), a syndrome that can strike just about anyone. Without the familiar pull of Earth’s gravity, spacefarers face an environment that challenges the sensory system to its limits and, in a world where up and down are nowhere to be found, space travelers often find themselves the worse for wear. For as long as there have been astronauts, SMS has wreaked havoc for mission planners because, despite the best efforts of space life scientists over the years, nobody has a grip on the problem. Despite all sorts of medications having been used over the years, astronauts still find themselves disorientated and queasy during the first couple of days of a mission. Typically, more than half of first-time astronauts suffer from SMS and symptoms usually resolve within 72 hours. Second-time flyers suffer fewer symptoms and third-time flyers are practically symptom-free, but sending astronauts into space repeatedly so they can adjust to the disorienting environment is a very, very expensive way of dealing with what is an intractable problem. So, if you want to avoid being the crewmember that has to spend half their mission wiping the contents of their stomach off the console, what do you do?

Well, firstly it is helpful to understand the problem as it applies to suborbital flight. We’ll begin with the hypotheses that have been put forward to explain why SMS occurs – the fluid shift hypothesis and the sensory conflict hypothesis. The first of these suggests SMS is caused by the fluid shift towards the head and chest region caused by the loss of hydrostatic pressure in the lower body when astronauts enter microgravity. This fluid shift, which can be as large as two liters, causes an increase in intracranial pressure (ICP) and an increase in cerebrospinal fluid (CSF) pressure, which exerts pressure on the vestibular system, thereby inducing SMS. The sensory conflict hypothesis on the other hand argues that SMS is caused by the conflict in vestibular and visual cues that occurs in microgravity (Figure 7.22). The end result of this fluid shift and/or sensory conflict is a range of symptoms (Table 7.3) that cause a major headache for an astronaut.
Table 7.3

Symptoms of motion sickness and criteria for grading motion sickness severity (Graybiel et al. 1968)


Pathognomonic (16 points)

Major (8 points)


(4 points)

Minimal (2 points)

AQS (1 point)

Nausea syndrome

Vomiting or retching

Nausea II, III

Nausea I

Epigastric discomfort

Epigastric awareness



Pallor III

Pallor II

Pallor I


Cold sweating






Increased salivation















Central nervous system



Eyes closed > II Eyes open III


Vestibular system. Credit: NASA

One serious consequence of an astronaut’s sensory system being out of sorts is responding to an emergency, since, with the crewmember’s perceptual-motor system compromised, the astronaut will find it difficult to perform tracking tasks, switch throws, and fine manipulation tasks. Given the negative operational consequences of SMS, it isn’t surprising that space agencies have spent a lot of time and resources in developing countermeasures. The Russians had some success with pre-flight stimulation of the vestibular system and similar pre-flight behavior modification techniques, which spurred an interest on the application of pre-flight adaptation training techniques such as virtual reality. The theory behind this type of training (see sidebar) is that devices such as virtual reality can simulate the sensory realignment that occur in microgravity that causes SMS: by repeatedly exposing astronauts to unusual environments, it should be possible for crewmembers to encode and adapt to these challenging stimuli.

Pre-flight Virtual Reality Training

Astronauts engaged in this type of training typically perform standard mission tasks, such as navigation or switch activation, in multiple orientations in a virtual environment in the hope that they will retain the skills acquired when it comes to the real thing. Over the years, the training has proven to be quite effective, with the number of crewmembers suffering from SMS symptoms reduced by half. Another similar training method is autogenic feedback training (AFT), which employs psycho physiological countermeasures to condition crewmembers to voluntarily control their physiological responses. The training, which takes place over several days, involves a lot of repetition and practice but, at the end of the conditioning, most people are able to exert a greater control over the physiological responses to motion stimulation.


If virtual reality and AFT fail, what can you do? Well, there is always the pharmacotherapy option. To reduce the effects of SMS, astronauts can be prescribed antihistaminic agents such as Meclizine, anticholinergic agents such as Scopolamine, and antihistaminic agents with anticholinergic effects such as Promethazine and Diphenhydramine. Unfortunately, these drugs cause side effects such as drowsiness and lack of concentration. Imagine spending US$150,000 on a trip of a lifetime and falling asleep! That would be a big downer! But being sick and spending the entire flight looking into the contents of a rapidly expanding vomit bag isn’t much fun either, so you need to do something that guarantees you an emesis-free flight and drugs might just be the answer. The trick to offsetting the side effects we just mentioned is to prescribe more drugs, only these concoctions counter the side effects of the first. For example, to counter the drowsiness induced by Scopolamine, Dexamphetamine is taken and, to offset the sluggish behavior caused by Promethazine, Ephedrine is given. While these combinations work for most, taking drug cocktails isn’t for everyone: some people taking these drug cocktails have reported feeling very jittery and others have experienced symptoms of rapid heart rate. But these drugs can be more effective if the intranasal mode of administration is taken. For example, it is known that the intranasal administration of Chlorphedra has no negative effects on cognitive performance. This is because the nasal method offers a more direct route to the central nervous system and bypasses the metabolism in the gut wall. The only noticeable side effect is nasal irritation.


In addition to the problems of barfing, suborbital astronauts must also contend with the effects on their perception, which is disrupted as a result of the illusory changes during the flight. For example, there may be many who will find the inversion illusion troubling, which means it will be important to focus on strong orientation cues and secure restraints, the latter of which won’t be a problem in the Lynx because the passenger and pilot are strapped in. But, even though our suborbital scientist will be strapped in, disruption of perception will cause a reduction in productivity, impaired performance, and an increased risk of mishap. So what can be done to deal with all these disruptions? Well, there are some strategies which are outlined below.

Strategies to minimize sensorimotor disruptions:
  1. 1.

    Adaptation and pre-adaptation

    The key to adaptation is repetition, which is why suborbital Lynx pilots will have an advantage over their passengers because they may be flying several times a week. This process of adaptation and re-adaptation, combined with suitable cognitive training, will benefit the pilots to the extent that they probably won’t have to rely on pre-adaptation and/or pharmaceuticals, which is a good thing given some of those side effects discussed earlier. For suborbital scientists who may be flying just once, the best way of adapting is by using parabolic flight (see later in this chapter). How many? It’s difficult to say because there is quite a range of rates of adaptation, but generally it takes up to three days or three to four minutes at zero-G per day for sensorimotor changes to develop. This means you will need to fly one parabolic flight sequence per day for three days before your Lynx flight. That’s an expensive way to adapt! An alternative is using short-radius centrifugation and/or unusual attitude training in a high-performance jet fighter, but there is still the cost issue.

  2. 2.


    Now you may be wondering how long the sensorimotor changes (see sidebar) that occur during the adaptation phase last. After all, there is no point embarking on a parabolic flight three weeks before your Lynx flight if those adaptive changes are only preserved for two weeks. Well, scientists have studied this and found that re-adaptation occurs within the first five days but then drops off after that time frame. So, if you want to ensure maximum re-adaptation, schedule your parabolic flight within five days of your Lynx flight. And, if you’re lucky enough to be flying a second flight, it will make sense to schedule it as soon after the first as possible, because those sensorimotor adaptations begin to degrade after a few weeks.



As the Lynx racks up flights, flight surgeons will get a better idea of the sort of sensorimotor disruptions that will affect suborbital astronauts. This database will be generated by the results of pre-screening and pre-, in-, and post-flight neurological function tests and assessment of any neurovestibular problems experienced during missions. Some of these conditions may have no bearing on flight safety, while others, such as benign paradoxical positional vertigo (BPPV) and vestibular migraine, may have longer-lasting consequences. No doubt, this database will form the basis of a research endeavor to correlate sensorimotor function with astronaut performance, the outcome of which will be a series of countermeasures specific to the deficits observed.


Astronauts working on board the ISS are constantly bombarded by radiation. In fact, the radiation environment in low Earth orbit (LEO) is so dangerous that space agencies have had to impose career limits. First there is the cosmic electromagnetic radiation, which includes gamma ray bursts, and then there is solar particulate radiation that includes solar flares. Being exposed to these types of radiation can cause long-term damage which is why the ISS is fitted with shielding but, for flights on board the Lynx, radiation shouldn’t be an issue because these missions will only reach comparatively low altitudes (Table 7.4) and exposure will be measured in minutes and not hours or days. Still, it’s helpful to understand the radiation environment, if for no other reason than to put your mind at rest when you strap into the right seat. As a radiation reference point, the general population is exposed to a background dose of radiation of about two to three millisieverts (mSv) per year. To put that in perspective, that annual radiation dose equates to the radiation exposure corresponding to around 300 suborbital flights per year. Another equivalent comparison is to use the X-ray, which corresponds to around 11 suborbital flights. So, even if you’re a hot-shot pilot who flies into space almost every day, radiation is an insignificant issue. For those who like to put a figure on risks, I’ve included a table (Table 7.5) showing the dose rates that are assumed for a flight on board the Lynx
Table 7.4

Terrestrial dose rates for radiation (equivalent whole-body dose)

Altitude (m)

Equator (mSv/day)

55° latitude (mSv/day)

























Table 7.5

Equivalent whole-body dose rates for radiation during suborbital flight

Altitude (m)

Equivalent whole-body dose (mSv/day)















As you can see, you have nothing to worry about, even if you’re rich enough to be flying every week. In fact, you would have to fly 188 flights per year before exceeding the dose limits for the general public. Now, if you happen to be a NASA astronaut, those limits are even higher because this group’s annual dose limit is 0.5 Sv. That equates to 94,000 suborbital flights per year. Like I said, radiation is not something you have to worry about if you are flying on the Lynx! Let’s move on.

Parabolic Flight

After all this talk of G-LOC (sidebar), and motion sickness, you may be wondering whether spaceflight is your cup of tea, so why not take a test run of sorts? We’re talking about parabolic flight (Figure 7.23). Although a zero-G flight only provides 20–24-second snapshots of what being weightless feels like, it is an invaluable training tool and also a great way of knowing whether your body is up to the real thing; if you spend most of your time projectile vomiting, then perhaps suborbital flight isn’t in your wheelhouse. But, if you have a blast, then you can look forward to the real thing.

Parabolic flight. Credit: ESA

G-LOC and Geasles

Tissue ischemia is a term used by physiologists to describe insufficient blood flow and it is a familiar term in the realm of acceleration physiology because it is ischemia that is the most significant effect of G. Since the eye’s retina is so sensitive to hypoxia, symptoms of sustained and/or increased G are usually manifested visually. As the Gs pile on, retinal blood pressure falls below the eye’s globe pressure and blood flow to the light-sensing receptors in the retina also falls, with the result that vision is lost progressively from the periphery. Tunnel vision progresses to grayout and then to blackout – a condition that is termed full retinal ischemia. The end result is G-LOC, a condition that may be accompanied by myoclonic convulsions, amnesia, and general discombobulation.

It isn’t just the circulatory system that suffers during sustained and increased G because the respiratory system also takes a hit. During +Gz, respiration is disrupted as the increased pressure of G collapses the small air sacs in your lungs, which obviously makes it difficult to breathe. And then there is the condition known as G-measles, or Geasles, which is caused by ruptured capillaries, which in turn results in unsightly red blotches.

Before we discuss the benefits of parabolic flight training, it’s worthwhile reminding ourselves of the distinction between free fall and weightlessness. Way back when the Shuttle was flying at an altitude of around 300 kilometers, gravity as measured on board the orbiter was only slightly less than measured at sea level (9.37 meters/second2 on board the Shuttle versus 9.81 meters/second2 at sea level). So, when terms such as microgravity, zero-G, and weightlessness are used to describe gravity in orbital flight, these terms are technically inaccurate because spacecraft are constantly falling towards Earth under the force of gravity; the reason vehicles remain in orbit is thanks to their velocity, and the reason astronauts perceive themselves as being weightless is due to the fact that they are falling under the influence of the gravitational field of the spacecraft. In reality, astronauts are in a perpetual state of free fall and this is something that can be replicated closer to Earth, albeit for much shorter periods. In parabolic flight (Figure 7.24), an aircraft flies a trajectory that provides up to 25 seconds of free fall.

European Space Agency (ESA) astronauts training in parabolic flight. Credit: ESA

Parabolic flight as a training tool for astronauts has a history dating all the way back to 1950 when the technique was tested by ace test pilots Chuck Yeager and Scott Crossfield at Edwards Air Force Base. Over the years, the technique was refined with the arrival of the F-94 fighter that permitted up to 30 seconds of free fall, and government organizations began to adopt parabolic flight programs for astronaut training and research. Here’s how NASA’s C-9B aircraft flies its parabolic trajectories. Once it reaches 350 knots of indicated airspeed (Mach 0.83) and an altitude of 7,300 meters, a gradual climb is initiated at full thrust, thereby generating vertical speed without sacrificing airspeed. During the gradual climb, the G-meter reads 1.5 Gs, but this reading increases to 1.8 Gs as the pitch angle increases to 45° (the “pull-up”). At 225 knots of indicated airspeed, with the aircraft closing in on an altitude of 10,000 meters, the pilots begin the zero-G parabola by pushing forward on the control yoke. This lowers the angle of attack of the wings, which in turn reduces wing lift. As the power is simultaneously reduced, airspeed falls as the aircraft reaches the top of the parabola, which it reaches at 10,000 meters (Mach 0.43 or 140 IAS/245 TAS – this speed isn’t much faster than the stall speed). This is the fun part when passengers and wannabe astronauts start floating around the cabin. It is also the point at which newcomers to the world of zero-G realize that working in weightlessness can be a bit of challenge because faulty proprioception leads to target overshoots, limb control disruption, and slower limb movements. Sadly, the parabola only lasts about 24 seconds, after which the aircraft pitches down (the “push-over”) and the Gs ratchet up to 2 Gs until the aircraft levels off for the longitudinal component before starting the next parabola.


  1. 1.

    X-15 pilots were awarded astronaut wings for flying above 80 kilometers, but the Fédération Aéronautique Internationale, FAI, the world governing body for astronautics records, defines space as an altitude above 100 kilometers. For those flying on Virgin Galactic, your ticket guarantees an altitude of 80 kilometers, which is not space, although Virgin Galactic passengers will be awarded astronaut wings – not FAI-branded, but Virgin Galactic-branded. The same applies to Lynx passengers incidentally: only the pilot will receive FAI/FAA-branded wings.

Copyright information

© Springer International Publishing Switzerland 2016

Authors and Affiliations

  • Erik Seedhouse
    • 1
  1. 1.Commercial Space OperationsEmbry-Riddle Aeronautical UniversityDaytona BeachUSA

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