HUMAN FACTORS IN COMMERCIAL SUBORBITAL FLIGHT

(Aerospace Medicine for Alt.Space Enthusiasts)

Dr. John M. Jurist

JMJSpace@aol.com

Part I: The Foundation

A number of alternative space start-up companies have expressly stated their intentions of developing and operating suborbital vehicles capable of carrying people.

With the successful completion of the Ansari X-Prize competition, interest in this potential market niche has increased. I will examine some human factors issues which need to be considered by entrepreneurs seeking to develop these vehicles. These factors will be considered in the context of the current regulatory environment and the assumption of risk by paying passengers riding suborbital vehicles. Specifically, what constitutes an informed consent in signing a waiver?

Later sections will consider the spacecraft cabin environment from a physiological standpoint, effects of acceleration during launch and re-entry, and the effects of potential failures and the way to deal with some of them. Then, the effects of microgravity or weightlessness, and radiation on cabin occupants and some of the implications of these various factors will be discussed.

Part II: Risk Assumption

There are three categories of human risks associated with suborbital flight: Those risks affect crew, passengers, and innocent bystanders. The FAA and its subsidiary AST (FAA/AST-2) has made it abundantly clear that commercial suborbital and eventually orbital operations must not expose the general public to any greater risks than do, for example, commercial aviation activities.

The rub comes with definition of risk to crew members and passengers. This has resulted in heated discussions in recent months. Generally, it is agreed that crew members are professional individuals capable of assessing the risks and benefits of suborbital flight and intelligently choosing to assume those risks.

Passengers are another matter entirely. Although waivers signed by people assuming risks of, for example, skydiving or scuba diving, are common, it is not clear how enforceable such waivers really are.

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Many space entrepreneurs assume that a person will be willing to pay up to several hundred thousand dollars in return for what essentially amounts to a joyride. People capable of paying such a price are frequently business owners whose insurance (not to mention boards of directors) place direct limitations on their non-business-related activities. More importantly, such people also have relatives and sizeable earning capacities. In the case of a suborbital flight gone awry, these relatives could well be willing to challenge waivers signed by a passenger on the grounds that grandma or grandpa could not truly make an informed consent to waive risks because the late relative was not in a position to realistically assess (or even fully comprehend) all of the associated risks of space flight.

The space flight operator must be able to prove that potential passengers were educated sufficiently to understand the risks they assume. These risks include the generic risks of space flight and the specific risks related to the specific vehicle and flight profile.

Analogies may be drawn from people consenting to potentially dangerous investigational medical procedures. A well-established set of rules related to almost all biomedical research governs the function of so-called Institutional Review Boards (IRBs). IRBs are composed of members representing the medical, scientific, and legal professions as well as members of the community at large. These boards are specifically tasked with reviewing research protocols in the context of assessing the risks and benefits of the proposed research both to society at large and to the individual research subject and assure that the overall risks are outweighed by the potential benefits in the context of local community standards. In addition, IRBs are required to evaluate the informed consent form (a waiver of liability) to assure that the risks and benefits are fairly explained in terms the average participant can understand.

What bodies or organizations fulfill the IRB role for commercial space flight? Should it be fulfilled by the appropriate governmental agency (AST, for example)? Who makes sure the rights of the paying passenger are observed in the explanation of risks to be waived? What is the conflict of interest if the waiver is written by counsel for an entrepreneurial space corporation without external review? Are these waivers enforceable in court or can they be broken by a savvy attorney representing the next of kin? What is an acceptable level of risk for a paying passenger? How do the risks change in, for example, a 65 year old woman with hypertension compared to a 35 year old business man who runs marathons as a hobby? One professional organization, the Aerospace Medical Association, has been working toward establishing medical screening criteria for commercial space flight, but has largely ignored the consent process in signing waivers of liability [Refs. 1,2]. Interested individuals have also considered the problem of medical screening for commercial manned space flight [Ref. 3].

An additional potential problem is dealing with catastrophe. Commercial suborbital flights will be highly visible activities – at least for the first few flights. If something goes wrong and crew members, passengers, or bystanders are injured, or property on the ground is destroyed, the commercial suborbital flight operators better have a public relations plan in place or the industry may be regulated into oblivion.

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References:

1. Medical Guidelines for Space Passengers. Aviation, Space, and Environmental Medicine, Vol. 72, No. 10, pp. 948-950, Oct. 2001.

2. Medical Guidelines for Space Passengers – II. Aviation, Space, and Environmental Medicine, Vol. 73, No. 11, pp. 1132-1134, Nov. 2002.

3. The Medical Implications of Space Tourism. R. Tarzwell, Aviation, Space, and Environmental Medicine, Vol. 71, No. 6, pp. 649-651, June 2000.

Part III: What Do I Breathe, and Why? A Small Dose of Pulmonary Physiology

The next few sections will deal with the spacecraft cabin, and the design compromises that result from the way the lungs work. Then the problem of decompression, either emergency or planned, will be considered.

Everybody knows that air pressure decreases with increasing altitude, and effectively approaches zero in space. What does this imply for the space craft cabin designer and how does gas exchange in the human lung affect these design considerations?

For the cabin designer, less is better. There is a direct high correlation between cabin weight for a given volume and the pressure differential the cabin must withstand. The key variable here is the pressure differential between the inside of the cabin and the outside.

At launch, external or ambient pressure is one standard atmosphere at sea level (about 760 mm Hg) – less if a high altitude launch site is used. In space, the external pressure is effectively zero and the internal pressure is fixed by design. In order to minimize cabin weight, the designer will want to minimize internal cabin pressure while in space. However, the cabin occupants require some pressure in order to survive and a necessary compromise must be reached. The cabin life support system will be designed to bleed cabin air during launch to equalize cabin pressure and ambient pressure up to a specified altitude. Above the critical altitude, the cabin will be sealed to maintain sufficient pressure to allow the occupants to survive and, presumably, function.

Ordinary air is about 21 percent oxygen. As altitude increases and pressure decreases, the effective amount of available oxygen in the air, usually characterized by the partial pressure of oxygen or pO2, decreases from a sea level value of 159 mm Hg (dry air) in rough proportion to total pressure.

The situation inside the lungs is different. The gases in the lung (alveolar air) are saturated with water vapor. The partial pressure of water vapor in the lungs is roughly fixed at 47 mm Hg at normal body temperature, and the other gases are proportionately

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distributed. The net result is that the available oxygen in the lung decreases disproportionately as altitude increases. For example, at sea level, the partial pressure of oxygen in the lungs (alveolar pO2) is about 103 mm Hg at a barometric pressure of 760 mm Hg. At an altitude of 20,000 feet, the barometric pressure is decreased by about 54 percent to 349 mm Hg, but the alveolar pO2 is decreased about 68 percent to 33 mm Hg. At about 63,000 feet, the barometric pressure is 47 mm Hg and there is theoretically no room in the lungs for oxygen.

Water vapor is not the only contribution to this problem. Assuming a constant metabolic rate and breathing activity at altitude, carbon dioxide plays a similar but lesser role than water vapor. For example, the amount of carbon dioxide in the lungs decreases from an average partial pressure of 40 mm Hg at sea level by about 25 percent to 30 mm Hg at 20,000 feet as air pressure decreases 54 percent.

Part IV: What Do I Breathe, and Why? Getting Adequate Oxygen

The way that water vapor, and to a lesser extent, carbon dioxide, displaces a disproportionate amount of the oxygen in the lungs as altitude increases has been discussed. Since we all need some oxygen to survive, this is a problem. How much do we need? Decompression to about 20,000 feet leaves a healthy normal young person (as in a sedentary military aviator) conscious for about 6 minutes. This time of useful consciousness decreases to between 2 and 3 minutes at 25,000 feet, and about 15 seconds at 45,000 feet. Some people will die at less than 20,000 feet, and most will eventually die at 28,000 feet while breathing air. The basic rule is to maintain the partial pressure of oxygen in arterial blood (after it is absorbed by the blood in the lungs) at 50-55 mm Hg. Keep in mind that there is an additional decrement in the partial pressure of oxygen between the gases in the lung and in the arterial blood of several mm Hg.

The two primary ways of dealing with this problem are to either breathe supplemental oxygen or to pressurize the cabin, or both. These approaches will be considered in reverse order.

Commercial airline cabins were required to be pressurized to a maximum equivalent altitude of 10,000 feet (523 mm Hg) until 1957, after which they were required to maintain a maximum allowed altitude of 8,000 feet (564 mm Hg). Recent investigations by the National Research Council have raised concerns that the current 8,000 foot limit might have an adverse affect on some passengers and crew. A recent study by Muhm at Boeing concludes that “… a substantial proportion of older passengers will [have arterial oxygen levels at 8,000 feet that fall] … below the threshold at which supplemental oxygen is recommended.”

At the commercial standard of 8,000 feet, Muhm’s estimate was that 44 percent of healthy passengers aged 65 years or more will have inadequate arterial oxygen levels while breathing air. His estimates for younger otherwise healthy people were that 27

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percent of those 55 years and older, and 14 percent of those 45 years and older fall into this category [Ref. 1].

Therefore, the cabin designer of a suborbital commercial spacecraft will have to perform a design tradeoff between minimizing cabin weight by pressurizing the cabin to the highest equivalent altitude possible while in space and considering the demographic characteristics of the paying passengers. The changes in oxygen exchange within the lungs in the general population as a function of altitude are not well characterized. The military has performed extensive studies of this nature in well-defined, younger populations that match their air crew demographics, but how many of these people can afford to buy a ticket to space?

Reference:

1. Predicted Arterial Oxygenation at Commercial Cabin Altitudes. J. M. Muhm, Aviation, Space, and Environmental Medicine, Vol. 75, No. 10, pp. 905-912, Oct. 2004.

Part V: What Do I Breathe and Why? The Limits of Supplemental Oxygen

Previously, I reported that an increasing percentage of aging normal people are inadequately oxygenated at 8,000 feet while breathing air.

A simple solution to the problem of decreased available oxygen with increased altitude is to supplement the breathing air with additional oxygen. This can be done up to a limit at which a person is breathing pure oxygen. Now, the spacecraft designer must include a breathing oxygen subsystem into the spacecraft cabin with its attendant weight penalty and associated fire hazards.

Breathing pure oxygen has limits. At 33,000 feet, breathing pure oxygen is roughly equivalent to breathing air at sea level. Below that altitude, an appropriate mix of oxygen and air can be used. Pure oxygen at 39,000 feet is roughly equivalent to air at 10,000 feet, and oxygen at 45,000 feet is roughly equivalent to air at 20,000 feet, and we are back to the physiological limits of a human being.

Actually, a person can go above about 40,000 feet by breathing pure oxygen at a higher pressure than the ambient pressure, but the effort is exhausting, has negative effects on the body, and only gains a few thousand feet before the required excess pressure becomes so great as to be dangerous.

At higher ambient altitudes, the solution is to wear either a partial or a full pressure suit. A full pressure suit is extremely expensive – currently over $1 Million and over $3 Million for a true space suit capable of EVA. (This is a potential business opportunity for the right type of alt.space enthusiast.)

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What about cabin depressurization? We are all familiar with potential use of supplemental oxygen with cabin depressurization in aircraft. What about spacecraft depressurization? If the cabin occupants are in a shirt sleeves, the depressurization better be reversed quickly. What if the occupants are wearing pressure suits? Because of mobility concerns, pressure suit designers prefer to keep the pressure differential between the inside and the outside of the suit as small as possible. The Shuttle EVA-type suit is pressurized to about 222 mm Hg, but it is purged with oxygen before use and the suit occupant must breathe oxygen for a period before using the suit in order to minimize the chances of developing decompression sickness (otherwise known as the bends, the chokes, or aeroembolism). The old Skylab suits were pressurized to about 200 mm Hg.

Since at least one existing alt.space company is considering the use of pressure suits for its passengers, prior military experience with these devices is relevant. Survey of more than 400 U-2 pilots found that more than ¾ reported symptoms of decompression sickness during their careers. More than 10 percent of the pilots reported that they altered or aborted their missions as a result. Most symptoms disappeared on return to the ground and breathing pure oxygen for a limited period. The U-2 cabin environment was designed to maintain an equivalent altitude of 29,000-30,000 feet, with the pilot wearing a pressure suit capable of maintaining an altitude of 35,000 feet and breathing pure oxygen during the flight and for a one hour period prior to the flight. Risk factors for developing decompression sickness include the effective exposure altitude, the rate of change of pressure to the exposure altitude, and fatigue, dehydration, and obesity. Incomplete washout of nitrogen during the preflight period of breathing oxygen is also a risk factor. Risks of decompression sickness in the general population have not been well characterized, and this is one of the factors for the spacecraft designer to consider.

The old partial pressure suits suffered from extreme wearer discomfort as wearing time increased. Heat dissipation was a great problem. The newer full pressure suits utilize liquid cooled undergarments for heat control. Spacesuit design is another set of problems we will consider later. Some members of the alt.space community have experimented with used pressure suits purchased from the old U.S.S.R. The viability of this approach is highly suspect for a variety of reasons which are beyond the scope of this paper.

Present commercial aircraft require a backup system in case of cabin depressurization (supplemental oxygen). It is reasonable to assume that future passenger-carrying commercial spacecraft will at least ultimately be required to provide for some type of backup (such as pressure suits during launch and re-entry) in case of depressurization. Keep in mind that commercial aircraft are also a very short time away from survivable atmospheric air pressure in case of emergency. A suborbital spacecraft is not. It is committed to the ballistic part of its trajectory from the latter portion of the rocket motor burn until it gets back down to breathable air. This process can take many minutes in the event of cabin depressurization during the burn and, depending on when depressurization were to occur, is well beyond the ability of a passenger to survive without as pressure suit.

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The previous discussion has touched upon some of the issues related to cabin environment. The problem of carbon dioxide absorption and humidity management is not as significant in suborbital flight as in orbital flight, but still exists. Other issues, such as accumulation of assorted toxic gases and vapors in cabin air, and growth of various bacteria, molds, and fungi in the cabin environment are relevant in discussions of orbital flight and beyond.

The potential problems of decompression sickness go beyond the preceding discussion. For example, symptoms covering the spectrum from discomfort to disabling pain result from expanding gases within the body (gastrointestinal system, ears, and even teeth) during rapid depressurization. These problems have not been discussed, but they exist.

Part VI: What Does Acceleration Do to the Human Body?

Because human tissues are viscoelastic (material properties vary with strain rate), the response of the body to acceleration varies with duration of exposure. In general, acceleration pulses of 0.2 seconds or less are considered to be “impacts,” while acceleration durations of more than perhaps two seconds are considered to be “prolonged.”

During impact accelerations, acceleration tolerance increases as the exposure duration decreases. Consequently, the best indicator of injury potential for impact accelerations is the so-called delta-V, or impact-related speed change. For prolonged acceleration exposures, body fluid shifts become relatively important, and tend to dominate the deleterious effects of acceleration.

Much of the modern understanding of acceleration effects comes from the pioneering work of the late John Paul Stapp during his career in the United States Air Force. An annual conference devoted to improving the understanding of crash injuries is named after Dr. Stapp. As is the case with many pioneers, his personality was, to say the least, interesting and tended toward irascible. Many of his remarks have become the stuff of legend. For example, during Congressional testimony in which he was asked to defend the experimental use of pigs and chimpanzees, he is quoted as saying “You wonder why I use hogs and chimpanzees? Well, man is somewhere between the hog and the chimpanzee. Some people are more like hogs; others are more like chimpanzees.” [Ref. 1] After observing that more USAF personnel were being killed in automobile crashes than in aircraft crashes, Stapp played a pivotal role in the adoption of automotive restraints and many thousands of lives have been saved as a direct result of his efforts.

The physiological effects of prolonged acceleration will be considered. Then, after some consideration of the effects of impact accelerations, various failure scenarios which may be relevant to commercial suborbital spaceflight will be discussed.

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Prolonged linear acceleration effects are usually simulated in a laboratory environment by the use of large-radius centrifuges. The first reference I have found related to human centrifugation described a 4 meter radius unit spun at 50 revolutions per minute located in Berlin’s Charite’ Hospital beginning in 1818. Interestingly, the device was used to subject mentally ill people to accelerations of up to 5 gravities, although the proposed therapeutic effect escapes me totally [Ref. 2].

A typical sitting human’s blood pressure, measured at the level of the heart, ranges from a gauge pressure of about 120 mm-Hg (systole) to about 75 mm-Hg (diastole). Although these pressure units are considered archaic in physics and engineering, they are still used in medicine. Sea level atmospheric pressure is about 760 mm-Hg. The systolic blood pressure maximum is attained as the main chamber of the heart (the left ventricle) completes its contraction and ejection of blood into the main artery (aorta). The diastolic or minimum pressure is attained just before the next contraction of the heart. It is a function of both heart rate and the peripheral flow resistance as the blood flows out into the body. An additional circuit from the right side of the heart pumps blood to the lungs through the pulmonary artery. Systolic pressure on this side of the circulation is typically about 20 mm-Hg and the diastolic pressure is about 7 mm-Hg.

Why is this relevant to any consideration of acceleration effects? A significant fraction of the total blood flow (defined as the cardiac output) is directed to the brain and is necessary for the brain to function. In a sitting human, that blood must be pumped uphill with a corresponding loss of pressure. At the level of the brain (perhaps 45 cm above the heart), the arterial blood would have a hydrostatic pressure drop of about 35 mm-Hg. The blood flow through the brain is related to the pressure driving the flow. If arterial pressure at the level of the brain drops, so does blood flow through the brain. The pressure drop of a fluid column, including a blood column, is proportional to the height of the column, the fluid density, and the acceleration of gravity. Under acceleration, the effective gravitational term is altered.

Assume that our sitting human is accelerated upwards. This is the so-called eyeballs down acceleration or plus Gz. If the acceleration level is, for example, three times the normal acceleration of gravity (3 times 9.8 meters/second2), a column height between the heart and the head of 45 cm will lead to an acceleration-induced pressure drop of about 105 mm-Hg at the brain instead of the typical one gravity drop of about 35 mm-Hg. If the flow resistance through the brain remains unchanged, then the brain blood flow would be reduced proportionally at +3 Gz compared to +1 Gz.

As the acceleration continues, the blood flowing downhill into the body from the heart – particularly into the abdomen and the legs, will tend to pool there because the normal venous return to the heart is impaired by the increased distance-acceleration product. This reduces the venous pressure at heart level from a typical value of perhaps 7 mm-Hg. The reduced venous pressure reduces the return flow of blood into the heart, so the blood pumped out of the heart is correspondingly reduced. The heart’s output per beat is reduced because of the reduced filling, and the output pressure in the arteries is

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reduced. This reduces the arterial pressure at brain level even more, and the process snowballs.

If the preceding were exactly true and no additional mechanisms were to come into play, the +3 Gz would become fatal as the brain blood flow spiraled to zero. The human body has some beautiful compensatory mechanisms which can be triggered. For example, as the heart’s output volume per beat is reduced, the heart rate, expressed in beats per minute tends to increase. In addition, there are pressure receptors in various locations in the circulatory system. They exist as part of several feedback mechanisms which tend to partially abate the effects of the acceleration. The most dominant of these mechanisms, the carotid sinus reflex, requires about 5 seconds to fully engage. If the onset of acceleration is sufficiently slow, the + Gz tolerance can be increased by up to about one G by this reflex. Any detailed discussion of these various compensatory mechanisms is far beyond the scope of this discussion. Interested (or masochistic) readers can refer to any of the many medical textbooks of circulatory physiology for more details.

Under acceleration the arterial pressure at the brain level is reduced as is the corresponding blood flow rate. A direct indication of this effect is the associated visual changes. Because the arterial pressure in the retina of the eye (the sensory portion) is typically less than that in the brain and vision begins to degrade at retinal systolic pressures somewhat below about 50 mm-Hg. Blood flow to the retina is reduced to symptomatic levels before brain blood flow. As a consequence, vision fails before consciousness is lost. This is the so-called “grey out” phase as peripheral vision progressively fails, central vision fails, and then consciousness is lost.

This circulatory effect is not the only problem faced by our sitting human as he or she is accelerated upwards. The more dense tissues of the body tend to be driven downwards. As a consequence, the liver sinks deeper into the abdomen, and the heart and great vessels also descend in the chest. The net effect of this process is to displace the diaphragm downward. This makes breathing progressively more difficult as + Gz

acceleration increases. In addition, any useful activities performed by the arms, such as reaching for switches, etc., becomes progressively more difficult. At +2 Gz, a person experiences a distinct feeling of heaviness. By +3 to +4 Gz, there is a marked dragging sensation in the chest and abdomen, and it requires great effort to move. By +6 Gz, it is extremely difficult to reach overhead. Depending on a person’s physical condition and stature, consciousness is generally lost at between +3 and +5 Gz in a sitting position. As blood pools in the legs, muscle cramping in the calves can occur. In fact, some of the blood can leak out of the smaller vessels and cause petechiae in the feet and legs.

Another interesting effect of acceleration is degradation of visual acuity. The acceleration forces distort the globe of the eye and reduce acuity.

What is the spacecraft designer to do? There are two approaches to abating these effects. The first is to decrease the uphill distance between the heart and brain by tilting the seat back, and the second is to apply counter pressure against the legs and abdomen to

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retard blood pooling there. The counter pressure can be generated intrinsically by grunting and straining the voluntary muscles to temporarily raise blood pressure and reduce blood pooling, or it can be generated extrinsically by a so-called G-suit.

Taking the tilted seat back approach to the maximum, the human will have the acceleration directed front to back (eyeballs in) or + Gx instead of eyeballs down or + Gz. With the heart and brain at more or less the same level, the hydrostatic pressure loss at the brain is largely abated. By bringing the legs closer to the heart level, blood pooling in the lower extremities is reduced. However, this doesn’t completely solve the problem. In general, the tolerance limit of +3 to +5 Gz is increased to perhaps +8 to +10 Gx. In this instance, tolerance is usually limited by chest pain and shortness of breath.

From the point of view of the designer, a sitting 6 foot human has a height of about 56 inches. If he or she is going to ride in a vehicle in which the primary acceleration is along the vehicle’s long axis and is + Gx for the seated occupant, the vehicle diameter will have to be at least 5 feet. If the rider is stretched out so the feet, head, and heart are all at the same level, the vehicle diameter will have to be more than 6 feet or so. This puts constraints on vehicle mass, air drag during the early part of the flight, and loading under cabin pressure during ascent as discussed previously.

A typical winged suborbital vehicle concept would entail longitudinal acceleration during the motor burn of about one Gx to about 4 Gx during a burn of perhaps 3 to 3½ minutes up to an altitude of about 170,000 feet. The ballistic phase of the flight would provide several minutes of microgravity. During re-entry to sensible atmosphere, dynamic loads would maximize at about 4 G (+3.7 Gz and -1.5 Gx).

What are some of the medical implications related to suborbital passengers as compared to crew? Military pilots of high performance aircraft are exposed to monitored acceleration both for familiarization and for screening purposes. Commercial spacecraft passengers are to have some type of medical screening yet to be determined. AST recommends collection of a medical history and having a physician sign off on the passenger.

I submit that this is inadequate. All passengers should be monitored and exposed to an acceleration profile similar to the proposed flight both for familiarization and for medical screening. The overall effects of + Gz, and, to a lesser extent, + Gx are very well documented in healthy populations and result in markedly increased heart work load and oxygen demand.

Heart rate increases and the vascular return pressure is reduced (decreasing preload) under acceleration. This basically starves the pump by decreasing filling of the atria during diastole. Muscle straining, grunting, activation of an anti-G suit, all increase resistance to circulation which tends to drive up the systolic pressure. If this process isn’t in exact synchrony with G loading, the heart afterload (peripheral flow resistance) fluctuates – possibly wildly.

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The effects of these events on the heart are not necessarily detected on an electrocardiogram, whether it is obtained during resting conditions or during exercise. Treadmill exercise protocols do not drop preload or cardiac output. Peripheral resistance to blood flow is decreased and output from the heart increases with aerobic exercise on a treadmill. Therefore, a treadmill test (stress electrocardiogram) will not assure that an individual can safely experience prolonged acceleration stresses because it does not develop analogous stresses.

Abnormalities of heart beat (arrhythmias) occur during acceleration. In a series of 1,180 centrifuge training sessions involving professional aeromedical course attendees at the USAF School of Aerospace Medicine, 47 percent resulted in arrhythmias. Of these, 4.5 percent resulted in (or should have resulted in) termination of the sessions. “Centrifuge training can provoke serious dysrhythmias in ostensible healthy individuals,…” [Ref. 3]. These arrhythmias can occur in prescreened individuals. For example, in a series of 195 male fighter pilots, Hanada and coworkers found relatively harmless physiological variant responses in 1/3 to ½ of the subjects, but also found a rate of 2.6 percent ventricular tachycardia, 1.5 percent paroxysmal supraventricular tachycardia, and 0.5 percent paroxysmal atrial fibrillation – all of which they considered indications to stop centrifuge training and initiate further medical studies [Ref. 4].

If a passenger candidate develops electrocardiographic abnormalities during centrifugation, the centrifuge can be stopped and, most likely, the irregular heart rhythm will revert to normal. If this problem manifests itself early during the acceleration phase of a flight, it can be life-threatening unless the flight is aborted. Remember that the reported incidence of these potentially dangerous findings is perhaps 4 to 5 percent of otherwise healthy people. Absent monitoring electrocardiographic activity of all passengers during flight, there is no effective way to gain the information necessary to abort the flight. The potential result is a dead passenger if he or she goes into complete cardiac arrest during the motor burn of up to several minutes. Remember that the vehicle is committed to the ballistic phase of the flight for a possibly prolonged period of time as well.

A lot of older men take aspirin daily to reduce potential clotting in diseased arteries. If that person’s blood pressure increases significantly during acceleration, the result can be bleeding. If that occurs in the brain, it is called a hemorrhagic stroke and can be catastrophic.

Previously, I alluded to the various compensatory mechanisms to maintain blood flow to the brain. As a general rule, the effectiveness of those mechanisms is reduced with increasing age. The acceleration tolerance characteristics of the population matching the demographics of potential paying suborbital spaceflight passengers have not been well defined.

What about negative accelerations – either – Gz (eyeballs up) or – Gx (eyeballs out)? Basically, the human body does not tolerate negative acceleration as well as positive acceleration. The basic reasoning described above holds except the blood pressure in the

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brain tends to be higher than in the heart under negative acceleration. Rather than the risk being loss of consciousness from inadequate brain blood flow, one risk is of hemorrhagic stroke from blood leaking out of vessels in the brain. Besides, for negative accelerations, the passenger will be hanging from his or her safety harness rather than be pushed into the seat with attendant discomforts associated with the concentrated loads under the straps.

Exposure to negative acceleration can produce so-called red eye, or bleeding into the conjunctiva (white part of the eye). This is usually not very serious.

References:

1. Man in Flight: Biomedical Achievements in Aerospace. E. Engle & A. Lott, Leeward Publications Inc., Annapolis, Maryland, 1979, page 103.

2. Op. cit., page 195.

3. Incidence of Cardiac Dysrhythmias Occurring During Centrifuge Training. I. McKenzie & K. Gillingham, Aviation, Space, and Environmental Medicine, Vol. 64, No. 8, pp. 687-691, Aug. 1993.

4. Arrhythmias Observed During High-G Training: Proposed Training Safety Criterion. R. Hanada et al., Aviation, Space, and Environmental Medicine, Vol. 75, No. 8, pp.688-691, Aug 2004.

Part VII: Impact Acceleration: An Extreme Skydiving Experience

The previous section discussed the effects of long term accelerations such as those experienced during motor burn or re-entry of a commercial space vehicle. This section will deal with the effects of short duration accelerations or impacts. These effects are relevant to dealing with possible in-flight emergencies or aborted flights.

Accelerations of short duration (under perhaps 200 milliseconds) do not involve significant fluid shifts within the body, nor do they involve the various reflex responses that can affect responses to longer duration acceleration. Instead, the forces applied to human tissues are proportional to the applied acceleration. If the tissue tolerances are exceeded, mechanical injury occurs. However, experiments with animals and unembalmed human cadavers have shown than the acceleration tolerance for a given impact scenario tends to decrease as the time of application of the acceleration increases. Below acceleration durations of perhaps 50 to 100 milliseconds, the acceleration tolerance versus time curve tends to follow lines of constant velocity change, or Delta-V. This is a result of the viscoelastic nature of most tissues. That is, the loading stress required to cause the tissue to fail increases with the applied strain rate. This effect can be quite significant. For example, the energy absorbed by a human tibia (shin bone)

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before it fractures is about 30 to 50 percent higher for loads applied within 50 milliseconds as compared to loads applied over many seconds.

What are the basic impact tolerances for well secured and well positioned humans? Work pioneered by the late John Paul Stapp and others over the past half century has established these limits for human subjects, cadavers, and assorted animal models. Plus Gz (eyeballs down) of a well positioned (not bent forward) person is up to about +30 Gz applied at rates of up to about 500 G/sec. Minus Gz (eyeballs up) can be tolerated up to about -15 Gz applied at up to about 200 G/sec. Plus Gx (eyeballs in) can be tolerated up to about +40 Gx for up to about 250 milliseconds. Minus Gx (eyeballs out) can be tolerated up to about -40 Gx for up to about 150 milliseconds and -25 Gx for up to about 1 second.

Dr. Stapp often acted as his own experimental subject. During one experiment in 1954 he rode a rocket sled from rest up to 937 feet per second (639 MPH) in about 5 seconds and then slammed to a stop in slightly more than one second at a peak of -40 G x

at an onset of about 600 G/sec. Among the effects of this experiment, Dr. Stapp was temporarily blinded with vision returning about 8 minutes later, experienced double vision for about 3 hours, and sported two impressive black eyes. This experiment demonstrated the extremes of acceleration tolerance than can be experienced by humans and increased confidence in the use of ejection seats in high performance aircraft.

Consider a commercial spaceflight passenger who bails out of a suborbital vehicle at apogee (effectively zero speed) and 350,000 feet above the ground and falls free. This could potentially be a result of some type of failure mode or elective as a form of extreme skydiving. Further, ignore the problems of maintaining stability but instead assume that the spread-eagled passenger either faces up to enjoy the sky and experience + Gx or faces down to enjoy the view of the approaching ground and experience – Gx. What kinds of dynamic loading does this individual experience as a result of air drag?

For the first 73 seconds, he or she experiences microgravity at less than one percent of normal gravity. During that time, he or she descends to about 267,000 feet and accelerates to a speed of about 2,280 feet/second. As the descent continues, the intrepid skydiver accelerates up to a maximum speed of about 3,340 feet/second (Mach 3.1) 114 seconds into the fall at an altitude of about 149,000 feet. He or she then decelerates until crossing 30,000 feet into modestly decent atmosphere 243 seconds into the fall at a speed of about 290 feet/second. The skydiver crosses 10,000 feet at 207 feet/second 325 seconds into the jump.

During the fall, 22 seconds are spent at acceleration levels above 2 G, 18 seconds are above 3 G, 13 seconds are above 4 G, and 6 seconds are above 5 G. Maximum acceleration is about 5.8 G at 137 seconds into the fall and an altitude of about 86,800 feet.

The basic altitude versus time relationship is shown in the first graph on the following page. The second graph shows the airspeed versus time relationship. The third graph demonstrates airspeed versus altitude.

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Skydive Height

0

100,000

200,000

300,000

400,000

0 50 100 150 200 250 300 350 400

Time, sec

Heig

ht, f

eet

Skydive Speed

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

0 50 100 150 200 250 300 350 400

Time, sec

Spee

d, ft

/sec

That would be quite a memorable ride, but thermal protection at extremes of cold and high stagnation temperatures (over 1000oF) for the 5 or 6 minute ride will be required. A pressure suit is also required. Stability during the free fall is a problem – particularly during the high acceleration phases of entry into denser atmosphere. If the skydiver enters a flat spin, blood is effectively centrifuged into the extremities. The human tolerance to a flat spin is about 140 RPM, and uncontrolled spin rates of up to about 200 RPM can occur. Spinning can produce vertigo and result in nausea and vomiting. This is both unpleasant and dangerous in a pressure suit and/or an oxygen mask.

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Skydive Speed vs. Height

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

0 100,000 200,000 300,000 400,000

Height, feet

Spee

d, ft

/sec

The first extreme skydiver was Joseph Kittinger in the late 1950’s as part of the USAF Project Excelsior. He jumped from balloons at altitudes of about 100,000 feet.

Part VIII: Failure Modes and Survival Strategies

As discussed previously, the risks of a “normal” suborbital flight on a healthy person are minimal but are not nonexistent. However, that may not be the case if an anomalous situation arises. Commercial suborbital operators are going to have to determine the various tradeoffs between passenger education, training, and familiarization for normal flight, abnormal flights, and emergency procedures against corporate liability, relative strength of the informed consent document, and costs.

In a recent interview on The Space Show®, Dr. George Nield of FAA/AST-2 appeared to describe AST’s position as essentially laissez faire – it is largely up to the suborbital operators to determine their policies and procedures in this arena [Ref. 1]. AST has issued guidelines and has until December, 2005 to develop proposed regulations and June, 2006 to develop final regulations. A key element is reliance on the principle of informed consent: Spaceflight is risky, but people can assume those risks as long as they understand the dangers involved. Informed consent was discussed previously and in a column in The Space Review® [Ref. 2].

If an accident occurs with damage to uninvolved people or property, or injury to commercial passengers, AST would most likely revisit this issue intensively. Given Rep. James Oberstar’s attempts during the creation of the recently enacted Commercial Space Launch Amendments Act of 2004 (CSLAA) to require commercial space flight to be essentially as safe as commercial aircraft operations, an accident would almost certainly

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result in rather hostile congressional scrutiny. Interestingly, CSLAA actually prohibits AST from issuing regulations to protect the safety of passengers, other than for medical screening or training, for the next eight years unless there is an accident that either results in or has the potential to result in, serious or fatal injuries [Ref. 3].

The various failure modes and their probabilities of occurrence must be defined before survival issues can be addressed rationally. There is not much rationality in spending, for example, several million dollars to protect against a one in a million failure possibility while ignoring a one in one thousand risk that can be abated for a few thousand dollars. This kind of failure analysis will be done by a prudent suborbital vehicle developer in any event. In addition, design efforts will be expended to assure graceful rather than catastrophic failures whenever feasible.

For example, one critical design decision is whether a commercial RLV is launched vertically (VTO) or takes off horizontally (HTO) like an aircraft. If a motor fails or is shut down during the first few seconds of flight in a VTO RLV, the vehicle will be lost. A HTO RLV with motor shutdown during the first few seconds of flight may be able to initiate a runway abort, a go-around procedure, or survive the event in some other fashion.

If a VTO RLV uses a cluster of motors, engine out capability may exist in the absence of a catastrophic failure. The probability of a motor failure, along with propellant leaks, etc., for a multiengine cluster is greater than for a single motor of similar reliability. If a single motor is 99.9% reliable for a given flight profile, the probability of a motor failure in a single mission is 0.1%. For a 5 motor cluster, the probability of a failure involving at least one motor is 1-0.9995 or about 0.5%. In order to increase odds of vehicle survival in a clustered VTO vehicle, designing in various motor shutdown scenarios to avoid catastrophic failures may be desirable.

For a VTO RLV, what are some human factors considerations? First, ejection seats are essentially useless during a very low altitude abort because the vehicle will be lost and the crew and passengers must be transported clear of the almost fully fueled vehicle’s potential fireball. This implies that, if abort capability is desired during this part of the flight envelope, some type of rocket-powered escape capsule must be used. That is why the Apollo system, for example, had an escape tower attached to the command module. The tower was ejected after the vehicle was outside its useful operating envelope. The only manned experience with an escape capsule during a launch abort occurred on September 26, 1983 with the abort of Soyuz T-10-1 as a consequence of a prelaunch booster fire. The 20 G escape of the capsule saved the crew.

During the intermediate portion of the flight profile, say up to Mach 2.5 or so, there is some limited usefulness for ejection seats in non-catastrophic vehicle failures (explosions).

Ejection seats have been designed for so-called zero-zero ejections (zero altitude, zero speed). These seats could be potential lifesavers for a HTO RLV emergency during

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the early phases of a flight, through climb out and through the intermediate portions of the near-vertical portions of the propulsion burn. In general, ejection seats are of limited value above about Mach 0.9 at sea level to perhaps Mach 3.7 at 65,000 feet because of high dynamic pressures and/or stagnation temperatures. Above 65,000 feet, high stagnation temperatures are problematic for survival without a capsule above speeds of about Mach 2.5 to 3.7

It is unlikely that a catastrophic failure would occur after the propulsion burn and before the recovery phase of the flight. Failure of the cabin environmental system is probably not going to occur catastrophically unless associated with a propulsion system failure.

After the propulsion burn, even if prematurely terminated, a suborbital RLV is committed to a ballistic trajectory for up to several minutes depending on how close the burn was to completion at termination. Staying with the vehicle is most likely a favored survival strategy during this phase of the flight even if cabin pressure is lost since the cabin provides some protection against the high stagnation temperatures encountered during return to denser atmosphere. This was mentioned previously. One scenario leading to cabin pressure loss near the end of the propulsion burn would involve a motor explosion with fragments penetrating the cabin. Provision of blast shielding around the motor(s) can abate this risk as can use of stored make-up gas to compensate for cabin leaks of limited magnitude.

If the cabin design involves use of a pure or nearly pure oxygen atmosphere at relatively low pressure, O2 prebreathing should take place before flight to reduce the risk of aeroembolism as discussed previously. If cabin depressurization at altitude is a significant risk, passengers should be equipped with either partial or full pressure suits or the environmental system must provide sufficient make-up gas to maintain pressures at no higher than perhaps 35,000 feet. If cabin depressurization occurs at altitude and goes above this value, passengers in shirt sleeves will die shortly.

During the coasting portion of the flight, the passengers could potentially be left free to float around in reduced gravity. Alternatively, they could remain strapped in their seats. A typical HTO suborbital concept might entail several minutes of microgravity. If passengers are free to unstrap from their seats at the completion of the propulsion burn and float around, how does one ensure that they are all back in their seats before the start of atmospheric re-entry? Does a commercial suborbital operator need a cabin attendant to deal with this issue? Will it work? This also affects marketing considerations in any business plan since experiencing a few minutes of microgravity while strapped into a seat may not be as attractive to a prospective paying passenger as floating free.

For suborbital commercial operations, the risk of a post-abort water landing must be considered and balanced against training any passengers for such an eventuality.

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Fire, smoke, and emergency egress after emergency landings should also be considered by the suborbital operator. Appropriate training and familiarization procedures should be incorporated into the pre-flight training period.

As discussed above, AST’s position is that it is up to the operators to make the appropriate decisions related to these factors in terms of passenger training and familiarization. Operators must be abundantly aware that although space flight is inherently risky, they will have battalions of lawyers (and members of congress) minutely examining every decision with the benefit of 20-20 hindsight if an accident results in injury or death to passengers or to people on the ground.

References:

1. George Nield, www.thespaceshow.com, interview on July 10, 2005.

2. John Jurist, Laying the foundation, www.thespacereview.com, article 320, February 14, 2005.

3. George Nield, personal communication, July 20, 2005.

Part IX: Radiation Exposure

The human radiation exposure during suborbital flight is considered for a typical winged (horizontal takeoff and landing) suborbital concept. These hazards are evaluated both in the context of the public at large (potential passengers) and occupational exposures (aircrew). The hazards are also considered in the context of typical background radiation exposures and medical exposures. Finally, recommendations are made regarding adherence to the ALARA (As Low As Reasonably Achievable) principle used by the Nuclear Regulatory Commission (NRC). For the purposes of this analysis, the following flight profile is assumed:

After motor ignition, the flight passes 25,000 feet altitude at 100 seconds, 50,000 feet at 120 seconds, and 100,000 feet at 160 seconds and reaches apogee of 390,000 feet at 310 seconds. Descent below 100,000 feet occurs at 440 seconds, and 50,000 feet at 460 seconds. At 600 seconds post-ignition, an essentially constant descent rate is established from 40,000 feet to about 10,000 feet at 1,600 seconds. This phase of the flight is followed by approach and landing.

This baseline flight profile assumes a total of 920 seconds exposure above 25,000 feet, 340 seconds above 50,000 feet, 280 seconds above 100,000 feet, and 220 seconds above 200,000 feet.

In addition to possible on-board radiation sources, which are not a factor in commercial suborbital flights unless radioisotope fluid level sensors are used, radiation exposure comes from the following potential sources during space flight:

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Energetic cosmic photons (including gamma ray bursts) This source of radiation varies markedly in intensity. In general, the atmosphere provides effective shielding at low altitudes. Cosmic electromagnetic radiation contributes to the general increase in measured exposure with increasing altitude. Gamma ray bursts are poorly understood short-duration plane waves of gamma radiation which pass through the solar system at rare intervals. They are not considered a significant problem in LEO operations.

Cosmic particulate radiation Effective shielding (deflection) of charged particles of both cosmic and solar origin is provided by the terrestrial magnetic field and the atmosphere. Secondary (bremstrahlung) radiation resulting from charged particle interactions with the atmosphere is responsible for the general increase of background radiation with geomagnetic latitude at low altitudes. Occasionally, extremely energetic high-Z particles (the so-called oh-my-God particles) enter the atmosphere and produce intense localized secondary radiation fluxes.

Energetic solar photons This is considered an insignificant threat or source of exposure.

Solar particulate radiation (including flare events) As with cosmic particulate radiation, the terrestrial magnetic field and atmosphere provide effective shielding from this radiation source at low altitudes and geomagnetic latitudes. Flare events, which are correlated with the solar cycle, can be predicted several hours in advance.

Trapped particulate radiation belts The Van Allen radiation belts are an insignificant source of radiation below altitudes of about 500 miles and are therefore an insignificant threat or source of exposure in suborbital operations.

Terrestrial background In addition to contributions of the above sources to ground level background radiation exposure, radionuclides present in the Earth’s crust provide some background radiation exposure. This is not regulated, nor is it biologically significant except under very limited circumstances. At ground level in the United States, the background dose for the general population typically averages 2 to 3 mSv per year. This dose is the total from all natural sources.

The rare particulate and gamma ray events of cosmic origin can be ignored in this discussion. Since HTO suborbital flights are assumed to occur at low altitudes except for very short times (less than 6 minutes above 50,000 feet and less than 16 minutes above 25,000 feet) and at relatively low latitudes (centering around Mojave, California at 35o

latitude and 2,500 feet altitude), solar flare events are also ignored. Radiation exposures will be converted into effective doses and expressed in Sieverts (Sv) or milliSieverts (mSv) in the following discussion.

As an approximation, the radiation background intensity tends to increase monotonically with geomagnetic latitude and with altitude up to about 65,000 – 80,000 feet. Above that altitude, intensity tends to decrease somewhat and then increase again (markedly if the Van Allen radiation belts are entered at altitudes above about 500 miles.

At low altitudes, the contribution of non-terrestrial sources to the daily background radiation dose rates can be summarized in the table on the following page:

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Equivalent whole body dose (mSv/day)============================Sea Level 6,500 feet======= ========

Equator 0.00066 0.0011050o Latitude 0.00075 0.00128

At higher altitudes, the following non-terrestrial daily dose rate contributions are observed:

Equivalent whole body dose (mSv/day)============================

Altitude (ft) Equator 55o Latitude========= ====== ========= 16,400 ½ 0.8 32,800 2 4 49,200 4 12 65,600 4 14½ 82,000 4 15 98,400 3 14 131,200 12½ 164,000 12

These dose rates do not include shielding from vehicle structure. For example, the old Concorde supersonic airliner and other long-haul airliners, flying at altitudes of about 33,000-50,500 feet, expose passengers to dose rates of 0.103-0.233 mSv/day. The Skylab missions provide approximate dose rates in LEO with structural shielding. Those doses ranged from about 0.6 to 0.9 mSv/day. The average was 0.77 mSv/day for a total of 171 days over three missions. Skylab orbited at an altitude of about 1,430,000 feet at an inclination of 50o.

The following conservative non-terrestrial occupant dose rates, including vehicular shielding, are assumed for suborbital flights:

Equivalent wholeAltitude (ft) body dose (mSv/day)========= ================ 25,000 0.17 50,000 0.24 100,000 0.29 150,000 0.32 200,000 0.35 300,000 0.40 400,000 0.46

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This is shown graphically below:

Dose vs Altitude(nonTerrestrial)

1

10

100

1,000

1 10 100 1,000 10,000

Altitude (thousands of feet)

Dos

e (m

icro

Siev

erts

/day

)

These dose rates are based on high estimates in the absence of solar flares and on reasonable aircraft structural shielding capabilities.

Given the projected flight altitude versus time profile for the assumed suborbital vehicle, the conservative whole body dose per flight estimate is no more than 0.0053 mSv. A typical 2 view chest x-ray examination provides a dose of about 0.06 to 0.25 mSv. Therefore, the typical chest x-ray corresponds to more than 11 suborbital flights, and the general population background dose of perhaps 2 mSv annually corresponds to more than 300 suborbital flights annually.

The NRC limits for the general public from radiation operations are 1 mSv per year and 0.1 mSv per year for minors. The ICRP limit for the general public is also 1 mSv per year. These limits are not exceeded for minors at 18 flights annually, and they are not exceeded for adults at a flight rate of 188 flights annually.

Another issue is the dose for suborbital crews. If the crews were limited to 188 flights annually, they would not exceed the dose limits for the general adult public.

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NASA considers astronauts as radiation industry workers. Thus, their annual dose limit is considered to be 0.5 Sv. This is equivalent to more than 94,000 flights annually. Assuming a maximum of 8 flights daily over a 5 day working week and a 50 week working year, a crew member would be exposed to a maximum of less than 11 mSv annually.

There are also career limits for radiation workers. They are:

Male Career Flight Limit atAge (yrs) Limit (Sv) 0.0053 mSv each======= ========= =============

25 1.5 283,018 35 2.5 471,698 45 3.2 603,773 55 4.0 754,716

As shown above, the career limits are not approached by suborbital crew member doses. Radiation exposure during orbital operations, particularly if prolonged, must be considered carefully in contrast to suborbital operations. A suborbital vehicle designer and operator might take cognizance of the following points when considering radiation exposure issues:

Some States have adoption agreements with the Federal government regarding radiation exposures of all citizens.

In other States, X-ray exposure is regulated locally and the NRC regulates radionuclide-related exposures. In these States, the NRC exposure regulations could well not be applicable unless the AST adopts them.

Depending on the regulatory environment under which suborbital operations will take place, voluntary adoption of the NRC general population and occupational dose limits may be advisable.

Treating suborbital crew as radiation workers would entail establishing a monitoring program, which would cost perhaps $1,000 annually for up to 10 workers exclusive of personnel time.

Fly a dosimeter (either TLD or film) on the first 10 flights without passengers to verify the estimates given above.

Do not fly in times of predicted high solar radiation exposures at altitude.

References:

1. Berry, Charles A.: Aeromedical preparations. Chapter 11 in Mercury Project Summary, NASA SP-45.

2. Bottollier-Depois, J. F.; Chau, Q.; Bouisset, P.; Kerlau, G.; Plawinski, L.; and Lebaron-Jacobs, L.: Assessing exposure to cosmic radiation during long-haul flights. Radiation Research.

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3. Cool, D. A.; and Peterson Jr., H. T.: Standards for Protection Against Radiation, 10 CFR Part 20 (NUREG-1446), October, 1991.

4. Dow, Norris F.: Structural implications of the ionizing radiation in space. Proceedings of the Manned Space Station Symposium, IAS, Los Angeles, 1960.

5. Levedahl, Blaine H.: A survey of radiobiology for engineers. Human Factors, pp. 1-68, August, 1959.

6. Nicogossian, Arnauld E.; Huntoon, Carolyn L.; and Pool, Sam L.: Space Physiology and Medicine, 3rd Ed., Lea & Febiger, 1994.

7. Schaefer, Hermann J.; and Golden, Abner: Solar influences on the extra-atmospheric radiation field and their radiobiological implications. Pp. 157-181 in Physics and Medicine of the Atmosphere and Space. Benson Jr., Otis O.; and Strughold, Hubertus (editors), John Wiley & Sons, New York, 1960.

Part X: Weightlessness

In the suborbital flight regime, weightlessness or microgravity is not a significant issue. First, a suborbital flight might subject crew and passengers to a maximum of perhaps 3½ minutes of microgravity. Second, the most significant risk related to brief exposure to reduced gravity is motion sickness or nausea. The remaining biological effects of reduced gravity conditions typically take exposures of hours to days to manifest themselves and of concern only during orbital or interplanetary operations.

The risk of space motion sickness or nausea is most significant during the first few days of orbital space flight and tends to manifest itself within an hour or so in susceptible people. Recovery generally occurs within 1½ to 2 days of flight.

The risk of nausea in reduced gravity is significantly abated if provocative motions, especially of the head, are avoided. During suborbital flights, the risk will be reduced if vehicle occupants remain strapped into their seats during the flight. During a short exposure of a few minutes, allowing passengers to unstrap from their seats and then return to their seats before deceleration commences may be impractical in any event.

Incipient motion sickness can be countered by holding the head in a fixed position. Odds of nausea can be reduced by various medications taken in advance of the flight and by prior familiarization with exposure to reduced gravity in aircraft flights. In a multipassenger vehicle, one passenger becoming nauseated can potentially trigger nausea in the others.

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If passengers are in pressure suits instead of a shirt sleeve environment during suborbital flight, response to nausea would require opening the helmet face plate to get a waste bag into position. Vomiting into a closed pressure suit helmet and/or oxygen mask is not only unpleasant, but is dangerous.

Acknowledgements

A slightly different version of this paper recently appeared as a series of columns in The Space Review® (www.thespacereview.com). I thank Drs. James S. Logan, David Livingston, and George Nield for helpful discussions.

The Author

John M. Jurist is a biophysicist with a long-standing interest in human factors. He earned academic degrees in physics, biophysics and nuclear medicine while at the UCLA Medical School. During his subsequent academic career, he held faculty positions in the Division of Orthopedic Surgery at the University of Wisconsin and in the Space Science and Engineering Center at that same institution. He has held adjunct or part time professorships in physics, engineering, and medical sciences in the Montana State University System. He currently serves as an Adjunct Professor of Space Studies in the Odegard School of Aerospace Sciences at the University of North Dakota and as an Adjunct Professor of Biophysics at Rocky Mountain College. Among other professional associations, he is currently a Life Member of the Aerospace Medical Association, a Life Member of the International Association of Military Flight Surgeon Pilots, and a Fellow of the Clinical Medicine Section of the Gerontological Society.

John has invested in alt.space start-ups, has supported other alt.space start-up activities with grants, and has funded propulsion projects in the Space Science and Engineering Laboratory at Montana State University and avionics projects in the Robotics Laboratory at Santa Clara University. Any opinions expressed herein are his own and do not necessarily reflect the opinions or policies of any entities with which he is affiliated.

Dr. Jurist lives in Billings, Montana and can be contacted at JMJSpace@aol.com.

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