© Christopher Earls Brennen


"Oh! I have slipped the surly bonds of earth
and danced the skies on laughter-silvered wings ... "

From "High Flight" by John Gillespie Magee Jr. USAF pilot,
killed Dec. 1941, the month in which I was born.


Maybe it all started when I was a small boy watching rivulets of sand sliding down the leeside of the dunes in Portstewart. I remember watching and wondering how one might ever understand such elegant yet simple movements. Many years later I studied the mechanics of fluids in college, even going on to earn a PhD in the subject. My thesis involved calculations of the shape and other features of fully developed cavity flows, that is to say the flow of a liquid around an object where the pressure in the wake falls below the vapor pressure and becomes a large vapor bubble attached to the object. But nowhere in all of these studies did I learn anything about the flow of sand!

Many years later when I was a young faculty member at the California Institute of Technology, a senior colleague approached me with an enlarged photograph. It showed the vertically-downward flow of sand around a cylinder. Below that cylinder where one would normally see a wake, was a void or cavity empty of sand. It looked for all the world just like the vapor-filled wakes I had studied for my PhD - and that was, of course, why my colleague had approached me. I became intrigued by that photograph and that fascination led to many interesting years of research into the flows of granular materials.

The pioneer of granular flow research had been a colonel in the British Army by the name of R.A. Bagnold. Colonel Bagnold spent a large part of the Second World War in the deserts of North Africa. Recognizing the need to improve the ability of vehicles to negotiate this wild and shifting terrain, Bagnold studied the dunes and their reaction to all kinds of vehicle traction. After the war he continued his research as a part time faculty member in England. He built a rotating viscometer to measure the "viscosity" of flowing granular material and the results he obtained were published in the Proceedings of the Royal Society. The paper contained not only his experimental measurements but also a qualitative theory that seemed to explain key features of the measurements. The paper became a classic, the first reference of almost all the papers published in the subject in the decades that followed.

Of course, in the years ahead there were other published measurements of the "rheology" of granular materials. And quite sophisticated theories were developed that supplemented and confirmed Bagnold's theoretical constructs. Then in the late 1990s, Melany Hunt discovered that though Bagnold's theories were valid, his experiments were flawed and did not support the conclusions he had drawn. In fact, the "rheology" he observed was caused by end effects in his device and not by the fundamental behavior of the granular suspension. Clearly the experiments would have to be repeated. Of course, in the interim other experiments had been performed. But a major problem with all such experiments is the complication caused by the difference in density between the particles and the fluid. This could only be overcome by eliminating gravity.

Thus it was that Melany and I (along with Jim Cory and Steve Hostler) designed and built an experiment to do these measurements in zero gravity in a research aircraft maintained by NASA specifically for zero gravity experiments. Fundamentally a structurally-strengthened Boeing 707, this aircraft, called the KC135, climbs and dives so as to produce a period of zero gravity as it is going over the top, transitioning from the climb to the dive. The duration of this zero gravity "parabola" is only about 25secs; it is preceded and followed by periods of 2g. The obvious potential consequences of such radical motions have earned the aircraft the nickname, the "vomit comet". Each flight of the KC135 consists of about 40 parabolas one after the other and each research opportunity typically involves flights on four successive days.

Most of the experiments aboard the KC-135 are conducted by the students or other young people involved in the project. It is rare that the older team members volunteer for this service. But, for me it seemed the opportunity of a lifetime and the closest I would ever get to outer space. Indeed, from the beginning I was resolved to fly on the vomit comet. Because of their knowledge of my outdoor adventures, this was no great surprise to Melany Hunt and my other colleagues. But there were several substantial impediments to be overcome before I could realize this ambition. First I would have to pass a medical examination. If successful, I would then have to travel to the NASA Johnson Space Center for a day long, FAA-approved course that involved testing in their high altitude chamber. This went by the euphemistic label of "physiological training". The medical examination presented a major hurdle for me since the FAA submission form clearly stated that a myocardial infarction was a disqualifying prior medical condition and I had suffered a mild heart attack about a year before these events. But it was worth a good shot and so I went to the medical examination armed with a strong letter from my cooperative cardiologist stating that I was in excellent physical shape and that he deemed there to be no reason that I should be disqualified because of my heart condition. The medical examination was conducted by a local, Pasadena eye doctor who was most helpful and promised to do what he could to get NASA approval. Several weeks later the answer was positive and I was set for the second hurdle, the "physiological training" at NASA Johnson in Houston, Texas.

So it was that Jim Cory, Steve Hostler and I flew to Houston, Texas, in February, 2003, for our "physiological training". By a bizarre and terrible coincidence, the Space Shuttle Columbia had tragically disintegrated on re-entry just a few days before our scheduled visit and a memorial service for the Columbia astronauts was planned for February 4, the very day of our scheduled training. Since all activity at the Johnson Space Center would cease during the memorial service (the President, cabinet members and other dignitaries would be in attendance), we expected that our training would be postponed. But that was not the case. Thus, as instructed, we showed up at NASA's "Neutral Buoyancy Laboratory" at the Johnson Space Center early that morning.

Being fitted for an oxygen mask In the high altitude test chamber
(Photo by Jim Cory)
(Photo by Jim Cory)
The first phase of our "training" consisted of a full morning of lectures on the effects of altitude, hypoxia (lack of oxygen) and low pressure on the human body. We learned many things that we would not have anticipated. For example, the name of the manoever one uses to clear one's ears during pressurization by holding one's nose, closing one's mouth and forcing air into the cavity behind these blockages. It is called a "valsalva". We also learned some of the unexpected disasters that can occur as a result of depressurization such as having a tooth explode because of a small air bubble trapped beneath a filling. During all of this we also completed a health questionaire in which we had to list our current medications. These were collected and a doctor behind the scenes was going over them as we sat in the lectures. I listed my heart medications and, throughout the morning, was in some trepidation waiting for the doctor to call me out and disqualify me. But that call never came. Instead, several young people attending the course received their dismissals.

At this point you may well be wondering (as we did) why high altitude instruction was deemed necessary since the interior of the KC-135 is pressurized just like a normal commercial airliner. The stated reason was that we would not have ready access to the usual emergency oxygen system with which commercial airliners are equipped. Consequently we would have to learn to use the more complex systems that pilots universally use. But I think a second, unstated reason was a desire on NASA's part to check us out in physically stressful circumstances in order to ascertain our ability to handle the enviroment of a KC135 flight.

Whatever the full complex of reasons, part of the morning was devoted to instruction in the use of the standard breathing apparatus used by pilots and, after lunch we were each fitted with a flight helmet and mask for the oxygen supply system. It was now time for the high altitude chamber test. The chamber is a large, steel pressure vessel containing a rectangular room measuring about 15 feet by 8 feet, with rows of seats down both sides and windows all around. At one end is a door that leads to a small antechamber about 4 feet by 8 feet with a door to the outside. Instructors sit at each end of the main chamber ready to give immediate aid to anyone in distress. The chamber operator and several medical personel sit outside at one end viewing the interior through a large window. We all took our assigned places and plugged in our oxygen supply and our intercom systems. We then sat breathing pure oxygen for about 30 minutes to clear all the nitrogen from our blood. I was surprised by my initial difficulty in breathing with the mask on at zero altitude; one has to work to pull the gas in. But I soon adjusted.

Steve in the test chamber KC135 on the tarmac at Glenn
(Photo by Jim Cory)
(Photo by Jim Cory)
Then it was time for altitude, the chamber was sealed and the pressure was lowered fairly rapidly as we "climbed" to 25000 feet. Before we got too high the man next to me had some serious ear problems and was removed to the antechamber for release back to ground level pressure. I was pleasantly surprised at my lack of any breathing or ear problems even when we reached 25000ft, which, incidentally, is not far short of the top of Everest. Once at that altitude, each of us in turn was required to take off our oxygen mask and breathe the rarified air. The purpose was to allow each of us to recognize our individual symptoms of hypoxia. I went first and had few symptoms, only a slight dizziness similar to that I had experienced climbing high mountains. I lasted the full five minutes at which point I was instructed to put on my own oxygen mask which I did without difficulty. However I was so intent in looking for physical symptoms that I forget to start the little mental tests (join the dots, arithmetic) that they had equipped us to attempt during the "flight". Thus I did experience reduced mental faculties.

My young colleague, Steve Hostler, also had few difficulties but two or three of the other 10 "students" showed signs of distress and had to be helped to put their masks on early. Once we had all completed the hypoxia test, the chamber pressure was increased gradually back to sea-level, a process that was accompanied by much valsalva-ing. After a brief pause outside the chamber we proceeded to the second phase of the testing in which we entered the antechamber in pairs with one instructor. The antechamber was then suddenly depressurized to 15000 feet (by opening a valve to the de-pressurized main chamber). The antechamber filled with condensation mist and we were then to don our masks and begin oxygen breathing as expeditiously as possible. My confidence was increasing rapidly as I could see a succesful end in sight and I had no difficulty with this second phase.

Thus we came to the end of the training and it was with considerable relief that I was among those who returned to the classroom for a final briefing and the presentation of certificates signifying our succesful completion of our "physiological training".

This special NASA airplane, the KC135 or "vomit comet", performs the following manoever to achieve 25secs of zero gravity. From level flight at about 26000ft and 510 knots (just about its maximum speed) it pulls up into a 45 degree inclination and, under full power, climbs to nearly 35000ft. There the pilot transitions into a parabolic trajectory during which the vehicle experiences the 25 seconds of zero gravity. The velocity at the top of this arc is only 325 knots, close to the stall speed of the aircraft. At the end of this manoever, its trajectory is a 45 degree descent and its speed increases back to 510 knots when it pulls out again into level flight at 26000ft. The g level during pullout is typically 1.8g. During the course of a flight it completes this manoever about 40 times thus producing 40 periods of zero gravity each of which are 25 seconds long. This test period lasts about an hour and a half.

Waiting to board Floating in zero gravity
(Photo by Jim Cory)
(Photo by Jim Cory)
Though the airplane is stationed at NASA Glenn just ouside of Cleveland, Ohio, flights are conducted out of both NASA Glenn and the NASA Johnson Space Center in Houston, Texas. In order to perform its radical manoevers and avoid other commercial and private aviation, the KC135 must seek permission to use restricted air space. When operating out of NASA Glenn in Cleveland it normally uses two such restricted air spaces. One is an east/west strip of space located over Lake Ontario and given the code name Misty. The other is over the thumb of Michigan and is known as Steelhead.

Our experiments were scheduled on a May 2003 test flight out of NASA Glenn and so the equipment was shipped there the preceding week. My arrival had been delayed so that Steve and Jim covered the first day alone. But I was there early on the morning of the second day in order to proceed with the pre-flight preliminaries. The first order of business was a visit to the medical office at Glenn for a final check-up. Yet again I was apprehensive that they would disqualify me. But the checkup was routine; I was merely asked whether there had been any change in my health since the previous medical examination and I could emphatically respond in the negative. Then I was issued the standard dose of motion sickness medication, two Sudanyl (30mg each) and either one or two Scopolamine (0.4mg each) for each flight.

The next stop was the regular preflight briefing where we met the pilot and captain, Stephen Feaster, the flight director John Saniec, and the flight doctor Dwight Peake. Thus briefed we headed for the airplane just outside the NASA hanger. Most of the interior of the KC135's passenger cabin is empty of seats, all facilities and partitions. The floor, walls and ceiling, indeed almost all surfaces, are lined with padded mats and, in place of windows, there are fluorescent lights that provide a brightly lit interior. The experiments, seven on our flight, are bolted to the floor. Only at the rear end does the cabin resemble a regular passenger airplane; there some 30 seats are installed and this is where all of us sat for the takeoff and the 30 minute flight from Glenn to Misty. Once we reached the restricted air space we left our seats and positioned ourselves on the floor next to our experiments.

The moment was at hand for the KC135 to begin its unique manoevers. We had, of course, been thoroughly briefed on what to expect and were closely watched and supervised by the NASA crew stationed at regular intervals along the length of cabin while the doctor roamed up and down watching for any sign of distress. Nevertheless the experience and sensations associated with the plane's roller-coaster ride were initially hair-raising. We began with a steep climb. Within the cabin, lacking any view of the horizon, it felt as though the plane were level at all times and only the magnitude of gravity was changing. The first sensation was of increased gravity and all such periods were the most unpleasant part of the flight for the inner ear does not like increased gravity. It is wise to remain still, kneeling or sitting on the floor. If you move your head too quickly, nausea comes quickly. Moreover, it feels as though your blood is all draining down to your feet. Then, after just a brief period of increased gravity, the pilot, in a matter of a few seconds, transitions into zero g. Your blood comes rushing back up into your head producing a headrush. At the same time, your stomach is moving upward rapidly. The first few times you encounter this transition it is an alarming experience though after four or five parabolas, knowing what to expect and knowing that you are not headed through the ceiling, one becomes accustomed to it. By comparison, the 25 seconds of zero g, are truly delightful. Initially one is reluctant to let go of the handrail hardly believing the environment. But soon you move immediately zero g arrives. I usually headed first for the safety of the handrail along the side of the ceiling. Floating in zero g is much like being underwater except that the resistance to tumbling or spin is much less. Consequently you must be careful at take-off not to impart too much spin for you can rapidly find yourself tumbling or spinning out of control. But once the zero spin take-off is perfected you find yourself embarking on longer and longer flights across and along the cabin. However, you must be ready for the signal from the NASA staff indicating that the zero g period is coming to an end. Indeed the staff regard the transition out of zero g as the most dangerous moment when people might come crashing down from their weightless flight. Consequently they yell "coming out" and this is the signal to take your place on the floor and prepare for non-zero g. All of this becomes quite routine as one progresses through the 40 parabolas which are interspersed with turns when the aircraft comes to the end of the restricted air space. After the first few parabolas devoted to adjustment, we had to get down to the business of the experiment and worked hard for the next hour or so as we swooped up and down through the skies. After 40 parabolas we were quite exhausted and glad to return to our seats in the rear of the aircraft for the flight back to Glenn.

Floating in zero gravity Colliding with the ceiling
(Photo by Jim Cory)
(Photo by Jim Cory)
There was much work to be done after we landed in order to prepare the experiment for the flight on the next day. That following day we repeated the whole agenda - and again the following day. But we had many difficulties with the experiment, troubles that now seem inevitable since it was not possible to predict how it would respond to a weightless environment and there was no way to test that ahead of time. Though we learnt alot from those first flights, it only allowed us to redesign the experiment for a more successful flight in the future.

On the other hand, from a personal perspective it had been a truly unique experience. I had come as close to an adventure in space as I ever could or would. I was born just a few years too early and born into a society only just entering the technological age. A few years later and I would have learnt of the real possibilitiy of venturing beyond the bonds of gravity and exploring the majesty of space. But I do not dwell on such hypotheticals. I am content for I have lived a life full of love and full of adventure beyond the wildest dreams of my youth. And I have tasted what the frontier of space might be like.

Last updated 3/5/04.
Christopher E. Brennen