Presentation of the Karman separation line, used as the boundary separating Aeronautics and Astronautics

Dr. S. Sanz Fernández de Córdoba
ICARE President
Last update: 21-06-2004

Preliminary remark: parts of this document have been written from old memories, and Dr. Sanz Fernández de Córdoba welcomes all comments and corrections from readers. Please send your feedback to the discussion list of the Commission at icare-info-l@fai.org.

1.- Background

a) Background in Astronautics

Sir Isaac Newton discovered the concept of what is commonly named nowadays "orbiting", long ago. This famous XVII-XVIII century scientist described what he named "everlasting free fall", the orbiting of today.

He was studying the firing of a cannon in horizontal position, considering no atmospheric effects (i.e. in vacuum). He then arrived to the right conclusion that the distance reached by the cannon ball was proportional to the initial (horizontal) speed of the ball, that is, to what is known as nozzle speed. This is a direct consequence of the fact that the time for the ball to reach the ground in free fall (no initial vertical speed and flat Earth) depends only from the height of the nozzle above the ground.

He then theorised that, if the nozzle speed could be made large enough, the distance reached would be very large as well. So much, that upon reaching certain limits (say a few thousands miles), the Earth could no longer be considered flat, as it could be done for the real distances that current artillery cannons were reaching at that time (or even nowadays). In other words, although the cannon ball will have "freely fallen" a certain distance towards the Earth’s centre, it would not hit the Earth’s surface, because this surface was further away from the initial horizontal line at the firing point. He then theorised that, the Earth being spherical, for certain initial speed, the free fall of the cannon ball will be equal at every moment in time to the distance that the Earth has receded from the initial horizontal line. Consequently, the cannon ball in "free fall", will never reach the Earth surface, and will be falling forever, in fact "orbiting" around Earth. He correctly calculated that initial speed, up to the knowledge then existing of the Earth’s radius magnitude, as being around 8 km/s in modern units (meter was defined long after Newton’s death).

It was no secret for Newton, nor for the rest of the scientists for many years, that this was a theoretical construction. First, there was little hope (at the time) of reaching such an initial speed. Even modern standard cannons do not approach a third of it. But at the same time, they did not doubt that, by other means than cannons, such speed would be eventually reached (the V2 rocket bomb developed by the Germans in WWII demonstrated they were not wrong). And second, and more serious, they did realise that the atmosphere represents such a resistance that the initial ball speed will be decreasing immediately, and then it will touch ground rapidly.

In other words, atmosphere existence was the major problem to demonstrate Newton’s theory or, in modern terms, to begin with astronautics.

b) Background in Aeronautics

Montgolfier demonstrated, long after Newton’s death, that it was possible for men to take off and flight in vehicles lighter than air. Much later, the Wright brothers demonstrated that it was possible for men to take off and fly in vehicles heavier than air. This historical order of discoveries in aeronautics is in itself a paradox, because men knew from prehistorically times that heavier than air things (birds to begin with) could take off and fly. But this is not what interests us here. The fact is that practical aeronautics was born.

It was not long afterwards when scientists and engineers started to put mathematical equations on the flight. By the first third of the XX Century, it was very well known that the aerodynamic forces that allow heavier than air bodies to fly and being controlled, are proportional to the density of the gas the vehicle is moving through (normally, air for aeroplanes), and to the square of the vehicle speed relative to the gas. Lighter than air bodies took off and flew, obviously, because there was air around; otherwise they will stay grounded.

Thus, to aeronautics, atmosphere was needed.

c) Background in Atmosphere knowledge

The scientific knowledge about the declining atmospheric density with increasing height above sea level was first recorded by Spanish missionaries sent to the Andes Mountains, in what is nowadays Ecuador, Peru and Bolivia. As it was discovered by Dr. Luis de la Serna Espina (MD), and published in front of the International Federation of Astronautics (IAF) Medical Symposium in the late 1960’s, a Spanish monk sent in the XVI Century to the then Spanish colonies in the Peruvian area, recorded a series of body malfunctions at high altitudes. He ascribed them to the "thinning" of the air at such altitudes, decreasing air density in modern terms. His reports, and several other reports from similar sources afterwards, were ignored and neglected as non-scientific.

In the XVIII-XIX Century however, several scientist studied the behaviour of gases. Names as Lavoisier, Guy-Lussac, Boyle, Mariotte, Cavendish, Avogadro, Torricelli and many others stand as cornerstones. Decreasing atmospheric density with height was equally (re) discovered. It was no long before scientist tried to see what happens with the apparently irregular behaviour of atmosphere. It is a gas and thus trying to occupy all available space (i.e. the complete universe); but apparently it did not. It is a gas not enclosed altitudewise, so any part of it should have equal density; but density decreased with altitude. And many more atmospheric properties, which seemed to contradict well proven gas laws.

All this needed explanation and they found it: Atmosphere is a not very big layer of gas, surrounding Earth, kept in place by Earth’s gravitational field, with density decreasing with height until it becomes negligible, i.e. atmosphere disappear above certain vertical distance from Earth surface.

2.- The separation of Aeronautics and Astronautics

a) The idea of separating both fields

All the above knowledge was available in the 1930’s. But it was in different areas: Aeronautics, Newton theories and atmospheric properties. So very few scientists had the vision of them all at the same time.

It took a most notable man, Hr. Theodore Von Karman to put things together. He was born at Budapest (nowadays in Hungary; then a part of the Austria- Hungarian Empire), the 11 of May 1881. It is difficult for modern readers to realise the international scientific prestige in aeronautics that Von Karman had accumulated by the early 1950’s. But so was it. He had such a prestige, that certainly no engineer or scientist in the world, interested in Aeronautics and/or Astronautics, would decline an invitation to co-operate with him, even, needless to say, without any reward in terms of money.

In the early 1950’s, Aeronautics and Astronautics were considered the same thing. In fact Astronautics, besides the "dreams" of a few scientists and engineers, was only a military endeavour, linked to Aeronautics in the Military Establishments of the time. But Von Karman had the feeling that there was a difference between the two. If such was the case, a line could be defined to separate them. The basics were there: Astronautics needed the lack of atmosphere to be viable; Aeronautics needed the presence of atmosphere. And atmosphere existed near the Earth’s ground, but did not exist far above the ground. In Astronautics, speeds impossible to maintain in atmospheric drag could be kept for very long periods without power applied to the vehicle. In Aeronautics (heavier than air vehicles) sustained flying without power is unthinkable. And so on. Thus, both disciplines could be separated in certain important aspects just because their dependence, on opposite ways, of atmosphere.

Thus, in the mid 1950’s, Von Karman got in touch with a series of (at the time) young leading scientist and engineers in Aeronautics and Astronautics with the view of defining a separation, as far as possible, between both disciplines. He had got to know them trough two international private, i.e. no government dependent, organisations. One was the recently created IAF (International Federation of Astronautics), which had held its first International Congress in 1950. The second, at the time by far more important, was the well known and very prestigious FAI (Fédération Aéronautique Internationale), organised in the first years of the XX Century, and which sanctioned and recorded all Aeronautic records.

It is for me impossible to name all the people that co-operated with Von Karman in this matter. Since there was no money involved, just free interchange of opinions trough private letters or conversations, a complete list can not probably be made. Von Karman, who never married, was fond of travelling all over the world. I remember him in Madrid in the late 1950’s (he died in 1963), as a guest in my father’s house, several times, for no other reason that being there, visiting Madrid, which he knew pretty well, and chatting with my father. I know he did the same with scientific friends in Rome, Milan and Paris at least. No record was kept of such meetings. So I can name only a few of those scientists, practically the ones I knew personally trough my father and, I may be even wrong about some of their names.

Members of that rather informal group were my father, Prof. Sanz Aránguez (from Spain; later Major General in the Corps of Aeronautical Engineers in the Spanish Air Force; co-founder and Vice President of the International Commission of Astronautical Records of FAI). Prof. Sheiffert (from the U.S., author of a book in Astronautics, "Principles of Orbit Mechanics", now out of print, which was the main source for orbit and orbit injection knowledge and calculation up to the end of the 1960´s; he was later full professor at Stanford University in California). Prof. Von Braun (from Germany, later in the U.S.; expert in rocket propulsion, involved in the V2 development, and later on U.S. orbit injectors). Prof. Eula (from Italy; involved in rocket propulsion and also in atomic research). Prof. Sokolsky (from the U.S.S.R.; probably one of the biggest names in the Soviet Astronautical development afterwards). Mr. Robert Genty (from France; later Colonel in the French Air Force and named Judge Unique Worldwide of Astronautic Records in the FAI). Prof. Dillaway (from the U.S.; later Delegate of the U.S. in the Commission of Astronautical Records of FAI). And, of course, some other scientists whose names I am sorry no to remember at this moment.

b) The Karman separation line: Scientific significance.

The interchange of ideas was finally conducted to a clear cut: In Aeronautics, level flying higher and higher meant to deal with less and less dense atmosphere, thus to the need of greater and greater speeds to have the flying machine controllable by aerodynamic forces. A speed so big in fact, that, above a certain altitude, could be close or even bigger than the circular orbital speed at that altitude (i.e. lift was no longer needed, since centrifugal force took over; and consequently aerodynamic flight was meaningless). Conversely, in Astronautics, lower and lower orbital flying led to encounter more and more dense atmosphere, so much that it would be impossible to keep the orbit for a number of turns around Earth without a significant forward thrust (thus making the free fall, or orbiting, concept meaningless). A lot of calculations were made, and finally it was reached the conclusion, accepted by all scientist involved, that around an altitude of 100 km. the boundary could be set. By the way, most calculations, which I could see at the time, were using nautical miles for altitude. That was probably because it was the only unit of length more or less common at the time (in fact, less common; even British and U.S. nautical miles, both at the time defined in feet, differed; European nautical mile, defined in meters, was also different; but the differences were small). So the altitude decided upon had a very uneasy number to remember. It was apparently Von Karman himself who realised, and proposed to the rest, the very round number of 100 km (very close to the calculated number). The rest of the people eagerly accepted it.

The 100-km altitude, ever since named the "Karman Line", came thus into existence as the boundary separating Aeronautics and Astronautics.

c) The Karman Line: Adoption in International Standards

Von Karman presented the result of this work in front of the IAF, which accepted it without much interest, because they were really not concerned with the problem of separating both fields. But the things worked differently for the FAI, deeply involved in human records for Aeronautics.

It was plainly clear at the time (mid 1950´s), that as soon as a man will get into orbit, most significant aeronautical records will be pulverised. Aeroplane’s altitude, speed, distance, time of flight, and many other parameters could no longer compete with an orbiting vehicle. Some of the people who had been working with Von Karman were somehow connected to FAI, among them, Sanz Aránguez from Spain, Dillaway from the U.S., several scientists from the U.S.S.R. (I do not remember their names; probably Sokolsky was among them) and Genty from France. They proposed to FAI to create a new category of flying machines, later named spacecraft in the FAI rules, which would have separated records. The FAI was more than willing to do so, and decided to create the International Commission of Astronautics (CIAstr; the name was changed in 1987 to International Commission of Astronautical Records, ICARE). Prof. Sanz Aránguez was made delegate of Spain, and Vice President of the Commission. Professor Dillaway was the Delegate of the U.S., and Col. Genty (from France) was named Secretary. These three people (as far as I know), together with a member of the U.S.S.R. delegation (I am not sure whether or not was the delegate himself, Mr. Skouridine) were the main ones involved in the drafting of the first set of rules for Astronautical Records. Several other countries, of course, named delegates whom, doubtless, contributed to the rules as well. The approved rules became Section 8 of the FAI Sporting Code. Naturally, they started by setting the rule that a flight could only be considered an Astronautical flight, and then qualify for a record under Section 8 of the FAI Sporting Code, when that flight goes beyond the 100 km line, i.e. the Karman Line.

3.- Demonstration of usefulness of Karman Line

Although the Karman Line had been a theoretical construction, it was later demonstrated to be a real thing. I do not mean there is anything magic about the exact 100 km., but it has been shown that, about that altitude, things change.

In the early 1960´s, the U.S. X-15 Aircraft was flown up to 108 km. In that part of the flight it was really a free falling rocket, with no aerodynamic control possible. In fact, it was considered an astronautical flight, and the pilot got, as a consequence, his "astronautical wings", i.e. the recognition of being an astronaut.

Later in the same decade (or very early in the next; Soviet information at the time was very scanty) the Soviet Union put in orbit an unmanned satellite, in very low orbit, whose attitude was controlled by aerodynamic forces. The real reason of such an experiment is not yet known. It is known however that it successfully described a few orbits just above the 100 km line (how much higher I do not know), but collapsed rapidly shortly after he crossed, or got too much close to, the 100 km. Karman line.