Introduction
While researching this article, I had the opportunity to visit the White Sands Missile Range to witness the launch of a scientific payload on a Black Brant rocket. The payload contained a new kind of optical telescope that could yield information about the atmosphere of a white dwarf star. The rocket took off without incident at its scheduled time (about 10 pm local time). However, within a minute, it was apparent that something was wrong. The rocket was drifting to the west and was projected to land off the range, violating safety rules; thus the flight controller was forced to initiate a small explosion in the rocket and cut the flight short. High-altitude winds had suddenly shifted in the 30 minutes before the flight, and the wind corrections, which had been entered just moments earlier, were not accurate. Fortunately, the instrument was recovered the next morning in excellent condition, so that it could fly again at a later date. So, even after 50 years, scientific rocketry remains a high-risk enterprise.
Everyone at White Sands that day witnessed the conjunction of two grand traditions—the striving of scientists to conduct research at increasingly higher altitudes and the development of the technology to carry instruments high into the atmosphere. In this article, the focus is principally on rockets that carry instruments for short durations, up to 5 minutes or so, before falling back to Earth. These rocket flights are also described as suborbital because the rockets do not achieve the high velocity needed to remain in an orbit around Earth.
Rockets that carry experiments are typically called ”sounding rockets.” ”Sounding” now means any observation of the properties of the ocean or atmosphere. Originally the term applied to the technique used to measure the depth of the water under a ship with a sounding line. The use of ”sound” probably derives from old English, where ”sund” meant sea. One of the first scientific uses of rockets was to measure the properties of the atmosphere, so naturally they were given the name sounding rockets, which has stuck.
High-altitude research has a long history. Blaise Pascal conducted a high-altitude experiment in the 1640s. To demonstrate that air had weight, he had a Toriccelli barometer carried up the Puy de Dome, an extinct volcano 1400 meters high. The importance of carrying scientific instruments to high altitudes for observing through the atmosphere was expressed eloquently by Sir Isaac Newton (1). In discussing the development of telescopes for astronomical purposes, he notes the limitation presented by the “twinkling” of the stars and writes, ”The only remedy is a most serene and quiet air, such as may be found at the tops of the highest mountains, above the grosser clouds.” By 1900, scientists were more or less routinely making expeditions to mountaintops and carrying instruments in balloons to conduct observations.
Rocketry has an even more ancient history, beginning with the discovery by the Chinese of gunpowder and its use in bamboo tubes in the thirteenth century to create projectiles of a sort. From a military perspective, gunpowder found its great utility in muskets and cannons, and it was not until around 1800 that practical rockets were used in warfare (and entered U.S. consciousness with the British bombardment of Fort McHenry during the War of 1812). Rocketry, or at least the human exploration of space, received a tremendous boost from the writers of the nineteenth century, especially Jules Verne, whose topic, From the Earth to the Moon, was widely read.
Science and rockets came together in the twentieth century. Major discoveries were made at the beginning of the century relating to high-altitude phenomena; specifically, the stratosphere, which demonstrated that the atmosphere was structured; cosmic rays, which were energetic radiation originating high in the atmosphere; and the long range transmission of radio waves that revealed the existence of a conducting layer in the atmosphere at high altitude. These and other advances led to pressure from the science community to perfect the means for increasingly sophisticated experiments at high altitude. Meanwhile, three extraordinarily talented physicists—Konstantin Tsiolkovsky in Russia, Hermann Oberth in German, and Robert Goddard in the United States—established the theoretical and technical basis for rockets. Unfortunately, it took a war and the search for an ultimate weapon by the Nazis to bring this all together in the development of the V-2 rocket by Dornberger and his team at Peenemunde. After World War II, scientists in the United States capitalized on the captured inventory of German rockets to conduct a variety of scientific observations and to develop rockets of their own, which formed the basis for the space science program in the United States and elsewhere.
Three sources were used principally in preparing this article: sites on the Internet, topics, and original articles. The topics include the following:
* Frank H. Winter, Rockets into Space, Harvard University Press, 1990 provides a summary of the history of the development of rockets in the twentieth century and its prehistory.
* David H. DeVorkin, Science With A Vengeance, Springer-Verlag, 1992, provides a detailed account of the emergence of rocket science following the development of the V-2 rocket.
* Bruce William Hevly, Basic Research Within a Military Context: The Naval Research Laboratory and the Foundations of Extreme Ultraviolet and X-ray Astronomy, 1923—1960, University Microfilms International, 1992, describes high altitude and rocket research at one of the centers where rocket science emerged.
• Homer E. Newell, High Altitude Rocket Research, Academic Press, 1953, provides a summary of rocket technology and of the status of the various science disciplines that used rockets up to about 1952.
* R.L.F. Boyd and M.J. Seaton (eds), Rocket Exploration of the Upper Atmosphere, Pergamon Press, 1954, describes the state of the science emerging from the use of sounding rockets up to mid-1953.
Only the Winter and DeVorkin topics are likely to be found in a general purpose library. The Newell and Boyd/Seaton topics are likely to be found only in major research libraries. The Hevly topic is his Ph.D. thesis and is of even more limited availability.
The discussion cited later of foreign rocket programs, information on the characteristics of individual rockets, a description of Pascal’s experiment with a barometer and of the Montgolfiers early balloon flights, a biographical memoir from Frank Malina—all these and much more information came from the Internet. I do not cite specific sites because they tend to be short-lived, but others take their place. My recommendation to the reader is to use one of the many web search engines with a few key words (e.g., Pascal and barometer) or an exact expression (e.g., discovery of ozone in the atmosphere).
The Development of Rockets
The physics and technology of rockets are discussed in other places in this topic. However, for the nontechnical reader, I note the following. The motive force for rockets is produced by expelling material, usually hot gas, at high velocity. This force is the result of Newton’s third law stating that for every action, there is an equal and opposite reaction. More precisely, it is Newton’s second law that shows that a forward force is produced on the rocket equal to the product of the rate at which mass is expelled and its velocity. This is in contrast to a gun in which the gas is contained in a barrel. The expansion of the hot gas in the gun’s barrel provides the forward impulse to the bullet. In fact, a key experiment performed by Robert Goddard, the American pioneer of rocketry, was to demonstrate that the performance of a rocket in a vacuum was better than that of a rocket in air. The extra ”push” caused by the expansion of the exhaust gas against the external medium is more than compensated for by the greater exhaust velocity of the expelled gas if it encounters no external resistance. So in rocket weapons, such as the American Bazooka or the Russian Katyusha, the barrel is left open in the backward direction to allow the exhaust gases to escape freely. Beyond this simple principle lies an enormous amount of technology, starting with the rocket engines needed to produce the high-velocity gas to the guidance systems that ensure controlled flight.
At the beginning of the twentieth century, rocket technology did not exist, nor was Newton’s third law generally recognized as the guiding principle of rocket propulsion. Even as late as 1920, the New York Times editorialized,
As a method of sending a missile to the higher, and even to the highest parts of the earth’s atmospheric envelope, Professor Goddard’s rocket is a practicable and therefore promising device. It is when one considers the multiple-charge rocket as a traveler to the moon that one begins to doubt… for after the rocket quits our air and really starts on its journey, its flight would be neither accelerated nor maintained by the explosion of the charges it then might have left. Professor Goddard, with his “chair” in Clark College and countenancing of the Smithsonian Institution, does not know the relation of action to re-action, and of the need to have something better than a vacuum against which to react … Of course he only seems to lack the knowledge ladled out daily in high schools. (2)
As frequently happens, the science and technology of modern rocketry arose independently in several places. Frank R. Winter’s topic, Rockets into Space (3), provides a summary of this early history. Konstantin Tsiolkovsky is generally given credit for the earliest work that led to contemporary rockets. He was born in Izhvesk, far to the east of Moscow, and was educated in Moscow. He spent his professional career in Kaluga, a town 160 kilometers from Moscow. He developed with mathematical rigor many of the basic relationships of rocket motion, for example, a rocket’s velocity as related to the exhaust velocity of the propellant. Although highly regarded as a scientist within the Soviet Union, he was elected to their Academy of Sciences in 1919, he seems to have had little influence on rocket development, even within his own country. He was essentially unknown in the West. It was not until 1940 that his works appeared in an English translation. Also, he did not actually build rockets or conduct experiments, nor was he associated with any groups that did.
Hermann Oberth, the pioneer of German rocketry, was born in Rumania to ethnic German parents. By the age of 15, he had designed his first rocket and in his early 20s, in 1917, had proposed to the German War Department the development of a rocket strikingly similar to the V-2. (The proposal was rejected.) In 1923, he published ”Die Rakete zu den Planeteraumen” (Rockets to Outer Space), an outgrowth of his Ph.D. thesis. The topic was very broad in its treatment of rockets and rocket flight and included problems related to manned space flight and to conducting science from spacecraft. In addition to attracting wide attention, it may have been the trigger for the international astronautics movement. Winter (3) remarks that because of his wide influence, Oberth deserves the title
”Father of the Space Age.”
However good Oberth’s ideas were, his rockets existed only on paper. Robert Goddard converted his own ideas to practice. He was born in Worcester, Massachusetts, about 100 kilometers west of Boston, educated in Worcester, and spent his professional career there. Goddard developed rockets through his interest in spaceflight. His diaries, beginning when he was 17, reveal a tortuous process, exploring every conceivable means of rocket propulsion until he arrived at the liquid-oxygen-fueled rocket in 1909 while a graduate student at Clark University. In that same year, he performed his first experiment related to rocket propulsion. By 1916, his work had advanced to the point where he was able to seek and receive support for rocket development from the Smithsonian Institution in Washington. His goal at that time was to produce a rocket that could carry instruments significantly higher than balloons could. Goddard achieved notable technical success with his research, but the performance of his rockets was modest compared to the much better supported German effort. The U.S. military never put significant resources into long-range rockets either before or during World War II. Goddard’s own wartime contribution was to the development of the JATO rockets, used to assist aircraft at takeoff.
Science at High Altitude
The nineteenth century witnessed the Industrial Revolution in which mechanical and electrical engines replaced humans, water, and wind as the principal motive forces. A similar revolution took place in science and exploration. Using the tools provided by the new industries, the scientific methodology developed by previous generations and their national wealth, investigators developed new understanding of the natural world and made astounding discoveries of phenomena never before observed. The new technology allowed the development of increasingly sophisticated instruments; it also allowed scientists to conduct expeditions far removed from their laboratories and even to leave the surface of Earth if necessary. One of the principal tools for high altitude research after 1900 was the balloon. It had its start in France more than a century earlier when two papermakers, Joseph and Etienne Montgolfier, noticed that burning paper and smoke rose up the chimney and wondered if they could apply that phenomenon to build a flying machine. They built their first balloon of paper and fabric and fueled it with a very smoky fire without understanding that it was the hot air, not the smoke, that caused the lift. The first manned flight occurred in Paris on 21 November 1783 before a crowd of 400,000. Soon afterward, the first sealed gas balloon was flown, using hydrogen. Ballooning became a way to perform high-altitude research and was a stepping-stone to science in outer space. In 1901, Reinhard Suring and Arthur Berson of the Royal Meteorological Institute in Berlin flew to an altitude of 10.5 km, still the record for an unmanned, open balloon flight. The flights of the Piccard brothers in sealed capsules during the 1930s were especially well publicized. Auguste Piccard, a particularly eloquent scientist, described the scene thus:
At an altitude of ten miles, the Earth is a marvelous sight. Yet it is terrifying, too. As we rose the Earth seemed at times like a huge disk, with an upturned edge, rather than the globe that it really is. The bluish mist of the atmosphere grew red-tinged, and the Earth seemed to go into a copper-colored cloud. Then it all but disappeared in a haze (4).
Effect of the Atmosphere on Scientific Investigations
Astronomers were among the first scientists to recognize the disturbing influence of the atmosphere on observations. Although the sky can appear perfectly transparent, it creates a number of deleterious effects. For starters, it must be remembered that there is a lot of material above us, amounting to about ten thousand kg/m2. It is remarkably transparent to visible light from blue to red, considering that the equivalent weight in glass would be about 4 meters thick. However, absorption becomes significant outside this band of colors in both the ultraviolet and infrared radiative bands. It is not the dominant constituents, nitrogen and oxygen, that are responsible for the absorption. In the infrared, the culprits are water vapor and carbon dioxide, better known now as greenhouse gases. In the ultraviolet, the culprit is ozone; it is the decrease of ozone as an atmospheric constituent and the subsequent increase of ultraviolet radiation at Earth’s surface that is of current concern to the climate change community. Further into ultraviolet (shorter wavelength), the atmosphere becomes highly opaque to radiation, as it is for particles that comprise cosmic rays.
Astronomers face absorption in the atmosphere and also refraction that causes light to bend. The degree of bending is a minor problem; however, small variations in density in the atmosphere make the bending very unsteady. To the human eye or to a small optical telescope, these changes cause images such as stars to move around and vary in brightness, to ”twinkle.” In a large telescope, the images enlarge beyond what the performance of the optics might allow.
Scattering is another effect of the atmosphere on radiation. Our blue sky is the result of scattering of sunlight, as is our red sunset. The effect of scattering is particularly noteworthy following a major volcanic eruption. The volcanic ash spewed into the atmosphere can result in brilliantly colored sunsets and sunrises for months or even years after such an event. Scattering is especially serious when trying to observe a faint object in the presence of a bright object. The scattered light from the bright object can overwhelm the brightness of the faint one.
Atmospheric Science
Labitzke and van Loon provide a thorough discussion of the history of investigations of the atmosphere (5). By 1900, the idea had been dispelled that the atmosphere was simply a gaseous mix of nitrogen and oxygen whose density and temperature decreased as one went to higher altitude. In 1896, Teisserenc de Bort, flying unmanned, instrumented balloons from Trappes near Versailles, discovered that after falling continuously with increasing altitude, the temperature began to rise above 11 km. De Bort, recognizing this sudden rise in temperature as evidence that the atmosphere was layered, coined the terms troposphere and stratosphere to describe the two layers. The discovery of ozone in the atmosphere was another great success of the early twentieth century. It had been found that solar radiation cutoff sharply in the ultraviolet, below about 3000 A. In 1880, W.N. Hartley proposed that the cutoff results from absorption by ozone in the atmosphere. The actual distribution ofozone high in the atmosphere was demonstrated by Lord Raleigh in 1917, based on observing the rising and setting sun. Then, in a remarkable precursor to experiments from rockets, Erich and Victor Regener measured the solar spectrum in a balloon, as it rose to a peak altitude of 31 km and fell back through the atmosphere. They were able to trace out the distribution of the ozone directly through its effect on the Sun’s radiation.
Ionospheric Investigations
Guglielmo Marconi was responsible for one of the astounding developments of the period when, in 1901, he transmitted a radio signal across the Atlantic Ocean. Within a few years, ”wireless telegraphy” was being used for practical communications across long distances on Earth. Also within a few years, it was recognized that there had to be a layer at high-altitude above Earth that could reflect radio waves and permit them to travel long distances. Otherwise, the radio waves would escape into space, and reception would be limited to 50 miles or so, depending on the height of the receiving and transmitting antennae. Oliver Heavyside wrote in 1902,
There may possibly be a sufficiently conducting layer in the upper air. The guidance(of radio waves) will then be by the sea on one side and the upper layer on the other. (6)
It was more than 20 years before scientists traced out the characteristics of this ”conducting layer,” specifically, that it was composed of electrons at high altitude. Robert A. Watson-Watt coined the term ionosphere to provide a continuous sequence with troposphere and stratosphere, the other layers in the atmosphere.
Radio propagation and the ionosphere attracted the attention of military organizations, none more so than the U.S. Navy because of its responsibility for maintaining a global fleet. Fortuitously, the Navy had created the Naval Research Laboratory in Washington, D.C., during the 1920s. Radio propagation was one of its major concerns, where it attracted the attention of E.O. Hulburt. Hulburt had broad interests as a scientist; his first research on the subject dealt with the characteristics of the ionosphere as a medium from which to derive radio propagation effects and vice versa. But then he asked the question, what caused the electrons to be there, and decided that ultraviolet radiation from the Sun was the likely culprit. It was more than just an inspired guess because it was believed that the Sun’s corona, well studied from eclipse observations, had to be composed of hot gas that would radiate copious amounts of ultraviolet radiation. Meanwhile, across town from the Naval Research Laboratory, at the Carnegie Institute, Gregory Breit and Merle Tuve had developed the technique of studying the ionosphere by radio sounding, sending up pulses of radio waves and watching for their return. This layer of the atmosphere was well above the altitude accessible to balloons. Hulburt, Breit, and Tuve knew of Goddard’s work on rockets and had discussed the possibility of using rockets to conduct observations at high altitude, but nothing much came of their interest until after World War II. Cosmic Rays
Bruno Rossi discussed the early history of cosmic rays and the role of high-altitude research in developing the discipline (7). Natural radioactivity was discovered in the laboratory by Henri Becquerel in 1896. By 1900, its distribution was being widely studied, including from above the surface of Earth. Scientists reasoned that if radioactive material was distributed throughout Earth’s crust like other minerals, the resulting radiation should decrease away from the surface because of attenuation in air. Such was the case as one left the surface, but then, inexplicably, the intensity of radiation began to increase. In 1912, Victor Hess, carrying his instruments in balloons floating above Austria, found that the intensity was four times greater at an altitude of 16,000 feet than at the surface. Hess hypothesized that there had to be a source of energetic particles falling upon Earth from above the atmosphere. A decade later, Robert A. Milliken proved the hypothesis to the satisfaction of the scientific community, based on measurements in mountain lakes in California. Milliken demonstrated that the intensity of the radiation at the bottom of such lakes was reduced by an amount corresponding to the absorption in the overlying water. It was Milliken who coined the term ”cosmic rays” for this new phenomenon. However, the radiation still had to penetrate more than half the atmosphere to reach even the highest mountains. Thus, there was little likelihood that the primary radiation impinging on the top of the atmosphere was actually being observed. So, to learn its true nature, it was necessary to get as high as possible in the atmosphere, and scientists continued to put instruments into high-altitude balloons. In one of the last prewar cosmic ray experiment in the United States, measurements were obtained in 1940 by scientists at the University of Chicago at an altitude of 70,000 feet, where only 3% of the atmosphere remains.
Astronomy and Solar Physics
Until fairly late in the nineteenth century, it was still common for astronomers to build observatories in or near their home institutions, even if it was a major city. However, in 1874, when James Lick’s Board of Trust in San Francisco began to consider the best possible site for an observatory, they investigated mountaintop sites despite the logistical problems and chose Mount Hamilton whose elevation is 4200 feet. Lick’s observatory (appropriately the Lick Observatory) became the first permanently occupied mountaintop observatory in the world. Although city lights were an important issue, the improved clarity of the sky was impressive. In 1909, a scientist at the French observatory on 2.9-km high Pic du Midi in the Pyrenees commented,
”The sky suddenly clears and I have before my eyes the most unimaginable scene that an astronomer can dream of. The Milky Way is sparkling, the stars shine like beacons. The sky is white with stars and their brightness is enough to light up the clouds that are at our feet.”
It also became apparent to astronomers that the Sun and the stars were producing radiation in the ultraviolet and at shorter wavelengths that was unobserved at Earth’s surface because of the opacity of the atmosphere. By 1900, the general nature of stars was well understood; in particular, it was known that the stars radiated because their surfaces were hot. In the case of the Sun, the temperature is about 5700 K and its radiation spreads from the red to the blue, peaking at about 4500 A in the green color range. However, observations of stars indicated that they existed in wide range of surface temperatures, exceeding 10,000 K. Most of the radiation of such hot stars would be in the ultraviolet range and would be absorbed in the atmosphere.
During the 1930s, the remarkable discovery was made that the temperature of the gas in the Sun’s atmosphere was several millions degrees. This was based on identifying spectral lines in the visual range from very hot atoms of elements such as iron and calcium. The lines themselves had been discovered more than 50 years earlier during eclipse observations. Such hot gas implied the emission of X rays from the Sun.
The Development of the V-2 Rocket
Ever since siege guns spelled the end of fortified cities, long-range artillery has been a staple of military inventories, certainly in Europe. Thus, the German Army recognized the potential of rockets as the basis for a new kind of cannon and in 1932 began their developing at a test station in Kummersdorf near Berlin, under the direction of a young Army officer, Walter Dornberger, who has written a memoir covering the development of the V-2 (8). Among his assistants was Wernher von Braun, then a 19-year-old student at the Berlin Institute of Technology. Their first project, a 650-pound thrust rocket engine, blew up. (Thrust, the standard measurement of rocket engine performance, is the product of the rate at which mass is expelled from the rocket and its velocity. By Newton’s laws, it is also the forward force on the rocket. It is expressed in pounds [English units] or Newtons [metric units]. One pound of thrust equals about 4.5 Newtons.) However by 1936, they had developed a working engine that had a thrust of 3500 pounds (the highest Goddard achieved was 825 pounds in 1941; the V-2 would attain a thrust of 55,000 pounds) and had attracted the attention of the German General Staff. With ensured support, the operation was transferred to Peen-emunde on the Baltic coast where it would remain through World War II. Although well supported, the program did not receive the highest priority for weapon development until 1943. The first V-2 rockets were fired at London in September 1994.
Dornberger’s group recognized that they did not have adequate information about the atmosphere at high altitude to calculate the trajectory of their rockets. In 1942, Von Braun and his staff briefed academic scientists around Germany on the program, including Erich Regener. Regener had headed a physics institute at Stuttgart and was a leading German scientist; his personal research involved using balloon-borne instruments to study cosmic rays and the properties of the upper atmosphere. His discovery of the ozone layer has already been noted. He did not have an easy time in Nazi Germany because he opposed the idea of Aryan science and had a Jewish wife. His son had fled Germany in 1936 and he himself lost his position at Stuttgart. By 1942, he had reestablished himself in Berlin and was continuing his research, supported in part by the German Air Ministry. He then began the development of instruments for the Peenemunde group that would fly on their rockets to measure the temperature, pressure, and absorption of solar radiation by ozone. In addition, he included an instrument for the German astronomer, Karl Kippenheuer, to measure solar ultraviolet radiation directly. Regener’s idea was to have these instruments contained in a capsule to be ejected from the rocket at its peak altitude and float to earth on a parachute, taking measurements on the way down. In spite of wartime pressure, the capsule and its instruments were fabricated and ready for installation in V-2 rockets by the end of 1944. But before preparations could be completed, the advancing Russian army forced the evacuation of Peenemunde.
It has been estimated that the development of the V-2 was as costly to the Germans as the atomic bomb was to the Americans. In addition to the facilities at Peenemunde, tens of thousands of concentration camp inmates, forced laborers from foreign countries, and German workers were employed in manufacturing the high-priority missiles at the Mittelwerk GmbH camp near the town of Nor-dhausen (Thuringia). It was certainly one of the great technical achievements of modern times.
Transition to the Postwar Era
The period between 1900 and 1945 set the stage for the scientific revolution that took place after the end of World War II, based on the use of rockets for scientific purposes. Scientists had developed compelling objectives in a variety of disciplines that required observations from high altitude. They had also developed the tools to do so. Of greater significance was that the scientists of the prewar period were still active in 1945. They, along with the scientists who emerged from the wartime laboratories, were instrumental in creating the revolution. DeVor-kin provides an detailed discussion of the postwar period of using sounding rockets for research purposes up to about 1955 (9).
Even before the end of the war, U.S. agencies were in Europe searching out information about German science and technology. The dramatic effect of the V-2 bombardments made rocket technology one of the focal points of interest. The U.S. Army Ordnance Department retrieved tons of V-2 parts and documentation from the Nazi factories at Mittelwerk and shipped them back to the United States. Ernst Krause from the Naval Research Laboratory and member ofa Navy group in Europe returned with the knowledge that sophisticated rockets could be built. The Alsos project, an effort focused on Germany’s atom bomb development, included Gerhardt Kuiper, an astronomer who had extensive experience in radio science. Kuiper learned about Regener’s wartime efforts to fly instruments on the V-2 and other upper atmospheric research in Germany. He transmitted that information to Donald Menzel at Harvard who distributed the information to other U.S. scientists, notably to Merle Tuve, E.O. Hulburt, and Leo Goldberg, then at the University of Michigan. Another activity, CIOS (Combined Intelligence Objectives Subcommittee) included Fritz Zwicky from the California Institute of Technology who came back a space enthusiast. His own desire was to study the effects of hypervelocity at high altitude by creating artificial meteors.
Following the end of the war in Europe, the United States engaged in a furious effort to assemble and fly V-2 rockets. The U.S. Army Bureau of Ordnance established a capability to rebuild V-2 rockets, having installed Von Braun and other members of the Peenemunde team at Fort Bliss in Texas. The rockets would then be flown from the newly established missile range at White Sands, New Mexico, under the direction of Colonel Holger Toftoy. Their primary directive was to gain experience in handling and firing missiles; their next objective was to conduct high-altitude research. To accomplish this latter objective, Toftoy sought out the nation’s military and civilian scientific organizations. Toftoy was no doubt impressed by the wartime partnership between the scientific and military communities and so naturally turned to scientists again to accept a role in this new endeavor. It is important to remember that in 1945, the United States did not have a formal process to conduct scientific research; both the National Science Foundation and NASA were more than a decade away. So individual agencies made individual arrangements. The Naval Research Laboratory convinced the Navy’s Office of Research and Invention, the forerunner of the Office of Naval Research, that it should have the lead in the Navy for conducting rocket research. The Applied Physics Laboratory, established during World War II under Merle Tuve, received sponsorship for rocket research and rocket development from the Navy’s Bureau of Ordnance. They were the major players, but other military agencies and major universities joined in. After much haggling, well described by DeVorkin (9), the V-2 Rocket Panel was formed with membership from NRL, APL, Cal Tech, Harvard, University of Michigan, and other organizations to oversee the allocation of space on V-2 rockets for high-altitude research. The research goals included radio and sound propagation in the atmosphere, properties of the atmosphere, cosmic rays, solar ultraviolet radiation, and various biological investigations. The first flight of a V-2 took place on 16 April 1946. The last V-2 was flown in 1952. By then, the Aerobee rocket, first flown in November 1947, had become the principal rocket used by American scientists. Figure 1 shows an assembled group of military personnel at White Sands preparing a V-2 for flight in 1947.
The Aerobee rocket was the culmination of work on rockets that had begun during the 1930s at the California Institute of Technology within a group headed by Frank Malina that eventually would become the Jet Propulsion Laboratory. Malina, then an engineering graduate student, was part of a group headed by Theodore Von Karmen, the great aerodynamicist. By 1939, the group obtained support from the Army Air Corps for JATO development. In January 1945, Malina and his colleagues proposed to develop a rocket that would carry a 25-pound payload to an altitude of 20 miles. In October 1945, they launched the first flight of their rocket—the WAC Corporal. It reached an altitude of 45 miles. It was America’s first sounding rocket, even though it could not compare in performance with the V-2. As an aside, Malina, in a biographical memoir, traces his interest in space exploration to reading Jules Verne’s, From the Earth to the Moon, when he was a boy of 12.
The WAC Corporal was built by the Aerojet Corporation, a private company that had emerged from the Cal Tech group in 1941 and built thousands of JATO units during the war. They were then asked by the Applied Physics Laboratory to build a new rocket that would carry 150 pounds to 50 miles and would become the Aerobee. The first one was launched in November 1947 and remained in service, with various modifications, through the 1980s. The Naval Research Laboratory, also recognizing the need for high-performance rockets, contracted with the Glenn L. Martin Company to build the Viking rocket to reach an altitude of 150 miles. The first Viking was flown in May 1949. It was too expensive to achieve wide use as a sounding rocket, but it did form the basis for the Vanguard program, the U.S. national program to put a satellite in Earth orbit. A remarkable aspect of these developments was their rapidity. In the case of the WAC Corporal and the Aerobee, less than a year was required to develop and fly the first rockets.
Figure 1. A V-2 rocket being prepared for flight at the White Sands Missile Range in 1947.
The importance of rockets for military and science applications was apparent to countries other than the United States. Work on rockets began in the United Kingdom in 1946 within the Royal Aircraft Establishment at Farnsworth. In 1955, U.K. scientists promoted the development of a rocket, which was to become the Skylark, that could lift 45 kg to 210 km. The first flight of the rocket took place in Woomera, Australia, during February 1957. The Skylark and the Woomera range have become staples in the British high-altitude research program. The Skylark has also been used by the Germans and the Swedes in their national research programs. France also began rocket development in 1946 in its Ballistics and Aeronautical Research Laboratory (LRBA) with first the EOLE, then Veronique, Vesta, Belier, Centaure, and Dragon. Launches from Hannaguir, Algeria, began in 1952.
Now, the U.S. sounding rocket program is managed by NASA and uses of thirteen different configurations, most of which are multistage. The earliest rockets fall into the lowest performance category of the current capability. In spite of the vast improvement in performance, science from rockets is still limited in time available for experimentation, which is intrinsic in the ballistic nature of the flight.
Rocket Performance
The first rocket scientists were primarily interested in achieving some level of thrust and altitude with their rockets. With the advent of the V-2, scientists and engineers had to achieve specific operational requirements, namely, to carry a 2300-pound warhead a distance of about 190 miles. To achieve that goal, the rocket would have to reach an altitude of about 60 miles. If there were no atmosphere, the rocket could achieve the 190-mile distance and reach an altitude of only about 48 miles. Atmospheric resistance requires that the rocket be fired more directly up than optimum, so that it spends less time in the lower atmosphere.
To conduct science, the V-2 and other rockets could be launched straight up. Because rocket ranges tended to be small, it was required that the rocket not fly very far from its launch site. For scientific purposes, V-2s achieved peak altitudes of more than 100 miles. For vertically launched rockets, the time in orbit depends only on the peak altitude, and the observation time for a given experiment was given by the altitude range in which data could be obtained. The size of rocket ranges and the cost of the rockets have limited their performance, so that even the latest rockets do not exceed about 300 miles in altitude and provide observing time of about 5 minutes for payloads of about 1000 pounds.
There has been one exception to this. After World War II, the range requirement of military missiles went from hundreds of miles to thousands of miles. To meet this requirement, the United States fired rockets from a range in California to islands in the South Pacific. These rockets allowed about 30 minutes of observation time. During the 1960s, a group at the Lawrence Livermore National Laboratory took advantage of these rockets to conduct astronomical observations (10).
Early Science Using Rockets
Science using sounding rockets proceeded at a furious pace in the years immediately after World War II. Between 1946 and 1950, the United States flew almost 100 V-2 and Aerobee rockets from the White Sands Missile Range with instruments that measured cosmic rays, solar radiation, atmospheric characteristics, and sky brightness. Cameras were flown that photographed Earth; even biological experiments were carried out. During this period, scientists were learning how to use rockets. The two outstanding problems that had to be dealt with were controlling the orientation of the rocket and retrieving the data and the instrument.
To achieve stable flight, rockets are generally spin stabilized. The rocket fins are tilted slightly so that aerodynamic forces induce a spin in the rocket as it ascends through the atmosphere. The effect is the same as the “rifling” in a gun barrel that imparts spin to a speeding bullet. Thus a rocket will maintain the same heading, more or less, during its flight as it had when it began at the ground. For most of us these days, the image we have of a rocket launch is NASA’s space shuttle. Such large rockets begin their flight too slowly to achieve much spin. They are held steady by gyroscopes that determine the orientation and small vernier rockets that keep the rocket headed in a set direction. For the earliest rockets observations, the rocket was pointed at a place such that a given object or region was in view during all or part of the flight taking into account the spin of the rocket. For some experiments, for example, measuring cosmic rays, pointing up was sufficient; for others, such as solar observations, lack of control limited the kinds of observations that could be conducted. The V-2 did use gyroscopic control because it was intended to impact a specific point and needed good control during the early phase of its flight.
Retrieving data by using radio communications to the ground was straightforward for data that could be in an electronic form. However, much of what scientists wanted to do, especially imagery, could not be reduced to electronic signals. So it was necessary to devise a means to recover parts of the rocket, at a minimum. Initially, the approach was brute force. The sensing portion of the payload would be built into a “crash-proof” container. Following some well-publicized instances where the rocket and its scientific payload produced a large crater and little of recoverable value upon its return to Earth, the V-2s were equipped with explosive charges that separated the science payload from the rocket, and the payload would then flutter down to Earth at lower speed. Eventually, Aerobees were equipped with parachutes that allowed recovery of the payload, more or less intact. Recovery of the payload also had the advantage of reducing the cost of reflights of the instrument.
A cosmic-ray experiment has the honor of being the first experiment performed from a sounding rocket. In April 1946 and twice again in May, single Geiger counters were flown by James Van Allen and his group at the Applied Physics Laboratory on V-2 rockets (11); in the first few years of rocket science, cosmic-ray measurements were the most popular experiments to be conducted. These experiments achieved a fair degree of sophistication. The first NRL experiment, flown in June 1946, used 10 Geiger counters, interspersed with 10-cm lead shielding, that weighed a total of 100 pounds (12). A number of important results emerged during the first several years of sounding rocket work. Van Allen and Singer (13) demonstrated that cosmic-ray intensity depends strongly on the strength of the local magnetic field. They reported that the intensity at the magnetic equator was about a factor of 3 lower than at White Sands, based on the flight of an Aerobee rocket from a U.S. naval vessel. The higher magnetic field at the equator sweeps away lower energy particles that can penetrate nearly to the surface of Earth at New Mexico. Going further north where the field is weaker, cosmic ray intensity increases further. Aside from measuring its intensity, scientists measured its atomic composition. Cosmic rays are mostly protons, but other heavier constituents were known to be present. In a 1950 V-2 rocket flight, the NRL group, using a detector assembly that had 48 Geiger counters, 2 ionization chambers, and about 8-inch thick lead slabs measured the presence of helium nuclei in cosmic rays and possible other elements as well, aside from the dominant proton contribution (14). Figure 2 illustrates the data obtained from this kind of experiment. These data came from X-ray detectors flown in 1962 that detect cosmic rays as well. On the ground, the counting rate is very low. As the rocket lifts off, the counting rate rises rapidly, reaches a peak at 52,000 ft (16 km), then falls to a steady value between about 117,000 and 250,000 ft. The peak is the Pfotzer maximum. It results because every cosmic ray that enters the atmosphere produces many particles when it interacts, which in turn produce more particles when they interact. This process continues until the average energy of the particles degrades, and multiple particles are no longer produced. So the number of particles increases and then decreases. At 250 seconds, the detectors, which are in the rocket and are shielded by a heavy aluminum door, are exposed to the outside environment and can respond to much lower energy radiation than represented by cosmic rays. The counting rate jumps abruptly and continues to rise as the rocket leaves the last vestige of the atmosphere. The process is repeated 300 seconds later as the rocket reenters.
Figure 2. Data from a sounding rocket flight showing the changes in counting rate in radiation detectors as the rocket ascended through the atmosphere and returned.
Cosmic-ray experiments, aside from being first, also illustrate the limitations of sounding rockets, namely, time at altitude and payload weight. The cosmic-ray intensity is very low, only about one per cm2 per second above the United States. They are also highly penetrating and require the use of lead or other heavy shielding as part of the experimental apparatus. The net result was that the cosmic-ray community had to await the development of large boosters and orbiting spacecraft before they could make significant advances in their discipline. Even today, cosmic-ray experiments strain the capability of the various national space agencies with their size and weight requirements.
Early solar observations were hampered by the fact that film was being used as the recording medium and needed to be recovered after the flight. Both NRL and APL developed spectrographs to measure the ultraviolet spectrum of the Sun, a subject of intense interest to astronomers of the day. The approach at that time was to put the film into an armored vessel, which eventually proved successful. Another problem facing the experimenters was that there was no good way to point continuously at the Sun. As a solution, both the NRL and the APL experimenters chose to place spectrograph apertures on opposite sides of the rocket and took short exposures to catch the Sun just at the moment when it shone in these openings, as the rocket spun on its axis. Tousey and his group at NRL were the first to obtain UV spectra during a V-2 flight on 10 October 1946 (15). During the next several years, both NRL and APL obtained successful spectra to a limiting short wavelength of about 2200 A that revealed hundreds of spectral lines. However, these results, which were important for the solar physics community, did not address a major issue of interest to NRL, namely, what was the radiation responsible for the ionosphere. To answer that question, it was necessary to search for X rays. The first attempt to do so by T.R. Burnight of NRL during an Aerobee rocket flight of 5 August 1948 used X-ray sensitive phosphors and film. As all too often occurred during these early days of rocket science, successful detection was claimed, but the result was ambiguous (16). Subsequently, other NRL scientists used thermoluminescent crystals to attempt to detect X rays. These are crystals that are excited by X radiation and subsequently glow when heated in proportion to the initial excitation. The issue was resolved decisively by Herbert Friedman and his colleagues at NRL, during a V-2 rocket flight of 29 September 1949 by using X-ray sensitive Geiger counters that could record individual X-ray photons (17). Friedman was a newcomer to atmospheric and space research. His background was laboratory X-ray analysis, and he had spent the war years applying his specialty to military needs. During that time, he had developed Geiger counters with very thin walls that were sensitive to X rays.
The requirements of the solar researchers led to the first important advance in rocket science since the emergence of the V-2 rockets themselves, namely, the development of a means to point the payload at a given target. In 1947, both NRL and APL built devices that tracked the Sun. About the same time, Marcus O’Day of the University of Michigan provided Air Force funding to a group at the University of Colorado to develop such a device. The Colorado device, a two-axis system, flew first in 1949 and became the standard unit for studies of the Sun. It also led to one of the most successful commercial “spin-offs” from rocket science. A few of the engineers from the University of Colorado formed a private company to provide support for the development of experimental rocket payloads, supported by financing from the Ball Brothers Corporation of Muncie, Indiana. The company is now Ball Aerospace Corporation.
Studies of the ionosphere were equally popular. Unlike cosmic rays, which could be measured in the atmosphere, the ionosphere begins at an altitude of about 100 km. Thus rocket-borne instruments provided the first direct reach into this atmospheric region. Most of the measurements, however, were not direct; rather, they relied on measuring the effects on the propagation of radio waves from the rocket to a ground receiver. However, these yielded detailed information on the region’s structure. A direct measurement of electron density was made by the University of Michigan group from a December 1947 V-2 rocket flight using a Langmuir probe, an instrument invented in the 1920s to measure the electrons produced in an electrical discharge. In this case, the ionosphere itself represented the discharge, and the electrons comprising the ionosphere were collected as a current. This instrument, in various forms, has become a staple of space research to record ambient electrons and their accompanying protons.
Studies of the ionosphere and companion measurements of the neutral atmosphere verified and extended the general understanding of the upper atmosphere. However, the limitation of rocket science here also became important. In contrast with cosmic-ray science, the problem was not weight limitation.
Instruments for measuring the atmosphere were small then and still are; rather, the problem was that a single rocket flight provides only a snapshot of a complex phenomenon that varies in space and time. Again, only the advent of orbiting spacecraft made it possible to extend measurements over the whole earth and through many seasons and allowed developing a thorough understanding of the underlying phenomena. The same is true of most geophysical phenomena.
An important area of applied research that attracted the attention of the early rocket scientists was photography of Earth from space. In 1947, groups from both APL and NRL were successful in obtaining photographs from V-2 rockets. Figure 3 shows a typical photograph from a rocket obtained during a V-2 flight in 1947. But because rocket cameras could take pictures for only a few moments in a restricted place, their importance as a practical reconnaissance or meteorological tool was recognized as limited in the same way as were other geophysical investigations. NRL did continue with a program of rocket photography, principally to determine where the rocket was pointing. However, in 1954, the group fortunately caught a hurricane in its view. As explained by the NRL researchers, rockets were seldom fired if the cloud cover exceeded 10% because that would make it difficult to photograph the rocket from the ground and to obtain photos of ground features from the rocket. However, they go on to note,
”A fortunate exception occurred on October 5 1954. Two rocket-borne movie cameras obtained pictures of towering clouds spiraling into a tropical storm near Del Rio, Texas.”
The resulting composite photograph, covering more than a million square miles of area clearly showed the utility of photographing storm clouds from the vantage point of a rocket (18).
Figure 3. Photograph of Earth taken in 1947 at an altitude of about 160 km from a V-2 rocket flown from the White Sands Missile Range.
In spite of the emergence of important new results, some university groups (Princeton, Harvard, Cal Tech) believed that the prospects were so limited that they abandoned the idea of using rockets for science projects. However, what was demonstrated was more important than any science. The several active groups showed that it did not take heroic efforts to build scientific payloads. These groups used instruments adapted from their laboratories or entirely novel instruments and flew them repeatedly on short schedules. Some of the basic requirements for rocket scientists were solved: notably retrieving data from space using radio technology and recovering payloads. A start was made in developing actively controlled rockets that could point at specific targets. The United States developed new rockets and improved their reliability enormously. By the 1950s, rockets, more often than not, did work, in contrast to the first few years, when the reverse was true. These advances had the effect of making rocket science more attractive to individual researchers and to funding agencies.
A Maturing Discipline
By the mid-1950s, many of the developments in contemporary rockets were in place. The Aerobee was replaced with the Aerobee-Hi, a rocket developed by the U.S. Air Force exclusively for sounding rocket research and designed to carry a parachute for payload recovery. The concept of controlling rockets had emerged with the successful development of the Sun seeker. A major scientific conference exclusively dedicated to rocket science was held in 1952 in Oxford, England, sponsored by the Rocket Research Panel of the United States and the Royal Society of London (19).
In 1957, events occurred that changed space science. The Soviet Union launched the world’s first artificial satellite in October of that year, as part of its contribution to the International Geophysical Year. The complementary U.S. effort was successful in the following year. The subsequent emergence of a national space agency (NASA) in the United States and the strong support of space science increased significantly the available funding and the number of participating groups. Other nations had also made commitments to national programs of space science. The focus of these programs was orbiting satellites, but the new interest did result in a significant increase in the number and quality of rockets available for research. There are a few new rockets, notably, the Canadian Black Brant, that replaced the Aerobee, and there has been considerable development of the means to control the rocket itself. In addition to high precision ”Sun followers,” ”star followers” are now available that can maintain subarc minute pointing precision.
There have been surprises, one of which was in the X-ray domain. The observation of X rays from the Sun had been one of the great successes of early rocket science, both from the perspective of the novel detectors that had been developed and the richness of the phenomena that had been observed. Thus, it was natural to consider looking for X rays from cosmic sources. The likelihood of observing Sun-like stars was extremely remote. Even the nearest stars are about a million times more remote from us than the Sun, and their radiation is reduced by 10 trillion times compared to that of the Sun. Nevertheless NRL’s Herbert Friedman did attempt to observe cosmic X rays from rockets without success. In 1960, he wrote,
Efforts to observe X-ray emission from celestial sources have yielded only negative results. On the basis of three experiments thus far, it can be stated that no fluxes stronger than … the 1-10 A and 44-80 A bands have been seen in fairly complete scans of the sky from White Sands, New Mexico. The observation of smaller fluxes will require longer observing times, available from satellite platforms rather than rocket probes. (20) Yet within a few years, X-ray astronomy from sounding rockets became an exciting new field of science.
In 1962, a group of scientists from American Science and Engineering, a private company in Cambridge, Massachusetts, flew an Aerobee sounding rocket from the White Sands Missile Range with Air Force sponsorship. The principal objective of the rocket was to look for X rays from the Moon that should result from excitation by solar X rays. A further objective was to survey the sky for celestial X-ray sources. The result was the detection of a very strong source of radiation near the galactic center and an indication of other sources as well (21). The source is now known to be Scorpius X-1, still the brightest steady X-ray source in the sky. This result was confirmed and extended, notably, by Friedman at NRL (21). Within a few years, X-ray observations of the cosmic sources became recognized as a distinct part of astronomy on a par with radio astronomy and cosmic rays. X-ray astronomy is an example of a discipline that emerged only because of the availability of rockets. Though X-ray observations are still carried out by rockets, satellite observations have dominated the discipline since 1970 when the first satellite dedicated to X-ray observations, UHURU, was launched into orbit.
The discovery of bright X-ray sources in 1962 marked another kind of transition. After it was established in 1947 as a service independent of the U.S. Army, the Air Force vigorously pursued a space mission, including sponsorship of basic scientific research from sounding rockets and satellites. Secondly, the research group at American Science and Engineering, Riccardo Giacconi, Frank Paolini, and the author, were in secondary school when the war ended and the first rockets were being flown for research purposes. This was a second generation from the perspective of sponsorship and personnel. The 1960s also marked the decade that NASA, founded in 1958, was creating a new constituency for space sciences. By 1964, NASA was committing substantial resources to the new discipline of X-ray astronomy.
Sounding rockets still fill an important niche in space sciences, especially in demonstrating new kinds of observational capability. NASA launched 20 rockets in 1999, 10 from the White Sands Missile Range. The image shown in Fig. 4 is an example of the sophistication that can be achieved from a rocket. It is an image of the Sun’s atmosphere in the radiation from hot hydrogen. This radiation emerges from the Sun’s chromosphere, immediately above the Sun’s surface. The spatial resolution of the image is about one-third of an arc second and is a tribute to the instrument developers at the Naval Research Laboratory and to the rocket developers at NASA. Results of this sort are still an important source of scientific results of high importance and a guide as to what is likely to make a productive space mission. Their cost is now well above the hundred thousand dollars or so that rocket experiments used to cost but still well below the hundred millions of dollars or so of satellite programs.
Figure 4. Image of a portion of the Sun’s lower atmosphere taken in the light of very hot hydrogen. The hydrogen is confined to the Sun’s magnetic field, yielding the strandlike appearance.
