Rockets

That turns out to be a very general principle: in any object or collection of objects, forces which only involve those objects and nothing else ("internal forces") cannot shift the center of gravity.

An astronaut floating in a space suit cannot shift his position without involving something else, e. g. pushing against his spacecraft. The center of gravity--or "center of mass"--is a fixed point, which cannot be moved without outside help (turning around it, however, is possible).

By throwing a heavy tool in one direction, the astronaut could get moving in the opposite direction, though the common center of gravity of the two would always stay the same. Given a bottle of compressed oxygen, the same result follows from squirting out a blast of gas (a scene that appeared in an early science fiction film). A rocket does much the same, except that the cold gas is replaced by the much faster jet of glowing gas produced by the burning of suitable fuel.

Launch of an   Atlas-Centaur.

The powerful rockets which lift hundreds or even thousands of tons off the launching pad depend on the same principle. If you ever watched a rocket lift off at Cape Canaveral, it is worth remembering that if you could somehow remove from the scene the launching pad, the atmosphere and the Earth, then the combined center of gravity of the rocket and its exhaust gases would always remain where it started, at the launching point.

It may seem like a round-about way for producing motion. And yet, rockets are (at least for now) the only practical means of leaving Earth and flying into space.

In 1915, as assistant professor at Clark University, Worcester, he began experiments on the efficiency of rockets. He bought some commercial rockets and measured their thrust using a ballistic pendulum, a heavy mass suspended by ropes, to which the rocket was attached. The rocket was fired, and the height to which the pendulum rose provided a measure of the total momentum (mass times velocity) imparted to it.

It can be shown from Newton's laws that the total momentum of a system free from outside forces is conserved; that is actually another formulation of the conservation of the center of gravity, mentioned in the discussion of rocket propulsion. Therefore the momentum given to the pendulum in one direction had to be equal to the momentum mv imparted to the rocket's gas jet and that momentum determined the length and height of its swing. By weighing the rocket before and after firing, Goddard could derive the mass m of the ejected gases and from that deduce v. For a 1-pound Coston ship rocket, he found that v was about 1000 ft/sec (300 m/sec).

De Laval's Nozzle

A rocket is essentially a heat engine, a device for converting the energy of heat (obtained from the chemical energy of the fuel) into mechanical energy--here the kinetic energy mv²/2 of its exhaust jet. Knowing m and v, Goddard could derive the kinetic energy given to the gas, and by burning a measured amount of the fuel, absorbing the heat (e. g. in water) and measuring the rise in temperature, the total amount of chemical energy converted to heat could also be obtained. The conclusion was rather disappointing: only about 2% of the available energy contributed to the speed of the jet.

Could this be improved? Luckily for Goddard, this problem had been solved by Gustav De Laval, a Swedish engineer of French descent. In trying to develop a more efficient steam engine, De Laval designed a turbine whose wheel was turned by jets of steam.

De Laval's turbine:   4 nozzles, one in  cross section.

The critical component, the one in which heat energy of the hot high-pressure steam from the boiler was converted into kinetic energy, was the nozzle from which the jet blew onto the wheel. De Laval found that the most efficient conversion occured when the nozzle first narrowed, increasing the speed of the jet to the speed of sound, and then expanded again. Above the speed of sound (but not below it! ) this expansion caused a further increase in the speed of the jet and led to a very efficient conversion of heat energy to motion. Nowadays steam turbines are the preferred power source of electric power stations and large ships, although they usually have a different design--to make best use of the fast steam jet, De Laval's turbine had to run at an impractically high speed. But for rockets the De Laval nozzle was just what was needed.

Goddard experimented on his ballistic pendulum with various nozzle designs, using a small metal combustion chamber filled with a type of gunpowder, ignited by electricity. The end of the chamber was threaded, so that nozzles of various designs could be screwed onto it and tested. Using a De Laval nozzle, he obtained jet velocities between 7000 and 8000 ft/sec and efficiencies of up to 63%. Later he replaced the ballistic pendulum with a more compact device, in which the thrust of the rockets did not lift a pendulum against gravity but compressed a calibrated spring. With that device he showed that (contrary to some popular claims) rockets worked just as well in a vacuum.

As Goddard himself noted, that made the rocket the most efficient of all heat engines, better than piston-driven steam engines (21%) and Diesel engines (40%). No wonder: from the second law of thermodynamics, the theoretically attainable efficiency of a heat engine increases with its operating temperature, and no other heat engine runs as hot as a rocket.

A rocket engine at the   Smithsonian, cut open to  show convergent-diver-  gent DeLaval nozzle.

After the US entered World War I, Goddard also worked for a while on military rockets, but none of his designs were implemented, though rockets somewhat similar to his design were turned in World War II into an effective weapon against tanks, known as the bazooka.

The idea of feeding the rocket with a continuous stream of solid charges also proved unfeasible, and in 1922 Goddard went back to his alternative idea, proposed independently by Hermann Oberth in Germany and also noted by Tsiolkovsky: a liquid-fuel rocket. It would have two lines running into its combustion chamber, one feeding fuel, the other oxygen, similar to the way a steel-cutting blowtorch operated, except here both lines carried liquids, not gases--in Goddard's design, gasoline and liquid oxygen.

Such a rocket promised very high efficiency, but also posed serious technological challenges. Both fluids had to be pumped at a steady rate, and one of them, liquid oxygen, was extremely cold. The high temperature of combustion in pure oxygen required heat-resistant materials, and to help overcome this, Goddard developed the technique of having the liquid oxygen cool the combustion chamber on its way from the fuel tank. This method is still use: in the picture on the right, the outer part of the nozzle is covered with a large number of metal pipes, through which the cold fuel flows on its way to the combustion chamber. Another completely novel problem which faced Goddard was the guidance and control of the rocket in flight.

Robert H. Goddard besides 1926 liquid- fueled rocket. The rocket is on top, recei- ving its fuel by two lines from the tank at the bottom.

On March 16, 1926, Goddard flight-tested his first liquid-fuel rocket. He thought stable flight could be obtained by mounting the rocket ahead of the fuel tank, with the tank shielded from the flame by a metal cone and the lines for fuel and oxygen pulling it behind the rocket: the design worked, but did not produce the hoped-for stability. The rocket burned about 20 seconds before reaching sufficient thrust (or sufficiently lightening the fuel tank) for taking off. During that time it melted part of the nozzle, while the camera with which Mrs. Esther Goddard was trying to record the flight ran out of film, so that no photographic record of that flight remains. Then it took off to a height of 41 feet, leveled off and later hit the ground, all within 2. 5 seconds, averaging about 60 mph.

Goddard's concept seemed validated, but he was still far from a practical design. Unfortunately, he worked in isolation, without the engineering resources of a major institution. In the years that followed he continued developing his rockets--controlling their motion by gyroscopes, steering them with small vanes thrust into their exhaust jet, and building larger and faster rockets. These were tested in test stands on the ground and sometimes also in free flight, mostly at a rocket lab he established in Roswell, New Mexico.

But the actual realization of his dream fell to others who enjoyed military or national support. Goddard, unfortunately, never lived to see the age of spaceflight. He died of cancer on August 10, 1945, in Baltimore.

The full story of rocket technology is too long to be covered here. Between World Wars I and II, especially in the 1930s, rocket enthusiasts and rocket clubs were active in Germany, the US, Russia and other countries. Experimental rockets were designed, tested and sometimes flown. Some of the experiments used liquid fuel, though solid-fuel rockets were also developed. In the latter, the fuel gradually burned off (as it did in early gunpowder rockets), and the entire fuel container was under pressure, supplying hot gas directly to the De-Laval nozzle.

Hermann Oberth

The hotbed of rocketry was Germany, where Hermann Oberth, a transplanted Romanian, vigorously promoted the idea of spaceflight, even though his doctoral thesis "The Rocket into Interplanetary Space" was rejected by the university of Heidelberg. Oberth was an early member of the "Society for Space Travel" (Verein fuer Raumschiffahrt or VfR) formed in 1927. In 1930 the VfR successfully tested a liquid fuel engine with a conical nozzle which developed a thrust of 70 newtons (about 10 newtons will lift 1 kg). By 1932 it was flying rockets with 600-newton motors.

The V2 Rocket

By that time, however, the German army had begun developing rockets for its own use, and in 1932 it enlisted the help of a young engineer named Wernher Von Braun. The military's rockets were larger and more ambitious, and the A2 which flew in 1934 developed a thrust of 16000 newton. This line ultimately led to the A4, designed and tested under Von Braun's supervision, a 12-ton rocket with a thrust of 250 000 newtons, a 1-ton payload and a range of 300 km (about 200 miles).

(Photo by Richard V. Wielgosz)

Renamed the V-2 ("vengeance weapon 2") by the German army, hundreds of rockets of this type were fired in late 1944 towards London, a target large enough to ensure serious damage even without precise guidance. Because these missiles flew much higher and much faster than any airplane, Britain had no way of intercepting them, and bombing their launch sites was also difficult, since the V-2 (like Iraq's missiles in 1991) employed mobile launchers. The attack only stopped when the German army was pushed beyond the rocket's range. Today a V-2 is on display at the National Air and Space Museum of the Smithsonian Institution in Washington (picture on the right).

In the USA

Theodore Von   Karmán

Meanwhile rocketry was developing in the US, quite apart from Robert Goddard's efforts. One noted pioneer was Theodore Von Karmán, a native of Hungary and the graduate of the Minta, one of the famed high schools of Budapest from which came a remarkable number of distinguished scientists. Karmán became an authority on aerodynamics and in the 1930s served as professor of aeronautics at Caltech, the California Institute of Technology in Pasadena, California.

Together with Frank Malina, one of his graduate students, Karmán began designing and building rockets at Caltech's Guggenheim Aeronautical Lab (supported by the Guggenheim family which also financed Goddard's work). Because rockets had a dubious "far out" connotation, they referred to their work as "jet propulsion. " Ultimately Karmán and Malina established at Caltech a laboratory devoted to rocket work, the Jet Propulsion Laboratory (JPL); today JPL is virtually part of NASA, a large lab specializing in the exploration of the solar system beyond Earth. Another distinguished student of Karmán was Hsue Shen Tsien, who later returned to China and helped establish that country's spaceflight effort.

Karmán's group built both solid-fuel and liquid-fuel rockets. During World War II one of the problems was getting heavily loaded seaplanes into the air, Karmán and his engineers solved this by designing the JATO rocket, for "Jet Assisted Take Off." It originally burned a mixture of roofing tar and perchlorate, an oxygen-rich compound similar to the one used by chemistry teachers for producing oxygen in classroom demonstrations: the tar was the fuel and the perchlorate provided the oxygen. (Robert Goddard designed an alternate liquid-fuel JATO rocket, but it was not successful. )

Later they designed the solid-fuel "Private" for military use, and a bigger liquid-fuel rocket, the "Corporal. " The latter was adapted for high-altitude research as the "WAC Corporal" (WAC stood for Women's Auxiliary Corps) which, with a thrust of 6700 newtons, reached in 1945 a height of 70 km; later a larger scientific rocket was developed from it, the Aerobee.

Military Uses

Apart from the V-2, the various armies in WW II used solid-fuel artillery rockets much in the way that Congreve had used them, for massive bombardments, to cover attacks or beach landings; the Russian army, for instance, had its famed "Katyusha".

The X-1 rocket airplane, the first  airplane to pass the speed of  sound. Effects of shock waves  are seen in the rocket exhaust.

In addition Germany developed rocket-powered fighter planes, whose engines only burned long enough to enable them rise and intercept American bombers, after which they glided to Earth, to land without any engine. These, however, were weapons of desparation, and the war ended before they could be used. After the war, in 1947, the US built and flew a rocket airplane, the X-1, and it became the first airplane to exceed the speed of sound in level flight, on 14 October 1947. The X-1, too, can be seen in the Smithsonian museum.

Rocket Staging and Technology

Each of the above rockets had a single engine, on which it rose until it ran out of fuel. A better way to achieve great speed, however, is to place a small rocket on top of a big one and fire it after the first has burned out.

Suppose one wanted to use a V-2 rocket to send a small payload--e.g., 10 kilograms--as high as possible. The usual payload of the V-2 rocket was one ton (1000 kg), and with that a height of about 100 km was possible. Reducing the payload to 10 kg would increase that height somewhat, but not by much, since the empty rocket, weighing about 3 tons, would also have to be raised to the full height.

The US army, which after the war used captured V-2s for experimental flights into the high atmosphere, used a more effective way. It replaced the payload with another rocket, in this case a "WAC Corporal," which was launched from the top of the orbit. Now the burned-out V-2, weighing 3 tons, could be dropped, and using the smaller rocket, the payload reached a much higher altitude. Such was the "Bumper" rocket (on the right) which in February 1949 reached an altitude of 393 km.

Today of course almost every space rocket uses several stages, dropping each empty burned-out stage and continuing with a smaller and lighter booster. Explorer 1, the first artificial satellite of the US which was launched in January 1958, used a 4-stage rocket. Even the space shuttle uses two large solid-fuel boosters which are dropped after they burn out (the "Challenger" disaster in 1986 occured when one of them failed).

The fuel for the shuttle's own engines--liquid hydrogen and oxygen--comes from a huge detachable tank. As that fuel is used up, the boosted mass decreases, and by Newton's 2nd law, the acceleration increases steadily (it is hard to reduce the engine's thrust, though the shuttle can do so to a limited degree). To reduce the acceleration and save the astronauts and the vehicle from excessive stress, at a chosen point in the flight two of the three engines are shut off. Even then, when the last fuel in the tank is burned, the acceleration reaches about 6g, pressing each astronaut down with an added force 6 times his or her body weight.

Persons not familiar with spaceflight rarely realize that almost all of a rocket's launch mass consists of fuel. The launch mass of the V-2 was about 75% fuel and 25% rocket, but as high as that may seem, it is not nearly good enough for spaceflight. In a 1948 article in the American Journal of Physics, titled "Can We Fly to the Moon? " the authors answered their question with a resounding "no! " They extrapolated the V-2 technology to larger rockets, estimated that 80% of the weight would be fuel, and concluded that a payload of 10 kg might be sent to the moon, but never a human being.

The Atlas Rocket

Launch of an   Atlas-Centaur.

Flights to the moon were only made possible by a technology in which the fuel formed a much bigger fraction of the mass. Of the mass of the Atlas missile, built in the 1950s and used by the first astronauts, about 97% was fuel. That rocket has been described as a "stainless steel balloon," keeping its shape with the help of pressurized gas in its interior, also used in pushing out the fuel. That was the vehicle with which on February 20, 1962 John Glenn became the first American to orbit the Earth. Because the fuel tank was so light, the Atlas only dropped two of its rocket engines at the end of the first stage of its flight, and like the shuttle continued on a third.

Glenn's flight was the first in "Project Mercury" which adapted the Atlas missile. It was followed (in the US) by "Gemini" whose two-man capsules were orbited by the more powerful Titan rocket. Then came the three-man "Apollo" missions, first flying around the Moon and then landing on its surface, on July 20, 1969. Altogether 6 successful landings on the moon were achieved, all using the giant (2700 ton) Saturn V rocket, powered by five huge F-1 rocket engines (and one more on the second stage). One such engine is displayed in the National Air and Space Museum (NASM) of the Smithsonian Institution in Washington, as is John Glenn's capsule, a Moon lander and many other mementos from the early days of spaceflight.

The space shuttle   in orbit.

Reentry

Any manned mission faces the problem of safe return to Earth, which requires getting rid of the huge amount of energy associated with orbital motion. A spacecraft in low Earth orbit moves at about 24 times the velocity of sound. Since the energy of motion ("kinetic energy") is proportional to the square of the velocity v, gram for gram (or ounce for ounce) such a spacecraft carries 24² = 576 times more energy than an object moving at the velocity of sound (in air), e.g. a typical pistol bullet.

Atmospheric friction converts this energy into heat, enough heat than to melt or even evaporate the re-entering material, even if it is a tough metal. To get rid of this heat, the vehicle re-enters the atmosphere at a shallow angle, stretching out the time spent in the more rarefied upper layers. It is also useful to present a blunt obstacle to the air, creating a strong shock front well ahead of the reentry vehicle, which dissipates much of the heat. For this reason the space shuttle starts re-entering the atmosphere bottom-first, and only after almost all its velocity is lost does it turn around to the nose-first attitude of an airplane.

Still, a great amount of heat reaches the spacecraft, requiring its forward-facing part to be lined with heat-resistant material. Mercury, Gemini and Apollo used shields which gradually wore down (ablated), sparing the spacecraft which then landed by parachute. The shuttle's bottom is lined with lightweight heat-resistant tiles of a special material. The Soviet Union built and in 1988 flew its own space shuttle, the Buran, but though its tests were successful, it was not used afterwards.

Picture on right: Wernher Von Braun and one of the F-1 rocket engines, the type that powered the Apollo moonflights. Such an engine is on display at the national Air and Space Museum of the Smithsonian, in Washington, DC.

Unmanned Spacecraft

So great is the variety of unmanned spacecraft that (as with manned spaceflight) it is impossible to cover all of it here. They can be divided into five groups, described separately in files linked to the list below: