26-04-2011, 12:55 PM
Submitted By:-
ANUP R. HINGMIRE
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CRYOGENIC ENGINE IN ROCKET PROPULSION
INTRODUCTION
What is Cryogenics ?
Cryogenics is the study of the production of extremely cold temperatures. This field of science also looks at what happens to a wide variety of materials from metals to gases when they are exposed to these temperatures. Cryogenics is a branch of physics concerned with the production of very low temperatures and the effects of these temperatures on different substances and materials. The temperatures studied in cryogenics are those below -243.67 degrees Fahrenheit (120 Kelvin); such low temperatures do not occur in nature.
These low temperatures have been used to liquefy atmospheric gases like oxygen, hydrogen, nitrogen, methane, argon, helium, and neon. The gases are condensed, collected, distilled and separated. Methane is used in liquid natural gas (LNG), and oxygen, hydrogen and nitrogen are used in rocket fuels and other aerospace and defense applications, in metallurgy and in various chemical processes. Helium is used in diving decompression chambers and to maintain suitably low temperatures for superconducting magnets, and neon is used in lighting.
Cryogenics is the study of how to get to low temperatures and of how materials behave when they get there. Besides the familiar temperature scales of Fahrenheit and Celsius (Centigrade), cryogenicists use other temperature scales, the Kelvin and Rankine temperature scale. Although the apparatus used for spacecraft is specialized, some of the general approaches are the same as used in everyday life. Cryogenics involves the study of low temperatures from about 100 Kelvin to absolute zero.
One interesting feature of materials at low temperatures is that the air condenses into a liquid. The two main gases in air are oxygen and nitrogen. Liquid oxygen, "lox" for short, is used in rocket propulsion. Liquid nitrogen is used as a coolant. Helium, which is much rarer than oxygen or nitrogen, is also used as a coolant. In more detail, cryogenics is the study of how to produce low temperatures or also the study of what happens to materials when you have cooled them down.
CRYOGENIC ENGINE INTRODUCTION
The use of liquid fuel for rocket engines was considered as early as the beginning of 20th century. The Russian K.E.Ziolkowsky, the American H.Goddard and the German-Romanian H.Oberth worked independently on the problems of spaceflight and soon discovered that in order to succeed, rockets with high mass-flow were mandatory. Already then the combustion of liquid fuels seemed the most promising method of generating thrust.
However it was not later until these pioneers made their attempts, the first big liquid powered rocket the German A-4 became reality in the mid-forties. This rocket became successful as the V-2 weapon. Liquid oxygen was used as the oxidizer and ethyl alcohol as the fuel which gave the rocket more than 300KN of thrust. It`s range was 300km.
As the development of rocket engines continued, higher thrust levels were achieved when liquid oxygen and liquid hydrocarbon were used as fuel. This allowed the construction of the first intercontinental rocket with a range of more than 10,000km.
Under normal atmospheric conditions, at temperature 300k and pressure 1 bar, these substances are in gaseous state. One cannot remedy the low density by increasing the pressure because the required tank structures would end being too heavy. The answer is to liquefy the fuels by cooling them down. This is why the fuels are also called cryogenic fuels.
In the sixties, the steadily increasing payload weights and the corresponding demand for more thrust of the launcher lead to the use of liquid hydrogen for the Centaur upper stage. At the peak of this development was the US Space Shuttle Main Engine (SSME).
In principle, cryogenic rocket engines generate thrust like all other rocket engines-by accelerating an impulse carrier to high speeds. In conventional aircraft engines the surrounding air is the main impulse carrier and fuel is the energy carrier. This is why such an engine requires the atmosphere not only to burn the fuel but also to generate thrust. But in rocket engines the impulse and energy carriers are identical and are present as fuel in the launcher. The chemical energy stored in the fuel is converted into kinetic energy by burning it in the thrust chamber and subsequent expansion in the nozzle, in the process creating thrust.
In order to compare a variety of fuel combinations, a quantity known as specific impulse, which determines the thrust per kilogram of emitted fuel per second, is used.
The favorite fuel and oxidizer combination used during the boost phase are Liquid Hydrogen(LH2) and Liquid Oxygen(LOX) which provide a specific impulse of 445 seconds, almost double that of hydrazine. The fuel is environmentally friendly, non-corrosive and has the highest efficiency of all non-toxic combinations.
To liquefy hydrogen has to be cooled to a temperature of minus 273C. It`s boiling point is 20K only just above absolute on the temperature scale.
Fuelling the booster rockets is a complex and hazardous process, for as soon as oxygen comes in contact with hydrogen, they spontaneously combust in a powerful explosion. Over the years cryogenic engines have become the backbone for boosters, used for placing heavy payloads in space, such as those used for the main engine for the space shuttle.
CRYOGENIC LIQUIDS:
Another use of cryogenics is cryogenic fuels. Cryogenic fuels, mainly liquid hydrogen, have been used as rocket fuels. Liquid oxygen is used as an oxidizer of hydrogen, but oxygen is not, strictly speaking, a fuel. For example, NASA's workhorse space shuttle uses cryogenic hydrogen fuel as its primary means of getting into orbit, as did all of the rockets built for the Soviet space program by Sergei Korolev. (This was a bone of contention between him and rival engine designer Valentin Glushko, who felt that cryogenic fuels were impractical for large-scale rockets such as the ill-fated N-1 rocket spacecraft.)
Russian aircraft manufacturer Tupolev developed a version of its popular design Tu-154 with a cryogenic fuel system, known as the Tu-155. The plane uses a fuel referred to as liquefied natural gas or LNG, and made its first flight in 1989.
