The key elements in designing a rocket are the propulsion system, which includes the propellant and the exit nozzle, and determining the number of stages required to lift the intended payload. Rocket navigation is usually based on inertial guidance; internal gyroscopes are used to detect changes in the position and direction of the rocket.Rocket Propellants
The most vital component of any rocket is the propellant, which accounts for 90% to 95% of the rocket's total weight. A propellant consists of two elements, a fuel and an oxidant; engines that are based on the action-reaction principle and that use air instead of carrying their own oxidant are properly called jets. Propellants in use today include both liquefied gases, which are more powerful, and solid explosives, which are more reliable. The chemical energy of the propellants is released in the form of heat in the combustion chamber.
A typical liquid engine uses hydrogen as fuel and oxygen as oxidant; a typical solid propellant is nitroglycerine. In the liquid engine, the fuel and oxidant are stored separately at extremely low temperatures; in the solid engine, the fuel and oxidant are intimately mixed and loaded directly into the combustion chamber. A solid engine requires an ignition system, as does a liquid engine if the propellants do not ignite spontaneously on contact.
The efficiency of a rocket engine is defined as the percentage of the propellant's chemical energy that is converted into kinetic energy of the vehicle. During the first few seconds after liftoff, a rocket is extremely inefficient, for at least two unavoidable reasons: High power consumption is required to overcome the inertia of the nearly motionless mass of the fully fueled rocket; and in the lower atmosphere, power is wasted overcoming air resistance. Once the rocket gains altitude, however, it becomes more efficient. as the trajectory, at first vertical, curves into a suborbital arc or into the desired orbit.
Although all known rockets currently in use derive their energy from chemical reactions, more exotic propulsion systems are being considered. In ion propulsion, a plasma (ionized gas consisting of a mixture of positively charged atoms and negatively charged electrons) would be created by an electric discharge and then expelled by an electric field. The engine could provide a low thrust efficiently for long periods; on a lengthy flight this would produce very high velocities, so that if there is ever a trip to the outer planets an ion drive might be used. Deep Space 1, a space probe launched in 1998 to test new technologies, was propelled intermittently by an ion engine. Even nuclear power has been considered for propulsion; in fact, a nuclear ramjet was developed in the early 1960s before it was realized that because the exhaust gases would be highly radioactive such a drive could never be used in earth's atmosphere.
A critical element in all rockets is the design of the exit nozzle, which must be shaped to obtain maximum energy from the exhaust gases moving through it. The nozzle usually converges to a narrow throat, then diverges to create a form which shapes the hypersonic flow of exhaust gas most efficiently. The walls of the combustion chamber and nozzle must be cooled to protect them against the heat of the escaping gases, whose temperature may be as high as 3,000°C—above the melting point of any metal or alloy.
Although early rockets had only one stage, it was early recognized that no single-stage rocket can reach orbital velocity (5 mi/8 km per sec) or the earth's escape velocity (7 mi/11 km per sec). Hence multistage rockets, such as the two-stage Atlas-Centaur or the three-stage Saturn V, became necessary for space exploration. In these systems, two or more rockets are assembled in tandem and ignited in turn; once the lower stage's fuel is exhausted, it detaches and falls back to earth. Soviet systems clustered several rockets together, operated simultaneously, to obtain a large initial thrust.