14. Are there different kinds of rockets? (A K2S Question)
From The Space Library
There are many different kinds of rockets, including: a. liquid-fueled chemical rockets b. solid-fueled chemical rockets c. hybrid-fuel rockets d. steam rockets e. cold gas rockets f. electric thermal rockets g. solar thermal rockets h. nuclear thermal rockets i. electric ion thrusters, and many more. a. Liquid chemical rocket engines typically have a combustion chamber where the fuel is mixed with an oxidizer and burned to produce high temperature and pressure. The hot gasses exit the combustion chamber through a nozzle throat and then expand in the nozzle to very high velocity, producing the vehicle thrust. The fuel and oxidizer must be pushed through an injector into the rocket combustion chamber at higher pressure than the pressure in the combustion chamber; otherwise the propellant will not go in. This is usually accomplished with pumps, but the pumps must be powered somehow. Some of the fuel and oxidizer is burned in a pre-burner to run a motor—a turbine— to drive the fuel and oxidizer pumps. The combination is referred to as a turbo-pump. For most propellant types an igniter is required to start the combustion reaction. To keep the engine from melting, the cold fuel is pumped through tubes around the combustion chamber, throat, and nozzle, keeping them cool before the fuel is injected into the combustion chamber to be burned, a process called regenerative cooling. The tanks usually need an additional pressurization system to push the fuel and oxidizer into the turbo-pumps fast enough to be pumped into the combustion chamber. Since all rocket propellants release large amounts of heat energy when they are combined and combusted, they are very dangerous and must be stored and handled very carefully. Liquid engines can be shut off when desired velocity and altitude conditions are reached or if emergency conditions demand thrust termination. Liquid rockets can be designed to be throttled, to reduce or control the thrust level. This is especially important for softly landing humans on the surface of the Moon or Mars. b. Solid rocket motors typically have all the propellant located inside the large combustion chamber with a hole down the center of the motor where the combustion takes place. The propellant is a careful mixture of fuel and oxidizer that burns as it boils off the propellant surface. Once the solid rockets have been ignited they are very difficult to turn off. As the solid propellant is burned away, the volume of the combustion region increases, which increases the stresses on the walls. The walls of the solid motor are quite thick to withstand these high combustion pressures and temperatures. Many of the military rockets are solids because they can be stored for long periods of time. Retro-rockets are small solid rocket motors that are used with multistage rockets to slow down a spent stage after separation before the next stage engines start. They are also used to slow down a satellite for re-entry into the Earth's atmosphere. Such retro-rockets were used on the Mercury and Gemini capsules to initiate their re-entry. c. Hybrid rockets use a combination of solid and liquid propellants. Solid fuel is located in the motor like the solid rocket motors, and the liquid oxidizer, stored in a separate tank, is pushed into the combustion chamber by a pressurization system or by pumps, driven by motors fueled by a separate fuel and oxidizer system. Hybrid rockets can be somewhat complex but they have an added safety feature. Like liquid rocket engines, they can be turned off when mission parameters or situations demand it. d. Steam rockets have been used in limited applications. Water in a very strong tank is heated to very high temperature and pressure by an external heat source just prior to launch. As the water exits the nozzle throat it changes into steam that expands against the walls of the nozzle providing thrust to the vehicle. Typically steam rockets cannot be shut down and they require a period of time for heating prior to launch. They can be stored hot for short periods by continuously adding heat. e. Cold gas rockets usually use very high-pressure nitrogen or carbon dioxide as propellant. There is no combustion involved. The propellant energy is provided by the pressure and volume of the high-pressure gas storage system. The high-pressure propellant is allowed to expand through a nozzle to provide thrust. Small cold gas jets were used on Sky Lab to occasionally provide thrust to assist in controlling the orientation of the vehicle. f. Electro-thermal rockets utilize electric heaters to increase the temperature of a fluid to well above its boiling point. This gas is then exhausted through a nozzle. The electricity must come from some source, such as solar panels. These devices were once suggested for providing thrust for the Space Station attitude control (orientation) using electricity from its solar panels for power and using biological waste fluids for the propellant. The resulting cloud of contaminants around the Station was a factor in not selecting this concept for this application. There are several types of electro-thermal rockets, including resisto-jets, arc-jets, induction-heated jets, and microwave heated jets. g. Solar thermal rocket concepts use a large inflatable mirror to concentrate sunlight into a thermal chamber to heat hydrogen gas to very high temperatures that then exits through a nozzle. These devices have been designed, built, and tested, but have never flown. These devices could be very effective for transporting cargo from Earth orbit to the Moon and back. However, the payload shrouds of our existing launch vehicles are not currently large enough to carry enough liquid hydrogen to make this concept competitive with alternatives. If larger shrouds become available, or if hydrogen depots are constructed, then solar thermal rockets may become very effective orbit transfer vehicles. h. Nuclear thermal rockets utilize a nuclear reactor to heat the propellant to provide thrust. Part of the hot gas is bled off to power the turbopumps. These devices were built and tested during the late 1960's. These engines may be the safest propulsion system for routinely sending humans to and from Mars. The high specific energy of the nuclear reactors can potentially shorten the travel time for the crew, reducing the crew exposure to hazardous space radiation. Radiation from the reactors can be shielded much more effectively than can the space radiation. Reactors do not become highly radioactive until they are turned on and begin operation, so they are safe to launch. The effectiveness of nuclear thermal rockets can be further enhanced if we can find a means for re-fueling using "in-situ" propellants for the trip home. i. Electric ion and plasma thrusters are small devices that accelerate very small amounts of propellant to extremely high velocities using high voltages or magnetic fields. There are many different types of electric thrusters. The electric power to operate these thrusters is currently provided by solar panels, and there are plans to someday use nuclear reactors to supply the propulsion power required. Sunlight availability in the vicinity of the Earth will provide these thrusters with power up to a megawatt. This may not be adequate for human applications but should be quite effective in moving cargo from Earth orbit to lunar orbit and return. Near Mars the sunlight intensity is half that at Earth, and at Jupiter it is only four percent that at Earth. Nuclear reactors have plenty of specific energy but current designs are limited in specific power. This can be improved if ways can be found to provide much hotter reactors for space propulsion.
Answer provided by John W. Cole
Question and Answer extracted from the book Kids to Space - by Lonnie Schorer
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