火箭设计-固体火箭

Solid-Fueled Rocket When the fuel in a solid-fueled rocket is ignited, the gases formed during combustion are forced out the nozzle and the rocket moves forward. The fuel is called the grain and is often formed with a hollow core for longer burning times. Solid rockets are rockets with a motor that uses solid propellants (fuel/oxidizer). The Chinese invented solid rockets and were using them in warfare by the 13th century. All rockets used some form of solid or powdered propellant up until the 20th century. Solid rockets are considered to be safe and reliable due to the long engineering history and simple design. Basic Concepts simple solid rocket motor consists of a casing, nozzle, grain (propellant charge), and igniter. The grain behaves like a solid mass, burning in a predictable fashion and producing exhaust gases. The nozzle dimensions are calculated to maintain a design chamber pressure, while producing thrust from the exhaust gases. Once ignited, a solid rocket motor cannot be shut off. Modern designs may also include; steerable nozzle for guidance, avionics, recovery hardware (parachutes), self destruct mechanisms, APU's, and thermal management materials. Design Design begins with the total impulse required, this determines the fuel/oxidizer mass. Grain geometry and chemistry are then chosen to satisfy the required motor characteristics. The following are chosen or solved simultaneously. The results are exact dimensions for grain, nozzle and case geometries; • The grain burns at a predictable rate, given its surface area and chamber pressure. • The chamber pressure is determined by the nozzle orifice diameter and grain burn rate. • Allowable chamber pressure is a function of casing design. • The length of burn time is determined by the grain 'web thickness'. The grain may be bonded to the casing, or not. Case bonded motors are much more difficult to design, since deformation of both the case and grain, under operating conditions, must be compatible. Common modes of failure in solid rocket motors are; fracture of the grain, failure of case bonding, and air pockets in the grain. All of these produce an instantaneous increase in burn surface area, and a corresponding increase in exhaust gas and pressure, and rupture of the casing. Another failure mode is casing seal design. Seals are required in casings that have to be opened to load the grain. Once a seal fails, hot gas will erode the escape path and result in failure. This was the cause of the Space Shuttle Challenger disaster. Grain Solid fuel grains are usually molded from a thermoset elastomer (which doubles as fuel), additional fuel, oxidizer, and catalyst. HTPB is commonly used for this purpose. Ammonium perchlorate is the most common oxidizer used today. The fuel is cast in different forms for different purposes. Slow, long burning rockets have a cylinder shaped grain, burning from one end to the other. Most grains, however, are cast with a hollow cross section, burning from the inside out (and outside in, if not case bonded), as well as from the ends. The thrust profile over time can be controlled by grain geometry. For example, a star shaped hole down the center of the grain will have greater initial thrust because of the additional surface area. As the star points are burned up, the surface area and thrust are reduced. Casing The casing may be constructed from a range of materials. Cardboard is used for model engines. Steel is used for the space shuttle boosters. Filament wound graphite epoxy casings are used for high performance motors. Nozzle A Convergent Divergent design accelerates the exhaust gas out of the nozzle to produce thrust. Sophisticated solid rocket motors use steerable nozzles for rocket control. Performance Solid fuel rocket motors have a typical specific impulse of 265 lbf·s/lb (2.6 kN·s/kg). This compares to 285 lbf·s/lb (2.8 kN·s/kg) for kerosene/Lox and ~389 lbf·s/lb (3.8 kN·s/kg) for liquid hydrogen/Lox1. For this reason solids are generally used as initial stages in a rocket, with better performing liquid engines reserved for final stages. However, the venerable Star line motors manufactured by Thiokol have a long history as the final boost stage for satellites. This is due to their simplicity, compactness and high mass fraction. The ability of solid rockets to remain in storage for long periods, and then reliably launch at a moments notice, makes them the design of choice for military applications. Amateur rocketry Solid fuel rockets can be bought for use in model rocketry; they are normally small cylinders of fuel with an integral nozzle and a small charge that is set off when the fuel is exhausted. This charge can be used to ignite a second stage, trigger a camera, or deploy a parachute. Designing solid rocket motors is particularly interesting to amateur rocketry enthusiasts. The design is simple, materials are inexpensive and constructions techniques are safe. Early amateur motors were gunpowder. Later, zinc/sulfur formulations were popular. Typical amateur formulations in use today are; sugar (sucrose, dextrose, and sorbitol are all common)/potassium nitrate, HTPB (a rubber like epoxy)/magnesium/ammonium nitrate, and HTPB or PBAN/aluminum/ammonium perchlorate. Most formulations also include burn rate modifiers and other additives, and also possibly additives designed to create special effects, such as colored flames, thick smoke, or sparks. A hybrid rocket propulsion system typically comprises a solid fuel and a liquid or gas oxidizer. These systems are superior to solid propulsion systems in the respects of safety, throttling, restartability, and environmental cleanliness. However, hybrid systems are slightly more complex than solids, and consequently they are heavier and more expensive. Common oxidizers include gaseous or liquid oxygen and nitrous oxide. The Reaction Research Society (RRS), although known primarily for their work with liquid rocket propulsion, has a long history of research and development with hybrid rocket propulsion. Several universities have recently experimented with hybrid rockets. BYU, the University of Utah and Utah State University launched a student-designed rocket called Unity IV in 1995 which burned the solid fuel Hydroxy-terminated polybutadiene (HTPB) with an oxidizer of gaseous oxygen, and in 2003 launched a larger version which burned HTPB with nitrous oxide. Portland State University also launched several hybrid rockets in the early 2000's. SpaceShipOne, the first private manned spacecraft, is powered by a hybrid rocket burning HTPB with nitrous oxide. The hybrid rocket engine was manufactured by SpaceDev. SpaceDev partially based its motors on experimental data collected from the testing of AMROC's (American Rocket Company) motors at NASA's Stennis Space Center's E1 test stand. Motors ranging from as small as 1000 lbf (4.5 kN) to as large as 250,000 lbf (1.1 MN) thrust were successfully tested. SpaceDev purchased AMROCs assets after the company was shut down due to lack of funding. Rocket fuel From Wikipedia, the free encyclopedia. Jump to: navigation, search Rocket fuel is the propellant which is burned with an oxidizer to produce thrust in rockets. Contents [hide] • 1 Overview • 2 Solid propellants • 3 Liquid propellants • 4 Hybrid propellants • 5 Mixture ratio • 6 Propellent density • 7 See also • 8 External links [edit] Overview Rockets create thrust by expelling mass backwards with velocity. Chemical rockets, the subject of this article, create thrust by reacting propellants into very hot gas, which then expands in a nozzle out the back. The thrust produced is the mass flow rate of the propellants multiplied by their exhaust velocity (relative to the rocket), as specified by Newton's third law of motion. It is the equal and opposite reaction that moves the rocket, and not any interaction of the exhaust stream with air around the rocket (but see base bleed). Equivalently, one can think of a rocket being accelerated upwards by the pressure of the combusting gases in the combustion chamber and nozzle. Rockets can move faster in outer space, because they do not need to overcome air resistance. The velocity that a rocket can attain is primarily a function of its mass ratio and its exhaust velocity. The relationship is described by the rocket equation: Vf = Veln(M0 / Mf). The mass ratio is just a way to express how much of the rocket is fuel when it starts accelerating. Typically, a single-stage rocket might have a mass fraction of 90% propellant, which is a mass ratio of 1/(1-0.9) = 10. The exhaust velocity is often reported as specific impulse. The first stage will usually use high-density (low volume) propellants to reduce the amount of volume exposed to atmospheric drag. Thus, the Apollo-Saturn V first stage used kerosene-liquid oxygen rather than the liquid hydrogen-liquid oxygen used on the upper stages (hydrogen is highly energetic per kilogram, but not per cubic metre). Similarly, the Space Shuttle uses high-thrust, high-density SRBs for its lift-off with the liquid hydrogen-liquid oxygen SSMEs used partly for lift-off but primarily for orbital insertion. There are three main types of propellants: solid, liquid, and hybrid. [edit] Solid propellants The earliest rockets were created hundreds of years ago by the Chinese, and were used primarily for fireworks displays and as weapons. They were fueled with black powder, a type of gunpowder consisting of a mixture of charcoal, sulfur and potassium nitrate (saltpeter). Rocket propellant technology did not advance until the end of the 19th century, by which time smokeless powder had been developed, originally for use in firearms and artillery pieces. Solid fuels (and really, all rocket fuels) consist of an oxidizer (substance providing oxygen) and a fuel. In the case of gunpowder, the fuel is charcoal, the catalyst is sulfur and the oxidizer is the potassium nitrate. More contemporary recipies employ such compounds as sodium or potassium chlorate and powdered aluminum. (This mixture is sometimes known as "white powder"; not only is it different in appearance than black powder, it has a considerably higher energy density.) However, white powder has insufficient specific impulse for orbital or near-orbital boosters. During the 1950s and 60s researchers in the United States developed what is now the standard high-energy solid rocket fuel. The mixture is primarily ammonium perchlorate powder (an oxidizer), combined with fine aluminum powder (a fuel), held together in a base of PBAN or HTPB (rubber-like fuels). The mixture is formed as a liquid at elevated temperatures, poured into the rocket casing, and cools to form a single grain bonded to that casing. Solid fueled rockets are much easier to store and handle than liquid fueled rockets, which makes them ideal for military applications. The LGM-30 Minuteman and LG-118A Peacekeeper (MX) missiles are four-stage rockets capable of intercontinental suborbital flights. The first three stages are solid fuelled, and in each case the last stage is a precision maneuverable liquid-fuelled bus used to fine tune the trajectory of the reentry vehicle. Their simplicity makes solid rockets a good choice whenever large amounts of thrust are needed and cost is an issue. The Space Shuttle and many other orbital launch vehicles use solid fuelled rockets in their first stages (solid rocket boosters) for this reason. However, solid rockets have lower specific impulse than liquid fueled rockets. It is also difficult to build a large mass ratio solid rocket because almost the entire rocket is the combustion chamber, and must be built to withstand the high combustion pressures. If a solid rocket is used to go all the way to orbit, the payload fraction is very small. (For example, the Orbital Sciences Pegasus rocket is an air-launched three-stage solid rocket orbital booster. Launch mass is 23,130 kg, low earth orbit payload is 443 kg, for a payload fraction of 1.9%. Compare to a Delta IV Medium, 249,500 kg, payload 8600 kg, payload fraction 3.4% without air-launch assistance.) Solid rockets are difficult to throttle or shut down before they run out of fuel. Essentially, the burning grain must be vented to lower the chamber pressure. Venting generally involves destroying the rocket, and is usually only done by a range safety officer if the rocket goes awry. The third stages of the Minuteman and MX rockets have precision shutdown ports which, when opened, reduce the chamber pressure so abruptly that the interior flame is blown out. This allows a more precise trajectory which improves targetting accuracy. Finally, casting very large single-grain rocket motors has proved to be a very tricky business. Defects in the grain can cause explosions during the burn, and these explosions can increase the burning propellant surface enough to cause a runaway pressure increase, until the case fails. [edit] Liquid propellants Main article: Liquid rocket propellants Liquid fueled rockets have better specific impulse than solid rockets and are capable of being throttled, shut down, and restarted. Only the combustion chamber of a liquid fueled rocket needs to withstand combustion pressures and temperatures. On vehicles employing turbopumps, the fuel tanks can be built with less material, permitting a larger mass fraction. For these reasons, most orbital launch vehicles and all first- and second- generation ICBMs use liquid fuels for most of their velocity gain. The primary performance advantage of liquid propellants is the oxidizer. Several practical liquid oxidizers (liquid oxygen, nitrogen tetroxide, hydrogen peroxide) are available which have much better specific impulse than ammonium perchlorate when paired with comparable fuels. Most liquid propellants are also cheaper than solid propellants. For orbital launchers, the cost savings do not, and historically have not mattered; the cost of fuel is a very small portion of the overall cost of the rocket, even in the case of solid fuel. The main difficulties with liquid propellants are also with the oxidizers. These are generally difficult to store and handle, either due to extreme toxicity (nitric acids), extreme cold (liquid oxygen), or both (liquid fluorine is a perennial favorite of wild-eyed enthusiasts). Several exotic oxidizers have been proposed: liquid ozone (O3), ClF3, and ClF5, all of which are unstable, energetic, and toxic. Liquid fuelled rockets also require troublesome and highly stressed pressurization systems, plumbing and combustion chambers, which greatly increase the cost of the rocket. Many employ turbopumps which raise the cost still more. Though all the early rocket theorists proposed liquid hydrogen and liquid oxygen as propellants, the first liquid-fuelled rocket, launched by Robert Goddard on March 16, 1926, used gasoline and liquid oxygen. Liquid hydrogen was first used by the Lockheed CL-400 Suntan reconnaissance aircraft in the mid-1950s. In the mid-1960s, the Centaur and Saturn upper stages were both using liquid hydrogen and liquid oxygen. The highest specific impulse chemistry ever test-fired in a rocket engine was lithium and fluorine, with hydrogen added to improve the exhaust thermodynamics (making this a tripropellant). The combination delivered 542 seconds (542 lbf·s/lb, 5.32 kN·s/kg, 5320 m/s) specific impulse in a vacuum. The impracticality of this chemistry highlights why exotic propellants are not actually used: to make all three components liquids, the hydrogen must be kept below -252 °C (just 21 K) and the lithium must be kept above 180 °C (453 K). Lithium and fluorine are both extremely corrosive, lithium ignites on contact with air, fluorine ignites on contact with most fuels, and hydrogen, while not hypergolic, is an explosive hazard. Fluorine and the hydrogen fluoride (HF) in the exhaust are very toxic, which trashes the environment, makes work around the launch pad difficult, and makes getting a launch license that much more difficult. The rocket exhaust is also ionized, which would interfere with radio communication with the rocket. Finally, both lithium and fluorine are expensive and rare, enough to actually matter. The common liquid propellant combinations in use today are: • LOX and kerosene (RP-1). Used for the lower stages of most Russian and Chinese boosters, and the first stage of the Saturn 5. Very similar to Robert Goddard's first rocket. This combination is widely regarded as the most practical for civilian orbital launchers. • LOX and liquid hydrogen, used in the Space Shuttle, the Centaur upper stage, the newer Delta IV rocket, and most stages of the European Ariane rockets. • Nitrogen tetroxide (N2O4) and hydrazine (N2H4), MMH, or UDMH. Used in military, orbital and deep space rockets, because both liquids are storable for long periods at reasonable temperatures and pressures. Hydrazine decomposes energetically to nitrogen and hydrogen, making it a fairly good monopropellant all by itself. This combination is hypergolic, making for attractively simple ignition sequences. The only inconvenience is that these propellants are toxic, hence require careful handling. [edit] Hybrid propellants A hybrid rocket usually has a solid fuel and a liquid or gas oxidizer. The fluid oxidizer can make it possible to throttle and restart the motor just like a liquid fuelled rocket. Hybrid rockets are also cleaner than solid rockets because practical high-performance solid-phase oxidizers all contain chlorine, versus the more benign liquid oxygen or nitrous oxide used in hybrids. Because just one propellant is a fluid, hybrids are simpler than liquid rockets. Hybrid motors suffer two major drawbacks. The first, shared with solid rocket motors, is that the casing around the fuel grain must be built to withstand full combustion pressure and often extreme temperatures as well. Modern composite structures handle this problem well. The primary remaining difficulty with hybrids is with mixing the propellants before burning. In solid propellants, the oxidizer and fuel are mixed in a factory in carefully controlled conditions (and even then it is tricky). Liquid propellants are generally mixed by the injector at the top of the combustion chamber, which directs many small fast- moving streams of fuel and oxidizer into one another. Liquid fuelled rocket injector design has been studied at great length and still resists reliable performance prediction. In a hybrid motor, the mixing happens at the surface of the melting or evaporating surface of the fuel. The mixing is not a well controlled process and generally quite a lot of propellant is left unburned, which limits the efficiency and thus the exhaust velocity of the motor. There has been much less development of hybrid motors than solid and liquid motors. For military use, ease of handling and maintenance have driven the use of solid rockets. For orbital work, liquid fuels are enough better than hybrids that most development has concentrated there. There has recently been an increase in hybrid motor development for nonmilitary suborbital work: • The Reaction Research Society (RRS), although known primarily for their work with liquid rocket propulsion, has a long history of research and development with hybrid rocket propulsion. • Several universities have recently experimented with hybrid rockets. BYU, the University of Utah and Utah State University launched a student- d