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Abstract What is propulsion? The word is derived from two Latin words: pro meaning before or forwards and pellere meaning to drive. Propulsion means to push forward or drive an object forward. A propulsion
system is a machine that produces thrust to push an object forward. On airplanes, thrust is usually generated through some application of Newton's third law of action and reaction. A gas, or working fluid, is accelerated by the engine, and the reaction to this acceleration produces a force on the engine.
A general derivation of the thrust equation shows that the amount of thrust generated depends on the mass flow through the engine and the exit velocity of the gas. Different propulsion systems generate thrust in slightly different ways. We will discuss four principal propulsion systems: the propeller, the turbine (or jet) engine, the ramjet, and the rocket. Why are there different types of engines? If we think about Newton's first law of motion, we realize that an airplane propulsion system must serve two purposes. First, the thrust from the propulsion system must balance the drag of the airplane when the airplane is cruising. And second, the thrust from the propulsion system must exceed the drag of the airplane for the airplane to accelerate. In fact, the greater the difference between the thrust and the drag, called the excess thrust, the faster the airplane will accelerate. Some aircraft, like airliners and cargo planes, spend most of their life in a cruise condition. For these airplanes, excess thrust is not as important as high engine efficiency and low fuel usage. Since thrust depends on both the amount of gas moved and the velocity, we can generate high thrust by accelerating a large mass of gas by a small amount, or by accelerating a small mass of gas by a large amount. Because of the aerodynamic efficiency of propellers and fans, it is more fuel efficient to accelerate a large mass by a small amount. That is why we find high bypass fans and turboprops on cargo planes and airliners. Some aircraft, like fighter planes or experimental high speed aircraft, require very high excess thrust to accelerate quickly and to overcome the high drag associated with high speeds. For these airplanes, engine efficiency is not as important as very high thrust. Modern military aircraft typically employ afterburners on a low bypass turbofan core. Future hypersonic aircraft will employ some type of ramjet or rocket propulsion. There is a special section of the Beginner's Guide which deals with compressible, or high speed, aerodynamics. This section is intended for undergraduates who are studying shock waves or isentropic flows and contains several calculators and simulators for that flow regime.
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1) Introduction Rocket propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages. However, most spacecraft today are propelled by forcing a gas from the back/rear of the vehicle at very high speed through a supersonic de Laval nozzle. This sort of engine is called a rocket engine. All current spacecraft use chemical rockets (bipropellant or solid-fuel) for launch, though some use airbreathing engines on their first stage. Most satellites have simple reliable chemical thrusters (often monopropellant rockets) or resistojet rockets for orbital station-keeping and some use momentum wheels for attitude control. Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north-south stationkeeping. Interplanetary vehicles mostly use chemical rockets as well, although a few have used ion thrusters and Hall Effect thrusters (two different types of electric propulsion) to great success.
2) history Just when the first true rockets appeared is unclear. Stories of early rocket like devices appear sporadically through the historical records of various cultures. Perhaps the first true rockets were accidents. In the first century A.D., the Chinese were reported to have had a simple form of gunpowder made from saltpeter, sulfur, and charcoal dust. It was used mostly for fireworks in religious and other festive celebrations. Bamboo tubes were filled with the mixture and tossed into fires to create explosions during religious festivals. lt is entirely possible that some of those tubes failed to explode and instead skittered out of the fires, propelled by the gases and sparks produced by the burning gunpowder.
Fig1: Chinese fire arrows arrows
It is certain that the Chinese began to experiment with the gunpowder-filled tubes. At some point, bamboo tubes were attached to arrows and launched with bows. Soon it was discovered that these
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gunpowder tubes could launch themselves just by the power produced from the escaping gas. The true rocket was born. During the latter part of the 17th century, the scientific foundations for modern rocketry were laid by the great English scientist Sir Isaac Newton (1642-1727). Newton organized his understanding of physical motion into three scientific laws. The laws explain how rockets work and why they are able to work in the vacuum of outer space. Newton’s laws soon began to have a practical impact on the design of rockets. Rocket experimenters in Germany and Russia began working with rockets with a mass of more than 45 kilograms. Some of these rockets were so powerful that their escaping exhaust flames bored deep holes in the ground even before lift-off. During the end of the 18th century and early into the 19th, rockets experienced a brief revival as a weapon of war. The success of Indian rocket barrages against the British in 1792 and again in 1799 caught the interest of an artillery expert, Colonel William Congreve. Congreve set out to design rockets for use by the British military. The Congreve rockets were highly successful in battle. Even with Congreve's work, the accuracy of rockets still had not improved much from the early days. All over the world, rocket researchers experimented with ways to improve accuracy. An Englishman, William Hale, developed a technique called spin stabilization. In this method, the escaping exhaust gases struck small vanes at the bottom of the rocket, causing it to spin much as a bullet does in flight. Variations of the principle are still used today.
3) Birth of modern rockets In 1898, a Russian schoolteacher, Konstantin Tsiolkovsky (1857-1935), proposed the idea of space exploration by rocket. In a report he published in 1903, Tsiolkovsky suggested the use of liquid propellants for rockets in order to achieve greater range. Tsiolkovsky stated that the speed and range of a rocket were limited only by the exhaust velocity of escaping gases. For his ideas, careful research, and great vision, Tsiolkovsky has been called the father of modern astronautic. Early in the 20th century, an American, Robert H. Goddard (1882-1945), conducted practical experiments in rocketry. He had become interested in a way of achieving higher altitudes than were possible for lighter-than-air balloons. He published a pamphlet in 1919 entitled A Method of Reaching Extreme Altitudes. In his pamphlet, Goddard reached several conclusions important to rocketry. From his tests, he stated that a rocket operates with greater efficiency in a vacuum than in air.
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Goddard also stated that multistage or step rockets were the answer to achieving high altitudes and that the velocity needed to escape Earth's gravity could be achieved in this way. Goddard’s earliest experiments were with solid-propellant rockets. In 1915, he began to try various types of solid fuels and to measure the exhaust velocities of the burning gases. While working on solid-propellant rockets, Goddard became convinced that a rocket could be propelled better by liquid fuel. No one had ever built a successful liquid-propellant rocket before. It was a much more difficult task than building solidpropellant rockets. Fuel and oxygen tanks, turbines, and combustion chambers would be needed. In spite of the difficulties, Goddard achieved the first successful flight with a liquid- propellant rocket on March 16, 1926. Fueled by liquid oxygen and gasoline, the rocket flew for only two and a half seconds, climbed 12.5 meters, and landed 56 meters away in a cabbage patch. By today's standards, the flight was unimpressive, but like the first powered airplane flight by the Wright brothers in 1903, Goddard's gasoline rocket was the forerunner of a whole new era in rocket flight.
Fig 2: Goddard rocket
A third great space pioneer, Hermann Oberth (1894-1989) of Germany, published a book in 1923 about rocket travel into outer space. His writings were important. Because of them, many small rocket societies sprang up around the world. In Germany, the formation of one such society, the Verein fur
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Raumschiffahrt (Society for Space Travel), led to the development of the V-2 rocket, which was used against London during World War II The V-2 rocket (in Germany called the A-4) was small by comparison to today's rockets. It achieved its great thrust by burning a mixture of liquid oxygen and alcohol at a rate of about one ton every seven seconds. Once launched, the V-2 was a formidable weapon that could devastate whole city blocks.With the fall of Germany, many unused V-2 rockets and components were captured by the Allies. Many German rocket scientists came to the United States. Others went to the Soviet Union.
Fig 3: German v-2 missile
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A few months after the first Sputnik, the United States followed the Soviet Union with a satellite of its own. Explorer I was launched by the U.S. Army on January 31, 1958. Soon, many people and machines were being launched into space. Astronauts orbited Earth and landed on the Moon. Robot spacecraft traveled to the planets. Space was suddenly opened up to exploration and commercial exploitation.. As the demand for more and larger payloads increased, a wide array of powerful and versatile rockets had to be built. Since the earliest days of discovery and experimentation, rockets have evolved from simple gunpowder devices into giant vehicles capable of traveling into outer space. Rockets have opened the universe to direct exploration by humankind.
4) General characteristics and principles The rocket differs from the turbojet and other ―air - breathing‖ engines in that all of the exhaust jet consists of the gaseous combustion products of ―propellants‖ carried on board. Like the turbojet engine, the rocket develops thrust by the rearward ejection of mass at very high velocity. The fundamental physical principle involved in rocket propulsion was formulated by Sir Isaac Newton. According to his third law of motion, the rocket experiences an increase in momentum proportional to the momentum carried away in the exhaust,
where M is the rocket mass, ΔvR is the increase in velocity of the rocket in a short time interval, Δt , m° is the rate of mass discharge in the exhaust, ve is the effective exhaust velocity (nearly equal to the jet velocity and taken relative to the rocket), and F is force. The quantity m°ve is the propulsive force, or thrust, produced on the rocket by exhausting the propellant,
Evidently thrust can be made large by using a high mass discharge rate or high exhaust velocity. Employing high m° uses up the propellant supply quickly (or requires a large supply), and so it is preferable to seek high values of ve. The value of ve is limited by practical considerations, determined by how the exhaust is accelerated in the supersonic nozzle and what energy supply is available for the propellant heating.
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Most rockets derive their energy in thermal form by combustion of condensed-phase propellants at elevated pressure. The gaseous combustion products are exhausted through the nozzle that converts most of the thermal energy to kinetic energy. The maximum amount of energy available is limited to that provided by combustion or by practical considerations imposed by the high temperature involved. Higher energies are possible if other energy are used in conjunction with the chemical propellants on board the rockets, and extremely high energies are achievable when the exhaust is accelerated by electromagnetic means
. Fig 4: action-reacton concept
The effective exhaust velocity is the figure of merit for rocket propulsion because it is a measure of thrust per unit mass of propellant consumed — i.e.,
Values of ve are in the range 2,000 – 5,000 metres (6,500 – 16,400 feet) per second for chemical propellants, while values two or three times that are claimed for electrically heated propellants. Values beyond 40,000 metres (131,000 feet) per second are predicted for systems using electromagnetic acceleration. In a typical chemical-rocket mission, anywhere from 50 to 95 percent or more of the takeoff mass is propellant. This can be put in perspective by the equation for burnout velocity (assuming gravity-free and drag-free flight)
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In this expression, M s / M p is the ratio of propulsion system and structure mass to propellant mass, with a typical value of 0.09 (the symbol ln represents natural logarithm). M p / M o is the ratio of propellant mass to all-up takeoff mass, with a typical value of 0.90. A typical value for ve for a hydrogen – oxygen system is 3,536 metres (11,601 feet) per second. From the above equation, the ratio of payload mass to takeoff M o) can be calculated. mass ( M pay /
A technique called multiple staging is used in many missions to minimize the size of the takeoff vehicle. A launch vehicle carries a second rocket as its payload, to be fired after burnout of the first stage (which is left behind). In this way, the inert components of the first stage are not carried to final velocity, with the second-stage thrust being more effectively applied to the payload.
4.1) principle of operation of a rocket Rocket engines produce thrust by the expulsion of a high-speed fluid exhaust. This fluid is nearly always a gas which is created by high pressure (10-200 bar) combustion of solid or liquid propellants, consisting of fuel and oxidiser components, within a combustion chamber.The fluid exhaust is then passed through a supersonic propelling nozzle which uses heat energy of the gas to accelerate the exhaust to very high speed, and the reaction to this pushes the engine in the opposite direction.In rocket engines, high temperatures and pressures are highly desirable for good performance as this permits a longer nozzle to be fitted to the engine, which gives higher exhaust speeds, as well as giving better thermodynamic efficiency.
4.2) Introduction of propellant Rocket propellant is mass that is stored, usually in some form of propellant tank, prior to being ejected from a rocket engine in the form of a fluid jet to produce thrust.Chemical rocket propellants are most commonly used, which undergo exothermic chemical reactions which produce hot gas which is used by a rocket for propulsive purposes. Alternatively, a chemically inert reaction mass can be heated using a high-energy power source via a heat exchanger, and then no combustion chamber is used .Solid
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rocket propellants are prepared as a mixture of fuel and oxidizing components called 'grain' and the propellant storage casing effectively becomes the combustion chamber. Liquid-fueled rocketstypically pump separate fuel and oxidiser components into the combustion chamber, where they mix and burn. Hybrid rocket engines use a combination of solid and liquid or gaseous propellants. Both liquid and hybrid rockets use injectors to introduce the propellant into the chamber. These are often an array of simple jets- holes through which the propellant escapes under pressure; but sometimes may be more complex spray nozzles. When two or more propellants are injected the jets usually deliberately collide the propellants as this breaks up the flow into smaller droplets that burn more easily.
4.3) Combustion chamber For chemical rockets the combustion chamber is typically just a cylinder, and flame holders are rarely used. The dimensions of the cylinder are such that the propellant is able to combust thoroughly; different propellants require different combustion chamber sizes for this to occur. This leads to a number *
called L :
Where:
V c is the volume of the chamber At is the area of the throat
L* is typically in the range of 25 – 60 inches (0.63 – 1.5 m).
The combination of temperatures and pressures typically reached in a combustion chamber is usually extreme by any standards. Unlike in air-breathing jet engines, no atmospheric nitrogen is present to dilute and cool the combustion, and the temperature can reach true stoichiometric. This, in combination with the high pressures, means that the rate of heat conduction through the walls is very high.
4.4) Rocket nozzles The large bell or cone shaped expansion nozzle gives a rocket engine its characteristic shape.
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Fig 5: Typical temperatures (T) and pressures (p) and speeds (v) in a De Laval Nozzle
In rockets the hot gas produced in the combustion chamber is permitted to escape from the combustion chamber through an opening (the "throat"), within a high expansion-ratio 'de Laval' nozzle. The exhaust speeds vary, depending on the expansion ratio the nozzle is designed to give, but exhaust speeds as high as ten times the speed of sound are not uncommon.
Fig 6: Rocket thrust is caused by pressures acting in the combustion chamber and nozzle. From Newton's third law, equal and opposite pressures act on the exhaust, and this accelerates it to high speeds.
About half of the rocket engine's thrust comes from the unbalanced pressures inside the combustion chamber and the rest comes from the pressures acting against the inside of the nozzle (see diagram). As the gas expands (adiabatically) the pressure against the nozzle's walls forces the rocket engine in one direction while accelerating the gas in the other.
4.5) Propellant efficiency For a rocket engine to be propellant efficient, it is important that the maximum pressures possible be created on the walls of the chamber and nozzle by a specific amount of propellant; as this is the source of the thrust. This can be achieved by all of:
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heating the propellant to as high a temperature as possible (using a high energy fuel, containing hydrogen and carbon and sometimes metals such as aluminum, or even using nuclear energy)
using a low specific density gas (as hydrogen rich as possible)
using propellants which are, or decompose to, simple molecules with few degrees of freedom to maximise translational velocity
Since all of these things minimise the mass of the propellant used, and since pressure is proportional to the mass of propellant present to be accelerated as it pushes on the engine, and since from Newton's third law the pressure that acts on the engine also reciprocally acts on the propellant, it turns out that for any given engine the speed that the propellant leaves the chamber is unaffected by the chamber pressure (although the thrust is proportional). However, speed is significantly affected by all three of the above factors and the exhaust speed is an excellent measure of the engine propellant efficiency. This is termed exhaust velocity, and after allowance is made for factors that can reduce it, the effective exhaust velocity is one of the most important parameters of a rocket engine (although weight, cost, ease of manufacture etc. are usually also very important).
4.6) Thrust vectoring Many engines require the overall thrust to change direction over the length of the burn. A number of different ways to achieve this have been flown:
The entire engine is mounted on a hinge or gimbal and any propellant feeds reach the engine via low pressure flexible pipes or rotary couplings.
Just the combustion chamber and nozzle is gimbled, the pumps are fixed, and high pressure feeds attach to the engine
multiple engines (often canted at slight angles) are deployed but throttled to give the overall vector that is required, giving only a very small penalty
Fixed engines with vernier thrusters .
High temperature vanes held in the exhaust that can be tilted to deflect the jet.
Rockets can be further optimized to even more extreme performance along one or more of these axes at the expense of the others.
4.7) Specific impulse The most important metric for the efficiency of a rocket engine is impulse per unit of propellant, this is called specific impulse . This is either measured as a speed (the effective exhaust velocity V e in
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metres/second or ft/s) or as a time (seconds). An engine that gives a large specific impulse is normally highly desirable. The specific impulse that can be achieved is primarily a function of the propellant mix (and ultimately would limit the specific impulse), but practical limits on chamber pressures and the nozzle expansion ratios reduce the performance that can be achieved.
Typical performances of common propellants
Propellant mix
liquid oxygen/ liquid hydrogen
liquid oxygen/ kerosene (RP-1)
nitrogen tetroxide/ hydrazine
Vacuum Isp (seconds) Effective exhaust velocity (m/s)
455
4462
358
3510
305
2993
Table 1: Typical performances of common propellants
4.8) Net thrust Below is an approximate equation for calculating the net thrust of a rocket engine:
where: exhaust gas mass flow effective exhaust velocity actual jet velocity at nozzle exit plane
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flow area at nozzle exit plane (or the plane where the jet leaves the nozzle if separated flow) static pressure at nozzle exit plane ambient (or atmospheric) pressure Since, unlike a jet engine, a conventional rocket motor lacks an air intake, there is no 'ram drag' to deduct from the gross thrust. Consequently the net thrust of a rocket motor is equal to the gross thrust (apart from static back pressure). The
term represents the momentum thrust, which remains constant at a given throttle setting,
whereas the
term represents the pressure thrust term. At full throttle, the net thrust of
a rocket motor improves slightly with increasing altitude, because as atmospheric pressure decreases with altitude, the pressure thrust term increases. At the surface of the Earth the pressure thrust may be reduced by up to 30%,depending on the engine design. This reduction drops roughly exponentially to zero with increasing altitude. Maximum thrust for a rocket engine is achieved by maximizing the momentum contribution of the equation without incurring penalties from over expanding the exhaust. This occurs when Pe = Pamb. Since ambient pressure changes with altitude, most rocket engines spend very little time operating at peak efficiency.
4.9) Energy efficiency
Fig 7: Rocket energy efficiency as a function of vehicle speed divided by effective exhaust speed
Rocket engine nozzles are surprisingly efficient heat engines for generating a high speed jet, as a consequence of the high combustion temperature and high compression ratio. Rocket nozzles give an excellent approximation to adiabatic expansion which is a reversible process, and hence they give efficiencies which are very close to that of th eCarnot cycle. Given the temperatures reached, over 60% efficiency can be achieved with chemical rockets.
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4.10) Thrust to weight ratio Rockets, of all the jet engines, indeed of essentially all engines, have the highest thrust to weight ratio. This is especially true for liquid rocket engines. This high performance is due to the small volume of pressure vessels that make up the engine- the pumps, pipes and combustion chambers involved. The lack of inlet duct and the use of dense liquid propellant allows the pressurisation system to be small and lightweight, whereas duct engines have to deal with air which has a density about one thousand times lower. Mass,
Jet or rocket thrust, Thrust-to-weight
kg
Kn
ratio
RD-0410 nuclear rocket engine
2000
35.2
1.8
J-58 (SR-71 Blackbird jet engine)
2722
150
5.2
3175
169.2
5.4
4621
1413
31.2
RD-0146 rocket engine
260
98
38.5
Space Shuttle's SSME rocket engine
3177
2278
73.2
RD-180 rocket engine
5393
4152
78.6
F-1 (Saturn V first stage)
8391
7740.5
94.1
NK-33 rocket engine
1222
1638
136.8
Jet or Rocket engine
Concorde's Rolls-Royce/Snecma
Olympus
593 turbojet with reheat RD-0750 rocket
engine,
three-propellant
mode
Table 2: Thrust to weight ratio
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Of the liquid propellants used, density is worst for liquid hydrogen. Although this propellant is marvellous in many ways, it has a very low density, about one fourteenth that of water. This makes the turbopumps and pipework larger and heavier, and this is reflected in the thrust-to-weight ratio of engines that use it (for example the SSME) compared to those that do not (NK-33).
4.11) ignition With liquid and hybrid rockets, immediate ignition of the propellant(s) as they first enter the combustion chamber is essential. With liquid propellants (but not gaseous), failure to ignite within milliseconds usually causes too much liquid propellant to be within the chamber, and if/when ignition occurs the amount of hot gas created will often exceed the maximum design pressure of the chamber. The pressure vessel will often fail catastrophically. This is sometimes called a hard start.Ignition can be achieved by a number of different methods; a pyrotechnic charge can be used, a plasma torch can be used, or electric spark plugs may be employed. Some fuel/oxidizer combinations ignite on contact (hypergolic) , and non-hypergolic fuels can be "chemically ignited" by priming the fuel lines with hypergolic propellants.Gaseous propellants generally will not cause hard starts, with rockets the total injector area is less than the throat thus the chamber pressure tends to ambient prior to ignition and high pressures cannot form even if the entire chamber is full of flammable gas at ignition. Solid propellants are usually ignited with one-shot pyrotechnic devices.Once ignited, rocket chambers are self sustaining and igniters are not needed. Indeed chambers often spontaneously reignite if they are restarted after being shut down for a few seconds. However, when cooled, many rockets cannot be restarted without at least minor maintenance, such as replacement of the pyrotechnic igniter.
6) Advanced propulsion systems 6.1) Ion thruster An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and accelerate the ions in the direction of the electric field. Electromagnetic ion thrusters use the Lorentz force to accelerate the ions. The term "ion thruster" by itself usually denotes the electrostatic or gridded ion thrusters.
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The thrust created in ion thrusters is very small compared to conventional chemical rockets, but a very high specific impulse, or propellant efficiency, is obtained. This high propellant efficiency is achieved through the very frugal propellant consumption of the ion thruster propulsion system. Ion thrusters use beams of ions (electrically charged atoms or molecules) to create thrust in accordance with momentum conservation. The method of accelerating the ions varies, but all designs take advantage of the charge /mass ratio of the ions. This ratio means that relatively small potential differences can create very high exhaust velocities. This reduces the amount of reaction mass or fuel required, but increases the amount of specific power required compared to chemical rockets. Ion thrusters are therefore able to achieve extremely high specific impulses. The drawback of the low thrust is low spacecraft acceleration because the mass of current electric power units is directly correlated with the amount of power given. This low thrust makes ion thrusters unsuited for launching spacecraft into orbit, but they are ideal for in-space propulsion applications. Various ion thrusters have been designed and they all generally fit under two categories. The thrusters are categorized as either electrostatic orelectromagnetic. The main difference is how the ions are accelerated.
Electrostatic ion thrusters use the Coulomb force and are categorized as accelerating the ions in the direction of the electric field.
Electromagnetic ion thrusters use the Lorentz force to accelerate the ions.
Power supplies for ion thrusters are usually solar panels, but at sufficiently large distances from the Sun, nuclear power is used. In each case the power supply mass is essentially proportional to the peak power that can be supplied, and they both essentially give, for this application, no limit to the energy.
Gridded electrostatic ion thrusters
Gridded electrostatic ion thrusters commonly utilize xenon gas. This gas has no charge and is ionized by bombarding it with energetic electrons. These electrons can be provided from hot cathode filament and accelerated in the electrical field of the cathode fall to the anode (Kaufman type ion thruster). Alternatively, the electrons can be accelerated by the oscillating electric field induced by an alternating magnetic field of a coil, which results in a self-sustaining discharge and omits any cathode (radiofrequency ion thruster). The positively charged ions are extracted by an extraction system consisting of 2 or 3 multi-aperture grids. After entering the grid system via the plasma sheath the ions are accelerated due to the potential
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difference between the first and second grid (named screen and accelerator grid) to the final ion energy of typically 1-2 keV, thereby generating the thrust. Ion thrusters emit a beam of positive charged xenon ions only. In order to avoid charging-up the spacecraft, another cathode is placed near the engine, which emits electrons (basically the electron current is the same as the ion current) into the ion beam. This also prevents the beam of ions from returning to the spacecraft and thereby cancelling the thrust.
Fig 8: A diagram of how a gridded electrostatic ion engine (Kaufman type) works
Gridded electrostatic ion thruster research (past/present):
NASA Solar electric propulsion Technology Application Readiness (NSTAR) NASA’s Evolutionary Xenon Thruster (NEXT)
Nuclear Electric Xenon Ion System (NEXIS)
High Power Electric Propulsion (HiPEP)
EADS Radio-Frequency Ion Thruster (RIT)
Dual-Stage 4-Grid (DS4G)
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Hall effect thrusters
Hall effect thrusters accelerate ions with the use of an electric potential maintained between a cylindrical anode and a negatively charged plasma which forms the cathode. The bulk of the propellant (typically xenon gas) is introduced near the anode, where it becomes ionized, and the ions are attracted towards the cathode, they accelerate towards and through it, picking up electrons as they leave to neutralize the beam and leave the thruster at high velocity. The anode is at one end of a cylindrical tube, and in the center is a spike which is wound to produce a radial magnetic field between it and the surrounding tube. The ions are largely unaffected by the magnetic field, since they are too massive. However, the electrons produced near the end of the spike to create the cathode are far more affected and are trapped by the magnetic field, and held in place by their attraction to the anode. Some of the electrons spiral down towards the anode, circulating around the spike in a Hall current. When they reach the anode they impact the uncharged propellant and cause it to be ionized, before finally reaching the anode and closing the circuit.
Fig 9: Schematic of a Hall Thruster
Field emission electric propulsion
Field emission electric propulsion (FEEP) thrusters use a very simple system of accelerating liquid metal ions to create thrust. Most designs use either caesium or indium as the propellant. The design consists of a small propellant reservoir that stores the liquid metal, a very small slit that the liquid flows through,
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and then the accelerator ring. Caesium and indium are used due to their high atomic weights, low ionization potentials, and low melting points. Once the liquid metal reaches the inside of the slit in the emitter, an electric field applied between the emitter and the accelerator ring causes the liquid metal to become unstable and ionize. This creates a positive ion, which can then be accelerated in the electric field created by the emitter and the accelerator ring. These positively charged ions are then neutralized by an external source of electrons in order to prevent charging of the spacecraft hull.
Electromagnetic thrusters
1) Pulsed inductive thrusters (PIT) 2) Magnetoplasmadynamic (MPD) / lithium Lorentz force accelerator (LiLFA) 3) Electrodeless plasma thrusters 4) Electrothermal thrusters 5) Helicon double layer thruster
6.2) Nuclear pulse propulsion Nuclear pulse propulsion (or External Pulsed Plasma Propulsion, is a proposed method of spacecraft propulsion that uses nuclear explosions for thrust. Project Orion was the first serious attempt to design a nuclear pulse rocket.. The idea of Orion was to
react small directional nuclear explosives against a large steel pusher plate attached to the spacecraft with shock absorbers. Efficient directional explosives maximized the momentum transfer, leading tospecific impulses in the range of 6,000 seconds (about twelve times that of the Space Shuttle main engine). With refinements a theoretical maximum of 100,000 seconds (1 MN·s/kg) might be possible. 6
Thrusts were in the millions of tons, allowing spacecraft larger than 8×10 tons to be built with 1958 materials. This low-tech single-stage reference design would reach Mars and back in four weeks from the Earth's surface. A number of engineering problems were found and solved over the course of the project, notably related to crew shielding and pusher-plate lifetime. There were also ethical issues with launching such a vehicle within the Earth's magnetosphere. Calculations showed that the fallout from each takeoff would kill between 1 and 10 people Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary
Society (BIS) to design a plausible interstellar unmanned spacecraft that could reach a nearby star within
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one human scientist's working lifetime (set to be 50 years. At the time fusion research appeared to be making great strides, and in particular, inertial confinement fusion (ICF) appeared to be adaptable as a rocket engine. ICF uses small pellets of fusion fuel, typically Li6D with a small deuterium /tritium "trigger" at the center. The pellets are thrown into a reaction chamber where they are hit on all sides by lasers or another form of beamed energy. The heat generated by the beams explosively compresses the pellet, to the point where fusion takes place. The result is a hot plasma, and a very small "explosion" compared to the minimum size bomb that would be required to instead create the necessary amount of fission. For Daedalus, this process was run within a large electromagnet which formed the rocket engine. After the reaction, ignited by electron beams in this case, the magnet funnelled the hot gas to the rear for thrust. Some of the energy was diverted to run the ship's systems and engine. In order to make the system safe and energy efficient, Daedalus was to be powered by a Helium-3 fuel that would have had to be collected from Jupiter. The "Medusa" design is a type of nuclear pulse propulsion which has more in common with solar sails than with conventional rockets. A Medusa spacecraft would deploy a large sail ahead of it, attached by cables, and then launch nuclear explosives forward to detonate between itself and its sail. The sail would be accelerated by the impulse, and the spacecraft would follow.
Fig 10: Conceptual drawing of a Medusa nuclear pulse propulsion type spacecraft, showing spacecraft capsule (A), tether winch (B), main tether (C), parachute canopy (E), and canopy riser tethers (D). Own work
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(George William Herbert), licensed to anyone under Creative Commons - Attribution Share-Alike License 2.5 and GFDL for Wikipedia and distribution there. Created in Visio, output as png. Georgewilliamherbert 09:04, 10 January 2007 (UTC)
6.3) Fusion rocket A fusion rocket is a rocket that is driven by fusion power. The process of nuclear fusion is wellunderstood and recent developments indicate this technology may be able to provide terrestrial based power within 30 year. However, the proposed reactor vessels are large and heavy, making them unsuitable to use on spacecraft in the foreseeable future. A smaller and lighter fusion reactor might be possible in the future when more sophisticated methods have been devised to control magnetic confinement and prevent plasma instabilities. For space flight, the main advantage of fusion would be the very high specific impulse, the main disadvantage the (probable) large mass of the reactor. In addition, a fusion rocket may produce less radiation than a fission rocket, reducing the mass needed for shielding. The surest way of building a fusion rocket with current technology is to use hydrogen bombs as proposed in Project Orion, but such a spacecraft would also be massive. To sustain a fusion reaction, the plasma must be confined. The most widely studied configuration for terrestrial fusion is the tokamak, a form of magnetic confinement fusion. Currently tokamaks weigh a great deal, so the thrust to weight ratio would seem unacceptable. The main alternative to magnetic confinement is inertial confinement fusion, such as that proposed by Project Daedalus. A small pellet of fusion fuel (with a diameter of a couple of millimeters) would be ignited by an electron beam or a laser. To produce direct thrust, a magnetic field would form the pusher plate. In principle, the Helium-3-Deuterium reaction or an aneutronic fusion reaction could be used to maximize the energy in charged particles and to minimize radiation, but it is highly questionable whether it is technically feasible to use these reactions. An attractive possibility is to simply direct the exhaust of fusion product out the back of the rocket to provide thrust without the intermediate production of electricity. This would be easier with some confinement schemes) than with others. It is also more attractive for "advanced fuels" . Helium-3 propulsion is a proposed method of spacecraft propulsion that uses the fusion of helium-3 atoms as a
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power source. Helium-3, an isotope of helium with two protons and one neutron, could be fused with deuterium in a reactor. The resulting energy release could be used to expel propellant out the back of the spacecraft. Helium-3 is proposed as a power source for spacecraft mainly because of its abundance on the moon. Only 20% of the power produced by the D-T reaction could be used this way; the other 80% is released in the form of neutrons which, because they cannot be directed by magnetic fields or solid walls, would be very difficult to use for thrust.
Fig 11: Magnetized target fusion (MTF)
Magnetized target fusion (MTF) is a relatively new approach that combines the best features of the more widely studied magnetic confinement fusion (i.e. good energy confinement) and inertial confinement fusion (i.e. efficient compression heating and wall free containment of the fusing plasma) approaches. Like the magnetic approach, the fusion fuel is confined at low density by magnetic fields while it is heated into a plasma, but like the inertial confinement approach, fusion is initiated by rapidly squeezing the target to dramatically increase fuel density, and thus temperature. MTF uses "plasma guns" (i.e. electromagnetic acceleration techniques) instead of powerful lasers, leading to low cost and low weight compact reactors A still more speculative concept is antimatter catalyzed nuclear pulse propulsion, which would use tiny quantities of antimatter to catalyze a fission and fusion reaction, allowing much smaller fusion explosions to be created.
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6.4) Bussard ramjet Bussard proposed a ramjet variant of a fusion rocket capable of fast interstellar spaceflight, using enormous electro-magnetic fields (ranging from kilometers to many thousands of kilometers in diameter) as a ram scoop to collect and compress hydrogen from the interstellar medium. High speeds force the reactive mass into a progressively constricted magnetic field, compressing it until thermonuclear fusion occurs. The magnetic field then directs the energy as rocket exhaust opposite to the intended direction of travel, thereby accelerating the vessel. A major problem with using rocket propulsion to reach the velocities required for interstellar flight is the enormous amounts of fuel required. Since that fuel must itself be accelerated, this results in an approximately exponential increase in mass as a function of velocity change at non-relativistic speeds, asymptotically tending to infinity as it approaches the speed of light. In principle, the Bussard ramjet avoids this problem by not carrying fuel with it. An ideal ramjet design could in principle accelerate indefinitely until its mechanism failed. Ignoring drag, a ship driven by such an engine could theoretically accelerate arbitrarily close to the speed of light, and would be a very effective interstellar spacecraft. In practice, since the force of drag produced by collecting the interstellar medium increases approximately as its speed squared at non-relativistic speeds and asymptotically tends to infinity as it approaches the speed of light (taking all measurements from the ship's perspective), any such ramjet would have a limiting speed where the drag equals thrust. To produce positive thrust, the fusion reactor must be capable of producing fusion while still giving the incident ions a net rearward acceleration (relative to the ship). The collected propellant can be used as reaction mass in a plasma rocket engine, ion rocket engine, or even in an antimatter-matter annihilation powered rocket engine. Interstellar space contains an average −21
of 10
kg of mass per cubic meter of space, primarily in the form of non-ionized and ionized hydrogen,
with smaller amounts of helium, and no significant amounts of other gasses. This means that the ramjet scoop must sweep 10
18
cubic meters of space to collect one gram of hydrogen.
The mass of the ion ram scoop must be minimized on an interstellar ramjet. The size of the scoop is large enough that the scoop cannot be solid. This is best accomplished by using an electromagnetic field, or alternatively using an electrostatic field to build the ion ram scoop. Such an ion scoop will use electromagnetic funnels, or electrostatic fields to collect ionized hydrogen gas from space for use as propellant by ramjet propulsion systems (since much of the hydrogen is not ionized, some versions of a scoop propose ionizing the hydrogen, perhaps with a laser, ahead of the ship.) An electric field can electrostatically attract the positive ions, and thus draw them inside a ramjet engine. The electromagnetic
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funnel would bend the ions into helical spirals around the magnetic field lines to scoop up the ions via the starship's motion through space. Ionized particles moving in spirals produce an energy loss, and hence drag; the scoop must be designed to both minimize the circular motion of the particles and simultaneously maximize the collection. Likewise, if the hydrogen is heated during collection, thermal radiation will represent an energy loss, and hence also drag; so an effective scoop must collect and compress the hydrogen without significant heating. A magnetohydrodynamic generator drawing power from the exhaust could power the scoop.
6.5) Solar sail Solar sailing is a way of moving around in space by allowing sunlight to push a spacecraft.A solar sail is a very large mirror that reflects sunlight. As the photons of sunlight strike the sail and bounce off,
they gently push the sail along by transferring momentum to the sail. Because there are so many photons from sunlight, and because they are constantly hitting the sail, there is a constant pressure (force per unit area) exerted on the sail that produces a constant acceleration of the spacecraft. Although the force on a solar-sail spacecraft is less than a conventional chemical rocket, such as the space shuttle, the solar-sail spacecraft constantly accelerates over time and achieves a greater velocity. Solar sails enable spacecraft to move within the solar system and between stars without bulky rocket engines and enormous amounts of fuel. When the spacecraft is in orbit around the Earth or sun, it is traveling in a circular or elliptical path at a given speed and distance. To go to a higher orbit (travel farther away from the object), you angle the solar sail with respect to the sun so that the pressure generated by sunlight is in the direction of your orbital motion. The force accelerates the spacecraft, increases the speed of its orbit and the spacecraft moves into a higher orbit. In contrast, if you want to go to a lower orbit (closer to the object), you angle the sail with respect to the sun so that the pressure generated by the sunlight is opposite the direction of your orbital motion. The force then decelerates the spacecraft, decreases the speed of its orbit and the spacecraft drops into a lower orbit. The pressure of sunlight decreases with the square of the distance from the sun. Therefore, sunlight exerts greater pressure closer to the sun than farther away. Future solar-sail spacecraft may take advantage of this fact by first dropping to an orbit close to the sun -- a solar fly-by -- and using the greater sunlight pressure to get a bigger boost of acceleration at the start of the mission. This is called a powered perihelion maneuver .
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6.6) Magnetic sail A magnetic sail or magsail is a proposed method of spacecraft propulsion which would use a static magnetic field to deflect charged particles radiated by the Sun as a plasma wind, and thus impart momentum to accelerate the spacecraft. A magnetic sail could also thrust directly against planetary and solar magnetospheres. The solar wind is a tenuous stream of plasma that flows outwards from the Sun: near the Earth's orbit, it contains several million protons and electrons per cubic meter and flows at 400 to 600 kilometres per second (250 to 370 mi/s). The magnetic sail introduces a magnetic field into this plasma flow, perpendicular to the motion of the charged particles, which can deflect the particles from their original trajectory: the momentum of the particles is then transferred to the sail, leading to a thrust on the sail. One advantage of magnetic or solar sails over (chemical or ion) reaction thrusters is that no reaction mass is depleted or carried in the craft. In typical magnetic sail designs, the magnetic field is generated by a loop of superconducting wire. Because loops of current-carrying conductors tend to be forced outwards towards a circular shape by their own magnetic field, the sail could be deployed simply by unspooling the conductor and applying a current through it. For a sail in the solar wind at 1 AU away from the Sun, the field strength required to resist the dynamic pressure of the solar wind is 50 nT . Zubrin's proposed magnetic sail design would create a bubble of space of 100 km in diameter (62 mi) where solar-wind ions are substantially deflected using a hoop 50 km (31 mi) in radius. The minimum weight of such a coil is constrained by material strength limitations at roughly 40 tonnes and it would generate 70 newtons (16 lb f ) of thrust, giving a mass/thrust ratio of 600 kg/N. It is not clear how such a coil would be cooled. The solar and magnetic sails have a thrust that falls off as the square of the distance from the Sun. When close to a planet with a strong magnetosphere, e.g. Earth or a gas giant, the magsail could generate more thrust by interacting with the magnetosphere instead of the solar wind, and may therefore be more efficient.
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Fig 11: Magnetic sail deployed
6.7) Beam-powered propulsion Beam-powered propulsion is a class of aircraft or spacecraft propulsion mechanisms that use energy beamed to the spacecraft from a remote power plant to provide energy. Most designs ar erocket engines where the energy is provided by the beam, and is used to superheat propellant that then provides propulsion, although some obtain propulsion directly from light pressure acting on a light sail structure, and at low altitude heating air gives extra thrust. The beam would typically either be a beam of microwaves or a laser. Lasers are subdivided into either pulsed or continuous beamed.Many proposed spacecraft propulsion mechanisms use power in the form of electricity or heat. Usually these schemes assume either solar-electric power, or an on-board reactor. However, both power sources are heavy. Therefore, one could instead leave the power-source stationary, and power the spacecraft with a maser or alaser beam from a fixed installation. This permits the spacecraft to leave its power-source at home, saving significant amounts of mass.
6.8) Alcubierre drive The Alcubierre drive, also known as the Alcubierre metric, is a speculative mathematical model of a spacetime exhibiting features reminiscent of the fictional "warp drive" from Star Trek , which can travel "faster than light", although not in a local sense. In 1994, the Mexican physicist Miguel Alcubierre proposed a method of stretching space in a wave which would in theory cause the fabric of space ahead of a spacecraft to contract and the space behind it to expand. The ship would ride this wave inside a region known as a warp bubble of flat space. Since the ship is not moving within this bubble, but carried along as the region itself moves, conventional relativistic effects such as time dilation do not apply in the way.
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Normally, Einstein's theory of relativity doesn't permit any object to travel faster than the speed of light, because accelerating up to that speed requires an infinite amount o fenergy. The Alcubierre drive gets around this by proposing that the drive would actually manipulate spacetime itself, causing the space in front of it to contract while the space behind it expands. This "warp bubble" allows the ship to reach a destination faster than a light beam traveling through "normal" spacetime.According to relativity, space is malleable, which is how the Alcubierre drive achieves this feat. (The early universe, for example, expanded faster than the speed of light because spacetime itself can expand faster, even though objects within spacetime cannot accelerate faster.) In this scenario, the ship containing the Alcubierre drive actually sits still and is carried along the warp bubble, kind of like a surfboard riding on an expanding wave. This means that time dilation and other relativistic effects aren't significant
Fig 11 : Embedded diagram of a Schwarzschild wormhole
7) conclusion Electromagnetic propulsion systems are one of the current areas of active research. The goal is to create an electrically powered spacecraft propulsion system. These engines accelerate ions by using electrostatic forces, and use a number of methods such as electromagnetic or electrostatic forces to directly accelerate the mass. Electric power is used to ionize the atoms and then to make a voltage gradient that is used to accelerate them to high exhaust velocities. These systems have not been able to produce sufficient force on their own to work in all cases, but they have been combined with nuclear electric systems to generate the appropriate amount of power to generate the appropriate amount of thrust. Other rocket propulsion systems that have been tested or are under research include electrothermal thrusters (that use electromagnetic fields to make a plasma to heat propellant which is then converted into kinetic energy), pulsed plasma thrusters, and pulsed inductive thrusters. Some of the rocket propulsion systems that remain science fiction but are not excluded from potential research include: a bias drive, disjunctive drive, differential sail, and a hyperspace drive based upon the Heim theory.Propulsion system to power spaceships for Inter planetary travel may become a reality in near future.
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Reference:
http://www.grc.nasa.gov/WWW/K-12/airplane/bgp.html http://www.sciencedaily.com/articles/s/spacecraft_propulsion.html http://en.wikipedia.org/wiki/De_Laval_nozzle http://en.wikipedia.org/wiki/Spacecraft_propulsion http://en.wikipedia.org/wiki/Category:Rocket_propulsion http://en.wikipedia.org/wiki/Rocket_engine
