The overall design of the X-20 Dyna-Soar was outlined in March 1960. It had a low-wing delta shape, with winglets for control rather than a more conventional tail. The framework of the craft was to be made from the René 41 super alloy, as were the upper surface panels. The bottom surface was to be made from molybdenum sheets placed over insulated René 41, while the nose-cone was to be made from graphite with zirconia rods.Due to the changing requirements, various forms of the Dyna-Soar were designed but with all variants sharing the same basic shape and layout. A single pilot sat at the front, while an equipment bay was situated behind. This bay contained either data-collection equipment, weapons, reconnaissance equipment, or in the X-20X shuttle space vehicle a four-man mid-deck.
After the equipment bay was the transition-stage rocket engine, which was used to maneuver the craft in orbit or fired during launch as part of an abort. This trans-stage would be jettisoned before descent into the atmosphere. While falling through the atmosphere an opaque heat shield would protect the window at the front of the craft. This would then be jettisoned after aerobraking so the pilot could see, and safely land.A drawing in SpaceAeronautics magazine from before the projects cancellation depicts the craft dipping down into the atmosphere, skimming the surface, to change its orbital inclination. It would then fire its rocket to resume orbit. This would be a unique ability for a spacecraft, for the laws of celestial mechanics mean that it is much more difficult for a rocket to do this once in orbit. Hence the Dyna-Soar could have had a military capacity of being launched into one orbit and rendezvousing with a satellite even if the target were to expend all its propellant in changing its orbit. Acceleration forces on the pilot, however, would be severe.Unlike the later Space Shuttle, Dyna-Soar did not have wheels on its undercarriage as it was thought that the rubber wheels would burn during re-entry. Instead Goodyear developed retractable wire-brush skis made of the same René 41 alloy as the air-frame.
Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit apoapsis by flying the vehicle through the atmosphere at the low point of the orbit periapsis, using drag to slow the spacecraft. Aerobraking saves fuel, compared to the direct use of a rocket engine, when the spacecraft requires a low orbit after arriving at a body with an atmosphere.When an interplanetary vehicle arrives at its destination, it must change its velocity to remain in the vicinity of that body. When a low, near-circular orbit is needed around a body with substantial gravity as many scientific studies require, the total required velocity changes can be on the order of several kilometers per second. If done by direct propulsion, the rocket equation dictates that a large fraction of the spacecraft mass must be fuel. This in turn means either a relatively small science payload or use of a very large and expensive launcher. Provided the target body has an atmosphere, aerobraking can be used to reduce fuel requirements by using a smaller burn to allow the spacecraft to be captured into a very elongated elliptic orbit. Aerobraking is then used to circularize the orbit. Due to the small effect of atmospheric drag, achieving the final orbit takes a long time e.g., over 6 months when arriving at Mars, and may require several hundred passes through the atmosphere of the planet or moon.
The kinetic energy dissipated by aerobraking is converted to heat and so a spacecraft using the technique needs to be designed to dissipate the heat generated. The spacecraft must also have suitable surface area and structural strength to produce and survive the required drag, but the deceleration and thus temperatures and pressures are not as significant as reentry or aerocapture. Simulations of the Mars Reconnaissance Orbiter aerobraking use a force limit of 0.35 N per square meter with a spacecraft cross section of about 37 m², and a maximum expected temperature as 340 °F 170 °C.In another article about Mars Observer the force on the whole spacecraft was compared to force of a 40 mph 60 kmh wind on a human hand at sea level on Earth.Another article on MGS quotes a force of roughly 0.2 N 0.04 lbf per square meter.Aerocapture is a related but more extreme method in which no initial orbit-injection burn is performed. Instead, the spacecraft plunges deeply into the atmosphere without an initial insertion burn, and emerges from this single pass in the atmosphere with an apoapsis near that of the desired orbit. Several small correction burns are then used to raise the periapsis and perform final adjustments. This method was originally planned for the Mars Odyssey orbiter, but the significant design impacts proved too costly.
Although the theory of aerobraking is well developed, utilising the technique is difficult as a very detailed knowledge of the character of the target planets atmosphere is needed in order to plan the maneuver correctly. Currently, the deceleration is monitored during each maneuver, modifying future plans accordingly. Since no spacecraft can yet aerobrake safely on its own, this requires constant attention from both human controllers and the Deep Space Network, particularly near the end of the process when the drag passes are only about 2 hours apart for Mars.Aerobraking was first used during the extended Venus mission of the Magellan spacecraft to circularize the orbit in order to increase the sensitivity of the measurement of the gravity field. The entire gravity field was mapped from the circular orbit during a 243 day cycle of the extended mission. After the gravity field was mapped, a windmill experiment was performed during the termination phase of the mission where atmospheric drag was used to deorbit the Magellan spacecraft.
In 1997, the Mars Global Surveyor MGS orbiter was the first spacecraft to use aerobraking as the main planned technique of orbit adjustment. MGS used the data gathered from the Magellan mission to Venus to plan its aerobraking technique. The spacecraft used its solar panels as wings to control its passage through the tenuous upper atmosphere of Mars to lower the apoapsis of its orbit over the course of many months. Unfortunately, a structural failure shortly after launch severely damaged one of MGSs solar panels, requiring a higher aerobraking altitude and hence one third the force than originally planned, significantly extending the time required to attain the desired orbit. More recently, aerobreaking was used by the Mars Odyssey and Mars Reconnaissance Orbiter spacecraft, in both cases without incident.
The hot gas produced escapes through a narrow opening the throat, into a high expansion-ratio de Laval nozzle. The nozzle dramatically accelerates the gas, converting most of the thermal energy into kinetic energy. The large bell or cone shaped expansion nozzle gives a rocket engine its characteristic shape. Exhaust speeds as high as ten times the speed of sound at sea level are not uncommon.Rocket thrust is caused by pressures acting in the combustion chamber and nozzle. From Newtons third law, equal and opposite pressures act on the exhaust, and this accelerates it to high speeds.Rocket thrust is caused by pressures acting in the combustion chamber and nozzle. From Newtons third law, equal and opposite pressures act on the exhaust, and this accelerates it to high speeds.A portion of the rocket engines thrust comes from the unbalanced pressures inside the combustion chamber but the majority comes from the pressures against the inside of the nozzle see diagram. As the gas expands adiabatically the pressure against the nozzles walls forces the rocket engine in one direction while accelerating the gas in the other.
Most rocket engines produce thrust by the expulsion of a high-temperature, high-speed gaseous exhaust. This is typically created by high pressure 10-200 bar combustion of solid or liquid propellants, consisting of fuel and oxidiser components, within a combustion chamber.Liquid-fueled rockets typically pump separate fuel and oxidiser components into the combustion chamber, where they mix and burn. Solid rocket propellants are prepared as a mixture of fuel and oxidizing components and the propellant storage chamber becomes the combustion chamber. Hybrid rocket engines use a combination of solid and liquid or gaseous propellants. Alternatively, a chemically inert reaction mass can be heated using a high-energy power source.
For a rocket engine to be propellant efficient, it is important that the maximum pressures possible be created by a specific amount of propellant acting on the chamber and nozzle. This can be achieved by all ofSince all of these things minimise the mass of the propellant used, and since pressure is proportional to the amount of propellant present to be accelerated as it pushes on the engine, the speed that the propellant leaves the chamber is constant. Thus the exhaust speed is an excellent measure of the engine propellant efficiency.For aerodynamic reasons the flow goes sonic chokes at the narrowest part of the nozzle, the throat. Since the speed of sound in gases increases with the square root of temperature, the use of hot exhaust gas greatly improves performance. By comparison, at room temperature the speed of sound in air is about 340ms while the speed of sound in the hot gas of a rocket engine can be over 1700ms much of this performance is due to the higher temperature, but additionally rocket propellants are chosen to be of low molecular mass, and this also gives a higher velocity compared to air.
Expansion in the rocket nozzle then further multiplies the speed, typically between 1.5 and 4 times, giving a highly collimated hypersonic exhaust jet. The speed increase of a rocket nozzle is mostly determined by its area expansion ratio the ratio of the area of the throat to the area at the exit, but detailed properties of the gas are also important. Larger ratio nozzles are more massive but are able to extract more heat from the combustion gases, increasing the exhaust velocity.Nozzle efficiency is affected by operation in the atmosphere because atmospheric pressure changes with altitude but due to the supersonic speeds of the gas exiting from a rocket engine, the pressure of the jet may be either below or above ambient, and equilibrium between the two is not reached.
For optimal performance the pressure of the gas at the end of the nozzle should just equal the ambient pressure if lower the vehicle will be slowed by the difference in pressure between the top of the engine and the exit, if higher then this represents pressure that the bell has not turned into thrust. To maintain this ideal the diameter of the nozzle would need to increase with altitude, giving the pressure a longer nozzle to act on and reducing the exit pressure and temperature. This increase is difficult to arrange. A compromise nozzle is generally used and some reduction in performance occurs. To improve on this, various exotic nozzle designs such as the plug nozzle, stepped nozzles, the expanding nozzle and the aerospike have been proposed, each having some way to adapt to changing ambient air pressure and each allowing the gas to expand further against the nozzle, giving extra thrust at higher altitude.A rocket engine is a jet engine that takes all its reaction mass propellant from within tankage and forms it into a high speed jet, thereby obtaining thrust in accordance with Newtons third law. Rocket engines can be used for spacecraft propulsion as well as terrestrial uses, such as missiles. Most rocket engines are internal combustion engines, although non combusting forms also exist.
Artificial satellites must be launched into orbit, and once there they must be placed in their nominal orbit. Once in the desired orbit, they often need some form of attitude control so that they are correctly pointed with respect to the Earth, the Sun, and possibly some astronomical object of interest.They are also subject to drag from the thin atmosphere, so that to stay in orbit for a long period of time some form of propulsion is occasionally necessary to make small corrections orbital stationkeeping.Many satellites need to be moved from one orbit to another from time to time, and this also requires propulsion.When a satellite has exhausted its ability to adjust its orbit, its useful life is over.Spacecraft designed to travel further also need propulsion methods. They need to be launched out of the Earths atmosphere just as satellites do. Once there, they need to leave orbit and move around.
For interplanetary travel, a spacecraft must use its engines to leave Earth orbit. Once it has done so, it must somehow make its way to its destination. Current interplanetary spacecraft do this with a series of short-term trajectory adjustments.In between these adjustments, the spacecraft simply falls freely along its orbit. The simplest fuel-efficient means to move from one circular orbit to another is with a Hohmann transfer orbit the spacecraft begins in a roughly circular orbit around the Sun. A short period of thrust in the direction of motion accelerates or decelerates the spacecraft into an elliptical orbit around the Sun which is tangential to its previous orbit and also to the orbit of its destination. The spacecraft falls freely along this elliptical orbit until it reaches its destination, where another short period of thrust accelerates or decelerates it to match the orbit of its destination.Special methods such as aerobraking are sometimes used for this final orbital adjustment.
Some spacecraft propulsion methods such as solar sails provide very low but inexhaustible thrustan interplanetary vehicle using one of these methods would follow a rather different trajectory, either constantly thrusting against its direction of motion in order to decrease its distance from the Sun or constantly thrusting along its direction of motion to increase its distance from the Sun.Spacecraft for interstellar travel also need propulsion methods. No such spacecraft has yet been built, but many designs have been discussed. Since interstellar distances are very great, a tremendous velocity is needed to get a spacecraft to its destination in a reasonable amount of time. Acquiring such a velocity on launch and getting rid of it on arrival will be a formidable challenge for spacecraft designers.
Spacecraft propulsion is any method used to change the velocity of spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by exhausting a gas from the backrear 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 such as the Pegasus rocket and SpaceShipOne have used air-breathing 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. While soviet bloc satellites have used them for decades, newer Western geo-orbiting spacecraft are starting to use electric propulsion for north-south stationkeeping. Interplanetary vehicles mostly use chemical rockets as well, although a few have experimentally used ion thrusters a form of electric propulsion with some success.
When in space, the purpose of a propulsion system is to change the velocity, or v, of a spacecraft. Since this is more difficult for more massive spacecraft, designers generally discuss momentum, mv. The amount of change in momentum is called impulse. So the goal of a propulsion method in space is to create an impulse.When launching a spacecraft from the Earth, a propulsion method must overcome a higher gravitational pull to provide a net positive acceleration.In orbit, any additional impulse, even very tiny, will result in a change in the orbit path.The rate of change of velocity is called acceleration, and the rate of change of momentum is called force. To reach a given velocity, one can apply a small acceleration over a long period of time, or one can apply a large acceleration over a short time. Similarly, one can achieve a given impulse with a large force over a short time or a small force over a long time. This means that for maneuvering in space, a propulsion method that produces tiny accelerations but runs for a long time can produce the same impulse as a propulsion method that produces large accelerations for a short time. When launching from a planet, tiny accelerations cannot overcome the planets gravitational pull and so cannot be used.
The Earths surface is situated fairly deep in a gravity well and it takes a velocity of 11.2 kilometerssecond escape velocity or more to escape from it. As human beings evolved in a gravitational field of 1g 9.8 ms², an ideal propulsion system would be one that provides a continuous acceleration of 1g though human bodies can tolerate much larger accelerations over short periods. The occupants of a rocket or spaceship having such a propulsion system would be free from all the ill effects of free fall, such as nausea, muscular weakness, reduced sense of taste, or leaching of calcium from their bones.The law of conservation of momentum means that in order for a propulsion method to change the momentum of a space craft it must change the momentum of something else as well. A few designs take advantage of things like magnetic fields or light pressure in order to change the spacecrafts momentum, but in free space the rocket must bring along some mass to accelerate away in order to push itself forward. Such mass is called reaction mass.