Air Breathing Rockets

1. INTRODUCTION

When most people think about motors or engines, they think about rotation. For example, a reciprocating gasoline engine in a car produces rotational energy to drive the wheels. An electric motor produces rotational energy to drive a fan or spin a disk. A steam engine is used to do the same thing, as is a steam turbine and most gas turbines.

If you have ever shot a shotgun, especially a big 12-gauge shot gun, then you know that it has a lot of "kick." That is, when you shoot the gun it "kicks" your shoulder back with a great deal of force. That kick is a reaction. A shotgun is shooting about an ounce of metal in one direction at about 700 miles per hour, and your shoulder gets hit with the reaction. If you were wearing roller skates or standing on a skateboard when you shot the gun, then the gun would be acting like a rocket engine and you would react by rolling in the opposite direction.

Fig.1

If you have ever seen a big fire hose spraying water, you may have noticed that it takes a lot of strength to hold the hose (sometimes you will see two or three firefighters holding the hose). The hose is acting like a rocket engine. The hose is throwing water in one direction, and the firefighters are using their strength and weight to counteract the reaction. If they were to let go of the hose, it would thrash around with tremendous force. If the firefighters were all standing on skateboards, the hose would propel them backwards at great speed!

When you blow up a balloon and let it go so that it flies all over the room before running out of air, you have created a rocket engine. In this case, what is being thrown is the air molecules inside the balloon. Many people believe that air molecules don't weigh anything, but they do (see the page on helium to get a better picture of the weight of air). When you throw them out the nozzle of a balloon, the rest of the balloon reacts in the opposite direction.

The jet propulsion devices are classified mainly in two types on the basis of would they rely on atmospheric oxygen or not. The main two types of jet propulsion devices are as follows,

1. Rockets:-

In rockets thrust is produced by ejecting stored matter, which is called the propellant. Such devices are generally classified according to the type of propellant/energy source used ( chemical , nuclear, electrical, solar, etc .). Since rockets are not dependent on using the surrounding medium as part of the propulsion system, they are well suited for space transportation systems.

2. Air-Breathing Engines or “duct propulsion devices”:-

In which the surrounding medium (air) is utilized as the “working fluid”, rather than the stored propellant. Such devices are classified according to the specific features and components of the thermodynamic cycle present in the engine (turbojets, turbofans, ramjets, turboprops, etc.). Since air breathing engines utilize the surrounding medium (air) and do not require the storage of all propellant components, they are well suited for aircraft transportation systems as well as ground-based power generation device. In both cases, the production of thrust for the engine takes place through the generation of very high exhaust velocities (high exhaust momentum) from the device.

2. Air-Breathing Engines

[a] Ramjet Engines

Ramjet Engines are conceptually the simplest of air-breathing engines, in that there are no rotating components. In the ramjet, air enters through a supersonic inlet where is it slowed and pressurized, then it is mixed with fuel and burned in the combustion chamber, then the gaseous products are expanded in the nozzle and exhausted from the vehicle at a speed exceeding that of the entering air. Ramjets typically operate in the flight Mach number range between 3 and 7, and are more often used for missile propulsion than for high speed aircraft propulsion. A schematic of the generic ramjet engine is shown in Fig.6,

Fig.2

[b] Turbojet Engines

Turbojet Engines have improved performance over the ramjet at subsonic as well as low supersonic speeds, although the complication and added weight associated with rotating components (compressor and turbine) are present in the turbojet. Here, in addition to the static pressure rise created by a diffuser or inlet, the compressor raises the air pressure and temperature prior to mixing and combustion with fuel in the combustion chamber. The combustion gases then enter the turbine, and are expanded through the turbine, doing work on the turbine, which is used in turn to drive the compressor.

An example of the generic turbojet engine with an afterburner is shown in fig3,

Fig.3

Further expansion of the gases occurs in the nozzle, where again the gas is exhausted at a higher velocity than that of air entering the engine. In some cases, an afterburner is added downstream of the turbine for additional thrust; this is generally used for high performance aircraft in which the added fuel consumption is not of significant concern.

[c] Turbofan or Turbo bypass Engines

Turbofan or Turbo bypass Engines are examples of means by which the fuel efficiency of the basic turbojet engine may be improved. Here there is a second turbine added downstream of the compressor-drive turbine, and this additional power from the second turbine is used to drive a fan that pumps air through a secondary burner-nozzle (turbo bypass) or nozzle (turbofan) system. This air then “bypasses” the main engine flow, but provides additional thrust for the overall engine.

[d] Turboprop or Turbo shaft Engines

Turboprop or Turbo shaft engines are also similar to the turbojet, except that they utilize a propeller to provide most of the propulsive thrust for the vehicle. The propeller and compressor are both driven by turbine(s). These engines are highly fuel efficient in comparison to the turbojet engine itself, although the speed and range of the vehicles are generally more limited. Turboprop engines are often used in small commuter aircraft, while turbo shaft engines are used for helicopter propulsion. A sample turboprop engine schematic is shown in fig.4,

Fig.4

[e] Scramjet:-Air-breathing engines have several advantages over rockets.

Because the former use oxygen from the atmosphere, they require less propellant--fuel, but no oxidizer--resulting in lighter, smaller and cheaper launch vehicles. To produce the same thrust, air-breathing engines require less than one seventh the propellants that rockets do. Furthermore, because air-breathing vehicles rely on aerodynamic forces rather than on rocket thrust, they have greater maneuverability, leading to higher safety: flights can be aborted, with the vehicle gliding back to Earth. Missions can also be more flexible

Fig.5

The ramjet is the most basic type of jet engine. In comparison to turbojets, they have no moving parts. They find use only in guided air launched missiles. The aeroplane firing them must be flying at supersonic speeds. Ramjets operate by subsonic combustion of fuel in a stream of air compressed by the forward speed of the aircraft itself, as opposed to conventional turbojet engines, in which the compressor section (the fan blades) compresses the air.

Fig.6

Scramjets (supersonic-combustion ramjets) are those in which the airflow through the whole engine remains supersonic (shown in fig.6). It is mechanically simple, but vastly more complex aerodynamically than a jet engine. In a scramjet powered aircraft, there must be tight integration between the airframe and the engine. Scramjet technology is challenging because only limited testing can be performed in ground facilities. Long duration, full-scale testing requires flight test speeds above Mach 8. X-43 Hyper-X, NASA's testbed for the scramjet, serves this purpose. To get the engine to that speed, some other power has to be used. In the Hyper-X, this will be provided by OSC's Pegasus booster. It must be noted here that scramjets are good only for sustaining hypersonic speeds, not for achieving them from zero.

3. AIR BREATHING ROCKET ENGINE

Space travel could be revolutionized if an experimental air-breathing rocket engine proves successful in ongoing tests. The unique engine, which can function as a rocket, ramjet or scramjet, uses air as an oxidizer. Compared to conventionally powered rocket engines, this technology would significantly reduce vehicle weight by eliminating a significant amount of onboard oxidizer.

Air-breathing propulsion is one of the most promising concepts we’ve seen for reaching NASA’s future-generation spaceflight goals. Meanwhile, the Marshall Center's academic partner, Pennsylvania State University of University Park, finished the first phase of its experimental work on air-breathing rocket engine development in mid-March and immediately started a second phase of activity. The experimental research now under way will examine the effect of two rockets in a duct and the use of hydrocarbon fuels, instead of hydrogen.

In conventional rocket engines, a liquid oxidizer (some form of oxygen) and a fuel are combined and burned to create a high-pressure, high-velocity stream of hot gases. These gases flow through a nozzle that accelerates them farther (8045 kilometers per hour [5,000 mph] is typical exit velocity), and then they leave the engine. This provides the thrust for the spacecraft.

Air-breathing rockets still use an oxidizer, but the source is oxygen from the atmosphere, rather than stored liquid oxygen onboard the craft. Intake vents allow the rocket to "breathe in" oxygen as the vehicle flies; this oxygen is combined with the rocket fuel, and combustion takes place. Current jet turbine engines use a similar process, but turbines have a compressor that generates pressure and can produce power even when stationary. Air-breathing rockets use rockets to provide the initial push to increase the speed of the vehicle until enough air is captured to provide adequate thrust for the vehicle. At that point, the rockets are turned off and the propulsion system uses the air to support the combustion process.

4. NECESSITY OF AIR BREATHING ROCKET ENGINES

Why change the way a rocket is powered? If you don't have to carry the oxidizer on the rocket, you can reduce the weight by up to 50 percent. Lighter vehicles are both cheaper to operate and easier to launch. NASA's goal is to reduce the cost of space flights by a factor of 100, and this is a way to help achieve that goal.

It's a little more complicated than that, of course. Air-breathing rockets are more technically called combined cycle rocket engines because they employ both conventional rockets and air-breathing technology. The initial push comes from a rocket; then ramjets start the air-breathing process (visualize ramming the air through the vents into the combustor), and when the speed gets up to Mach 6, the scramjet takes over (scram jets use supersonic combustion; ram jets use subsonic combustion). Once the speed reaches Mach 15, the scramjets are turned off, the rockets go back on, and the vehicle goes into orbit.

Conventional rockets launch vertically- straight up- to exit the atmosphere as quickly as possible. Air-breathing rockets, because they need oxygen from the atmosphere, stay in the atmosphere as long as they can to inhale as much oxygen as possible.

Rather than launching vertically, air-breathers can be launched either vertically or horizontally. They fly much like an airplane, cruising at high altitudes, taking in oxygen until the proper speed is reached for orbit. Getting off the ground is the most expensive part of any mission to low-Earth orbit, and reducing a vehicle's weight decreases cost significantly.

5. ENGINE CONCEPT

The diagram given below of the Cosmos Mariner is an excellent representation of how a craft having both jets and rocket engines might be constructed. The jets engines have been tightly integrated into the main body since they will pose a problem while re-entry and also spoil the aerodynamics of 'spaceship'. Efficiency is critical to performance. Note that the only cargos on the craft are the passengers.

Fig.7 (HTOL 1.5STO)

Weight is very crucial in the launch business. Currently in takes 10,000 bucks per pound to get stuff to orbit. Getting to orbit is very difficult, and every ounce counts. Even if money were no problem, the hardware required gets exponentially complex with size. 'One and a Half Stages' is an innovative HTOL design which uses in-flight refuelling of a single-stage vehicle before shooting for space. Such a Spacecraft will take off with no liquid oxygen in its tanks, using jets to reach a high altitude. By this time most of the jet fuel has been used thus making it lighter. It will then rendezvous with a tanker which will supply it LOX in flight. The Spacecraft can then effectively "take off" with full tanks at high altitude. The liquid Hydrogen or other fuel will be on the vehicle before take-off.

Fig.8 Performance of air-breathing engines burning hydrogen fuel

The concept is innovative and practical. It does reduce problems due to take-off weight to a large extent. In-flight refuelling is practiced routinely by airforces around the world and so safety should not be a problem. However, the technology to transfer fuel at such an extremely low temperature does not exist (LOX has to be stored at extremely low temperatures to liquify it).

6. AIR BREATHING ROCKET ENGINE TESTING:

Taking another step toward making future space transportation more like today's air travel, NASA's Marshall Space Flight Center in Huntsville, Ala., and its industry partners have completed a series of successful tests on air-breathing rocket engines. The latest ground testing focused on engine performance during low-speed portions of the flight, when high thrust levels are needed to push the air-breathing rocket through Earth's atmosphere. An air-breathing -- or rocket-based, combined cycle -- engine inhales oxygen from the air for about half the flight, so it doesn't have to store the oxygen on board. That reduces the vehicle's weight at launch, resulting in significant cost savings.

Fig.9 Air breathing engine successfully tested

At launch, the engine is powered by specially designed rockets strategically placed in a duct that captures air. Once the vehicle reaches twice the speed of sound, the rockets are turned off and the engine relies totally on oxygen in the atmosphere to burn its fuel. When its speed increases to about 10 times the speed of sound, the engine converts to a conventional rocket-powered system for the final push to orbit.

Fig.10 Air breathing engine successfully tested

Fig.11 Wind tunnel tests show good aerodynamic and propulsion performance for the Hyper-X configuration. Shown here is a Mach 7 test of the full-scale model with spare flight engine in Langley’s 8-Foot High Temperature Wind Tunnel.

Similar testing by Aerojet Corp. of Sacramento, Calif., and Rocketdyne of Canoga Park, Calif., showed that recent modifications to the engine's internal geometry improved performance. Aerojet conducted tests at its newly refurbished facility in Sacramento, while Rocketdyne conducted tests at the General Applied Sciences Laboratory (GASL) on Long Island, N.Y.

7. LIFTOFF

As efficient as air-breathing rockets are, they can't provide the thrust for liftoff. For that, there are two options being considered. NASA may use turbojets or air-augmented rockets to get the vehicle off the ground. An air-augmented rocket is like a normal rocket engine, except that when it gets a high enough speed, maybe at Mach two or three, it will augment the oxididation of the fuel with air in the atmosphere, and maybe go up to Mach 10 and then change back to normal rocket function. These air-augmented rockets are placed in a duct that capture air, and could boost performance about 15 percent over conventional rockets.

Fig.12 Magnetic levitation tracks could one day be used to launch vehicles into space

Further out, NASA is developing a plan to launch the air-breathing rocket vehicle by using magnetic levitation (maglev) tracks. Using maglev tracks, the vehicle will accelerate to speeds of up to 600 mph before lifting into the air. Following liftoff and after the vehicle reaches twice the speed of sound, the air-augmented rockets would shut off. Propulsion would then be provided by the air-breathing rocket vehicle, which will inhale oxygen for about half of the flight to burn fuel. The advantage of this is it won't have to store as much oxygen on board the spacecraft as past spacecraft have, thus reducing launch costs. Once the vehicle reaches 10 times the speed of sound, it will switch back to a conventional rocket-powered system for a final push into orbit. Because it will cut the weight of the oxidizer, the vehicle will be easier to maneuver than current spacecraft. This means that traveling on an air-breathing rocket-powered vehicle will be safer. Eventually, the public could be travelling on these vehicles into space as space tourists. The Marshall Center and NASA's Glenn Research Center in Cleveland are planning to design a flight-weight air-breathing rocket engine in-house for flight demonstration by 2005. That project will determine if air-breathing rocket engines can be built light enough for a launch vehicle.

8. ADVANTAGES

1. Because the former use oxygen from the atmosphere, they require less propellant--fuel, but no oxidizer--resulting in lighter, smaller and cheaper launch vehicles.

2. To produce the same thrust, air-breathing engines require less than one seventh the propellants that rockets do.

3. Furthermore, because air-breathing vehicles rely on aerodynamic forces rather than on rocket thrust, they have greater maneuverability, leading to higher safety flights.

4. As they are efficient and safe ordinary people can travel into the space.

5. The vehicles powered by air breathing rocket engine rely on aerodynamic forces rather than on rocket thrust, they have greater maneuverability, leading to higher safety.

6. The vehicles powered by air breathing rocket engine are completely reusable and can take off and land on regular air plane runways.

9. ADVANCED SPACE TRANSPORTATION PROGRAM

(PAVING THE HIGH WAY TO SPACE):

Going to Mars, the stars and beyond requires a vision for the future and innovative technology development to take us there. Scientists and engineers at NASA's Marshall Space Flight Center in Huntsville, Ala., are paving the highway to space by developing technologies for 21st century space transportation.

The high cost of space transportation coupled with unreliability is a virtual padlock on the final frontier. But, imagine the possibilities when space transportation becomes safe and affordable for ordinary people. Whether it’s living and working in space, exploring new worlds or just leaving the planet for vacation, the opportunities for business and pleasure on the space frontier are endless.

Our dreams of everyday life in space and its promise for a better life on Earth are hostage to the high cost of space transportation. That’s why Marshall Center scientists and engineers are pushing a variety of cutting-edge technologies – from simple engines to exotic drives – to reduce the cost of space transportation and open the final frontier.

Dramatic improvements are required to make space transportation safer and more affordable. Future space launch vehicles must be safer, more reliable, simpler and highly reusable. The Advanced Space Transportation Program is developing technologies that target a 100-fold reduction in the cost of getting to space by 2025, lowering the price tag to $100 per pound. As the next step beyond NASA's X-33, X-34 and X-37 flight demonstrators, these advanced technologies would move space transportation closer to an airline style of operations with horizontal takeoffs and landings, quick turnaround times and small ground support crews.

This third generation of launch vehicles — beyond the Space Shuttle and "X" planes — depends on a wide variety of cutting-edge technologies, such as advanced propellants that pack more energy into smaller tanks and result in smaller launch vehicles. Advanced thermal protection systems also will be necessary for future launch vehicles because they will fly faster through the atmosphere, resulting in higher structural heating than today's vehicles.

Another emerging technology – intelligent vehicle health management systems – could allow the launch vehicle to determine its own health without human inspection. Sensors embedded in the vehicle could send signals to determine if any damage occurs during flight. Upon landing, the vehicle's onboard computer could download the vehicle's health status to a ground controller's laptop computer, recommend specific maintenance points or tell the launch site it's ready for the next launch.

Magnetic levitation

Magnetic levitation or maglev - technologies could help launch spacecraft into orbit using magnets to accelerate a vehicle along a track. Just as high-strength magnets lift and propel high-speed trains and roller coasters above a guide way, a maglev launch-assist system would electromagnetically drive a space vehicle along a track. The magnetically levitated spacecraft would be accelerated at speeds up to 600 mph and then shift to rocket engines for launch to orbit. A 50-foot track was built at Marshall in mid-1999 for testing and design analysis of maglev concepts for space propulsion. Scaled demonstrations of maglev technology will be conducted on a 400-foot track also planned at Marshall.

Lasers and microwaves

Lasers and microwaves are among the beamed-energy propulsion concepts the Advanced Space Transportation Program is pursuing. If the energy to propel a spacecraft doesn’t have to be carried on board the vehicle, significant weight reductions and performance improvements can be achieved. Beamed-energy propulsion uses a remote energy source — such as the Sun, a ground- or space-based laser or a microwave transmitter — to send power to the vehicle via a "beam" of electromagnetic radiation. Presently, beamed energy is the most promising technology to lower the cost of space transportation to tens of dollars per pound. Research into this technology is a joint effort of the Marshall Center, the Air Force Research Laboratory Propulsion Directorate at Edwards Air Force Base, Calif., and Rensselaer Polytechnic Institute of Troy, N.Y.

NASA-Marshall plans to use electrodynamic tethers for the first demonstration of a propellant-free space propulsion system, which could lead to a revolution in space transportation. An electrodynamic tether works as a thruster as a magnetic field exerts a force on a current-carrying wire. When electrical current flows through a tether connected to a spacecraft, the force exerted on the tether by the magnetic field raises or lowers the orbit of the satellite, depending on the direction the current is flowing.

The Fastrac engine

The Fastrac engine is a 60,000-pound-thrust engine that will be used for the first powered flight of NASA’s X-34 technology demonstrator. Fastrac is less expensive than similar engines because of an innovative design approach that uses commercial, off-the-shelf parts and fewer of them. Fastrac uses common manufacturing methods, so building the engine is relatively easy and not as labor-intensive as manufacturing typical rocket engines.

NASA began full-engine, hot-fire testing of the Fastrac rocket engine in March 1999. The Marshall Center designed and developed the Fastrac engine. Full-engine testing is being conducted at NASA’s Stennis Space Center, Mississippi, Fastrac component testing continues at NASA Marshall.

The Advanced Space Transportation Program also is developing pulse detonation rocket engine technology that could lead to lightweight, low-cost rocket engines. Like an automobile engine, pulse detonation rocket engines operate by injecting fuel and oxidizer into long cylinders and igniting the mixture with a spark plug. The explosive pressure of the detonation pushes the exhaust out the open end of the cylinder, providing thrust to the vehicle. Marshall’s industry partners have designed built and successfully tested subscale pulse detonation rocket engines using hydrogen and oxygen gas as propellant.

Exotic, high-energy propulsion will be required to travel to the outer planets and other star systems. Antimatter propulsion could leap from science fiction to scientific fact. Antimatter has propelled science fiction vehicles at "warp speed" for years, and could actually power spacecraft in the new millennium. Because of its superior energy density, antimatter annihilation is often suggested as the ultimate source of energy for propulsion. Antimatter is identical to matter except that particles’ electrical charges are reversed. A proton is positive, whereas an antiproton is negative. When regular matter collides with antimatter, they annihilate each other and produce phenomenal energy. In an antimatter engine, the charged particles would be channeled out the back of a spacecraft to produce thrust. In mid-1999, the Marshall Center built a High Performance Antimatter Trap, which will store antiprotons for a 10-day lifetime. The Trap will be used in future antimatter experiments for space propulsion.

The Marshall Center is also developing fusion and fission propulsion concepts to tap their potentially high-performance capabilities. These exotic technologies could be used for human missions to Mars and beyond. An enormous amount of energy is released by fission — the splitting of one atomic nucleus into two atoms. NASA is also investigating fusion as a space propulsion alternative. The opposite of fission, fusion combines two or more lighter atoms to form one heavier atom, producing a tremendous amount of energy in the form of heat. The energy efficiency of fusion compares to a car traveling 7,000 miles on one gallon of gas.

The Marshall Center is leading NASA’s propulsion research for interstellar precursor missions that would venture over 23 billion miles from Earth. NASA is considering the launch of such a precursor mission by 2010. In addition to fission, potential propulsion concepts for interstellar missions include sails. Thin, reflective sails could be propelled through space by sunlight, microwave beams or laser beams – just as the wind pushes sailboats on Earth. Sails in space would have a very large surface area – almost a half-mile wide – but could be thinner than cellophane. While sails are not being considered for human missions, they offer low-cost propulsion for robotic probes.

The Advanced Space Transportation Program is sponsoring basic research on the leading edge of modern science and engineering, such as gravity manipulation, space and time warping and theories that might enable faster-than-light travel. NASA is examining futuristic technologies in search of a breakthrough in space transportation, similar to the silicon chip breakthrough that revolutionized the computer industry and made desktop computers part of everyday life.

10 FUTURE DEVELOPMENTS

Space Shuttle (STS) (OPERATING)

Space Shuttle (Space Transportation System) operates since April 12th 1981. It starts vertically and lands horizontally. The reusable parts are the orbiter and the two solid rocket boosters. The fuel tank burns in the atmosphere. The current fleet counts four orbiters. The Space Shuttle is currently the main (and only) US piloted launch system since the end of the Apollo flights (1975). The Shuttle is capable to carry cargo up to approx. 30 tons to and from orbit. It plays one of the key roles in building of the International Space Station

Energiya & Buran (PAUSED or CANCELED)

This equivalent of the Space Shuttle system was developed in former USSR. The orbiter (named Buran) successfully flew into space in 1988 without crew and automatically landed. The program was then paused (and probably cancelled) because of the financial problems. The central fuel tank had its own hydrogen-oxygen engines; therefore the rocket system (Energiya) wasn't depending on the piloted orbiter and could be used for carrying of very heavy and large cargo to orbit.

X-33/VentureStar (PROJECT UNDER DEVELOPING)

The Reusable Launch Vehicle (RLV) will use the aerospike engine to reach the orbit without any additional stages or rocket boosters (one stage vehicle). The vehicle will start vertically and land horizontally. Currently the half-scale demonstrator X-33 is built. The demonstrator will be purposed for unpiloted suborbital tests, the full-size version would be the pure Earth-to-orbit and orbit-to-Earth piloted space transportation system.

X-40A/X-37(PROJECT UNDER DEVELOPING)

The unmanned X-40A is used to test systems for a reusable spacecraft X-37. X-40A is an 85 percent scale model of the X-37, which eventually will be launched aboard the space shuttle and return to Earth like an airplane.

X-38/Crew Return Vehicle (PROJECT UNDER DEVELOPING)

The X-38 Advanced Technology Demonstrator is the important step to build the Crew Return Vehicle (CRV). The first space test is planned for summer 2001 from a space shuttle. The CRV will be permanently docked to International Space Station as an emergency shuttle, capable to get all 7 astronauts (the Soyuz spaceship can take 3 astronauts). The X-38 is shaped as a lifting body.

X-43 (Hyper-X) (PROJECT UNDER DEVELOPING)

The program Hyper-X has to developed, test and demonstrates "air-breathing" engine technologies for future hypersonic aircrafts and reusable space launch vehicles. It will be launched "on the nose" of the Pegasus rocket.

HYPERSOAR (UNDER DEVELOPMING)

HyperSoar is the concept of the hypersonic intercontinental passenger "jetliner" flying on the edge of the atmosphere. Jet engines are using oxygen from air to accelerate and throw the vehicle to suborbital space jump. The flight has to consist of several such jumps.

SAENGER

The German Saenger (Sänger) is the concept of a two-stage reusable space transportation system. It consists of the carrier Saenger and the piloted orbiter Horus. The second configuration has to launch the non-reusable unpiloted cargo orbiter Cargus instead of Horus.

Hermes (PROJECT CANCELED)

Hermes was the European project of the small piloted shuttle launched on top of the Ariane-5 rocket. The project was canceled in 1992.

Hope/Hope-X/Hope-XA (PROJECT UNDER DEVELOPING)

The Japan project Hope had to be an unmanned supply carrier for Japan module of the International Space Station. The original concept was restricted to a new, smaller 16 meter long Hope-X, which has to fly on top of the H-2A rocket. The Hope-XA, a slightly larger cargo-carrying capable shuttle based on Hope-X, is also mentioned. The Hope-X will test the technologies needed to develop a single-stage rocket plane by 2010. (A conceptual study proposes the 50 meter long, a 20 meter wingspan reusable vehicle capable of carrying 10 tons to a 200 km orbit or to higher orbit with an upper stage.)

11.CONCLUSION

Air breathing rocket engines has successfully completed testing. Air breathing rocket engines are more efficient and safe .They breathes oxygen from atmosphere hence reducing the weight of onboard oxygen, which in turn reduces the weight of vehicle. Air-breathing rocket engine technologies have the potential of opening the space frontier to ordinary folks.

Air-breathing rocket engines could make future space travel like today's air travel, said Hueter, manager of NASA's Advanced Reusable Technologies project. The spacecraft would be completely reusable, take off and land at airport runways, and be ready to fly again within days.

An air-breathing rocket engine inhales oxygen from the air for about half the flight, so it doesn't have to store the gas onboard. So at take-off, an air-breathing rocket weighs much less than a conventional rocket, which carries all of its fuel and oxygen onboard. Getting off the ground is the most expensive part of any mission to low-Earth orbit, and reducing a vehicle's weight decreases cost significantly.

This unconventional approach to getting to space is one of the technologies NASA's Advanced Space Transportation Program at the Marshall Center is developing to make space transportation affordable for everyone from business travelers to tourists. NASA's goal is to reduce launch costs from today's price tag of $10,000 per pound to only hundreds of dollars per pound.

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