Air Breathing Rocket
As it has been explained in the earlier section, a liquid oxidizer and fuel are combined and burned to produce high pressure, high velocity stream of hot gases in conventional rocket engines
Air breathing rockets use oxidizer but the source is the oxygen from the atmosphere rather than stored liquid oxygen. The spacecraft is built with intake vents that “breathe in” oxygen as the vehicle flies. This oxygen is combined with the rocket fuel and combustion takes place.
Initially rockets are used to propel until the air breathing rockets reach twice the speed of sound. At this stage, atmospheric oxygen would mix with fuel to propel the vehicle 10 to 12 times the speed of sound. Air breathing rockets are launched horizontally so that they can capture sufficient air required to break through the earth's atmosphere.
Air-augmented rockets can be used to launch the air breathing rockets. After the lift-off the rockets are turned off and the propulsion system is used to support the combustion process.
Air –augmented rockets are normal rocket engines with the exception that at high speeds it enhances the oxidization of the fuel with the air in the atmosphere. They are placed in ducts, which capture air.
The idea behind the design of air-breathing rockets is to reduce the cost of placing the spacecraft in the orbit, cut weight and increase efficiency. Currently, it costs about $10,000 per pound to launch an object into space. The aim is to reduce this price to about few hundred dollars per pound. The weight of the vehicle would be reduced by 50 percent, as it carries no oxidizer.
“Air-breathing rocket engines could make future space travel like today's air travel”. Hueter, manager of NASA's Advanced Reusable Technologies, quoted this statement.
The next propulsion technique is antimatter propulsion.