How Air-breathing Rockets Will Work

In a conventional rocket engine, a liquid oxidizer and a fuel are pumped into a combustion chamber where they burn to create a high-pressure and high-velocity stream of hot gases. These gases flow through a nozzle that accelerates them further (5,000 to 10,000 mph exit velocities being typical), and then leave the engine. This process provides thrust for the spacecraft.

If you've read the article on How Rocket Engines Work, then you know that the space shuttle needs 143,000 gallons of liquid oxygen, which weighs about 1,359,000 pounds. When empty, the shuttle itself only weighs 165,000 pounds, the external tank weighs 78,100 pounds, and the two solid rocket boosters weigh 185,000 pounds each. That's a total of 613,000 pounds. When you add fuel and oxidizer, the total weight of the vehicle jumps to 4.4 million pounds.

NASA has determined that it can easily drop the weight of a vehicle at launch if they were to take away the liquid oxidizer, which would quickly drop the weight of the vehicle to about 3.1 million pounds. That's still a heavy vehicle, but it would mean a huge reduction in cost of launching a vehicle into orbit.

So, if you remove the liquid oxygen, wouldn't the fuel be unable to combust and provide thrust? You have to think outside the normal operation of a conventional rocket engine. Instead of using liquid oxidizer, an air-breathing rocket, as its name suggests, will take in air from the atmosphere. It will then combine it with the fuel to create combustion and provide thrust.

An air-breathing rocket engine, also called a rocket-based, combined cycle engine, is very similar to a jet engine. In a jet engine, air is sucked in by the compressor. The engine then compresses the air, combines it with a fuel, and burns the product, which expands and provides thrust. A jet engine can only be used for up to Mach 3 or 4 before its parts will begin to overheat. In a supersonic combustion ramjet, or scramjet, an air inlet draws in air. The air is slowed and compressed as the vehicle speeds through the atmosphere. Fuel is added to the supersonic airflow, where the two mix and burn. Fuels most likely to be used with the air-breathing rockets include liquid hydrogen or hydrocarbon fuel.

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.

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.