Spacecraft propulsion is a general term used to describe the various methods that have been used, are currently used and which may be used in the future to enable a spacecraft to accelerate and decelerate while in the earth’s atmosphere or while traveling in outer space. Current and historic spacecraft propulsion systems generally fell into one of two categories; solid fuel systems and liquid fuel systems. Future spacecraft may be powered by nuclear, electromagnetic, or ion propulsion systems.
Many believe that solid fuel rockets were first used as weapons as early as the 13th century and all of the rockets developed until the early 20th century were powered by solid fuels. Solid fuel propulsion systems are generally less volatile than liquid fueled propulsion systems, which can make them easier to store for extended periods of time and make them safer to work with. The downside of solid fuel systems is that once they are ignited, they cannot be shut down until all of the propellant has been burned.
The inability to turn off the engine when needed has prevented the use of solid fuel systems as the primary basis for spacecraft propulsion systems, which generally require the ability to start and stop the engines when needed. However, solid fuel systems have found a consistent place as part of a spacecraft’s launch propulsion system. Solid rocket boosters have consistently been a component of the launch systems for the Russian space program since the launch of Sputnik I in 1957. The United States has also used solid fuel boosters for its unmanned spacecraft program since the late 1950s and the Space Shuttle system utilized the largest solid rocket boosters used for manned spaceflight to date.
The first liquid fueled rocket was launched by American scientist Robert Goddard, considered to be the father of modern rocketry, in the late 1920s. Goddard believed that liquid-fueled rockets provided more power and were more efficient than their solid fueled counterparts. The liquid fueled rocket paved the way for the development of larger and more powerful rocket engines and the propulsion systems that would one day usher in the Space Age. Liquid fuel propulsion systems use a fuel, such as liquid hydrogen, kerosene or alcohol and an oxidizer such as liquid oxygen. The oxidizer provides the oxygen necessary to ignite and burn the fuel, which in turn enables a spacecraft’s propulsion system to work in the oxygen-less environment of space.
Many experts agree that the manned exploration of the solar system will require future spacecraft propulsion systems that are based on technologies such as ion or nuclear power that may be more effective and efficient and require less fuel than current spacecraft propulsion systems. Ion engines essentially create an electric field by ionizing a gas. The ions, or charged atoms, are then forced out, creating thrust. Nuclear spacecraft propulsion systems would work through the use of a nuclear reactor that heats a liquid fuel, such as liquid hydrogen, and forces it out of the engine, creating the necessary thrust.