Three interplanetary propulsion systems are bipropellant liquid rockets, ion engines, and VASIMR. Bipropellant rockets are preferred for interplanetary maneuvering due to their ability to turn on and off, improved specific impulse, and top speed. Ion engines have a high specific impulse and maximum speed but produce too little thrust to ascend from Earth’s gravitational field. VASIMR is the most advanced and promising system, using radio waves and magnetic fields to accelerate a propellant and shorten the journey to Mars.
There are three proposed means of interplanetary travel that get the most attention: bipropellant liquid rockets, electric motors, especially the ion engine or ion thruster, and the more experimental (but extremely promising) VASIMR, a variable specific impulse magnetoplasma rocket. , which uses radio waves and magnetic fields to accelerate a propellant. These interplanetary propulsion systems have exhaust speeds (i.e. maximum speeds) of 3 – 5 km/s, 30 – 50 km/s and 10 km/s – 300 km/s, respectively. Specific impulse (miles per gallon, basically) varies on a similar scale. To initially descend from the planetary surface, solid-fuel rockets are often used.
Bipropellant rockets have been used extensively throughout the space program and have landed humans on the moon as well as being used as the primary means of interplanetary travel for satellites. The ion engines are new and were only tested in space for the first time in 1998, aboard Deep Space I. The VASIMR engines are even newer and have only been tested on the ground. The first tests in orbit should take place in 2010.
Bipropellant rockets are preferable to solid rockets for interplanetary maneuvering for multiple reasons, the most obvious of which is that bipropellant rockets can be turned on and off whereas solid rockets are a one-time deal. They also offer improved specific impulse and top speed. The downside is that they are more expensive than solid rockets due to the pumping machinery. Solid rockets work on a more “fire and forget” principle, just like bottle rockets: you fire them, they just fire. Bipropellant rockets have many more moving parts. However, for interplanetary propulsion, they are usually considered standard. These rockets combine a fuel and a high-pressure oxidizer using turbopumps to produce thrust.
Ion engines work in a similar way to particle accelerators: they ionize some substance, usually argon, mercury or xenon, and accelerate it out of a nozzle using a powerful electric field. This takes advantage of the charge-to-mass ratio of the ions to produce thrust. Ion drives are intended only for interplanetary travel because they produce too little thrust to ascend from Earth’s gravitational field. Unlike the ion engines depicted in Star Wars, real ion engines take many weeks or months to accelerate to a reasonable speed, but they have a very high specific impulse and maximum speed compared to conventional rockets, making them attractive.
VASIMR is the most advanced, yet fully feasible proposed interplanetary propulsion system. As stated earlier, the system uses radio waves and magnetic fields to accelerate a propellant, usually hydrogen. A magnetically induced “choke” allows for a variable nozzle flow, hence the variable specific impulse component of the VASIMR acronym. These systems are the most promising, able to accelerate continuously and could shorten the journey to Mars from years to about 8 months. The VASIMR was originally developed during nuclear fusion research.
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