As we have seen, the primary rocket engines used on launchers need to be big to produce enough thrust to lift the weight of the vehicle. Generally speaking, launching things vertically from Earth's surface tends to be a brute force affair, with thrust levels measured in multiples of MegaNewtons, and flight times confined to a few minutes. However, once in orbit around Earth, or in interplanetary cruise, we have a bit more flexibility, in that vehicle weight is not so much a factor so that thrust levels can be smaller and burn times longer.
To date, the vast majority of unmanned exploration of the solar system has been achieved using chemical space propulsion, and doing a simple calculation it can be shown that the best AV (change in speed) we can get from an unstaged chemical propulsion system is around 10 km/sec (6.2 miles/sec). This is on the basis of a spacecraft that consists of only the rocket engine, the fuel tanks, and the propellant; it has no payload! Although this is a rather pointless spacecraft, nevertheless it makes the point that currently spacecraft and mission design is significantly constrained by the existing status of propulsion technology. When we look at future missions to the planets, such as landing robotic and human explorers on the surface of Europa, the AV requirements are well in excess of 10 km/sec. This is a fundamental obstacle that needs to be overcome by technical developments in space propulsion, and in this section we take a brief look at some of these. Currently, there are two main contenders: nuclear electric propulsion and nuclear thermal rockets.
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