12. How fast do rockets fly? What is the maximum speed? (A K2S Question)
From The Space Library
All of the propulsive energy of a rocket vehicle comes from the propellant and its exhaust velocity. The maximum, or ideal, velocity that can be obtained by a single stage rocket is roughly twice the exhaust velocity, depending on the ratio of the fuel mass to the total vehicle mass (this mass ratio is called the "propellant mass fraction"). A propellant mass fraction of 86.5 percent will provide an ideal velocity of twice the exhaust velocity, and is typical of launch vehicle stages. 63.2 percent will provide an ideal velocity equal to the exhaust velocity and this is typical of space transfer stages. Forty percent will provide an ideal velocity of about half the exhaust velocity, and this is typical of spacecraft using electric propulsion engines. The ideal velocity assumes the rocket burn takes place in space beyond the influence of gravity. It is usually calculated using the rocket equation, derived and first published by Tsiolkovsky in 1903. Multistage rockets can achieve higher velocities by adding the velocities of the upper stages to that provided by the lower stages. Even with many stages it is difficult to get more than 65,600 feet per second from a chemical rocket. However, a spacecraft swinging by a planet can get a gravity-assist and increase its velocity. Future nuclear electric rockets may be able to achieve 164,000 feet per second and, even further in the future, fusion rockets may be able to achieve 656,000 feet per second. If this is ever achieved, then humans will have the capability to travel anywhere in the solar system and return within several years of travel time.
Answer provided by John W. Cole
Question and Answer extracted from the book Kids to Space - by Lonnie Schorer
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