Specific impulse

A term used by rocket engineers to describe the efficiency of fuel. Impulse is the thrust multiplied by the burn time. Specific impulse is the impulse per kilogram of fuel.

Rockets are of two types; solid fuel - nitroglycerine (dynamite), and liquid fuel (2H2 +O2).

Liquid hydrogen and liquid oxygen are very difficult to handle and must be stored at very low temperatures. They are used because the right 2:1 mixture of H2 and O2 has a higher specific impulse than any other chemical fuel.