meaningful for air-breathing engines; nevertheless, it is useful for comparison with other types of engines. The highest specific impulse for a chemical propellant ever test-fired in a rocket engine was with a tripropellant of lithium, fluorine, and hydrogen. However, this combination is impractical. Lithium and fluorine are both extremely corrosive, lithium ignites on contact with air, fluorine ignites on contact with most fuels, and hydrogen, while not hypergolic, is an explosive hazard. Fluorine and the hydrogen fluoride (HF) in the exhaust are very toxic, which damages the environment, makes work around the launch pad difficult, and makes getting a launch license that much more difficult. The rocket exhaust is also ionized, which would interfere with radio communication with the rocket. Nuclear thermal rocket engines differ from conventional rocket engines in that energy is supplied to the propellants by an external nuclear heat source instead of the heat of combustion. The nuclear rocket typically operates by passing liquid hydrogen gas through an operating nuclear reactor. Testing in the 1960s yielded specific impulses of about 850 seconds (8,340 m/s), about twice that of the Space Shuttle engines. A variety of other rocket propulsion methods, such as ion thrusters, give much higher specific impulse but with much lower thrust; for example the Hall-effect thruster on the SMART-1 satellite has a specific impulse of but a maximum thrust of only . The variable specific impulse magnetoplasma rocket (VASIMR) engine currently in development will theoretically yield , and a maximum thrust of . See also Jet engine Impulse Tsiolkovsky rocket equation System-specific impulse Specific energy Standard gravity Thrust specific fuel consumption—fuel consumption per unit thrust Specific thrust—thrust per unit of air for a duct engine Heating value Energy density Delta-v (physics) Rocket propellant Liquid rocket propellants Notes References External links RPA - Design Tool for Liquid Rocket Engine Analysis List of Specific Impulses of various rocket fuels Rocket propulsion Spacecraft propulsion Physical quantities Classical mechanics Engine technology