Heavy Lift Launch Vehicle

A Heavy Lift Launch Vehicle, or HLLV, is distinguished from Medium Lift Launch Vehicles (MLLV) by the mass that they can lift into Low Earth Orbit (LEO). While there is currently no specifically stated mass, it is generally agreed that rockets like the Titan IV, Atlas V, Ariane 5, Proton, and Delta IV are all considered "Heavy Lift" launch vehicles, as they're capable of lifting more than 30,000 lb (14,000 kg) to LEO, and more than 10,000 lb (5,000 kg) to geostationary transfer orbit (GTO).

Orbital requirements

HLLVs are the only rockets capable of moving heavier satellites into geostationary or geosynchronous orbit. The capability of achieving geostationary transfer orbit is critical to the placement of modern satellites, as well as to the success of future space programs going to the Moon and to Mars.

Fuel requirements

Several different fuel combinations have been used in heavy lift launch vehicles. The earliest ones were simply liquid hydrogen and liquid oxygen, which, when combined, produces a very significant amount of thrust, and whose only combustion byproducts are heat and water vapor. This approach was used for the upper stages of the Saturn V, and is still used for the main engines on some HLLVs, such as the Ariane 5, Delta IV and the Space Shuttle. It is also often used for upper-stage motors, due to its high specific impulse.

Other fuel options include those used by solid-fuel rockets, combinations of various liquid fuels such as RP-1 and liquid oxygen as used in the Atlas 5, and hypergolic fuels, such as unsymmetrical dimethyl hydrazine and nitrogen tetroxide, used in the Proton rocket.

The advantages of liquid hydrogen and liquid oxygen include the largest thrust to mass ratio (which equates to the highest specific impulse), and the absence of any toxic byproducts. On the other hand, both liquid hydgrogen and liquid oxygen must be constantly replenished as they boil off, and the time the rocket can remain on the launch pad is short due to the cryogenic handling required.

Solid-fuel rockets are generally unthrottleable, but recent advances in catalyst triggers make hybrid rockets as throttleable as liquid-fuel rockets.

The main advantage of fuels such as dimethyl hydrazine and nitrogen tetroxide is that they require no cryogenic handling, and can thus sit on the launch pad for long periods of time. Their main problem, however, is that they're highly toxic, and thus require special handling.

ee also

*Comparison of heavy lift launch systems
*Rocket
*Spacecraft propulsion

External links

* [http://www.nasa.gov NASA] - National Aeronautics and Space Administration

Further reading

* Mallove, Eugene F. and Matloff, Gregory L. "The Starflight Handbook: A Pioneer's Guide to Interstellar Travel", Wiley. ISBN 0-471-61912-4.