Gas heated by a chemical reaction provides thrust. Cargo transported by rockets is called payload. The ratio of cargo mass to the total mass of the rocket including its cargo and propellant is called payload fraction. Its value ranges from 6 percent for liquid propellant rockets to 0.2 percent for solid propellant rockets. The minimum mass is 10 tons.


If we ignore gravity and aerodynamic drag, the final velocity of a rocket equals:

V = (exhaust_gas_velocity) natural_logarithm (cargo_mass / total_mass)

The total_mass includes structural parts, propellant, and cargo. According to the above formula, which is know as the rocket equation, a high velocity of exhaust gas is needed to launch massive cargo. Rocketeers often talk of specific impulse, which is measured in seconds and is proportional to the exhaust gas velocity. A specific impulse of one second corresponds with the exhaust gas velocity of 9.8 m/s. The maximum velocity of the exhaust gas is about twice its

speed of sound:

Umax = A0(2/(G-1))0.5

A0 is the initial speed of sound of the exhaust gas
G is the ratio of specific heat at constant pressure to specific heat at constant volume
The high exhaust gas velocity calls for a hot gas having low molecular mass. The extreme temperature of the exhaust gas is the main cause of the high cost and high failure rate of rocket launchers. To maximize the specific impulse, some researchers attempt to build rockets propelled by pure hydrogen heated either by electric current, or a laser, or microwaves, or a nuclear reactor.

There are five types of chemical rockets:

  1. Liquid propellant rockets burn a mixture of liquid fuel and liquid oxidizer, e.g., hydrogen and oxygen. They have a high specific impulse (350-540 seconds) but require expensive turbopumps to feed fuel and oxidizer at a high pressure to the combustion chamber. The thrust-to-weight ratio of the Space Shuttle main engine is about 70. Russian NK-33 engine's thrust-to-weight ratio is approximately 125.

    Profile of liquid propellant rocket engine

    Profile of liquid propellant rocket engine

    Roton is a liquid propellant rocket which substitutes centrifugal force for the expensive turbopumps. The entire rocket rotates about vertical axis. It looks like the Hero engine, except that it has helicopter-like blades which provide lift during flight through the atmosphere. The specific impulse is only 300 seconds because the centrifugal force is too weak to pressurize low density liquids, e.g., liquid hydrogen.

    Roton profile

    Roton profile

    More information is available at:,

    Another unique design is the catalytic decomposition rocket engine. It uses a single chemical: liquid hydrazine. Hydrazine is very toxic and unstable at high temperatures. In the presence of a catalyst, hydrazine decomposes into nitrogen, ammonia, and hydrogen. The specific impulse is 240 seconds. This reliable rocket engine controls the attitude of communications satellites and the roll of the upper stages of rocket launchers.

  2. Solid propellant rockets burn a solid block made of fuel, oxidizer, and binder (plastic or rubber). The block is called grain. Ammonium perchlorate oxidizer and other chlorine compounds are toxic, corrosive, and damage the ozone layer. Ammonium nitrate oxidizer is hygroscopic, but is usually more desirable, because it is safe, cheap, and smokeless. Solid propellant rockets are inexpensive, but have a low specific impulse (200-260 seconds), and cannot be throttled. In other words, the rocket cannot be stopped; it burns until all the grain is exhausted. When used in outer space, they may produce space junk in the form of micrometer-size aluminum oxide particles and centimeter-size slag.

    Solid propellant rocket profile

    Solid propellant rocket profile

  3. Hybrid rockets burn a mixture of solid fuel and liquid or gaseous oxidizer, usually synthetic rubber and oxygen. The rubber is perforated to ensure thorough mixing of the fuel and oxidizer. Hybrid rockets are exceptionally safe. They almost match the high specific impulse of liquid propellant rockets, and require only half the number of expensive turbopumps. Most designs forgo turbopumps; liquid oxygen is fed into the combustion chamber by tank pressure.

    Hybrid rocket profile

    Hybrid rocket profile

    Hybrid rocket page at AspireSpace.
    Hybrid rocket page at Marshall Space Flight Center.
  4. Inverse hybrid rockets burn a mixture of solid oxidizer and liquid or gaseous fuel. They are much less popular than hybrid rockets because the liquid fuel is highly flammable.
  5. Pulse detonation rockets periodically detonate a mixture of liquid fuel and liquid oxidizer in a straight tube that has one end closed. Because the mixture is injected into the tube at a low pressure, turbopumps are not needed. Detonations do not bode well for the durability of these novel rockets. The specific impulse is about 10 percent higher than that of the liquid propellant rockets.


K. K. Kuo and M. Sommerfield, (eds.) Fundamentals of Solid-Propellant Combustion, AIAA, 1984.

George P. Sutton, Rocket Propulsion Elements, 5th edition, Wiley-Interscience, 1986, ISBN 0-471-80027-9.

Y. M. Timnat, Advanced Chemical Rocket Propulsion, Academic Press, 1987.

Atmospheric Effects of Chemical Rocket Propulsion, AIAA, 1992.

Dieter K. Huzel and David H. Huang, Modern Engineering for Design of Liquid-Propellant Rocket Engines, AIAA, 1992, ISBN 1-56347-013-6.

There are four newsgroups devoted to rocket launchers:
History of rockets:
W. Von Braun, F. I. Ordway III, and D. Dooling, Space Travel. A History, Harper and Row, 1995.
A brief history of rockets by James M. Dumoulin.
History of rocket launchers by Mark Wade.

Curator: Al Globus
If you find any errors on this page contact Al Globus.
Space Settlement hompage

This site was hosted by the NASA Ames Research Center from 1994-2018 and is now hosted by:

National Space Society