CHEMICAL ROCKET LAUNCHER
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
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
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
microwaves, or a
- A0 is the initial
speed of sound of the
- G is the ratio of specific heat at constant pressure to specific
heat at constant volume
There are five types of chemical rockets:
- 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
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.
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.
- 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
Solid propellant rocket profile
- 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 page at AspireSpace.
Hybrid rocket page at Marshall Space Flight Center.
- 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.
- 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
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
- 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.