Liquid oxygen was the earliest, cheapest, safest, and eventually the preferred oxidiser for large space launchers. Its main drawback is that it is moderately cryogenic, and therefore not suitable for military uses where storage of the fuelled missile and quick launch are required. Liquid oxygen, as normally supplied, is of 99.5 percent purity and is covered in the United States by Military Specification MIL-P-25508. High purity liquid oxygen has a light blue colour and is transparent. It has no characteristic odour. Liquid oxygen does not burn, but will support combustion vigorously. The liquid is stable; however, mixtures of fuel and liquid oxygen are shock-sensitive. Gaseous oxygen can form mixtures with fuel vapours that can be exploded by static electricity, electric spark, or flame. Liquid oxygen is obtained from air by fractional distillation. The 1959 United. States production of high-purity oxygen was estimated at nearly 2 million tonnes. The cost of liquid oxygen, at that time, ex-works, was $ 0.04 per kg. By the 1980's NASA was paying $ 0.08 per kg.

Solid propellants have the fuel and oxidiser embedded in a rubbery matrix. They were developed to a high degree of perfection in the United States in the 1950's and 1960's. In Russia, development was slower, due to a lack of technical leadership in the area and rail handling problems. The disadvantages of solid propellants include:

• Slightly higher empty mass for the rocket stage

• Slightly lower performance than storable liquid propellants

• Transportability issues: Solid propellants are cast into the motor in the factory, unlike liquid fuel rockets which can be fueled at the launch pad. This means they have to either be: 1) limited in size to be transportable (as for the Delta and Ariane strap-on motors); 2) cast in segments, with the segments assembled at the launch base (as for Titan and the Space Shuttle); or 3) cast in a factory at the launch site (actually done for large test motors intended for Saturn V upgrades).

• Once ignited, they cannot be easily shut down or throttled. Thereafter they have to be pre-cast or milled out for a specific mission.

• Nearly always catastrophic results in the event of a failure

Advantages of solid rocket motors, many of which make them ideal for military applications:

• High density and low volume

• Nearly indefinite storage life

• Instant ignition without fuelling operations

• High reliability