Liquid nitrous oxide (N2O / dinitrogen monoxide / 'laughing gas') is the oxidiser of choice for hybrid rocket motors because it is benign, storable, and self-pressurising to 48 atmospheres at 17 deg C. The combination of HTPB or PMMA solid fuel and N2O is totally benign and non-toxic. It is difficult to find a rocket motor safer than one using rubber/plastic and laughing gas. It is non-explosive. The rubber / LOX combination has been rated by the Vandenberg range as a 0 lb of TNT equivalent (i.e. non-explosive), and N2O is even safer (it is used as a pressurant for whipped cream). The fuel has to be vaporized in the presence of an atomized oxidizer with a high temperature igniter in order for it to burn. Even in failure mode, it’s safe. Flow oxidizer without vaporized fuel and nothing happens. Vaporize fuel without oxidizer and nothing happens. Ignition only occurs when fuel is vaporized in the presence of an atomized oxidizer with a high temperature igniter. Principal uses are as an anesthetic in short-duration surgical operations (prolonged inhalation can cause death) and as a propellant for food aerosols. It is most commonly produced by decomposition of ammonium nitrate.

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