In short, hydrogen yields more power per gallon. Fact Sheets Top Hypergolic propellants are fuels and oxidizers which ignite on contact with each other and need no ignition source. This easy start and restart capability makes them attractive for both manned and unmanned spacecraft maneuvering systems. Another plus is their storability they do not have the extreme temperature requirements of cryogenics. It is similar to ammonia.
Nitrogen tetroxide is a reddish fluid.
It has a pungent, sweetish smell. Both fluids are highly toxic, and are handled under the most stringent safety conditions. Hypergolic propellants are used in the core liquid propellant stages of the Titan family of launch vehicles, and on the second stage of the Delta. The higher efficiency of the OMS system is attributed to higher expansion ratios in the nozzles and higher pressures in the combustion chambers. The solid-propellant motor is the oldest and simplest of all forms of rocketry, dating back to the ancient Chinese.