Sentence:  Although the LF<sub>2</sub>-LH<sub>2</sub> cryogenic combination is <WSD>hypergolic</WSD>, the toxicity of the fuel and the products of combustion have made the use of this system impractical. In general, the commonly used <WSD>hypergolic</WSD> propellants are considered conventionally storable, that is, they do not need special refrigeration equipment to keep them liquid. 
Meaning of the word marked with <WSD> tags:  Igniting spontaneously upon contact with an oxidiser.