An RDX-base explosive SPX was developed that satisfies Navy requirements for a booster no more sensitive than tetryl and can be initiated by explosive charges of very small dimensions. SPX is finely divided RDX mean particle size, 10 mu coated with 1.5 calcium stearate. The Spooner small- scale gap test SSSGT showed SPX to have nearly the sensitivity of CH-6 and significantly less sensitivity than tetryl. SPX can usually be initiated by MDF loaded with 1 grainft PETN and is reliably initiated by 5 grainsft MDF. In contrast, CH-6 cannot be initiated by 10 grainsft MDF.