Abstract: By using electric probe array with different lengths each other, just in the shape of hedgepig, the free surface （inner surface) velocity history of cylindrical metal tube was measured which was driven by sliding inward detonation. The relation of free surface velocity v and radius of metal tube R was obtained elementarily, and some discussions were also given. The analysis elemen?tarily reveals the influence of explosive thickness on free surface velocity, and the detonation pressure increasing phenomenon under thicker nitromethane (40 mm) condition may be the cause of Digger ini-tial free surface velocity v0 than computed value based on C-J detonation theory. The result above lays a good foundation for further quantitative measurement of the free surface velocity of axial symmetric structure under inward detonation.

Key words: detonation mechanics , inward detonation , cylindrical metal tube , electric probe , free sur?face velocity