Detonation of a thin explosive layer in evacuated tubes

Detenation of thin layers of dispersed primary and secondary high explosives (HE) on the outer surface of glass and plastic tubes $0.6$–$3$ mm in diameter was examined at an initial air pressure inside the tube of $0.1$ MPa to $30$ Pa. It is shown that, under these conditions, the air practically does not influence the detonation velocity, which for secondary explosives (PETN, RDX, and HMX), is lower than or approximately equal to the Chapman–Jouguet detonation velocity $D_{\mathrm{CJ}}$ for a homogeneous mixture of the same substances. Experiments with a primary HE (lead azide) revealed regimes with a wave velocity higher than $D_{\mathrm{CJ}}$ and a varying reaction zone pattern. When tubes containing a layer of a secondary HE were filled with an explosive gas mixture, waves of a hybrid detonation with a velocity both higher and lower than that in the evacuated tubes was observed. In tubes with diameter $2$–$3$ mm, detonation proceeded in a spinning regime over the entire range of the initial pressure and at a velocity higher than $D_{\mathrm{CJ}}$. It is concluded that in the evacuated tubes with a thin HE layer on the walls, ignition is transferred by the stream of hot detonation products moving at the head of the detonation wave.