Detonation wave velocity and curvature of a plastic-bonded, nonideal explosive PBXN-111 as a function of diameter and confinement

Abstract

Detonation velocities and wave front curvatures are measured on various diameter cylinders of PBXN-111 (RDX/AP/Al/HTPB binder with 20/43/25/12 weight percent). The cylinders are either unconfined or encased in 5-mm-thick brass tubes. In all experiments with brass tubes (diameters from 19 to 100 mm) the detonation velocity of PBXN-111 was affected by the confinement. Steady detonation waves propagated in brass encased charges with diameters as small as 19 mm, which is about half of the 37.1 mm failure diameter for unconfined PBXN-111. The radii of curvature at the center of the detonation wave fronts ranged from 52 to 480 mm for charge diameters from 25 to 100 mm, respectively. Detonation velocity as a function of radius of curvature at the wave’s center is represented by a single curve for both cased and uncased cylindrical charges. The difference in the axial position of the detonation wave at the center of the charge and at the edge of the charge (i.e., lag distance) are between 1.6 and 6.7 mm. The angles between the detonation wave fronts and the brass/charge interfaces are between 73° and 82° while the angles at the cylindrical free surface for the uncased charges are between 61° and 64°. Calculation of this angle for brass encased charges using oblique shock equations and assuming no reaction in the shock front resulted in angles 8° higher than measured except for the 100-mm-diam charge which was in agreement. The calculated angles for the uncased charge are in agreement with the measured values.

Steady Detonation Waves in Right Circular Cylinders of Non-ideal Explosives

“…These a angles are the same on both cylinder edges as long as the detonation wave is symmetrical around the cylinder axis, which may not be exactly true for highly heterogeneous HE's [19]. A streak record of the detonation wave reaching the flat end of a PBXN-111 cylindrical charge [13,14] is given in Fig. 10.8, along with a graph of its break out distance-time results.…”

“…This approach also works for HE cylinders inside confinement tubes. Table 10.1 and Figure 10.6 give D for cased and uncased cylinders of PBXN-111 [13,14] with r o ¼ 1.79 g/cm 3. Notice that the D values for cased and unconfined PBXN-111 fit different straight lines for D versus 1/diameter. Clearly there is a large size effect on D for both of these.…”

“…10.8, along with a graph of its break out distance-time results. The wave shape can be measured accurately and a Bessel function was chosen that gives a good fit of the data [13,14]. The wave curvature at any point can be determined from the data fit but the radius of curvature at the centerline is what is used in the 2-D Wood-Kirkwood [4] detonation theory to obtain the sonic plane distance behind the detonation front which will be discussed later.…”

“…No accurate theory exists for predicting failure diameters [17]. However, [13] hydrocodes with reactive flow models that have the reaction zone correctly will accurately calculate the failure diameter [18] of near-ideal HE's.…”

“…This fact is often neglected leading to a misunderstanding of why non-ideal HE's perform better at larger sizes. It is easy to measure the steady detonation wave curvature [13,14], in a right circular cylinder, as shown in Fig. 10.7 by using a streak camera to record the wave arrival at the end of the cylinder.…”