Detonation Initiation by Hypervelocity Projectiles
J. Belanger
University of Minnesota
Minneapolis-St. Paul, MN USA
M. Kaneshige, J. E. Shepherd
California Institute of Technology
Pasadena, CA 91125 USA
Detonation wave experiments have been carried out in the T5 free-piston reflected-shock tunnel at the California Institute of Technology. T5 has been modified by the addition of a launch tube, extending from the nozzle throat into the dump tank, and a test section/target section assembly, which are mounted on the downstream door of the dump tank. The overall assembly of the gun tube and test section are shown in Fig. 1.
Figure 1: Overall view of T5 with gun modification and detonation test section
The high-enthalpy gas generated by T5 accelerates a 25 mm diameter nylon
sphere (about 10 grams) through the 3 m long launch tube. Passing through
the T5 dump tank, the sphere ruptures a mylar diaphragm and enters a test
section mated to the downstream door of the dump tank. The present series
of tests have used a mixture of H 
and O with N
diluent in the test
section. The kinetic energy of the projectile is absorbed by a special
catcher assembly in the target section, downstream of the test section.
Using a shock speed of 5000 m/s and helium gas in the shock tube section of
T5, the projectile muzzle velocity is approximately 3000 m/s. Within the
test section we record the projectile
velocity, blast wave pressures and image the wave field around the projectile
using a differential interferometer illuminated with a 10 ns pulse of light from a
Q-switched laser. An interferogram of the shock wave and flow field
around the projectile propagating in nitrogen at Mach 9 is shown in Fig. 2a.
Very specific conditions (see Shepherd 1994) are needed to initiate and stabilize a detonation wave on a blunt body. In previous experiments using projectiles, stabilized detonation waves have not been observed because the projectiles were either too slow or too small. In the present tests, we used a very sensitive mixture (stoichiometric hydrogen-oxygen) diluted with a small amount of nitrogen to reduce the Chapman-Jouguet wave speed (2400 m/s) to significantly less than the projectile velocity. The estimated detonation cell width of this mixture for an initial pressure of 1 bar is approximately 2 mm, a factor of ten less than the projectile diameter. An interferogram of the detonation wave and associated flow field around the projectile is shown in Fig. 2b. The transverse wave instability of the detonation is clearly visible.
Figure 2: a) Blast wave produced by projectile in nitrogen. (b)
Detonation wave produced by projectile in 2H + O + N mixture.
Projectile is 25 mm diam. sphere moving at 2900 m/s.
These results are dramatically different than the previous reacting flow results reported in the literature. Apparently, the entire flow field surrounding the projectile is subsonic. There is an enormous increase in the shock standoff distance in comparison to the inert case and a very slow relaxation of the wave toward the CJ state. There are clearly very significant implications for combustion processes in ram accelerators and other hypersonic propulsion schemes.
Acknowledgement
We are indebted to H. G. Hornung for his avid interest, advice and generosity with the T5 facility. Bahram Valiferdowsi expertly operated T5 and the staff of the GALCIT shops made many timely contributions.
References