1. The Development of a New Small-Bore Taylor-Impact Launcher
Tyler DiStefano1 | The Cooper Union for the Advancement of Science and Art
Dr. Daniel Eakins2 | Institute of Shock Physics at Imperial College London
IREU: Georgia Institute of Technology School of Materials Science and Engineering
INTRODUCTION AND BACKGROUND
Light Gas Guns (LGGs) are used to launch metallic-plated sabots at high
velocities towards parallel metal alloy plates. The resulting collision
between the sabot and the metal plate generates shock waves that
longitudinally propagate through the material of interest. Analysis of the
wave propagation reveals material properties and can result in an
experimental thermodynamic equation of state through the Rankine-
Hugoniot equations.1
The Institute of Shock Physics has a newly built meso-scaled LGG that
launches 12.95-mm small-bore sabots. The impacting sabot velocity is
critical information for the use of the Rankine-Hugoniot relations. System
analysis and optimization is necessary to obtain predictable impact
velocities of the sabots. The analysis is focused on the time response and
optimization of the piston located within the breech of the small-bore
LGG.
EXPERIMENTAL METHODS
Experimental analysis of the current system was performed using the
LineVISAR system and HetV tools to observe the impact velocity of the
Al-1060 plate and 3.98-g sabot, respectively. The sabot projectile had a
Copper flyer plate that was lapped to ensure flatness and to eliminate any
effects of tilt within the collision. The LineVISAR system was set up using
a combination of optical mirrors and lenses with a 2.2-W laser.
Furthermore, the streak camera captured a shifted interference pattern
that indicates an impact velocity of the parallel metal alloy at the end of
the barrel. The HetV tool was installed by a system of probes at the end
of the barrel, which uses Doppler-shifted light to calculate the velocity of
the sabot as it travels along the barrel from the instant of firing. The HetV
tool was also used to track the velocity of the piston along the breech
without the presence of a sabot and target plate, where the front-loading
chamber was set to 35-bar and the evacuating reservoir was initially set
to 40- bar.
The experimental results were then compared to a series of 3D
simulations run through ANSYS Workbench/Mechanical, which modeled
the time response of the piston. Pneumatic modeling of the breech’s
internal ballistics provided a representative time constant of the system,
including realistic conditions such as friction and system jitter.2 The
simulations ran through a succession of decreased time constants, which
reflected numerous prototypes of the current system that included design
changes. ANSYS Mechanical also simulated a novel three-chamber
alternative design for the breech and was then compared to the current
results.
RESULTS AND DISCUSSION
Analysis of the current ballistics provided motivation for a redesign of the breech
with different initial conditions than the current ones used to launch the sabot. The
HetV data indicated that the piston currently displaces 5-mm backwards. This
displacement does not clear the entire pressurized valve and shows that the sabot
does attain its full potential velocity with the previously used initial conditions.
The solutions from the simulations provided insight to fill the dual-pressure system
proportional to the effective surface area corresponding to the force in the sliding
direction. The proportion between the front-loading chamber and the evacuating
reservoir is a 1.8x differential. Furthermore, this change ensures that there is a
faster time response after firing the gas gun because the operating pressures are
now near equilibrium conditions of the piston.
As the simulations were sampled across lower time constants, it reflected design
changes to the physical system. It was determined from analytical pneumatic
modeling that increasing the valve diameters from 6-mm to 10-mm increases gas
flow rate by 2.7x, thereby decreasing the previous time constant by that amount.
Moreover, the three-pressure system was deemed to have a considerably lower time
constant (τ=2-ms) than that of the dual-pressure system, where the front-loading
and reservoir chambers are pressurized to 4-bar and 𝑃 (𝑡) = 4𝑒^-t/τ, until P(t)
reaches 1-bar, respectively. As shown in Figure 4, the lower time constant results in
a faster displacement response of the piston and consequently a more predictable
sabot velocity.
SUMMARY AND CONCLUSION
A mathematical analysis and optimization of the breech’s internal ballistics
was performed using the 3-D Explicit Dynamics suite from ANSYS Mechanical/
AUTODYN. Following numerous experimental and simulated tests, it was
determined to redesign the breech of the meso-scaled gas gun from a two-
pressure system to a three-pressure system, in which the driving pressure of
the sabot is separate from the pressurized chambers that launch the piston
backwards. This change optimizes the current performance of the gas gun
such that initial pressure conditions can yield controlled collision velocities.
Future work includes the physical fabrication of the breech’s new design and
implementation into the meso-scaled LGG. The new installment will involve an
extra gas valve in the laboratory because of the switch from two-pressure
chambers to three-pressure chambers. Moreover, it is critical to calibrate the
internal ballistics of the breech’s new design to corresponding impact
velocities of the sabot in order to ensure experimental control.
ACKNOWLEDGEMENTS
A very special thank you to Dr. Daniel Eakins and the Institute of Shock
Physics for their continued advisement throughout the duration of this
summer project, and their warm welcome to Imperial College London.
Furthermore, thank you Dr. Naresh Thadhani and Dr. Valeria Milam for their
substantial support, and to the NSF for the organization and funding of the
SURF/IREP program.
REFERENCES
[1] Winter, Ron. AWE. Key Concepts of Shock Hydrodynamics. 2011.
[2] Hong, Ing. Tessmann, Richard. The Dynamic Analysis of Pneumatic Systems
using HyPneu. 1998.
Figure 4—Piston time response within the new three-pressure breech design
across varying time constants of the system.
Figure 2—Section view of the previous dual pressure system.
Figure 3—Section
view of the new three
pressure breech
Rankine-Hugoniot Relations
Figure 1—Experimental shock response from previously existing LGG
design.