Single-Stage Modifications for the 2SLGG
In the Hypervelocity Impact Laboratory (HVIL), the two-stage light gas gun (2SLGG) has a minimum muzzle velocity of approximately 2.0 km/s for most launch scenarios. To guarantee proper functioning, a baseline performance level is necessary. This velocity range is typically adequate for addressing many hypersonic and hypervelocity impact problems. However, the impact response of a material depends heavily on projectile impact velocity, as well as other factors. Some impact effects and damage features only become visible within specific velocity ranges (often below 2 km/s). To address these cases, the HVIL 2SLGG can be transformed into a single-stage gas gun (SSGG) or a single-stage powder gun SSPG. The SSGG modification lowers the velocity range from 2.0–8.0 km/s to approximately 0.1–1.5 km/s. This modification uses the 12.7 mm bore of the 2SLGG to optimize existing infrastructure and experimental capacity. The SSPG modification can launch projectiles at speeds between 1.0 km/s and 2.7 km/s, also utilizing the 12.7 mm launch tube. These two modifications broaden the HVIL’s operating velocity range to 0.1–8.0 km/s, allowing for more extensive research possibilities in ultra-high strain rate and impact physics. Moreover, these modifications combined with the 2SLGG facilitate systematic studies of transition phenomena as velocities vary from low to hypervelocity. For reference, the figure below summarizes the working velocities of each launch configuration.
Laser velocimeter station
As the projectile travels from the blast tank to the target tank, it passes through a series of laser intervalometers (laser curtains) projected by the laser velocimeter system (LVS). The projectile disrupts the constant signal between the laser transmitter and receiver allowing the system to determine a time interval between these two interruptions. Because the coupled signals are separated by a known, fixed distance, an accurate velocity (within 2 m/s at 8 km/s) of the projectile can be determined. This system serves as the primary instrument for determining the projectile velocity.
Dynamic delay generator
One of the many challenges encountered when dealing with hypervelocity events is establishing a trigger for high-speed videography. At speeds in the hypervelocity regime and the associated MHz frame rates, slight variations in velocity can cause drastic differences in the required delay when the original trigger happens a meter away. This means that capture the full event of interest at high frame rates, a system needed to be developed that took in the real time velocity is calculated a dynamic delay time to trigger the camera.This was achieved using by inputting the signal from two laser velocimetry gates at a known spacing into a house developed code that calculated the optimal delay and output the appropriately timed camera trigger signal. Using LabVIEW’s GUI features, the code was made more user friendly by incorporating buttons for reset and ways to quickly change the target location value, as well as the option to input slight offsets to the user’s preference, and help indicators showing important information. This system can also be expanded to include additional trigger outputs at different times for future diagnostic equipment.
Ultra-high speed Camera(s)
The HVI event is captured using a series of high-speed imaging systems. The primary camera(s) is the Shimadzu Hyper Vision HPV-X2, which is capable of recording events at up to 10 million fps for 128 or 256 consecutive images. These images can then be played back and analyzed as a digital video. The imager can record 200 ns exposures every 200 ns at 1 million fps, 110 ns exposures every 200 ns at 5 million fps, and 50 ns exposures every 100 ns at 10 million fps. The camera accepts all Nikon lenses and features no ghosting. The video from this system can be used to determine projectile velocity, observe debris cloud formation, identify debris cloud expansion rates, examine shock wave development in clear/soft gel targets, and observe high-rate atmospheric events (e.g., shock-shock, shock-projectile, shock-particle interactions) .
Flash X-ray System (450 kV)
Accompanying the ultra-high and high-speed cameras, a Scandiflash dual-head, flash X-ray (FXR) system is configured to capture the HVI event. The FXR system can penetrate debris (e.g., smoke, metal, and plastics), capturing images of the HVI event in 20 billionths of a second. This is accomplished by the FXR system emitting a high-penetration and short-exposure “burst” of X-rays, producing stop-motion radiography. The FXR system will include a 300 kV pulser (Figure A), an X-ray tube (Figure B), and a control tower with a PDU 4 power distribution unit, four FXRC 4 Flash X-ray channel controllers, and one SU 8 – an 8-channel Ethernet switch (Figure C).
Ultra-high speed Schlieren Imaging
Schlieren is a proven technique that is used to study density gradients in gaseous environments. Such density gradients can be caused by a number of factors and allow for the visualization of flow features such as turbulence and shockwaves. Schlieren is achieved by the preferential blocking of refracted light from the density gradients allowing for them to be visualized in the image through the resulting intensity variations. While many variations of Schlieren systems have been developed, a simple lens type system has proven to be a powerful tool for the analysis of hypervelocity flow structures. The system developed by our team has been used to study the bow shock and expanding shock layers that are formed by projectiles travelling at hypervelocities in partial atmosphere. In addition, the interaction of this bow shock with objects in the surrounding environment, such as water droplets, have been studied. Finally, we have used our schlieren system to examine the individual shock structures formed by fragments in HVI debris clouds as well the resultant shock-fragment and shock-shock interactions.
Novel Particle Tracking Algorithms
Due to the extreme experimental conditions associated with HVIs, in-situ diagnostic capabilities must be applicable to environments with limited optical access and large standoff distances. Because high speed videography can be setup with limited equipment in a relatively simple system that provides understandable data for a broad range of experimental conditions, it is one of the most commonly used diagnostic tools in high-rate facilities. However, videography alone only provides qualitative data, meaning that the large numbers of videos produced often go without reaching their full potential. To address this issue, we have developed processing algorithms that extract data from the projectile, debris, and other objects present during HVI events. Currently, these processing codes provide time resolved sizes and 2D velocities for the objects captured. With this information estimations of the kinetic energy and momentum of both projectile and debris can be found, providing further insight to the energy transfer that takes place.
