The ongoing pursuit of space and hypersonic flight continues to expose critical gaps in the understanding of material behavior under hypervelocity impact (HVI) and hypersonic flow conditions. Such limitations pose serious risks for aerospace vehicles, spacecraft, hardened structures, defensive systems, etc. Consequently, the development of materials and systems that can endure HVIs and hypersonic flight is a major obsta- cle in the quest for sustainable space exploration, reusable air-breathing hypersonic vehicles, and enduring protective structures. HVIs ( ≥3.0 km/s) can induce severe material deformation, erosion, fracturing, frag- mentation, melting, vaporization, and sublimation. At the same time, hypersonic ( ≥Mach 5) vehicles may be subjected to intense thermal and mechanical loads. Addressing these grand challenges requires a multi- faceted and interdisciplinary approach, combining well-designed experiments with physics-based analytical and numerical modeling. Studying material behavior under HVIs and hypersonic conditions has been facili- tated by two-stage light gas gun (2SLGG) aeroballistic ranges for almost seven decades. This current study surveys over 90 2SLGG aeroballistic ranges operational since 1990 to assess global launch and experimental capabilities. The 2SLGG’s origins and research applications are explored, highlighting its significance in various fields, including shock physics, planetary science/defense, military defense, nuclear physics, hyper- sonic vehicle survivability and performance, and spacecraft micro-meteoroid/orbital debris protection. A summary of relevant HVI phenomena is presented to underscore the importance of 2SLGGs and to elucidate similarities and differences among various 2SLGG aeroballistic ranges and their supporting methods/tools. The 2SLGG’s working principles are explained, and configurations and operations are compared. Modifi- cations resulting in “three-stage light gas guns” are briefly mentioned for completeness. The full range of current 2SLGG performance capabilities is assessed with impact kinetic energies ranging from ∼10 joules to nearly 100 megajoules, and the facility survey results are used to explain the variations in aeroballistic ∗Corresponding author. J. Mike Walker ’66 Department of Mechanical Engineering, Texas A&M University, College Station, Texas, 77843, United States. Email address: telacyjr@tamu.edu (Thomas E. Lacy, Jr.) Preprint submitted to International Journal of Impact Engineering October 22, 2023 range tankage, experiment types, research applications, and diagnostic systems. Finally, an overview of 2SLGG performance prediction methods is provided, featuring notable empirical, analytical, and numerical approaches. Keywords: two-stage light gas gun (2SLGG), aeroballistic range, hypervelocity impact, hypersonics, ballistics, terminal ballistics, high-speed imaging, flash X-ray imaging, photon doppler velocimetry (PDV), planar impacts, aerothermophysics, hypervelocity launchers, nuclear/pellet injection Contents 1 Introduction and Motivation 2 2 A Brief Overview of Hypervelocity Impact Phenomena and Research 8 3 The Two-Stage Light Gas Gun (2SLGG) 13 3.1 The 2SLGG Working Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2 A Configurational and Operational Comparison of 2SLGGs . . . . . . . . . . . . . . . . . . . 21 3.3 Notable Modifications to the 2SLGG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 4 A Brief Comparison of 2SLGG Performance Capabilities 31 5 Differences in 2SLGG Aeroballistic Ranges 34 5.1 2SLGG Aeroballistic Range Tankage Assemblies . . . . . . . . . . . . . . . . . . . . . . . . . 35 5.2 2SLGG Experiment Types and Research Applications . . . . . . . . . . . . . . . . . . . . . . 39 6 Diagnostic Tools and Techniques 44 7 Performance Prediction Methods for Two-Stage Light Gas Guns 49 7.1 Empirical Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 7.2 Closed Form Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 7.3 Physics-Based Numerical Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 8 Conclusions 53 1. Introduction and Motivation For many centuries, free flight ballistic ranges were used to study high-velocity projectile dynamics and 1 impact physics. Such ranges nearly all employed some form of launch tube in which an energetic powder 2 2 was burned to generate the pressures necessary to accelerate projectiles to high velocities ( e.g., a cannon or 3 gun). The first gun originated around the 12th century in Europe or Asia [1]. Formal scientific study of 4 the physics of classical internal ballistics, projectile flight characteristics, and terminal impact effects did not 5 begin in earnest until around the beginning of the 19th century. Throughout history, a number of eminent 6 scholars worked on these ballistic problems, including Galileo, Newton, Robins, Lagrange, Poisson, R´ esal, 7 H´ elie, Serrau, Moisson, Hugoniot, Gossot and Liouville, Charbonnier, R¯ oggla, Love and Pidduck, Cranz, 8 Fowler et al. , Corner, Thornhill, Hunt, and Nelson, among many others [2]. The earliest recorded existence 9 of a single-stage laboratory gun (cannon) was in 1742 when Benjamin Robins used a ballistic pendulum 10 to measure the muzzle velocity of a projectile [3]. His experiments proved that aerodynamic effects on 11 the projectile were nonnegligible and that the existing prevailing theories for predicting projectile ballistics 12 needed significant modifications. 13 Over the next two centuries, many studies focused on optimizing projectile shapes to minimize aerody- 14 namic drag ( cf. [2, 3]). These were conducted with single-stage compressed gas or nitrocellulose powder 15 guns. In general, a single-stage gun consists of three main components: a pressure breech containing the 16 driver gas, a projectile, and a launch tube (barrel) as shown in Fig. 1a. These launchers can have overall 17 dimensions on the order of centimeters ( e.g., small firearms) to 10–50 m ( e.g., Schwerer Gustav [4]) and can 18 accommodate projectile diameters and masses ranging several orders of magnitude. Hypervelocity phenom- 19 ena were not widely considered until the 1930s when astronomers began to study planetary impacts. This 20 work gained little traction until the late 1940s [5] when the advent of supersonic jets, hypersonic rockets, 21 and missiles at the end of the Second World War necessitated the study of hypervelocity impacts (HVIs) 22 (circa. 1945) [6]. The introduction of ballistic missile technology and spacecraft shielding required laboratory 23 launch velocities of up to 6.0 km/s and >7.0 km/s, respectively. Conventional, single-stage compressed gas 24 and nitrocellulose powder guns, though, could only accommodate launch velocities up to 2.75 km/s (roughly 25 Mach 8) [6]. Significant improvements in launcher design were required in order to develop these emerging 26 technologies. 27 Rocket motors, jet engines, shock tubes, and wind tunnels all appeared to be viable tools to extend 28 the launch velocity ceiling but possessed significant limitations in fully replicating projectile/vehicle flight 29 dynamics [6]. Several launching methods were designed, developed, and implemented to accelerate objects 30 up to ∼10 km/s ( e.g., Van de Graaff accelerators, plasma guns, and electromagnetic rail guns). Other 31 launching techniques, such as magnetically launched flyers, reached velocities well beyond the scope of this 32 work (15–44 km/s) [7]. In contrast, the basic structure and working principles of the single-stage powder gun 33 provided many advantages in generating the required projectile launch conditions: ( i) the solid propellant 34 was relatively stable and non-toxic, ( ii) the propellant ignition was more reliable compared to certain rocket 35 3 fuels, ( iii) the high-pressure combustion gases were safely contained within the gun barrel, ( iv) the muzzle 36 velocity was easily controllable and repeatable, ( v) the projectile base pressure could be maintained for 37 a relatively long duration, and ( vi) the high achievable projectile accelerations enabled launchers to be 38 relatively small and easily manufacturable [8]. 39 During the 1940s and 1950s, Crozier and Hume [2, 6], Charters et al. [8], and Slawsky et al. [9] 40 conducted groundbreaking research that involved adding an extra stage to a single-stage gun. This innovation 41 dramatically raised the launcher’s velocity ceiling while retaining its original benefits. The resulting novel 42 apparatus soon became known as the two-stage light gas gun (2SLGG). In the nearly eight decades since its 43 inception, the 2SLGG has been reliably used to launch objects to velocities ranging roughly 2.0–10.0 km/s. 44 Even higher velocities have been reached with modifications to the 2SLGG, creating so-called “three-stage 45 light gas guns” (3SLGGs). The single-, two-, and three-stage guns are all pressure-driven launchers. Single- 46 stage launchers rapidly propel projectiles down a launch tube (barrel) using high-pressure gas, generated 47 either through controlled combustion or mechanical compression and initially contained within a pressure 48 breech [2, 10]. 2SLGGs employ the single-stage launching technique to accelerate a (usually) consumable 49 piston down a “pump tube,” rapidly compressing a light gas to extremely high pressures and temperatures. 50 This high pressure “working” gas is then released viaa single-use, rapidly opening valve and accelerates 51 a projectile to hypervelocities (Fig. 1b). A comprehensive discussion of 2SLGG operating principles is 52 included later in this paper. The 3SLGG (or “modified 2SLGG”) uses yet another stage (see Section 3.3) 53 to achieve even higher velocities (Fig. 1c) [11]. The conceptual schematics presented in Fig. 1 highlight key 54 similarities and differences between each gun by clearly indicating all components, including the pressure 55 breech, driver gas, projectile, launch tube (barrel), pump tube, piston, working gas, diaphragm, and flyer 56 plate launcher [6, 12, 13]. The achievable impactor velocity-scale ranges for single- and multi-stage guns are 57 compared to some other relevant launching methods in Fig. 2 [7, 14]. To date, multi-stage gas guns remain 58 the predominant technique for launching macroscale projectiles to velocities exceeding roughly 5 km/s. More 59 details on these multi-stage launchers can be found in Refs. [6, 12, 13, 15]. 60 Early ballistics, hypersonics, and HVI research led to over 70 years of scientific advances, starting with the 61 inception of the 2SLGG [5]. Swift [6] loosely categorized this period into five eras based on the significant 62 scientific contributions and corresponding political environment: the Early Days (1945–1957), the Space 63 Race (1958–1969), the “Dark Ages” (1970–1977) the Cold War (1978–1991), and the Commercial Space Age 64 (1996–present) [6]. Figure 3 displays a graph summarizing the findings of a previous study that examined the 65 temporal distribution of high-velocity and hypervelocity related publications over a period of 70+ years [5]. 66 The figure has been adapted to emphasize significant 2SLGG-related historical milestones, as they correlate 67 well with scientific advancements made during this period [6, 11, 13, 16–32]. In the Early Days, research 68 4 Figure 1: Conceptual schematics of a (a) single-, (b) two- [6], and (c) three-stage gun (“modified two-stage gun”) with key elements/components highlighted to indicate differences [12, 13]. was primarily focused on developing and optimizing the 2SLGG, with an emphasis on maximizing muzzle 69 velocity. The first peak in scientific productivity was largely motivated by the Space Race, during which the 70 first NASA 2SLGG was developed [17], the Aeroballistic Range Association (ARA) was founded [18], and the 71 largest 2SLGG to date was constructed [19]. This surge in scientific output was succeeded by a significant 72 dip that began and lasted throughout much of the Cold War. Only near the end of the Cold War was there 73 another strong peak in scientific productivity due to the declassification of relevant data [5]. This nearly 25 74 year period of maximum scientific discovery aligned with the founding of the Hypervelocity Impact Society 75 (HVIS) [22] and the establishment of two commercial 2SLGG manufacturers, Thiot Engineerie and Physics 76 Applications, Inc. (PAI) [23, 24]. The noticeable decline in the number of publications after approximately 77 the year 2000 likely resulted from a shift in defense funding towards the War on Terror, a change that was 78 largely driven by the September 11, 2001, attacks on the World Trade Center. Following the Cold War 79 and post-Cold War periods, 2SLGG research has been primarily motivated by commercial space efforts, 80 hypersonic technological developments, and the pursuit of ever-higher velocities. The need to develop next 81 generation protective structural concepts and materials essential to ensure safe space travel and enable 82 5 Figure 2: Impact velocity as a function of impact size for common impact testing techniques, with lines indicating constant characteristic strain rates. Techniques include single-stage gas gun (SSGG), single-stage powder gun (SSPG), two-stage light gas gun (2SLGG), three-stage light gas gun (3SLGG), Van de Graaff accelerators (VDF), laser-driven flyers (LDF), laser-induced particle impact tests (LIPIT), rocket sleds, rail guns, and plasma guns. The hatched shaded regions of the same color as the all-caps labels represent the specific velocity-scale domains in which the corresponding techniques are used. Adapted from Ref. [14]. hypersonic vehicle operations is anticipated to lead to an increase in scientific productivity over the next 83 decade and beyond [33–42]. 84 Ultimately, the launcher is but one part of an “aeroballistic range.” Range tankage, instrumentation, data 85 acquisition systems, and various supporting equipment make an aeroballistic range a powerful tool to study 86 a variety of scientific phenomena. Depending on the research application, typical launch packages can range 87 from simple geometries, such as spheres, cylinders, and cubes, to complex scale models of spacecraft with sizes 88 ranging from 100 microns in diameter and weighing a few micrograms [43] to 175 mm in diameter weighing 89 several kilograms [44]. Velocities of 7 km/s are routinely achieved, and specially designed launch packages 90 have been reportedly accelerated to roughly 11 km/s [31]. Unique projectiles have carried on-board diagnostic 91 instrumentation, multiple bodies have been packaged to launch simultaneously, and even liquids and powders 92 have been successfully launched [6]. These special use cases would not be possible without a unique feature 93 of the 2SLGG: the ability to optimize the loading parameters to vary the projectile acceleration profile 94 while still producing the same muzzle velocity. Instrumentation, diagnostic equipment, and data acquisition 95 systems are constantly evolving due to technological advancements and are typically specific to the types 96 6 Figure 3: A temporal distribution of scientific publications on the experimental characterization, theoretical modeling, and numerical simulation of high-velocity and hypervelocity impacts, with an emphasis on key historical milestones. The figure was adapted from previous work by Signetti et al. [5] and Swift [6]. Key events were sourced from Refs. [6, 11, 13, 16–32]. of experiments that a given facility was designed to conduct. Some currently employed diagnostic methods 97 include ultra-high-speed videography, schlieren and shadowgraphic imaging, photon Doppler velocimetry 98 (PDV), flash X-ray radiography (FXR), velocity inteferometer system for any reflector (VISAR), and laser 99 velocimetry (see Section 6) [45–49]. The sensitivity and function of the instruments along with the data they 100 provide determine the quality and usefulness of the actual scientific findings produced by a given facility. 101 Establishing a 2SLGG aeroballistic range is particularly challenging from time and cost perspectives due 102 to the intrinsic complications associated with the operational aspects of the 2SLGG itself, safety concerns (use 103 of explosives, high-pressure gas, impact ejecta, etc.), and experimental turnaround time. These difficulties 104 are exacerbated by the need for megahertz-rate triggering, timing, and diagnostic systems to capture impact 105 events in-situ . 2SLGG performance largely depends on interactions between pump tube piston mass, driver 106 gas pressure and composition, initial light working gas pressure, projectile release pressure, and projectile 107 mass. Additionally, structural features of the launcher such as chamber volume, pump tube and launch tube 108 length and diameter, and convergence angle of central breech play a role. Peak light gas pressures can be 109 intense enough to shatter projectiles or even deform/rupture gun components [6]. These considerations do 110 not include the time and cost invested in designing experiments (targets, projectiles, test matrices, etc.) 111 and additional experimental capabilities or performing pre- and post-impact characterization. Of course, 112 7 these difficulties grow nonlinearly with the scale of the range and complexity of the experiment. Hence, a 113 significant portion of 2SLGG-related research efforts have been dedicated to designing, developing, installing, 114 calibrating, using, and improving equipment and capabilities (see, e.g., [50–56]). 115 Due to the vast array of potential research applications of 2SLGGs, there is a considerable diversity of 116 aeroballistic range capabilities and configurations. Furthermore, the research demands and experimental ca- 117 pabilities are in a state of rapid evolution, with new developments and advancements emerging continuously. 118 Several previous studies have attempted to catalog aeroballistic ranges, diagnostics, and research associated 119 with 2SLGGs worldwide (see, e.g., [6, 57, 58]). These studies, however, were either limited to a few select 120 aeroballistic ranges or were conducted over 50 years ago. Consequently, the present work aims to serve as an 121 extensive review of 2SLGG aeroballistic ranges operational as of 1990 to expand the scope of known facilities 122 and capabilities. A more complete understanding of the available launching and diagnostic capabilities facil- 123 itates and motivates scientific discovery. In addition, relevant research activities are briefly summarized to 124 better understand the diversity in aeroballistic ranges. Sources for this survey include journal publications, 125 conference proceedings, technical reports, textbooks, websites, and dissertations/theses, with a percentage 126 breakdown of the sources provided in Figure 4. Relevant information from over 90 2SLGG aeroballstic ranges 127 was compiled to provide a comprehensive overview and reference of modern aeroballistic ranges as of 2023. 128 HVI phenomena are first briefly discussed. Physical limitations with single-stage guns are used to motivate 129 the working principles of 2SLGGs. The method by which 2SLGGs accelerate projectiles to hypervelocities 130 is then presented in detail with