Visible and Near-Infrared Spectra of the Secondary Combustion of a 152 mm Howitzer BRYAN J. STEWARD,* GLEN P. PERRAM, and KEVIN C. GROSS Department of Engineering Physics, Air Force Institute of Technology, 2950 Hobson Way, Wright Patterson Air Force Base, Ohio 45433-7765 Visible and near-infrared spectra were collected from 53 firings of three different propellants from a 152 mm howitzer. The observable muzzle flashes had 50–100 ms durations and were collected with approximately 0.75 nm spectral and 10 Hz temporal resolutions. Atomic and molecular emissions were primarily limited to contaminant species including K, Na, Li, CaOH, CuOH, and CuO. A relatively strong continuum baseline was observed from hot particulate matter and plume-scattered solar radiation. A radiative transfer model was used to demonstrate self-absorption in an optically thick plume as the likely source of significant broadening of the potassium 4 2 P 3/2,1/2 –4 2 S 1/2 doublet. Indeed, the entire O 2 (X-b) absorption band is evident in the blue wing. This feature is used to estimate range to source to within 4–9% for individual firings and 0.5% with multiple observations. Ratios of atomic line intensities were used to differentiate munitions configurations, yielding ratios of between-class variance to within-class variance of 8.6 to 18.2 using 1 to 4 atomic lines. Further reduction of the number of features by characterizing the relative atomic line intensities by a temperature parameter (T ¼ 7900–8900 K) significantly degrades class discrimination. Index Headings: Muzzle flash spectra; Optical trapping; Passive ranging; Discriminant analysis. INTRODUCTION The firing of gun systems, to include both small-arms and large-caliber artillery, results in optical and acoustic signatures that may be remotely detected at great distances. 1,2 However, in the engineering design of large-caliber gun systems, often the weapons engineer’s focus is to optimize the gun’s ballistics performance and meet operational requirements, such as projectile range, firing cadence, barrel life, etc. Firing signatures are also considered but are primarily limited to (1) minimizing the muzzle blast because of its harmful effect on nearby structures and health risks to the firing team and (2) suppression of muzzle flash. 1 The latter is a practical concern because gun firing often results in emissions that are easily visible and pose risks for detection and localization by hostile forces. 1 The term muzzle flash can refer to a number of temporally and spatially distinct phenomena, which have been character- ized previously and are only summarized here. 1–4 In typical fuel-rich gun systems, partially combusted propellant gases begin to flow out of the barrel immediately after the projectile and emit visible radiation because of their high temperatures. This is the primary flash and, because the gases cool quickly as they expand, it is localized to a very small spatial region at the muzzle. Expansion against atmosphere results in downstream shock structure that can reheat the expansion-cooled propellant gases and cause them to self-luminesce, forming the interme- diate flash. The third region – and the one typically referred to as muzzle flash or secondary combustion – results from the combustion of propellant gases after mixing with atmospheric oxygen. Of the three regions, only the occurrence of secondary combustion is not assured due to its dependence on a source of re-ignition. When it does occur, secondary combustion is the greatest source of radiation in both size and duration. 1,2 Although there has been a significant amount of research into the occurrence and suppression of secondary combus- tion, 5–7 the available literature on the spectral characteristics of muzzle flash is limited. Early studies commissioned by the U.S. Army found that less than 1% of radiated energy is in the visible and identified that the principal emissions in the visible result from electronic transitions in atomic potassium, sodium, calcium, and copper; band emissions from calcium and copper oxides and hydroxide molecules; and continuum from particulates such as soot. 2,3 Work by Klingenberg et al. confirmed that line, band, and continuum emissions result from excitation due to shock heating and exothermicity in the combusting plume. 1,4,8,9 Carbon monoxide and cyanide band emissions were also identified, and with the atomic and molecular emissions previously identified, account for nearly all of the non-continuum emissions. 4 No modern characterizations of the spectra resulting from gun firing could be found in the literature. Because of the increasing preponderance and fidelity of remote observation systems, a study of the visible spectral characteristics of the muzzle flash of a large-caliber gun is warranted. We focus on secondary combustion signatures for practical applications including monocular passive ranging and munitions discrim- ination. EXPERIMENTAL Visible through near-infrared emission spectra and visible imagery was observed for 201 firings of a 152 mm howitzer during 10–19 October 2007. Imagery of the gun firing and muzzle plumes was reported previously, 10 and various flash geometries observed with the imager are shown in Fig. 1. An Ocean Optics HR4000CG UV-NIR grating spectrometer collected spectra on 165 of the firings at 0.75 nm spectral resolution over a 200–1100 nm spectral range. The spectrom- eter’s 5 lm entrance slit was fiber-coupled to a ;30 cm diameter, ;4.5 mrad full field of view Cassegrain telescope. Instrumentation was located with a view perpendicular to the firing azimuth at a distance of 429 m, providing the telescope a full field of view of approximately 2 meters. The spectrometer acquired spectra at nearly 10 Hz with an integration time of 100 ms per spectra. Five munitions configurations were fired during the test. All munitions configurations consisted of a steel-cased, high- explosive warhead with a copper driving band, black powder igniter, and were distinguished only by composition and mass Received 23 August 2011; accepted 22 September 2011. * Author to whom correspondence should be sent. E-mail: bryan. steward@us.af.mil. DOI: 10.1366/11-06445 Volume 65, Number 12, 2011 APPLIED SPECTROSCOPY 1363 0003-7028/11/6512-1363$2.00/0 Ó 2011 Society for Applied Spectroscopy