The ignition of the liquid gun propellant (LGP) 1846 or XM46 by 1-μm neodymium (Nd):glass laser light has been explored
under conditions of confinement. Variation in ignition delay with energy and initial pressure are explored. Good repeatability,
millisecond delay times, and reasonable pulse energy requirements characterize the observations.
The challenges of developing laser ignition for an artillery cannon in which the black powder (BP) of the charges must be ignited are described. Laser ignition threshold behavior for BP is quantified for hot and cold temperatures and compared to ambient behavior. The threshold behavior for single grains of BP is compared to that of multiple grains more similar to that encountered in the gun. In addition to characterization of the BP ignition parameters, the influence of other materials is discussed briefly.
Metastable intermolecular composites (MIC) consisting of nanometer-scale aluminum and molybdenum trioxide have been proposed as fast initiators. A compound of this class of material was evaluated as a potential environmentally friendly replacement pyrotechnic material for lead styphnate for use in the primer of the M230 medium-caliber automatic cannon. In addition to removing the lead hazard, laser ignition would also reduce or remove certain hazards due to electrostatic or radio frequency radiation. This study was conducted with both a flashlamp-pumped Nd+3:YAG laser and a fiber-coupled diode laser. The measured threshold ignition energies of the MIC and two other inorganic pyrotechnic compounds are presented. The low ignition threshold, advances in diode laser technology, and compact size of the diode laser indicated that laser diode technology could be an ideal candidate ignition source for the M230 cannon. The candidate pyrotechnic compounds were also evaluated for suitability in laser initiation via measurement of time-to-first-light. This metric provided a measurement of the potential for achievement of the necessary action time required for proper cannon operation.
An experimental laser ignition system has been developed for the M230 cannon used on the Apache helicopter. This high-rate of fire (625 rounds/minute) chain gun provided many design challenges including a complex optical path, limited available space, high levels of shock and vibration and a maximum allowable cartridge functioning time of 4 ms. In order to satisfy this time limit with laser ignition, a subprogram was developed to explore laser sensitive, fast-acting energetic materials with good ignition transfer properties. Annual consumption rates in excess of 500,000 rounds per year in training dictated that ultimate ammunition production costs be considered in all design alternatives. Environmentally benign ("green") materials are being required by federal and state environmental regulations and standards and were factored into the new optical ignition system design. Both newly developed nano energetics and other pyrotechnic materials have been explored to fill this requirement. The development of the system and components is described in this paper from the firing of Mann barrel (single shot) fixtures to optimize cartridge performance, single-shot firing of the cannon using fiber-optic and optical-train light paths, culminating in a demonstration firing of the automatic cannon at its full rate of fire for a 10-round burst. The remaining technical challenges and future direction of the program are presented.
Medium caliber cannons, such as the Army's M230 chain gun, currently utilize a high current electrical pulse to initiate the propellant. While electrical ignition is reliable, electrical based primers are susceptible to premature ignition from EMI, EMP, or other stray or directed electromagnetic sources. Laser ignition of medium-caliber cannon systems has several advantages over the current electrically initiated ignition system. In addition to removing hazards due to electrostatic or radiated electromagnetic energy, lasers are an ideal ignition source for new primer compounds, such as Metastable Intermolecular Composites (MIC), that are potentially environmentally friendly replacements for lead styphnate containing compounds. This paper will describe our efforts to develop a laser source suitable for the M230 medium-caliber automatic cannon as used on the Apache AH-64 helicopter. We will describe early proof of concept laser systems including a fiberoptic-coupled flashlamp-pumped Nd:YAG source, direct Nd:YAG laser sources, and a full rate of fire demonstration laser that was mounted directly on the M230 housing. We will also discuss our plans and designs for a direct semiconductor laser ignition source for the M230.
In the application of laser ignition to large caliber cannons, a critical element is the window into the cannon chamber to admit the laser energy. This window must repeatedly withstand a particularly harsh environment of highly reactive high temperature combustion products from the gun propeller at pressures up to 440 MPa. Failure of the window can be caused by either thermal gradients in the window or mechanical force, or a combustion. Previous successes with single-crystal sapphire have sometimes been limited by window deterioration modes suggestive of crystalline behavior. Samples of ALON have been fabricated in the same design as the standard sapphire windows and were qualified for gun testing. This process involves a series of experiments in a closed chamber where gun propellant is burned to generate an environment similar to that inside the gun. Windows mounted in two methods have been tested. One of these windows has survived the full pre-gun test series with no visible damage.
A 14-bit scientific CCD camera has been used as a medium-speed streak camera through software modifications that preserve the high dynamic range and excellent sensitivity characteristic of these cameras. Line shift speeds as short as 8.1 microsecond(s) ec are found to have no detrimental effects on image quality as long as light intensity is kept below saturation levels. Examples of streak images from shock tube studies of reactive systems are presented.