The Blast Wave Accelerator - Feasibility Study

This paper describes a new concept for propelling a projectile to hypervelocities and demonstrates the feasibility by numerical simulations. The concept employs imploding blast waves to accelerate a projectile which travels down a launch tube. The launch tube can be open to the atmosphere or sealed and maintained at some low pressure to minimize drag. The launch tube has a liner that contains a suitable explosive or energetic material. The explosive is configured with inert annular rings so as to prevent upstream detonation. A suitable trigger detonates each explosive ring sequentially as the projectile passes. The resulting blast wave causes an elevated pressure on the aft-end of the projectile. The acceleration continues until the projectile leaves the tube. In theory, launch velocities exceeding 6 km/s are possible. INTRODUCTION Numerical simulations are conducted to demonstrate the feasibility of using the Blast Wave Accelerator (BWA) concept to meet the mission requirement defined by NASA. These mission requirements produce the following operational envelope for the BWA. • Launch mass (kg): 1000 2000 (max payload = 500) • Launch velocity (km/s): 7-9 • Peak acceleration (Kilogees [kG]): 50 250 The launcher design is constrained by economics, material strength, and explosive properties. The requirements are listed below. • Tube diameter (m): 0.5 1 • Tube length (m): 400 • Launch mass density (kg/m): 1000 4000 • Launch mass length to diameter ratio (L/D): 3-5 • Peak pressure(GPa): 2 • Explosive energy density (MJ/kg): 5 • Explosive density (kg/m): 1600 This is an initial study of the feasibility of the BWA concept. We are interested in whether the launch velocity can be obtained under the constraints. We are also interested in determining the effect of initial launch tube pressure on the acceleration. MATHEMATICAL MODEL AND SOLUTION METHOD For simplicity, the mathematical model ignores the launch tube boundary condition at the breech, i.e. we assume that the length of the launch tube is infinite. A fixed domain is used to solve the moving boundary problem by fixing the computational coordinate system on the projectile. The detonation of the explosion is assumed to be instantaneous, so that the energy and mass are released in a short time. The masses of the projectile and each explosive charge are fixed at mp = 1000 kg and me = 10 kg, respectively. The axial spacing between explosive charges is le = 0.3 m, and CP552, Space Technology and Applications International Forum-2001, edited by M. S. El-Genk © 2001 American Institute of Physics 1-56396-980-7/017$ 18.00 589 the energy density of the explosive is e = 5 MJ/kg. In this work, a constant me is assumed for all charges. In future work, le and me may change for different charges, to achieve higher efficiency. We also assume that the flow is axisymmetric and the detonation products and the surrounding gas are perfect and inviscid. The specific-heat ratio /is 1.4 everywhere. The barrel and the projectile are assumed to be rigid. The friction between the barrel and the projectile is neglected. Unless specified, the launcher is located at 3000 m above sea level, which sets the initial pressure and density of the launch tube at: p^ = 0.070117 MPa and p^ = 0.90895 kg/m. Modeling of the Explosive Charge The explosive is modeled as a mass and energy source that is released to the surrounding flow instantaneously. The volume of the explosive ring is determined by the density and mass of the explosive charge. The energy of the explosive ring is determined by the energy density and mass of the explosive charge. We assume that the mass and energy source is uniformly distributed inside the volume of the explosive ring. Projectile Geometry Two projectile shapes are considered in this initial study. The 96% subcaliber shape (Figure 1) is used for the calculations in this report. The full caliber shape (Figure 2) is used to study of the effect of the high base pressure. The dimensions shown in Figure 1 and 2 are in meters. Based on the geometry, the volume of the subcaliber projectile is 0.4975 m, and of the full caliber projectile is 0.5307 m. This yields projectile densities of 2010 kg/m and 1884 kg/m, respectively. The rear ramp angle of the projectile base is 19.5° from the tube axis.