Survivability Assessment of Current Aircraft Designs Using State﷓of﷓the﷓Art Technologies in Hydrodynamic Ram and Fuselage Deco

Table of Contents

1.0 Introduction........................................................................................................................... 2

2.0 Elements of Hydrodynamic Ram............................................................................................ 2

2.1 Shock Phase

2.2 Drag Phase

2.3 Cavitation Phase

3.0 Use of Modern Hydrocodes.................................................................................................. 3

4.0 State-of-the-Art in Hydrodynamic Ram Analysis.................................................................... 3

5.0 Proposed Research Work..................................................................................................... 4

6.0 References............................................................................................................................ 4

The hydrodynamic ram effect in fuel tanks is identified as one of the important factors in aircraft vulnerability. Hydrodynamic ram occurs due to the high pressures that are developed within a fluid when a high speed projectile penetrates a tank. The passage of the projectile through the fuel causes an intense pressure pulse to propagate in the fuel and strike the walls of the tank. This large internal fluid pressure on the walls causes severe petaling of the walls, usually at the entrance and exit points of the projectile.

Hydrodynamic ram effects have proven to be a major combat related threat to the modern aircraft. Since the fuel tanks of tactical aircraft have the largest exposed area of all the vulnerable components, engineering estimates of fuel tank response to penetrating ballistic projectiles are required in order to design more survivable tanks. Accurate simulations of hydrodynamic ram, including failure mode prediction, are useful in enhancing survivability and in guiding pretest specimen setup to ensure projectile strike and exit at critical locations, thereby minimizing the cost of expensive development tests.

The second item on the aircraft vulnerability agenda is the subject of decompression or depressurization. If for any reason the pressurization system fails, or a break occurs in the aircraft structure due to a missile or internal explosion, the result will be a decompression. A slow decompression may occur where, for example, a door seal fails, resulting in a gradual escape of internal pressure. If a rapid or explosive decompression occurs, the sudden equalization of air pressure poses a safety hazard to the aircraft.

2.0 Elements of Hydrodynamic Ram

Accurate simulation of structural damage requires accurate modeling of the energy transfer from the projectile to the fluid and structure. This requires accurate simulation of projectile kinetics and kinematics, which in turn requires accurate simulation of fluid dynamics as well as fluid‑structure interactive dynamics. There are three primary sequential phases to model in hydrodynamic ram analysis: the shock phase, the drag phase, and the cavitation phase. Each of the three phases of hydrodynamic ram contributes to structural damage and is therefore critical to account for in ballistic analysis.

The shock phase is initiated when the projectile penetrates the wall of the tank and impacts the fluid. As the impact energy is transferred to the fluid, a hemispherical shock wave centered at the point of impact is formed. This creates an impulsive load on the inside of the entry wall in the vicinity of the entry hole causing the entry wall to crack and petal.

As the projectile travels through the fluid, its kinetic energy is transformed into fluid motion as the projectile is slowed by viscous drag. A pressure field is generated as fluid is displaced from the projectile path. In contrast to the pressure field developed in the shock phase, the fluid is accelerated gradually rather than impulsively. The gradual acceleration causes the peak pressure to be much lower, and the duration of the pressure pulse to be longer.

During the drag phase, a radial velocity is imparted to the fluid and a cavity is formed behind the projectile. The subsequent expansion and collapse (oscillations) of the cavity constitutes the cavity phase of hydrodynamic ram. Significant pressure pulses can accompany the collapse of the cavity.

Until recently, it was difficult to predict the complex and dynamic interactions among threat, structure, and adjacent fuel tanks. The development of modern hydrocodes made it possible to model both solids and fluids and allowed the simulation of highly transient behaviors and large nonlinear effects. Hydrocodes offer a promising means of gaining physical insight into the overall behavior of a structure-fluid interface. The problem with prior models is the uncertainty surrounding their approximation of reality. Vulnerability models have been traditionally based on empirical data, and as more experimental data was added, models had to be reevaluated. There was also a lack of understanding of ram‑related phenomenology and damage mechanisms. Modern hydrocodes take advantage of physics‑based hydrocode technology, advanced modeling technology by considering dynamic material properties, and improved failure criteria to provide effective techniques to accurately model the hydrodynamic ram problem for aircraft design and analysis applications.

In the 1980's empirical codes such as ERAM and nonlinear analytical codes such as ABAQUS were coupled to give a complete hydrodynamic ram analysis. ERAM was used to determine hydrodynamic ram loads and ABAQUS was used as a nonlinear quasi‑static analysis tool taking the peak pressures from ERAM. The problem with this method of performing the analysis was that the ERAM analysis method can only approximate fluid loads, and these loads are then simply used as applied loads, in ABAQUS, ignoring the true coupling that actually occurs between the fluid and the structure. Using the method of images, reflections of the walls of the structure are also accounted for. The pressure due to the hydrodynamic ram effect is computed and transmitted to the ABAQUS model for the structural response.

In the 1990's, completely coupled fluid-structure interaction codes have been developed, such as DYTRAN, ALE3D, and PRONTO‑SPH, to perform the hydrodynamic ram analysis. Any hydrocode should possess the following qualities in order to accurately simulate the hydrodynamic ram problem:

1. Compressible and Incompressible Multi‑Fluid Dynamics

2. Structural Dynamics

3. Detonation Physics

4. True Fluid-Structure Interaction

5. Advanced Constitutive Models ( such as large strain metal plasticity and failure, and composites modeling)

6. Mixed Multi Materials

It is proposed to implement the current state‑of‑the‑art hydrocodes to assess the survivability of currently used aircraft designs. These hydrocodes utilize Coupled Euler‑Lagrange (CEL) techniques, such as Arbitrary Lagrange‑Euler (ALE), and Smoothed Particle Hydrodynamics (SPH) methodologies to perform hydrodynamic ram and fuselage decompression analyses. Although these new analysis approaches are computationally intensive, they provide direct modeling of the fluid and structure interaction as the failure of the structure proceeds during the impact event. In addition, the new generation of cost-efficient computers, which were not available during the times when currently used aircraft designs were developed, added in new dimensions to the analytical capabilities of modern hydrocodes.

An attempt will also be made during the course of study to develop and demonstrate structurally efficient fuel containing wing designs that contain hydrodynamic ram damage. New designs should mitigate hydrodynamic ram forces by decoupling the pressure waves between the fluid and structure. Laboratory tests will also be performed to validate new designs and concepts.

1. Bronisz, C.L., and Hirt, C.W. “Hydrodynamic Ram Simulations Using FLOW‑3D”, Flow Science, Inc. report, May 1990 (FSI‑90‑49‑1).

2. Freitas, C.J., Anderson, C.E., Walker, J.D., Littlefield, D.L., “Hydrodynamic Ram: A Benchmark Suite”, ASME Pressure Vessel Piping Conference and Symposium on Structures Under Extreme Loading Conditions, Montreal, Canada, July 1996.

3. Heitz, R.M., and Bischoff, G.H., “Small‑Arms Fire Vulnerability and Protection of Aircraft Integral Fuel Tanks and Fuel Lines”, Hydrodynamic Ram Seminar, Technical Report AFFDL‑TR‑77‑32, May 1977.

4. Hill, A.T., “Hydraulic Ram Ballistic Testing of Low Cost Composite Wing Structure”, Presented at the 37th AIAA / ASME / ASCE / AHS / ASC Structures, Structural Dynamics, and Materials Conference. Salt Lake City, Utah, April 15‑17, 1996.

5. Lee, T. W., “Preliminary Hydrodynamic Ram Investigations at Denver Research Institute”, Hydrodynamic Ram Seminar, Technical Report AFFDL‑TR‑77‑32, May 1977.

6. Lundstrom, E.A., and Fung, W.K., “Fluid Dynamic Analysis of Hydraulic Ram, IV, User's Manual for Pressure Wave Generation Model”, Prepared for Joint Technical Coordinating Group for Aircraft Survivability, Naval Weapons Center, China Lake, California, Report Jl CG/AS‑74‑T‑018, Sept. 1976.

7. Lundstrom, E.A., “Fluid Dynamic Analysis of Hydraulic Ram”, NWC TP 5227, Naval Weapons Center (Unclassified), China Lake, CA, July 1971.

8. Nunn, K., Dompka, R., Jones, L., “Design and Manufacture of Low Cost Composites ‑ Bonded Wing, Phase II, Final Report”, WPAFB WL‑TR‑93‑8009, Manufacturing Technology and Flight Dynamics Directorates. Wright Laboratory, Wright Patterson Air Force Base, Ohio. June l993.

9. Proceedings of the FAA Aircraft Hardening and Survivability Symposium, Atlantic City, New Jersey, August 1992.

Hawthrone, CA 90250-3277