Technical note | Small Scale Drop Tower Test for Practice Torpedo Impact Modelling

ABSTRACT

 A small scale drop tower arrangement was used to simulate the dynamics of a practice torpedo impacting a submarine's pressure hull. This experiment provides a set of reference data against which numerical impact models can be tested. To ensure structural similarity, the model hull form used in the experiment is of a T-stiffened cylindrical section. The hull form and the impacting nose shell of the model torpedo have both been designed to deform under drop tower impact loads. To broaden the parameter range of this experiment and thus present a stronger test for numerical impact models, three impact velocities were used with three model hull forms - steel plate with and without stiffeners and aluminium plate without stiffeners. A qualitative comparison of the results for the three model hull forms shows that stiffeners tend to limit the extent of the dent, that aluminium plate has a greater elastic response than that of both stiffened and unstiffened steel plate, and that the nose is flattened for impact against the steel hull form but dimpled for impact against the aluminium hull form. Experimental data presented in this report includes the tensile properties of the nose, hull and stiffener materials, dimensional scans of the deformed noses and hull plates, cross-sections taken through the impact dents, and high speed video and kinematic data of the fall, impact and rebound of the model torpedo nose from the hull form. A supplementary digital data-set is available for the numerical modelling of this experiment.

EXECUTIVE SUMMARY

Practice torpedoes may be used as part of the training system for undersea warfare. A practice torpedo behaves in most respects as its warshot counterpart but it is not fitted with an explosive warhead and it is designed not to hit its target. However, if an accidental impact does occur, the concentrated transient load may pose a safety risk to the target vessel.

Critical safety assessments for practice torpedo impact include the response of valves and penetrators, control surfaces, ballast tanks, frame mounted equipment and the pressure hull. This report confines itself to a modelled pressure hull impact.

The simplest method to assess impact safety is to conduct a full scale test. An impractical and expensive option, the results of any one test are only applicable to a limited range of hull and torpedo combinations. An alternative and more general approach is to numerically predict the consequences of an impact.

A small scale drop tower arrangement was used to simulate the dynamics of a practice torpedo impacting a submarine's pressure hull. This experiment provides a set of reference data against which numerical impact models can be tested.

To ensure structural similarity, the model hull form used in the experiment is of a T-stiffened cylindrical section. The hull form and the impacting nose shell of the model torpedo have both been designed to deform under drop tower impact loads. To broaden the parameter range of this experiment, and therefore to present a stronger test of a numerical model, two additional model hull plate variants were used - steel and aluminium plate without stiffeners. The hull forms used with three drop heights for three impact velocities present seven unique impact combinations (not all combinations were conducted).

A qualitative comparison of the results for these three model hull forms shows that stiffeners tend to limit the extent of the dent, that aluminium plate has a greater elastic response than that of both stiffened and unstiffened steel plate, and that the model torpedo nose is flattened for impact against the steel hull form but dimpled for impact against the aluminium hull form. Experimental data presented in this report includes the tensile properties of the nose, hull and stiffener materials, dimensional scans of the deformed model noses and model hull forms, cross-sections taken through the impact dents, and high speed video and kinematic data of the fall, impact and rebound of the model torpedo nose from the model hull form section.