L-299 New Electromagnetic Spectrum Concerns for HERO: Considerations of Material Heating, the Traditional Bridgewire Effect, and Waveform Effects on EIDs

Technological advances have resulted in the development of next generation radars that radiate extremely high levels of electromagnetic energy which is hazardous to ordnance. These advances, coupled with the trend to use more sensitive, low-power electronic circuits in the design of ordnance systems, can cause premature actuation of ordnance components known as Electrically Initiated Devices (EIDs). The electromagnetic environments (EMEs) created by these new systems are challenging the traditional test methodologies for evaluating the Hazards of Electromagnetic Radiation to Ordnance (HERO). This report provides background discussions on energy transfer, safety margins, the Maximum Allowable Environment (MAE) derived from HERO testing, and the derivation of the HERO curves as an introduction to new electromagnetic spectrum concerns for HERO. These new concerns include the applicability of the traditional bridgewire response using the half-wave dipole model as a function of frequency, material heating as a hazard, emerging radar waveforms necessitating analysis of the response times of EIDs, and considerations in the evaluation of Exploding Foil Initiators (EFIs) during HERO testing.

The half-wave dipole model plays a major role in much of the guidance provided by HERO Safe Separation Distance (SSD) curves and Maximum Allowable Environment (MAE) calculations. For the frequency range 80 MHz to 100 GHz., the MAE values are inversely proportional to the theoretical induced current on an EID configured as a matched half-wave dipole, which decreases as a function of frequency due to the effective area of an antenna. This effectively creates a cut-off frequency above which a bridgewire will become HERO SAFE.

The electromagnetic heating of materials is shown to potentially increase with frequency. This interaction has previously received little attention and existing information is limited due to its complex nature. The hazard that this effect presents to ordnance pertains to the energetic materials they contain and the material properties of the munition itself. To assess their risk to heating, the physical properties of the materials and the overall design of the ordnance system can be analyzed for the electromagnetic environment to which they will be exposed.

The longer pulse widths and higher duty cycles associated with next generation radar waveforms could allow HBW EIDs to respond to single pulses (peak-power sensitive) or exhibit a thermal stacking effect. The specific thermal response time of EIDs must be considered when evaluating these environments. Finally, while it is not considered possible to unintentionally initiate an EFI, they are susceptible to damage/degradation. The radar waveforms differ significantly from the standard signal used in determining the Maximum No Damage Current (MNDC) level during EFI qualification. Consequently, a signal replicating the radar waveform should therefore be used to establish a proper MNDC for a HERO test.