Effects of the Trapping Criteria on the Severity of the Resultant Yield Histogram for Blast DFO Risk Assessment

Abstract

Blast Distant Focusing Overpressure (DFO) risk is one of the standard risks that are routinely analyzed and mitigated by Range Safety for Expendable Launch Vehicle (ELV) launches. The blast DFO risk, in terms of probability of casualty and casualty expectation, is calculated on the day of launch based on the prevailing meteorological conditions, and it is then compared against the pertinent launch commit criteria to determine the Range status, i.e. Green/Go vs. Red/No Go. While there are many inputs affect the blast DFO risk results, there is only one input that pertains to the launch vehicle itself. This input is called yield histogram. The yield histogram depends primarily on the launch vehicle configuration and contains information on the expected TNT equivalent yields and their respective probability of occurrence for different failure modes such as Malfunction Turns (MT), Loss-Of-Thrust (LOT), Catastrophic-On-Trajectory (COT), and Random Attitude (RA) failure modes. Typically, flight failures result in intact impacts of an entire launch vehicle, Flight Termination System (FTS) breakup, overpressure/explosion breakup, or aerodynamic breakup. With the exception of the intact impact response mode, other response modes are expected to violate at least one of the established mission limits that were designed to “trap” the launch vehicle when flying on an anomalous trajectory. These criteria are called trapping criteria. Examples of the standard trapping criteria are Present Position Azimuth Limits, Present Position Elevation Limits (destruct angles), Maximum Altitude, Minimum Altitude, Maximum Straight-Up Time, Q-Alpha Limit, and the Range Safety Officer (RSO) reaction time (used for trapping obviously erratic flights). Trapping criteria selection, development, and implementation require meticulousness, luculent understanding of the launch vehicle aerodynamics, awareness of the Range Safety implementation of mission rules, knowledge of the FTS, and cognizance of the launch vehicle propellant yield modeling. Since a more sever yield histogram translates to low launch availability, an accurate development of the yield histogram based on veridical trapping criteria should increase launch availability by eliminating undue conservatism and unnecessarily launch delays. This paper identifies the most significant trapping criteria, elucidates the interrelation between the trapping criteria and the severity of the yield histogram, and proves the direct effect of the trapping criteria on the yield histogram. Additionally, this paper will illustrate the notable parameters and assumptions that significantly affect the trapping criteria and the resultant breakup state vectors. Finally, recommendations will be offered to accurately develop yield histograms by selecting and implementing well-reasoned trapping criteria based on industry best practices to avoid overestimating or underestimating blast DFO risk.