DOW-UAP-D48, Department of the Air Force Report, 1996

On September 10, 1996, the Research Triangle Institute submitted a technical study to the Department of the Air Force titled "Modeling Unlikely Space-Booster Failures in Risk Calculations." The report, prepared by James A. Ward Jr. and Robert M. Montgomery for the 30th and 45th Space Wing safety offices, establishes methods for predicting where launch vehicle debris would impact if catastrophic failures occurred during flight, with particular focus on unusual failure modes that could cause vehicles to deviate significantly from their intended flight path.

The document originated as part of a broader effort to assess launch-area safety risks from ballistic missiles and space vehicles. The Air Force's DAMP (Facility Damage and Personnel Injury) risk-analysis program models six possible failure response modes that affect ground safety. While most vehicle malfunctions produce debris impacts near the launch point or along the intended flight line, a small subset of failures can cause a vehicle to execute a sustained turn away from the planned trajectory. These unlikely events, classified as Mode 5 failure responses, pose risks to population centers located miles uprange or far to the sides of the launch corridor. The report examines how to mathematically model these rare but consequential failure modes.

The study documents actual historical examples of Mode 5 failures drawn from launch records spanning decades. In 1961, an Atlas missile lost stability 161 seconds after launch, resulting in impact 1,316 miles downrange and 215 miles crossrange from the target. A 1962 Atlas experienced erroneous guidance commands that produced 60 degrees of yaw deviation, eventually requiring deliberate destruction by range safety. A 1968 Atlas with booster engine failures led to asymmetric thrust that drove the vehicle into a retrofire attitude. The report details seven such cases showing that none could be explained by simpler failure classifications, demonstrating the genuine need for Mode 5 analysis. A particularly illustrative recent example involved a Joust solid-rocket vehicle that experienced aft-skirt structural failure at 14 seconds, executing a radical pitch-up maneuver that would have placed impacts far from the flight line had range safety not intervened.

To estimate failure probabilities, the authors evaluated empirical data from 532 Atlas flights, 232 Delta flights, 337 Titan flights, and 85 Thor flights conducted from the Eastern and Western Ranges through August 1996. Rather than relying on theoretical engineering estimates of component reliability, which the authors argue contain inherent subjectivity and K-factors accounting for untested environmental conditions, they employed statistical analysis of actual launch outcomes. Using exponential filtering techniques that weight recent test data more heavily than older results to account for design improvements and learning curves, the researchers computed overall failure probabilities for different flight phases. For Atlas during the first 280 seconds of flight, they estimated a 3.1 percent failure probability. Delta showed lower failure rates at 1.3 percent, while Titan showed 6.4 percent. Among the relatively rare failures observed, Mode 4 responses (near-flight-line tumbles) dominated at 86 percent of failures during early flight, while Mode 5 responses accounted for approximately 8 percent of failure cases.

The core technical challenge addressed in the report concerns two shaping constants, designated A and B, that control the Mode 5 impact density function. This mathematical function defines, for any given launch failure time and vehicle malfunction, where debris would be distributed across all possible compass directions and ranges. The authors used extensive computer simulation of vehicle trajectories to test how impacts would scatter if vehicles experienced either random-attitude failures (where thrust suddenly points in an arbitrary direction) or slow turns (where engine nozzle gimbal failures cause sustained curved flight paths). Running 270,000 to 290,000 simulated trajectories per vehicle type at ten-second intervals throughout the powered flight, they calculated impact distributions and compared these to theoretical predictions using various A and B values.

Results proved highly sensitive to assumed vehicle breakup characteristics. Testing three constant dynamic pressure limits defining when aerodynamic forces would tear apart the vehicle (5,000, 10,000, and 20,000 pounds per square foot), the researchers found that lower breakup thresholds produced higher concentrations of impacts near the launch pad. Atlas IIAS configurations showed breakup percentages ranging from near zero before 30 seconds to over 80 percent between 40 and 105 seconds when subjected to the 20,000 limit. The shaping constant A varied from 1.90 for no-breakup cases with B equal to 1,000 to 6.30 for high-B scenarios with low breakup strength. By contrast, shaping constant B proved relatively insensitive for risk calculations, provided A values were correspondingly adjusted. For example, risks computed using B values ranging from 1,000 to 5,000,000 differed by only 12 percent when A was properly chosen.

The report establishes recommended shaping constants for specific vehicles under assumed breakup conditions. For mature Atlas and Delta vehicles during early flight phases, the study recommends B equals 1,000 with A equals 3.0 to 3.5, depending on assumed vehicle strength. For Titan IV, similar values apply. The analysis also addresses how these theoretical parameters translate to actual launch-area safety. Risk calculations for sample launches show that Mode 5 risks remain highly dependent on aerodynamic breakup strength, with risks varying by factors of 20 or more depending on whether vehicles are assumed to break up at 5,000 or 20,000 pounds per square foot of dynamic pressure.

Distribution restrictions on the original document limit access to U.S. government agencies and their contractors. The file remained classified for administrative and operational reasons until declassification in May 2026. The detailed appendices contain complete launch and failure histories for Atlas, Delta, Titan, and Thor programs, with narrative descriptions of anomalous flight behavior and system failures recorded from program inception through 1996. These raw historical datasets formed the empirical foundation for all probability estimates and response-mode classifications discussed throughout the study.