Is the jet effect fact or fiction?

Kenneth A. Rahn, Sr.
Center for Atmospheric Chemistry Studies
Graduate School of Oceanography, University of Rhode Island
Narragansett, RI 02882-1197

18 August 1998

Submitted to Lancer’s Conference "JFK: His Life, His Death, His Legacy"
November 19-23, 1998, Dallas Grand Hotel

    Since Nobel laureate physicist Luis Alvarez first proposed the "jet effect" as a mechanical way to explain JFK’s backward lurch after being hit by the fatal bullet, it has been extremely controversial within the community of assassination researchers. I have followed this discussion very carefully because the lurch was the thing that first interested me in the assassination. Here I wish to present the results of five years of study.
    My data base is Josiah Thompson’s classic measurements of JFK’s movements published in "Six Seconds in Dallas." I first recognized the inevitable experimental noise and removed part of it by carefully smoothing the data. This revealed five sequential accelerations of JFK (forces that acted on him) from Zapruder frames 310 through 330: a quick snap forward at 313, an equally quick lurch rearward at 314, a slow, steady acceleration rearward in 315–318, a forward push in 319–324, and an increasing forward acceleration in 326–330. By multiplying the masses of head or body by the accelerations, it was easy to determine which of these forces were quick enough and strong enough to have come from bullets. The quick forward snap in 312–313 was among them.
    To see whether this snap was numerically compatible with a Mannlicher-Carcano 6.5-mm bullet fired from 100 yards away and 60 feet above the road, one need only insert the appropriate values for the variables into the equation for conservation of linear momentum, or better yet, the corresponding equation for angular momentum. The results of these calculations will form the second part of the talk.
    The core of the presentation, however, will be an assessment of whether the two backward lurches, the quick one in 313–314 and the longer, slower one in 315–318 could have been caused by a shot from the knoll or vicinity or by a jet effect from a rear hit. For various reasons, these problems are much trickier to address than the forward snap. They will be treated separately, first the range of effects from a frontal or side shot with more than 300 types of rifle and handgun ammunition, and then the range of jet effects that could have been created by a Mannlicher-Carcano fired from the Depository. To add some spice to the talk, I will briefly examine whether the handgun claimed to have been fired by James Files from the knoll could have created the lurch.
    The calculations for the jet effect require that the simultaneous equations for conservation of momentum and energy be set up and solved in both the linear and angular domains. I have done this in seven steps of progressively increasing complexity and realism, and will show the results. The last steps reproduce—for the first time, I believe—the magnitude of the actual jet effect. My talk will conclude by discussing this magnitude, how much it contributes to the overall lurch, and the various assumptions that are required to produce such a jet effect. Only then can the reality and the significance of the jet effect be properly evaluated. The audience will then know whether the jet effect is fact or fiction.

    Kenneth A. Rahn has a B.S. in Chemistry from MIT (1962) and a Ph.D. in meteorology from the University of Michigan (1971). He is an atmospheric chemist at the University of Rhode Island, where he has been since 1973. He has been interested in the JFK assassination, and particularly its scientific aspects, since 1992. His interdisciplinary scientific research regularly uses chemistry, geochemistry, physics, meteorology, mathematics, and statistics.