Taming the Jet Effect
Kenneth A. Rahn, Sr.
Center for Atmospheric Chemistry Studies
Graduate School of Oceanography
University of Rhode Island
Narragansett, RI 02882-1197
14 May 1997
Prepared for submission to COPA’s
Fourth Annual National Conference
13–15 June 1997, Washington, D.C.
Through the years, the jet effect has been given a
rocky road by members of the JFK critical community. Proposed originally by
Nobel laureate Luis Alvarez as a mechanical way to explain the president’s
backward lurch after being hit by the fatal bullet, the jet effect has been
repeatedly analyzed by physicists and laymen alike and declared either
unimportant or absent altogether. Physical simulations such as those of Dr. John
Lattimer have also been declared wanting. I have followed this discussion with a
great deal of interest because JFK’s lurch and the reasons for it were the
things that originally interested me in the assassination. Here I wish to
present the surprising result of five years of study: Far from being negligible
or absent, the jet effect is actually calculated to be so large that the problem
is to understand why it is as small as it is.
My data base is Josiah Thompson’s classic measurements of JFK’s movements published in "Six Seconds in Dallas." The first critical step is to recognize the inevitable experimental noise and remove part of it by smoothing the data. This reveals a succession of five forces acting on JFK 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. Only the first two of these are quick enough and strong enough to have come from bullets.
Wound ballistics tells us that the quick forward snap can only have come from a bullet from the rear. Is the snap compatible with a Mannlicher-Carcano 6.5-mm bullet from 100 yards away and 60 feet high? A simple comparison shows that this bullet would have had more than enough forward momentum to have created the snap. In fact, all reasonable calculations predict a greater velocity of the head than is observed: roughly 5 ft s-1 vs the observed 3.3 ft s-1. The difference is consistent with a greater distance of travel than shown in 312–313 and a shorter time. This snap could also have been produced by many other weapons of similar power.
Could the quick backward lurch of 313–314 have come from a shot from the knoll or vicinity? A similar approach reveals problems caused by a bad angle (60° ) and the momentum required to move the entire body rather than just the head. A survey of more than 300 types of rifle and handgun ammunition reveals that this movement—a rearward velocity of 0.4 ft s-1—is right on the edge of the capability of handguns but easily within the range of about one-half the rifle shells. On the other hand, the next stage of the lurch—average rearward velocity of 0.8 ft s-1—is out of the range of more than 90% of the rifle shells and all of the handguns. The last stage of the lurch—average rearward velocity of 2.3 ft s-1—cannot be produced by any rifle that I could find. To spice up the talk, I will examine whether the handgun allegedly used by James Files could have done what he claimed to do.
Could this same backward lurch have been mechanical recoil from the first shot—a "jet effect"? However improbable this may seem to many, physics is clearly on its side, and with a vengeance. The second critical step to understanding these motions is to set up the simultaneous equations for conservation of momentum and energy and solve them for the two variables hardest to estimate—usually the mass of the cloud of fragments and the average velocity of the fragments. I will show the results of seven calculations of progressively increasing complexity, done with both linear and angular equations. (The third critical step is recognizing that the angular approach is the correct one.) The initial results, with the simpler scenarios, inevitably yield mechanical lurches that far exceed the observed lurch—sometimes by up to an order of magnitude or more. The jet effect truly has to be tamed! Only when the cloud of fragments is made three-dimensional—the fourth critical step—and the forward snap is calculated simultaneously and allowed to use up much of the 54.6 milliseconds of Z frame 312-313—the fifth critical step—is the magnitude of the jet effect reduced to that actually observed.
Thus we have two competing explanations for the quick backward lurch—a jet effect from a rear shot versus a direct effect from a front/side shot. Which are we to accept? Two separate lines of reasoning work toward the jet effect: "Occam’s razor" (the Principle of Parsimony), which dictates that the simplest explanation be chosen first; and the total lack of physical evidence for a second shot (no damage to the left hemisphere; fragments confined to right hemisphere; only one set of entrance and exit wounds; all analyzable fragments traced by ballistics and NAA to Oswald’s rifle; etc.).
So when all is said and done, we have the forward snap being created by a shot from the rear, the first, quick part of the rearward lurch being mechanical recoil from that same shot, and the second and third parts of the lurch being created by other forces entirely, none of which can be a second shot.
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.