Since the company that founded the Tennis Server was a graduate of the NASA Johnson Technology Commercialization Center, we were quite excited when we saw the following item in the March 1995 issue of NASA Tech Briefs:
One of the unexpected benefits of research in outer space is a design concept for a better performing tennis racquet. The essence of the concept is to taper the strings in such a way as to shift the center of percussion (also called the "sweet spot") toward the toe (the outer end of the racquet, farthest form the player's hand).
This concept originated more than 20 years ago in NASA studies of networks of strings for connecting platforms in outer space. Researchers found that changing the diameters of strings could radically change the stability of a network against vibration. This idea was inspired by an experiment in which a space-borne spider was coaxed to spin a good web and was observed to make its web with threads of nonuniform size.
The major difficulty in designing a high-performance tennis racquet arises from the difference between the position of the maximum speed (the toe) and the position of optimum vibration (the sweet spot). In a typical older design, the sweet spot is at the geometric center of the strung area of the racquet. The impact of the ball on the sweet spot causes minimal kick or jar to be transmitted to the player's hand. However, the average player tends to strike the ball farther out toward the toe, and to obtain maximum power on serves, it is necessary to strike the ball as close as possible to the toe. Thus, performance can be improved by moving the sweet spot outward.
In addition to increasing power on serves, tapering the strings to move the sweet spot outward improves the player's control and the feel of the racquet in the player's hand. An important element of improved control is increased stability: the racquet is less likely to twist in the player's hand on off-center shots. An important element of better feel is better absorption of vibrations; this is expecially important for players who have chronic arm problems.
The optimization of the tapers of the strings for a given racquet design is based on the fundamental physics of vibrating strings. A computer performs the optimization numerically, by use of a genetic algorithm. As its name suggests, a genetic algorithm implements a mathematical evolutionary process that resembles biological evolution in some respects and that arrives at an optimized design (disribution of mass along each string) by following a sequence of random design changes and preserving those changes that improve performance.
The string material can be nylon, animal gut, or other naturally or artificially spun threads. The sting can be attached to a conventional racquet frame. The required distribution of mass along each string can be achieved by spinning the string thicker or bonding different string materials along a central braid in the string.
A more detailed report on this technology is also available from NASA. Request NASA Tech Brief MFS-28922. This work was done by David A. Noever of Marshall Space Flight Center. Inquiries for additional information or concerning rights for the commercial use of this invention should be addressed to the Marshall Space Flight Center.