Colino, Barbero, and Tapiador reply: We thank Alfonso Gañán-Calvo, Alan Spero, and David Wright for their careful, detailed comments on our Quick Study. Gañán-Calvo’s point about sonic sheets is pertinent; it shows that the topic is too subtle to be judiciously covered in the limited space of a Quick Study. We thank him for expanding on it for Physics Today readers. Also, because the real flow structure was unknown, we simplified the plots: The offending panel b of the figure, for which “downstream” is incorrectly used instead of “upstream,” is obviously not a precise depiction of the aerodynamics but a sketch intended for a wide audience. For a discussion of such issues as low-intensity shock waves around the suit and the complex flow patterns that expansion waves, shocks, and turbulence form past a body—downstream—at transonic or supersonic regimes, see the more detailed treatment given in references 1, 2, 3.
Spero and Wright bring attention to two other free falls that we think can hardly be compared with Baumgartner’s. The SR-71 pilot and reconnaissance system officer could have experienced a fall with a presumably horizontal, probably supersonic initial speed. Nevertheless, it is hard to tell without ejection trajectory data, fall dynamics details, or any other information except the tragic outcome.
We also know little about the jumps associated with the Vostok program. Neither example can be compared with the jump by Baumgartner—or even the one by Joseph Kittinger in 1960—not only because the two jumpers began their descent at nearly zero speed but especially because precise speed and location measurements were available. Indeed, we should thank the Red Bull Stratos project for making such quality data available.
