Autzoo.1619 net.space utzoo!henry Mon May 3 19:08:58 1982 Saturn 5 plume The underexpanded-exhaust explanation is, I think, the correct one. There are several reasons why a rocket engine may not expand its exhaust to the ambient pressure, like constraints on how long and heavy the nozzle can be, but the big limit is that any fixed nozzle is necessarily right for only *one* ambient pressure. This means that as the rocket climbs, underexpansion inevitably occurs as the outside pressure drops. In fact, I seem to recall that there was an optional nozzle extension designed for the F-1. I don't think it ever got used, but its existence suggests that the F-1 may not have been optimally expanded even at sea level. There are engine concepts that are optimally expanded over a considerable range of pressures, but they are very different from orthodox nozzles, and as far as I know none of them has ever been used "for real". ----------------------------------------------------------------- gopher://quux.org/ conversion by John Goerzen of http://communication.ucsd.edu/A-News/ This Usenet Oldnews Archive article may be copied and distributed freely, provided: 1. There is no money collected for the text(s) of the articles. 2. The following notice remains appended to each copy: The Usenet Oldnews Archive: Compilation Copyright (C) 1981, 1996 Bruce Jones, Henry Spencer, David Wiseman.