Su http://astronautix.com/Mambo/ ho trovato un interessante critica all’originale progetto dello Shuttle .Si è sempre detto che con un orbiter con un sistema di lancio interamente riutilizzabile tutti gli attuali problemi non sussisterebbero.Secondo l’autore dell’intervento postato più sotto invece se si fosse seguita questa strada il programma sarebbe stato chiuso negli anni 80! E’ una critica sensata dal punto di vista tecnico? mi piacerebbe sapere che ne pensate.
“Everything would have been OK if NASA had only been allowed to develop the 100% recoverable two-stage original shuttle design instead of the compromised partially-recoverable design. One can assuredly say that if the 100% reusable shuttle had been built, the program probably would have been cancelled in the 1980’s. The SSME shuttle main engine was supposed to be 100% reusable, good for ten missions without inspection and 50 missions between overhauls. Instead it turned out to have a shorter component life than the ‘expendable’ F-1 and J-2 engines of the Saturn V that it replaced, and require 100% inspection and repair after each mission. The compromised shuttle design that flew used only three of these engines. The 100% recoverable monsters used 14. The SSME’s have flown 345 times (counting three per mission) and experienced problems resulting in engine shutdown 2 times (not counting pad aborts). That means that the 14-engine design would be experiencing an engine shutdown in 8% of the missions. Can anyone imagine the expense, the headaches, the failure modes, the unending billions that would have been expended in trying to solve such problems”?The reusable design had other severe problems. NASA’s foam problems are at least partly related to fuelling and defuelling the main tank several times in tests or scrubbed launches prior to the real thing. Each fuelling cycle brings the tank and its insulation down to -259 deg C, then back to Cape Canaveral local temperature and humidity. These thermal cycles cause expansion and contraction of the metal, freezing and unfreezing of humidity in foam voids. This cannot be avoided. What kind of problems would a 100% reusable shuttle have in going through 100’s of fuelling and defuelling cycles? Add to that re-entry cycles with a hot structure after landing? What kinds of metal or insulation failure problems would all of these contraction and expansion cycles create? It is hard to believe that this would not have been a major problem, requiring replacement of the entire booster or orbiter at regular intervals.Then there is the heat shield issue. The orbiter of the fully-recoverable design would have been had a much lower density on re-entry due to the volume of the empty hydrogen tanks. This would have lowered the load on the orbiter’s heat shield, making the engineering problem less acute. But the design NASA favored still used the same heat tiles selected for the orbiter design that flew. NASA imagined that these could be standard and glued onto the lower wing surface like bricks. It turned out, for aerodynamic reasons, each of these 27,000 tiles had to be individually shaped and machined. The problems with this heat shield design are, and remain, one of the main issues of the shuttle program. Now for the fully recoverable design, the under wing surface would have been four times greater - and the number of tiles, and the problems, that much more. This alone would have resulted in the cancellation of the fully recoverable design.
Finally, the 800 pound gorilla in the recoverable designs were the abort scenarios. The booster and orbiter were designed for vertical takeoff, and then to fly back to base after they had consumed their propellant. In case of an abort on the way to orbit, they would have to be flying with most of their propellant aboard. In abort scenarios this would have to mean wing loadings many times the design loads - undoubtedly the wings would have snapped off. This is not even to mention the problems with separation of the piggy-back vehicles in the case of max-Q or other aerodynamic loads or conditions except for the optimum design conditions for separation.
Finally, these spacecraft would have the same fundamental problem as the shuttle, being designed using the same technology and design margins, meaning even if the several-times-worse SSME and tile problems could be overcome, they would still also fail around 1% of the time. And like the shuttle, they had no provision for crew escape, and therefore would have been just as deadly.
So in retrospect the shuttle design that was chosen was indeed the correct one, at least in comparison to the fully recoverable design. What all of these reflect is the hubris of NASA at the time of the shuttle decision. NASA believed that their reliability engineering process would make rocket technology that had only hundreds of hours of operating time behind it, as reliable as aircraft technology, that had millions of hours of operating time and corrective action, and higher design margins. This belief was the triumph of hope over reason. The high success rate of a dozen Saturn launches had lulled NASA into a false sense of confidence in the maturity of the technology.