karn@eagle.UUCP (07/28/83)
I have been watching this discussion with a fair amount of amusement. A few observations are in order: 1. The reliability of Ariane in terms of delivering payloads to the nominal orbit, bad as it may be (4/6 success rate if you ignore the little problem with the Oscar-10 separation) is better than the Shuttle/IUS system (0/1 success rate so far.) Of course, this isn't really fair as both rates are bound to improve. I expect that the success rates for both future IUS and Ariane launches will be quite good. 2. The Ariane provides a geostationary transfer orbit, which is what the shuttle combined with the first stage of the IUS gives you. The shuttle and the IUS first stage worked just fine on TDRS-A; it was the apogee kick motor (which you would still need with either launch method) which failed. 3. Ariane-1 (the current configuration) has a payload capability of 1,500 kg into TRANSFER orbit, with a net capability of 750 kg into the final geostationary orbit. (This takes into account the smaller apogee kick motor required due to the lower transfer orbit inclination.) This is far less than the mass of TDRS, making the suggestion moot. A dedicated STS flight carrying the DOD/NASA two-stage IUS (mass 14,515 kg) has a capacity of 2,268 kg to geostationary orbit. Not until ESA flies the Ariane-5 model (probably in the 1990's) will its payload capability be comparable. 4. Whatever the political realities, ESA and NASA aren't as polarized toward each other as you might think. There is a considerable amount of cooperation on such projects as Spacelab (ESA payload on a NASA launch) and Ariane launch support (NASA facilities for ESA launches). When the Ariane launch schedule was slipped earlier this year because of the L-5 failure, the Exosat experiment was removed from an Ariane and launched on a Delta instead. 5. The only part I can't understand about the IUS is why the Air Force decided on a three-axis stabilized approach. This seems to be just unnecessary complexity to me as I can't see any advantages over the more common spin-stabilized method used by the PAM for commercial payloads. Three axis control just makes the system more complex and subject to failures (e.g., thrust vector control) and the payload thermal design more difficult. I suppose it's easier to turn the payload around for the second firing during the five hour coast to apogee if you're not spinning, but I can't see how it's worth all the hassle. References for payload capacities are the NASA Space Transportation System User Handbook and the paper Ariane Launch Vehicle: A European Program. Both are rather old, so the exact values may be different. Phil