I can't say under what exact circumstances you would jettison your external tanks. Out pilots did during several air-to-ground Desert Storm missions where they were engaged with multiple SAMs - including the Package Q mission to Baghdad where we lost 228 and 257. For more information and inflight video of the mission, please see our website at:
http://www.lucky-devils.net/baghdad.htmlFuel tanks though cost $$, and like almost everything else, you have to consider the use of your available assets in compairson with what you'll get by using them. We had a limited number of fuel tanks going into the war, and while we had a crew that built more from kits almost nightly, but they were still a limited resource, so we always thought of it as a 'drop them if you have to' but if you drop them too often, there was always the possibility of their not being replacements.
The concept of utilization of assets also folds into why the US doesn't use conformal tanks on their 16's. The USAF generaly uses the Viper as a short ranged, general purpose fighter / attack aircraft. The conformal tanks give the acft a pretty good deep strike capability, but that mission is, in general, filled by the F-15E's. I don't have experience with CFT equipped aircraft, so this is just opinion, but I would think that at least the Block 30 and above aircraft could likely be modified to carry the tanks with a structural and fuel system modification program. While I assume that it has been considered on at least a limited scale, I would guess that it was decided against going forward based upon the fact that you would be spending what are today very limited dollars to give an aircraft capabilities that we already posess in the Strike Eagle.
Yes, 15's use both CFT's and underwing drop tanks, but in general C's carry only drops, while the E's are rarely (if ever) seen without the CFT's.
Mike