It would be cool if we had atmospheric-only engines which have a much higher fuel economy - if you don\'t need to carry oxidizer then your fuel will go much further. Modern turbofans are spectacularly efficient and of course, you wouldn\'t see many long-haul flights if those aircraft had to carry their oxygen with them. A rocket running on liquid oxygen and RP-1 (which is basically kerosene, so very similar to jet fuel) requires 2.7 times (by mass) as much liquid oxygen as fuel. Plus, insulation to help keep it cool - you don\'t need that for kerosene. So, if you want to try and build an atmospheric flight vehicle, its fair to say that you can legitimately improve fuel economy by a factor of 3.7 (pretending you have no oxidizer in those fuel tanks). In addition, one could reasonably go to a factor of 5 or 6 to compensate for the differences in thermal efficiency between a rocket and a turbofan engine. If you\'re making a fighter plane improve your fuel efficiency by maybe a factor of 4 because a turbojet is not as efficient as a turbofan by a long shot. I have 'atmospheric flight only' versions of a couple different fuel tanks that do not change in mass as they empty - in the interest of fairness I just set their dry mass to their wet mass - this keeps my flight characteristics consistent throughout my burns. I have my fuel economy (thrust same, burnrate 20%) in my 'atmospheric flight only' engines increased by a factor of 5. IMO these are reasonable changes and don\'t feel like cheating to me. I think that expanding Kerbal Space Program\'s capacity to include more interesting atmospheric flight possibilities is maybe a more interesting (from my point of view) angle than more space stuff. Kerbal Air and Space Program?
