> Even military aircraft tend to only go supersonic when needed because it guzzles fuel. Yep. In fact, this is one of many things that makes the Concorde so sp

> Even military aircraft tend to only go supersonic when needed because it guzzles fuel. Yep. In fact, this is one of many things that makes the Concorde so special: being able to cruise for a long time at Mach 2+. Supercruise has only come to fighter jets fairly recently, and there it is usually in the Mach 1.4-1.6 range. This means that if the Concorde has even a small head-start, it can outrun most fighter jets that might want to intercept it. The SR-71 could keep it company but is unarmed. Maybe the Mig-31, although its range also diminishes rapidly with increased velocity. > efficiency is what has kept civilian supersonic flight from being a thing This was an issue, but less than you might suspect, for example it didn't actually burn that much more than its contemporaries. The planned Concorde B [1] was to have more efficient engines that would have allowed the same performance without afterburners. Along with having larger fuel tanks, this would have led to a dramatic increase in range and therefore in the possible routes. Alas, the US effectively killed Concorde before that could happen by not allowing overflight, due to noise, particularly the sonic boom, and one suspects also the fact the the US SSTs weren't making much progress. And of course the oil price shock. Concorde tickets were actually quite a bit cheaper than the public expected, BA (and AF, presumably) later raised prices to match public perception, with a nice boost to profitability. > extreme altitude where the air is thinner Jets in general (~35K feet) and Concorde in particular (~60K feet) already do this. IIRC, jet engines are actually not very efficient, but they continue to operate well at high altitudes where the air is so much thinner that the plane becomes more efficient overall, whereas both piston engines and the propellers they drive become significantly less efficient at altitude. [1] https://www.heritageconcorde.com/concorde-b https://www.heritageconcorde.com/concorde-b