[trhway]

first it makes the flight longer while not decreasing the cost of the flight thus damaging the economics of such an operation.

second - those are very different modes. We don't know how to design a plane flying efficiently (and thus achieving a good range, and the range is already an issue for supersonics) in both modes, subsonic and supersonic (well, we do have B-1 and Tu-160 though that is well beyond commercial reach and i'd say those planes more like illustrate the issue than showcase the solution). Concorde for example was using afterburner to get to supersonic mode as quickly as possible, and while afterburner is very inefficient, such approach was overall still most efficient for Concorde because the plane was designed for the supersonic mode.

[Theodores]

Just to clarify, Concorde used afterburners for take-off and to get to supersonic speeds. When at speed it was able to supercruise without the afterburners.

What this meant in reality was that anyone in South West London (near to Heathrow but not necessarily just west of Heathrow) had this part of the day when the noise was terrible. You could be outside, sat down enjoying beers and all conversation would have to stop until this thing went overhead. Ear splitting was the word and there was no doubt the plane was Concorde.

Further down the road in the West Country the Concorde (there was only one!) would be able to be heard but it would not be announcing its presence and demanding that you stop everything that you were doing to look at it. At this stage it was high altitude but not on the supersonic super-cruise.

Then, over the Bristol Channel, Concorde would hit the afterburners good and proper to get up to full speed. People in Devon, Cornwall, South Wales and the south of Ireland would hear the boom albeit not at full intensity. It was still a 'boom' though.

If you look at the map and the size of the Bristol Channel then you can get an idea as to how big of an overland corridor you would need. It is huge, even for somewhere like Tibet.

[galaxyLogic]

I see. But so was there any technical necessity for Concorde go supersonic already over the land, rather than say 50 miles off the cost?

Or was that only an economic necessity?

[trhway]

My understanding it is economics, range and time. Technically Concorde of course could fly subsonic. It is just that fuel efficiency was lower due to supersonic design of wings and engines. Its engines were designed with Mach 2 ram effect in mind - that ram pressure on top of mechanical compression resulted in high compression ratio and thus provided high efficiency - and without that additional ram pressure (which is much much lower at subsonic speeds) the pure mechanical compression ratio of its engines was pretty low and thus low efficiency. It naturally didn't have that switch from mechanical compression to pure ram like SR-71 :) The Concorde supersonic wings have low lift-to-drag ratio at low speeds which naturally means high thrust - high fuel consumption and resulting high noise - to keep it in the air at those low speeds.