Seeing as we were talking about the UK we can also talk about the fact that increased injuries of loose drivers/passengers if they don't manage to die will affect others by the increased cost and use of resources in the NHS.

I'm not sure trains are a good model either; Diesel-electric trains are effective because the torque you need for starting & driving a train doesn't easily come from a diesel motor without lots of gearing and clutches that are complex, inefficient and potentially unreliable.

Electric motors can give you all the torque you want from a standing start and so they make it easier to use diesels, avoiding the need to electrify your rail network (partly the reason Britain went with Diesel-electric trains in the '50s - they didn't have the capital to electrify).

With aircraft it's less clear where the advantage is going to come from since the kerosene motor + generator combination (and its associated losses) in an aircraft isn't solving a clear problem like lack of torque in a train and power requirements with lots of peaks and troughs as in a car.

However, a big advantage (from an environmental point of view) could be the ability to take electrical power for flight - once you have this you can gradually feed in alternative or low carbon energy into your mix. This type of aircraft could be a first step in that direction.

It's not beyond current capability even in civil aviation, in fact Boeing offered a folding wing option for the 777 but (so far) no-one's bought it.

This is partly due to lack of driver (gates at airports were either wide enough or made wide enough) but also because any driver to wing folding's got to be pretty strong to overcome the weight penalty

If you were to see a _big_ increase in span for aircraft of these capacities I'd imagine that folding wings might become more popular...

You're correct that drag has a large influence, engines have a part to play also.
Turbofans have a 'bucket' speed where their efficiency (specific fuel consumption or the fuel they burn per second per pound of thrust) is best*
The result is that, when the aerodynamics and engine efficiency are combined, there will be a best efficiency speed (best range speed) that's not far below the theoretical 'design' speed. However many airlines fly faster than this, depending on their balance of fixed vs. hourly costs.
Generally you can get higher efficiency by flying slower but you have to make changes to the aircraft, as seen here where much of the efficiency probably comes from the lower lift dependent drag that you can get from the larger spans of these aircraft. They probably get quite a lot of gain from engine improvements also, perhaps half.

*all bets are off with open-rotor or propellor engines, broadly these like to fly slower overall and you lose efficiency steadily the faster you fly.

The issue of requirements is one that I've always found interesting.

There always seems to be an assumption that customers know how to write requirements. Personally (from the position of a hobby coder who needs to use the services of professionals to get real applications written) I've always found it difficult to write intelligent requirements.

Don't get me wrong, I know that this is my fault, but I find that I need assistance from people who actually understand the ways that things _could_ be done and know the implications of the things that may be asked for. I always prefer to plan for a significant activity just to find out what I should be asking for. Motivation of the people who I ask for advice is important. If I (or the company I work for) pays the developer I know I can expect that they want to get the best results for the company as a whole.

I work in the aircraft industry and I've seen enough poorly chosen requirements for aircraft to know that this isn't a solely IT issue...

The problem with your comparison of Janitors and IT works is visual. If the Janitors are all laid off, it is noticeable (trash cans overflow, dust begins to settle, glass becomes smudgy, etc), whereas if the IT department is laid off, much of their work may go unnoticed immediately. People tend to ignore IT until something breaks.

No, it doesn't work that way, thats a typical misunderstanding of the way it works because you're confusing observations from your perspective with reality.

The dot isn't moving at all, and you wouldn't actually have a dot painted on the surface of the moon. From your perspective on Earth it may appear that way, but appearances are often deceiving.

What you end up with is something that would resemble a dimmer (than the dot would be if stationary) line or blur on the surface of the moon, spread out over vast distances, that would appear to the observer on the Earth as though it was moving faster than it actually is. When the reflection of the light from the laser returned to your eye, the line would appear as a dot again, all due to perspective of the viewer.

The engines-on-the-side configuration is a way to try to deal with engine weight changes. Huge trim issues arise if your engine isn't as light (or heavy) as you think it will be.

Spaceplanes with engines at the back face a real struggle with balance if _anything_ changes in the engines - they tend to be very heavy compared to anything else in the 'dry' structure of the vehicle and a small error either way can leave you with depleted uranium bulkheads to pull your CG back if it's too late in the development programme to change the configuration.

You really can't get away with stuff like that since single-stage-to-orbit is _really_ hard to do.

Couple that with the big aerodynamic centre changes that you tend to get over such a large flight regime and you may end up with a lot of mass budget being taken up by control surfaces.

I understood from a couple of lecturers at Uni and past co-workers that this would have been one of the really big problems that HOTOL would have had to deal with had it not been cancelled... (which project Alan Bond of Reaction Engines was also involved in)

Commercial jets are at lift/drag of around 18-20 now
As an engineer working with fixed wings it is my firm belief that helicopters fly because they are so ugly that the ground repels them - on that basis this thing is getting to the moon.