Question of the Week Podcast

Question of the Week episode

Mon, 24th Nov 2014

Will flights ever get faster?

Aeroplane (c) Flickr user, Axwel

14 hour flights sound like your idea of hell? Paul Jenn wrote in asking whether it was possible to speed up plane journeys, so we went to Neil Scott, head of engineering at Airbus, to find out.

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Law of diminishing returns applies to making aircraft go faster... RD, Tue, 17th Jun 2008

Yes, but this can be solved by flying higher, which is what Concorde did. I generally agree though, we have not got the technology yet to both speed up the flights AND make it cheaper. graham.d, Tue, 17th Jun 2008

The Boeing 7E7, which eventually emerged as the 787, was originally planned to be a highly area-ruled design, capable of flying at just below the speed of sound for the same fuel burn as the then current range of airliners.  However, this aspect of the design was eventually dropped in favour of flying at the same speeds as other airliners, but with reduced fuel costs, the main reason being that the airline businesses could charge the same price for the same journey time at a reduced fuel cost to themselves.  Reducing the journey time would have seen the same fuel costs - guess which option was most attractive to the airline businesses?

Modern jet airliners rely upon high-bypass turbofan engines to achieve their economy but these engines produce relatively low-speed exhaust thrust - only a relatively small proportion of the air drawn in by the fan is actually routed through the gas generator and burnt (which produces high-speed thrust) - the majority of the air is simply accelerated by the fan, just like a propeller, and bypasses the engine core entirely.

Supersonic aircraft need high-speed exhaust thrust engines, which means thrust from burnt fuel, and although the engines used in supersonic military jets now are bypass types, they have low bypass ratios.  The bypass air can help with subsonic cruise but can't contribute much at high speeds.

Both noise and complexity is an issue with supersonic capable engines.  Higher thrust speeds mean more noise, but as well as this, the air entering turbine type engines must do so subsonically.  This means that the engine nacelles have to incorporate additional mechanisms for slowing down the airflow before it gets to the engine compressors.  Depending on the type of nacelle, it's usually done with either a moving centerbody i.e. the cones you see in the engines of the MiG21, EE Lighting and SR-71, or internally with a series of movable ramps i.e. Concorde, F-15.

Flying supersonically is much more costly in terms of design, manufacture, maintenance and fuel - OK for the military but not for airlines who are primarily concerned with making a profit. LeeE, Tue, 17th Jun 2008

yeah i would imagine it being really expensive and polluting  and if its too fast it may give people heart attacks so there may be a speed limit for a aeroplane to be able to made up to benep, Tue, 17th Jun 2008

Aiplanes at present flying altitude have reached the maximum efficieny level. As can be seen be the abadoned Concord.

The hypersonic plane soon to be revealed will be a hybrid rocket/ jet plane and fly high above the atmostphere at enormuos speeds.

For example New York City To Sydney in little over six hours. Alan McDougall, Sun, 29th Jun 2008

One version of this "soon to be revealed" project died in 1988.
Getting a plane to that speed, then slowing it down again is horribly expensive in energy terms. Oil isn't getting cheaper at the moment. I don't expect to see this project happen any time soon. Bored chemist, Sun, 29th Jun 2008

Air travel gets slower and slower the time actually spent in the aircraft is often about half the of the actual journey time  syhprum, Tue, 25th Nov 2014

If you've time to spare, go by air.

I love flying, to the extent that I fly myself for any journey of more than 100 miles (or less if it involves crossing the London area) and less than 1000. What I resent is the 3 hour trip to Heathrow and a 4 hour "security" checkin for a 6 hour flight to Boston, followed by an hour waiting for immigration and then two days waiting for the suitcase: the best bit is sitting in the plane, eating, sleeping, or watching a movie, with no worries or frustrations at all. Flying to the Far East is great - a whole day in a plane, even if someone else is driving it! Until "they" improve the ground transit and handling facilities at airports, I see no point in flying any faster than Mach 0.9, and the British Isles are just beautiful at 5000 ft and 150 mph.

If I really need to talk urgently to someone in Australia, or on the Moon, I pick up the phone. Otherwise, enjoy the trip.  alancalverd, Tue, 25th Nov 2014

The next step may be sub-orbital rocket-assisted space hops. 
Yes, it will come.  Hopping from Britain to Australia in a few hours. 

Not that it will be cheap.  You may be spending a million dollars a ticket. 

How much is it worth to save an hour of travel time?  $100,000 per hour saved?  Even if you are sleeping most of that saved travel time?

Don't forget spending hours getting to the couple of space airports sparsely scattered around the globe. CliffordK, Tue, 25th Nov 2014

Even if we get suborbital flights that can reach anywhere on Earth in a few hours (most of which is spent getting the last 50km through the atmosphere), there is still the jet lag, which is sometimes quoted as 1 day elapsed time to fully recover from each hour of timezone change. evan_au, Wed, 26th Nov 2014

I suppose you could minimize your jet lag by flying from Britain to South Africa, or Australia to Korea.  Just choose your destinations carefully.

I'm not sure about the day per time-zone recovery.  However, what you do around the time of the international flight may be crucial to jet lag recovery.  In particular, one should try to start the destination sleep schedule while in flight, or perhaps even before leaving. 

So, with that in mind, the time in the air isn't entirely wasted, although not particularly productive either (full internet access would help some).

Just think how times have changed.  Not too many generations ago, and people were sailing across the Atlantic, and taking wagon trains across the Great Plains. CliffordK, Wed, 26th Nov 2014

I think airline pilots gain brownie points for saving fuel and often fly the aircraft much slower than the technology alows syhprum, Sat, 29th Nov 2014

The object of airline flying is to meet a schedule: missing a landing slot can be very expensive and screws up everyone else's day too. Private pilots and bush pilots on the other hand tend to fly for maximum range. The speed you fly, in either case, depends on the expected wind vector. Flying slowly is rarely the answer unless you have a hell of a tailwind. You can reduce your fuel consumption to "idle" but in the case of a small piston engine you may end up going backwards (it's quite good fun in a Piper Cub), and in the case of a jet the burn rate at stall speed can actually be higher than the cruise consumption.   

The difference between permitted takeoff and landing weights for an airliner is significant, so arriving overhead with much more fuel than planned (say due to an unexpected jet stream) can mean having to divert to burn off the excess.

Most airliners are optimised to cruise at Mach 0.9 or thereabouts.They won't survive supersonic flight and really don't like going much slower because to do so you need to deploy flaps and slats, which increase drag. alancalverd, Sat, 29th Nov 2014

The following isn't much help for crossing oceans, but on all other paths it would be better to use a system like the hyperloop - maglev capsules running through vacuum tubes on a narrow track. Faster than planes and environmentally friendly too. David Cooper, Sat, 29th Nov 2014

This sounds like the theory in the early days of motor vehicles: If you travel faster than 15 miles an hour, your brain will turn to jelly.
...Perhaps, on the roads of the day, it may have felt like that!

What matters are the G-Forces involved, and too much vibration can make you feel ill  .
But speed itself causes no problems - if you didn't look out the window, you wouldn't know how fast you were travelling. evan_au, Sat, 29th Nov 2014

High speed rail is already being used in many places around the world, except in the USA. 

Perhaps the future is a maglev rail in a vacuum, but that may be dangerous as traumatic decompression could be lethal.  And, of course, the infrastructure to build it would be extreme. 

The British have already put rail under the ocean...  well the channel, and I can imagine a Gibraltar-Spain tunnel or bridge will come in the future.  An underwater Atlantic or Pacific crossing is possible, but would be expensive, and would require some pretty extreme engineering to deal with pressures and safety margins. CliffordK, Sat, 29th Nov 2014

It's the wrong kind of high-speed rail: highly polluting, destructive and noisy. HS2 will have to be banned on environmental grounds soon after it's been built.

Decompression in a plane isn't great either, but it's rare, but in a tube with a vacuum in it you'd be able to let air back in in an emergency while you slow all the capsules to a halt over the course of half a minute or so, and that would make it fully survivable.

Narrow tubes for narrow capsules (no room for passengers who are highly overweight) shouldn't be a great challenge. Aeroplanes are more extreme.

The best bet would be to have something that floats. A thin tube for the maglev capsules to go through would be held higher than the tallest waves and would bridge between floating towers which would have their bulk kept deep underwater below the bottom of the deepest troughs. Alternatively, the tube could be kept at a deep level, perhaps a hundred metres below the surface. It may never happen at all though as there's little need for such a thing. It might be possible to go from London to New York via Siberia using a hyperloop system and get there faster than a plane which flies a direct path. David Cooper, Sun, 30th Nov 2014

A Moscow to Beijing and on to Alaska high speed rail line is already in the planning stage and will get built if wars don't intervene syhprum, Sun, 30th Nov 2014

Narrow tubes for narrow capsules (no room for passengers who are highly overweight) shouldn't be a great challenge. Aeroplanes are more extreme.

This statement is baffling. What infrastructure? The beauty of air travel is that all you need is a bit of flat grass (though concrete is nice) at each end and nothing but air in between. No problem of demolishing houses or destroying communities to connect a thousand destinations in any order: you take off, then fly in a great circle direct to wherever you are going. GPS, radio and radar are useful but the capital investment is negligible compared with railways and the flexibility of air travel is unbeatable. I recently flew from Cambridge to Swansea for a routine job, but literally as I got out of the plane I got a call to an emergency in Dublin. No new infrastructure, no need to re-route anyone else or wait for tomorrow's train - just get back in the plane and fly direct across the water.

The economics of HS2 are absurd. It would be cheaper, quicker, and have ten times the passenger capacity, to invest the money in aircraft to fly between Leeds, Manchester, Birmingham and London.  alancalverd, Mon, 1st Dec 2014

Narrow tubes for narrow capsules (no room for passengers who are highly overweight) shouldn't be a great challenge. Aeroplanes are more extreme.

This statement is baffling. What infrastructure?

I can see why you find it baffling, but I wasn't talking about the infrastructure when I said aeroplanes are more extreme. What's extreme about planes is that you have to tie yourself into a bomb and blast through lots of thick atmosphere before you get to altitude, and even there you've got masses of air to punch through, so the pollution involved is astronomical.

If everyone on the planet flew about the way that you do, the pollution would lead to a collapse of ecosystems all by itself, and then the mass-starvation of billions. Planes are not the solution - they are part of the problem and we need to get rid of them. Sending small capsules through narrow tubes would have a tiny impact and you wouldn't have to demolish much to make room for them. By having a network of them and giving capsules the ability to switch track without slowing down, journeys from any town to any other could be made in minutes with very little energy used, so it would be almost as good as a teleport. Capsules are the future of transport, not just for people but also for goods. We have idiots wanting the sky filled with angry buzzing as drones deliver things at high energy cost, but a network for sending capsules would enable people and goods to be sent from place to place for little energy cost and at much higher speed. That is where all our efforts should be going instead of building more roads, planes and high-speed trains. It all comes down to energy. For high speed travel you need maglev and a vacuum, and you'd have that for the intercity links. For lower speed travel to get to actual houses, wheels may be more appropriate, but it should be organised around the idea of capsules and packet switching, using extremely lightweight tracks (covered to keep the weather out) that can be build overhead to keep them away from existing infrastructure. That is where the future will inevitably take us, so it's a pity we have to watch so much of the planet being destroyed while the idiots at the top in politics insist on trying everything else first. David Cooper, Mon, 1st Dec 2014

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