Supersonic aircraft

[jrphilps]

Ever since the retirement of the concorde back in the early 2000s, supersonic commercial aircraft have become an extinct class. Many have spoken on just how undesirable such a transportation gap is. My question is, rather than developing an entirely new aircraft from the ground up, wouldn't it be possible to alter some existing model (like a boeing 767, or airbus a300) to break the sound barrier? In theory, all this would require is some structural reinforcement, increasing the wing sweep, and slapping on a larger pair of engines. So why haven't any of the large airliner companys got around to doing this?

[relin]

As to the "why," there's really not a big market for it. Getting across the Atlantic in a couple hours is nice, but the market never really supported it. And considering how difficult it is in the US for airlines, I don't see supersonic aircraft coming back until there's a reason for that kind of travel. I wouldn't begin to predict the scenario that would usher supersonic flights back.

It wouldn't really be possible for existing passenger aircraft to be retrofitted for supersonic flight. Structural reinforcement, altered wings and bigger engines aren't enough to make a craft go supersonic, and it certainly isn't enough to it safe enough for commercial traffic. That's not even including the training for pilots: once supersonic, flight controls invert and getting commercial pilots the hours they'd need to make them (and governments) comfortable is immense.

[PakledHostage]

In theory, all this would require is some structural reinforcement, increasing the wing sweep, and slapping on a larger pair of engines. So why haven't any of the large airliner companys got around to doing this?

Although there was a DC-8 that was intentionally flown to Mach 1.01 in a dive back in 1961, it would take a lot more than what you are describing to build an airliner that could do it safely and economically in every day operation.

Boeing had proposed to build the Sonic Cruiser about 12 years ago or so, but the airlines weren't interested in the extra bit of speed and pushed Boeing to design a more conventional but more fuel efficient aircraft instead. The result was the 787.

[MaverickSawyer]

I suspect that the next time passengers go supersonic will be on a suborbital, Mach 5+ hop across the Pacific (Tokyo-LAX in an hour, anyone?)

[Nibb31]

In theory, all this would require is some structural reinforcement, increasing the wing sweep, and slapping on a larger pair of engines. So why haven't any of the large airliner companys got around to doing this?

It's much more complicated than that. The Concorde was an extremely complex aircraft, way in advance of its time, with all sorts of unique solutions to all of the problems of covering subsonic, transonic, and supersonic flight envelopes. For example, it had a super complicated fuel ballast transfer system in order to reconfigure the center-of-mass during flight:

Do you think they added all that stuff just for fun?

Simply modifying a subsonic airframe by adding reinforcements would just add extra take-off weight to the plane. It wouldn't be efficient at all.

[Nuke]

what good is it to cross the atlantic in 2 hours if you spend 4 hours at airports.

there is of course the this.

http://en.wikipedia.org/wiki/Reaction_Engines_A2

its a derivative of the skylon/sabre tech. it will have the precoolers and run on lh2, but the rocket bits will be replaced with a more conventional turbofan bits. it would be able to push mach 5.

Edited January 25, 2014 by Nuke

[Tex_NL]

I suspect that the next time passengers go supersonic will be on a suborbital, Mach 5+ hop across the Pacific (Tokyo-LAX in an hour, anyone?)

But at what cost? Are people really prepared to pay 5 times as much for a ticket that will only get you there a few hours earlier?

Nowadays people are still prepared to suffer for hours, being packed like sardines in economy class while they could sit in luxury in business class. Just to safes them a few coins.

what good is it to cross the atlantic in 2 hours if you spend 4 hours at airports.

Good point. But it would still be faster than flying for 8 hours and STILL spend 4 hours at the airport.

[jrphilps]

As to the "why," there's really not a big market for it. Getting across the Atlantic in a couple hours is nice, but the market never really supported it. And considering how difficult it is in the US for airlines, I don't see supersonic aircraft coming back until there's a reason for that kind of travel. I wouldn't begin to predict the scenario that would usher supersonic flights back.

Yeah, you've got a point. As beautiful as the concorde was, there just wasn't the financial incentive for it to survive. I just can't believe we're not going to see supersonic travel anytime soon.

It wouldn't really be possible for existing passenger aircraft to be retrofitted for supersonic flight. Structural reinforcement, altered wings and bigger engines aren't enough to make a craft go supersonic, and it certainly isn't enough to it safe enough for commercial traffic. That's not even including the training for pilots: once supersonic, flight controls invert and getting commercial pilots the hours they'd need to make them (and governments) comfortable is immense.

Okay, maybe supersonic flight is out of the question. But these mods should definitely be able to increase the top speed by 100 kmh or so, right? Enough to run down those fancy gulfstreams (and the corporate snobs who ride in them)?

[Tex_NL]

...

Okay, maybe supersonic flight is out of the question. But these mods should definitely be able to increase the top speed by 100 kmh or so, right? Enough to run down those fancy gulfstreams (and the corporate snobs who ride in them)?

If it was that easy it would probably have been done by now.

Modern airliners all cruise somewhere near Mach 0.8. Small business jets like the Gulfstream G series are much smaller and more streamlined. As a result they can reach speeds round Mach 0.9 but typically fly at the same Mach 0.8.

Both the Boeing 767 and Airbus A300 cruise at the lower edge of transonic speed (typically Mach 0.8 - Mach 1.23). Increasing the speed by 100 km/h would put them right the danger zone. Adding structural support to withstand those forces would significantly increase the weight resulting in a plane that might not even be able to reach those speeds in the first place.

[Kerbin Dallas Multipass]

The Gulfstream G650 was test flown at mach 0.995.

You don't really want to cruise at that speed in any subsonic design because in case you hit tubulences you could easily break mach 1 - likely resulting in bits falling off your plane

[Nibb31]

Okay, maybe supersonic flight is out of the question. But these mods should definitely be able to increase the top speed by 100 kmh or so, right? Enough to run down those fancy gulfstreams (and the corporate snobs who ride in them)?

Airliners already fly just below Mach 1, because that is the optimal speed. If you go beyond Mach 1, you need a whole different flight envelope and things get way more complicated, and expensive. Going at Mach 1.01 is pointless, because you get very little benefit for a much higher cost.

[softweir]

Concorde never made it as a commercial transport facility, it only carried on for so long as an "Experience" flight in combination with a trans-Atlantic cruise - and it's days were always numbered, even before the crash at Charles de Gaulle Airport.

The number of people who would be prepared to pay enough cash in order to fly supersonic isn't great enough. The vast majority of people want the flights to be cheap, which is why bucket-class outfits like Ryanair can operate. Supersonic flight is expensive and can never be cheap, because there are so very many additional technical issues over and above those of sub-sonic flight. A supersonic craft needs very many systems in addition to those of a subsonic craft, and many of those systems it has in common have to be more complex so the construction and maintenance costs are much higher, and the fuel costs are VERY much higher.

Maybe, if we get out of the global economic downturn we are slogging through and have a significant upturn then there might be a sufficient number of people with the disposable income wanting quick and high-ticket flights that supersonic airliners will become viable. But maybe not.

[PakledHostage]

Okay, maybe supersonic flight is out of the question. But these mods should definitely be able to increase the top speed by 100 kmh or so, right? Enough to run down those fancy gulfstreams (and the corporate snobs who ride in them)?

The Boeing Sonic Cruiser proposal that I mentioned previously wasn't going to be much more than 100 km/h faster than conventional airliners, and the airlines already weren't interested in that design.

Another thing that needs to be considered is how you integrate faster aircraft into the air traffic control system. Large regions of the globe (i.e. most of the oceans and arctic regions) are not covered by radar. Aircraft flying in these regions are slotted into tracks. Separation in the track system is maintained by specifying the speed that the aircraft must be flown at. It doesn't matter that your aircraft can cruise at Mach 0.87 if the guy in front of you can only do Mach 0.8. You can request an alternate altitude that doesn't have a slow poke¹ in it, but that alternate altitude may be unfavorable for some other reason like wind direction/speed or turbulence. And while it is true that a lot of business jets are designed to fly faster than commercial airliners, they are also designed to fly higher to give them the freedom to do so.

¹This is the source of the joke among Boeing pilots that the A340 is the only airplane certified for birdstrikes from behind...

[Themohawkninja]

Supersonic aerodynamics are different than subsonic, so large low-swept wings like that of a modern jetliner would warp and break (especially during transonic flight). That is why the Concorde has delta wings. They are much more stable at high velocities and low-to-mid mach ranges. Once you hit hypersonic (mach 5 and above), then you get into temperature issues, as the atmosphere heats up around the aircraft and causes the aircraft to reach very high temperatures. Similarly, at such speeds the engines start to get very hot as the air coming in is slowed to subsonic speeds in mere fractions of a second. All of that kinetic energy is turned into thermal energy, hence why things like the SABRE engine need complex, and very high-tech pre-coolers.

I don't know enough to explain specifics, but that's basically why you can't just put a 747 into a dive to reach mach 1.

[K^2]

Another thing that needs to be considered is how you integrate faster aircraft into the air traffic control system. Large regions of the globe (i.e. most of the oceans and arctic regions) are not covered by radar. Aircraft flying in these regions are slotted into tracks. Separation in the track system is maintained by specifying the speed that the aircraft must be flown at.

Supersonic flight makes more sense at higher flight levels than subsonic flights. So I don't think that's going to be a big problem. If all your subsonic traffic is around 10km, and your supercruisers are above 15km, you're in the clear. And by the time they need to descend, they'd be in radar-covered area.

[PakledHostage]

@K^2: True enough, but there remains the issue of economic viability. Higher cruise altitudes require that the fuselage be designed for higher differential pressures, and must therefore be heavier. I don't recall what altitude the Boeing Sonic Cruiser would have operated at, but it doesn't stand out in my memory as being a particularly high altitude aircraft. Limited time of useful consciousness in the event of depressurization is already a concern at the upper operating limits of commerical airliners, and you don't have to go too much higher before pressure suits would be required. Obviously it is impractical to operate at those types of altitudes.

The Concorde had (and many business jets currently have) the luxury of operating in uncongested airspace, but I think the OP was talking about a time when faster aircraft would be more common place. They too would have to be shoehorned into the same airway system that sub sonic transports currently follow, above the currently congested flight levels but in the same ~10 000 foot thick block of airspace that are already occupied by sub-sonic business jets today. The same challenges would eventually occur in those higher flight levels as do today.

Edited January 25, 2014 by PakledHostage

[K^2]

At 10km you are looking at a little over 25% of sea level pressure. At 15km, it's going to be a little under 15%. That does not make a principal difference for aircraft structure. Time of useful consciousness is reduced, of course, but it's not a problem for trained crew. It is a problem for passengers, but again, it's something that can be addressed with the right procedures. While useful consciousness is going to be less than 10 seconds, actual consciousness time is going to be longer, and revival with oxygen will be effective longer still. So as long as you have crew moving around assisting people, you shouldn't have serious problems.

Finally, while this is bellow partial pressure of oxygen at ground level, pure oxygen is still breathable at 15km, and the systems that feed oxygen at a bit of a pressure bring it to an almost comfortable breathing level. (Though, at an expense of effort to exhale.)

15km is pretty much at the limit of how high an aircraft can go and still save the majority of passengers in event of depressurization, but it's still within such a limit. That said, we will have to venture beyond these limits some time. There are a lot of failures in modern aircraft that you cannot have a backup plan for. And when we finally graduate to hypersonic, and then, hopefully, suborbital and orbital flights, loss of pressure will have to be such a thing. We'll just have to make sure that loss of pressure does not happen in flight.

[PakledHostage]

...

With all due respect, I didn't realise that being a particle physicist made you an expert on aircraft structures engineering and regulatory compliance? It is my professional opinion as an aerospace engineer that these things do matter. Economic viability, safety and operational considerations must be taken into account in aircraft designs. Indeed they are and they have served to influence the evolution of commercial airliners into their current configurations and operating regimes. Attempts have been made with aircraft like the Concorde and Boeing Sonic Cruiser to break out of that niche, but after 50 years of jet airliners being in existence, we are still plodding along sub-sonically below ~42000 feet¹. Any aircraft design that is intended to operate outside of that niche will have to deal with the problems that exist there, and those may render the design too unsafe, too impractical or too costly to be viable.

¹Some business jets operate above that level, but for the people that can afford to fly in them, the convenience of uncongested airspace is probably worth the extra cost of certification, structural weight and operating restrictions.

[Kerbin Dallas Multipass]

@PakledHostage

Don't you get higher fuel efficiency in the FL400-500 range, possibly counterbalancing the increased cost and weight?

I recall this as one of the reasons why several business jets are designed for silly altitudes up to 51k feet

BTW: From what I read the typical cruise altitude for the sonic cruiser would have been around 45k feet

[K^2]

With all due respect, I didn't realise that being a particle physicist made you an expert on aircraft structures engineering and regulatory compliance? It is my professional opinion as an aerospace engineer that these things do matter.

If you are telling me that a 10% change in pressure differential is going to make a significant difference to the design of a structure that's meant to take more significant stresses from everything ranging from normal loading, to shears and twists, to vibrations of every kind, then I will take you word for it. There can certainly be nuances that I cannot foresee, but it is rather unexpected. So I hope you'll forgive me for jumping to conclusions.

And I'm definitely not prepared to discuss what is and isn't economically viable. I know that's sort of what OP was going for, but if I'm talking about viability of a certain approach, you can pretty much assume that I'm ignoring any economical factors. So, like, if I make assumption about crew being trained to deal with depressurization, I assume they have military-like training and can deal with it in under ten seconds. In practice, it might not be possible for airlines to keep crews that are this well trained due to economical realities.

If we need to fly supersonic airliners, I can't imagine any physical reason why we couldn't work around all of the problems. Feel free to correct me, but I'd be completely shocked to learn otherwise. But I can easily believe that all of these fixes would cost too much for this to be worth it for passenger transport.