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Comment by BorgHunter | Hacker News Reader

Even if the noise problem has been fixed, supersonic travel has another huge problem: Fuel consumption. Concorde positively guzzled fuel compared to the jets of its day, and this (among other things) made it uneconomical to operate. The authors of this article never even touched this topic. Has it improved considerably?

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Subsonic flight is cutting through cheese with a sharp knife. Supersonic flight is cutting through cheese with a hammer. There is no way it will ever be commercially economical with gas turbine engines and afterburners. Even super cruise without afterburners is incredibly inefficient and has very few uses outside of tactical aviation.

Drag in supersonic flight is not nearly as big an issue as you would think. The Concorde was inefficient mostly because it used afterburning turbojets that had poor efficiency at low speed.

https://en.wikipedia.org/wiki/File:Qualitive_variation_of_cd...

That is coefficient of drag. You have to remember that goes into the drag equation which includes velocity squared.

force_of_drag = .5 * mass_density * (velocity^2) * coefficient_drag * area.

As that coefficient starts tapering off after transonic drag decreases velocity squared is there to ramp up the drag.

That's true, but supersonic planes also travel higher (Concorde -> 18 km, XB-70 -> 22 km). The real problem is that L/D decreases at supersonic speeds. For example, the XB-70 had a L/D of about 7, while the 707 had a L/D over 18...

As lift is also proportional to density, area and square of velocity, that whole term cancels out from the equation of fuel usage per distance, basically only L/D or cL/cD matters.

In practice it might be a bit more complicated.

EDIT: indeed here we can see that Concorde had lift to drag ratio of only 7.5 at Mach 2, while Boeing 747 has 17.7 at Mach 0.9 or so. https://en.wikipedia.org/wiki/Lift-to-drag_ratio

But you should also factor in their comparative passenger capacities (e.g. impact per passenger-mile).

Perhaps better would be aircraft weight to number of passengers ratio. Revenue wise the supersonic airliner flies faster, meaning you get more seat miles per day per seat.

It's worth pointing out that Concorde didn't cruise using its afterburners; it only lit them briefly to punch through the transonic flight regime.

At supersonic cruise (M2) Concorde used fuel at the same rate as a Jumbo, a quick google search reveals this:

"BA said a 747 cost $100,000 to fly each way across the atlantic in wages, fuel, maintenace etc, before fixed costs. At the same time Concorde was costing $120,000".

Having read in the past the various Concorde threads on pprune.org, one view was that it was killed of by Airbus who didn't want to keep on supporting it as the manufacturer so helped get rid of the certificate of airworthiness. see this url http://www.concorde-spirit-tours.com/concorde.htm for at least one view.

The 747 carries ~450 passengers. The Concorde carried ~100. So, the Concorde is 4.5 times worse on a per capita basis.

EDIT: Actually, ~5.4 times worse if you take into account the 1.2x difference in cost.

British Airways' 747-400 carry between 275 and 345 passengers, depending on configuration. But still. Similar in principle :)

http://www.britishairways.com/en-gb/information/about-ba/fle...

Depends on the airline, I suppose. Air Canada used to fly 747-100s with 400 seats:

http://www.aircanada.com/en/about/fleet/historical/b747-100....

You're right; let's go nuclear:

https://en.wikipedia.org/wiki/Nuclear-powered_aircraft

I actually would think that could be done "safely". No sarcasm. I am expecting that if fusion ever becomes a reality we will go with electric motors which will certainly always be sub-sonic.

> electric motors which will certainly always be sub-sonic

Why is that?

Electric motors have a top speed which is governed by the forces generated by induction. At higher speeds they spend more and more torque countering those forces. Contrast this with turbines, where a higher inlet speed translates into a higher output speed - the amount of energy a turbine uses to keep itself spinning is constant rather than proportional to airspeed. This is why electric motors would have issues flying at high speeds.

Can you not simply use overdrive gearing to keep motor speed down?

... or spin the motor with another motor. It could be motors all the way down . . .

What may still be simpler than the fusion reactor some posts up.

If you vary the inlet and outlet geometry of a ducted fan you can have your fan running at subsonic, while the plane moves supersonic.

You could also heat the airstream after the compressor.

For a plane, make that a neutron-less kind of fusion.

After fusion becomes commercially viable, and the entire world concerts, we'll still be about as far as neutron-less fusion as we are from fusion right now.

Airplanes will go nuclear, but not with onboard reactors. There is simply no way to make onboard reactors sufficiently safe, especially in a crash. Instead nuclear power plants will be used to produce synthetic hydrocarbon liquid fuel for airplanes.

Are you sure there's no way? Or do you mean that's not economically sensible to do so?

RTGs are designed to survive uncontrolled reentry. I am aware that they are much simpler devices, but we are talking about containment.

There's no way to get the public to accept that though.

The most powerful RTG ever flown had a power output of 3kW.

A typical commercial jet engine (on a SUBSONIC aircraft) is on the order of 50MW.

Might as well try to power an ocean-going freighter with a hand-held sterling engine.

The market can figure that out. If the noise problem is solved (I can't read the article because it says I need to sign in), then allow it and let the airlines decide if they can make money doing it.

Click the "web" link above, under the article title. Sometimes you need to use an incognito window, but not always.

It's not so much the cost of fuel that is a concern, but the environmental impact. Unlike power generation and ground transport, global emissions from aviation are increasing rapidly at a time of growing concern about climate change.

While increased speed is nice if it can be done efficiently, innovation in aviation needs to remain focused on improving efficiency.

Banning a particular form of transportation seems a poor method for reducing emissions. Just tax the fuel properly and it will sort itself out. Flying is currently much too cheap compared to its environmental impact imho. How can it be cheaper to fly from Berlin to Munich than taking the car (or the train)?

"tax it properly" Define properly.

We are afraid of possible feedback loops destabilizing ice sheets much earlier than predicted? Maybe the price should be 5x what it current is?

If emitting CO2 causes unknown massive harm, then it seems sane to figure out a CO2 neutral method to create these liquid fuels that jets require. That would seem very expensive, with current technology.

Defining "properly" when related to taxes is too difficult for a HN comment. Also, I'm neither a climate scientist nor do I consider myself an expert in economy. I'm rather concerned about climate change, so my numbers would likely be much higher than necessary.

But I'd suggest putting a bunch of scientists in a room for a while to figure out the environmental impact and define some acceptable limit of yearly pollution. Then raise taxes until we're under the limit. Or something similar.

Have them do it every 2 years to account for changing science.

Of course, then you have a de facto regulatory body, which everyone hates, and thus a new problem.

Sounds good to me.

Difficult to define it, but we can be pretty sure we're nowhere close with our current tax policies.

In theory you set the tax rate to counteract the net negative externalities. It's estimating those externalities that's the source of contention (as you hint at re: feedback loops). So assign ten teams to estimate it and take the median?

Properly, in this context, simply means tax it like you tax fuel/energy for other modes of transportation.

Airlines have been very successful at getting special treatment by scaremongering "no more connections for you". Which is certainly a mostly empty threat, because connections will only go down by as much as demand elasticity reacts to the price difference.

I agree that the fuel should be taxed but Concorde flew a lot higher than other commercial jets, this could change the environmental impact.

There were instances of U2s being asked to change course to stay out of the way of a Concorde.

>How can it be cheaper to fly from Berlin to Munich than taking the car (or the train)?

I can't answer that specific case, but in general it's pretty simple, especially if you're comparing cars and planes: when you drive yourself in a car, you're not only transporting yourself and luggage, you're also taking a 3000-pound hunk of steel. On a plane, it's just you and some bags. You can pack a bunch of people and their crap into an aluminum tube when you give them sardine-sized seats. You get big economies of scale when you do things at larger sizes. And with an airplane, you don't have to take windy, indirect routes and stop at stoplights.

With trains, it's not so clear-cut: trains are generally more efficient than road-going vehicles, because they have lower friction (steel on steel rather than rubber on asphalt), less acceleration/deceleration (no stoplights), and economies of scale (lots of people can fit into a train). One possible problem is the tracks don't go the best route, as they have to conform to both local geography and also history, and tracks are expensive to build (airplanes go directly between airports and the only infrastructure they need is the airports, nothing in between).

Google Maps is ridiculously slow where I am so I'm not going to try finding the driving time between Berlin and Munich, but I'm guessing it's around 3-4 hours? Why a plane might be cheaper (I'm taking your word for it) probably comes down to differences in regulation between airlines and trains in Germany. I agree, it shouldn't be that way; trains should generally be cheaper. Here in the US, trains are generally terrible. Amtrak is actually OK in the northeast corridor, but outside that it mostly sucks: it's very slow, and typically very expensive compared to airfare. The trains frequently don't even have WiFi (outside the NE). They're really comfortable, though, I'll give them that. The coach seats are like 1st-class seats on airlines. But there's probably a bunch of reasons for all this: airlines are largely deregulated here in the US, and there's a ton of competition driving prices down. There's only one Amtrak, with zero competition. Amtrak has to rent space on the tracks from freight companies that own the tracks, and their trains take last place on the tracks (yep, freight gets priority here over passenger rail!). And their trains are generally horribly slow, since we don't have anything resembling high-speed rail, so they're not too attractive to anyone that values their time: no one really wants to spend 3 days (not exaggerating) on a train trip that they can do in 4 hours by airplane.

The competition thing alone is probably one big reason you're seeing that different in Germany. Government-owned agencies just cannot compete against private industry in most cases. When there's no competition and it's nearly impossible to get fired, that leads to an organizational culture which is bloated, ineffective, and overly expensive. I don't know if it's this bad in Germany, but here in the US, if you work at a government agency and need to just buy an office chair, it can take almost a year. Private companies don't work that way because they'd go out of business if they did.

Berlin–Munich is 5.5 hours, according to Google Maps.

The Concorde had never been optimized for fuel consumption. Even for a jet of its time, it was quite wasteful of fuel. I once was told by someone who had some insight into Boing, that they had a much more efficient supersonic airliner in development in the competition to the Concorde, when the project was cancelled. So yes, supersonic airplanes would use more fuel than subsonic ones, but how much we can't know until someone builds a modern supersonic aircraft.

> but how much we can't know until someone builds a modern supersonic aircraft.

You really think modern aircraft manufacturers are surprised by the fuel consumption (or any other performance characteristic) of their aircraft prior to them actually being built?

Oh, by the time they start to assemble physical parts they of course know exactly the performance figures, but that is pretty close to the end of the design process. Only after the design process you know these and I don't know how much of that is being done now.

That's true but there's enough history of supersonic aircraft design that it's not really much of a mystery what the general numbers are going to look like given speed, range, capacity, etc. targets. Sure, the designers won't know at a detailed level until they actually design the aircraft to some degree. But it's pretty much a certainty that they have (at a minimum) a back of the envelope idea of how things will look.

http://www.nasa.gov/press-release/nasa-begins-work-to-build-...

NASA is working on this - both the sound and the fuel use.

In fact, x-plane development is going to be funded by NASA Aeronautics for the production of a number of next-gen flight demonstrators, included a supersonic low-boom demonstrator.

Truss-braced wing configuration, hybrid-electric and all-electric are among the other contenders.

The article does touch on how newer designs could reduce sonic booms by making lighter aircraft. If a new supersonic jet is manufactured that is a fraction of the weight of a Concorde, it could be much more fuel efficient as well.

The article does mention that the size of the aircraft affects the size of the boom.

Concorde was more-or-less viable for BA (though not for AF). Specialized maintenance for such a small fleet (because it effectively only operated the transatlantic route due to regulations) was the thing that killed it economically.

It was viable in that it broke even on its running costs, but towards the end it wouldn't have covered the cost of actually acquiring the aircraft, nor any major expenditures such as upgrades required for safety or compatibility with newer systems.

Concorde used to serve a lot more than just the NYC-London/Paris route. It used to serve Bahrain, Rio de Janeiro, Dakar, Caracas, Washington DC, Miami, and Mexico City. Those faded away due to a lack of demand. (Ignoring Dallas and Singapore, as those failed due to restrictions on supersonic flight.)

I'm sure a new design could be a lot more economical. Of course, regular airliners have become a lot more economical as well, so it's a tougher bar to clear.

  Concorde was more-or-less viable for BA (though not for AF).

Can you elaborate, or direct me to a source?

I'm genuinly interested.

There was an extended forum thread (on pretty much all aspects of Concorde - went on for tens or hundreds of pages) that was linked from here once - afraid I don't have it to hand.

If this thread is anywhere, I would bet its on flyertalk (awesome air travel forum with great info).

Maybe this one: http://www.pprune.org/tech-log/423988-concorde-question.html

Very interesting.

Thank you very much.

Yes, the Concorde actually flew with afterburners, which are notoriously fuel-inefficient. Modern engines can do Mach 2.2 without afterburners.

This was only for take off and acceleration through the trans-sonic regime. It certainly didn't use them for super-cruise at Mach 2; at this speed the engines were operating at their peak efficiency.

Gaz guzzling is not a foregone conclusion. For instance, recall that the SR-71 actually got better fuel mileage the faster it flew, due to insightful use of the energy found in the compression wave.

If you are concerned about fuel consumption you are probably not sufficiently wealthy. You probably even drive a Corolla or a Civic instead of a nice Hummer :-)

Nobody who's actually wealthy would describe a Hummer as "nice".

Right :-) I forget to add the "sarcasm" tag.

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