How Big Can Airplanes Get?

[The Odyssey]

Any discussion of large aircraft is not complete without mention of the CL-1201;

http://aviationtrivia.blogspot.com/2010/06/in-aviation-those-who-dare-to-dream-are.html

Looks like someone has been playing too much Ace Combat

Seriously now, I can't really see any drastic PoDs justifying building a monster like that, mostly because I'd think it would be really vulnerable to SAMs and enemy fighters.

 

[Gunnarnz]

Seriously now, I can't really see any drastic PoDs justifying building a monster like that, mostly because I'd think it would be really vulnerable to SAMs and enemy fighters.

No more vulnerable than a tanker or AWACS aircraft, and we already have plenty of those. There are also some advantages to larger-size platforms - for example the most effective radar systems require a great deal of power and payload capacity, which makes them impractical to mount on most aircraft. Not an issue for something like this, of course...

 

[Nietzsche]

Any discussion of large aircraft is not complete without mention of the CL-1201;




http://aviationtrivia.blogspot.com/2010/06/in-aviation-those-who-dare-to-dream-are.html

...

Why not just go for broke and make the Heli-Carrier from Avengers?

 

[LeX]

I keep looking around at this thread again because that plane is just too beautiful. It's not especially pretty the way something like the SU-27 family is, but the scale of it is just divine.

 

[danderson]

Maybe something that would require huge power requirements, so much so that your standard 747 can't provide it. Maybe some kind of laser based missile defense system? Have it orbit under likely ICBM flight paths waiting to shoot them down. And I guess the crude on board computers would both be power and weight intensive, so the need might be there. Or some kind of ultra high bandwidth communications and intelligence platform. Either way you'd get around the problem of infrastructure since you'd only need one or two bases and have it fly the rest of the way.

Civilian side, I just can't see the economics working out. The infrastructure is just too expensive and I can't imagine costs per mile being that much better than a 747.

 

[Thande]

You need to find something that only that can do, and then the military will fund it. Like how they were ready to fund a moonbase in order to have an unreachable nuclear retaliation site until they found out you can do that more cheaply with nuclear submarines.

Incidentally, you may be interested in a short story by Timothy Zahn in the collection "Time Bomb and Zahndry Others", in which he posits the idea of the USA being covered by five giant flying aircraft carriers that are constantly doing a stately cycling loop over the country, and the idea is you fly a normal passenger jet up to these carriers and then relax in hotel-like surroundings there until you reach your destination and fly the plane down to the airport.

 

[asnys]

You need to find something that only that can do, and then the military will fund it. Like how they were ready to fund a moonbase in order to have an unreachable nuclear retaliation site until they found out you can do that more cheaply with nuclear submarines.

The non-gargantuan atomic-powered airplane gets funded that way to start with. The military is initially looking for a bomber, but they end up with an ALBM carrier (that's basically what happened IRL, except they never finished it IOTL). After that, atomic engines gets used on ABM carriers, C3I aircraft for use during nuclear war, possibly airborne sensor/RADAR platforms, and eventually transports. If you're not worried about expense, atomic aircraft are just perfect as missile platforms and C3I planes, since they can stay up there for weeks at a time. These vehicles would be in the 0.5-2 million lbs. weight class, not true monsters like the CL-1201.

We then have a convenient oil crunch, government subsidies to nuclear power to deal with said oil crunch, and readily available mass-produced aircraft reactors. I'm not yet sure this can actually make atomic aircraft cost competitive, but I'm hoping it can over long haul routes. Once civilian A-planes are flying, that leads to a natural pressure to build bigger and bigger planes for economies of scale, because reactor mass scale sublinearly with reactor power. This is helped along by the military looking for cost-effective ways to do air-basing of *Peacekeepers and similar tasks, which leads them in the same direction towards Giant Superplanes. I'm not sure how big they can actually get; that's the purpose of this thread. But examples in the 20,000,000-lb. weight class were considered by NASA studies in the 60s and 70s.

So that's the progression, A (atomic planes) --> B (civilian atomic planes) --> C (gigantic atomic planes) --> D (flying atomic aircraft carriers). I'm 90% sure I can pull off A. I'm not sure if I can pull off B or C, and I'm hoping to get a handle on the plausibility of C given B in this thread.

Incidentally, you may be interested in a short story by Timothy Zahn in the collection "Time Bomb and Zahndry Others", in which he posits the idea of the USA being covered by five giant flying aircraft carriers that are constantly doing a stately cycling loop over the country, and the idea is you fly a normal passenger jet up to these carriers and then relax in hotel-like surroundings there until you reach your destination and fly the plane down to the airport.

That was based on a real-life NASA proposal circa 1979. Although I don't remember if the passengers were supposed to shift to the carrier, or if the carrier was just a tug.

I eagerly await this timeline. Also, I need to update my own at some point...

Thanks. Sadly, it's going to be a while. Frigging grad school, getting in the way of the internet.

 

[e of pi]

In theory, there's no limits on the size of the aircraft in terms of physical capabilities. The constraint equations are weight-independent--if a plane with a T/W and wing loading (W/S) can fly with a loaded weight of 1m lbs, then one with 2m or 10m lbs can fly just as well--the question is market viability. There is, though, a few other things. First is the longer distances--think of how the nose of the plane settles during landing as the aircraft touches down on the main gear then the pilot drops the nose onto the nose gear. If the aircraft was at ~10 degrees, then that vertical drop will be about 17% of the distance between the gear. Not bad on a Cessna with 6 feet or so, not terrible even with 40 feet between the gear, but dropping 17% of, say, a hundred feet or so...that could get more interesting than most passengers would prefer.

Similar stuff happens with flying wings--imagine a plane rolling degrees due to a gust before the pilot compensates. Not exactly uncommon, and no big deal for passengers in seats only 10 ft off the centerline--it's only about 0.85 ft. However, passengers out 100 ft along a flying wing would suddenly drop or jump by about 8 ft, which would tend to make people complain. Not to mention banking by 30 ft or so for a turn...you'll need more roll stability, and in general need to maneuver less radically.

 

[Astrodragon]

There is actually one size limit, which is the wingspan. Wings droop and flex. Better/stiffer materials help reduce this, but at some point the flexure is going to be a limitation - having the wings go up and down and hit the ground on takeoff isnt too good...

Flying wings help, the wing is thicker, but eventually there is a finite limit. Different wing shapes help as well. And even when it doesnt touch the ground, the larger the flex the more strain it puts on the wing root. Which eventually breaks.

Wing flexing is already an issue with large planes - there are wind speed limits above which they cant fly because of this.

There are also undercarriage problems. There is only so far you can take multiple tyres...

And of course there is the non-trivial detail of how long it takes to stop...

 

[von hitchofen]

And of course there is the non-trivial detail of how long it takes to stop...

and the small matter of how long the runway needs to be for it to take off safely - the Bristol Brabazon syndrome...

 

[MattII]

could flying wings or BWBs get any bigger (ie heavier) than conventional airliners are now? And could monstrous seaplanes (especially ground-effect seaplanes) become commercially viable?

There are also undercarriage problems. There is only so far you can take multiple tyres...

From what I understand though, BWBs and flying wings just use more sets of wheels (3-4 main sets, as opposed to 2 for modern commercial jets)

And of course there is the non-trivial detail of how long it takes to stop...

Air-brakes plus wheel brakes, plus maybe some reverse thrust from the engines if it can be done.

 

[LeX]

What about airliners and jumbo jets with double fuselages?

 

[Mike Stearns]

What about airliners and jumbo jets with double fuselages?

LOL! I'd love to see a tandem fuselage A380

 

[Thande]

That was based on a real-life NASA proposal circa 1979. Although I don't remember if the passengers were supposed to shift to the carrier, or if the carrier was just a tug.

Ah, now that's interesting--it did seem a rather random idea for him to have thought it up himself. What was the logic behind the proposal, do you know?

 

[asnys]

Ah, now that's interesting--it did seem a rather random idea for him to have thought it up himself. What was the logic behind the proposal, do you know?

I haven't dug through the study, I'm afraid, just seen concept art and some abstracts. If I had to guess, my guess would be something like this: the fuel and operating costs of an atomic engine are fairly small compared to the cost of building it in the first place, so to make it economical you want to maximize its capacity factor, that is, keep it in use as much as possible. At the same time, bigger engines are proportionally cheaper, because system mass scales sublinearly with system power, so a bigger atomic plane has a much higher cargo fraction than a smaller one. So the best kind of atomic plane is a huge one that never lands, but that's kind of overkill on most routes - unless you do the hookon/dropoff thing, letting the atomic giant "fly" many routes simultaneously.

 

[Dathi THorfinnsson]

Firstly, of course, there's the Boeing Pelican.

Secondly. Have you guys done the math on a nuclear aircraft? Take the lightest nuclear reactor in use now, calculate how much power it produces, figure out how much power e.g. that Pelican takes to fly. Putting on board that many reactors, and it never gets off the ground, let alone carries payload.

The OTL atomic planes were really nasty. Some were nuclear jets with the air heated by the core. Ouch. Some used conventional power generation but had very highpower reactors, that I think used near bomb grade fuel. Again, I say, Ouch.

 

[zoomar]

Firstly, of course, there's the Boeing Pelican.

The OTL atomic planes were really nasty. Some were nuclear jets with the air heated by the core. Ouch. Some used conventional power generation but had very highpower reactors, that I think used near bomb grade fuel. Again, I say, Ouch.

Wait? As far as I know, there never were any functional nuclear airplanes OTL. The closest thing was a proposal to put a reactor in the fueslage of a B-36 just to see how the concept might work. The plant would not have powered the engines. We have no evidence regarding what might have been the actual disadvantages or advantages of a nuclear-powered super giant plane.

 

[zoomar]

Although I risk coming off as the Zeppelin fan that I am, I fail to see how anyone would seriously propose super gigantic aircraft that did not incorporate at least some aspects of lighter-than-air technology.

 

[asnys]

Firstly, of course, there's the Boeing Pelican.

Secondly. Have you guys done the math on a nuclear aircraft? Take the lightest nuclear reactor in use now, calculate how much power it produces, figure out how much power e.g. that Pelican takes to fly. Putting on board that many reactors, and it never gets off the ground, let alone carries payload.

The OTL atomic planes were really nasty. Some were nuclear jets with the air heated by the core. Ouch. Some used conventional power generation but had very highpower reactors, that I think used near bomb grade fuel. Again, I say, Ouch.

Nuclear reactors currently in use are not remotely weight-optimized. They aren't supposed to be. That's like saying an oil-powered plane could never get off the ground because the power/weight ratio of a coal-fired electricity generator is too low.

The direct-cycle nuclear turbojet - engine exhaust heated by passing through the core - is only a health risk if you are operating under the Linear No-Threshold hypothesis (and even there the risk is pretty small). For the record, I think we should use LNT in the real world, but it's not a proven fact. Under the Hormetic hypothesis the system is safe during ordinary operation. Furthermore, direct-cycle is only a stepping stone to indirect-cycle engines using pressurized helium or other intermediate coolants to carry heat from the core to a heat exchanger in the turbojet. Indirect-cycle engines, aside from eliminating entirely fission product loss during flight, are also much more efficient because you can make the core more compact, and therefore make the shield lighter. None of these systems bear any significant resemblance to the Light Water Reactors the US uses today. (The ANP project did very briefly consider a "Supercritical Water Reactor" design that would work similarly to an LWR, but it was rejected early in the program due to being a terrible idea.)

All of these would require the use of weapons-grade fissiles. Which, while I certainly would not endorse that in real life, is excellent for the purposes of fiction. Also, let me note that stealing fuel rods from an A-plane is a highly non-trivial endeavor, since they're radioactive enough to kill you fast after the engine's been operated - such as during the flight from the heavily guarded refueling center to wherever they're actually being used. And the fuel is likely to be a ceramic or other non-metallic chemical form; processing that into a metal suitable for use in a weapon is not something you can do in your average garage. Making bombs out of plane reactors is something a state might do, but not a terrorist cell.

 

[asnys]

Wait? As far as I know, there never were any functional nuclear airplanes OTL. The closest thing was a proposal to put a reactor in the fueslage of a B-36 just to see how the concept might work. The plant would not have powered the engines. We have no evidence regarding what might have been the actual disadvantages or advantages of a nuclear-powered super giant plane.

There are stories the Soviets built one, using an inadequately-shielded direct-cycle engine which killed most of the crew. As far as I can tell this story originated with a Discovery channel documentary that got so much else wrong I suspect they just made it up. But I'm not certain it didn't happen, and we know they were experimenting with the idea, so...

The US did build a few atomic-powered turbojets that we ran on static tests in the 50s, and we flew a 1 MWth testing reactor around in that B-36, but that's as far as we got.