I’m not sure it really can be - materials behave differently at different scales - note, this is not an argument that it is possible at any scale, but it seems to me that making a large rigid shell is not the same as just scaling up a small one in every dimension. Ostrich eggs are not just big hen eggs - they are built differently, and I suspect for the reason that not all factors controlling strength of a rigid shell scale in the same way.
The first-order estimate for these kinds of things usually involves looking at the scaling laws. For instance, we know we can’t just scale up an elephant by a factor of 2, because the leg cross-section only goes up by a factor of 4 while the volume and mass go up by 8x. So the legs end up with twice the force and the animal can’t even stand. You need to change some other factor if you want a larger animal.
But for a vacuum balloon the factors are the same. Double the diameter and the buoyant force goes up by 8x. But the surface area of the hull goes up by 4x and the required hull thickness by 2x, so the total mass goes up by 8x as well. The fundamental strength requirements are thus scale-independent.
That said, there are buckling forces and other factors that come into play as well, but the point is that if your materials aren’t strong enough at one scale, just making it bigger or smaller won’t help.
Well, at truly extreme scales the scaling might break down: For instance, materials with truly flawless crystalline structures are often much stronger than those with a typical amount of flaws, and it might be possible to more practically manufacture those flawless materials on a very small scale than on a larger scale. This is why, for instance, graphene is so much stronger than graphite, even though graphite is just a bunch of layers of graphene.
But the best we can do in that direction, with current tech, is those graphene aerogels that Francis Vaughan already looked into, which were still only 2% of the needed strength.
Just read this today, sort of a footnote to “non-lighter-than-air” airships:
[…pressure in] the cabin area, usually at levels around 12 psi (about equivalent to atmospheric pressure at 7,000 feet). Why 12 psi instead of something like sea-level pressures of about 14.7 psi? 12 psi is sufficient for the majority of passengers while simultaneously reducing the structural strain on the aircraft itself over something like sea level atmospheric pressures.
Poor writing, or poor comprehension on my part: “structural strain” is a result of internal pressure? (Opposite of this thread, BTW.)
ETA, cx to above: passenger jetliner
Leo: The “structural strain” is caused because we are pumping air into the interior at a higher pressure than is outside.
At cruise we may have 12psi absolute pressure inside but the air pressure outside is down around 5 depending on altitude. So the difference is 7ish pounds pushing outwards on every square inch of the surface of the aircraft cabin. Each window, roughly 12" square, has 71212 = 1000 pounds of force trying to shove it out of the aircraft. Doors have 8-15 *tons *of force trying to shove each of them open.
And every bit of the outer skin is being pushed on, being stretched, by the forces within trying to get out. Just like a toy party balloon, the aircraft (slightly) inflates in size on every flight then deflates back upon landing. Do that inflate/deflate cycle enough times and, just like repeatedly bending a paperclip, some metal will tear someplace.
The durability of the materials versus this inflate/deflate cycling is typically the ultimate life-limiting factor of modern aircraft. We can replace engines, update the avionics and interiors, but once the structure has been inflated / deflated umpteen thousand times, it’s worn out and the airplane is now only suitable for being melted down for beer cans.
Late add: in engineering terminology, “stress” is a load applied to piece of structure. “Strain” is the amount of bending or stretching that occurs as a result of the stress. The stress/strain ratio is “stiffness.”
So the difference between interior and exterior air pressure applies a stress of, say, 7 psi to the skin surface. Which results in a strain of, say, 2/1000ths of an inch of stretching in the skin circumference.
Thanks. I understood, at least, the principle of the popping balloon, but the numbers you give are eye-opening.
And what you say about the natural last trump for any airplane. Fascinating…how does the word “duty cycle” in engineering fit in to this? Doesn’t that mean "number of times for design function under normal operating conditions "–so Boeing will have that number scoped out – for internal or sales use–of the airframe itself?
ETA: Something about the SR-71 being so loosely put together on the ground it leaks fuel, so that when pressurized it balloons out/stiffens up…?
I don’t know. You should at least consider that your engines won’t be running at full power all the time so they won’t generate the maximum waste heat all the time. Also, there maybe times when you don’t need as much heat as the engines are outputting so you still need to be able to cool the engines without relying on the helium as your cooling medium. You may also have problems sealing your envelope against all the things you want running through it, like the air intake, exhaust and propeller drive.
That said, though, “use the waste heat from the engines to heat the lifting gas” is at least an idea that could in principle work, and which might be worth looking into. I strongly suspect that someone has in fact already looked into it, and found that it turned out to have a worse cost/benefit than other techniques, but it’s something that would take serious engineering to rule out, as opposed to fundamental physics objections, or back-of-the-envelope engineering.
Ref **Leo **a couple posts above …
In general engineering parlance “duty cycle” is something different. E.g. a heating element that operates 25% of the time while the machine is running and is shut off the other 75% of the time it’s running is said to have a 25% duty cycle. Many things like motors, compressors, or actuators have a duty cycle limit. e.g. If you run our model XYZ-123 gizmo more than 10% of the time or for longer than 25 minutes at a stretch it’ll overheat. That’s a “10% duty cycle limit”
Airframe life is designed and tracked in “cycles”. Which correspond to one flight = one take off & landing. As an example, one of our oldest & relatively rattiest 767s just flew to the graveyard yesterday. It had 18,534 cycles. Conversely we recently retired a narrow-body aircraft with 46,600 cycles. Different aircraft are designed for different lifetimes measured in both cycles and in hours.
Both those numbers are well below the design lifespan for the respective type. Either aircraft might end up converted to freight or soon be flying in some dark corner of the world for another decade or more.
I don’t know of a good public reference for the absolute cycle limit of any type. I’d ballpark it at 60,000 cycles for a narrow-body = short haul aircraft and 30,000 for a wide-body = long haul aircraft, but that’s spitballing off some numbers I vaguely remember from various accident reports.
Generally what happens is the manufacturer specifies increasingly complex and expensive inspections ever deeper into the structure once the airplane gets to the equivalent of human age 60. At which point it becomes increasingly uneconomical to perform the next big inspection. When the savings from ever lower cost to lease no longer offsets the ever increasing cost to inspect/maintain & buy fuel vs. later more efficient types, the economic life of the aircraft is over even if the structural life has a bit more to live.
SR-71s leaked fuel because the airplane structure expands from skin friction heating due to high speed flight. So they built it to fit together nicely when hot in flight, not when cold on the ground. Or at least that’s the story passed down in aviation lore; I’ve always been a bit suspicious of it. Or at least suspicious that it was only half the story,
A famous incident that’s relevant here is the 1988 explosive decompression event on Aloha Airlines Flight 243. This B737 had nearly 90,000 flight cycles.
Probably should have been retired a bit sooner.
Except there is a bunch of developing countries in Asia, Africa and South America that don’t have many laws about sticking to manufacturer recommendations so it will end up in one of those until it falls apart. Not blaming anyone for this, thats just the economics of it.
One thing about aluminum is that is has no fatigue limit. What this means is that no matter how gently you apply stress to the material, there is still a finite number of cycles before it fails. It builds up microfractures and loses strength until failure. This is in comparison to materials like steel, which have effectively infinite life as long as the stress is kept below a certain value. As long as planes are made from aluminum, there will be a cycle limit. I don’t know about composites.
Well there you go. Thanks for going to the trouble of looking that up. My half-remembered WAG was a bunch low.
Clearly 90,000 was within the Boeing cycle limits in effect for the 737 at the time. Though as you say, equally clearly those limits were too high for that particular aircraft in that particular operation.
IIRC there was a lot analysis after that accident that found the very short flights in the very salty corrosive environment had worn Aloha’s airplanes out much faster than Boeing had predicted for general usage. Several other jets of theirs made one last flight to the mainland graveyard not long after the 243 accident.
Seems like the best use of lighter than aircraft would be for drones, specifically amazon delivery drones. Also the government does use them today when they need a stationary eye in the sky like at the Mexican boarder crossing.
Why use aluminum when you can use lead instead? https://www.youtube.com/watch?v=HZSkM-QEeUg
Well, the new blimp already has the engines attached to the internal frame and protruding through the pressure envelope, so I don’t think running things through the envelope is a problem.
Some balloons including the Breitling a Orbiter have heated their lifting gases, so it is clearly feasible. I don’t think it would be beyond reason to think that same technology couldn’t be applied to a powered blimp, if someone wanted. In such an application it would have to be pretty flexible, as you say. It could use the waste engine heat when the engines are running and the envelope needs heating, vent it when it doesn’t. When power isn’t needed but heating is perhaps the engines could be fun in a less efficient mode to maximize the heat they produce.
These things don’t seem to difficult from an engineering standpoint to accomplish.
I do suspect that someone in blimp design already has. I’d imagine the reason it hasn’t been implemented isn’t because of the difficulty of doing so. More likely it’s just that they don’t need to.
In balloons, this heating is most useful in long distance, endurance or overnight travel. At night, it gets cooler and, there’s no sunlight and a lighter than air balloon cools and shrinks and loses lift. To remain aloft you have to dump ballast. The next when it heats up again the balloon is lighter and rises to high, so you have to vent gas to remain at your proper height. The next night you have to dump ballast again. So, if you want to travel distance overnight or nights you are constantly dumping ballast or venting gas. One uses up a lot of capacity carrying all this stuff which is just going to be dumped or vented.
The blimps in use today generally aren’t going on long distance overnight or multi day trips, so it’s not worth doing, is my guess.
Well, some (e.g. Goodyear blimp) cover considerable distances. They typically adjust buoyancy with ballonets (air bags that can be inflated and deflated).
Why would you think that?
A balloon big enough to lift a mere 20lb package is the size of a big pickup truck. That’s not very convenient to maneuver a delivery to somebody’s front door. especially not compared to a multi-copter drone the size of a pizza box.
In any kind of wind it takes insane amounts of horsepower to control a balloon/blimp. Power that could instead be used to hover and to fly 5-10x faster. For a delivery service, speed is key. All else equal, a vehicle that travels twice as fast delivers almost 2x as many packages per workday. A productivity difference of 5x or 10x is a total game-changer.
Conversely, your point about the government aerostats is spot-on. When you want something substantially stationary in the sky for a long term, buoyant lift is tough to beat. The only potentially competitive tech is still in the R&D phase now, and that’s solar-powered high altitude unmanned sailplanes that can stay aloft for weeks without needing to land for maintenance. But the key word here is “stationary”; the cable to the ground is what makes the aerostats practical. If they had to use conventional engines to stay in position (i.e. if they were free-flying blimps) they wouldn’t work.