nother cliffhanger ending, courtesy of Republic Serials! Another black-and-white biplane crash, another jetpack ride to the undercarriage of a zeppelin!
When I was growing up in Colorado Springs in the late 1970s, the local NBC affiliate used to crowd every weekday afternoon, betwixt the end of school and the start of the evening news, with matinee serials from the 1930s, '40s and early '50s. With names like Zombies of the Stratosphere and G-Men vs. Black Dragon, these shows had the sort of abysmally low budgets and laughable acting we expect today only from Mystery Science Theater 3000 and the Power Rangers. Channel 4 showed them for exactly this reason: because they were cheap, and Colorado Springs was a small town in those days. But they did have a few intriguing quirks, including a fetish for the hip new technologies of the pre-WWII era: aircraft and rockets. The technical details were all wrong, but the enthusiasm was so infectious that its spirit lives on today, in retrofuturistic media properties like The Rocketeer (1991, based on Dave Stevens' 1980s comic-book series of the same name) and Crimson Skies (Microsoft's 2000 PC game, based on a 1990 role-playing game from FASA).
Now Paramount is getting in on the action with the end-of-summer blockbuster Sky Captain and the World of Tomorrow. Kerry Conran's script offers up a 20th-century parallel universe of goofy flying machines, machine-gun-rattling dogfights, wing-walking aerobatics and other good clean fun, but this time the special effects are top-notch. As a result, the sky candy looks at least as real as the grainy World War II dogfight footage on The History Channel, and it begs the question: Could a world like this really have happened? Refracted through the twin lenses of hindsight and 21st-century sensibility, some of these aircraft look ridiculous. But then again, looking forward from 1930 you could say the same thing about the single-winged Fokker monoplane, which had been rather decisively shot out of the skies in World War I. Who'd've guessed that monoplanes would soon consign the trusty biplane to a few marginal roles, like crop dusting?
The simple fact is, while the laws of aerodynamics never change, the human fashions in exploiting them certainly do. History is littered with experimental aircraft that worked just fine, or mostly fine, but never became popular. So what's the deal? Could these World of Tomorrow behemoths really fly? Let's find out!
A future full of hot air
Ever since the first hot-air balloon lifted off in 1783, people have dreamed of lighter-than-air gunboats keeping peace among the clouds. The word "balloon" refers to any gas-filled bag, and most balloons are shaped like an inverted raindrop, because that's the lightest and lowest-stress design. It also provides a big area for the wind to push against, making a sort of aerial sailboat. Balloons are almost impossible to steer, though, unless you add some sort of fan to propel them. And once you do this, you'll quickly find you need to streamline the thing to reduce drag and improve maneuverability, until you have something that looks pretty much like a ship. Hence the term: airship.
Traditionally there are two kinds of airships: blimps, with soft, balloonlike hulls, and zeppelins, with balloon skins stretched tight over a rigid skeleton. The former were invented in France and the latter in Germany, where the engineering was so rigorous that some designs are still in use today. All the seminal reference works on airship design date back to the '30s and '40s and draw heavily on German sources. More recently a third type of airship has emerged, with a fully rigid hull. The problem with all of these is buoyancy. At sea level, a cubic meter of helium can lift about 800 grams of material, including the airship structure itself. A typical blimp, like Goodyear's Spirit of America, is 65 meters long and 17 meters wide, holds 7,500 cubic meters of helium, weighs about 5,800 kilograms and carries nothing but a pilot and cameraman.
But we want to arm it, right? Battleship guns are clearly too heavy for a blimp, but for the sake of argument let's say we want to carry a 20mm Vulcan cannon of the sort used on fighter planes. Including a power supply and a full drum of ammunition, this weighs about 400 kilograms. Recoil would be a serious issue, so at a bare minimum we should throw in another 400 kg for ballast and structural reinforcement, which means our blimp needs an additional 1,000 cubic meters of helium. Unfortunately, this increases the blimp's size, which increases its weight, which means we've got 775 kg more balloon material to lug round, which means we need still more helium to get it off the ground. ... When it all settles down, we've got a blimp 15 to 20 percent larger than Goodyear's. Actually firing the gun would be a wild ride, but there's nothing impossible about it.
Still, if we want more guns, or heavier guns, or a bigger load of ammunition, or enough buoyancy to rise more than a few feet off the ground, the size of the airship increases exponentially. Putting a real World War II-era battleship in the air would require a kilometer-wide balloon! This hardly seems practical, although we could gain an additional 8 percent lift by filling the bag with hydrogen instead of helium. An explosion risk, yes, but in my own brief foray into airship design, I found that hydrogen diluted with 40 percent helium would not ignite even under the flame of a welding torch. I'd call that an acceptable hazard, whose lift benefit is a respectable 5 percent. (The legal monster prompts me to add: Don't assume this is safe just because I got away with it.) If we're willing to heat this gas to a few hundred degrees, we can gain another 20 percent, which ain't bad.
Can we do better still? The only thing more buoyant than hydrogen is the vacuum of empty space, which could technically lift about 1.2 kilograms per cubic meter if you could somehow enclose it in a rigid balloon. Unfortunately, the air presses inward on this vacuum like a fist around an empty beer can, with crushingly similar results. Any container strong enough to withstand the pressure is too heavy to be lifted by the buoyancy. As an airship design, it simply doesn't work. Or anyway, not with metal or plastic. In the future it may be possible to build lightweight airship hulls out of diamond, filled with nothing at all and boasting 50 percent more lift than a humble blimp of the same size. You could pack some definite armament on that: three to six Vulcan cannons or perhaps one small howitzer. Still, since this technology is beyond us in the 2K-aughts, it would be really out of place in a 1930s cliffhanger. Let's look elsewhere.
Winging it into tomorrow
A spinning golf ball generates lift through something known as the Magnus effect, and in the late 1970s the Van Dusen Development Company built a special sort of rigid airship to take advantage of it. In fact, the ship looked like a helium-filled golf ball (the lifting sphere) clutched between the upraised wings of a manta ray (the gondola). At rest, the vehicle was heavier than air, but as the ball began to spin, the whole contraption would lift off the ground like magic. The test model was just over six meters long and was capable of carrying several kilograms of payload, but the full-scale version would have measured 120 meters and could have carried a whopping 70 tons (call it 90 Vulcan cannons, or one full-sized howitzer with a hefty load of ammo). This vehicle was intended for use in the Canadian timber industry, but alas, no one ever ponied up the money to build one. Which is too bad, because in my personal opinion it would have been the world's slickest airship by far, and one of the most mind-boggling sights in all of history. Bling bling, indeed.
Anyway. In Paramount's movie we can clearly see flying aircraft carriers supported by helicopter-like outboard engines. Would this work in the real world? Sure. Unfortunately, a typical propeller can generate only enough thrust to lift 25-50 kilograms per square meter of fan area. This is why helicopters have rotors with really long blades; any heavier-than-air skyship would look more like the two-rotor Chinook (the U.S. Army's beloved heavy-lift "Mixmaster") than like the aircraft carrier U.S.S. Ronald Reagan and would burn a lot of fuel very quickly. But with added lift gas, we can make a hybrid airship of it. A Goodyear-sized blimp filled with heated hydrogen-helium mix and supported by four-meter-wide engines at each of its four corners would be capable of lifting more than 3,500 extra kilograms. That's another four to nine Vulcan cannons, and if you scale the whole thing up until it's as long as the Reagan—almost exactly a third of a kilometer—you really could slap a modest flight deck and some fighter planes on top and sail the thing into battle.
But here we run into the sad reality of the combat airship: It's a sitting duck. "Armored balloon" is a contradiction in terms; barring some sort of self-healing material, a single bullet is sufficient to bring the airship down. Of course it might take weeks for all the helium to leak out through one small hole, but we're also talking about a really big target that could absorb stray fire from any direction. Even if all combatants tried their damnedest to aim away from the ship, it wouldn't likely survive in the midst of a real dogfight. Airships are also painfully vulnerable to extreme weather. But hey, in a Hollywood sense they look really cool coming down.
One last item: What's up with those flappy-wing aircraft? In technical terms these are called "ornithopters," and the first serious designs for them date back to Leonardo da Vinci. The first actual flying model was built by the German architect Karl Friedrich Meerwein in 1781, and actually succeeded in carrying him for a short distance down the bank of a hillside, mainly by gliding. Bird-sized ornithopters have been flying ever since, but generating positive lift with a 'thopter large enough to carry a human is not so easy. It requires large wings that have to be flapped very quickly, which in turn requires a powerful energy source, which must be light enough to be carried by the aircraft itself. This is why birds and flying dinosaurs grow only so large and no larger (see "Great Flaming Reptiles" for more details). In 1990, Russian scientist Vladimir Toporov and his students succeeded in building a four-winged, dragonfly-shaped ornithopter called Giordano, which carried a furiously pedaling pilot aloft for a journey of 300 meters. If Toporov and his pictures are to be believed, the vehicle is purely muscle-powered, takes off vertically, can achieve horizontal flight at speeds up to 50 kilometers per hour and has since been surpassed by even better designs.
Still, these Kitty Hawk clunkers are a long way from the deadly graceful machines of the World of Tomorrow, which might just work if they had a magic power plant of some sort. Those metal machines would never get off the ground flapping as slowly as they do in the film, though; in flight the wings would be as blurry and as loud as the blades of a propeller. They'd need to be very strong and very light, but not ridiculously so. Metal skin over a fiberglass skeleton might even suffice, albeit barely. Certainly graphite should do the trick.
My review? Overall I'd have to say that Sky Captain's world—however unlikely—does fall more or less within the realm of physical possibility. Which should help us appreciate the film all the more, for the men and women braving the skies in these iffy contraptions must be very courageous indeed. Happy landings!
Sources used for writing this column can be found here.
Wil McCarthy is a rocket guidance engineer, robot designer, nanotechnologist, science-fiction author and occasional aquanaut. He has contributed to three interplanetary spacecraft, five communication and weather satellites, a line of landmine-clearing robots and some other "really cool stuff" he can't tell us about. His short writings have graced the pages of Analog, Asimov's, Wired, Nature and other major publications, and his book-length works include the New York Times notable Bloom, Amazon "Best of Y2K" The Collapsium and most recently Lost in Transmission. His acclaimed nonfiction book, Hacking Matter, is now available in paperback.
