Lab Notes
Iron Manic, Lithium Depressive

Stronger than steel, more powerful than an M1 tank, able to leap tall buildings and defeat bad guys ... Who among us has never dreamed of being a superhero? Unfortunately, Superman is from another planet, Spider-Man was bitten by a radioactive arachnid, and Batman combines the resources of an aerospace corporation, the intellect of a Sherlock Holmes or a Thomas Edison and the physical prowess of both an Olympic gymnast and a UFC cage-fight champion. Hard to imagine you and I could measure up against a standard like that!

But when a wounded Tony Stark first climbed inside his Iron Man armor in 1963 (possibly inspired by Robert Heinlein's Starship Troopers, 1959), he offered up the possibility that with the right equipment, any ordinary person (well, any billionaire genius) could grow up to be a bulletproof, cement-wall-smashing, crime-fighting rocket jock.

Created by comic legends Stan Lee, Jack Kirby and Larry Lieber, the Iron Man character was conceived as a playboy businessman with a crime-busting secret identity. DC's Bruce Wayne has never been cited as an inspiration, but I'd be very surprised if that thought wasn't kicking around somewhere in the brains of Batman's competitors over at Marvel. In any case, the new Iron Man movie—written by Mark Fergus and Hawk Ostby (the same team that brought us Children of Men) and directed by John Favreau (better known as an actor, whose credits include "Gutter" from the 1994 college comedy PCU and a recurring boyfriend role on Friends)—moves the story firmly into the 21st century with issues of substance abuse, corporate ethics, terrorism and military adventurism taking front-and-center status.

An intro to exos

I won't bore you with the question of whether powered exoskeletons are possible, because they've already been invented and are on the verge of being deployed in the real world—much like Ripley's "loader" in the 1986 classic Aliens. But if you look closely at the demonstration videos (see, for example, this) you may notice heavy wires connecting these suits to an external power supply somewhere out of the frame. Sure, your cell phone battery may last a couple days between charges, but as any Prius driver can tell you, the juice doesn't last very long when it's running the motors to push a heavy machine around. Any discussion of powered battle armor is, first and foremost, a discussion about the power itself—where it comes from, how it's stored, and how it can be harnessed without blowing the aforementioned battle armor to Kingdom Come, along with its occupant and any unfortunate bystanders.

A kilowatt-hour is roughly the energy required to run a hair dryer for 30 minutes, or nurse an electric scooter six kilometers up a gentle hill. Unfortunately, today's best lithium-ion batteries store only about 0.1 kilowatt-hours for every kilogram they weigh. The technology is moving fast, though, and if we imagine that a latter-day Howard Hughes figure like Tony Stark has access to the top-secret best of the best, then for the sake of argument let's assume he's got 10 times the storage capacity of a plug-in hybrid Prius, or about 15 kilowatt-hours.

Assuming the suit consumes 25 kilowatts during superhero-type activity (about half as much as a Harley Davidson), then his battery power will last a little over an hour and a half before he's forced into standby mode. That's fine for short smash-and-grab-type missions, or to get Stark through a single quick firefight or daring escape, but sheesh, most of us spend longer than that just commuting to work and back. How useful is the strength of Superman when it's coupled to the endurance of Deputy Dawg?

There's a reason we started using gasoline for cars, and a reason we still use it today. Gasoline stores 12 kilowatt-hours per kilogram—over a hundred times as much as a battery. Diesel fuel and kerosene are better still, but not by much. Anyway, assuming a decent engine or generator, a few gallons of petroleum-based fuel should be enough to power Iron Man through the better part of a day. But what's a decent engine? Will it make as much noise as a chainsaw motor? A turbine engine would certainly be quieter than internal combustion, but even better would be a scheme for direct conversion of gasoline into electricity through some sort of controlled chemical reaction. Possible? Sure. If nothing else, you could strip the hydrogen off the gasoline molecules and react it with oxygen in a fuel cell.

A more sophisticated gizmo would make use of the carbon as well, and greenhouse effects be damned. This kind of flameless combustion generator is not something you can buy at the hardware store, but again, Stark's access to the best military technology makes this more plausible, at least within the time frame of the next 10 years or so.

Heavy mettle and nuclear power

And how about those rockets in the soles of his feet? Nope. Sorry. For a typical rocket engine, it would take at least a kilogram of propellant to lift a man 100 feet in the air and lower him gently to Earth again. Depending how much Iron Man actually weighs, this number could rise by quite a bit. Most of that propellant is oxidizer, too; the space shuttle's external fuel tank carries seven times as much oxygen, by weight, as hydrogen. However, jets engines simply burn their fuel with a steady supply of air rushing through them, meaning they don't have to carry any oxidizer. The weight savings are tremendous, which is why we use jet engines, not rocket engines, to push airplanes around. A simple rocket engine is much easier to miniaturize than a jet, but recent improvements in computer-aided design, computer-aided manufacturing, 3-D stereolithography, microelectromechanical systems (MEMS) and materials science make miniature turbojet engines a real possibility.

If we mount one on the outside ankle of each iron boot, and run them on the same liquid fuel that powers the exoskeleton motors themselves, then we're simply left with a control problem; how does Stark keep his balance with a force of several hundred pounds pushing against his feet? Still, this strikes me as a learnable skill. I really don't think Iron Man will get very far, though; every minute of flying would probably cut two or three hours off his total endurance. Much more plausible is the Starship Troopers model, where jets are used to "bounce" around on the battlefield rather than to fly above it, Superman-style.

What about nuclear power, you ask? Well, we're talking about a lot of energy here, and the shielding required to operate the reactor safely—especially right next to human skin!—would weigh Iron Man down even more than he already is. Tellingly, no one has ever succeeded in building a nuclear-powered aircraft of any kind.

Overall I give Iron Man a C-minus for science—poor but passing. It's certainly possible, but the limitations and technical challenges are formidable and have traditionally been glossed over with little regard for plausibility, practicality or even any real sense of drama. Call me crazy, but the weaknesses of a real exoskeleton could be a lot more exciting than the simple knowledge that good will always throw evil through a wall. But maybe that's too much to hope for in a movie based on comic books.

Sources:
The Internet Movie Database (www.imdb.com): "Iron Man"
www.rottentomatoes.com: "Iron Man"
Wikipedia (en.wikipedia.org): "Iron Man", "Gasoline"
McCarthy, Wil: Hacking Matter, Basic Books, 2003
Glover, Thomas J.: Pocket Ref, Sequoia Publishing, 1989
Conroy, Mike: 500 Comic Book Villains, Barrons Educational Series, 2004
howstuffworks.com: "Harley Davidson Engine"
http://eng.usna.navy.mil/~bruninga/FD-Prius-Power.html