Patriotic geeks who hoped to build a fully functional imperial walker out of the "Star Wars" universe have run up against Lucasfilm's intellectual property rights. But at least the indefinite delay gives them time to consider the massive engineering challenges of re-creating a 50-foot-tall robotic vehicle.

Making a modern-day robotic walker is not impossible, said Heiko Hoffman, a robotics expert at HRL Laboratories in Malibu, Calif., but it easily could cost $100 million or more.

The elephantine four-legged walkers used to attack a snowy Rebel Alliance base in the film "The Empire Strikes Back" were officially known as AT-AT walkers, for "All Terrain Armored Transport."

"[The cost] would likely be much higher for a seriously armored vehicle," Hoffman said of the cost. "If we just build an AT-AT that looks cool, it could be much cheaper."

Hoffman spoke as an outside expert with no connection to the grassroots "AT-AT for America" project. That effort began as an idea on the blog of one man, Mike Koehler, but quickly attracted hundreds of volunteer emails and donations through the crowd-sourcing website Kickstarter.

That project has shut down. "I don’t want anyone to feel that this was a hoax or a stunt," Koehler wrote April 17. "I genuinely felt that we could do this. With so much help, I was sure of it."

Walk, don't run

The AT-AT walker lumbers along like a mechanical elephant, lifting just one foot at a time. That "statically stable" walking style works for a heavy vehicle, because the center of mass always sits above a "triangle" created by keeping three feet on the ground, Hoffman said.

But building a huge 50-foot-tall walker is challenging because structural strength does not increase on par with sheer mass, Hoffman explained. A vehicle 10 times the size of a smaller model might have a supporting beam 10 times larger, but it would have to support a mass 1,000 times greater.

The AT-AT walker also must deal with huge stress on its leg joints, which makes running virtually impossible.

"In the running dog, a hind limb exerts 1.5 times the force of the body weight," Hoffman told InnovationNewsDaily. "Imagine a single AT-AT leg exerting a force 1.5 times the weight of the armored AT-AT."

Tank-like tracks might prove more suitable for a vehicle the size of an AT-AT, Hoffman said. He added that making the legs shorter also could improve the walker's safety, because its center of mass currently sits too high for comfort.

Leggy robot lineage

Science fiction has not ignored how robot legs can be a liability. Luke Skywalker's Rogue Squadron takes down one of the AT-AT walkers in "The Empire Strikes Back" by entangling its legs. And Peter Singer, a defense analyst at the Brookings Institute in Washington, D.C., notes that even small, furry Ewoks are able to take out a smaller variety of imperial walkers by using primitive log traps in "Return of the Jedi."

"Robotic legs remain incredibly complex and expensive, and not that capable, especially the bigger they get," Singer wrote in "Wired for War: The Robotics Revolution and Conflict in the 21st Century" (Penguin Press HC, 2009).

Back in reality, Japan's Sakakibara Kikai Co. created a two-legged "Land Walker" that resembles a futuristic 11-foot-high mech and wields two air cannons that fire rubber balls. But by shuffling along on wheels hidden inside its feet, it barely moves as fast as an elderly person with a walker, Singer said.

Perhaps the most visually impressive robot legwork comes from Boston Dynamics. The U.S. company developed a four-legged "Big Dog" robot that prances with deer-like grace while sounding like an angry chain saw. It even stays on its feet after a human gives it a hard shove, which might improve on the fatal flaw in the Empire's AT-AT Walker

None of the walking robots is anywhere close to battlefield-effective yet, Singer said in an email.

Not dead yet

Of course, even a fully functional imperial walker can't scatter rebel scum in style without laser beams attached to its head. In reality, modern laser weapons just don't have the destructive power to live up to the "Star Wars" ideal of laser cannons.

Still, all the engineering challenges don't seem to have dampened the enthusiasm created by the "AT-AT for America" concept, even if the project has been voluntarily shut down due to the intellectual property concerns involved in re-creating a "Star Wars" vehicle.

Koehler wrote a blog post entry April 17 about having an idea for how to move the project forward in cooperation with Lucasfilm. He pointed to the swarm of volunteers who emailed to express willingness to help.

"I also ask for your ideas," Koehler wrote. "What do you do with hundreds of passionate people who want to do something to make our country awesome?"

You can follow InnovationNewsDaily Senior Writer Jeremy Hsu on Twitter @ScienceHsu.