Joe Johnston explains the secret origins of The Empire Strikes Back's new Star Wars vehicles

The story of Star Wars is, particularly in the early days of the franchise, a story of innovation. George Lucas and his team were constantly striving to make the kind of sci-fi adventure film they wanted to see, and that very often meant inventing things that didn't exist before, or at least making something that looked like nothing else in sci-fi cinema at the time. That spirit extends into every aspect of the original trilogy, but it was often particularly true of the many vehicles designed for the film. As The Empire Strikes Back turns 40, one of the key visual effects minds behind arguably the greatest Star Wars movie ever is taking a look back at how some of the film's iconic designs came to be.

Joe Johnston has since gone on to become a celebrated director in his own right, but back in the early Star Wars days he was just beginning to make the transition to film from a background in industrial design. By the time of Empire, he was working as a visual effects art director at Industrial Light & Magic, which meant one of his key tasks for the film was designing new ships and vehicles. And of course, when we think of new ships and vehicles in The Empire Strikes Back, we immediately think of the Battle of Hoth. In a new StarWars.com oral history of how that celebrated planetary struggle came to be, Johnston explained that the sequence started with a sense of real freedom, as Lucas wasn't usually keen on laying out specific battle action in his scripts.

"George had ideas about how to do things but he really wanted us to explore the best ways to figure out the action. Specifically, to the Battle of Hoth, I just remember him saying, 'Just come up with shots. They don't even have to be connected. Just come up with cool shots that would show some ideas about how this ragtag band of Rebels on this snow planet defend their base from an overwhelming force of Imperial soldiers.' At that point we hadn't designed the walkers yet. We knew they were going to have some kind of mechanized equipment, but it wasn't specifically a walker."

That "mechanized" equipment began in Johnston's original storyboard sketches as a massive Imperial tank with treads instead of feet. As the conceptualizing evolved into something more Star Wars-y, though, Johnston hit upon an idea inspired by a series of conceptual drawings by Syd Mead that were distributed to design students by US Steel. Though popular legend would have that the AT-ATs were inspired by construction cranes around the bay in Oakland, Johnston claims Mead's designs as the spark that led to the now-iconic vehicles.

"One of the illustrations in there was a truck that walked on four legs," Johnston explained. "The illustration of it was walking through this snowy forest and I thought, 'That is really cool.' It was a truck, it looked like some kind of equipment hauling vehicle or something — I don't remember what the intended purpose of it was. But it was definitely a truck.

"So I took the idea of this machine that walks on four legs and I made it, basically, a military vehicle with a separate head. As I remember it, the Syd Mead illustration was sort of all one piece. It was the truck body and sort of a cab up front. I don't think it was a separate head. I took that and started working up designs using that basic idea of a tank that walks on four feet through the snow, and that became the snow walker.

"We never referred to it as the AT-AT. We hated that name. [Laughs] We just called it the snow walker."

Next, it was time to come up with a new kind of fighter for the Rebels to battle the Imperial assault. As they worked on what would become the snowspeeders, Empire's design team decided that the ships would be in some way driven by the Rebel Alliance's need to recycle what they had on hand. As it often did with the look and feel of Star Wars, the best idea came from legendary concept artist Ralph McQuarrie.

"One of the ideas was, what if they took the cockpit off of a Y-wing, for instance, and powered just the cockpit, and made a little speeder out of that? We actually built a prototype of that," Johnston explained. "It was Ralph who said, 'If these rebels are having to build this stuff themselves, if they were building them from scratch, they would use all flat panels. They would just cut up some steel or whatever material the rebels had, and they would build these things out of flat panels.'"

Using the idea of a ship made entirely of "flat panels" as the jumping-off point, McQuarrie produced the first snowspeeder sketches, which Johnston and his team later refined.

"I would have to say that the original germ of an idea came from Ralph and his notion that the rebels didn't have the facilities to create anything that had compound curves on it," Johnston recalled. "They're just welding stuff out of sheet steel. [Laughs] The final design was following those design rules."

Though it wouldn't see a lot of action until Return of the Jedi three years later, The Empire Strikes Back also featured the debut of the AT-ST "chicken walker," the two-legged scout transport that accompanied the AT-ATs in the background of their assault. According to Johnston, the idea for the AT-ST grew out of his own research into military vehicles, and the realization that the assault equipment was always accompanied by smaller scout vehicles. To build the actual model, Johnston went not to the drawing board but to parts from model kits.

"It's one of the models that was designed through kit bashing," Johnston said. "I didn't do any drawings of it before I had a three-dimensional model of it. I just used model kits and planes and there was a model by — I forget the company name, it was a bulldozer. It had a lot of great parts.

"I basically was making the prototype and making it in three-dimensional form instead of doing sketches first. I finished the model, I showed George, he liked the design a lot. He said, 'That's great, but we don't have any time.' This was very near the end of the VFX process, so we didn't know how we were actually going to have time to get this thing into any shot in the movie. But George says, 'You know, we'll use it in the sequel. We'll use it in the next film,' which at the time was called Revenge of the Jedi.'"

Lucas made good on his promise and featured the AT-ST prominently in the third Star Wars film, but stop-motion animator Phil Tippett was also able to break down Johnston's model and rebuild it around a stop-motion armature in time to insert the "chicken walker" into a few shots of Empire.

For more on the making of the Battle of Hoth, from stop motion to sound design, check out the full oral history at StarWars.com.