Firing my laser

The exterior of the F-1 was meticulously photographed and then mapped with a structured light scanning rig, which uses a projector to paint a pattern of stripes onto the surface being scanned. Mounted on the side of the projector are two cameras which each record how the pattern falls on the surface being scanned. For every exposure, the projectors and camera capture sixteen different stripe patterns.

The structured light rig can focus on an area ranging from 65mm in size all the way out to 1.5 meters, so getting a surface scan of the entire F-1 engine required a lot of crawling around and manually aiming the scanner rig. It wasn't immediately obvious how a bunch of handheld scan images could maintain coherency—how do you indicate to the scanner that picture 2 is linked to picture 1?

The answer was both simple and brilliant. "You notice these little targets? These little stickers?" said Shape Fidelity engineer Rob Black, who was demonstrating the equipment for me. Black indicated the small white-on-black circular dots pasted on the test material on the table in front of us. All around us were disassembled F-1 engine pieces, and I noticed that every single component was peppered with the little dots. "We stick these things on by hand, and the scanner sees these targets, so when we move from one position to the next, it can see what's coming... and stitch everything together. That way, we don't have to use encoders or robots."

Each part gets tiny dots hand-applied in what is effectively a random, unique pattern. The structured light software can use the unique layout of dots to stitch together all pictures of the object being scanned, without requiring the camera to be mounted in a motion controlled rig.

"We took just a regular digital camera and walked around the engine and took photographs," Black said. "The software took all those photographs and built a 3D coordinate for each of the targets, and what you get is a very sparse data set—it's basically the X-Y-Z value of the center of these points."

After the point map was assembled, Black performed a detailed structured light scan of the entire outside surface of the engine. "But what we wanted was a scan of the inside—the vanes, the clearances, all the definition of the interior," explained Black.

Taking the F-1 apart to get at its insides was always part of the plan, but as the team proceeded, it became obvious that actually cracking the thing open without breaking it was going to require specialized tooling—tooling that might have existed 40 years ago but which has long since been destroyed or lost.

The exterior scan was therefore used to develop the specialized tooling needed to fit the F-1's nuts, bolts, and fasteners. Some of the bolts were annoyingly unique—Betts noted that at least one high-torque bolt in the turbopump assembly required its own special torque adapter to remove.

The team was able to use the structured light scan of that particular bolt and, in less than half a day, to fabricate a tool using an additive manufacturing method called electron beam melting to quickly "print" 3D projects out of metal powder. Armed with this and other custom tools, Case, Betts, and Coates took the engine apart, down to its tiniest components.

"So what that let us do was scan the parts—all the individual pieces and parts," Black said. He pulled up a PowerPoint presentation on his laptop and pointed at one particular slide. "This is an example of one of the scanned pieces. You'll notice the gray is the scanned data, like we got on the screen here, but it also maps to points. Well, those points are the same points that were mapped in the assembly [the initial scan]. There's only one way that part will fit into that constellation of points, and that's what you see on the lower right."

"And so what you get now is a true 3D definition, inside and out, of all the relationships—not just the part geometry, but the relationship between the parts. And we did this for all the parts that you see on the shelves here," Black added.

Touching the past

The result was a complete and highly accurate CAD model of the entire F-1 rocket engine, down to its tiniest bolt. The fidelity was so good that the scanner even picked up tiny accumulations of soot left on the turbine blades from the engine's previous test firing back in the 1960s. The engineers removed the soot and re-scanned, but even this seemingly trivial accumulation yielded valuable data—sooting is a problem with kerosene-powered engines, so understanding how it builds up inside the engine could reduce its occurrence.

"Because they didn't have the analytical tools we have today for minimizing weight, everything was very robust," noted Betts, when I asked what they found as they tore down the engine. "That's apparent in really every aspect of the engine. The welds—"

"Oh, the welds!" interrupted Case. "The welds on this engine are just a work of art, and everything on here was welded." The admiration in his voice was obvious. "Today, we look at ways of reducing that, but that was something I picked up on from this engine: just how many welds there were, and how great they looked."

"You look at a weld that takes a day," he continued, "and there are thousands of them. And these guys were pumping engines out every two months. It's amazing what they could do back then and all the touch labor it took."

"Their ability to withstand imperfection, too," said Betts. "There were a few things on the engine that we disassembled, where today you may throw that part away because of the imperfections, but it goes to show that they fully understood what the big drivers were in their design. That's one thing we were trying to get knowledge on: what imperfections were OK to live with versus what imperfections are going to give us problems?"

"Like with the injector," said Case, speaking of the 44-inch (1.1 meter) metal plate that spewed the propellent into the engine's nozzle. "There are hundreds of holes drilled into the main injector—all drilled by hand, too. And one of the holes you can actually see where the drill bit came down at the wrong spot, and the guy just stopped—you can see where he moved over to where the hole was supposed to be and finished drilling the hole. They kept that and would have flown with that engine. Those kinds of things were pretty neat."

"One thing I notice when I look back at older engines," commented Coates, the senior engineer, "was just like Nick and Erin were alluding to: the complexity of the welds. You didn't have the kind of advanced manufacturing we had today, so quite honestly, these were hand-made machines. They were sewn together with arc welders, and it's pretty amazing to see how smooth and elegant it came out. Today, you'd look at doing precision casting, not these thousands of welds."

Lighting a 40-year old candle

The engine disassembled by Betts, Case, and Coates was number F-6090, assembled in December 1968 just as Apollo 8 was carrying three astronauts further away from Earth than any human being had ever before traveled. F-6090 had been test-fired for 240 seconds and then mounted on the S-IC stage of the Saturn V that would have flown as Apollo 19, but the engine was eventually pulled and placed into storage at MSFC. As the team methodically stripped engine F-6090 down, it became obvious that a test-fire of some of the engine's components was within the realm of probability.

With F-6090 being torn apart to learn from, the team turned to engine F-6049, which had served for years as a display engine at the Udvar-Hazy Center at the Smithsonian National Air and Space Museum. F-6049 was in even better condition than F-6090, but simply firing the entire F-1 engine straight away wasn't practical. For one thing, though the F-1s were originally tested at MSFC in the 1960s, that test infrastructure has since been repurposed. In addition, the city of Huntsville has grown up considerably since the Apollo era; lighting off an engine the size of an F-1 at Marshall today would likely blow out every window in the entire city.

Instead, the team decided to start with a series of firings on F-6049's gas generator. An engine like the F-1 is sort of like two separate rocket engines: one small, one large. The smaller one consumes the same fuel as the larger, but its rocket exhaust is not used to lift the vehicle; instead, it drives the enormous turbopump that draws fuel and oxidizer from the tanks and forces them through the injector plate into the main thrust chamber to be burned.

As with everything else about the F-1, even the gas generator boasts impressive specs. It churns out about 31,000 pounds of thrust (138 kilonewtons), more than an F-16 fighter's engine running at full afterburner, and it was used to drive a turbine that produced 55,000 shaft horsepower. (That's 55,000 horsepower just to run the F-1's fuel and oxidizer pumps—the F-1 itself produced the equivalent of something like 32 million horsepower, though accurately measuring a rocket's thrust at that scale is complicated.)

Getting the gas generator ready for firing would be a huge step in teaching Betts and Case about LOX/RP-1 engines, and it would provide modern data on just how well the old components operate. Betts, Case, and Coates pulled the gas generator, the gas generator injector, and the gas generator combustion chamber from F-6049, along with one of the ball valves for the propellent. Every "soft good" in the gas generator—every seal and gasket—had to be recreated from scratch, since all had hardened or rotted. In the process, the team had to spend quite a bit of time ensuring that they were creating functional seals and gaskets, since plastics technology had changed considerably since the 1960s. Just creating the soft goods required a lot of chemistry work.

As the preparation for the gas generator tests continued, though, something happened that caused the exercise to shed its academic roots and turn very, very practical.