Hitting the Books: Elon Musk and the quest to build a better rocket engine

It took 15 years of iteration to quadruple the Merlin's power output.

Putting people safely into orbit is no small feat, especially without the seemingly limitless R&D budgets afforded to national space programs. However that has done little to dissuade a new generation of the private spaceflight companies from looking to the heavens above and thinking, "yeah, I can get up there, no sweat." In Liftoff: Elon Musk and the Desperate Early Days that Launched SpaceX, senior Ars Technica space editor, Eric Berger chronicles the company's tumultuous 20-year effort to field reliable and reusable rocket technologies, culminating in a heady decade-long run at the forefront of private spaceflight.

Liftoff by Eric Berger

From the book LIFTOFF: ELON MUSK AND THE DESPERATE EARLY DAYS THAT LAUNCHED SPACEX by Eric Berger. Copyright © 2021 by Eric Berger. Published on 3/2/2021 by William Morrow, an imprint of HarperCollins Publishers. Reprinted by permission.

In the spring of 2016, Amazon founder Jeff Bezos invited a handful of reporters into his rocket factory in Kent, Washington. No media had been allowed inside before, but Bezos’s secretive, fifteen-year-old space company named Blue Origin was finally beginning to reveal the full scope of its plans. Like Musk, Bezos had identified low-cost access to space as the key hurdle standing between humans and moving out into the Solar System. He, too, had begun building reusable rockets.

Over the course of three hours, Bezos led a tour through his glossy factory, at turns showing off Blue Origin’s tourist spacecraft, large rocket engines, and large 3D printers. He also shared his basic philosophy of Gradatim ferociter, Latin for “Step-by-step, ferociously.” Rocket development begins with the engine, explained Bezos, who was then working on his fourth-generation engine, known as the BE-4. “It’s the long lead item,” he said, casually strolling through the factory, wearing a blue-and-white-checkered shirt and designer jeans. “When you look at building a vehicle, the engine development is the pacing item. It takes six or seven years. If you’re an optimist you think you can do it in four years, but it still takes you at least six.”

In the fall of 2019, as we talked on board his private Gulfstream jet, I related this story to Musk. It was a Saturday afternoon, and we were flying from Los Angeles to Brownsville, Texas. This interview had originally been scheduled for early evening the day before, at SpaceX’s factory in Hawthorne, California. An hour past the scheduled time on Friday, his apologetic assistant texted that a crisis had come up. Musk felt terrible, she said, but we would have to do the interview at a later date. I returned to my hotel, preparing to fly back to Houston, when the assistant called back that evening. Musk had decided to visit the company’s Starship build site in South Texas that weekend and wanted to know if I cared to tag along. We could do the interview during the flight.

Three of Musk’s sons joined their dad for the trip, along with their dog Marvin (as in Marvin the Martian). A well groomed and mannered Havanese, he adored his master. With Marvin at Musk’s feet, we had gathered around a table at the back of the plane, for the interview. Clad in a black “Nuke Mars” T-shirt and black jeans, Musk wanted the boys to hear Dad’s stories about the old days.

Musk laughed when told about Jeff Bezos’s timeline for engine development. “Bezos is not great at engineering, to be frank,” he said. “So the thing is, my ability to tell if someone is a good engineer or not is very good. And then I am very good at optimizing the engineering efficiency of a team. I’m generally super-good at engineering, personally. Most of the design decisions are mine, good or bad.” Boastful? Maybe.

But SpaceX built and tested its first rocket engine in less than three years with Musk leading the way. Musk and Bezos, at least, would agree on this much: the process of building a rocket begins with the engine. After all, engine is the root word of engineer. In principle, a rocket’s propulsion system is simple: An oxidizer and a fuel flow from their respective tanks into an injector, which mixes them as they enter the combustion chamber. Inside this chamber, the fuels ignite and burn, producing a superhot exhaust gas.

The engine’s nozzle channels the flow of this exhaust in the opposite direction a rocket is meant to go. Newton’s Third Law of Motion — for every action, there is an equal and opposite reaction — does the rest.

Alas, the reality of building a machine to manage the flow of these fuels, control their combustion, and channel an explosion to lift something toward the heavens is staggeringly complex. And that’s not to mention fuel efficiency. A rocket engine’s thrust depends on the amount of fuel burning, its exit velocity, and pressure. The greater each of these variables are, the more thrust an engine produces, and the heavier payload it can power into orbit. Conversely, if it takes too much fuel to produce a large enough thrust, or the engine is too heavy, a rocket will never leave the ground.

Musk recognized early on that when it came to propulsion, [founding member of SpaceX and American aerospace engineer Tom] Mueller was not a good engineer—he was a great one. For the Falcon 1 rocket Musk wanted a lightweight, efficient engine that produced about seventy thousand pounds of thrust. This, he reasoned, should be enough to get a small satellite into orbit. Mueller had helped design and build several engines at TRW, some more powerful than this, and some less so. The Merlin engine would draw upon some of these concepts and ideas, but Mueller said he and Musk began with a “clean sheet” design.

Few of Mueller’s friends in the industry believed building a brand-new, liquid-fueled rocket engine without government backing was possible. “All these guys told me a private company can’t build a booster engine, that takes the government,” Mueller said. SpaceX did not invent the Merlin engine out of whole cloth. As with almost all rocket engines, the Merlin drew on previous work. For example, although Mueller had developed a lot of different engines, he lacked experience with turbopumps. Rockets use a staggering amount of fuel, and a turbopump is the machine that feeds propellant into a rocket engine as fast as possible. Inside the Falcon 1 rocket, liquid oxygen and kerosene fuels would flow from their tanks into a rapidly spinning pump, which would spit out this propellant at high pressure, delivering fuel into the combustion chamber primed to produce the maximum amount of thrust. One of the first issues Mueller had to address was how to build a turbopump.

In the late 1990s, NASA had developed a rocket engine nearly as powerful as the proposed Merlin engine called Fastrac. There were other similarities. Fastrac used the same mix of fuels, liquid oxygen and kerosene, a similar injector, and had the potential for reuse. Despite a series of successful test firings, NASA canceled the program in 2001. Given these commonalities, Mueller thought SpaceX might be able to use the turbopumps NASA built for the Fastrac engine. He and Musk visited NASA’s Marshall Space Flight Center in Alabama shortly after Fastrac’s demise in 2002, and asked if they could have them. Yes, they were told, but SpaceX would have to go through NASA’s procurement program, which could take a year or two. This was too slow for SpaceX, so Musk and Mueller moved on to Barber-Nichols, the contractor that had built the turbopumps.

Barber-Nichols, it turned out, had had a devil of a time building the Fastrac turbopump. To work with the larger Merlin engine, Barber-Nichols would need to do a lot of redesign work. They went back and forth with the SpaceX engineers. During one visit to the Colorado-based company, a designer there happened to suggest a name for the engine to Mueller. Musk had chosen the Falcon name for the rocket, but said Mueller could name the engine, stipulating only that it shouldn’t be something like FR-15. It should have a real name. One Barber-Nichols employee, who was also a falconer, said Mueller should name the engine after a falcon. Then, she began listing various species of the bird. Mueller chose the merlin, a medium-sized Falcon, for the first-stage engine. He named the second-stage engine after the smallest of falcons, the kestrel.

When Barber-Nichols finally delivered the redesigned turbopump to SpaceX in 2003, it still had major problems. This forced Mueller and his small team to begin a crash course in turbopump technology. “The bad news is that we had to change everything,” Mueller said. “The good news is that I learned everything that can go wrong with turbopumps, and really how to fix them.” Because the pressurization of rocket fuel allows an engine to squeeze out a maximum amount of thrust, good turbopumps are essential. This would become one secret to SpaceX’s eventual dominance of the global launch market. Mueller said the original pump from Barber-Nichols weighed 150 pounds, with an output of about 3,000 horsepower. Over the next fifteen years, SpaceX engineers continued to iterate, changing the design and upgrading its parts.

The turbopump in the modern-day Falcon 9 rocket’s Merlin engine still weighs 150 pounds, but produces 12,000 horsepower.