No choice was more critical than the Merlin rocket engines used to power Falcon 9. SpaceX propulsion chief Tom Mueller and his team selected an engine type called the pintle that was pioneered by Mueller’s former employer, TRW, which used it for the descent stage of the Apollo lunar module. Unlike most rocket engines, in which droplets of fuel and oxidizer are sprayed into the combustion chamber through an injector plate resembling a shower head, the pintle uses a needle-like injector that’s more like the nozzle on a garden hose. It’s not only less expensive to make, Mueller says, but it is also less susceptible to combustion instability, a runaway buildup of energy within the thrust chamber that has vexed engineers since the dawn of the Space Age (it added years and many millions of dollars to development of the giant F-1 engines for the Saturn V moon rocket, for example). Combustion instability can make an engine undergo what veterans dryly call an RUD, for Rapid Unscheduled Disassembly; civilians would call it blowing up. Even the Merlin had a couple of RUDs in the early days of development. “There are a thousand things that can happen when you go to light a rocket engine,” Mueller says, “and only one of them is good.”
That, of course, was hardly news by the time SpaceX got started; studies had shown that over the previous two decades, the vast majority of rocket failures were due to engine malfunctions. And so, before attempting the multi-engine Falcon 9, Musk began with the smaller and less expensive Falcon 1, which uses a single Merlin engine in its first stage. Test launches of this 70-foot rocket, beginning in 2006, were SpaceX’s baptism by fire. Only after three failed attempts did the Falcon 1 become the first privately built liquid-fuel vehicle to reach orbit, in September 2008. Musk and his team were both elated and sobered. “We knew it would be hard,” Mueller says, “but it was harder than we thought.”
The rest of the aerospace world took notice. With the early Falcon 1 failures, says Stern, Musk “showed spine, showed he would spend his own money, showed he would stick with it.” And the lessons learned from Falcon 1 smoothed the path for Falcon 9, whose successful maiden launch, in June 2010, impressed observers accustomed to watching other would-be rocket startups, from AMROC in the 1980s to Kistler in the 1990s, fail before getting anything into space.
The Falcon 9 was designed from the beginning to be human-rated, meaning an increased focus on reliability. The rocket’s avionics and controls are triple-redundant (as will be some sensors in the human-rated version of the Atlas V), and the flight computers, which run on Linux, will “issue the right commands even if there’s severe damage to the system,” Musk says. The choice of nine engines for the first stage was made with reliability in mind: From the moment of liftoff, Falcon 9 can suffer an engine shutdown and keep flying; after about 90 seconds, it can tolerate a second engine shutdown. Even if an engine explodes, says Mueller, the others will not be affected.
Of course, SpaceX goes to great lengths to prevent such a scenario. Part of the Merlin’s qualification testing involves feeding a stainless steel nut into the fuel and oxidizer lines while the engine is running—a test that would destroy most engines but leaves the Merlin running basically unhindered. Every Falcon upper and lower stage is test-fired in Texas before it’s cleared to fly. “It’s very common to do component and system-level testing…. That’s very typical in aerospace, ” says Alan Lindenmoyer of Houston’s Johnson Space Center, who has been working with SpaceX since 2005 as manager of the agency’s Commercial Crew and Cargo program. “But to actually put a vehicle together and do system-level testing of the rocket is not. That’s a level of rigor you don’t typically see.” On the pad at Cape Canaveral, Florida, the rocket undergoes an additional brief firing a few days before launch. And just before liftoff, for a few moments after the engines are lit, their performance is analyzed by the Falcon’s computers before hold-down clamps are released and the rocket is allowed to rise.
The Merlin engine itself has undergone a number of improvements, including reducing the number of parts and increasing its power and efficiency. According to Mueller, the 140,000-pound-thrust Merlin 1D, designated the production model for Falcon 9, has the highest thrust-to-weight ratio of any rocket engine ever made.
Significantly, the Merlin engines—like roughly 80 percent of the components for Falcon and Dragon, including even the flight computers—are made in-house. That’s something SpaceX didn’t originally set out to do, but was driven to by suppliers’ high prices. Mueller recalls asking a vendor for an estimate on a particular engine valve. “They came back [requesting] like a year and a half in development and hundreds of thousands of dollars. Just way out of whack. And we’re like, ‘No, we need it by this summer, for much, much less money.’ They go, ‘Good luck with that,’ and kind of smirked and left.” Mueller’s people made the valve themselves, and by summer they had qualified it for use with cryogenic propellants.
“That vendor, they iced us for a couple of months,” Mueller says, “and then they called us back: ‘Hey, we’re willing to do that valve. You guys want to talk about it?’ And we’re like, ‘No, we’re done.’ He goes, ‘What do you mean you’re done?’ ‘We qualified it. We’re done.’ And there was just silence at the end of the line. They were in shock.” That scenario has been repeated to the point where, Mueller says, “we passionately avoid space vendors.”
In a few cases, SpaceX has even been able to advance the state of the art. For the Dragon’s heat shield, the company chose a material called PICA (phenolic impregnated carbon ablator), first developed for NASA’s Stardust comet-sample-return spacecraft. Rejecting the prices they were getting from the manufacturer, they took advantage of help from NASA’s Ames Research Center to make it themselves. According to Mueller, SpaceX’s material, called PICA-X, is 10 times less expensive than the original, “and the stuff we made actually was better.” In fact, says Musk, a single PICA-X heat shield could withstand hundreds of returns from low Earth orbit; it can also handle the much higher energy reentries from the moon or Mars.
Musk, who is SpaceX’s chief designer as well as its CEO, is involved in virtually every technical decision. “I know my rocket inside out and backward,” he says. “I can tell you the heat treating temper of the skin material, where it changes, why we chose that material, the welding technique…down to the gnat’s ass.” And he pushes his people to do more than they think is possible. “There were times when I thought he was off his rocker,” Mueller confesses. “When I first met him, he said, ‘How much do you think we can get the cost of an engine down, compared to what you were predicting they’d cost at TRW?’ I said, ‘Oh, probably a factor of three.’ He said, ‘We need a factor of 10.’ I thought, ‘That’s kind of crazy.’ But in the end, we’re closer to his number!”