SpaceX celebrated the first flight of its Falcon 9 rocket in over four and a half months on Saturday, with a remarkably smooth launch of the vehicle from California. The Falcon 9 had previously been grounded since September, after one of the rockets exploded on a launchpad in Florida during a routine fueling procedure. Though the stakes were high for Saturday's launch, the mission's success doesn't relieve the pressure on SpaceX.

Questions about the accident's cause have been circling the company for months, and the company only disclosed the probable source of the explosion two weeks ago. The failure was apparently complex, involving an elaborate interplay of materials in an extremely cold propellant tank. That's not all: SpaceX uses its propulsion materials in a way that's different from its competitors, and this may have partially contributed to the failure.

The Federal Aviation Administration has accepted the explanation SpaceX gave for the failure, and the company says it's taken steps to fix it. So SpaceX needed last weekend's launch to go well, to regain its customers' confidence. But in the space business, confidence isn't built on one launch alone. The September explosion was the second major failure for the company in just over a year, bringing down the Falcon 9's reliability rating.

SpaceX is scheduled to carry people aboard its rockets next year for NASA. When that happens, failures could potentially have more than just financial repercussions; people's lives will be at stake, and the company must ensure there are no more hiccups before passengers start flying on the Falcon 9. SpaceX will need to consistently launch its rockets over the coming year in order to establish that its vehicles are dependable again.

What happened
The September explosion began in the rocket's upper liquid oxygen tank. That's where the "super-chilled" liquid oxygen propellant is stored, which is needed to fuel the engine in the upper part of the rocket. Liquid oxygen is a common propellant used by aerospace companies, but the kind that SpaceX uses is even colder than most liquid oxygen propellants, which hover around -298 degrees Fahrenheit. SpaceX keeps its oxygen extra cold - at -340 degrees Fahrenheit, close to its freezing temperature - since it makes the propellant much more dense. That way more of the oxygen can fit into the tank, increasing the amount of propellant SpaceX rockets can use, according to CEO Elon Musk.

The problem is that the cold makes liquid oxygen temperamental. For one, it warms up pretty quickly and runs the risk of boiling off. That's why SpaceX loads its propellant half an hour to 45 minutes before launch - to prevent as much of it from boiling away as possible. The procedure, known as "load and go," has resulted in a few launch delays: SpaceX launches have been halted due to rising liquid oxygen temperatures before takeoff.

In the September explosion, the super-chilled liquid oxygen had a bad reaction with something inside the propellant tank - one of three small vessels that hold very cold helium. These tanks are called composite overwrapped pressure vessels, or COPVs, and they're an important part of the Falcon 9 propulsion system: they help to maintain pressure in the liquid oxygen tank. The helium stored in the COPVs is used to fill up the empty space left behind when the liquid oxygen exits the tank during flight.

Engineers like using COPVs because they're fairly lightweight. In SpaceX's case, the COPVs consist of an aluminum liner wrapped in carbon fibers. This way, when the pressurized helium is loaded into the vessel, the aluminum liner can expand, while the carbon overwrap keeps the entire thing from bursting. Engineers have used all-metal vessels to store pressurized helium in rockets before, but that kind of design is heavier than a COPV. And lighter is always better for escaping Earth's gravity. "The carbon overwrapped pressure vessels are the lightest weight ones you can build, and have a long history of successful performance," Glenn Ecord, a retired NASA engineer who worked with COPVs on the Space Shuttle program, tells The Verge.

The problem for SpaceX, however, is that the COPV design may have been partially to blame for the accident. Something caused the aluminum liner of the vessel to scrunch up, or buckle, creating hills and recesses in the metal. Then liquid oxygen seeped between the carbon fiber overwrap and the aluminum liner - something that's not supposed to happen. The liquid oxygen then pooled in the recesses, right up against the liner. That became a problem. The helium in the COPV got so extremely cold that it may have caused the nearby liquid oxygen to solidify, according to SpaceX, and solid oxygen is much more likely to ignite due to friction. Friction from the carbon fiber overwrap may have caused the spark needed to set the oxygen on fire, leading to a chain reaction that ultimately destroyed the rocket.

Why SpaceX is unique
COPVs have long been used in aerospace engineering and are common in many rockets. They're used by other major rocket manufacturers, such as the United Launch Alliance, to pressurize propellant tanks. Even NASA used up to 24 COPVs on the Space Shuttle to store pressurized gases.

Typically, COPVs are kept outside of a tank and are rarely kept at such cold temperatures. But SpaceX's use of COPVs is different. It puts its COPVs inside its super-cold liquid oxygen tank. "It is certainly a unique feature," Michael Kezirian, an adjunct associate professor of astronautics at the University of Southern California, tells The Verge. "There are probably good reasons why they designed the system in that manner, but it's not something I recall encountering before with respect to the use of COPVs."

SpaceX may do this to save on weight. Storing the COPVs inside the liquid oxygen tank allows the helium vessels to be extra cold. This allows them to be built smaller and, therefore, lighter. But keeping the COPVs inside such a cold environment has another side effect. Such low temperatures can cause the carbon overwrap of the COPV to expand while the aluminum liner contracts, creating a gap between the two liners. That may be how the oxygen is getting trapped between the overwrap and the aluminum, according to Kezirian.

"You manufacture the vessels in such a way that the liner in no way separates from the overwrap, so there is no opportunity for fluid to enter between the liner and the overwrap," says Kezirian. "As the temperature falls, the tendency in any COPV is for the metallic material to contract and the carbon fiber to expand. A consequence of a very low temperature could be to create an opportunity for this compression to be lost." This is, however, informed speculation; SpaceX hasn't said how the oxygen was trapped between the liners.

We do know from the Space Shuttle program that buckling can occur in COPVs. NASA tested each COPV after it was made by simulating the pressure of a flight, in a process called "proof testing." The tests strain the liners, and when the pressure is relieved, the COPV may buckle as it returns to its normal shape. It's possible that SpaceX, by avoiding proof tests, can dodge the buckling, says Ecord. "If you don't buckle the tank during the proof test, then any oxygen getting in there can't puddle," says Ecord.

SpaceX declined to comment on how it would prevent buckling. The company also didn't say how it would keep the oxygen out of the COPV liners, citing proprietary concerns and restrictions from International Traffic in Arms Regulations. But the company said it has identified "credible causes" for the failure and would address these design concerns in the long-term. In the meantime, however, SpaceX will load the helium at a warmer temperature, and return to an older way of loading that's been successful before. That should prevent the oxygen from turning into a solid, lowering the chances of the propellant catching fire.

It's unclear how SpaceX will prevent COPV buckling or stop oxygen from getting between the composite overwrap and the metal liner. "I personally would recommend no oxygen between the overwrap and liner," says Ecord. "If oxygen can't get into the buckles, it can't happen again. I don't know what they've done about that; maybe changing the thermal conditions will keep that tight." Ecord says he's confident, though, that SpaceX has fully addressed the issue.

Concerns about propellant loading
The September accident also raised concerns about SpaceX's loading procedures. In November, a panel of expert advisors to NASA expressed anxiety about SpaceX's plans to load propellant into the Falcon 9 with people on board. In the past, NASA astronauts boarded the Space Shuttle only after propellant had been loaded. But when astronauts fly with SpaceX, the plan is for them to board the crew capsule on top of the rocket before the propellant is loaded. This unusual order of events is required so that the super-chilled liquid oxygen doesn't warm up too much on the launchpad.

The advisory panel's anxieties were echoed in a recent report from NASA's Aerospace Safety Advisory Panel, which also discussed the need to better understand the cold temperatures associated with SpaceX's loading procedures.

"We believe that the focus of the investigation must not be solely to identify and fix the specific cause of this mishap," ASAP wrote in its report. "It must focus also on improving the understanding of how the system functions in the dynamic thermal environment associated with 'load and go' so that other previously unidentified hazards can be discovered. This is not a trivial effort."

It'll take time to see if the problem is truly fixed. The best thing SpaceX can do now is launch, successfully - a lot. The more successful launches, the higher the Falcon 9's reliability rating grows, and as that rating grows, confidence in the rocket does, too. SpaceX is poised to have a busy year ahead: Recent financial documents obtained by The Wall Street Journal show that SpaceX hoped to launch 20 times in 2016, but only managed to get eight missions up. So now SpaceX has a backlog in missions, making it likely that Falcon 9s will launch frequently this year. There's just little room for error.

This article first appeared in The Verge and is published with permission