SpaceX fingers helium as cause of Falcon 9 rocket explosion

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The explosion involved a Falcon 9 rocket similar to this one used to launch Orbcomm 2(Credit: SpaceX)

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SpaceX has released the preliminary findings into the Falcon 9 accident on September 1 that destroyed the unmanned rocket and the Amos-6 satellite on Launch Complex 40 (LC-40) at Cape Canaveral Air Force Station, Florida. According to the company, the "anomaly" that caused the explosion is still uncertain, but it has narrowed things down to the cryogenic helium system used to feed the second stage engine.

The explosion of the Falcon 9 occurred not during flight or launch, but on the launch pad while engineers were fueling the rocket in the run up to a routine static firing, where the first-stage Merlin engines are fired while the booster is held to the ground. The engines were never fired, but, without warning, the rocket did blow up. No one was injured or killed in the accident, but the explosion destroyed the rocket and payload and seriously damaged the launch complex.

In the three weeks since, an accident investigation team from SpaceX, the US FAA, US Air Force, industry experts, and NASA has been studying the debris that was gathered and catalogued, 3,000 channels of telemetry data, and audio/video recordings. SpaceX founder and CEO Elon Musk even had to make a personal appeal to the public on Twitter to obtain the latter because SpaceX didn't have any cameras running at the moment of the accident.

According to SpaceX, the entire episode occurred in a mere 93 milliseconds from the moment the anomaly first registered until all data feeds from the rocket cut out. Though the launch pad was extensively damaged by a fire so intense that it fused the sand into glass, the rest of the fueling and control systems were too far away to be affected, as was the SpaceX hangar and SpaceX's second launch site at LC-39A.

In tracing the cause of the explosion, the investigators concluded that it was different from the CRS-7 midair explosion, which was caused when a substandard strut failed and a gas bottle broke loose in flight. Instead, the September 1 launchpad accident was due to a fault in the cryogenic helium system for the liquid oxygen (LOX) tank in the second stage.

SpaceX hasn't said exactly how the helium system failed, but the catastrophic nature of the explosion points to a likely scenario.

For the sake of simplicity and economy, the Falcon 9's second stage is a shortened version of the first stage, standing only 13.8 m (45 ft) high, yet retaining a diameter of 3.66 m (12 ft). It's powered by a modified Merlin rocket engine designed to operate in the vacuum of space.

Inside the second stage, the Falcon 9 carries 64,820 kg (141.713 lb) of super-chilled LOX and 27, 850 kg (61,398 lb) of RP-1, which is basically a super-refined grade of kerosene. To further save weight and keep the design simple, the fuel and oxidiser aren't kept in completely separate tanks, but in one large tank with an insulated bulkhead keeping the LOX in the upper half and the RP-1 in the lower half. The tank halves aren't pressured and while on the ground have an internal pressure equal to about that of a soda can.

Along with the propellants is the cryogenic helium system, which are insulated gas bottles filled with liquid helium that sit submerged in the LOX to keep them at a frosty −452.2° F (−269° C, 4 K, ). When the Falcon 9 second stage fires, turbopumps suck the LOX and RP-1 into the engine's combustion chamber, but the pumps can't do the job alone because the second stage fires in zero gravity, so the propellants need to be forced in at ignition. Furthermore, the pumps would soon cause a vacuum to form inside the tanks and the propellants wouldn't flow properly. This is where the helium comes in.

The liquid helium is run through a heat exchanger that uses the engine's heat to warm the helium back to a gas. This is fed into the LOX and RP-1 tanks, pressurizing them, and keeping a constant flow to the turbopumps, so the engines run smoothly.

The Falcon 9 second stage seen on the Orbcomm 2 mission(Credit: SpaceX)

Based on the investigation team's preliminary identification of the helium system as the source of the problem and the catastrophic nature of the second-stage explosion, the most likely scenario is that one of the helium bottles in the LOX tank suffered a sudden, massive failure, such as bursting open or a valve failing. The helium flashed into gas in the warmer liquid oxygen and the tank suddenly overpressurized.

This caused another failure between the LOX tank and the RP-1 section. Because the failure was so fast, the bulkhead most likely ruptured or shifted aside. At this point, the LOX and the RP-1 mixed. The RP-1 that the Falcon 9 uses is super-chilled to 20° F (−7° C, 266° K), but the LOX is cooled to −340° F (−207 °C, 66° K). When they mix, the LOX immediately boiled and the pressure in the tank skyrocketed.

It's likely that either the tank blew from the pressure and the mixture ignited, or it ignited in the tank due to an electrical spark, friction, or some other cause. The rest was a matter of combustion and gravity as the first stage, then the propellants in the satellite payload exploded, and the remaining LOX flooded into underground passages to feed more fires.

The September 1 accident involves much more than the loss of a rocket and an expensive payload. It has serious implications for SpaceX as well as the future of NASA's manned commercial spaceflight project.

According to a report in AmericaSpace, there hasn't been a preflight rocket accident in the US where the bird was destroyed on the pad since 1959. Having suffered a mishap that hasn't been seen at Cape Canaveral or any other US launch site in almost 60 years does not reflect well on SpaceX. Worse, it calls into question SpaceX's Crew Dragon launch procedures that it plans to use to send astronauts into space. One possible outcome of the current accident investigation is that SpaceX may need to carry out an extensive redesign or end up scratched from manned flights for safety reasons.

A recent NASA audit has already shown that both SpaceX and Boeing are slipping behind schedule as they develop systems to ferry crews to the International Space Station with the first flights unlikely before the end of 2018. This explosion has a strong chance of putting SpaceX even further behind because its design is based on using supercooled LOX instead of more conventional LOX. This has to be loaded onto the rocket in the last half hour before launch or it heats up too much, so for Crew Dragon, the astronauts would have to be put onboard before fueling, which is exactly the opposite of standard NASA procedure and may no longer be acceptable.

In a statement, SpaceX said, "At SpaceX headquarters in Hawthorne, CA, our manufacturing and production is continuing in a methodical manner, with teams continuing to build engines, tanks, and other systems as they are exonerated from the investigation. We will work to resume our manifest as quickly as responsible once the cause of the anomaly has been identified by the accident investigation team. Pending the results of the investigation, we anticipate returning to flight as early as the November timeframe."

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