A crewed mission to the International Space Station that was set to depart from Kennedy Space Center on Halloween has been pushed back at least several weeks as NASA and SpaceX investigate an issue with the company’s Merlin rocket engine. But the problem in question wasn’t actually discovered on the booster that’s slated to carry the four new crew members up to the orbiting outpost. This story starts back on October 2nd, when the computer aboard a Falcon 9 set to carry a next-generation GPS III satellite into orbit for the US Space Force shut down the engines with just two seconds to go before liftoff.

The fact that SpaceX and NASA have decided to push back the launch of a different Falcon 9 is a clear indication that the issue isn’t limited to just one specific booster, and must be a problem with the design or construction of the Merlin engine itself. While both entities have been relatively tight lipped about the current situation, a Tweet from CEO Elon Musk made just hours after the GPS III abort hinted the problem was with the engine’s gas generator:

As we’ve discussed previously, the Merlin is what’s known as an “open cycle” rocket engine. In this classical design, which dates back to the German V-2 of WWII, the exhaust from what’s essentially a smaller and less efficient rocket engine is used to spin a turbine and generate the power required to pump the propellants into the main combustion chamber. Higher than expected pressure in the gas generator could lead to a catastrophic failure of the turbine it drives, so it’s no surprise that the Falcon 9’s onboard systems determined an abort was in order.

Grounding an entire fleet of rockets because a potentially serious fault has been discovered in one of them is a rational precaution, and has been done many times before. Engineers need time to investigate the issue and determine if changes must be made on the rest of the vehicles before they can safely return to flight. But that’s where things get interesting in this case.

SpaceX hasn’t grounded their entire fleet of Falcon 9 rockets. In fact, the company has flown several of them since the October 2nd launch abort. So why are only some of these boosters stuck in their hangers, while others are continuing to fly their scheduled missions?

Certified Pre-Flown Boosters

Since 2018, an increasing number of SpaceX missions have utilized previously flown boosters. This is thanks to the introduction of the newest, and final, revision of the rocket known as Block 5. After gaining valuable flight experience with earlier versions of the booster, SpaceX engineers were able to identify the components that were the most heavily damaged during flight, reentry into the Earth’s atmosphere, and landing.

Falcon 9 Block 5

With these weak points addressed in Block 5, SpaceX believes the Falcon 9 should be able to fly at least 10 missions with only minor refurbishment. With occasional overhauls, each fuselage could potentially make as many as 100 flights, though it will be years or even decades before that theory can be put to the test. To date, no individual Falcon 9 has made more than six flights.

Out of the 18 successful launches that SpaceX has conducted so far in 2020, all but two of them have been on reused Block 5 boosters. Most of these rockets, and their engines, were originally constructed in 2018 or 2019. Newly manufactured boosters are generally only used at the customer’s specific request at this point, and the two entities that have consistently asked to fly on fresh rockets just so happen to be NASA and the Space Force.

Consequently, the rockets that were scheduled to launch the Crew Dragon Resilience on October 31st and the GPS III satellite Sacagawea on October 2nd were both built around the same time earlier this year. In fact they even have sequential serial numbers, referred to by SpaceX as B1061 and B1062, respectively.

In light of this, it becomes clear why SpaceX hasn’t grounded the entire fleet. We aren’t looking at an issue that impacts the Falcon 9 itself, merely the ones that have been manufactured in 2020.

Age Versus Experience

As the design of the Falcon 9 Block 5 and Merlin engine have been frozen since 2018 to facilitate human-rating the vehicle by NASA, we know no new or redesigned components have been introduced. But that doesn’t mean SpaceX couldn’t have received a bad batch of components from a supplier which found their way into B1061 and B1062. Such a scenario wouldn’t be without precedent.

In 2015, the CRS-7 mission ended with a complete loss of the vehicle just over two minutes after liftoff when a high-pressure helium bottle inside the Falcon 9’s second stage broke loose. An examination of the stainless steel eye bolts used to hold the bottles in place found that, despite being rated by the manufacturer for a load of 10,000 pounds, some failed at just 2,000 pounds. A later investigation by NASA determined that the onus for the mishap was ultimately on SpaceX as they failed to independently verify the manufacturers claims before flight.

CRS-7 disintegrating over the Atlantic Ocean

While pre-flight testing is of course important, there’s no substitute for the real thing. SpaceX has always maintained that a rocket designed for reusability will be just as reliable, if not more so, than one designed for a single flight. With each subsequent flight and inspection, the company believes the overall reliability of the vehicle is improved.

Ancillary parts that appear to be wearing down are replaced, while core vehicle systems that have proven their reliability remain in place. SpaceX’s stated goal is to implement a maintenance schedule for their rockets not unlike what’s used with commercial aircraft, and eventually, be able to return an individual booster to flight within a few days or even hours.

In requesting a fresh rocket for their missions NASA and the Space Force believe they’re getting a more reliable vehicle, but this would appear to be at odds with the facts. To date, new and reused Falcon 9 Block 5 rockets have a cumulative 100% success rate. There’s no evidence to support the assertion that one is inherently more reliable than the other, whereas the current situation seems to indicate that the use of flight-proven boosters would have prevented delaying the Resilience and Sacagawea missions.

Test Like You Fly

Of course, a delay is preferable to a failure. When human lives are at stake, it’s always better to take the extra time to make sure everything is working perfectly. According to Kathy Lueders, Associate Administrator of NASA’s Human Exploration and Operations Mission Directorate, SpaceX is currently replacing one of the engines in the ISS-bound Falcon 9 that exhibited gas generator behavior similar to the October 2nd anomaly. They will also replace an engine in the next rocket that came off of the assembly line, serial number B1063, which is currently scheduled to launch a joint NASA/ESA Earth observation satellite before the end of the year.

Endeavour will fly on a reused booster in March of 2021

Which brings up an interesting point. Using a standardized booster for all payloads, whether it’s cargo for the Space Station, communication satellites, or human occupants, allows for more opportunities to catch potential flaws than NASA ever had during the Space Shuttle era.

If the crewed mission wasn’t set to fly on a rocket that was virtually identical to the one carrying the Sacagawea GPS III satellite, the gas generator issue may not have been detected in time. While the Crew Dragon is ready and able to pull its human occupants to safety should the booster rocket fail, it’s not a contingency anyone is particularly eager to utilize.

Between post-flight examinations and maintaining the same vehicle configuration regardless of payload, the Falcon 9 is quickly working its way towards becoming one of history’s most reliable orbital boosters. Which is likely why NASA has decided to allow crew flights on flight-proven Falcon 9 boosters next year. The agreement specifies the agency will only allow astronauts to fly on lightly used rockets, but at least it’s a start.

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