A manned mission to the International Space Station that was supposed to depart Kennedy Space Center on Halloween has been postponed for at least several weeks as NASA and SpaceX investigate a problem with the company’s Merlin rocket engine. But the problem in question was not actually discovered on the booster that is supposed to carry the four new crew members up to the outpost in orbit. This story begins on October 2, 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 its engines with just two seconds before takeoff.
The fact that SpaceX and NASA have decided to reject the launch of a different Falcon 9 is a clear indication that the problem is not limited to just one specific booster and must be a problem with the design or construction of the Merlin engine itself. Although both entities were relatively limited on the current situation, a Tweet from CEO Elon Musk made a few hours after the GPS III was discontinued suggested that the problem was with the engine’s gas generator:
As we discussed earlier, the Merlin is what is known as an “open loop” rocket engine. In this classic design, which dates back to the WWII German V-2, the exhaust of what is essentially a smaller, less efficient rocket engine is used to spin a turbine and generate the power needed to pump propellants into the main combustion chamber. A higher than expected pressure in the gas generator could lead to catastrophic failure of the turbine it drives, so it’s not surprising that the Falcon 9’s onboard systems determined that an abortion was needed.
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 in the past. Engineers need time to investigate the problem and determine if changes need to be made to the rest of the vehicles before they can safely fly back. But this is where things get interesting in this case.
SpaceX hasn’t grounded its entire fleet of Falcon 9 rockets. In fact, the company flew several after the October 2 launch disruption. So why are only some of these boosters stuck in their hooks, while others keep flying on their scheduled missions?
Certified pre-flown boosters
Since 2018, an increasing number of SpaceX missions have used previously flown boosters. This is thanks to the introduction of the most recent and definitive revision of the rocket known as Block 5. After gaining invaluable flying experience with previous versions of the booster, SpaceX engineers were able to identify the components that were the most severely damaged during the flight, re-entry into the Earth’s atmosphere and landing.
With these weaknesses addressed in Block 5, SpaceX believes the Falcon 9 should be able to fly at least 10 missions with only minor renovations. With occasional revisions, each fuselage could potentially make up to 100 flights, though it will take years or even decades before that theory can be put to the test. To date, no Falcon 9 has made more than six flights.
Of the 18 successful launches that SpaceX has conducted so far in 2020, all but two have been reused in Booster Block 5. Most of these rockets, and their engines, were originally built in 2018 or 2019. The boosters again production are generally used only at the specific request of the customer at this point, and the two entities that have constantly asked to fly on fresh rockets just so happen to be NASA and Space Force.
Consequently, the rockets that were supposed to launch the Crew Dragon Resilience October 31 and GPS III satellite Sacagawea on 2 October they were both built around the same time earlier this year. In fact, they also 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 are not looking into an issue that impacts the Falcon 9 itself, only those that were produced in 2020.
Age versus experience
As the design of the Falcon 9 Block 5 and Merlin engine has been frozen since 2018 to facilitate human evaluation of the vehicle by NASA, we know that no new or redesigned components have been introduced. But that doesn’t mean SpaceX can’t have received a bad batch of components from a supplier that has made its way into the B1061 and B1062. Such a scenario would not be unprecedented.
In 2015, the CRS-7 mission ended with a complete loss of the vehicle just over two minutes after takeoff, when a high-pressure helium cylinder inside the Falcon 9’s second stage broke off. An examination of the stainless steel eyebolts used to hold the bottles in place found that, despite being rated by the manufacturer for a 10,000-pound load, some failed at just 2,000 pounds. A subsequent investigation by NASA determined that the burden of the crash was ultimately on SpaceX as they failed to independently verify the manufacturers’ claims prior to the flight.
While pre-flight testing is obviously important, there’s no substitute for the real thing. SpaceX has always argued that a rocket designed to be reusable 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 that the vehicle’s overall reliability has improved.
Auxiliary parts that appear to be wearing are replaced, while major 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 that used with commercial aircraft and, ultimately, be able to get an individual booster back into the air within days or even hours.
In requesting a new rocket for their missions, NASA and the Space Force believe they are getting a more reliable vehicle, but this would seem contrary to the facts. To date, new and reused Falcon 9 Block 5 rockets have a cumulative success rate of 100%. There is no evidence to support the claim that one is inherently more reliable than the other, while the current situation seems to indicate that the use of proven boosters would have prevented delaying the Resilience is Sacagawea missions.
Try as if you were flying
Of course, a delay is preferable to a failure. When lives are at stake, it’s always best to take the extra time to make sure everything is running smoothly. According to Kathy Lueders, associate administrator of NASA’s mission direction for human exploration and operations, SpaceX is currently replacing one of the engines in the ISS-bound Falcon 9 which showed gas generator behavior similar to the October 2 anomaly. They will also replace an engine in the next rocket that has rolled off 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.
Which raises an interesting point. Using a standardized booster for all payloads, be it Space Station cargo, communications satellites, or human occupants, offers more opportunities to detect potential defects than NASA ever had during the era of Space Shuttle.
If the manned mission was not set up to fly in a rocket, it was virtually identical to the one carrying the Sacagawea GPS III satellite, the gas generator problem may not have been detected in time. Although the crew dragon is ready and capable of rescuing its human occupants in the event of a booster rocket failure, it is no coincidence that no one is particularly eager to use.
Between post-flight examinations and maintaining the same vehicle configuration regardless of payload, the Falcon 9 is quickly making its way to become one of the most reliable orbital repeaters in history. Which is probably why NASA decided to allow crew flights on the tested Falcon 9 boosters next year. The agreement specifies that the agency will only allow astronauts to fly slightly rockets used, but at least it’s a start.