The crew mission to the International Space Station, which was scheduled to depart from the Kennedy Space Center on Halloween, was postponed for at least a few weeks while NASA and SpaceX investigated a problem with Merlin’s rocket engine. But the problem in question was not actually found on the booster, which was planned to take the four new crew members to the orbital outpost. This story begins on October 2, when a computer aboard the Falcon 9 was set to carry a new generation GPS III satellite into orbit to shut down the engines just two seconds before takeoff.
The fact that SpaceX and NASA have decided to reject the release of a different Falcon 9 is a clear indication that the problem is not limited to one particular booster and should be a problem with the design or construction of the Merlin engine itself. While both entities are relatively close to the current situation, a tweet from CEO Elon Musk, made just hours after the GPS III was interrupted, hinted that the problem was with the engine̵
As we discussed earlier, the Merlin is what is known as an “open-cycle” rocket engine. In this classic design, dating back to the German V-2 of World War II, the exhaust gases from a substantially smaller and less efficient rocket engine are used to rotate the turbine and generate the power needed to pump the propellants into the main combustion chamber. . Higher than expected pressure in the gas generator can lead to a catastrophic failure of the turbine that drives, so it is not surprising that the on-board systems of the Falcon 9 determined that the abortion was in order.
Grounding an entire fleet of missiles, as a potentially serious damage was found in one of them, is a rational precaution and has been done many times before. Engineers need time to study the problem and determine if changes need to be made to other vehicles before they can safely return to flight. But in this case, things get interesting.
SpaceX did not ground its entire fleet of Falcon 9 rockets. In fact, the company operated several of them after the October 2 shutdown ceased. So why are only some of these boosters stuck in their hangers while others continue to carry out their planned missions?
Certified boosters before flight
As of 2018, an increasing number of SpaceX missions have previously used flight boosters. This is thanks to the introduction of the latest and most recent 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 most damaged during on a flight, re-entering the Earth’s atmosphere and landing.
With these vulnerabilities discussed in Block 5, SpaceX believes that the Falcon 9 should be able to perform at least 10 missions with only a minor upgrade. With random repairs, each fuselage can potentially make up to 100 flights, although it will be years or even decades before this theory is tested. To date, no single Falcon 9 has flown more than six flights.
Of the 18 successful launches that SpaceX has conducted so far in 2020, all but two of them have been re-used Block 5 amplifiers. Most of these rockets and their engines were originally designed in 2018 or 2019. Newly built boosters are usually used only at the specific request of the customer at this time, and the two organizations that have always wanted to fly fresh rockets, that’s exactly what happens to be NASA and the space forces.
Hence the missiles that were supposed to launch Crew Dragon Sustainability on October 31 and the GPS III satellite Sakagawa on October 2 were built at the same time earlier this year. In fact, they even have serial numbers listed by SpaceX as B1061 and B1062, respectively.
In light of this, it is clear why SpaceX has not grounded the entire fleet. We are not looking at a problem that affects the Falcon 9 itself, but only those produced in 2020.
Age versus experience
Since the design of the Falcon 9 Block 5 and the Merlin engine have been frozen since 2018 to facilitate the assessment of the car by NASA, we know that no new or redesigned components have been introduced. But that doesn’t mean that SpaceX couldn’t get a bad batch of components from a vendor that found its way to 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 take-off when a high-pressure helium cylinder in the second stage of the Falcon 9 was released. A study of the stainless steel eye bolts used to hold the bottles in place found that although they were estimated by the manufacturer for a load of £ 10,000, some failed for just £ 2,000. A later investigation by NASA found that the severity of the accident was ultimately on SpaceX, as they failed to independently verify the manufacturers’ claims before the flight.
Although pre-flight tests are of course important, there is no substitute for the real thing. SpaceX has always argued that a reusable rocket would be just as reliable, if not larger, than one designed for a single flight. With each subsequent flight and inspection, the company believes that the overall reliability of the car improves.
Additional parts that appear to be worn are replaced while the main vehicles, which have proven their reliability, remain in place. SpaceX’s stated goal is to implement a maintenance schedule for its rockets, as opposed to what is used with commercial aircraft, and ultimately be able to return a single booster in flight within days or even hours.
When ordering a new rocket for their missions, NASA and the Space Forces believe they are getting a more reliable vehicle, but this seems to contradict the facts. To date, the new and reusable Falcon 9 Block 5 missiles have been a 100% cumulative success. 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 flight-proven boosters would prevent delays. Sustainability and Sakagaveya missions.
Test as you fly
Of course, delay is preferable to failure. When human life is at stake, it is always better to take extra time to make sure everything works perfectly. According to Cathy Luders, assistant administrator of NASA’s Directorate of Human Research and Exploitation, SpaceX is currently replacing one of the engines in the ISS-linked Falcon 9, which demonstrates the behavior of the gas generator similar to the October 2 anomaly. They will also replace an engine in the next rocket to come 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 brings up an interesting point. Using a standardized booster for all payloads, whether space station cargo, communications satellites, or human occupants, provides more opportunities to capture potential shortcomings than NASA ever had during the space shuttle.
If the crew’s mission was not set to fly a rocket that is almost identical to the one carrying Sakagaveya Satellite GPS III, the problem with the gas generator may not have been detected in time. As long as the Crew Dragon is ready and able to pull its people to safety if the launch vehicle fails, this is no coincidence that anyone particularly wants to use.
Between in-flight tests and maintaining the same vehicle configuration, regardless of payload, the Falcon 9 is quickly making its way into one of the most reliable orbital accelerators in history. Which is probably why NASA decided to allow the crew to fly with proven Falcon 9 booster flights next year. The agreement specifies that the agency will only allow astronauts to fly slightly used missiles, but at least it’s a start.