Mass Value Report for July 2024

The first in flight anomaly for Falcon 9 in many years raises the question - how much further can this system really be pushed?

It had to happen sooner or later - there has been a Falcon 9 anomaly in flight. The second stage started to show signs of a leak during its first engine burn, and then exploded during its second burn, leaving the payload of Starlink satellites at too low an altitude. Their ion propulsion was not able to produce enough thrust to raise their orbits fast enough to avoid their decay, and they all reentered the atmosphere and were destroyed.

Ramping up the cadence of a system is a way to discover random failure modes that might feasibly not occur at lower rates. Falcon 9 is not a perfect system and it was near inevitable that something would happen sooner or later. The particular fault was identified as an Oxygen leak. SpaceX states:

Post-flight data reviews confirmed Falcon 9’s first stage booster performed nominally through ascent, stage separation, and a successful droneship landing. During the first burn of Falcon 9’s second stage engine, a liquid oxygen leak developed within the insulation around the upper stage engine. The cause of the leak was identified as a crack in a sense line for a pressure sensor attached to the vehicle’s oxygen system. This line cracked due to fatigue caused by high loading from engine vibration and looseness in the clamp that normally constrains the line. Despite the leak, the second stage engine continued to operate through the duration of its first burn, and completed its engine shutdown, where it entered the coast phase of the mission in the intended elliptical parking orbit.

They have managed a swift return to flight by removing the system in question, and inspecting similar ones on new stages

SpaceX engineering teams have performed a comprehensive and thorough review of all SpaceX vehicles and ground systems to ensure we are putting our best foot forward as we return to flight. For near term Falcon launches, the failed sense line and sensor on the second stage engine will be removed. The sensor is not used by the flight safety system and can be covered by alternate sensors already present on the engine. The design change has been tested at SpaceX’s rocket development facility in McGregor, Texas, with enhanced qualification analysis and oversight by the FAA and involvement from the SpaceX investigation team. An additional qualification review, inspection, and scrub of all sense lines and clamps on the active booster fleet led to a proactive replacement in select locations.

What I want to talk about here the impact of this event on how many flights Falcon 9 and Falcon Heavy will make this year. I’ve been tracking flight data for some time, and over the past few years it has followed an exponential trajectory.

The recent rate of increase of Falcon flights, roughly 43%, is obviously not sustainable forever. True exponential processes don’t exist in nature, never mind business, and all of them must level off eventually. This is why I’ve been modelling the flight rate as an S curve for some time; if the current rate is to be maintained, as it has to be in order to reach Musk’s 2050 target for mass-to-orbit, then Starship will have to pick up the baton for this kind of growth.

The Cadence Question

SpaceX rightly get huge credit for being able to refly first stage boosters dozens of times, and using this resume capability to achieve a roughly biweekly cadence. The second stage, on the other hand, tends to be seen as a fairly mundane item. But to manufacture two such stages a week is also a startling achievement.

The Soviet/Ukrainian Zenit rocket has a second stage of similar size to Falcon 9, which also uses the same propellant combination. In the 32 career year of that rocket, 84 such stages were flown. SpaceX manufacture and flew more second stage than that in 2023 alone. Likewise, there have only been 104 Centaur III upper stages (used on the Atlas V) ever flown over two decades, a number that Falcon 9 will likely exceed in this calendar year.

In an interview with Lex Fridman last year, Jeff Bezos made the following statement when asked if there was a path to making the second stage of New Glenn reusable:

There is, and we know how to do that. Right now we're gonna work on manufacturing that second stage to make it as inexpensive as possible. Sort of two paths for a second stage, make it reusable, or work really hard to make it inexpensive so you can afford to expend it. And that trade is actually not obvious which one is better.

Even though with Starship SpaceX are going for full reuse, in the Falcon 9 program they are still pushing the other side of the trade Bezos is talking about. With first stage boosters achieving ever higher reuse rates whilst still being manufactured at a more or less constant pace, the second stage is one of the program’s main constraints (along with the availability of launch pads and droneships).

So how will the anomaly impact cadence? The month of July saw only 6 flights due to the 15 day stand down, compared to a more typical 11 flights in June. This is a significant gap, but not an entirely unprecedented one in recent times, if we look at this year alongside 2022 and 2023: