This afternoon we were less than a minute away from Starship’s Eighth Flight Test when a few complications caused a scrub. Despite the fact that this is a test program with a long list of changes each flight, Starship launch scrubs are actually quite rare.
This time around, updates from SpaceX confirmed that the reason had to do with a low ground spin start pressure among a few other complications, both with the ship and booster.
Launch Scrub
As both the upper stage and booster were being filled with propellant, everything was going as planned until around T – 23:00 minutes. Here, in the background of the livestream, you could hear an individual in mission control call hold hold hold to troubleshoot due to an unsuccessful transition. It’s hard to hear as the commentators were speaking when the call came in, so the audio overlaps. The commentators then clarified that an operator hold got in and that at that point they were troubleshooting at least one issue.
For reference, T – 40 seconds is the designated hold point. This means that with the complication arising around T – 23 minutes, they have time to try and fix the issue. If they’re successful, they can pass right through T – 40 seconds and continue toward a launch.
Later at around T – 10 minutes one of the commentators was quoted saying, “If you tuned in and heard earlier, we did hear talk about a flight controller hold going in. Just a reminder, all that means is that we wouldn’t be able to pass that T – 40 second mark. They’re literally talking about it right now on the loops, that they think they’ve got the problem figured out, it was an issue on the booster. Where we think we will be clear by around the T – 2:00 mark, and we’ll be able to move right through that T – 0:40, hopefully” he said.
Eventually, the clock hit T – 40 seconds and there was a hold. They then confirmed that the booster issue was solved, but they were now working on a late-breaking issue with the ship. About 5 minutes later, the clock started again just for a few seconds before resetting back to T – 0:40 for another hold. They confirmed that in the few seconds the clock started back up, a couple more holds had tripped and that it wasn’t something they could change with small config tweaks. From here they began offloading propellant.
Not long after Elon provided more context in a tweet saying, “Too many question marks about this flight and then we were 20 bar low on ground spin start pressure. Best to destack, inspect both stages and try again in a day or two.”
For context, when he mentions being 20 bar low on ground spin start pressure, he’s referring to the Raptor engine turbopumps. As far as what exactly is causing the low pressure, it’s possible even SpaceX doesn’t know and won’t until they take a closer look at the vehicle. Notably, unlike previous Starishp launches, this time around they didn’t complete a wet dress rehearsal and stacked the rocket within the last 24 hours. That being said, the lead up to a launch is inevitably a wet dress rehearsal. The downside being, if you find an issue, you have to scrub the launch.
As for the timeline, in Elon’s tweet he mentioned trying again in a day or two. The soonest they can be back on the pad ready to launch would be in 24 hours. However, realistically based on everything we’ve heard and seen, there were a few more issues than just one which could extend that timeline at least by a few days. They’ll destack the rocket and investigate any of the anomalies that popped up. Depending on how easy they are to fix and or address, the sooner Starship will be back on the pad and ready.
After the scrub SpaceX also tweeted saying, “Standing down from today’s flight test attempt. Starship team is determining the next best available opportunity to fly.” Not long from now, we should get more info from the company regarding the next launch date.
A Lot of Upgrades
It’s important to point out that each time Starship launches or attempts to launch, we are almost seeing a partially new rocket. In the time since flight test 7, several hardware and operational changes were made to increase the reliability of the upper stage.
In regard to the last flight, the company released a statement saying, “Starship’s six second stage Raptor engines powered the vehicle along its expected trajectory. Approximately two minutes into its burn, a flash was observed in the aft section of the vehicle near one of the Raptor vacuum engines. This aft section, commonly referred to as the attic, is an unpressurized area between the bottom of the liquid oxygen tank and the aft heatshield. Sensors in the attic detected a pressure rise indicative of a leak after the flash was seen.”
“Roughly two minutes later, another flash was observed followed by sustained fires in the attic. These eventually caused all but one of Starship’s engines to execute controlled shut down sequences and ultimately led to a loss of communication with the ship. Telemetry from the vehicle was last received just over eight minutes and 20 seconds into flight.”
“Contact with Starship was lost prior to triggering any destruct rules for its Autonomous Flight Safety System, which was fully healthy when communication was lost. The vehicle was observed to break apart approximately three minutes after loss of contact during descent. Post-flight analysis indicates that the safety system did trigger autonomously, and breakup occurred within Flight Termination System expectations.”
They go on to say, “The most probable root cause for the loss of ship was identified as a harmonic response several times stronger in flight than had been seen during testing, which led to increased stress on hardware in the propulsion system. The subsequent propellant leaks exceeded the venting capability of the ship’s attic area and resulted in sustained fires” they said.
In order to address flammability potential in the attic section on Starship, additional vents and a new purge system utilizing gaseous nitrogen were added to the current generation of ships to make the area more robust to propellant leakage. Future upgrades to Starship will introduce the Raptor 3 engine, reducing the attic volume and eliminating the majority of joints that can leak into this volume.
If that wasn’t enough, Starship’s forward flaps have been upgraded to significantly reduce their exposure to reentry heating while simplifying the underlying mechanisms and protective tiling. Redesigns to the propulsion system, including a 25 percent increase in propellant volume over previous generations, add additional vehicle performance and the ability to fly longer duration missions. And the vehicle’s avionics underwent a complete redesign, adding additional capability and redundancy for increasingly complex missions like propellant transfer and ship return to the launch site.
Flight 8 will also include several experiments focused on enabling Starship’s upper stage to return to the launch site. They highlight that a significant number of tiles have been removed from Starship to stress-test vulnerable areas across the vehicle. Multiple metallic tile options, including one with active cooling, will test alternative materials for protecting Starship during reentry.
On the sides of the vehicle, non-structural versions of Starship’s catch fittings are installed to test the fittings’ thermal performance, along with a section of the tile line receiving a smoothed and tapered edge to address hot spots observed during reentry on Starship’s sixth flight test. Starship’s reentry profile is designed to intentionally stress the structural limits of the upper stage’s rear flaps while at the point of maximum entry dynamic pressure. Finally, several radar sensors will once again be tested on the launch and catch tower’s chopsticks with the goal of increasing the accuracy when measuring distances between the chopsticks and a returning vehicle.
When Flight 8 does eventually happen, Starship is set to deploy four Starlink simulators, similar in size to next-generation Starlink satellites, as the first exercise of a satellite deploy mission. The Starlink simulators will be on the same suborbital trajectory as Starship and are expected to demise upon entry. A relight of a single Raptor engine while in space is also planned.
In a final quote SpaceX says, “Developmental testing by definition is unpredictable. But by putting flight hardware in a flight environment as frequently as possible, we’re able to quickly learn and execute design changes as we seek to bring Starship online as a fully and rapidly reusable vehicle”.
Conclusion
This afternoon SpaceX ended up scrubbing Starship Flight 8 due to a low ground spin start pressure. It’s possible that in just a few days the vehicle is back on the pad ready to launch but it could also easily take longer as SpaceX works to find and fix the issues identified today.