As of right now, we could be just over a week away from the fourth launch of Starship. SpaceX has said that the launch could be as soon as June 5th, with the only thing possibly holding them back being regulatory approval which is still pending. However, this approval process has only improved over time and when combined with the company’s confidence to set a launch date so soon, June 5th seems like a realistic date.
With that being said, it brings up questions regarding SpaceX’s plans for this fourth flight. Flight 3 was a big improvement but still failed to complete some of the overall objectives. That’s why SpaceX has made some changes both to the vehicle itself and part of the mission profile for this next test flight. Here I will go more in-depth into some of the improvements made to Flight 4 hardware, the mission profile, what went wrong on the last flight, and more.
Super Heavy Improvements
SpaceX has a different objective going into Flight 4 than they did for the last flight attempt. Flight 3 featured a host of in-space objectives including propellant transfers, using the payload door, and eventually the reentry and recovery process. While those ending objectives were not fully successful, that is now the sole focus of Flight 4. In a statement, the company said, “The fourth flight test turns our focus from achieving orbit to demonstrating the ability to return and reuse Starship and Super Heavy. The primary objectives will be executing a landing burn and soft splashdown in the Gulf of Mexico with the Super Heavy booster, and achieving a controlled entry of Starship.” they said.
With these altered objectives and a lot of new data from the last flight, some ‘changes and upgrades were made to the vehicle and hardware. For example, in another statement, SpaceX highlights that “To accomplish this, several software and hardware upgrades have been made to increase overall reliability and address lessons learned from Flight 3.” They went on to say, “The SpaceX team will also implement operational changes, including the jettison of the Super Heavy’s hot-stage following boostback to reduce booster mass for the final phase of flight.”
In other words, the large ring that sits on top of Super Heavy and between the booster and ship will be jettisoned and discarded just over a minute after stage separation. With Starship working to become a fully reusable vehicle, it’s possible that SpaceX plans to jettison the hot-stage ring only for some of the initial Integrated Flight Tests to gather data and achieve a successful landing. It’s also worth pointing out that during Flight 3, the booster experienced a few different issues while returning through the atmosphere including oscillations that could have to do with the hot stage ring. Either way, we can expect to see that ring ejected soon after stage separation.
Besides that change, there were a few very important upgrades made related to specific problems that occurred on Flight 3. On the last test, following stage separation, Super Heavy initiated its boostback burn, and all 13 engines ran successfully until six engines began shutting down, triggering a benign early boostback shutdown.
SpaceX confirmed that the booster then continued to descend until attempting its landing burn, which commands the same 13 engines used during boostback to perform the planned final slowing for the rocket before a soft touchdown in the water, but the six engines that shut down early in the boostback burn were disabled from attempting the landing burn startup, leaving seven engines commanded to start up with two successfully reaching mainstage ignition. The booster had lower than expected landing burn thrust when contact was lost at approximately 462 meters in altitude over the Gulf of Mexico and just under seven minutes into the mission.
In a new statement, SpaceX said, “The most likely root cause for the early boostback burn shutdown was determined to be continued filter blockage where liquid oxygen is supplied to the engines, leading to a loss of inlet pressure in engine oxygen turbopumps. SpaceX implemented hardware changes ahead of Flight 3 to mitigate this issue, which resulted in the booster progressing to its first ever landing burn attempt.” As far as upgrades, they said, “Super Heavy boosters for Flight 4 and beyond will get additional hardware inside oxygen tanks to further improve propellant filtration capabilities. And utilizing data gathered from Super Heavy’s first landing burn attempt, additional hardware and software changes are being implemented to increase startup reliability of the Raptor engines in landing conditions.”
Upper Stage Improvements
Not only is the booster improved for this next flight but the upper stage is getting some upgrades as well. With the focus of Flight 4 being a controlled reentry of the upper stage, they had to make some alterations to try and ensure this process goes well on the next launch. During flight 3, several minutes after Starship began its coast phase, the vehicle began losing the ability to control its attitude. Starship continued flying its nominal trajectory but given the loss of attitude control, the vehicle automatically triggered a pre-planned command to skip its planned on-orbit relight of a single Raptor engine.
SpaceX said in a new statement that “The lack of attitude control resulted in an off-nominal entry, with the ship seeing much larger than anticipated heating on both protected and unprotected areas. High-definition live views of entry and a considerable amount of telemetry were successfully transmitted in real time by Starlink terminals operating on Starship. The flight test’s conclusion came when telemetry was lost at approximately 65 kilometers in altitude, roughly 49 minutes into the mission. The most likely root cause of the unplanned roll was determined to be clogging of the valves responsible for roll control. SpaceX has since added additional roll control thrusters on upcoming Starships to improve attitude control redundancy and upgraded hardware for improved resilience to blockage.”
These changes are hoping to solve the problem and allow Starship to much more efficiently and less violently reenter the atmosphere in one piece. Following the flight test, SpaceX led the investigation efforts with oversight from the FAA and participation from NASA and the National Transportation and Safety Board (NTSB). During Flight 3, neither vehicle’s automated flight safety system was triggered, and no vehicle debris impacted outside of pre-defined hazard areas. Pending FAA finding of no public safety impact, a license modification for the next flight can be issued without formal closure of the mishap investigation.
They were quoted saying, “Upgrades derived from the flight test will debut on the next launch from Starbase on Flight 4, as we turn our focus from achieving orbit to demonstrating the ability to return and reuse Starship and Super Heavy. The team incorporated numerous hardware and software improvements in addition to operational changes”
Besides those changes, the flight profile is similar to what we have seen in the past. At T-49 minutes before liftoff SpaceX will begin loading propellant into the ship. Around 10 minutes later that process will start for the booster. With a few minutes left in the count the vehicle will be full of propellant and ready to launch. At T-10 seconds water from the flame selector will begin shooting out in preparation for booster ignition. Finally, Starship will ignite its engines and lift off for a fourth time.
At T+1:02 Max Q will occur, the moment of peak mechanical stress on the vehicle. A minute and a half later the main engines will cutoff. At T+2:45 hot staging will commence and the two vehicles will separate. The booster will then start its boostback burn and won’t shut off its engines until T+3:52. Interestingly, just seconds after the boostback burn shutdown is when the host stage ring will be jettisoned according to SpaceX’s Flight 4 flight profile. By T+6:39 the booster is transonic and getting ready to attempt its landing. A few seconds later it will begin its landing burn until T+7:04. At that point, assuming everything has gone according to plan, the booster will have splashed down in the Gulf of Mexico in one piece.
Focusing on Starship, it’s not expected to begin reentry until 47:25 into the flight. By T+1:03:11 the upper stage is transonic. Within the next three minutes, it will slow and become subsonic, attempting a flip and landing burn. This all leads to an expected landing at T+1:05:48 into the flight. All of which are set to happen very soon. If the specific date of June 5th wasn’t enough, a few days ago on the 23rd Elon tweeted saying, “Flight 4 in about 10 days.” It’s clear that SpaceX is ready and confident the FAA is going to provide approval soon.
If SpaceX is successful and manages to complete reentry and landing milestones on this flight, it would mark a significant accomplishment for the program. With those steps complete they could begin working toward actual landing and reuse attempts rather than disposal in the ocean. Something to look forward to in the coming weeks.
Conclusion
SpaceX has made some changes to both the physical Starship vehicle itself as well as the launch plan. These come after new data was gathered during Flight 3 in areas where issues arose. These improvements could be put to the test as soon as June 5th. We will have to wait and see how it progresses and the impact it has on the space industry.