First soft splashdown of Starship, Super Heavy Booster on Flight 4 by SpaceX.

SpaceX launched a test flight of its giant Starship rocket from Starbase, its research site in southern Texas, for the fourth time in little over a year. The launch, known as Flight 4, moves the vehicle closer to its objective of being a mostly reusable rocket.

Like the first three flights, Flight 4 went on a suborbital trajectory without a cargo. In contrast to the previous missions, the Super Heavy Booster (Booster 11) and the Starship upper stage (Ship 29) made a soft splashdown on Flight 4. At the end of a 120-minute window, at 7:50 a.m. CDT (8:50 a.m. EDT, 1250 UTC), the rocket was launched.

On Wednesday, SpaceX assembled the 121 m (397 foot) Starship rocket by stacking Ship 29 on Booster 11. Elon Musk, the founder of SpaceX, said that “the main goal of this mission is to get much deeper into the atmosphere during reentry, ideally through max heating” in a post on X, previously Twitter, on June 1.

He praised Starship’s return “despite [the] loss of many tiles and a damaged flap” after the flight. Flight 3 saw the top stage slide out of control, making it impossible for the vehicle to relight one of its six Raptor engines.

But since it could connect to SpaceX’s other company, the Starlink satellite internet network, the rocket was able to broadcast back high definition camera footage of its descent through a layer of plasma. In a blog post after launch, SpaceX said, “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.”

It was found that blockage of the roll control valves was the most probable cause of the unintended roll. Since then, SpaceX has enhanced hardware for increased resistance to obstruction and added more roll control thrusters to forthcoming Starships to improve attitude control redundancy.

Six of the thirteen Raptor engines that were utilized in the boostback burn were prematurely shut down by the Super Heavy Booster from the previous mission, and these engines remained offline when the booster tried to execute a landing burn.

“When contact was lost at approximately 462 meters in altitude over the Gulf of Mexico and just under seven minutes into the mission, the booster had lower than expected landing burn thrust,” according to SpaceX.

“Continuous filter blockage where liquid oxygen is supplied to the engines was found to be the most likely root cause of the early boostback burn shutdown, resulting in a loss of inlet pressure in engine oxygen turbopumps.”

“To significantly increase propellant filtering capabilities, Super Heavy boosters for Flight 4 and beyond will get new hardware within oxygen tanks.”

Now that Flight 4 has succeeded, Musk has hinted at an audacious goal for Flight 5: snagging the Super Heavy Booster using the launch tower’s dubbed “chopsticks.”

Eyes on the Moon

Flight 4 was a significant mission for NASA as well as SpaceX. When the agency launches the Artemis 3 mission, which is now scheduled for September 2026, the rocket will be the main attraction.

The manager of the Human Landing System program, Lisa Watson-Morgan, and her staff are still collaborating with SpaceX to comprehend the advancement of the rocket that will act as the Moon lander for the astronauts who have not yet been identified for the Artemis 3 and Artemis 4 missions. Prior to the launch of Flight 4, she had an interview with Spaceflight Now.

“Observing how the lessons learned from [flights] one and two were applied to manufacturing, production, and flight operations during Flight 3 was fantastic,” Watson-Morgan said. “Raptor was not the subject of any problems. To be honest, there have been no fires and a great deal of consistency around the engines. For us, it was a big triumph when you got all those engines to start up.

She pointed out that even if the goal of having the Raptor relight on the upper stage during Flight 3 was not achieved, there is still plenty of time to reach that goal. Watson-Morgan said that they would need a demonstration in the latter part of 2024 or the first half of 2025.

According to Watson-Morgan, “SpaceX is making modifications, adjustments, and changes as they go through their design and development process, which is how they’re continuing to mature their Raptors.” “And an updated build sequence will incorporate all of that.”

The propellant transfer, which moved the ship’s liquid oxygen (LOX) from the header tank to the main upper stage LOX tank, was one of the accomplishments that Watson-Morgan and NASA recognized. The Space Technology Mission Directorate (STMD) of the National Aeronautics and Space Administration (NASA) had a $53.2 million Tipping Point contract that required proof of transfer of 10 metric tons of propellant, and that was the intended outcome.

Even though the HLS office wasn’t directly engaged, Watson-Morgan noted that officials from STMD agreed with SpaceX that the demo was successful. One of the primary components of SpaceX’s objective for the Artemis lunar missions is the ability to execute ship-to-ship fuel transfers, and this is the first step toward that goal.

One idea put forward by SpaceX is to send a tanker variant of the ship’s upper stage into low Earth orbit. Next, a succession of ships would be sent out to dock with the tanker, transfer fuel into it, and then transfer it to the HLS Starship before it sets sail for the Moon.

Prop transfer is the genuine deal when it comes to opening the door to the cosmos. Indeed, it is. “We’re doing all we can to get ready for that, to help SpaceX with that,” Watson-Morgan said, adding that it is also the key to Mars and the South Pole, and that it truly is their long pole. “Plus, Blue Origin also has that in their concept, so we’re doing everything we can to assist them with it.”

Watson-Morgan said that there is currently no definitive estimate for the number of fuelling flights that will be directed to the tanker due to the uncertainty around the amount of propellant that has to be transferred.

The six tanks are the determining factor. The amount we want to transfer determines this. What additional goals do we want to demonstrate during the flying test, and for how long do we intend to do so? Declared Watson-Morgan. So, it may be a few or it can be more than that. Therefore, our goals are the determining factor.

“One thing that I really like about SpaceX is their willingness to be flexible and open with their goals. They are also open to additional objectives, depending on the timing, if NASA thinks we need them.”

Starship expansion

Whether SpaceX can launch many Starship missions from different launch towers is a factor in the timeliness of the propellant transfer. Down in Starbase, the corporation is constructing a second tower.

Their facilities at NASA’s Kennedy Space Center in Florida produced extra segments and components for this purpose, and they floated them down to Texas. Two more tower pieces, the so-called “chopsticks” and the supports for its elevator system were added to the four that were sent earlier this year. This week, two more parts were carried aboard the barge.

Although NASA is keen on ensuring that Starship launch capabilities are operational at KSC as well, according to Watson-Morgan, the propellant transfer mission may be executed from two towers at Starbase.

The FAA (Federal Aviation Administration) will hold public scoping hearings next week to get feedback on the possibility of permitting around 44 Starship launches annually from the historic Launch Complex 39A.

At the same time, the Department of Air Force is conducting an evaluation of a similar kind for Starship flights from either Space Launch Complex 37, which was formerly used as the launch site for the Delta 4 Heavy rocket that was operated by United Launch Alliance (ULA), or from a new launch pad that is being considered for use and is referred to as SLC-50.

That is something that we can’t wait to see. “We need to see it by the uncrewed demo for sure, and it is abundantly clear that we would like to see that before in order to make certain that everything is in order,” Watson-Morgan said. According to the statement, “We will proceed with pad checkouts and all of that, as well as operational readiness reviews in advance of it.”

In accordance with the HLS agreement that SpaceX has with NASA, the company will carry out an unmanned landing of Starship on the Moon before to the delivery of the Artemis 3 mission.

Humans in a loop

During the process of creating the human-rated version of Starship, they are also soliciting feedback from the Astronaut Office, which is situated at the Johnson Space Center of the National Aeronautics and Space Administration. The office provides insights and comments on the functioning of specific components of the vehicle, such as the interface, control system, and position of handles, according to Watson-Morgan, who referred to them as “the crew” for brevity.

She said that the HLS office mostly collaborates with astronauts Raja Chari and Randy Bresnik, the latter of whom has been a part of the process “since the very beginning.” According to Watson-Morgan, there are also representatives from the astronaut office functioning as members of their control board.

In her words, “We have a Human Landing System control board, where any requirements changes or updates or how things are implemented get to go through their formal board actions, and the crew is a voting member,” she said.

During the first integrated test of Axiom Space’s pressurized spacesuits, which took place on April 30 at the offices of SpaceX in Hawthorne, California, NASA astronaut Doug “Wheels” Wheelock and Axiom Space astronaut Peggy Whitson completed the test alongside models of a Starship elevator and the airlock.

In a statement, Logan Kennedy, who is the lead for surface activities in NASA’s HLS Program, stated, “Overall, I was pleased with the astronauts’ operation of the control panel and with their ability to perform the difficult tasks they will have to do before stepping onto the Moon.”

Kennedy went on to say that he was impressed with the astronauts’ ability to execute the demanding duties. “The test also confirmed that the amount of space available in the airlock, on the deck, and in the elevator is sufficient for the work that our astronauts plan to do,” the test said.