Starship finds success on fourth flight test

by Adrian Beil

Just over a year after the first integrated flight test, SpaceX successfully launched the fourth flight of Starship on Thursday. For the first time, Super Heavy completed a successful landing burn while Starship made it through reentry – despite extensive burn damage to a forward flap – and softly landed in the Indian Ocean. This marks a major step in the Starship program and paves the way for future, more intensive test flights.

Overview of the mission

Flight 4 of Starship lifted off from the Orbital Launch Pad at SpaceX´s Starbase site in Boca Chica, South Texas at 7:50 AM CDT. Notably, one outer Raptor engine shut down just after liftoff, something not seen since the first flight of Starship in April 2023.

After the third flight reached atmospheric reentry without attitude control, this flight aimed to push further into reentry, proving the thermal protection system (TPS) on Ship 29.

However, this mission would not only make it further into reentry but completely pass through it. Starship’s onboard control thrusters were able to maneuver the ship before and during reentry, keeping the vehicle stable throughout. The ship passed through peak heating and maximum dynamic pressure successfully.

The excitement promised by SpaceX did not end there, though. For a currently unknown reason, plasma began burning through the hinge of Ship 29’s right-side forward flap. Given that Starship had never made it this far into reentry, something like this occurring was not out of the question. And although large chunks of the flap were torn off, seemingly including the aftmost hinge, the control surface was still able to actuate and help the vehicle execute its signature “flip-and-burn” maneuver – quickly moving from a horizontal orientation to a vertical one – not seen since SN15 flew in May 2021.

A then-live view of Ship 29 during atmospheric reentry from a forward flap, with signal provided by SpaceX’s Starlink constellation. Credit: SpaceX on X

With a still-red-hot forward flap and enough control still available, Ship 29 was able to reignite its sea-level Raptor engines and softly touch down in the Indian Ocean, bringing the fourth Starship flight test to a close. The ship likely broke up after tipping over.

On the booster side, Flight 4 aimed to test fixes for the blockage issues that caused early shutdowns of several Raptor engines on the previous booster, Booster 10, as it attempted to land in the Gulf of Mexico. These fixes seemed successful, as Booster 11 touched down softly in the Gulf before tipping over – even though one of the 13 engines failed to reignite for landing. Based on comments from Elon Musk after the flight, SpaceX may choose to attempt a catch on the next mission.

Further changes to the flight profile included the jettison of the hot staging ring just after the end of the boostback burn. The hot stage ring allows the exhaust of the ship’s engines to be safely directed away when they ignite while still attached to the booster.

The hot stage ring on the flight four stack. Credit Mary (@bocachicagal) for NSF

Since the ring features heavy shielding, it adds a considerable amount of mass to the booster. This is why SpaceX has opted to jettison the ring to increase the chances of booster landing success during this test phase. When it debuts at a yet-unknown date, Starship V2 will feature a much lighter hot stage ring permanently fixed to the forward end of the booster.

The propellant transfer and the opening of the payload door, which were present on the last flight, were not included on this flight. According to SpaceX, the propellant transfer demo was successful on the last mission, while the payload door test is most likely seen as non-critical for this mission, and SpaceX refocused on the recovery of the vehicles.

Booster 11

The booster featured 33 Raptor engines, which were tested in a static fire test on April 5.

The booster was similar to its predecessor, featuring updates to ensure a successful return to the designated landing zone. This included redundancy updates to blockage prevention, which most likely meant changes to the internal filter system that filters liquid oxygen and liquid methane to prevent solid matter from being pulled into the engines.

Including the hot staging ring, the booster stands 71 meters tall, with a primary diameter of nine meters, not including the chimes or stainless steel grid fins. It can hold up to 3,400 tons of propellant, although SpaceX has not fully loaded the vehicle on these test flights.

All 33 Raptor engines start up at liftoff in a staged ignition, producing up to 7,590 tons of force.

At the top of the booster, below the hot staging ring, are the four stainless steel grid fins used to guide the booster at the later parts of the landing. In contrast to Falcon 9, SpaceX removed the ability to deploy and store the grid fins to the side of the vehicle, which means these grid fins are always deployed.

The topmost of the two tanks stores liquid methane, connected to the Raptor engines using a large transfer tube that goes through the lower liquid oxygen tank. A separate, smaller tank is featured at the bottom of the LOX tank, which stores methane solely for landing.

A large manifold at the bottom of the booster supplies the Raptor engines with methane. After Flight 2 of Starship demonstrated problems with propellant sloshing, a slosh baffle was expected to be installed right above that manifold. A weld line in the area makes it appear that this feature was copied from Falcon 9 and integrated into Super Heavy. This baffle prevents sloshing at the bottom of the vehicle, which could cause control issues.

Of the 33 Raptor engines, the outer ring is spun up and started from the pad, while the inner 13 can start themselves. This is needed as these inner 13 are used to perform the vehicle’s boostback and landing burns, while the outer 20 are only used for the ascent stage of the flight.

Ship 29

Ship 29 was part of the same build version as Ship 28, although it did feature a handful of minor changes. As with all “orbital” Starships, its six raptor engines are divided into three sea-level Raptor engines in the center and three vacuum Raptor engines around these sea-level engines. This combination of engines allows the Ship to feature enough efficiency and thrust to perform its mission.

The ship performed a spin prime of all six engines on March 11, followed by two static fires on March 25 and 27. The first static fire tested all six Raptor engines, while the second only demonstrated deorbit capabilities with one single Raptor engine.

Ship 29 stood 50 meters tall, with the same nine-meter core diameter as the booster. This width did not include the flaps of the vehicle. The ship featured forward and aft flaps, which help steer it during the reentry and increase the surface area during reentry to lower the ship’s terminal velocity.

The ship featured 1,200 tons of propellant and could produce up to 1,500 tons of thrust. Down the line, Starship is expected to be able to carry over 100 tonnes of payload into low-Earth orbit, which current prototypes are not capable of.

The Second WDR

The tank configuration is the same as on the booster, with methane in the upper tank and liquid oxygen in the lower tank. At the very top of the ship is the header tank, which is used during the flip-and-burn maneuver to prevent propellant sloshing.

This version of the ship features a dispenser that will eventually be used on Starlink missions, allowing it to deploy larger variants of the satellites. In the future, different versions of the ship are expected which can carry larger satellites or even humans back to the Moon for the NASA Artemis program.

The Countdown

Several hours before the timeline on the SpaceX website even started, the road was closed in Boca Chica and the village was evacuated. This is due to an increased hazard area around the launch site during a flight. The orbital tank farm, pre-chilling valves, pumps, and everything else also kicked into gear hours before the ship and booster moved into propellant loading.

Around T-1:15:00, SpaceX conducted a poll to verify GO for propellant loading. After all systems were verified as ready for flight, liquid methane loading began at T-49 minutes followed by liquid oxygen loading at T-47 minutes.

The tank farm from the air, via Jack Beyer for NSF

These early loads start with the ship. This is due to the current tank farm configuration, which can fuel a booster much faster than a ship. Seven minutes after the ship, the booster started loading 40 minutes before the predicted T0.

At T-19:40, the 33 Raptor engines below Booster 11 and the six engines on the ship began to be chilled. This prepares the material against a sudden cryo-shock once full propellant flow begins just before ignition.

Around T-3:20 for the Ship and T-2:50 for the Booster, the propellant load was wrapped up and the tanks were taken up to flight pressure. At 30 seconds before liftoff, the SpaceX launch director verified GO for launch; at T-10 seconds, the massive deluge system at the bottom of the Orbital Launch Pad was activated.

This system was installed after Flight 1 and is designed to prevent the rapid destruction of the launch site as 33 Raptor engines ignite. It fires water against the thrust of the Raptors to remove the enormous energy that these engines release at liftoff.

IFT-3 launch via Mary (@bocachicagal) for NSF

At T-3 seconds, the Raptors ignited. Note that Starship does not feature hold-down clamps at this point anymore. These are released well before ignition. The stack will lift off as soon as the engines throttle up and the stack reaches a thrust-to-weight ratio of over 1.

The stack reached max-Q at T+1:02, which was the moment of highest mechanical stress on the rocket. As the air gets thinner, the speed increases on the whole stack. This was also roughly the time when Starship went supersonic.

Starship Flight 2 passes through max-Q. Credit: Tyler Gray for NSF.

At T+2:41, the booster performed an event called MECO. For most rockets, this means “main engine cut off,” but for Starship, this translates to “most engines cut off”, as all but a few engines are shut down. This is also when the Starship ignited its own six engines in the hot staging maneuver, separating both stages while both booster and ship engines were still running.

The booster then started up ten additional engines for the boostback burn, which continued for over a minute until T+3:52. Just two seconds later, at T+3:54, the hot-stage ring was jettisoned as the booster began its flip.

At T+6:43, the booster performed the final startup of 13 Raptor engines for the landing burn startup, before shutting down all but the inner three engines. This burn ran for 20 seconds and – for the first time – ended in a soft splashdown in the Gulf of Mexico. As previously stated, only 12 of the 13 engines ignited, however, the booster seemingly was able to compensate for the lower thrust.

No further recovery was planned for the booster. SpaceX would try to sink it by opening all valves to flood the tanks, and if that did not work, it would use explosive devices to sink the vehicle.

Ship 29, meanwhile, continued firing until T+8:23 when its Raptors were cut off. This placed the ship on a near-orbital trajectory, with a perigee just below the Earth’s surface. The ship then entered a 40-minute coast phase to the reentry location.

At T+47:25, the ship began to re-enter Earth’s atmosphere, resulting in plasma forming around its surfaces. This reentry continued for about 15 minutes before it bled off at enough speed to be transonic. This saw immense plasma damage to one of the ship’s forward flaps, although the vehicle was still able to keep itself mostly under control.

Past Missions

Starship has already performed several low- and high-altitude hop tests and three Starship integrated flight tests.

At the beginning of the program, SpaceX used the platform Starhopper as a Raptor engine testbed. This system was initially expected to use up to three Raptor engines for higher-altitude flight tests. Still, in the end, it only performed a low-altitude hop and a 150-meter hop before being retired in favor of more advanced flight prototypes.

After several test setbacks, the first Starship to fly again was SN5. SN5 was a prototype that flew successfully to 150 meters, becoming the first full-scale tank section to fly. SN6 repeated that flight profile just a month later in 2020.

The prototypes SN8/SN9/SN10/SN11 and SN15 performed a high-altitude flight test to test the belly-flop maneuver of the Starship system. Only the prototypes SN10 and SN15 achieved a landing, although SN10 exploded shortly after.

After that, SpaceX moved to the integrated flight tests.

Flight 1 of a full Starship stack on April 20, 2023 with Booster 7 and Ship 24 experienced several Raptor engine failures on the way up. This was due to a fire in the engine section of the vehicle, which eliminated more and more Raptor engines during the ascent. Flight 1 did not achieve successful separation of the vehicle stages.

Flight 2 followed a few months later with an upgraded deluge system on the pad, updated fire extinguishing capabilities, and the hot staging maneuver as a newly added capability. Hot staging worked as planned, and the second stage successfully ignited. However, the booster failed shortly after the beginning of the boostback burn, and the ship did not reach a full-duration burn as planned. The vehicles flown on this mission were Ship 25 and Booster 9.

For the last flight, with Booster 10 and Ship 28, the ship made it to reentry, and the booster got significantly further into its boostback burn. However, thanks to blockages of vehicle systems, both stages were lost: the booster shortly above the Gulf of Mexico and the ship during reentry.

(Lead Image: Starship Flight 4 ignites its engines ahead of liftoff. Credit: Sean Doherty for NSF)

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