SpaceX is preparing to launch the fourth flight of Starship on Thursday, during a two-hour window that opens at 7 AM central time. Booster 11 and Ship 29 are the pairing for the latest test flight, with primary goals of a pinpoint soft water landing for the Booster, and additional re-entry data for the Ship.
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Overview of the mission
The mission will lift off from the Orbital Launch Pad at SpaceX´s Starbase site in Boca Chica, South Texas. The pad for the mission is the same as the one used for the previous three missions. After reaching the reentry part of the flight profile without attitude control on the last flight, this mission aims to reach further into the reentry part of the flight, proving the Thermal Protection System (TPS) on Ship 29.
For the booster, the aim is to fix the blockage issues that caused the early shutdowns of several Raptor engines on Booster 11´s return to the Gulf of Mexico. The booster aims to reach the Gulf of Mexico for a soft touchdown, which then would simulate a potential booster catch on a software-demonstrated launch tower. This simulation would potentially clear flight five, for an attempt to catch Booster 12.
Further changes to the flight profile include the jettison of the Hot Staging Ring. This device allows the thrust to escape during the rocket’s hot staging maneuver, which ignites the second-stage engines before stage separation.
Since the ring features heavy shielding, it most likely raises the center of mass of the booster significantly. This is the most likely reason why SpaceX has opted to jettison the ring to increase the chances of booster success during this test phase.
The HSR on Flight 4 – via Mary (@bocachicagal).
Another change is the potential use of the previously tested belly flop and flip maneuver that SpaceX tested on the early Starship hop tests. This involves the Ship reigniting its three Raptor sea-level engines after the reentry phase of the flight before changing its orientation about 90 degrees and performing a soft water splashdown.
The propellant transfer and the opening of the payload door, which were present on the last flight, are not included on this flight. Based on SpaceX and NASA statements, it seems the propellant transfer demo was successful on the last flight, while the payload door test is most likely seen as non-critical for this mission, and SpaceX is refocusing on the recovery of the vehicles.
SpaceX previously wanted to perform the landing burn to demonstrate its ability to deorbit a Starship, but it will not be featured on this flight. Instead, the trajectory will bring the Ship to the Indian Ocean.
Booster 11
The booster features 33 Raptor engines, which were tested in a static fire test on April 5.
The booster is similar to its predecessor, featuring updates to ensure its flawless return to the designated landing zone. This includes redundancy updates to blockage prevention, which most likely means changes to the internal filter system that filters liquid oxygen and liquid methane so the liquid can reach the booster.
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 it is expected that SpaceX might not fully load the Booster again.
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 top of the two tanks is the methane tank, connected to the Raptor engines using a giant transfer tube that goes through the lower tank, which features oxygen. A separate tank is featured at the bottom of the LOX tank, which stores propellant for the landing.
A giant manifold at the bottom of the Booster supplies the Raptor engines. 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 likely that this feature was copied from Falcon 9 and integrated at Super Heavy. This baffle prevents huge sloshing at the bottom of the vehicle.
Of the 33 Raptor engines, the outer ring is ground-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 burn, while the outer 20 are only used for the ascent stage of the flight.
Ship 29
Ship 29 is part of the same build version that Ship 28 was part of. It does feature minor changes, but overall, it is similar to its predecessor. 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 while also having enough thrust to perform landing maneuvers.
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 stands 50 meters tall, with the same nine-meter core diameter as the Booster. This does not include the flaps of the vehicle. The Ship features forward and aft flaps, which help steer it during the reentry and include the surface area during reentry to lower the ship’s terminal velocity.
The Ship features 1,200 tons of propellant and can produce up to 1,500 tons of thrust. Down the line, it is expected to carry over 100 tons of payload into low-Earth orbit, which is impossible on current prototypes.
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 could be used on Starlink missions, as it can deploy small flat satellites. Later, different versions of the Ship are expected, which can bring bigger 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 starts, the road is closed in Boca Chica, and the village will be 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 will also kick into gear hours before the Ship and Booster move into propellant loading.
Around T-1:15:00, SpaceX will conduct a poll to verify GO for propellant loading. If successful, this poll will lead to the start of liquid methane loading at T-49 minutes and the start of 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 will also start loading 40 minutes before the predicted T-0.
At T-19:40, the 33 Raptor engines below Booster 11 and the six engines on the ship will be chilled for ignition. This prepares the material against a sudden cryo-shock once t-0 is reached.
Around T-3:20 for the Ship and T-2:50 for the Booster, the propellant load will wrap, which means the tanks can be prepared for flight pressure. At 30 seconds before liftoff, the SpaceX launch director will verify GO for launch; at T-10 seconds, the giant flame deflector at the bottom of the Orbital Launch Pad will activate.
This system was installed after Flight 1 and is designed to prevent the rapid destruction of the launch site if 33 Raptor engines hit the ground. 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 will ignite. Note that Starship does not feature hold-down clamps at this point anymore. These are released way earlier before ignition. The stack will lift off as soon as the stack reaches a thrust-to-weight ratio of over 1.
The stack reaches Max Q at T+1:02, which is the moment of highest mechanical stress on the rocket. As the air gets thinner, the speed increases on the whole stack. This is also roughly the time when Starship will go supersonic.
IFT-2 through MaxQ – via Tyler Gray for NSF.
At T+2:41, the Super Heavy stack will perform a maneuver 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 shutting down. This is also when the Starship will start to ignite these six engines in the hot staging maneuver, separating both stages while both Booster and Ship engines are still running.
The Booster then starts up ten additional engines for the Super Heavy boostback burn, which burns for over a minute until T+3:52. Just two seconds later, at T+3:54, the hot-stage ring will be jettisoned. This most likely happens during the booster’s reorientation flip.
At T+6:43, the booster will then perform the final startup of 13 Raptor engines for the landing burn startup. This burn runs for 20 seconds and hopefully ends in a soft splashdown in the Gulf of Mexico. It is expected that SpaceX will down-select the Raptor engines later in the landing burn and land on just three Raptor engines.
No further recovery is planned for the booster. SpaceX would try to sink it by opening all valves, and if that did not work, it would use explosive devices to sink the Booster.
The Starship engines will run until T+8:23, when they will be cut off. The ship will then enter a 40-minute coast phase to the reentry spot.
At T+47:25, the Ship will start to re-enter Earth’s atmosphere, resulting in plasma forming around its surfaces. This reentry is expected to run for about 15 minutes before it bleeds off at enough speed to be transonic. It would then ignite its engine for the landing flip at T+1:05:38, hopefully resulting in a touchdown just ten seconds later.
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 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, and SN10 exploded shortly after.
After that, SpaceX moved to the Integrated Flight tests.
Flight 1 of a full Starship full 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 of the stack. It achieved the hot-staging, and the second stage successfully ignited. However, the Booster failed shortly after the beginning of the boostback burn, and the Ship did not reach full trajectory as planned. The vehicles were Ship 25 and Booster 9.
For the last flight, with Booster 10 and Ship 28, the vehicle actually achieved reentry on the Ship section, and the Booster got significantly further into its Boostback burn. However, thanks to blockages on both systems. Both stages were lost. The Booster shortly above the Gulf of Mexico and the Ship during reentry.
(Feature Image: Starship Flight 4 Full Stack. By NSF/Mary (@bocachicagal))
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