Success of Starship’s fourth flight: B11 splashes down and S29 survives re-entry

The world’s most powerful rocket—though not yet the one with the largest payload, which is still NASA’s SLS—has taken off again. Fourth test mission of the Starship IFT-4 system (Integrated Test Flight 4) began on June 6, 2024 at 12:50 UTC when the constellation consisting of Launch Vehicle 11 and Ship 29 departed the orbital launch pad at Starbase (Boca Chica, Texas). The mission, essentially a repeat of IFT-3, began just half an hour later and was overall a success. B11 landed softly in the Gulf of Mexico after performing a stop-fire burn, while S29 was able to control its position on a “quasi-orbital” trajectory and survived re-entry, albeit with significant structural damage, and landed in the Indian Ocean.

The entire mission went very smoothly, perhaps in part because the surprise factor of the first missions was left behind. The set lifted off the ramp flawlessly, despite the failure of one of the twenty attached Raptors on the outer ring three seconds after takeoff. The delicate stage separation proceeded without apparent incident, and Super Heavy shut down all but the center three Raptors to maintain climb acceleration while Starship S29 fired its six engines. The two launcher elements then separated after 2 minutes 53 seconds while flying at a speed of 5,500 km/h and an altitude of 72 kilometers.

As S29 continued on its suborbital trajectory, the Superheavy turned around and the thirteen internal Raptors that could move ignited again without issue to perform a reverse ignition (combustion acceleration). The hot separation ring, one of the mission’s new features, was then jettisoned from B11 to lighten the vehicle and make the huge rocket easier to control. The maneuver was executed perfectly. At an altitude of one kilometer, the sequence of turning on the thirteen central Raptors for ignition retardation began, but one of the engines did not come to life. Against all odds, the Heavyweight dramatically slowed its speed and landed 7 minutes and 24 seconds into the mission, gently reducing the momentum of the three center Raptors. The giant rocket then fell on its side into the sea, unharmed. Of course, during the descent we could see ejected debris and very noticeable vibrations in at least one of the aerodynamic grilles, in addition to the engines not starting, so there is clearly room for improvement.

S29 turned off its six engines and was launched into a suborbital trajectory with an apogee of 213 kilometers and a perigee of about -10 kilometers, with an inclination of 26.8°. Throughout the flight phase, S29 transmitted telemetry via Starlink satellites, although communication with external cameras was momentarily lost for reasons that are not entirely clear. Unlike the previous mission, S29 had a much more stable flight. During the transfer we saw the C29 expelling a significant amount of fuel, was there something planned to keep the attitude and pressurization of the tanks under control or a failure of the propulsion system? We do not know. The spacecraft approached atmospheric entry, the most critical stage of the mission, with the correct orientation and, apparently, maintained it perfectly during it. S29 experienced maximum heat and maximum aerodynamic pressure – two events that are separated in time during reentry, contrary to what many people believe – although it was deliberately launched without two tiles to test the strength of the fuselage (in section “Made of steel” Starship is able to withstand higher temperatures than aluminum ships and therefore withstand greater damage to the heat shield).

On re-entry, the camera was pointed at the forward starboard airfoil and was seen to have suffered severe damage as hot air penetrated the base and tiles and parts of the fuselage fell off. However, the surface held up without peeling off. It is unknown whether other parts or surfaces of the craft suffered similar damage, although if the damage had been very severe, the craft would have been unable to control its position during descent. Reflections on the fuselage – it was night in the landing zone – suggest that there was some kind of fire or flash on the S29 (in the engine area?). The ship tried to move vertically –flip maneuver— and turn on the center Raptors for landing, another new feature of the mission. Again, reflections from the fuselage, as well as telemetry data, indicate successful ignition, although to what extent is unknown. S29 finally reached the surface of the Indian Ocean northwest of Australia an hour and six minutes after launch.

In the absence of knowledge of critical data such as the accuracy of B11’s landing, the degree of structural failure of S29, or the level of Starship’s aerodynamic control during descent and final braking, the fourth launch mission of the world’s most powerful system was a success, as it demonstrated that the Starship was capable of maintaining its orientation during quasi-orbital flight and re-entry times are two necessary requirements before entering orbit to complete a mission (SpaceX cannot risk leaving the starship in orbit like a piece of space junk). Therefore, the next priority goal is to demonstrate the feasibility of ignition retardation. The fact that the S29 could more or less restart its engines at low altitude over the water gives some peace of mind at this point. No doubt the surprise of the mission is a lesson in the strength and stability of the starship during re-entry, but logically changes will need to be made to the heat shield, especially where the airfoils are attached. Although Elon Musk wants the next mission to try to lift Super Heavy using the tower’s Mechazilla system, I doubt they will dare try until the second Boca Chica launch tower is ready. Overall, the mission leaves a pleasant aftertaste – perhaps this is the first thing that can be considered a “success” without many nuances – and the feeling that the system is constantly, little by little, progressing.