Delving Deeper: Super Heavy thrust and counting down to flight

by Adrian Beil

The orbital test flight of Starship is just around the corner.

Starship has conducted a 31-engine static fire, a single-engine spin prime test, and multiple Booster quick disconnect (QD) tests in the last few weeks. However, the question remains if the ground service equipment (GSE) can handle almost double the thrust of the static fire, which will happen during the actual launch attempt – currently targeting March. Furthermore, SpaceX is preparing more mitigations to reduce the damage to the launch equipment.

Looking at Thrust Numbers

Initially, the 31-engine test was planned to be a 33-engine static fire. This was the first attempt to fire all the engines below the Super Heavy Booster. SpaceX later confirmed that one engine was turned off before the test, and one stopped during the firing.

According to Elon Musk, this test was rated at about 50 percent throttle of the full stack, while the flight will be about 90 percent throttle. Additionally, SpaceX tweeted that this test was conducted with a 7.9 million pound-force thrust, translating to roughly 35 meganewtons.

Knowing that the final stack will have about 74.58 meganewtons of potential thrust based on each raptor producing 2.26 meganewtons at 100 percent throttle, a rating of 90 percent brings the thrust of Super Heavy at lift-off to 67.12 meganewtons. This would be almost double the thrust of the 31-engine test.

This does mean that as of the Static Fire test, Starship did not overtake SLS, with its roughly 39 meganewtons of liftoff thrust. However, it is now ahead of Saturn V, which produced approximately 34.5 meganewtons of thrust.

Furthermore, it means that for now, the Starship pad only had to endure about half the forces it will have to resist during the orbital flight. While it successfully handled this test, it will have to manage a lot more power during the orbital flight test in a few weeks.

Historically, it should be added the N1 is still the rocket with the most thrust ever produced to date on paper, with its 45.4 meganewtons. However, it is questionable how much of this thrust was ever reached, as the rocket was plagued with early flight engine issues and multiple engines shutting down early in its flights.

Countdown Overview of the WDR and 31-Engine Static Fire

Come the big day of the orbital test flight, which has multiple public and internal references to March, operations will begin after the road closure and after the initial chill down of the GSE. Cooling down the plumbing before fueling can take several hours and multiple recycles. As such, the current data from previous countdowns would not be sufficient to analyze the countdown behavior.

Full Stack during the wet dress rehearsal. (Credit: Nic Ansuini for NSF)

 After multiple recycles to load the rocket, the final “OLM Vent” started at 1:43:40 during the wet dress rehearsal (WDR). This vent most likely indicates that the lines to the rocket are in chill down in preparation to let cryo-cooled liquids flow through them.

The OLM vent is a vent that originates from the mount. It is a large vent of most likely nitrogen that eventually ceases, a point that provides visual clues that the rocket has entered propellant loading.

During the WDR, the OLM vent stopped at 1:52 PM local time, with this mark followed by 75 minutes to T0. However, for the static fire, it was 82 minutes.

The difference might be the multi-minute hold that was observable on SpaceX’s webcast of the test. However, as there was no webcast for the WDR, it can’t be entirely determined if such a hold was present on the test or if this is the reason the countdowns are different.

The duration of both tests was likely the same, as the countdown is not decided by the overall fuel load but by the percentage load on individual tanks. Both WDR and static fire loaded the LOX tank full, while the methane tank stayed mostly empty for the static fire to mitigate the risk of an explosion, while it was full during the WDR.

Overall, this means that the duration of fueling for the orbital stack during the launch will probably be close to 75 to 90 minutes. However, it also hints that SpaceX can quickly perform multiple restarts of the GSE chill process.

What remains unknown is Starship’s hold and recycle capability after fueling has begun. While the rocket could hold for a more extended period of time during the static fire test, it might be different with a full pressurized load in both tanks during the flight attempts.

Damage from the 31-Engine Static Fire

Not much damage was observed during the firing of 31 engines. Besides some flying concrete and charred legs on the OLM, the overall pad infrastructure handled the static fire well.

Toasty OLM legs. (Credit: Jack Beyer for NSF)

It was also observed on previous static fires that the legs would suffer damage to their protective coating, which happened again during this latest test. However, in the past, SpaceX could repaint the coating to repair the OLM.

To mitigate further damage to the pad, SpaceX is preparing two significant protections.

The deluge system, which arrived from Florida a few weeks ago, is now being prepared to be installed as SpaceX is digging spaces for the massive water pipes to be placed.

This deluge system, in contrast to the current FireX system, will provide a huge amount of water to protect the pad and other infrastructure against damage from the fire. However, SpaceX has not confirmed if the deluge system is ready for the orbital test flight, and with the test approaching rapidly, this seems more and more unlikely.

The second mitigation is additional shielding on the ring of the OLM. The opening two panels of this shielding have been installed over the past few days and will be finished before the flight test.

As for the two raptors that did not fire, the recent spin prime test was likely the test of one of these troublesome engines.

The Raptor work platform has also been moved back to the OLM again, which could indicate that SpaceX is at least considering replacing one or two raptor engines of Booster 7 before the orbital flight. So far, no engine has been removed from the vehicle since the static fire.

The Path to Orbit

Currently, SpaceX has not commented on how many tests remain on the path to orbit, but on a SpaceX Starlink webcast, it was indicated that the 31-engine fire was one of the few last major tests before the flight.

SpaceX could decide to repeat this test until all 33 engines work flawlessly, but so far, no indication has been given that this is the plan for the static fire.

Meanwhile, Booster 7’s partner for this historic test, Ship 24, had its crane attachment points removed and tiles installed.

Ship 24 waiting for rollout back to the launch site. (Credit: Jack Beyer for NSF)

It is being prepared in the rocket garden as the second stage of the flight. Its roll back to the launch site could come anytime over the coming weeks.

After the stacking, SpaceX could perform a few more checkout tests and final steps before the FAA license and hardware readiness would clear the path for the orbital launch.

Photos from Jack (@jackbeyer), Mary (@bocachicagal), and Nic (@NicAnsuini).

(Lead image: Booster 7 fires 31 Raptor engines. Credit: Mary for NSF)

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