Thread by @RyanHansenSpace

Here's a detailed thread that breaks down some of the information suggesting that Starbase Tower B will be used to catch the first Starship and not Tower A.

Let's start by looking at the catch points attached to #Starship S33 before Starship Flight 7. While I understand that these catch fittings were non-structural, their primary purpose was to analyze their thermal performance. You'd only get valid data if you used the hardware you intended to use later with structural versions. In this image from @StarshipGazer, we can see the catch fitting covered in tiles and the contact point hanging below the arm.
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In various Discord chats and RGV Starbase Weekly episodes, I've repeatedly pointed out that this hardware was incompatible with the arms on Tower A. Specifically, the contact point. This slightly curved rectangular metal plate on the underside of the arm does not extend down far enough from the arm to create a gap large enough for the lip on the landing rail to not contact the underside of the arm. Using this hardware on Tower A would lead to the catch fitting getting damaged by the lip on the landing rail, resulting in tiles being crushed and the weight of the ship possibly damaging the lip on the landing rail during a catch attempt.

For comparison, here are screenshots from my "Super Heavy Booster Catch Explained" video showing the size and clearance for the small cylinder on the booster's catch fitting compared to the lip on the landing rail. As you can see, the height of the cylinder on the booster's load point barely clears the lip, and the rectangular plate on S33's catch fitting is about 3x smaller than the cylinder on the booster, so it definitely would not work.

Given the differences in vehicle contact point design and the incompatibility with landing rails, something would need to change. I was pretty sure that the ship catch hardware was representative of the actual design hardware, so I assumed this meant they would shave down the lip on the landing rails to make it work for the Tower A catch arms.

Since #SpaceX moved the new arms for Tower B to the launch site last night, @StarshipGazer got some nice up-close pictures of the landing rail hardware today (I highly recommend becoming a supporter to see exclusive images). It is very easy to see that the lip on the landing rails for Tower B has been shaved down to a height that would make it more compatible with the ship catch hardware seen on S33. I would estimate that its roughly 1/4 - 1/5 the size without measuring. This is easier to see in supporter-exclusive images.
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In addition, the scheduling of work at Pad B and supporting hardware has seemed to slightly prioritize the arms and tower over the past few weeks. The SQD arm is far behind schedule in comparison to the arms. The rush to get the arms and carriage out to the tower so soon after Flight 7 also seems to indicate that SpaceX wanted this done ASAP, as if they planned to use the arms on Flight 8 or 9. Now, it's evident that a catch on Flight 8 is not going to happen after the outcome of Flight 7, but it's looking like the arms could be operational on the tower by Flight 9.

If SpaceX wanted to catch a booster and ship on the same flight while only having one operational pad, the booster would likely have to be removed from the OLM before the ship could come in for a catch. I'll touch on this briefly later, but just assume this is true for now. In the future, having a booster on the OLM for a ship catch might be possible, but the orientation of Pad A likely makes this challenging.

During Flight 7, I closely watched the timing of post-catch booster operations. I was expecting B14 to be placed on the OLM and detanked much quicker than B12 was. This would allow SpaceX to return to the pad and remove B14 from the OLM ASAP to see how long it would take to get a booster away from Pad A for a ship catch to take place. While B14 operations appeared faster than B12, they weren't as fast as I expected.

This is where I have to be a little careful with some information, but I have to mention it to get the point across. The NSF L2 member forums have had information regarding ship duration in orbit before a catch attempt for nearly a week, but I obviously cannot include that information in this post. However, combining post-catch timing with the L2 forum information provides essential context to this topic. I would highly recommend becoming an L2 member to gain access to additional information on various spaceflight activities.

When it comes to Pad A, booster catches occur over the OLM because that's the optimal location for the return trajectory of boosters from the Gulf of Mexico. The arms close to point in the direction of the returning booster, and the booster slides between them. The booster must also be in the proper orientation for the load points to make contact with the landing rails during the final setdown.

At this point, we understand that a ship returning for a catch at Starbase will target the Gulf of Mexico and then use the flaps to circle back toward the launch site once in the belly flop position. Regardless of the flip direction, the ship would likely do something similar to boosters and slide between the closing arms after flipping vertically while also ensuring the vehicle orientation is correct for the contact points. The arms would still need to rotate toward the ship after being open by some amount.

Depending on the flip and burn direction, the ship catch position would either be over the OLM, in which case the ship's exhaust plume would cook the top of the previously landed booster, or off to the side of the OLM, which has VERY limited swing distance for the north catch arm and one unlucky grid fin would get some extra heat. Considering these factors, I believe that catching a ship with Tower A while a booster is on the OLM is not an option, for now.

When considering all hardware details, public information, and aspirational timelines, it seems logical that if Flight 9 is intended to be the first attempt to catch a ship, it would be a launch from Pad A that would result in Tower A catching the booster and Tower B catching the ship. We know there are proposals for a catch-only tower in Florida, so the idea that the ship does not return to the tower it launched from is not farfetched. This is the only option based on the hardware that I currently see but things could change in the next few months.