Everything to know about SpaceX's mid-air booster "catch"

SpaceX successfully conducted a fifth test flight of Starship—and managed to catch the first-stage booster in mid-air back at the launch site. I don’t feel the need to go over the whole thing in detail because a lot of what there is to say has already been said. But the aspect I have been getting the most questions about—and what I want to focus on—is that booster landing.

Specifically, why a “catch” in mid-air, rather than a landing on a droneship or landing pad like the Falcon 9? Did the chopstick arms really catch the spacecraft, or did it just dock back at the launch pad? Why not use parachutes to slow the booster down? Why did it look like the rocket was on fire during the landing burn?

The test flight was a success

I’m not really here to debate whether the flight was a success because I think it was absolutely a success. That doesn’t mean it was perfect or the ship is ready to be operational, just that for what they were trying to achieve, SpaceX nailed this flight. The two big things we were watching on this flight were the booster landing and a pinpoint splashdown of the upper stage. They both went very well.

Okay, but why should I care?

There are a lot of people who have (understandable) mixed feelings about SpaceX for myriad reasons — everything from the CEO to space junk to satellite pollution. So if you’re wondering why this test launch in particular is important in terms of a broader view—it’s crucial for NASA’s plan to return humans to the moon.

NASA’s current plan is to land humans on the moon in September of 2026 (they likely will not make this deadline, but hopefully it will happen by the end of the decade.) There are many reasons for the possible delay to landing, but one of those is partner hardware.

The Human Landing System, or HLS, will take humans from lunar orbit to the surface, remain on the surface during the landing, and then return the astronauts to orbit. HLS is a modified version of Starship (HLS will also need to be refueled in space, hence why SpaceX has been working on those demonstrations—the next one is currently planned for 2025, when two Starships will dock in orbit, with one transferring propellant to the other).

But back to Artemis, to be able to land on the moon, NASA needs Starship operational. I can tell you that they were watching this launch closely and likely breathed a sigh of relief at the success here.

Here’s the thing: They landed a 20-story building

If you're still wholly unimpressed by this, I'm actually not quite sure why you're still here (hate reading will take years off your life, don't do it, it’s not worth it!) but I'd tell you this to put things into perspective.

Super Heavy (not the entire Starship/Super Heavy combo, we’re just talking about the booster here) is 233 feet tall (or 71 meters). That’s taller than the average 20 story building. And they’re sending it to the edge of space, and then bringing it back and catching it with mechanical arms attached to a launch tower.

The livestream play-by-play: Launch to “catch”

At this point, I’m going to go through the livestream. This is really easiest if you just watch, rather than read, (the analysis in my video starts here.) However, if you’d really just rather read, I’ll do my best to insert screencaps and GIFs to show a bit of what I’m talking about.

You see here Starship on the launch pad. The upper stage, Starship, looks black here, while the booster Super Heavy looks white on the bottom, which makes it easier to see what's going on and the part of the rocket we’re focusing on.

All of the Raptor engines lit upon liftoff, so no engine failures for the booster, which is great. The curving as seen below is normal, pitching downrange to cut through the atmosphere to get to the thinner parts of the upper atmosphere more quickly and using less propellant.

The vehicle made it through Max Q beautifully. Max Q is the moment of maximum aerodynamic pressure on the vehicle, basically where the launch vehicle is the most stressed, so there's always a sigh of relief when that happens.

One thing that's important to note is you can really see the blue methalox flame. Without getting too technical, Starship uses liquid methane as propellant and liquid oxygen as the oxidizer, it's more efficient and cleaner burning than the traditional liquid oxygen and rocket grade kerosene that other rockets, including Falcon 9, use. I'll talk about methalox a little more later, but this is a great view of the flame below.

The next big thing to talk about is stage separation.

Starship uses hot staging—basically Starship, the upper stage, ignites its engines to move away from the Super Heavy booster before separation is complete. That's a tweak that SpaceX made after that first test flight when the two stages didn't separate.

First Super Heavy's boosters shut down—this is what SpaceX calls "Most engines cut off" instead of "Main engine cut off," the traditional term, because three engines remain lit (the lights on the bottom left and right show which engines are lit—Starship the upper stage is on the right, while the Super Heavy booster is on the left.)

Then the stage separation happens, and the booster’s engines light back up for the boostback burn. Starship, the upper stage, also lights its engines.

Now, for the purposes of this video only, we're saying goodbye to Starship, though it continued on into space for its suborbital flight and survived re-entry and splashdown mostly intact, and then after it splashed down it exploded in a pretty spectacular fashion. This may have been a controlled detonation or a result of a fuel leak as it tipped over and fell into the ocean.

But, back to the booster. At this point, Super Heavy is making its way back to the launch site in Boca Chica, Texas, with the boostback burn. On every previous flight test for Starship, SpaceX has dropped the booster into the Gulf of Mexico. This time they wanted to land it back at their base, but it's not what you think of as a traditional booster landing. The idea is to maneuver it into place with what they call chopsticks—giant mechanical arms on their launch gantry, called Mechazilla. Super Heavy has pins on it that you can actually see below (where the arrow is pointing), and these are what need to slid into place for these giant arms to catch the Super Heavy booster.

Before the booster could return, SpaceX had to perform automated checks on it to ensure that it was healthy and ready. When everything checked out, they sent the command for Super Heavy to try and return back to base. If it hadn't checked out, they would have dumped it in the Gulf and tried again next time. But luckily, the tower was "go" for catch.

Now the boostback burn is complete, all engines are shut down, and you can see the thrusters on the side firing just to maneuver the rocket as it returns back to the launch pad. You can see the hot staging ring falling away.

Because they are trying to land the booster here, they jettisoned the hot staging ring, which isn't new, that happened on the last test flight. This is just to take off as much mass as possible while they're trying to figure out the booster's return, it's around 20,000 lbs, that's about 9 metric tons.

One interesting thing below—that glow at the bottom of the booster is the engines themselves, not the engines firing. There's no burn happening. That is some immense heat, from the friction with re-entry. We did learn after the flight that there was some outer engine warping from the heat—no huge deal, but interesting to see.

The landing burn goes in stages. It starts with 13 Raptor engines to slow down its speed. While it’s descending though, keep an eye out for the flame just above the engine burn and the black smoke that goes with it. There have been a lot of questions about what’s going on here, and I’ll get to that.

We're in the next stage of the landing burn and down to the final three engines (this will become important), Super Heavy is maneuvering here. I know this looks really close on the feed, like Super Heavy was about to run into the launch pad, but if you look at it from other angles, which I'll show you later—it's close but not AS close as it seems here.

And....success!!!!

Let’s talk about the black smoke and fire

Okay, so let's go back over that and address some of the many questions that I've gotten about this: what is on fire, is that normal, what's the black smoke from, was this really a catch versus just a mid-air landing, is the catch necessary or just all for show, why aren't they just landing this like they do with a Falcon 9.

SpaceX provided multiple camera angles for this over the past couple of days, which has really been helpful to parse what's going on. Let's talk about that fire first. I want to flag that I talked with rocket engineer Gokul Das, who's worked on ISRO missions, to parse out what we think is happening.

Keep in mind that unless SpaceX tells us directly what's going on, everything you hear and see about this is conjecture. I've seen a lot of people talking very authoritatively about this, as if they know exactly what's happening and unless they work for SpaceX or have heard it directly from someone who works at SpaceX, there's not really authority there just because we aren't entirely certain. But there's a lot we do know, and quite a bit we can deduce.

Let’s look at this alternative shot, where we can get a better idea of where the flame and smoke are coming from.

The secondary fire, I think, is from fuel venting. Basically the vehicle vents its fuel before the catch in order to avoid overpressurization. Simple, right? Well there is actually more going on than that in my opinion.

When Super Heavy hits 1 km, it starts its landing burn. That's 13 engines, three center engines and the ring of 10 around it. That's certainly a lot of thrust to slow the rocket down, which they need to do quickly. But once they slow it down, that many engines are not good for fine control, which is why they shut down those 10 after they achieve the desired deceleration and we're left with three engines.

But when those 10 engines shut down, there's what Das described as a "flame fold back"—basically when the engines are shut down, the flame folds back because of air flow and that ignited the methane, and because there's additional methane spewing around in the engine compartment, this might have ignited the actual engine compartment as well. I’ve slowed down the video so you can see it in the GIF below—watch what happens to the flame as the engines (bottom left display) shut down.

There's been some speculation that there was more venting than was intentional, that this may have been a leak and not just an intentional vent. My thinking it was probably an intentional vent, maybe they vented more than they intended, I'm not sure they wanted it to catch on fire, but it's not a huge deal that it did.

The black smoke is more complicated, simply because just burning methalox fuel wouldn't cause that. But because liquid fueled engines take awhile to shut down (you have to do it slowly), there's still a lot of methane as well as exhaust floating around, so as I mentioned, the engine compartment might have ignited as well. There may be something in the engine compartment that's burning to give off this black smoke, but we don't really know what.

My suspicion here is that it's some sort of thermal protection on the engines that's burning off. It's very possible that there's an internal fire going on that's charring through some stuff. It's not great, but this is a test flight. It's certainly not terrible, and not taking away from the overall accomplishment—I don't want my focus on this to be interpreted as it being a serious issue. I just was personally very curious about what might be going on here and wanted to dive in.

This would need to get fixed at some point but also...depending on what happened here, this heating may not even be an issue.

Raptor 3, the next iteration of the Raptor engines don't actually require a heat shield so this may all be a moot point because the internal plumbing on these includes regenerative cooling. These are currently in production—but also SpaceX does often overpromise when it comes to timing on these things, so I don't want to just say it's not a problem because Raptor 3 will take care of it soon.

But also, if the engine compartment catches on fire, and the internal plumbing isn't well protected, and Raptor engines are cooled by methane, and that internal plumbing catches on fire . . . well, let's just say fires in an engine compartment generally aren't ideal, and this is something SpaceX will need to look at.

Is this a catch or a docking?

It is a catch—the giant Mechazilla arms do move to grab the rocket. It's hard to see in that wide main shot, but in this other video video SpaceX provided from the launch tower, you can actually see the arms moving to grab the booster (GIF is below). It is a catch.

Is the mid-air catch just for show?

Is this just something SpaceX is doing for theatrics? That's a hard no. This is a very actually smart and elegant solution to the problem of building landing infrastructure. The reason you can't use parachutes, and the reason they didn't want to land the thing on the ground is the same—Super Heavy is HUGE. Remember, I said it's the height of a 20- or 25-story building. It does not have landing legs, and the amount of weight landing legs would add to the booster in order to be able to support its weight and the shock of landing would be . . . well, it'd make it that much harder to land.

Why not use parachutes?

Parachutes to slow it down have the same issue—the thing is so big and heavy (the thing roughly weighs about 200 metric tons, which is around 440,000 lbs, as compared to the Crew Dragon which is slowed by parachutes, which weighs around 12,000 kg or 26,577 lbs ) that parachutes wouldn't make much of a dent. Also parachutes are relatively inaccurate. They're fine when you have a landing zone a few kilometers wide but they aren't super useful for a pinpoint landing at a launch pad. Plus parachutes are heavy, and they don't want to add to the mass of the booster even more.

What about landing on a droneship or on land?

It can be hard to find the precise numbers, but from what I can tell from my research the Falcon 9 booster weighs around 25,000 kg. The weight of the landing legs alone is 2,000 kg, or around 8 percent of the weight. You can see the four of them per booster below.

Starship would need bigger ones, and maybe more—possibly 6 instead of 4. Even making the legs as lightweight as possible, which if anyone can do it at this point SpaceX can, that's still a lot of extra weight. Plus they'd need to be sturdy enough to support the weight of Super Heavy which might mean making them even stronger and therefore heavier.

What's more, the landing infrastructure would need to be extensive. The landing infrastructure is often damaged by the rocket. It's not a huge deal with the Falcon 9 because it's not an especially powerful rocket. But Super Heavy could do a great deal of damage to landing infrastructure when it's coming in, not to mention stuff could get kicked back up and damage the booster.

Super Heavy does have the ability to hover, which also helps with the catch (but would also damage landing infrastructure even further if it hovered right above a pad). Falcon 9 can't get its thrust low enough to hover, but this does mean that Super Heavy can minimize the shock absorption necessary for any landing structure—but Mechazille and the chopstick arms do have shock absorption built in.

Starship and rapid reuse

The idea for rapid reuse of Starship includes not having to refit the booster in between uses. They want to be able to do basic checks on it, refuel it, and use it again, possibly a turnaround of hours not even days. (This is also where landing legs are a problem. On the Falcon 9, they need to be refurbished between uses). That means landing infrastructure that can handle a lot of use, as well as rockets that can land without damaging themselves or what's around them. That means catching the rocket in midair, instead of letting it land on the ground, protects everything on the ground while still bringing the rocket back.

Plus landing it back at the launch site means they can literally lower it to the ground, refuel it, and go. They don't even need to take the time to transport it back like they do with Falcon 9s landed on drone ships.

Environmental concerns? Here are mine.

Before I close this newsletter out, I do want to mention that venting of methane into the atmosphere before the catch. Methane is a greenhouse gas, and while it's cleaner burning than traditional liquid oxygen and rocket grade kerosene, I don't love the venting here for environmental reasons. (There are other environmental issues to get into with Starship, the flame deluge system is currently under investigation, that's a whole separate topic and this is long already. I'll cover all of these at some point)

I want to point this out, not because it's a big deal now—this is a test flight, it's a one off, most of the methane burned off whether intentional or not (though the byproducts are water and . . . carbon dioxide, again not great for climate change). But if we're talking about rapid reusability, if we're talking about one day sending Starship into the sky multiple times a week and landing Super Heavy back at the base—that's going to add up, and I hope that's something that will get addressed because we don't need to add to our environmental woes with this.

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