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Comment by ChuckMcM | Hacker News Reader

Pretty impressive. The engineering team at SpaceX is really something. Some thoughts;

The 'chopsticks catch' was amazing to watch. Seems like it adds a lot of risk and clearly the booster needs additional fire suppression systems :-) perhaps the tower could mount something that sprays the booster like the barges have for the F9 boosters.

The heatshield held out for a much longer time, the asymmetric heating on the flaps was interesting. I had guessed that all four flaps would have equivalent heating based on an approach that was basically that side of the rocket perpendicular to the flow but it seems like that isn't the case. Still it seems like they are close to having something workable here.

The detonation at the end was pretty spectacular too, but I suspect that structurally the tanks failed as the rocket hit the water vs anything that was an engineering failure. Engineering it to be strong enough to land on water would presumably compromise the cargo to orbit number.

The use of Starlink was really interesting. The ability to get live video for the entire re-entry is pretty game changing for engineering. I'd guess there are even more 'views' than they showed (there would be if I were running things :-)) but overall that capability is something that really helps evaluate the changes made.

I can easily imagine that flight 6 will be nominal end to end without any unintended damage. That would enable, perhaps, one of their 'massive' Starlink missions to test cargo delivery. It will also start to give us some better numbers on exactly how much cargo Starship can put in orbit in 'full reuse' mode which is essential if they want to create a fueling station on orbit for the Artemis program.

Again, hats off to engineering at SpaceX, y'all did good.

View on HN

As to whether catching the booster adds risk - I'm not sure it does.

First, to the extent the booster is out of position in the x (sideways) direction, the chopsticks can move to accomodate error. But actually I think this dimension is the easiest of the three, as the booster has plenty to time to null any error in this dimension.

In the y direction (direction of travel towards the tower), the rails on the chopsticks can cope with the booster touching down along quite a long distance. But importantly, they appear to be smooth, so the pins can initially skid along them and then the booster can swing if it has not fully nulled any horizontal movement. In contrast, if the booster used legs and has not yet fully nulled any horizontal motion at touchdown, there is a greater risk of breaking a leg or simply tipping over.

And in the z direction, it should be possible for the chopsticks can absorb more vertical motion than legs can absorb, because you can easily build in huge springs/dampers/etc into the ground equipment without concern about mass.

Catching also puts the booster in tension rather than compression - it's easier to be rigid in tension than compression.

Finally, if legs were used, the engines would have to get close to the ground during landing, so reflected shock from the ground could cause damage. I know Falcon 9 does this, but the area of the base of Starship is much greater, so there's effectively less room for the reflected energy to escape. Catching completely removes this risk.

On balance, I think they would have better chance of success for each mission by catching. The main downside would be if you fail to catch, you may need to build a new tower, whereas a flat pad would be cheaper and easier to repair.

I am really curious what the maximum wind speed allowed for a booster landing will be. Upon landing, it has a lot of windage and not nearly as much mass as during take-off.

I have experience with docking large boats and it does seem to be a bit similar. In the case of boats, wind is a big deal, and the booster has nothing "below the waterline" to slow down the effects of wind.

I doubt this is an issue.

Not because wind wouldn't affect an empty booster; it certainly would.

But since the booster returns within 8 minutes of the launch, the weather in which a booster lands is restricted to the weather in which they will launch a rocket.

But then wouldn't the limiting factor for launch be the maximum allowed wind for landing?

The value of what is going up (which includes the booster) will be greater than the value of the empty booster. Factors of safety would be based on the launch rather than the catch. In other words, if it's deemed safe to launch the calculation for safe to land is easier to pass. Especially when you are taking passengers on launch. Wind is already a significant factor in launch.

That's true, except it neglects the cost of the launch tower itself. If you botch a catch and need to rebuild the launch tower, that could get very expensive, both in immediate costs of rebuild, plus in opportunity cost of missed launches. So in the end, whichever has the lowest wind limit, launch or landing, will likely determine whether they fly.

Ah, excellent point! They wouldn't ignore one hazard because another is less severe. And you are correct, I wasn't considering hazards to the launch tower itself. I think you are absolutely right, either would cause a flight to be scrubbed. I wonder if the two wind limits would be different.

This is often the case with airplanes; it's not a new concept.

Definitely not, and I am not trying to be a doomsayer here. It's just interesting. Now that I think about it more, I believe a Falcon 9 Starlink launch was once delayed due to weather conditions at the drone ship.

That value is unlikely to be significantly different than safe takeoff conditions. Yes the booster is lighter at landing, but launch is way more dangerous with larger error margins and more conservative condition requirements.

And with a barge recovery the legs must have enough spring to deal with waves. The tower doesn't move, they can get a lower landing velocity.

The most challenging axis in my opinion is the roll axis of Super Heavy, if there is a roll angle error, the pins could not sit properly on the chopsticks and the whole booster slides off.

Just slap a buncha RCS on there, maybe a reaction wheel or two, and presto.

Source: Kerbal Space Program

Might as well chuck a full size engine on the side pointing in the yaw direction to be safe. I mean as long as you tie enough magical struts and cables to it I'm sure you are fine /s

Seems like it adds a lot of risk

I'm actually wondering about that. If I understand correctly, the arms can move up and down, and pivot around the tower. This allows them to correct for some error in the rocket trajectory and also (presumably) "soften" the final contact. Between the nozzles and the arms, it gives SpaceX a lot of degrees of freedom in the final seconds (you can see how the booster kind of "hovered" right at the end) and in certain respects might even offer more forgiveness than the hard ground.

Could it smash into the tower? For sure. Would that be more dangerous than smashing into the pad? I don't know.

It's a new technique with which we don't have a lot of experience.

My thought (admittedly not well developed) is that smashing into a landing pad of concrete can damage that pad but it can be quickly repaired without affecting the ability to launch future rockets. If you damage the launch tower significantly you're going to have to suspend launches from it until you fix it. So the "higher risk" is more critical assets offline in the event of a non-optimal return.

Understood, although weren't they out of commission for a really long time anyway the last time they damaged their launchpad?

That was a launch pad, not a recovery pad. The launch pad has to be engineered to survive full thrust from all the engines, and for a rather long time as the loaded vehicle accelerates upward.

Thanks for clarifying!

Apparently they are heavily investing in having multiple towers ready to go to be able to do multiple successive launches. Presumably with that approach, one being damaged for a while will be annoying but not project-stopping.

There are already 2 launch towers (2nd one not quite complete but getting there.)

Heh Plus one at Kennedy in Florida, LC-39A

It helps enormously that unlike Falcon-9 this rocket can dial down the thrust of its engines low enough to be able to actually hover or to move arbitrarily slowly in the final meters before touchdown.

It can arrive to the designated intermediate point with some already good accuracy, and then take some time to trim the remaining errors to the noise level more slowly, possibly with feedback from the ground sensors.

The chopsticks also include rails with shock absorbers, the action of which can be seen in the view from the tower during the landing [1], so the required accuracy is probably relatively modest, provided one plans the maneuver carefully.

[1] https://youtu.be/Ysx4t7ICO58?t=678

The main takeaway from Scott's commentary is that the chopsticks allow the ommission of landing legs and all their subsequent systems and saves a ton of weight. The added risk to dial this technique in is likely worth it in the long run from a sustainability stand point.

Except that the landing legs allow you to land anywhere with a flat pad of concrete, whereas this requires comparatively enormous infrastructure investment.

You'd still have to recover the rocket. I imagine the number of places you could land and economically recover the rocket is pretty small.

unless it can be refueled from a truck and reused without inspection.

That is so many iterations in the future, it's not really meaningful to design for now.

Super Heavy and its 33 raptor engines really needs a specific launch pad - on the first launch they tried to see what happens when they just fire it over a regular concrete slab (but still way above in a launch mount) and ended up with a massive crater. While Starship might be able to hop from unimproved landing sites, that is not really an option for Super Heavy, even with low fuel and short hops IMHO.

That's why it is good in the long run, assuming the infrastructure can't get annihilated in a crash.

There's always a risk, but at the same time, they've designed the infra now, they can rebuild it from the plans (and iterate on any flaws). There's a (imo unnecessary) idea of doing a lot of launches, for which you'd need multiple liftoff and landing sites.

>comparatively enormous infrastructure investment Any infrastructure that can remain on the ground and doesn't have to be on the rocket is worth whatever the investment cost.

You note of course that instead of legs, which have mass and have to have a structure in the stage which distributes the loads the SH has those parts with pins which were locked eventually with Mechazilla's arms, and those parts also have mass and need to have corresponding distribution of loads.

How different those consoles with pins are from possible landing legs, and how much savings they provide is an interesting question. It's quite possible they provide some savings - but it would be nice to know some details.

It seems pretty likely that those pins are going to save considerable mass and volume compared to articulating landing legs.

Why so? Pin parts need to withstand similar loads - and if amortizing rails of Mechazilla may soften the contact, the direction of loads for pin parts is less favorable than for legs. Legs don't need to be big or too numerous - effectively legs are those pin parts moved to the engine compartment and turned for an angle.

Because the pins can be much shorter. Take a look at the falcon 9 legs. They are enermous both in absolute terms and relative to the whole rocket. They need to be that long to provide a stable platform and enough clearance for the nozzles and the residual plume as the engines shut down.

Not just that. Landing legs need to actuate, which means having actuators - another part that can fail. Pins are just dumb bits of steel.

IIRC at least one returning Falcon 9 stage was lost to a landing leg collapsing.

Don't forget that between certification and catch attempt that catching infrastructure is subject to the launch of the most powerful rocket man has ever created. It seems that the consideration about another part to fail is not valid here as the parts to fail have not disappeared but rather moved to the tower. They could still fail - in fact it seems that there are now many more recovery-critical parts.

That is, unless the falling rocket could abort a tower catch and move to a secondary nearby tower if a failure is detected in time.

> and move to a secondary nearby tower if a failure is detected in time.

They will have a tower in the Cape. It’s conceivable they could land there depending on return trajectory and save some mass for payload with that maneuver. I am also quite sure they will be a dozen towers in Boca Chica and I wouldn’t be surprised if they build a couple in California for Southward launches.

I think it is very much valid if the entire context is taken into account.

The tower is used for various stacking and craning operations between launches. There is a better chance to detect any developing anomalies outside the launch context.

Also, being on Earth and flying nowhere, it can be sturdier and heavier than any flight hardware. Much like Roman aqueducts, it can be overbuilt a bit to ensure some extra resilience.

Plus, more towers at the same site, as you say. If one malfunctions, another one can act as a backup. In contrast, every single landing leg is a mission-critical component and cannot be replaced in-flight by another one.

They do checks of the tower systems before using it, and have abort contingencies in case something goes wrong during final approach. I'm not sure if they intend (or have fuel budget) for last-second aborts to other towers, or if they just ditch in the ocean (remember there are no humans on the booster).

I'm curious how late in the catch sequence they can still abort.

Compression and tension are quite different loads. There have been rockets in history that would collapse under their own weight unpressurized. Neutron's second stage is a hung tank for similar reasons. Bucking is a pain. Super Heavy can obviously support its own weight, but tension is always going to be the easier load path.

The booster needs lift points anyway for manufacturing and moving it to the launch mount. You can’t engineer those out, really.

This seems a bit like removing landing gear from aircraft and telling airports to shoulder the added cost of accommodating them. You've simply shifted complexity elsewhere. I understand that people are dazzle-eyed over the science fiction appeal, but IMO this feels like a distraction. The rocket's already reusable and already the largest rocket ever built, this doesn't add any fundamentally new operational capability while also burning a lot of engineering cycles and adding complexity and uncertainty.

What you're missing is the rocket equation. The less weight, the less fuel you need and the larger your payload. We should trust the decisions of these experienced engineers who are deeply familiar with the tradeoffs involved in spaceflight more than our own intuition.

Yup, the rocket equation is truly brutal. Anyone who thinks legs are superior to a tower hasn't played Kerbal Space Program--and remember that stock KSP is easy mode. You don't need anything like the mass ratio that Earth rockets need.

Removing unnecessary systems that have mass is a big part of making reusable rockets work. It's why propulsive landing is superior to landing with wings, for example.

What you're describing with airplanes already happened. Large airplanes used to land on the water, which incurred a mass and aerodynamic penalty for the airplane but was very cheap to operate airfields ("fields"?) for; it only required a flat lake or harbor which was already there. The switch to landing gear allowed airplanes to be more optimized but requires more infrastructure expenditure for large aircraft.

Not a distraction at all.

It is not just about shift of complexity from A and B. Anything that stays on Earth permanently can be built without particular regard to its weight, e.g. much stronger, much more resilient, with bigger safety factors etc.

With any flight hardware, you need to make painful tradeoffs between reliability/sturdiness and weight.

If anything out of the ordinary happens, massive steel chopsticks can take a lot more strain than a landing leg which needs to be carried to the edge of space and back.

The booster is always returning to the general vicinity of the launch tower (either the tower itself or a barge). It isn’t used anywhere unimproved, and in particular is not used on Mars. So what scenario would it be helpful to be able to land the booster on a flat concrete pad?

It sounds like sci-fi thinking tbh, but at the same time, Musk has hinted at using rockets for intercontinental travel. But even then, it wouldn't be just a concrete pad, it'd need disaster recovery systems and infrastructure in place.

You're talking about Starship, not the booster.

Starship will get landing legs.

Only for Moon & Mars landings. The plan is to use the catch tower, Same as the booster, when landing on Earth.

Just build another tower, like you build another airport

Not just a hint. Here's a 2 minute video produced by SpaceX called "Earth to Earth".

https://www.youtube.com/watch?v=zqE-ultsWt0

The booster does not travel more than ~50 miles.

The first stage doesn't really need to land anywhere, it launches from a known location that already has a comparatively enormous infrastructure investment.

The second stage might want to land in other places. Not as a satellite launching bus or fuel truck though, that just wants to go up and down in an uncomplicated and unsurprising way, and that's where the vast majority of their launches will come from.

For inter-planetary missions yes, but they have different second stage designs for those that aren't made for tower landings. If it gets used as a military transport, then similarly it will be a different second stage design.

The military transport option is going to provide so much capital.

It's the C5 Galaxy, but with only a couple of hours needed to deploy into any theater in the world.

There's some work needed to have the launch flexibility though. Ie lead time to launch, multiple launch locations.

In comparison, simplified, if you have a bunch of things you need to send somewhere, you can go to the nearest airstrip and call a bunch of C5:s from somewhere a couple of hours away.

If we’re talking about sending equipment / supplies, you can prelaunch into orbit and then re-enter on demand.

That could actually probably be worse as the orbital path would not likely go near the wanted landing site, potentially in days. And anyway how do you know in advance what you are going to need (if it's not a nuke)?

Instead, with a near-future rocket, you could have some sort of assortment of "most likely stuff needed" stored near a launch site and be ready to pack and launch in an hour.

The starship can use the atmosphere to change its orbit.

The capability hasn't been tested but why wouldn't it work if it works on a x-37?

It’s called cross-range capability, and yes Shuttle was able to do this (albeit for different reasons).

Starship has a lot less cross-range capability than X-37.

Exactly. Cross-range capability is expensive mass wise. X-37 is very heavy for its payload, as was the Space Shuttle.

https://fastcompanyme.com/technology/this-startup-has-a-plan...

With maybe totally different requirements on the G forces and vibrations that the equipment and people inside a Starship must withstand compared to flying. Not necessarily all the existing equipment can survive a Starship launch and not necessarily all military personnel can fly in a rocket. Of course they can select the personnel, like they do select paratroopers. Fixing the equipment or developing new one might be costly.

Thankfully, military stuff that is field deployed typically already has insane shock and vibration requirements. We build military stuff at our facility and it all has to go through lots of shock, vibration, and temperature testing. The military really wants to be sure things don't fail on the battlefield (which could also be aptly called "the-shock-and-vibration-field")

Just want to add that a lot of military equipment is already designed to be airdropped in addition to any other expected battlefield stresses, so they’re probably some of the best candidates for rocket transport in existence.

Landing legs are out of the question for Super Heavy anyway. If the engines come that close to the ground, the reflected sound from the ground will tear the engines apart.

That's fascinating!

Do you have a source for this where I can read more?

The 3 RS-25 (1860 kN each)[1] used for the Space Shuttles had 300,000 Gallons of water output per 41 seconds [2] when it launched. On landing, the Falcon Super Heavy used 5 or so [3] of the Raptor Engines (2750 kN each [4]). I'm making a few assumptions based on Napkin Math, but the parent comment seems about right since the engineering required (and the payload weight lost due to the weight/space requirements of landing feet for the FSH), would be too high to withstand the vibration reflection of landing on solid landing pad.

[1]https://en.wikipedia.org/wiki/RS-25

[2]https://en.wikipedia.org/wiki/Sound_suppression_system#NASA [3]https://www.youtube.com/watch?v=pIKI7y3DTXk&t=6850s

[4]https://www.spacex.com/vehicles/starship/

To me, it's not risk reduction that they're after with that booster catching mechanism, but weight reduction. Those landing legs that we've seen before (and the mechanism related to them) are costly weight that is absolutely necessary only on the rocket itself, because that is expected to land on its own somewhere on a bare rock. For booster however, it makes sense to have as much of such launch and landing weights externalized, considering booster's reduced use-case of starting from a spaceport and very soon ending up back there too.

The way the trajectory is designed is that it has to scoot over to the tower at the last second, and it only does that if it's really really sure it can make it, otherwise it crashes off to the side.

A good explanation of how it works, from before the catch: https://www.youtube.com/watch?v=ub6HdADut50

Yes, we should probably assume that the SpaceX engineers have considered all of the risks HN readers are able to come up with in a few hours. And that they have evaluated alternatives like the added weight etc of having foldable legs on the booster.

The catchzilla solution is an example of their amazing ability to think out of the box. This solution, and things like the rapid evolution of the Raptor engine (see picture, story here: https://medium.com/@futurespaceworld/the-evolution-of-spacex...), dynamic engine configuration (33, 13, 3, zero, up again) and control is almost magically impressive. This is the stuff of Sci-Fi, brought to life.

But did they consider a giant spider web?

We can probably assume yes: https://www.youtube.com/watch?v=qz7Eb8WiiWA

> The detonation at the end was pretty spectacular too, but I suspect that structurally the tanks failed as the rocket hit the water vs anything that was an engineering failure.

It's possible that SpaceX programed the AFTS to trigger some time after the rocket touched down in the water. Just to make sure that it completely submerges quickly.

> I can easily imagine that flight 6 will be nominal end to end without any unintended damage.

I think it depends on what you mean by "nominal". SpaceX ultimately wants to catch the 2nd stage as well. I suspect that they are a ways off of that, since it would have to approach over land. The FAA is going to need to have very high confidence that it will do exactly what it's designed to do before they're going to allow that.

> Just to make sure that it completely submerges quickly.

Why would they want to do that? (genuinly curious).

I reckon there would be a lot of useful data left if they could recover or even just inspect the remains. The remains are one big tank, so it would have floated.

No doubt it would be very useful to recover. SpaceX isn't the only one who could pluck it out of the Indian ocean though. You don't want to leave a prototype for the most advanced Spacecraft ever made just sitting around for competitors to grab (most notably China which is currently speed running SpaceX-like designs).

It landed right next to their own camera-bearing buoy. You can bet their own recovery ship was right nearby. And with access to radio control too. Likely with a couple US military ships on hand too.

I certainly hope so. Ocean are polluted enough and although such a ship is just a, well, drop in the ocean, the ideq of accepting to pollute more is unbearable to me, especially for a world class company like SpaceX...

Other than some electronics it's actually pretty clean vehicle - methane will gas off, steel will quickly disintegrate in water, there isn't tons of plastic or paint.

Looking at the composition of the ship, it won't be polluting the ocean much. No people on board to produce trash etc., mostly just plain stainless steel and a bit of ceramics that will make great hiding spot for the abyssal fish for a few years.

It might not be that simple - I've read an article how they recovered one of the solid rocket boosters from the first successful Ariane 5 flight to check all was fine. IIRC it was a slog, they had to tow it back very very slowly, avoid it sinking, fighting all kinds of weather and tow line issues, etc. Have not found the article, but there is a picture how it looked like[0].

With Starship it could have been similar & possibly worse given the size and more complex shape (various voids that might fill/drain & the thing is not really built for floating). Also you are in the middle of an ocean (Indian in this case) with potential for all kinds of weather on the way. Towing might again be very slow, so you might need to stage a massive submersible transport ship or something similar to make a recovery successful. And then the thing might still tip over and explode anyway - meaning all this was in vain.

I think is most likely they won't bother and instead just stream as much data as possible over Starlink in real time (or heck, even via WiFi once the buoy is in range) for analysis. They want to catch the shop eventually anyway, so manual post flight analysis will wait.

They can now check all over the first recovered booster anyway. :)

[0] https://www.flickr.com/photos/europeanspaceagency/3700131835

But, couldn't they wait for like, 6 hours? Take a good look when it's cooled down a bit. Send some drones over to film it, and then sink it?

I get that towing is probably too expensive for what it's worth. But I'm surprised they don't even go pick up some tiles before kablooing it.

Attach a tether to one of the fin pods, then blow up the rest to sink it cleanly and bring back the important part.

Possible yes but still, this is a prototype with new fin configuration, materials and lots of detail to be understood from inspecting it in detail. An inspection would be very useful.

At the same time, this is SpaceX and they have a few others ready to launch already. Perhaps they indeed can keep it somewhat coarse and wait for detailed inspection until one of them makes it right back to solid ground?

You wouldn't need to tow it; if you really wanted to you could use one of those deep see platform recovery ships that sink themselves. The rocket is big but it's tiny compared to ocean-going vessels.

The previous ship did not come down where it was supposed to. I don't think they wanted humans anywhere near where it was coming down, at least until they can reliably do pinpoint landings. Even the Falcon 9, as accurate as it is, doesn't have humans anywhere near the landing location.

Yep this will be the reason. And lets not forget that Bezos was able to find and recover the Apollo 11 Saturn V engines from point nemo. If that was relatively simple you can bet plucking a freshly dropped entire starship from the indian ocean would be a doddle, especially when sat views likely show exactly where it landed.

I'm fairly certain the Apollo first stage engines were recovered from a location relatively close to Florida/Bahamas, just east of the launch site. Not point nemo.

That is correct, off FL. The recovered engines were from the first stage they would never have made it half way around the world. Point Nemo is used to stash spacecraft that were in orbit.

Also a yacht of a fishing boat could run into it in the night and sink - eq it could become an unmarked floating hazard.

correct me if I’m wrong but I don’t think the second stage is meant to be caught. It will have legs to land on Earth/Mars without any landing infrastructure.

It will have legs for Mars but they plan on catching it on Earth.

It is expected to land astronauts on the Moon as well this decade, which will certainly require legs.

That is a different version. The ones launching/landing on Earth are supposed to be caught by arms similar to the booster.

The ones for other celestial bodies are also planned to return to Earth... Being inside one as it does the belly flop will be quite the experience.

Pretty sure the Artemis lunar missions are going to use a different vehicle for Earth to LEO and LEO to Earth, so I don’t think anyone will be inside a Starship belly flop for quite some time

Or have the tower on the moon catch it.

Why is SpaceX choosing to land the booster on the Mechazilla arms instead of performing a soft ground landing like the Falcon 9 booster?

1) legs are heavy 2) empty rockets are stronger in tension than compression

the scale of Superheavy is such that the above two items are making the arms scheme make sense. The number of engines also gives this rocket the ability to hover, which probably makes the scheme easier to pull off.

It's my understanding that these engines can be driven with variable power, which also makes deceleration and controlled hover more feasible.

This was also true for this engine's predecessor, the Merlin 1D. However that engine's rocket (the Falcon 9) can't hover, as the power of a single engine throttled to its lowest setting still overcomes the weight of a nearly-empty booster.

All high performance engines tend to only throttle in a range in the upper half of the engine's performance, the difference making hovering possible in this case is that the Super Heavy has _so many_ engines that it can turn off. This is a sort of secondary throttling, or meta-throttling, and the rocket can use the combination of engine throttling and engine-off to hover comfortably (while near-empty) with three engines going.

Thanks for the clarification!

> 2) empty rockets are stronger in tension than compression

True, but wouldn't the deceleration burn be putting much more compression onto the near-empty rocket than the landing?

The engine load is probably a steady, consistent magnitude. While a landing load is rapid and variable. Also, you need to design legs for wind loading after landing, which can be high if you want to launch often.

3) rapid reuse by landing on launch tower

Spent rocket stages are empty of fuel, but not necessarily of the ullage gases, the pressure inside could be e.g. 3 atmospheres and that could be enough to provide some stiffness in the direction perpendicular to the axis.

> It's possible that SpaceX programed the AFTS to trigger some time after the rocket touched down in the water. Just to make sure that it completely submerges quickly.

The SpaceX host on the stream said that they were going to try and touch it down on the water at more of an angle than the previous flight to attempt to get it to survive the initial splash-down so they could get some more data and video footage.

Obviously this wasn't guaranteed to succeed, but it indicates that they weren't planning to immediately detonate the ship on touchdown.

As I recall it was common on the early Falcon 9 landing tests that splashed down in the ocean to also explode after tipping over and smacking the water. Once they're actually landing them on a pad, tower, or ship that should be much less of an issue.

Yeah, I don't think SpaceX planned for a RUD on tip-over (even if they did anticipate it).

I think they've towed stuff in the sea before so I would have thought that'd be their preference if it were possible.

Yeah. It wasn't planned, but was a likely outcome.

I'm not sure if they would have actually attempted to tow this one somewhere given its location in the Indian Ocean, but they might have taken the opportunity to do some inspections before sinking it.

They don't want the booster actually landing in the water. They managed to do one test so well it survived--and then it became a hazard to navigation.

I imagine that they will be their own first customer - putting Starlink satellites into orbit while they are gaining confidence in the reliability of the system for external customers.

They have to prove out the landing of the second stage, either with another catch or with landing gear, which they need for the lunar lander anyway.

I think that has always been the plan. the V2 starlink sats were designed to fly on Starship. When Starship wasn't coming along as planned they shoe horned the guts of the V2 on to a smaller sat that became the V2 Mini.

If Im not mistaken, they already delivered a payload in one of the previous flights, because they cut the transmission for a while and didn’t show the payload bay like one previous flight.

I’m not sure they are, if only because of the altitude. They only took Starship up to 200km, Starlink are nearly double that so while they do have thrusters that’s quite the distance.

Starship is not really in a true orbit, I forget the exact terminology but the trajectory is designed to re-enter regardless of what happens with the the flight.

It is in an suborbital trajectory but with enough energy to be equivalent to reaching orbit.

Think of it as instead of thrusting perpendicular to gravity for the most optimal energy usage Starship instead points its nose up gaining more altitude but now lacks the speed to miss earth as it comes down.

They actually made use of this this flight again - eq. they still did not perform a deorbit burn this flight & let the trajectory to pull them down to the atmosphere.

They showed 4 streams at once during some of the reentry. One view of each control surface. They may have had still more views but just that 4 was a first.

Imagine, linking that video into a 360° virtual cockpit As if you were inside a booster made from glass.

Wait, that was 5 views no? Facing four fins, plus one fin from the front.

I'm wondering if they'll be using the vertical equivalent of arresting gear on aircraft carriers[0]. See when fighter jets land on aircraft carriers? There's a cable that decelerates them. That, but for vertical landing.

The way these chopsticks are set forces the booster into a dangerous, snake like maneuver (a SnakeX maneuver) at the last second from a vertical setting, to get into the chopsticks. This maneuver is due to the fact chopsticks are short and the booster has to land on one point in space. No degrees of freedom.

Now, imagine if the chopsticks were long. The booster wouldn't have to land at one specific point, but it could now land on a line. One degree of freedom.

Now replace the long chopsticks with cables, and then add another pair of cables perpendicular to them. So you have a pair of parallel cables perpendicular to a second pair of parallel cables. Now the booster doesn't have to land on a line, but can land anywhere in the grid that's covered by the cables. Two degrees of freedom.

Pushing this thought leads to having a sort of iris diaphragm, like the ones in optics, but an iris diaphragm of cables. The diaphragm is open when the booster is about to land, then closes in quickly. Imagine this[1], but it's cables cinching in.

Now, it's a diaphragm of cables, not a diaphragm of rigid beams, so I imagine the deceleration to be even smoother as the cables elongate, and an additional system of springs and dampers to counter the weight of the booster.

The booster is vertical and stays vertical. Granted, an unstable equilibrium, but it beats doing the SnakeX maneuver to get to the chopsticks, and that's another story.

Now, imagine the iris cable diaphragm can move up and down like equipment handling containers, and now you have three degrees of freedom. That's less control to worry about on the booster's side at the worst possible moment, landing, where you can't make adjustments anymore.

This not only means being more forgiving on mistakes related to position, but also on speed and angle. The diaphragm catches the booster at any angle, and given that it cinches way above the center of gravity, the booster goes back to a more stable vertical position.

For the fire, maybe you just need a big hole down there.

- [0]: https://en.wikipedia.org/wiki/Arresting_gear

- [1]: https://en.wikipedia.org/wiki/File:Iris_Diaphragm.gif

SpaceX likes to simplify and not to complexify things. And they demonstrated they can do landing accurately.

Moreover I think Elon discusses the mechazilla arms in one of Tim Dodd interviews. They dont want longer arms, ratger shoeter.

Think about physics involved for longer arms and how much more stress you will putt on the connection points.

>SpaceX likes to simplify and not to complexify things. And they demonstrated they can do landing accurately.

Yes. Something falling into a web of cables. I'd say it may be simpler than optimally controlling that last SnakeX maneuver to be hugged by a short-armed T-Rex.

>Think about physics involved for longer arms and how much more stress you will putt on the connection points.

That's why I wasn't talking about arms, but cables, as explained by most of the reply.

Good luck not ruining rocket with that cable web. Now you got other problems to target into some cell. Moreover cables could be lingering depending on thrust. Or maybe thrust will just cut em easily.

>Good luck not ruining rocket with that cable web.

What do you mean?

>Now you got other problems to target into some cell. Moreover cables could be lingering depending on thrust. Or maybe thrust will just cut em easily.

It's an iris diaphragm that's open but cinches on the booster.

Did you think about how you support the cables ? Surely they must be tensionned or else they will hang. How do you tension them ? Of course you know that the more the cable is tensionned, the more force you need. So you need a big-ass structure to hold all of these cables. They must be able to circulate around the perimeter, while being in tension, while not collapsing the structure that holds them, etc..

I'm sure you think it's easy, but I'm sure some people thought a bit more

>Did you think about how you support the cables ? Surely they must be tensionned or else they will hang. How do you tension them ?

Mechanical advantage. The booster weighs 250 tons. A 40' shipping container has a max payload of about 30 tons. There are cranes that can lift 250 tons, and it won't be one, but many. Have you seen gantry cranes?

>So you need a big-ass structure to hold all of these cables.

Similar to the big-ass structure holding the 250 ton booster with the T-Rex arms?

>They must be able to circulate around the perimeter, while being in tension, while not collapsing the structure that holds them, etc..

Not circulate, but translate. The cinching in is a result of them translating. Again, see cranes and gantry cranes. Or, just see the actual chopsticks: circulating, not collapsing the structure that holds them, while being in tension, holding the booster at the free end.

Are you seeing the shear and moment diagram of that cantilever beam with point load? (I know, it's an extension of a supported beam, but still cantilever).

>I'm sure you think it's easy

I'm not sure I think it's easy, I can't see how you're more sure than I about my own thoughts.

>but I'm sure some people thought a bit more

You are people, too. Nothing prevents you from thinking as well, if for nothing than to have a civil conversation on a forum.

Now, that's all fun. Imagine they keep it the way it is, but they duplicate the setting to make a circle, so the booster lands in the middle of many chopsticks... What do you think about that?

> so the booster lands in the middle of many chopsticks...

And how exactly this requires less precision? I see multiple issues:

- No way to escape/last second abort away from tower once you dipped into that net.

- The booster arms must be longer/heavier, the tower support structure need to support more weight.

- Cables have a lot less thermal mass than the tower/arms - if the torch coming out of raptor engines will touch the cable, it may either cut or soften the cables and they will behave in a different way.

I mean we could have had this discussion previously but now they demonstrated on the first try that they can catch the booster... why bother?

>I mean we could have had this discussion previously but now they demonstrated on the first try that they can catch the booster... why bother?

Because these things must work every time, not just the first time. Because why not talk about it, it's an interesting topic and musing about it is amusing. Because it's not a bother. I'm surprised one needs a reason, but here are three already.

The crane lifts on the same axis as gravity, so the force is the same as the object. If the cable is horizontal-ish the the force is X/cos(a), which can be many times higher than the object

https://en.wikipedia.org/wiki/Mechanical_advantage

https://en.wikipedia.org/wiki/Crane_(machine)

https://archive.org/details/constructioncra00markgoog

This is a solution for a problem that's already solved, that is, booster maneuverability and accuracy. But they've just demonstrated that the booster is accurate and controlled enough to land on the chopsticks, and they have over a decade of experience in making rockets land accurately in a specific zone.

Next year, China says it will be testing a concept very similar to what you've described, with its Long March 10A rocket:

https://www.youtube.com/watch?v=27TvGDpPLNw

It will be interesting to see how these two approaches fare vs. one another.

That's exactly it, thank you! What worries me with the current one that landed is the gradient: because the arms are short and the spot is tight, the burden is on the booster to make very sharp corrections (especially on pitch) to get caught when there's literally a few meters left, with an increased risk at the worst time (no altitude, and structures around), as opposed to leaving the pitch as is and landing vertically. In other words: the possibility to screw a perfect launch by introducing irremediable risk in the last few seconds and meters.

Then again, I just watched the launch and that was the first thing that popped into my mind, and the first "design" that popped into my mind as I was replying to the thread, so not much thought went into it.

One thing to note is that the "Snake" maneuver was designed to keep the tower safe during the test, and not forced on the rocket by the chopsticks.

The rocket was set to come down on the pad just in front on the tower and launch mount until the final 3-engine landing burn started. This kept the infrastructure safe until the last moment, but also required that lateral translation you referred to.

That's how the Chinese are planning to do it:

https://youtu.be/OYWBmu6H0ik?feature=shared&t=35

What’s the advantage of catching the booster over landing it in the ground? Catching it seems like an extra complication.

You save the mass of the landing systems, you get to have all that mass on the ground and not have to lift it into space. Dramatically improves the performance of the rocket.

I can speculate ...

I think the primary reason would be that landing legs are heavy, and it wastes performance to carry them. If your landing mechanism is mostly on the ground, you get that performance back.

Secondary reasons might include that it's simpler to get the booster right back to the pad. Once things have settled into an operational cadence, it's likely feasible to lower and lock the booster onto the stool, stack a ship on top, refuel, and relaunch -- no more messing around with barges, transport, weather issues, etc.

To my knowledge, you're right, but in reverse order. I believe the driving force is time, rather than mass overhead, but certainly both play a large part.

Your primary reason matches what SpaceX themselves have said. You either need to be strong enough to handle the shock of inpact, or need to spread the impact over time. Building either into the rocket adds a lot of mass.

Landing on a device that spreads out the shock moves that weight to the static landing platform.

It's worth noting that at least 1 Falcon 9 core probably got trashed not from the landing, but from rough seas - so cutting out the ground logistics chain adds resiliency.

Falcon 9 can land on land just fine (and it was done multiple times on less demanding missions). Landing on barge gives it extra performance.

The rocket equation implies that if you want to maximize the delta-v a rocket gets out of a certain amount of fuel, then you should get the dry mass as close to zero as possible. Eliminating landing legs helps a lot.

Why send the landing mechanism to space when it isn't needed there? Whatever kit you put on a rocket has to be brutally miniaturized to limit how much you eat into the payload mass. Also has to be rugged enough to withstand tremendous vibrations and thermal stresses. That adds cost and more points of failure. You want to move as much of the complexity off the rocket as possible. Then doesn't matter if the catching mechanism on the launch tower is big and heavy.

I thought it was so they can name make the booster body thinner relying more on the fuel for structural rigidity.

1) legs are heavy 2) empty rockets are stronger in tension than compression 3) the booster is large enough to make (1) and (2) matter more than they did for Falcon 9.

The reflected sound of the engines is enough to destroy the engines, ironically. That's also why the launch mount is so high. You'd need truly enormous legs, which wouldn't work for weight.

The load for landing and almost empty booster would be less, but otherwise yes, it would be much more than the single Merlin engine on the Falcon 9, with all associated issues (local scorching/spalling of the pad, acoustic issues, extra weight, longer turnaround, etc.).

>The 'chopsticks catch' was amazing to watch. Seems like it adds a lot of risk and clearly the booster needs additional fire suppression systems :-) perhaps the tower could mount something that sprays the booster like the barges have for the F9 boosters.

There's no reason to reinvent the airport firetruck.

The airport fire truck can’t control fires 250ft in the air

It's pushing the limits but it's close enough to existing capability to be a pretty easy ask.

Unless you're gonna make it fully automated, it's not gonna work here as it can't be within kilometers of the landing site during landing in case there's a catastrophic failure.

> no reason to reinvent the airport firetruck.

There is. The booster is high above, and is larger than basically any aircraft. There's no flat concrete airfield around either.

I would suggest blowing CO2 or nitrogen through pipes positioned at the right height on the tower.

It’s not going to burn like that in production.

Why would you use a firetruck if you could also run a hose up the tower and get it right where it's needed?

>The heatshield held out for a much longer time, the asymmetric heating on the flaps was interesting. I had guessed that all four flaps would have equivalent heating based on an approach that was basically that side of the rocket perpendicular to the flow but it seems like that isn't the case. Still it seems like they are close to having something workable here.

Heat shielding didn't look relevant at this section of the flight at all. The booster didn't have any shielding.

They're talking here about Starship, not the booster.

How does this logically make sense? They said "it looks like the heat shield held out".

Where was a heat shield holding out?

On starship. Believe it or not, Chuck’s comment was about both booster and starship

And I’m saying he’s wrong no heat shield held out to anything.

The booster never travelled fast enough to need heat shielding ever. Only the starship needs it.

If chuck is talking about both likely he doesn’t know what he’s talking about.

Dude you are having trouble reading. His comment first talked about booster and then talked about starship. It’s not that difficult to follow.

The booster actually does have heat shielding behind the engine bells, to protect against aerodynamic heating on the return. In some of today's footage you can see it glowing yellow.

However, Raptor 3 is supposed to obviate the need for this shielding.

Where did it glow yellow? What time in the video?

The booster doesn’t travel fast enough to need heat shielding ever.

It makes zero sense to use heat shielding behind the engine bells. Why? Think about it. If the booster renters the atmosphere engine bells first the engine bells would burn up BEFORE the air even touches the heat shield. If there are actually tiles there it’s just there to protect the booster from the heat of the exhaust.

> However, Raptor 3 is supposed to obviate the need for this shielding.

Not completely getting this. So there’s a case for the booster entering the atmosphere at orbital velocity engine bells first? I would think if they did this then THEY want the booster to burn up. Anything entering the atmosphere at that velocity needs to be made aerodynamically stable as it will be traveling faster then the speed of sound. This is what causes the heat. If you send something at the speed of sound engine bells first that’s not stable and is unlikely they will do that at all, with or without heat shielding.

I don’t think parent knows what he’s talking about.

Here's a timestamped link to Scott Manley's commentary video where he points out that the yellow glow at the bottom of the booster is the heat shielding behind the engines.

https://youtu.be/Ysx4t7ICO58?t=550

You ought to do some basic research before making a post like this. Honestly might be the most baffling comment I've ever seen on HN---what kind of mindset does it take to have this kind of overriding confidence in one's own lay speculation?

The engine bells are made of different material than what's behind them (a material that has to directly withstand hot exhaust) and the aerodynamics of the ass end of the rocket are complex; some combination of these factors means this heating isn't an issue for the bells but is a concern for what's behind them. The booster does not re-enter from orbital velocity, but does come down engines first at supersonic speed. Stability in this orientation throughout the descent is a problem SpaceX solved with Falcon 9, and SH works the same way.

See the other reply to your comment for timestamped video of aerodynamic heating behind the engine bells.

Is it possible you're confusing the booster with the second stage (Starship)? They are not the same thing.

>You ought to do some basic research before making a post like this. Honestly might be the most baffling comment I've ever seen on HN---what kind of mindset does it take to have this kind of overriding confidence in one's own lay speculation?

Don't appreciate this at all. I can be wrong, but there's no need to make personal comments on my mindset.

I agree that HN etiquette is generally more polite. However, this was in direct response to your comment to him/her:

> I don’t think parent knows what he’s talking about.

We can all do better.

that's different. Not knowing what he's talking about is different from a comment about character. But no point in arguing about this.

> Where did it glow yellow? What time in the video?

This is a better view than the sibling comment linked. It's a greater close-up and you can clearly see the yellow glow behind the engine bells. This view is from Cosmic Perspective, a partner of Everyday Astronaut, whose video is linked:

https://www.youtube.com/live/pIKI7y3DTXk?si=LI4-xQ7UhnvITTiG...

In case you get confused due to lack of context, the booster shot is a replay. When Tim goes to split-screen view, the right-side image is a live view of the second stage ("Starship") as it re-enters from orbital speed. It is not a different angle on the booster that is shown on the left.

Later commentary explains that the heating behind the engine bells is due to atmospheric compression and SpaceX specifically orients the drop of the booster to focus heating in this spot.

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