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Comment by rkagerer | Hacker News Reader

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.

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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/

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

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.

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

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

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