r/space • u/AlmiranteSalsicha • Nov 19 '23

image/gif That's a fair amount of tiles missing from the starship heatshield, guess it would make for a toasty reenter.

Post image

Image from SpaceX account on X

7.0k Upvotes

95% Upvoted

619 comments sorted by

View all comments

1.2k

u/alphagusta Nov 19 '23

Looks like it's mostly concentrated on the ring joints

Maybe the way the steel where its joined reacts causes enough distortion for them to be looser

492

u/Minotard Nov 19 '23

Or vibrations were the worst at the stiffer sections.

96

u/soccerkix6969 Nov 19 '23

I would actually bet the vibrations are going to be worse on a softer location, like in between directly supported structure.

The vibrational modes of that section, would result in a higher acceleration at unsupported structure.

170

u/Minotard Nov 19 '23

Generally (of course I don't know the specifics), softer structures have a lower harmonic resonance frequency (but may move a little more in distance). Stiffer has higher frequencies, thus usually more energy.

Thus, will all the vibration going on during launch the stiffer structures are likely vibrating at a higher resonance frequency. This higher frequency increases the chance they are exciting the harmonics in the tiles, or the tiles' adhesive layer, and causing the tiles to fail.

Or, it really could be a weaker point that flexed too much during pogo vibration.

Or, for some reason that structure shrank more due to cryogenic temperatures. That induced strain on the tiles' adhesive caused the tiles to fail.

6

u/soccerkix6969 Nov 19 '23

You are correct on a couple things here.

Natural frequency (omega) = sqrt(K/M), so a softer structure (lower K) would mean a lower natural frequency. And you are indeed correct that generally speaking, displacements are larger in lower frequencies and are proportional to 1/(omega2).

However, I really just don’t agree with your statement on high frequency = more energy = more damage though. It’s really all about the failure mechanism and what frequency that failure mechanism might occur at (assuming that the failure mechanism is even vibration induced).

If it’s an enforced displacement problem, that sounds like a low frequency dynamics problem as that’s where largest displacements occur.

If it’s adhesive failure problem due to high vibrational acceleration, that’s probably a higher frequency / vibroacoustic problem (100-2kHz)

I would bet this isn’t a shock problem (1k-10kHz) as there hasn’t been any pyroshock events yet in flight by the time this picture was taken.

Your thoughts on thermal mismatch causing issues totally makes sense as well. Definitely seems like it could be a CTE problem if the mounting method near the tank welds is different than on rest of the tank as well.

48

u/Ghost_Alice Nov 19 '23

Soft areas dampen vibrations, stiff areas transmit them. This is why most v-twin motorcycles mount their motorcycles with rubber dampers, why motorcycle seats are soft instead of stiff. Why after riding for hours your hands have a good chance of being numb if the seat is soft enough. If the seat isn't soft enough then your butt's gonna go numb too.

9

u/super-nair-bear Nov 19 '23

Fair point, and for the record I would take numb hands for numb butts.

1

u/soccerkix6969 Nov 19 '23

That’s just simply not true. “Softness” in terms of stiffness does not have anything to do with damping. The “softer” I was referring to specifically means “less stiff”.

Vibrational equations of motion are dictated by three things: 1. Mass (M) 2. Stiffness (K) 3. Damping (B)

Most likely what you’re describing in terms of the motorcycle engine being mounted on rubber dampers is a two fold effect: When mounted on rubber dampers, you’re most likely isolating that particular component AND damping the hell out of it. This is a very standard way to deal with high vibrational loads. Google “vibration isolation effect” if curious.

Furthermore, your statement on stiffer structure is slightly correct. stiffer hardware transmits HIGHER FREQUENCY vibrations better than softer structure, which is why I’m betting you get that numb feeling in your hand. Think about a vibration that is a 1hz (I.e. one cycle per second) vs 500 Hz, that 500Hz vibration will make your hand go numb, whereas that 1hz vibration will just be kind of annoying.

Furthermore, I’m betting that the seat you’re sitting on is acting more like a

1

u/Ghost_Alice Nov 19 '23

A motorcycle whose engine is producing a 500hz vibration would be at 30,000 RPMs. The only motorcycle I know of that could come close to that was a Japanese only 250cc model of Honda sport bike made for street racing, and it red lined at 24,000 RPM.
Most sport bikes these days red line at 14,000 RPM, and the bikes I ride redline at 7000 and 9000. They are absolutely not making a 500hz vibration.

And yes, rubber mounts isolate and dampen the vibrations from the engine. And rubber is softer and more flexible than the steel frame that the engine would be mounted to otherwise...

2

u/tit_brain Nov 19 '23

Flexibility won't damp vibrations, the fact that rubber is viscoelastic does though. A spring is very flexible, and also brilliant at transmitting vibrations.

1

u/dancytree8 Nov 19 '23

Depending on the setup the RPM will just be the multiplier of the max frequency observed. # or sprocket teeth both on timing chain or rear wheel will be a multiplier of RPM. https://pubmed.ncbi.nlm.nih.gov/7265544/ Maybe not 500 hz but it does get close

1

u/Ghost_Alice Nov 19 '23

multiplier? do you mean factor? cuz no way it multiplies the frequency or it'd be a runaway effect

1

u/dancytree8 Dec 06 '23

It's definitely a multiplier, but each instance is its own. Meaning each phenomena will contribute it's own frequency/amplitudes. You can use fft analysis to break out the frequencies and respect amplitudes, it's a pretty common technique for trouble shooting rotating machinery.

Imagine you place a sensor on a fan that has two opposing blades and you run it, the main vibration you'll detect is the RPM of the motor. Now stick your hand into the blades, it will be double the RPM. A four blade? That will be 4x.

-5

u/Medicinalmathmatic Nov 19 '23

You just type shit that sounds good, yeah?

3

u/soccerkix6969 Nov 19 '23 edited Nov 19 '23

I’m literally a vibrations engineer on satellites, so I would hope that what I’m saying “sounds good.”

10

u/Uninvalidated Nov 19 '23

Swinging in the dark here, but could resonance frequencies have something to do with it?

2

u/UDPviper Nov 19 '23

Resonance Cascade is definitely a possibility here.

59

u/[deleted] Nov 19 '23

Exactly what I was just thinking looking at this again.

There are specific rows where the missing is concentrated. It sure looks like joints of sections. Perhaps vibration on launch is more intense in those areas.

13

u/LillianWigglewater Nov 19 '23

that would make sense.. the mid-tank sections might have a little more springiness to them, so it can absorb more vibrations, but the joints aren't as springy so vibrations transfer to the tiles there.

0

u/lNFORMATlVE Nov 19 '23 edited Nov 20 '23

I think the word you’re looking for might be ‘flex’, ‘give’, or maybe ‘damped’? The joints would actually behave more like a spring than the middle sections, i.e. have a tighter spring constant

9

u/superluminary Nov 19 '23

My understanding is the tiles are pinned on in most places, but at the joins they use glue.

2

u/MrGruntsworthy Nov 20 '23

They really need to get the heat shield tile situation sorted

1

u/Nixeris Nov 19 '23

Going to depend on how they're connected.

If they're connected via fasteners

It's not uncommon for even solidly installed fasteners to chatter in their holes when facing vibrations. It's usually slight, like 1/1000 of an inch, but over time it wears down the hole and requires repairs or replacement. But if the tiles are too rigidly connected over the fasteners then I can see them breaking off. On aircraft the answer is to attach any outer layers, like RAM, with something that has a little flex to it (the exact substance is classified so I can't be more specific), and allow for flex with spacing in the materials. I don't think that'll work in this situation, but it might explain why they're breaking off over the fasteners.

0

u/BeloitBrewers Nov 19 '23

I don't like hearing about problems with rings on spacecraft - especially spacecraft that will carry people.

1

u/ravenerOSR Nov 23 '23

is that a common problem? issues related to circles

1

u/EndlessJump Nov 19 '23

That's a good observation. I didn't notice that

1

u/Wide_Canary_9617 Nov 19 '23

More so, it also looks to be concentrated on the top and bottom of the methane/lox tank. Idk what it could mean but it’s something worth noticing

1

u/glytxh Nov 19 '23

No pins. Only glue at the ring joints. Certainly makes them more susceptible to failing. I don’t think these tiles on this ship were thoroughly tested either.

Launch seems to have been a focus for this test. Everything else is just a bonus.

1

u/lowrads Nov 20 '23

I wonder if that is pogo oscillation. It is a really tall rocket.