Author Topic: Reusability of BFS Heat Shield  (Read 11614 times)

Offline envy887

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Re: Reusability of BFS Heat Shield
« Reply #40 on: 05/29/2018 01:42 PM »
Pica-X comes in tiles. I believe this is one major reason why Pica was was not selected for the Orion heat shield. The tiles are much larger though and should have less issues dropping off. Also with a mostly symmetric shape the number of different tiles will be much smaller than with the Shuttle.
The Orion heatshield is now made of avcoat tiles after the monolithic shield had issues on EFT-1. IIRC PICA was not chosen due to issues to thermal cycling during a longer mission, but PICA-X is reformulated to avoid this problem.

Offline Cheapchips

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Re: Reusability of BFS Heat Shield
« Reply #41 on: 05/29/2018 02:23 PM »
I'm curious as to how big the BRFs tiles will be.  I'm sure I read up to 1m2 for PICA-X tiles? 

(and at what size does a tile become a segment?)

Offline john smith 19

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Re: Reusability of BFS Heat Shield
« Reply #42 on: 05/29/2018 10:25 PM »
Ok, in terms of BFS, I agree, there will likely be a large number of identical tiles. The issue with the shuttle tiles was that they had to be completely replaced for each landing.
Incorrect. Roughly 10% of the tiles needed replacing per flight. OTOH there were about 24 000 of them (in total) and NASA were quite proud of the fact nearly everyone was unique.  I doubt the (theoretical) improvement in performance was worth the massive PITA in mfg complexity.

Carnegie Mellon had a plan to build a robot with heads that could scan the vehicle, identify damaged tiles and another head that could replace them. NASA did not proceed, citing worries about how the vehicle would crash into the orbiter and damage it before it could be stopped.
Of course it would have resulted in substantial job losses, and reduction in Shuttle support costs.
I strongly doubt SX will be so worried. I think BFS also has substantially fewer tile shapes, as it's much more cylindrical than Shuttle. But both have complex nose and wing areas.
Quote from: Semmel
And on top of that, the tiling was impossible to automate. Each tile had to be placed and each gap had to be filled... by hand. This was one of the main reasons shuttle got expensive. For BFS, if they have chance at all, the tiles have to stay on the vehicle for many launches.
It was a management decision not to automate tile inspection/repair. The inspection technology existed. The replacement technology would be made to work.

People seriously underestimate the precision modern robot "end effectors" are capable of, such as the placing of an artificial cherry on top of an individual cake (without indenting the cake or damaging the cherry).

Quote from: Semmel
This reduces the workload compared to shuttle tremendously independent on the shape of the tiles. Then, if they can do the tiling automatically, this becomes feasible. A robot will not care if the tiles are all identical, the manufacturing tolerances are much tighter though. The actual shape of the tiles is less an issue I would imagine, they just need to have a mold for each shape. Identical tiles help but I would think thats not the deciding factor.
I think Lockheed reckoned it was better to make a standard billet, cut it into blocks and machine them to shape than cost them to shape. In fact IIRC Boeing took out a patent to "slip cast" such blocks to a more near-net-shape than previously.

Another technique that was looked was the use of modified passive RFID chips (quite like the ones for licensing pets). It was possible to create a plated link that would melt to give an open circuit at the tiles do not exceed temperature. Running a suitable RFID reader over the inter tile gaps (where they could be inserted) would let the repair team quickly identify tiles needing replacement regardless of surface damage.

Potentially the real game changer was invented by NASA in the early 80's, when they found a way to mechanically fasten (IE Bolt) a brittle, low (and ansiotropic) TCE material to a ductile high TCE material (like Aluminum alloy) without inducing severe stress due to a temperature rise.

This technology is now long out of patent, meaning that in principal tiles could (safely) be much larger. Shuttle tiles were mostly 6"x6"

But if you can move to blankets. They were 1/4 the cost of the tiles per m^2.
« Last Edit: 05/30/2018 08:43 AM by john smith 19 »
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Online Elmar Moelzer

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Re: Reusability of BFS Heat Shield
« Reply #43 on: 05/29/2018 11:36 PM »
I'm curious as to how big the BRFs tiles will be.  I'm sure I read up to 1m2 for PICA-X tiles? 

(and at what size does a tile become a segment?)
I am pretty sure that those PICA- X tiles can be almost(!) any size and shape. I assume that they will size and shape them to separate areas with the most and the least (expected) wear. So that tiles with the most wear can be thicker and be replaced more often while tiles that have little wear can be thinner and will probably be rarely replaced.

Offline edzieba

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Re: Reusability of BFS Heat Shield
« Reply #44 on: 05/30/2018 09:24 AM »
People seriously underestimate the precision modern robot "end effectors" are capable of, such as the placing of an artificial cherry on top of an individual cake (without indenting the cake or damaging the cherry).
Precision and handling dexterity is not the problem, it's control and sensing (both optical and kinaesthetic/proprioceptive) that are the tricky bits. That's why robots that can casually wave about several tons of metal and weld them together with sub-mm accuracy were in operational use over half a century ago, but a robot that can pick a strawberry without damage is still cutting edge technology.
The orbiter tiles were individually unique, fragile, and fitted to a variable substrate (felt and caulking) and required tens of hours of fixation for the adhesive to cure. Correct manual fitting required careful placement and direct moderation of applied force. Even assuming a manual pass by a human to prepare the tile location to a known state (rather defeating the purpose) fitting the tile would have required a new motion set to have been recorded for each tile position, using the automation technology (open-loop control using pre-set motions) available at the time.

Offline Semmel

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Re: Reusability of BFS Heat Shield
« Reply #45 on: 05/30/2018 10:34 AM »
[snip] [talk on Shuttle tiles] [snip]

Thanks for clearing this up, I obviously have some lack of knowledge on Shuttle refurbishment and its history. Anyway, back on topic. :)

Offline biosehnsucht

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Re: Reusability of BFS Heat Shield
« Reply #46 on: 05/31/2018 04:54 AM »
Clearly, it was not doable during development of the shuttle, and by the time it was (if it was) during it's lifetime, it wasn't necessarily worth the investment for the remaining flights.

Between improvements to heatshield design itself, and robotics, software (instead of manually training each motion for each tile, can likely let software figure that out on it's own by just giving it the right information on where everything is and needs to be etc), etc ... probably entirely feasible with modern technology to have robots handle any necessary refurbishing.

Though I'm sure the idea is that the BFS won't need it in any regular basis and probably will have much larger segments...  or perhaps smaller segments but removable in bulk by removing what they're attached to, allowing for offline refurb and just installing a whole super-segment at once?

Offline guckyfan

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Re: Reusability of BFS Heat Shield
« Reply #47 on: 05/31/2018 05:51 AM »
or perhaps smaller segments but removable in bulk by removing what they're attached to

That would be the BFS airframe. I hope it will not be necessary to remove that.

Edit: Elon mentioned the tiles are directly attached to the frame.
« Last Edit: 05/31/2018 05:52 AM by guckyfan »

Offline john smith 19

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Re: Reusability of BFS Heat Shield
« Reply #48 on: 05/31/2018 07:58 AM »
Clearly, it was not doable during development of the shuttle, and by the time it was (if it was) during it's lifetime, it wasn't necessarily worth the investment for the remaining flights.
There were 100s (if not 1000s) of jobs involved in tile maintenance. It was a significant cost item.
Quote from: biosehnsucht
probably entirely feasible with modern technology to have robots handle any necessary refurbishing.
Some off the issues with Tesla suggest a mix of human and automation is the best way to handle complex tasks. But it depends on the end effector. The "big hardware" example was car windshields. Heavy,fragile, and more or less transparent. Car assembly line robots lacked the precision to place them, so they got them approximately into position before the "hand" did fine X,Y positioning before dropping it onto the frame for applying mastic (a process not dissimilar to fitting a tile).
Quote from: biosehnsucht
Though I'm sure the idea is that the BFS won't need it in any regular basis and probably will have much larger segments...  or perhaps smaller segments but removable in bulk by removing what they're attached to, allowing for offline refurb and just installing a whole super-segment at once?
Possibly. I don't think SX have given enough detail on the TPS to really say. That there hiring people with ceramic tile experience suggests this is still under discussion.
or perhaps smaller segments but removable in bulk by removing what they're attached to

That would be the BFS airframe. I hope it will not be necessary to remove that.

Edit: Elon mentioned the tiles are directly attached to the frame.
At present. That could change.
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Offline MikeAtkinson

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Re: Reusability of BFS Heat Shield
« Reply #49 on: 05/31/2018 10:16 AM »
Do we have a primary source that the heat shield will be PICA-X?

The nearest I can find is from the AMA

Quote
Question:
Will the BFS heat shield be mounted on the skin, or embedded?

Will the BFS PICA-X heat shield be mounted on top of a common, single piece of 9m diameter cylindrical carbon-fiber outer tank skin additively, or will it be an integrated part of the outer BFS skin?

Answer: ElonMusk
The heat shield plates will be mounted directly to the primary tank wall. That's the most mass efficient way to go. Don't want to build a box in box.

That is not a direct confirmation of a PICA-X heat shield, for instance Elon Musk may not want to correct a wrong assumption for competitive reasons. [His use of "plates" is a bit strange for PICA-X, tiles, panels or segments would in my opinion be more likely for PICA-X.]

Offline Cheapchips

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Re: Reusability of BFS Heat Shield
« Reply #50 on: 05/31/2018 10:40 AM »
The only other bit he'd said was at IAC 2016, which again isn't a confirmation, just a suggestion they'd evolve what they have:

Quote
We've been refining the heat-shield technology using our Dragon spacecraft, and we're now on version three of PICA, which is 'phenolic impregnated carbon ablator, ' and it's getting more robust with each new version, with less ablation, more resistance, less need for refurbishment.

The heat shield's basically a giant brake pad. So it's like, how good can you make that brake pad against extreme reentry conditions, and minimize the cost of refurbishment. And make it so that you could have many flights with no refurbishment at all.

Online eriblo

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Re: Reusability of BFS Heat Shield
« Reply #51 on: 05/31/2018 12:21 PM »
... and iterated at IAC 2017:

Quote from: Elon
So you come in, you're entering very quickly, going seven and a half kilometers a second. For Mars, there will be some ablation of the heat shield. So it's just like a sort of brake pad wearing away. It is a multi-use heat shield, but unlike for Earth operations, it's coming in hot enough that you really will see some wear of the heat shield.

This does certainly not rule out any work on ceramic (non-ablative?) TPS for Earth use but I'd say that all hints are that Pica-X is the current baseline.

Offline MikeAtkinson

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Re: Reusability of BFS Heat Shield
« Reply #52 on: 05/31/2018 12:26 PM »
Thanks, I was hoping for some more definitive reference (edit: to PICA-X)  I had missed.
« Last Edit: 05/31/2018 12:26 PM by MikeAtkinson »

Offline speedevil

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Re: Reusability of BFS Heat Shield
« Reply #53 on: 05/31/2018 01:26 PM »
... and iterated at IAC 2017:

Quote from: Elon
So you come in, you're entering very quickly, going seven and a half kilometers a second. For Mars, there will be some ablation of the heat shield. So it's just like a sort of brake pad wearing away. It is a multi-use heat shi

I note that 'seven and a half kilometers a second' doesn't quite match up with the rest of the quote, orbital mechanics and the supposed transit time, if you read it as 'aerobraking into Martian atmosphere'.

Hundred day or so transits seem to end up more like 15km/s than 7.5.
To get to 7.5km/s entry, you need to slow down to 200 days or so.
But if you're doing that, you end up with a payload in Martian capture orbit of some 400 tons, as you're needing about 3.5, not about 5km/s from LEO. (and can aerobrake with 'no' heating over the next few weeks into LMO).

He may have been instead meaning the entry to earth, when 'doing Mars' - as part of the Earth-Mars-Earth service wears the heatshield.


Online eriblo

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Re: Reusability of BFS Heat Shield
« Reply #54 on: 05/31/2018 02:50 PM »
[...]

I note that 'seven and a half kilometers a second' doesn't quite match up with the rest of the quote, orbital mechanics and the supposed transit time, if you read it as 'aerobraking into Martian atmosphere'.

Hundred day or so transits seem to end up more like 15km/s than 7.5.
To get to 7.5km/s entry, you need to slow down to 200 days or so.
But if you're doing that, you end up with a payload in Martian capture orbit of some 400 tons, as you're needing about 3.5, not about 5km/s from LEO. (and can aerobrake with 'no' heating over the next few weeks into LMO).

He may have been instead meaning the entry to earth, when 'doing Mars' - as part of the Earth-Mars-Earth service wears the heatshield.
Yes, I've noticed that the numbers don't add up for fast transits, my guess is that the example is an early slower transit. But it is certainly an example Mars entry as seen in the presentation (BTW, how do you get an 7.5 km/s entry on return to Earth?  ??? ). Also note the "Leverages ablative heat shield materials developed for Dragon vehicles." which is another suggestive but inconclusive quote regarding PICA-X on BFS  :P

Offline speedevil

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Re: Reusability of BFS Heat Shield
« Reply #55 on: 05/31/2018 04:04 PM »
Yes, I've noticed that the numbers don't add up for fast transits, my guess is that the example is an early slower transit. But it is certainly an example Mars entry as seen in the presentation (BTW, how do you get an 7.5 km/s entry on return to Earth?  ??? )

To clarify - 15km/s or so for somewhat fast entry into Mars, or 15km/s for pretty slow to Earth.
You're not going to get to 7.5km/s entry to Earth, but if you read the comment on wear as an aside explaining the wear on the whole system when going E-M-E, then it works.

(well, at least in one pass, in principle you can do most of a hohmann transfer ending up in capture orbit, and gradually brake down - at least for cargo not crew)

Offline john smith 19

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Re: Reusability of BFS Heat Shield
« Reply #56 on: 05/31/2018 06:21 PM »
To clarify - 15km/s or so for somewhat fast entry into Mars, or 15km/s for pretty slow to Earth.
You're not going to get to 7.5km/s entry to Earth, but if you read the comment on wear as an aside explaining the wear on the whole system when going E-M-E, then it works.

(well, at least in one pass, in principle you can do most of a hohmann transfer ending up in capture orbit, and gradually brake down - at least for cargo not crew)
Didn't either the 2016 or 2017 presentation have a slide with a spread of transit time running from (IIRC) about 110 days to about 150? Not sure about 200+ though.
Orbital entry from LEO is more like 7800m/s, lunar about 1.41x that.

Obviously the most data they have on TPS ablation is from the ISS orbit at about this entry velocity.

This is another of those trade off situations. At this entry speed they have known values of ablation, so go with that or go with shorter transit times and a higher ablation rate. Obviously that will have an impact on TPS reuse.

It's pretty clear that aerocapture is the big technology if you want to shorten transit times.

BTW one technology that's does not seem to have been really talked about is flexible SIRCA and PICA (or XIRCA and PICAX for SX versions) variants.
This variant has the potential for reducing TPS installation to (essentially) laying down TPS using something that looks like a large tape gun, with openings accessed by cutting round them with a sharp blade.
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Offline RDMM2081

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Re: Reusability of BFS Heat Shield
« Reply #57 on: 06/01/2018 05:05 PM »
There must be a better resource for information on SIRCA than:  https://en.wikipedia.org/wiki/Atmospheric_entry#SIRCA

Mainly I am interested in what the downsides are for a SpaceX application.  I can understand that it probably doesn't have much flight heritage (Why bother using a re-usable(it's what the R in SIRCA stands for!) heat shield material when there haven't been "production" re-usable rockets/stages/spacecraft up until now) and presumably is more expensive than PICA.  Is it more or less brittle than PICA/PICA-X?  Is there any resource about these hypothetical(?) flexible variants?  Surely there must be some trade offs.

ISTM that SpaceX reusability goal with BFS is to build on the promise of block 5: multiple flights with minimal refurbishment.  So whatever TPS they end up with for BFS should be resilient enough to withstand several flights without refurbishment (automated inspection is likely another story).  I can't see them being satisfied with a BFS solution that requires refurbishment between every flight, that would simply destroy their flight rate, and this thing needs a very high flight rate.

Offline rsdavis9

Re: Reusability of BFS Heat Shield
« Reply #58 on: 06/01/2018 09:21 PM »
There must be a better resource for information on SIRCA than:  https://en.wikipedia.org/wiki/Atmospheric_entry#SIRCA

Mainly I am interested in what the downsides are for a SpaceX application.  I can understand that it probably doesn't have much flight heritage (Why bother using a re-usable(it's what the R in SIRCA stands for!) heat shield material when there haven't been "production" re-usable rockets/stages/spacecraft up until now) and presumably is more expensive than PICA.  Is it more or less brittle than PICA/PICA-X?  Is there any resource about these hypothetical(?) flexible variants?  Surely there must be some trade offs.

ISTM that SpaceX reusability goal with BFS is to build on the promise of block 5: multiple flights with minimal refurbishment.  So whatever TPS they end up with for BFS should be resilient enough to withstand several flights without refurbishment (automated inspection is likely another story).  I can't see them being satisfied with a BFS solution that requires refurbishment between every flight, that would simply destroy their flight rate, and this thing needs a very high flight rate.

From the wiki:
1. Silicone
2. Ceramic
3. Machinable

I think a continuous blanket of some material would be ideal. No tiles and gluing.
For refurbishment grinding down and reapplying more would be great. 
With ELV best efficiency was the paradigm. The new paradigm is reusable, good enough, and commonality of design.
Same engines. Design once. Same vehicle. Design once. Reusable. Build once.

Offline speedevil

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Re: Reusability of BFS Heat Shield
« Reply #59 on: 06/03/2018 10:25 AM »
I can't see them being satisfied with a BFS solution that requires refurbishment between every flight, that would simply destroy their flight rate, and this thing needs a very high flight rate.
Crappy heat-shield affects nothing till you need to start large campaigns - probably considerably post 2026, or truly massive P2P.
It mildly increases launch costs.

This is because BFS with shielding that can manage very low heat flux repeatedly, and a hundred tons of fuel reserve for retroburn still leads to 50 tons a launch, and you don't care about this tripling of launch cost at $5M/launch.
Nor does it impact initial P2P trials where you're not trying to pack a thousand people onboard.

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