Author Topic: Reusability of BFS Heat Shield  (Read 17402 times)

Offline JonathanD

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Reusability of BFS Heat Shield
« on: 04/02/2018 03:02 pm »

Sorry if I missed a topic on this elsewhere, but it seems like the heat shield on BFS is sure going to take a beating, especially at interplanetary velocities and especially coming back from Moon/Mars and landing on Earth.  The heat shield on Dragon is relatively small, but still looks pretty complicated (https://www.nasa.gov/images/content/215574main_spacex_heatshield_226x170.jpg) in terms of # of tiles and fitment.  And we know from Shuttle the complicated nature of maintaining the integrity of a large thermal management system.  Even at LEO velocities it's no joke.  I'm curious how reusable this portion of BFS is realistically, and how that may impact the design architecture of using multiple tanker trips to orbit for refilling.  Seems like it would take extensive inspection at the least, if not frequent maintenance.  Are my concerns overstated?

Offline Elmar Moelzer

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Re: Reusability of BFS Heat Shield
« Reply #1 on: 04/02/2018 04:02 pm »
For LEO it should be fine, but for interplanetary velocities, it will need frequent refurbishment. I think Musk talked about 12 mars launches between major refurbishments. The tanker will have 100 reuses, since that only goes to LEO. So that should give you an estimate of the capability of the TPS.

Offline Alkan

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Re: Reusability of BFS Heat Shield
« Reply #2 on: 04/03/2018 08:53 am »
For LEO it should be fine, but for interplanetary velocities, it will need frequent refurbishment. I think Musk talked about 12 mars launches between major refurbishments. The tanker will have 100 reuses, since that only goes to LEO. So that should give you an estimate of the capability of the TPS.

I think you should try to find out where that is, because I feel like I've heard it all about ITS and BFR, scouring everything, and I never heard reuse numbers. I just heard roughly "there will be some ablation of the heat shield on Mars."

Offline niwax

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Re: Reusability of BFS Heat Shield
« Reply #3 on: 04/03/2018 09:00 am »
For LEO it should be fine, but for interplanetary velocities, it will need frequent refurbishment. I think Musk talked about 12 mars launches between major refurbishments. The tanker will have 100 reuses, since that only goes to LEO. So that should give you an estimate of the capability of the TPS.

I think you should try to find out where that is, because I feel like I've heard it all about ITS and BFR, scouring everything, and I never heard reuse numbers. I just heard roughly "there will be some ablation of the heat shield on Mars."

Well, 12 reuses to Mars and back is a timeframe of over 25 years when using every minimum energy opportunity. Even traditional airframes have a lifetime on the same order of magnitude, so the heat shield won't be the only thing that needs refurbishing.
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Offline JonathanD

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Re: Reusability of BFS Heat Shield
« Reply #4 on: 04/03/2018 02:00 pm »
Well, 12 reuses to Mars and back is a timeframe of over 25 years when using every minimum energy opportunity. Even traditional airframes have a lifetime on the same order of magnitude, so the heat shield won't be the only thing that needs refurbishing.

I've got to think they are going to do a very thorough inspection/refurbishment after every Mars trip before they send it "there and back again" with more kerbals on board.

Offline speedevil

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Re: Reusability of BFS Heat Shield
« Reply #5 on: 04/03/2018 04:28 pm »
For LEO it should be fine, but for interplanetary velocities, it will need frequent refurbishment. I think Musk talked about 12 mars launches between major refurbishments. The tanker will have 100 reuses, since that only goes to LEO. So that should give you an estimate of the capability of the TPS.

I think you should try to find out where that is, because I feel like I've heard it all about ITS and BFR, scouring everything, and I never heard reuse numbers. I just heard roughly "there will be some ablation of the heat shield on Mars."

From memory, the 100 reuses comes from numbers from IAC2016, where it was in the context of tanker reuse, and 'we are aiming at 100 reuses before major refurbishment'.
This is because for ITS as it was then, 100 reuses is all that was needed for Mars ambitions, before it impacted cost.

Recent statements have not made comment more than 'minimal wear', with the wear implied to be during reentry to earth from Mars transit if you read carefully.

P2P of course means you want to get rework of the heatsheild averaging well below $50K/flight or so, or it starts to become a noticable line item.

Offline Elmar Moelzer

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Re: Reusability of BFS Heat Shield
« Reply #6 on: 04/03/2018 04:49 pm »
Well, 12 reuses to Mars and back is a timeframe of over 25 years when using every minimum energy opportunity. Even traditional airframes have a lifetime on the same order of magnitude, so the heat shield won't be the only thing that needs refurbishing.
Unless they use it for something else between mars launch opportunities.

Offline KelvinZero

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Re: Reusability of BFS Heat Shield
« Reply #7 on: 04/07/2018 09:59 am »
I have wondered if they will produce some highly automated refurbishment method, eg scanning each inch, repairing cracks and 3d printing shielding back on where it is needed. (if that makes any sense for something like pica-x)

With enough automation refurbishing the shield might be just another mundane operation between each flight like refilling the tanks.

Offline speedevil

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Re: Reusability of BFS Heat Shield
« Reply #8 on: 04/07/2018 06:04 pm »
I have wondered if they will produce some highly automated refurbishment method, eg scanning each inch, repairing cracks and 3d printing shielding back on where it is needed. (if that makes any sense for something like pica-x)

With enough automation refurbishing the shield might be just another mundane operation between each flight like refilling the tanks.

You probably don't want to re-do the whole thing if you can possibly avoid it.

Scanning the whole surface was developed for shuttle after the incident, but at that time was lots more expensive.

How repairable any defects are is a fun question, as is the tolerance for any hidden defects, never mind scanning for damage in orbit and repair.

Offline Slarty1080

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Re: Reusability of BFS Heat Shield
« Reply #9 on: 04/07/2018 06:42 pm »
How easy would it be to accurately test or simulate the wear on a BFR heat shield?
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Offline Elmar Moelzer

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Re: Reusability of BFS Heat Shield
« Reply #10 on: 04/07/2018 08:34 pm »
How easy would it be to accurately test or simulate the wear on a BFR heat shield?
I think the BFS- grasshopper flights planned for next year are meant to test that out (among other things).

Offline Robotbeat

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Re: Reusability of BFS Heat Shield
« Reply #11 on: 04/08/2018 12:16 am »
How easy would it be to accurately test or simulate the wear on a BFR heat shield?
Easy, just use an arcjet. Ames has a bunch.
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Offline the_other_Doug

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Re: Reusability of BFS Heat Shield
« Reply #12 on: 04/08/2018 03:36 am »
At the FH Demo post-launch briefing, Musk discussed the heating profiles that are coming up in the computer simulations for BFS entries, at both Mars and Earth.

He was discussing it in such a detailed fashion that I can't imagine SpaceX isn't pretty solidly aware of what their TPS is going to have to deal with.  As it's supposed to be PICA-X based, and SpaceX is already familiar with that material, again I would imagine they have the various specifications for its application and refurbishment pretty well patted into shape already... ;)
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Offline Slarty1080

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Re: Reusability of BFS Heat Shield
« Reply #13 on: 04/08/2018 10:10 am »
How easy would it be to accurately test or simulate the wear on a BFR heat shield?
I think the BFS- grasshopper flights planned for next year are meant to test that out (among other things).
So it would appear difficult to test as it needs to be tested in situ high in the atmosphere with a special vehicle (grasshopper flights). What about simulation? How good is that likely to be?
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Offline Cheapchips

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Re: Reusability of BFS Heat Shield
« Reply #14 on: 04/08/2018 11:50 am »

SpaxeX have sophisticated simulation software.  Like any model, they'll still need to refine with actual test data..  This video from a while ago is mostly engine simulation. There are some Dragon bits in the video too though.


Offline Elmar Moelzer

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Re: Reusability of BFS Heat Shield
« Reply #15 on: 04/08/2018 04:33 pm »
So it would appear difficult to test as it needs to be tested in situ high in the atmosphere with a special vehicle (grasshopper flights). What about simulation? How good is that likely to be?
They are going to gradually increase altitude and speed over multiple flights all the way to orbit. BFS without BFR can go all the way to orbit. So they can and will test that.

Offline mgfitter

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Re: Reusability of BFS Heat Shield
« Reply #16 on: 04/20/2018 10:23 pm »
Will BFS's winglets have control surfaces?

-MG

Offline Lars-J

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Re: Reusability of BFS Heat Shield
« Reply #17 on: 04/20/2018 10:47 pm »
Will BFS's winglets have control surfaces?

-MG

Yes, the renderings we have seen from behind show two body flaps.
« Last Edit: 04/20/2018 10:48 pm by Lars-J »

Offline john smith 19

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Re: Reusability of BFS Heat Shield
« Reply #18 on: 05/09/2018 08:54 am »
At the FH Demo post-launch briefing, Musk discussed the heating profiles that are coming up in the computer simulations for BFS entries, at both Mars and Earth.

He was discussing it in such a detailed fashion that I can't imagine SpaceX isn't pretty solidly aware of what their TPS is going to have to deal with.  As it's supposed to be PICA-X based, and SpaceX is already familiar with that material, again I would imagine they have the various specifications for its application and refurbishment pretty well patted into shape already... ;)
If so then why are they advertising for someone with "heritage" tile TPS experience?

Run the numbers.

Check what the KE and PE for a Kg of mass at full orbital speed WRT to that at which the F9 booster separates. IE 1/2 x 1 x V(orbital m/s)^2 + 1 x h(metres) x g(proper g, with altitude variation factored in)

Orbital is not a little bit more energy to dissipate, it's a lot more energy to dissipate.
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Offline speedevil

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Re: Reusability of BFS Heat Shield
« Reply #19 on: 05/09/2018 10:24 am »
If so then why are they advertising for someone with "heritage" tile TPS experience?
Low priority work on solutions that seem non-favoured can result in the ability to rapidly pivot if things change.

As a stupid example, perhaps it turns out that with 3d printing, accurate milling of tile blanks on demand, rapid automated scanning enabled by the explosion in LASER metrology and better cameras and computers, maybe you can repair defects in a vehicles shuttle-like tiles in a couple of hours.

Lack of hiring in an area you think they should be hiring in can also mean they think they've nailed it internally, rather than a move in an unexpected direction.

Perhaps this job is even far down the road, and they're working on how to make a $20 reentry package to throw a million of at Mars.

Offline biosehnsucht

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Re: Reusability of BFS Heat Shield
« Reply #20 on: 05/09/2018 10:38 pm »
With modern drone tech (to remove old tile and install new one), combined with space versions of all of the above (laser, printing, etc), you might even be able to do all that on-orbit without another flight sent up.

Ideally you'd have a system that has no need for this, but that doesn't mean knowing how might not be useful ...

Offline john smith 19

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Re: Reusability of BFS Heat Shield
« Reply #21 on: 05/10/2018 08:43 am »
Low priority work on solutions that seem non-favoured can result in the ability to rapidly pivot if things change.
Or you could go with Occams razor that they've accepted PICAX can't hack it for upper stage recovery without unacceptable loss of payload and are going with another solution.

I reckon the ratio of energy loss from LEO compared to 1st stage separation for F9 is about 20:1 and there's a lot more surface to protect to begin with.

One of the other lessons of Shuttle maintenance was that while the acreage TPS was annoying the detail  stuff, around doors and joints was an enormous PITA. Aerosurfaces especially, requiring +/- 45deg from their centreline while stopping the ingress of very hot air. Seals and bearings were a serious pain to mfg and maintain.

OTOH the BFS shape seems much  simpler for most of its area. IIRC NASA said Shuttle had 24 000 different shaped tiles. The ability to mfg them all was viewed as a mfg triumph, rather than a design failure given the logistics of testing and repairing them.  :(

BFS's much simpler overall shape suggests a lot of it could be done as "staves," like a barrel. the joker will remain the differential thermal expansion coefficient between the integral CRFP tank structure and the selected TPS material, although NASA solved (and patented) fasteners to cope with large differential CTE's in the early 80's and I'd guess the technology is long out of patent.

An interesting development strategy for the TPS would be to cover the whole vehicle with a uniform layer of ablator, add whatever mass you could and still make orbit, then fly the re entry and see what thickness had been ablated away. Knowing the material properties of PICAX would give you a pretty good idea of how much enthalpy it had been exposed to.  This would give you a map as accurate as the positioning accuracy of the coordinate measuring hardware would allow.
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Offline speedevil

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Re: Reusability of BFS Heat Shield
« Reply #22 on: 05/10/2018 10:11 am »
Or you could go with Occams razor that they've accepted PICAX can't hack it for upper stage recovery without unacceptable loss of payload and are going with another solution.
See the 'party balloon' tweet.
F9S2 seems headed in a different direction entirely.

Offline Patchouli

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Re: Reusability of BFS Heat Shield
« Reply #23 on: 05/10/2018 02:45 pm »
Or you could go with Occams razor that they've accepted PICAX can't hack it for upper stage recovery without unacceptable loss of payload and are going with another solution.

I think a possible solution for BFS since it should have a fairly low ballistic co efficient might be the metallic thermal protection system from the X-33.
For interplanetary return it would need an abalator or some form of actively cooled TPS on the hottest parts of the vehicle.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040086762.pdf

The X-33s TPS requires fewer panels and was supposed to be much lower maintenance than the Shuttle's TPS.
« Last Edit: 05/10/2018 02:46 pm by Patchouli »

Offline Elmar Moelzer

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Re: Reusability of BFS Heat Shield
« Reply #24 on: 05/10/2018 06:43 pm »
PICA-X is really light weight. So unless they assume it would ablate too much for rapid reusability, it should be fine.

Offline john smith 19

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Re: Reusability of BFS Heat Shield
« Reply #25 on: 05/10/2018 10:24 pm »
PICA-X is really light weight. So unless they assume it would ablate too much for rapid reusability, it should be fine.
Depends. The NASA original was tested over a rang of specific gravities (relative to water) of 0.22-1.0.

Over a 58m long 9m wide cylinder that's a bit over 9 tonnes for a 1 inch layer.
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Offline Elmar Moelzer

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Re: Reusability of BFS Heat Shield
« Reply #26 on: 05/11/2018 12:02 am »
Over a 58m long 9m wide cylinder that's a bit over 9 tonnes for a 1 inch layer.
It is only 48 meters long and tapered towards the front. In return we have a small amount of wing surface that also needs to be protected. PICA-X is a little lighter than the original PICA at 0.2 grams per cm3.
I am assuming an average thickness of 4 cm. The whole ship wont need the full 4 cm of thickness. In return one can probably assume a thicker layer around the bottom half of the ship.
With all this in mind, I am estimating a very conservative 9.5 tonnes for the whole spaceship covered in PICA-X with an average thickness of 4 cm. I think that is pretty decent at 12% of the dry mass. I normally would assume about 20% of the dry mass to go to TPS. So there is still some margin there.

Offline john smith 19

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Re: Reusability of BFS Heat Shield
« Reply #27 on: 05/12/2018 05:18 pm »
Over a 58m long 9m wide cylinder that's a bit over 9 tonnes for a 1 inch layer.
It is only 48 meters long and tapered towards the front. In return we have a small amount of wing surface that also needs to be protected. PICA-X is a little lighter than the original PICA at 0.2 grams per cm3.
I am assuming an average thickness of 4 cm. The whole ship wont need the full 4 cm of thickness. In return one can probably assume a thicker layer around the bottom half of the ship.
With all this in mind, I am estimating a very conservative 9.5 tonnes for the whole spaceship covered in PICA-X with an average thickness of 4 cm. I think that is pretty decent at 12% of the dry mass. I normally would assume about 20% of the dry mass to go to TPS. So there is still some margin there.
So about 11.1% of the structural mass of the BFS.

Again the question is wheather it's good enough to cope with dissipating a combined kinetic and potential energy per Kg of structure that's about 20x the F9 booster copes with.

And AFAIK it can't.
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Online envy887

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Re: Reusability of BFS Heat Shield
« Reply #28 on: 05/12/2018 06:05 pm »
Over a 58m long 9m wide cylinder that's a bit over 9 tonnes for a 1 inch layer.
It is only 48 meters long and tapered towards the front. In return we have a small amount of wing surface that also needs to be protected. PICA-X is a little lighter than the original PICA at 0.2 grams per cm3.
I am assuming an average thickness of 4 cm. The whole ship wont need the full 4 cm of thickness. In return one can probably assume a thicker layer around the bottom half of the ship.
With all this in mind, I am estimating a very conservative 9.5 tonnes for the whole spaceship covered in PICA-X with an average thickness of 4 cm. I think that is pretty decent at 12% of the dry mass. I normally would assume about 20% of the dry mass to go to TPS. So there is still some margin there.
So about 11.1% of the structural mass of the BFS.

Again the question is wheather it's good enough to cope with dissipating a combined kinetic and potential energy per Kg of structure that's about 20x the F9 booster copes with.

And AFAIK it can't.

PICA-X works fine for Dragon which has much higher heat flux and peak temperature than BFS will have, due to Dragon's higher ballistic coefficient and lower hypersonic L/D.

Offline Eerie

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Re: Reusability of BFS Heat Shield
« Reply #29 on: 05/12/2018 06:23 pm »
Over a 58m long 9m wide cylinder that's a bit over 9 tonnes for a 1 inch layer.
It is only 48 meters long and tapered towards the front. In return we have a small amount of wing surface that also needs to be protected. PICA-X is a little lighter than the original PICA at 0.2 grams per cm3.
I am assuming an average thickness of 4 cm. The whole ship wont need the full 4 cm of thickness. In return one can probably assume a thicker layer around the bottom half of the ship.
With all this in mind, I am estimating a very conservative 9.5 tonnes for the whole spaceship covered in PICA-X with an average thickness of 4 cm. I think that is pretty decent at 12% of the dry mass. I normally would assume about 20% of the dry mass to go to TPS. So there is still some margin there.
So about 11.1% of the structural mass of the BFS.

Again the question is wheather it's good enough to cope with dissipating a combined kinetic and potential energy per Kg of structure that's about 20x the F9 booster copes with.

And AFAIK it can't.

Why are you comparing it with F9 booster at all? You should compare it with Dragon.

Offline tdperk

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Re: Reusability of BFS Heat Shield
« Reply #30 on: 05/18/2018 11:39 pm »
Over a 58m long 9m wide cylinder that's a bit over 9 tonnes for a 1 inch layer.
It is only 48 meters long and tapered towards the front. In return we have a small amount of wing surface that also needs to be protected. PICA-X is a little lighter than the original PICA at 0.2 grams per cm3.
I am assuming an average thickness of 4 cm. The whole ship wont need the full 4 cm of thickness. In return one can probably assume a thicker layer around the bottom half of the ship.
With all this in mind, I am estimating a very conservative 9.5 tonnes for the whole spaceship covered in PICA-X with an average thickness of 4 cm. I think that is pretty decent at 12% of the dry mass. I normally would assume about 20% of the dry mass to go to TPS. So there is still some margin there.
So about 11.1% of the structural mass of the BFS.

Again the question is wheather it's good enough to cope with dissipating a combined kinetic and potential energy per Kg of structure that's about 20x the F9 booster copes with.

And AFAIK it can't.

I know of no reason even to suspect it can't, since it already has.

No part of the F9 booster even has any PICA-X on it, an the PICA-X v1 thru v3 is in use on the Dragon or, has been thoroughly tested for such usage.

Offline john smith 19

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Re: Reusability of BFS Heat Shield
« Reply #31 on: 05/19/2018 07:01 am »

Why are you comparing it with F9 booster at all? You should compare it with Dragon.
Did you not know that the BFS is basically a stage with wings (whose nearest resemblance is to the space shuttle) while Dragon is basically a cone which enters base first?

So yes the F9 US is the relevant item to compare BFS with in terms of shape and surface area.

And both have a lot of surface area to protect.

Which is where the mass per unit area (Kg//m^2) starts to become important.
What's acceptable over 14m^2 may not be at 678 m^2.
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Offline JamesH65

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Re: Reusability of BFS Heat Shield
« Reply #32 on: 05/21/2018 11:54 am »

Why are you comparing it with F9 booster at all? You should compare it with Dragon.
Did you not know that the BFS is basically a stage with wings (whose nearest resemblance is to the space shuttle) while Dragon is basically a cone which enters base first?

So yes the F9 US is the relevant item to compare BFS with in terms of shape and surface area.

And both have a lot of surface area to protect.

Which is where the mass per unit area (Kg//m^2) starts to become important.
What's acceptable over 14m^2 may not be at 678 m^2.

The difference is velocity. This alone makes them incomparable.

Offline john smith 19

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Re: Reusability of BFS Heat Shield
« Reply #33 on: 05/25/2018 04:40 pm »

Why are you comparing it with F9 booster at all? You should compare it with Dragon.
Did you not know that the BFS is basically a stage with wings (whose nearest resemblance is to the space shuttle) while Dragon is basically a cone which enters base first?

So yes the F9 US is the relevant item to compare BFS with in terms of shape and surface area.

And both have a lot of surface area to protect.

Which is where the mass per unit area (Kg//m^2) starts to become important.
What's acceptable over 14m^2 may not be at 678 m^2.

The difference is velocity. This alone makes them incomparable.
In fact neither is a very good comparison. Dragon is way too small and can protect most of the structure in the lee of the reentry (typcial reentry angles are about 5 deg below local horizontal) while the F9 booster sees nothing like the air velocity of re-entry.

In fact BFS's closest match is either Shuttle or the X37b,with all the issues that implies.

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Offline johnfwhitesell

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Re: Reusability of BFS Heat Shield
« Reply #34 on: 05/25/2018 09:18 pm »
In fact BFS's closest match is either Shuttle or the X37b,with all the issues that implies.

The BFS doesn't have tiles, thanks to PICA/PICA-X, doesn't have solid boosters and doesn't attach on the side.  They learned from the shuttle.
« Last Edit: 05/25/2018 09:20 pm by johnfwhitesell »

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Re: Reusability of BFS Heat Shield
« Reply #35 on: 05/26/2018 12:05 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.

Offline john smith 19

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Re: Reusability of BFS Heat Shield
« Reply #36 on: 05/29/2018 06:25 am »
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.
Correct.  Although I think NASA have grudgingly admitted that a tiled heat shield is survivable at Lunar reentry (although ARD demonstrated titled heat shield survival from orbit in 1998).

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Re: Reusability of BFS Heat Shield
« Reply #37 on: 05/29/2018 10:59 am »
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.

No, the tiles are not symmetric. Thats because the capsule enters at an angle and the center of influx air is not at the center of the capsule.

Look at the Dragon heat shield:
https://i.imgur.com/s7yORbX.jpg
https://forum.nasaspaceflight.com/index.php?topic=41016.msg1593168#msg1593168

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Re: Reusability of BFS Heat Shield
« Reply #38 on: 05/29/2018 12:04 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.

No, the tiles are not symmetric. Thats because the capsule enters at an angle and the center of influx air is not at the center of the capsule.

Look at the Dragon heat shield:
https://i.imgur.com/s7yORbX.jpg
https://forum.nasaspaceflight.com/index.php?topic=41016.msg1593168#msg1593168

I was talking about the BFS heatshield. Possibly your argumentapplies to that as well. But I would expect that there would be a large number of mostly identical tiles at least on the cylincrical part of the body.

Edit: Maybe that was not quite clear because I mentioned the Orion heatshield.
« Last Edit: 05/29/2018 12:06 pm by guckyfan »

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Re: Reusability of BFS Heat Shield
« Reply #39 on: 05/29/2018 01:01 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.

No, the tiles are not symmetric. Thats because the capsule enters at an angle and the center of influx air is not at the center of the capsule.

Look at the Dragon heat shield:
https://i.imgur.com/s7yORbX.jpg
https://forum.nasaspaceflight.com/index.php?topic=41016.msg1593168#msg1593168

I was talking about the BFS heatshield. Possibly your argumentapplies to that as well. But I would expect that there would be a large number of mostly identical tiles at least on the cylincrical part of the body.

Edit: Maybe that was not quite clear because I mentioned the Orion heatshield.

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. 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. 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.

Online 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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Offline 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.

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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?

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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.

Offline 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.


Offline 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.

Offline CorvusCorax

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Re: Reusability of BFS Heat Shield
« Reply #60 on: 07/18/2018 10:17 am »
I remember reading somewhere that PICA/PICAX has an issue with soaking up water and for that reason is initially painted over on Dragon with a silvery paint. (Having water - even in small amounts - in the heatshield during reentry is obviously a bad thing. It would turn to steam, expand drastically and in doing so blows the material apart from within - highly increasing the ablation rate.)

Obviously this paint layer does not survive reentry, it would ablate away.

Dragon currently gets multi-month refurbishment between flights, including - up to now - reapplying a brand new heatshield and new paint.

For BFS this is not an option. It might be an option for in between mars flights, but not for the tanker/cargo variant when going to LEO. Not only needs the ablation of ablative parts of the heatshield be so small that it can go douzends of flights, but the heatshield also needs to remain in a state unaffected by the environment - including moisture, rain, and slightly corrosive coastal winds at the cape.

This would mean, either SpaceX is not using PICAX on BFS but a much more resilent ceramic material which would have to require neither inspection nor refurbishment between LEO flights. Or they have a new version of PICAX which no longer soaks up moisture and is much more resilent to the environment.

Ideally - for maximum ultraquick reuse - a heatshield would not ablate at all during LEO reentries, but dissipate heat radiatively similar to how the shuttle did - but would become ablative when facing much hotter reentries- for example when doing direct interplanetary reentry OR when reentering ballistic in cases of emergency/malfunction. In the latter case the heatshield would have to be replaced.


Offline jpo234

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Re: Reusability of BFS Heat Shield
« Reply #61 on: 07/18/2018 11:16 am »
I remember reading somewhere that PICA/PICAX has an issue with soaking up water and for that reason is initially painted over on Dragon with a silvery paint. (Having water - even in small amounts - in the heatshield during reentry is obviously a bad thing. It would turn to steam, expand drastically and in doing so blows the material apart from within - highly increasing the ablation rate.)

Obviously this paint layer does not survive reentry, it would ablate away.

Dragon currently gets multi-month refurbishment between flights, including - up to now - reapplying a brand new heatshield and new paint.

For BFS this is not an option. It might be an option for in between mars flights, but not for the tanker/cargo variant when going to LEO. Not only needs the ablation of ablative parts of the heatshield be so small that it can go douzends of flights, but the heatshield also needs to remain in a state unaffected by the environment - including moisture, rain, and slightly corrosive coastal winds at the cape.

This would mean, either SpaceX is not using PICAX on BFS but a much more resilent ceramic material which would have to require neither inspection nor refurbishment between LEO flights. Or they have a new version of PICAX which no longer soaks up moisture and is much more resilent to the environment.

Ideally - for maximum ultraquick reuse - a heatshield would not ablate at all during LEO reentries, but dissipate heat radiatively similar to how the shuttle did - but would become ablative when facing much hotter reentries- for example when doing direct interplanetary reentry OR when reentering ballistic in cases of emergency/malfunction. In the latter case the heatshield would have to be replaced.

Different from Dragon, BFS is not supposed to splash down in the ocean.

That said, we know from job postings that SpaceX investigates ceramic heat shield materials to allow hundreds of reuses.
« Last Edit: 07/18/2018 08:17 pm by jpo234 »
You want to be inspired by things. You want to wake up in the morning and think the future is going to be great. That's what being a spacefaring civilization is all about. It's about believing in the future and believing the future will be better than the past. And I can't think of anything more exciting than being out there among the stars.

Offline RoboGoofers

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Re: Reusability of BFS Heat Shield
« Reply #62 on: 07/18/2018 05:27 pm »
I remember reading somewhere that PICA/PICAX has an issue with soaking up water and for that reason is initially painted over on Dragon with a silvery paint. (Having water - even in small amounts - in the heatshield during reentry is obviously a bad thing. It would turn to steam, expand drastically and in doing so blows the material apart from within - highly increasing the ablation rate.)

Obviously this paint layer does not survive reentry, it would ablate away.

Dragon currently gets multi-month refurbishment between flights, including - up to now - reapplying a brand new heatshield and new paint.

For BFS this is not an option. It might be an option for in between mars flights, but not for the tanker/cargo variant when going to LEO. Not only needs the ablation of ablative parts of the heatshield be so small that it can go douzends of flights, but the heatshield also needs to remain in a state unaffected by the environment - including moisture, rain, and slightly corrosive coastal winds at the cape.

This would mean, either SpaceX is not using PICAX on BFS but a much more resilent ceramic material which would have to require neither inspection nor refurbishment between LEO flights. Or they have a new version of PICAX which no longer soaks up moisture and is much more resilent to the environment.

Ideally - for maximum ultraquick reuse - a heatshield would not ablate at all during LEO reentries, but dissipate heat radiatively similar to how the shuttle did - but would become ablative when facing much hotter reentries- for example when doing direct interplanetary reentry OR when reentering ballistic in cases of emergency/malfunction. In the latter case the heatshield would have to be replaced.

I was wondering the same, mainly about P2P. there's no structures shown in any of the renderings as BFS hangers to keep them out of the rain.

I suspect that P2P BFS will evolve away from the interplanetary version, and vice versa, with the interplanetary ships preferring weight and the P2P extreme reusability.

Online cppetrie

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Re: Reusability of BFS Heat Shield
« Reply #63 on: 07/18/2018 07:38 pm »
I remember reading somewhere that PICA/PICAX has an issue with soaking up water and for that reason is initially painted over on Dragon with a silvery paint. (Having water - even in small amounts - in the heatshield during reentry is obviously a bad thing. It would turn to steam, expand drastically and in doing so blows the material apart from within - highly increasing the ablation rate.)

Obviously this paint layer does not survive reentry, it would ablate away.

Dragon currently gets multi-month refurbishment between flights, including - up to now - reapplying a brand new heatshield and new paint.

For BFS this is not an option. It might be an option for in between mars flights, but not for the tanker/cargo variant when going to LEO. Not only needs the ablation of ablative parts of the heatshield be so small that it can go douzends of flights, but the heatshield also needs to remain in a state unaffected by the environment - including moisture, rain, and slightly corrosive coastal winds at the cape.

This would mean, either SpaceX is not using PICAX on BFS but a much more resilent ceramic material which would have to require neither inspection nor refurbishment between LEO flights. Or they have a new version of PICAX which no longer soaks up moisture and is much more resilent to the environment.

Ideally - for maximum ultraquick reuse - a heatshield would not ablate at all during LEO reentries, but dissipate heat radiatively similar to how the shuttle did - but would become ablative when facing much hotter reentries- for example when doing direct interplanetary reentry OR when reentering ballistic in cases of emergency/malfunction. In the latter case the heatshield would have to be replaced.

Different from Dragon, BFS is not supposed to splash down in the ocean.

That said, we know from job postings that SpaceX investigates ceramic head shield materials to allow hundreds of reuses.
Water from the ocean isnít the primary issue.   Itís water vapor from the air being absorbed into the material while itís being processed and sitting on the pad.

Offline Robotbeat

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Re: Reusability of BFS Heat Shield
« Reply #64 on: 07/19/2018 12:49 am »
I remember reading somewhere that PICA/PICAX has an issue with soaking up water and for that reason is initially painted over on Dragon with a silvery paint. (Having water - even in small amounts - in the heatshield during reentry is obviously a bad thing. It would turn to steam, expand drastically and in doing so blows the material apart from within - highly increasing the ablation rate.)

Obviously this paint layer does not survive reentry, it would ablate away.

Dragon currently gets multi-month refurbishment between flights, including - up to now - reapplying a brand new heatshield and new paint.

For BFS this is not an option. It might be an option for in between mars flights, but not for the tanker/cargo variant when going to LEO. Not only needs the ablation of ablative parts of the heatshield be so small that it can go douzends of flights, but the heatshield also needs to remain in a state unaffected by the environment - including moisture, rain, and slightly corrosive coastal winds at the cape.

This would mean, either SpaceX is not using PICAX on BFS but a much more resilent ceramic material which would have to require neither inspection nor refurbishment between LEO flights. Or they have a new version of PICAX which no longer soaks up moisture and is much more resilent to the environment.

Ideally - for maximum ultraquick reuse - a heatshield would not ablate at all during LEO reentries, but dissipate heat radiatively similar to how the shuttle did - but would become ablative when facing much hotter reentries- for example when doing direct interplanetary reentry OR when reentering ballistic in cases of emergency/malfunction. In the latter case the heatshield would have to be replaced.
Inspection may be acceptable if it can be done quickly and cheaply. Actually a good spot for computer vision systems. High resolution scans (via crane or drone). Problem areas are identified and sent to humans for detailed analysis.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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