Starship heat shield

[ZachF]

Also, I can't seem to find a clear source on whether that 3t full reuse figure for Stoke is RTLS or Droneship... honestly with RTLS I can't really make the math work well unless I use an upper stage empty mass estimate that seems unrealistically low (ie, weighs less than a AV Centaur balloon tank upper stage in spite of having much higher thrust, landing legs, an attached PLF, heat shield and also not a balloon tank)

[StraumliBlight]

I recently came across Blue Origin's Metal encapsulated ceramic tile patent, which attempts to reduce water absorption.

A metal encapsulated ceramic tile thermal insulation system for rockets and associated methods is disclosed. A representative system includes a launch vehicle having a first end and a second end generally opposite the first end and includes a heat shield positioned at the second end. The heat shield includes a plurality of thermal protection apparatuses, where individual of the thermal protection apparatuses include ceramic tiles encapsulated by inner and outer metal layers, which are positioned on opposing top and bottom surfaces of the ceramic tiles. The plurality of thermal protection apparatuses includes a plurality of pins positioned within corresponding holes drilled through the ceramic tiles and are secured to the metal layers. The outer metal layer can protect the ceramic tile from tool strikes and debris and can also prevent water from reaching and being absorbed by the ceramic tile.

[...]

In some embodiments, the porous ceramic material can be capable of readily absorbing water (e.g., water from the atmosphere such as rain, snow, humidity, etc. or water used in cooling or noise suppression systems). However, water is dense and saturating the ceramic tile with water can increase the weight of the thermal protection apparatus. To restrict or prevent water ingress, the outer metal layer can act as a hermetic waterproof barrier and can prevent most of the water incident on the outer surface from reaching and being absorbed by the ceramic tiles. To further reduce the amount of water capable of being absorbed by the ceramic material, the thermal protection apparatus can include waterproofing applied to the ceramic tile. In some embodiments, the waterproofing can be applied to the ceramic tile by submerging the ceramic tile in the waterproofing material for a suitable amount of time.

In some embodiments, the waterproofing can be applied to the bottom surface and the edge portions of the ceramic tile. In other embodiments, the waterproofing can be applied through the entire thickness T. However, the high temperatures at points within the ceramic tile near the top surface can cause the waterproofing near the top surface to burn off while the waterproofing near the bottom surface can remain intact throughout the launch and landing. Because some of the waterproofing remains within the ceramic tile and because the outer metal layer can prevent most of the water from reaching the ceramic tile, the amount of water that can be absorbed by the ceramic tile can be reduced and the thermal protection apparatus can be used for multiple launches and landings without having to apply waterproofing between launches.

[InterestedEngineer]

I recently came across Blue Origin's Metal encapsulated ceramic tile patent, which attempts to reduce water absorption.

A metal encapsulated ceramic tile thermal insulation system for rockets and associated methods is disclosed. A representative system includes a launch vehicle having a first end and a second end generally opposite the first end and includes a heat shield positioned at the second end. The heat shield includes a plurality of thermal protection apparatuses, where individual of the thermal protection apparatuses include ceramic tiles encapsulated by inner and outer metal layers, which are positioned on opposing top and bottom surfaces of the ceramic tiles. The plurality of thermal protection apparatuses includes a plurality of pins positioned within corresponding holes drilled through the ceramic tiles and are secured to the metal layers. The outer metal layer can protect the ceramic tile from tool strikes and debris and can also prevent water from reaching and being absorbed by the ceramic tile.

[...]

In some embodiments, the porous ceramic material can be capable of readily absorbing water (e.g., water from the atmosphere such as rain, snow, humidity, etc. or water used in cooling or noise suppression systems). However, water is dense and saturating the ceramic tile with water can increase the weight of the thermal protection apparatus. To restrict or prevent water ingress, the outer metal layer can act as a hermetic waterproof barrier and can prevent most of the water incident on the outer surface from reaching and being absorbed by the ceramic tiles. To further reduce the amount of water capable of being absorbed by the ceramic material, the thermal protection apparatus can include waterproofing applied to the ceramic tile. In some embodiments, the waterproofing can be applied to the ceramic tile by submerging the ceramic tile in the waterproofing material for a suitable amount of time.

In some embodiments, the waterproofing can be applied to the bottom surface and the edge portions of the ceramic tile. In other embodiments, the waterproofing can be applied through the entire thickness T. However, the high temperatures at points within the ceramic tile near the top surface can cause the waterproofing near the top surface to burn off while the waterproofing near the bottom surface can remain intact throughout the launch and landing. Because some of the waterproofing remains within the ceramic tile and because the outer metal layer can prevent most of the water from reaching the ceramic tile, the amount of water that can be absorbed by the ceramic tile can be reduced and the thermal protection apparatus can be used for multiple launches and landings without having to apply waterproofing between launches.

Wonder what kind of metal doesn't completely slag away on re-entry from 8km/sec

[DanClemmensen]

Wonder what kind of metal doesn't completely slag away on re-entry from 8km/sec

Maybe tungsten. Tungsten carbide might be better but technically is not a metal. Anything refractory enough for this will need some exotic method of application since you cannot simply melt it.

[Twark_Main]

Wonder what kind of metal doesn't completely slag away on re-entry from 8km/sec

Paper, naturally.

Plain ordinary printer paper has a sufficiently low area density that the peak heating is below the smoke point of paper.

I'm joking of course, but the serious point is that "can you re-enter from 8 km/s" is really a whole-vehicle system-level design question, not just a one-dimensional material science question.

[InterestedEngineer]

Wonder what kind of metal doesn't completely slag away on re-entry from 8km/sec

Paper, naturally.

Plain ordinary printer paper has a sufficiently low area density that the peak heating is below the smoke point of paper.

I'm joking of course, but the serious point is that "can you re-enter from 8 km/s" is really a whole-vehicle system-level design question, not just a one-dimensional material science question.

the metal is right on the outer surface of the heat shield.  It's not a whole vehicle problem at that point - that's how you got to the "put the metal on the outside of the heatshield" idea in the first place.  because of system design issues, it seems like a good idea to protect tiles with metal.

But magic wands don't solve the problem of a metal being heated to 1600K in a plasma.

So it is a material science question.   Which really is the fundamental limit for this problem space.

[meekGee]

Wonder what kind of metal doesn't completely slag away on re-entry from 8km/sec

Paper, naturally.

Plain ordinary printer paper has a sufficiently low area density that the peak heating is below the smoke point of paper.

I'm joking of course, but the serious point is that "can you re-enter from 8 km/s" is really a whole-vehicle system-level design question, not just a one-dimensional material science question.

To be fair, when discussing "which metal can survive etc", it's in the context of "while coating a ceramic tile of a re-entering vehicle", and the range of possibilities decreases by a very large amount.  For example, paper is right out.

And actually, even if only discussing LEO re-entry, I don't know which metallic coating can survive.  Remember that it's not just soaking in that temperature, it's blasted by high speed plasma that will erode it mechanically even if it just softens up.

If it's deposited on a highly insulating tile, the temperature of the coating layer will approach that of the radiative environment, not that of the bulk tile. Think about a tent wall on a sunny day.

I'm not saying it's impossible, just that the shock wall outside is a lot hotter than the tile itself.

[Twark_Main]

Wonder what kind of metal doesn't completely slag away on re-entry from 8km/sec

Paper, naturally.

Plain ordinary printer paper has a sufficiently low area density that the peak heating is below the smoke point of paper.

I'm joking of course, but the serious point is that "can you re-enter from 8 km/s" is really a whole-vehicle system-level design question, not just a one-dimensional material science question.

the metal is right on the outer surface of the heat shield.  It's not a whole vehicle problem at that point - that's how you got to the "put the metal on the outside of the heatshield" idea in the first place.  because of system design issues, it seems like a good idea to protect tiles with metal.

But magic wands don't solve the problem of a metal being heated to 1600K in a plasma.

So it is a material science question.   Which really is the fundamental limit for this problem space.

"1600K" isn't a constant of nature, or even a constant of Earth. It's a variable that changes with the rest of the vehicle design.

The current Starship design is hardly "maxed out" on its ability to add surface area. And of course, simply scaling the design up or down changes its Ballistic Coefficient. If SpaceX really does encounter "fundamental" material science limits that apply to the entire problem space, then they can simply re-scale Starship and/or alter its OML to minimize the BC, rather similar to solving systems of parametric equations when sizing airplane subsystems.

[InterestedEngineer]

Wonder what kind of metal doesn't completely slag away on re-entry from 8km/sec

Paper, naturally.

Plain ordinary printer paper has a sufficiently low area density that the peak heating is below the smoke point of paper.

I'm joking of course, but the serious point is that "can you re-enter from 8 km/s" is really a whole-vehicle system-level design question, not just a one-dimensional material science question.

the metal is right on the outer surface of the heat shield.  It's not a whole vehicle problem at that point - that's how you got to the "put the metal on the outside of the heatshield" idea in the first place.  because of system design issues, it seems like a good idea to protect tiles with metal.

But magic wands don't solve the problem of a metal being heated to 1600K in a plasma.

So it is a material science question.   Which really is the fundamental limit for this problem space.

"1600K" isn't a constant of nature, or even a constant of Earth. It's a variable that changes with the rest of the vehicle design.

The current Starship design is hardly "maxed out" on its ability to add surface area. And of course, simply scaling the design up or down changes its Ballistic Coefficient. If SpaceX really does encounter "fundamental" material science limits that apply to the entire problem space, then they can simply re-scale Starship and/or alter its OML to minimize the BC, rather similar to solving systems of parametric equations when sizing airplane subsystems.

This isn't starship, this a BO patent.

The question is, how is it practical to have a metal covered tile that needs to take peak heating of 1400-1600K in plasma?

It sounds like unobtanium to me.  You can skirt around the question all sorts of ways like BC or whatever, but the patent is the patent and that problem has to be solved for the idea to be useful

[Twark_Main]

Wonder what kind of metal doesn't completely slag away on re-entry from 8km/sec

Paper, naturally.

Plain ordinary printer paper has a sufficiently low area density that the peak heating is below the smoke point of paper.

I'm joking of course, but the serious point is that "can you re-enter from 8 km/s" is really a whole-vehicle system-level design question, not just a one-dimensional material science question.

the metal is right on the outer surface of the heat shield.  It's not a whole vehicle problem at that point - that's how you got to the "put the metal on the outside of the heatshield" idea in the first place.  because of system design issues, it seems like a good idea to protect tiles with metal.

But magic wands don't solve the problem of a metal being heated to 1600K in a plasma.

So it is a material science question.   Which really is the fundamental limit for this problem space.

"1600K" isn't a constant of nature, or even a constant of Earth. It's a variable that changes with the rest of the vehicle design.

The current Starship design is hardly "maxed out" on its ability to add surface area. And of course, simply scaling the design up or down changes its Ballistic Coefficient. If SpaceX really does encounter "fundamental" material science limits that apply to the entire problem space, then they can simply re-scale Starship and/or alter its OML to minimize the BC, rather similar to solving systems of parametric equations when sizing airplane subsystems.

This isn't starship, this a BO patent.

If it isn't somehow related to Starship, why are you bringing it up in the "Starship heat shield" thread? 

If it is somehow related to Starship, then, exactly, what are you hassling me for? 

It sounds like unobtanium to me.  You can skirt around the question all sorts of ways like BC or whatever...

What is this, Contradiction Tuesday?  "It's impossible. Totally unobtainable. Unless you do this thing over here, but like I said it's impossible." 


Anyway, here's the melting point of metals. Even relatively boring metals like Inconel won't melt at those temperatures.

[Action]

Anyway, here's the melting point of metals. Even relatively boring metals like Inconel won't melt at those temperatures.

Yes, as you say, this is a systems problem not a fundamental physics problem.

There are many refractory metals that are useful at those temperatures.  The big problem is usually that they are very dense and it's hard to make light structures with them.  I don't know the details of BO's idea, but I could imagine clever design and the use of a non-metal insulator backing making a metal skin more practical.

The heat shield design space is poorly explored in practice.

[OTV Booster]

Sputtering or direct vapor deposition might be a way of coating a metal onto ceramic. If it's nice and shiny in the IR, so much the better.


IMO, chemical scouring, not thermal, would be the hard part.

[Vultur]

Is softening temperature rather than melting point the issue, then? 1600K isn't that extreme as metals go - titanium melting point is 1941K, and it's fairly low density unlike a lot of really high melting point metals (like tungsten, tantalum, molybdenum, etc).

Or is the primary problem for metal TPS oxidizing (like the metal tiles that rusted up a previous Starship) rather than melting?

[InterestedEngineer]

I recently came across Blue Origin's Metal encapsulated ceramic tile patent, which attempts to reduce water absorption.

They also mention the outer metal layer temperature "generally depends on the speed of the first stage as it descends, where the speed is generally dependent on the altitude at which the first and second stages separated."

This is for a first stage heat shield.  The LEO-return heat shield is far more speculative.

Also they admit that their emissitivity is only 0.6 - 0.7, so it rejects 0.7/0.9  = 78% of the heat as a ceramic tile so it'll run hotter.

As far as using this for a return-from-LEO heat shield cover:

Inconel won't work - that close to melting temperature the structure will fall apart as grain boundaries start to change dramatically.   The surface oxidation won't stay on and it'll get blasted off by the plasma, and it can't reform in that thin of air.  You'd have to re-treat it on the ground with pure O2 after every few launches.  The functioning temperature of Inconel is closer to 1200K.

highly refactoyr materials such as

TZM - molybdenum alloy (Mo-0.5Ti-0.1Zr)
C-103 - niobium alloy (Nb-10Hf-1Ti)

can handle the 1600K easily but have horrible oxidation properties.  You wouldn't want to use them at 1600K outside of a neutral atmosphere environment.  Certainly not one with nitrogen and oxygen ions bombarding you.  These don't have self-protecting oxidation layers like Inconel does.

(same goes for titanium)

Yes, material science is the bottleneck, and yes, it is relevant for the Starship heat shield when a competitor proposes something radical and potentially breakthrough.

Only it ain't. 

BO's lower stage is aluminum, they need protection for even a Mach 4 entry by their booster.  so that's what this is probably for (same problem as Falcon-9, I forget what they use)

[Action]

Inconel won't work - that close to melting temperature the structure will fall apart as grain boundaries start to change dramatically.   The surface oxidation won't stay on and it'll get blasted off by the plasma, and it can't reform in that thin of air.  You'd have to re-treat it on the ground with pure O2 after every few launches.  The functioning temperature of Inconel is closer to 1200K.

Generally to use nickel alloys in hot oxidizing environments, you coat them with a thin layer of ceramic.  It's done in jet engine turbines, for example.  They have long lives.

But inconel isn't really pushing the limits for metals in hot-structure concepts; it's just a commonly used and less fussy material.  When people propose exposed inconel on reentry vehicles, they generally propose backside cooling for it.

[Action]

Inconel won't work - that close to melting temperature the structure will fall apart as grain boundaries start to change dramatically.   The surface oxidation won't stay on and it'll get blasted off by the plasma, and it can't reform in that thin of air.  You'd have to re-treat it on the ground with pure O2 after every few launches.  The functioning temperature of Inconel is closer to 1200K.

Generally to use nickel alloys in hot oxidizing environments, you coat them with a thin layer of ceramic.  It's done in jet engine turbines, for example.  They have long lives.

But inconel isn't really pushing the limits for metals in hot-structure concepts; it's just a commonly used and less fussy material.  When people propose exposed inconel on reentry vehicles, they generally propose backside cooling for it.

These ceramic layers are extremely thin in practice.

I have no idea if BO is doing something like this, but I could imagine encapsulating a bulky insulating tile in a tantalum or rhenium box, and then coating the outside of that box with an extremely thin ceramic coating.  You don't worry about that coating getting wet because it's so thin it can't really absorb any mass, the refractory metal keeps the bulk tile isolated from the atmosphere, and the bulk tile insulates the body of the craft.

Oftentimes these proposed metal shingle structures are quite complicated.  The downside is usually thought to be that they are difficult to make light.

[meekGee]

Is softening temperature rather than melting point the issue, then? 1600K isn't that extreme as metals go - titanium melting point is 1941K, and it's fairly low density unlike a lot of really high melting point metals (like tungsten, tantalum, molybdenum, etc).

Or is the primary problem for metal TPS oxidizing (like the metal tiles that rusted up a previous Starship) rather than melting?

I don't think that's the temperature the metal will see.  That's the temperature tiles (bulk ceramic) operate at. The radiative environment right outside is much hotter. A thin metallic coat on the outside of a tile may be at a different temperature altogether.

Also, that super-fast flow is really trying to create little ripples on the surface. I can see the thin hot metal a lot more susceptible to that than bulk ceramic.

[TomH]

We continue to see test flights with tiles intentionally missing to gather data. One thing we do not have data for is a SS with an entirely intact TPS. What are your predictions regarding when we will see a SS fly with a fully intact set of tiles? (And please don't give snarky answers like, "When SX feels like they are ready to," or "When SX has gathered all the data they can get with tiles missing.")

[Twark_Main]

We continue to see test flights with tiles intentionally missing to gather data. One thing we do not have data for is a SS with an entirely intact TPS. What are your predictions regarding when we will see a SS fly with a fully intact set of tiles? (And please don't give snarky answers like, "When SX feels like they are ready to," or "When SX has gathered all the data they can get with tiles missing.")

The snarky answers are the right ones, so...

[eriblo]

We continue to see test flights with tiles intentionally missing to gather data. One thing we do not have data for is a SS with an entirely intact TPS. What are your predictions regarding when we will see a SS fly with a fully intact set of tiles? (And please don't give snarky answers like, "When SX feels like they are ready to," or "When SX has gathered all the data they can get with tiles missing.")

The snarky answers are the right ones, so...

Indeed, we don't know enough about their plans to have more than complete guesses if snarky answers are eliminated. My would be "They'll probably fly their first 'best attempt' heat shield on the first Starship they intend to refurbish and refly." And that's far from a given...