Author Topic: SpaceX Starship (BFS) Engineering General Thread 2  (Read 175452 times)

Offline meekGee

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #940 on: 02/11/2019 09:25 pm »
Yes the differential expansion seems to me like a really fundamental challenge... 

The outer skin is, well, outer...  Things like expansion joints don't play well with the idea of keeping plasma out of the interior....

But maybe that's exactly the trick...  Something like scales, with vapor coming out of the cracks to keep the plasma out...
Presumably the whole cooling approach is dependent on the coolant being at a higher pressure than the external plasma.
Of course.

The quandary was how to make an outer skin that doesn't buckle on expansion since it is hotter, and thin.

You could make it from sections with overlapping "shingled" lips but that's a leak path for the plasma.  But...  What if the vapor guards that path by escaping from between the shingles?

Floating outer skin.
Only attached at edges.
Not attached to stringers.
Maybe some sort of squishable gasket to hold the floating outer skin in place so it doesn't rattle.
I haven't done the calc but it seems a couple of mm's extra radius should make up for the expansion.
That skin, thin as you'd want it, had to see all the aero loads...  Which are pretty high, non-uniform, and transient.

It can't be dismissed as "non structural".
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Offline JonathanD

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #941 on: 02/11/2019 09:31 pm »
Tangential question, I'm wondering how they plan to damp the vibrations coming from SH booster rattling up through Starship?  It's gonna be brutal.

Offline cuddihy

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #942 on: 02/11/2019 09:36 pm »
Lots of fuel mass and tankage on SH/SS will damp a lot. Also, with 19-31 engines, would think a lot of the vibration is going to dampen itself— or rather drown itself out, in “white noise” vibration that is not as disruptive as say 5 big F-1s behind you.

Online RoboGoofers

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #943 on: 02/11/2019 11:33 pm »
The pore size will grow as the hot side expands. Could this lead to some self-regulation?

Online Lars-J

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #944 on: 02/12/2019 12:06 am »
One thing that I'm wondering about the active heat shield that vents: (my apologies if it has already been asked)

Elon did mention that they could use water, but methane was even better - and they will have some on board. So that seems great. But... Won't venting a fuel such as methane cause it to instantly combust in the re-entry heat, and thus make it even hotter? (In Earths atmopshere where oxygen is relatively plentiful)

Or does it not matter, since it has already done its job carrying the heat off the ship, and any combustion will happen in the plasma trail behind the Starship?

Offline IainMcClatchie

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #945 on: 02/12/2019 12:09 am »
Floating outer skin.
Only attached at edges.
Not attached to stringers.
Maybe some sort of squishable gasket to hold the floating outer skin in place so it doesn't rattle.
I haven't done the calc but it seems a couple of mm's extra radius should make up for the expansion.

In the 1960s they used Omega joints as expansion joints.  The problem between the inner and outer skins is that you need to expand in two directions.

The outer skin could have Omega joints in one direction, say, longitudinally, and the inner skin could have Omega joints circumferentially.  The joints act as stringers as well, so you end up with stiffness in two directions as well as CTE mismatch.  Since the inner skin doesn't change temperature as much, it ends up constraining the vehicle's overall length change.

The problem with this idea is which Omega goes on which skin.  If you put the circumferential Omega on the inner skin, the vehicle diameter changes with inner temperature, and it's clear how to attach the tank domes.  But then the outer skins end up defining the growth of the vehicle in length.  Since the leeward heating is very different from the windward heating, the vehicle is going to bow significantly, which is going to stress everything inside.  You end up putting Omega joints on all the pipes running end-to-end, but you were going to do that anyway.

You can fix the bowing problem by putting the circumferential Omega on the outer skin.  But now the inner skin has the longitudinal Omega joints, and the diameter of the fuselage changes with temperature.  How do you attach a tank dome to this?

I wonder about the utility of having a twin wall skin at all.  Why not just have an inner tank which holds propellant for landing.  Propellant from the inner tank is sprayed onto the inside of the since outer wall, where it vaporizes.  The vapor overpressure from the larger tanks are vented through gas-gas engines firing into the edges of the bow shock, increasing the effective radius of curvature of the vehicle and causing the bow shock to stand off further.

Offline Wolfram66

Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #946 on: 02/12/2019 12:46 am »
One thing that I'm wondering about the active heat shield that vents: (my apologies if it has already been asked)

Elon did mention that they could use water, but methane was even better - and they will have some on board. So that seems great. But... Won't venting a fuel such as methane cause it to instantly combust in the re-entry heat, and thus make it even hotter? (In Earths atmopshere where oxygen is relatively plentiful)

Or does it not matter, since it has already done its job carrying the heat off the ship, and any combustion will happen in the plasma trail behind the Starship?

 I believe EM also stated that there won’t be enough oxygen at the altitude of heating and interfacing with the atmosphere to cause combustion. This very question was asked in the QA where he discussed the CH4 transpiration cooling concept

Offline spacenut

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #947 on: 02/12/2019 03:40 am »
Methane needs between 4-14% air to gas mixture to ignite.  To much methane, it won't ignite.  Too little it won't ignite.  I worked in the natural gas industry for 40 years.  I've been in schools showing this with a small scale house showing what can happen. 

I too have read that the highest temperature is at high altitude with very little air.  Once it gets through the upper atmosphere, they turn off the methane and flip to vertical to slow down like their F9 booster. 

Offline livingjw

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #948 on: 02/12/2019 03:49 am »
Atlas sheet metal. All resistance welded from flat sheet less than .5 mm thick It all came into the factory as great rolls about 3 ft wide. The one shows what can happen when you allow the internal pressure to go too low. Notice that the slosh baffles are only connected to the tank wall by a few tabs.

John
« Last Edit: 02/12/2019 03:51 am by livingjw »

Offline QuantumG

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #949 on: 02/12/2019 06:35 am »
And while building large things out of stainless steel is a known quantity, building large things out of carbon composite is a known quantity too. Boeing builds entire airliners out of carbon composites, on a production line, so it's a mature building material for large structures.

I'm just hoping that you're reading that last sentence in a sarcastic voice.  ;D

The idea that stainless steel work is on-par with carbon composites work is comical. I mean, if you were comparing it to "composites work" then yeah... a lot of people make boats out of the stuff. There's no shortage of good fibreglassers. When it comes to stainless steel work, that's the pinnacle. That's where their industry ends. Stainless steel is an understood material... and unlike woodworking, which still has some of the mystique of the craftsman left in it, stainless steel working is "just plumbing". Which is funny, when you think about it.

Society internalises knowledge just like each of us individuals do. The class system of engineering... where being a "grease monkey" is a stereotype we believe about people who work on machinery that defines our whole existence. How high regard to we hold the carbon composites magician over the lowly welder...

Anyone who manages to flip those tables, is okay with me.

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Offline OxCartMark

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #950 on: 02/12/2019 03:08 pm »
Atlas sheet metal. All resistance welded from flat sheet less than .5 mm thick It all came into the factory as great rolls about 3 ft wide.
So I'm curious, was there no step to form the cut flat gores to be portions of a spherical surface?  Or did they weld them flat and then pressurize the structure to stretch form it into a more true hemisphere?  Or is it in the end still a number of panels that are flat in one dimension (at least when not pressurized)?

Offline livingjw

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #951 on: 02/12/2019 04:00 pm »
Atlas sheet metal. All resistance welded from flat sheet less than .5 mm thick It all came into the factory as great rolls about 3 ft wide.
So I'm curious, was there no step to form the cut flat gores to be portions of a spherical surface?  Or did they weld them flat and then pressurize the structure to stretch form it into a more true hemisphere?  Or is it in the end still a number of panels that are flat in one dimension (at least when not pressurized)?

- I am pretty sure I saw a picture of a gore stretch former which grabbed two ends of a piece of sheet with hydraulic clamps and pulled it over a rounded surface.

- It also appears that, at some point, they must pressurize the tanks, because the wrinkles you see during manufacturing disappear when they are done.

John

« Last Edit: 02/12/2019 04:03 pm by livingjw »

Offline su27k

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #952 on: 02/12/2019 04:01 pm »
Lots of Atlas factory photos can be seen here: https://www.flickr.com/search/?user_id=49487266%40N07&view_all=1&text=atlas%20factory

Although without good captions it's hard to know what is what.

Offline Coastal Ron

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #953 on: 02/12/2019 04:21 pm »
And while building large things out of stainless steel is a known quantity, building large things out of carbon composite is a known quantity too. Boeing builds entire airliners out of carbon composites, on a production line, so it's a mature building material for large structures.

I'm just hoping that you're reading that last sentence in a sarcastic voice.  ;D

No, I was using my straight and serious voice...

{self-edit snip}

Suffice it to say you are imagining things I never said.
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Online rsdavis9

Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #954 on: 02/12/2019 04:41 pm »

I wonder about the utility of having a twin wall skin at all.  Why not just have an inner tank which holds propellant for landing.  Propellant from the inner tank is sprayed onto the inside of the since outer wall, where it vaporizes.  The vapor overpressure from the larger tanks are vented through gas-gas engines firing into the edges of the bow shock, increasing the effective radius of curvature of the vehicle and causing the bow shock to stand off further.

When I first heard about steel and active cooling this was my first thought of how to design it. It seems that spraying the liquid methane at the tank walls is the simplest way to do it.

One complication is what to do about the cargo section.
1. Have a double wall there with enough room to spray coolant. (seems doable)
2. Have a methane tank that extends the full length of the vehicle.
a. hard to design a inter tank dome vertically the length of the ship.

Wouldn't you need to vent the methane along the windward centerline to get the best effect from transpirational cooling?
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Offline RobLynn

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #955 on: 02/12/2019 07:12 pm »
Floating outer skin.
Only attached at edges.
Not attached to stringers.
Maybe some sort of squishable gasket to hold the floating outer skin in place so it doesn't rattle.
I haven't done the calc but it seems a couple of mm's extra radius should make up for the expansion.

We've had hints of a design using a much hotter SS310 outer skin from Elon tweets and SS has a pretty high expansion ratio, so probably has to be overlapped foil scales/shingles if that is the case - though seems to me that would make the overlapped areas dicey for cooling.  Or could have overlaps in axial direction only, with wavey corrugation circumferentially to accommodate differential expansion (like parts of SR71A skin used) - that would have the secondary benefit of stiffening the scales in the axial direction for greater distance between supports using thinner 310 foil.  Gets rather complicated around curvature of nose and fin roots.

I am a bit surprised that (if there is) a foil outer hot skin makes sense, radiative heat transfer at those temperatures is so high that intuitively it would save very little cooling, 10's of kW/m² will be radiated through to the second layer, and it makes construction far more complex, though I guess they could employ a layer of ceramic fibre insulation between them to stop that radiative heat transfer (more complex again).  A near-isothermal sandwich construction wall that doesn't suffer from differential expansion stresses but uses a bit more methane coolant seems like an easier option to get flying sooner.
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Online rakaydos

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #956 on: 02/12/2019 07:21 pm »
Floating outer skin.
Only attached at edges.
Not attached to stringers.
Maybe some sort of squishable gasket to hold the floating outer skin in place so it doesn't rattle.
I haven't done the calc but it seems a couple of mm's extra radius should make up for the expansion.

We've had hints of a design using a much hotter SS310 outer skin from Elon tweets and SS has a pretty high expansion ratio, so probably has to be overlapped foil scales/shingles if that is the case - though seems to me that would make the overlapped areas dicey for cooling.  Or could have overlaps in axial direction only, with wavey corrugation circumferentially to accommodate differential expansion (like parts of SR71A skin used) - that would have the secondary benefit of stiffening the scales in the axial direction for greater distance between supports using thinner 310 foil.  Gets rather complicated around curvature of nose and fin roots.

I am a bit surprised that (if there is) a foil outer hot skin makes sense, radiative heat transfer at those temperatures is so high that intuitively it would save very little cooling, 10's of kW/m² will be radiated through to the second layer, and it makes construction far more complex, though I guess they could employ a layer of ceramic fibre insulation between them to stop that radiative heat transfer (more complex again).  A near-isothermal sandwich construction wall that doesn't suffer from differential expansion stresses but uses a bit more methane coolant seems like an easier option to get flying sooner.

We may be overthinking it. a foot of expansion over the length of the rocket is like 4 inches over the width of the rocket, 2 inches of expansion to each side. That's not too bad for a flexible connection.

Offline JonathanD

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #957 on: 02/12/2019 07:35 pm »
I get the re-entry cooling concept for the main body of Starship, what in the world are the fins and canards are going to be made out of?  They are going to be subject to some pretty rough re-entry temps.

Offline king1999

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Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #958 on: 02/12/2019 08:29 pm »
I get the re-entry cooling concept for the main body of Starship, what in the world are the fins and canards are going to be made out of?  They are going to be subject to some pretty rough re-entry temps.

Maybe they can use SX500 for the fin and canard edges?

Online rsdavis9

Re: SpaceX Starship (BFS) Engineering General Thread 2
« Reply #959 on: 02/12/2019 08:31 pm »
I get the re-entry cooling concept for the main body of Starship, what in the world are the fins and canards are going to be made out of?  They are going to be subject to some pretty rough re-entry temps.

I would assume more of the same. More cooling flow.

So many options of how to use outer metal skin and cool it.
It will be very interesting to see what they finally decide on.

EDIT:
1. double skin. Let outer skin get hot. Increases radiative cooling.
2. Single skin. Cool from inside.
3. Inject cool gas to outside to deflect reentry plasma stream.


« Last Edit: 02/12/2019 08:44 pm by rsdavis9 »
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.

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