Author Topic: Elon has changed BFR design again - what does this mean  (Read 520663 times)

Offline Mongo62

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Re: Elon has changed BFR design again - what does this mean
« Reply #1020 on: 12/09/2018 04:07 am »
So I'm wondering if the switch to stainless is enabled by the switch to skydiver reentry..

Do you have information that the switch is to stainless steel, specifically?

The paper that Elon liked on Twitter:

Enhanced strength and ductility in a high-entropy alloy via ordered oxygen complexes

The report is paywalled, but the following is from the publicly viewable abstract:

Unlike traditional interstitial strengthening, such ordered interstitial complexes lead to unprecedented enhancement in both strength and ductility in compositionally complex solid solutions, the so-called high-entropy alloys (HEAs). The tensile strength is enhanced (by 48.5 ± 1.8 per cent) and ductility is substantially improved (by 95.2 ± 8.1 per cent) when doping a model TiZrHfNb HEA with 2.0 atomic per cent oxygen, thus breaking the long-standing strength-ductility trade-off.

Of course, the alloy(s) to be used are likely to be different from the example alloy above, but Elon's remark about it being a heavy metal suggests a dense but extremely strong non-ferric metal alloy to me.
« Last Edit: 12/09/2018 04:23 am by Mongo62 »

Offline speedevil

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Re: Elon has changed BFR design again - what does this mean
« Reply #1021 on: 12/09/2018 04:24 am »
Of course, the alloy(s) to be used are likely to be different from the example alloy above, but Elon's remark about it being a heavy metal suggests a dense but extremely strong non-iron-based material to me.
Stainless Steel  is very heavy compared to either Al-Li or carbon composite.
Also, very exotic novel alloys with no demonstrated use at large tank scale are not very compatible perhaps with 'flying in 2019'.

Offline meekGee

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Re: Elon has changed BFR design again - what does this mean
« Reply #1022 on: 12/09/2018 04:42 am »
So I'm wondering if the switch to stainless is enabled by the switch to skydiver reentry..

Do you have information that the switch is to stainless steel, specifically?

The paper that Elon liked on Twitter:

Enhanced strength and ductility in a high-entropy alloy via ordered oxygen complexes

The report is paywalled, but the following is from the publicly viewable abstract:

Unlike traditional interstitial strengthening, such ordered interstitial complexes lead to unprecedented enhancement in both strength and ductility in compositionally complex solid solutions, the so-called high-entropy alloys (HEAs). The tensile strength is enhanced (by 48.5 ± 1.8 per cent) and ductility is substantially improved (by 95.2 ± 8.1 per cent) when doping a model TiZrHfNb HEA with 2.0 atomic per cent oxygen, thus breaking the long-standing strength-ductility trade-off.

Of course, the alloy(s) to be used are likely to be different from the example alloy above, but Elon's remark about it being a heavy metal suggests a dense but extremely strong non-ferric metal alloy to me.
Do not....  Was speculated upthread, but this seems to fit.

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Offline meekGee

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Re: Elon has changed BFR design again - what does this mean
« Reply #1023 on: 12/09/2018 04:48 am »
This switch didn't happen overnight, right?  This must have been a plan B all along, and at some point recently plan B became plan A.

Which means that somewhere SpaceX has advanced proof articles for this method of construction...

But now it appears they may have built an entire flight article.  Without anyone knowing.

Where?  How will it get to BC?

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Offline niwax

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Re: Elon has changed BFR design again - what does this mean
« Reply #1024 on: 12/09/2018 05:05 am »
Or they (or Elon) think they can build a flight article in a month. They now make F9 at something like 20 days each, so it's theoretically possible if they have machinery from previous tests. i think we'll get something like the dearmoon section shot as well as an updated render.
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Offline MATTBLAK

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Re: Elon has changed BFR design again - what does this mean
« Reply #1025 on: 12/09/2018 05:11 am »
If they're switching from composites to stainless steel; does that reduce the net payload of BFR to low Earth orbit from about 100 metric tons to less than 90? Or even 80?! :(

...Still, if less than 90 tonnes; that would still kick SLS butt because of full reusability...
« Last Edit: 12/09/2018 05:13 am by MATTBLAK »
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Offline tater

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Re: Elon has changed BFR design again - what does this mean
« Reply #1026 on: 12/09/2018 05:15 am »
Or Elon is less persnickety about using the word "render" (instead of "pics") than people in this forum.
« Last Edit: 12/09/2018 05:22 am by tater »

Offline oldAtlas_Eguy

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Re: Elon has changed BFR design again - what does this mean
« Reply #1027 on: 12/09/2018 05:18 am »
If they're switching from composites to stainless steel; does that reduce the net payload of BFR to low Earth orbit from about 100 metric tons to less than 90? Or even 80?! :(

...Still, if less than 90 tonnes; that would still kick SLS butt because of full reusability...
You cannot make the assumption that dry weight will increase.

One of the reasons for going this direction is weight reduction. Counter intuitive is that a heavier material results in total lower mass.
« Last Edit: 12/09/2018 05:22 am by oldAtlas_Eguy »

Offline GWH

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Re: Elon has changed BFR design again - what does this mean
« Reply #1028 on: 12/09/2018 05:25 am »
Why does everyone keep saying stainless steel? Is there any ACTUAL evidence of that?
Titanium is much heavier than Aluminum,  could be that or any other material until we actually hear something definite....

Offline MATTBLAK

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Re: Elon has changed BFR design again - what does this mean
« Reply #1029 on: 12/09/2018 05:31 am »
If they're switching from composites to stainless steel; does that reduce the net payload of BFR to low Earth orbit from about 100 metric tons to less than 90? Or even 80?! :(

...Still, if less than 90 tonnes; that would still kick SLS butt because of full reusability...
You cannot make the assumption that dry weight will increase.

One of the reasons for going this direction is weight reduction. Counter intuitive is that a heavier material results in total lower mass.
Yes true. But I somehow doubt that they'll be going down the the 'Atlas balloon' tank road. Maybe Elon has just encountered the sheer difficulties of trying to fabricate gargantuan 'bubble-free' composites and has decided to take a leaf out of aircraft and ship building techniques? I'm just sayin' - I'm not a trained engineer, but my speculation is not entirely uninformed... Maybe if it results in a - pick a figure from the aether - only 10% or 15% percent increase in dry mass; Elon would be correct in assuming/believing that would be worth it? Certainly if it could speed up development and prototyping somewhat.
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Offline oldAtlas_Eguy

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Re: Elon has changed BFR design again - what does this mean
« Reply #1030 on: 12/09/2018 05:38 am »
If they're switching from composites to stainless steel; does that reduce the net payload of BFR to low Earth orbit from about 100 metric tons to less than 90? Or even 80?! :(

...Still, if less than 90 tonnes; that would still kick SLS butt because of full reusability...
You cannot make the assumption that dry weight will increase.

One of the reasons for going this direction is weight reduction. Counter intuitive is that a heavier material results in total lower mass.
Yes true. But I somehow doubt that they'll be going down the the 'Atlas balloon' tank road. Maybe Elon has just encountered the sheer difficulties of trying to fabricate gargantuan 'bubble-free' composites and has decided to take a leaf out of aircraft and ship building techniques? I'm just sayin' - I'm not a trained engineer, but my speculation is not entirely uninformed... Maybe if it results in a - pick a figure from the aether - only 10% or 15% percent increase in dry mass; Elon would be correct in assuming/believing that would be worth it? Certainly if it could speed up development and prototyping somewhat.
A small case in point. The F9 is a 50/50 stiff structure and balloon tank structural design.

It is strong enough to be unpreassurized in 1g but is pressurized for flight to handle flight loads.

Offline meekGee

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Re: Elon has changed BFR design again - what does this mean
« Reply #1031 on: 12/09/2018 05:48 am »
Or Elon is less persnickety about using the word "render" (instead of "pics") than people in this forum.
Because you don't typically announce you're about to release the renders of the suborbital first test article...

How many renders of Grasshopper have you seen?

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Offline gin455res

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Re: Elon has changed BFR design again - what does this mean
« Reply #1032 on: 12/09/2018 05:48 am »
Does this make it possible for the mass of the initial Starships (left on Mars) to be used as raw material for base construction?  Easier to cut into sheet or even melt down?


Additionally, if the Super Heavy is also stainless, does this mean staging higher is possible, as the Super Heavy is more heat tolerant and can do sportier re-entries?

Offline lrk

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Re: Elon has changed BFR design again - what does this mean
« Reply #1033 on: 12/09/2018 06:02 am »
Additionally, if the Super Heavy is also stainless, does this mean staging higher is possible, as the Super Heavy is more heat tolerant and can do sportier re-entries?

Doing RTLS does impose a limit to the practical S1 dV.  It would already be flying a rather lofted trajectory to help minimize the boostback burn, but at a certain point you really start to eat the losses from the rocket equation (and a worse propellant mass fraction doesn't help.)  So I really doubt the constraint is re-entry. 

Offline oldAtlas_Eguy

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Re: Elon has changed BFR design again - what does this mean
« Reply #1034 on: 12/09/2018 06:06 am »
Additionally, if the Super Heavy is also stainless, does this mean staging higher is possible, as the Super Heavy is more heat tolerant and can do sportier re-entries?

Doing RTLS does impose a limit to the practical S1 dV.  It would already be flying a rather lofted trajectory to help minimize the boostback burn, but at a certain point you really start to eat the losses from the rocket equation (and a worse propellant mass fraction doesn't help.)  So I really doubt the constraint is re-entry.
The SH would likely share the same tank design and tooling so reentry would not be the problem.

Offline meekGee

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Re: Elon has changed BFR design again - what does this mean
« Reply #1035 on: 12/09/2018 06:14 am »
Additionally, if the Super Heavy is also stainless, does this mean staging higher is possible, as the Super Heavy is more heat tolerant and can do sportier re-entries?

Doing RTLS does impose a limit to the practical S1 dV.  It would already be flying a rather lofted trajectory to help minimize the boostback burn, but at a certain point you really start to eat the losses from the rocket equation (and a worse propellant mass fraction doesn't help.)  So I really doubt the constraint is re-entry.
The size of the tanks on BFS is also dictated by Mars flight dV considerations...  So it might be larger than optimal for just LEO considerations.

And then there's the tanker..  will it have dedicated transfer tanks or just stretch tanks? Or will they all have stretch tanks?

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Offline docmordrid

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Re: Elon has changed BFR design again - what does this mean
« Reply #1036 on: 12/09/2018 06:29 am »
Summary,

Everyday Astronaut
@Erdayastronaut
Replying to @elonmusk and 3 others
It seems like youve made some really big changes recently. Is this why we didnt get a big technical rundown at #dearmoon or IAC this year? So whats with the big mandrels at the port? Or those carbon tanks shown off for the past couple years?
|
Elon Musk ✔ @elonmusk
Yes
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Elon Musk ✔ @elonmusk
But cool pics of the demo Starship that will fly suborbital hops coming in ~4 weeks
|
Malcolm Head @Malcolmmarsman
Replying to @elonmusk and 4 others
Is Super Heavy in development at all, or just starship right now?
|
Elon Musk ✔ @elonmusk
Both, but demo Starship is being built now, whereas Super Heavy hardware will start getting built in spring
10:42 PM - Dec 8, 2018

https://twitter.com/elonmusk/status/1071610949432356864
« Last Edit: 12/09/2018 06:31 am by docmordrid »
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Offline mikelepage

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Re: Elon has changed BFR design again - what does this mean
« Reply #1037 on: 12/09/2018 06:31 am »
The thought just occurred to me.  If you're building Starship out of stainless steel, how much easier (conceptually at least) does it become to imagine building Starship on Mars:o  Or at least being able to repair it.
« Last Edit: 12/09/2018 06:47 am by mikelepage »

Offline docmordrid

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Re: Elon has changed BFR design again - what does this mean
« Reply #1038 on: 12/09/2018 06:39 am »
I don't think we can ignore those high entropy alloys in the PhysOrg article Musk liked,

https://phys.org/news/2018-11-strength-ductility-high-entropy-alloy-oxygen.html
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Offline meekGee

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Re: Elon has changed BFR design again - what does this mean
« Reply #1039 on: 12/09/2018 07:05 am »
I don't think we can ignore those high entropy alloys in the PhysOrg article Musk liked,

https://phys.org/news/2018-11-strength-ductility-high-entropy-alloy-oxygen.html
I didn't get the impression that these alloys were mature enough to be available for manufacturing already.

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