Author Topic: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)  (Read 609627 times)

Offline Lar

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1300 on: 02/22/2018 08:36 pm »
I think Zubrin just wants to live to see boots on Mars. So anything that can go there NOW rather than in 10 years will have his support. Because he has seen too many of the 10 year estimates always remain 10 years away.

Anyway, I think he is wrong and Musk is clearly right. This time 10 years might really be just 10 years away.

10 MUSK years but those are shorter than NASA years...
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline raketa

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1301 on: 02/22/2018 08:54 pm »
No this is the first time we will have a transport system that we could afford without blowing USA budget.

Offline speedevil

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1302 on: 02/22/2018 09:09 pm »
Good point. We have had the tools since the 60's to go to Mars. But not the political will to spend the money.

I would somewhat argue there has been the political will to spend the money, (the current and past NASA budget).
Even the occasional interest.

The toxic group-think driven by contractors that would like to make $500M, not $50M rockets, and history driven by a program 'wasting anything but time' lead to any serious effort into making Mars happen dying.

And by serious effort, I do not mean proposing ISRU, nuclear, or complex technical schemes to reduce launched mass, but reducing the cost of launch in non-fragile ways.

x-33/venturestar is perhaps the best example of this, a vehicle which can only work if three untried at the time technologies had worked well.

(metallic TPS, linear aerospike, conformal composite hydrogen tanks).

By non-fragile, I mean not requiring any single technology to work well beyond the state of the art.

BFS - at least the 2017 variant is lots less fragile systemically than V*.
There are several major risks that might be considered.

Graphite structure:
Swap to Al/Li, for either stage, or end up at double the target structural mass
Raptor:
If you get no better thrust than has already been announced, and it explodes occasionally.
Tanks:
End up needing to line the tanks.
Heatshield:
Ends up only capable to 5km/s.

None of these alone, or even some combinations happening kill the project, and it is still an enormous advance in $/kg, and $/launch.

V*, shuttle, ... all had such narrow margins of success that one subsystem underperforming badly would kill the project, meaning 'failure is not an option' and exploding the costs even before questions of efficiency of contractors or cost+ or ... is raised.



Offline oiorionsbelt

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1303 on: 02/22/2018 10:12 pm »
10 MUSK years but those are shorter than NASA years...
It's good to hear it put this way. The whole "time dilation/Elon time" criticism is tiresome and this gives perspective.

10 MUSK years but those are shorter than NASA years...
It's good to hear it put this way. The whole "time dilation/Elon time" criticism is tiresome and this gives perspective.
Many of his 'aspirational' targets are for things that wouldn't even be on the horizon (or happen at all) if it wasn't for his companies. That's why I'm a bit baffled when people dismiss him entirely for the delays.
The worst go from 'This will never happen, how can you take him seriously?' to 'Yeah, it happened, but it took 4 years instead of 2, how can you take him seriously?'
Some people like to only see the negative side I guess, even when the positive prevails...
Failure is not only an option, it's the only way to learn.
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Offline Kansan52

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1305 on: 02/22/2018 11:03 pm »
Speedevil, you make good points regarding the budget that has prevailed for decades.

The post Lunar Landing NASA budget was contracting. We had already paid for most of the development. NASA wanted to continue to fly the Saturn V with improvements, NERVA was on deck for orbital tests, a space station that could be used a Mars staging area, and so on.

Not only was there no political will to continue to Mars it was forbidden to even make plans to go to Mars. So the Saturn V was eliminated. NERVA was eliminated. It was shuttle or space station but not both. No more Moon. No Mars attempt. The budget was slashed.

Ever since, start, stop, don't listen to engineers, sorry, starting to rant. IMO on all of the post.

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1306 on: 02/22/2018 11:14 pm »

Zubrin, along with most of the Space old guard just don’t take BFR/BFS seriously, it’s too outside the paradigm. What SpaceX says it’s doing is mass producing general purpose Spaceships the size of Jumbo airliners that cost about the same to build and operate. They don’t buy that.

The BFR/BFS approach doesn’t scale down to what would have seemed reasonable, starting with a few people and small specialized very expensive ships. If it works, it works by entirely transforming the scale and assumptions of the industry. Otherwise it just crashes and burns.
Just a reminder. "Mars Direct" baseline was a 120t payload Shuttle Derived LV and he would have liked the 140t of a Saturn V, but obviously that was not viable.

So BFS at 150t seems like a pretty good fit for how he thinks a Mars programme could (or should) be done.

FH --> Bit too small BFS--> Just right.

x-33/venturestar is perhaps the best example of this, a vehicle which can only work if three untried at the time technologies had worked well.

(metallic TPS, linear aerospike, conformal composite hydrogen tanks).
And still haven't been flight tested.
Quote from: speedevil
By non-fragile, I mean not requiring any single technology to work well beyond the state of the art.

BFS - at least the 2017 variant is lots less fragile systemically than V*.
There are several major risks that might be considered.

Graphite structure:
Swap to Al/Li, for either stage, or end up at double the target structural mass
Raptor:
If you get no better thrust than has already been announced, and it explodes occasionally.
Tanks:
End up needing to line the tanks.
Heatshield:
Ends up only capable to 5km/s.

None of these alone, or even some combinations happening kill the project, and it is still an enormous advance in $/kg, and $/launch.

V*, shuttle, ... all had such narrow margins of success that one subsystem underperforming badly would kill the project, meaning 'failure is not an option' and exploding the costs even before questions of efficiency of contractors or cost+ or ... is raised.
I think you severely underestimate how inter related all the aspects of BFS are. 
VTOL systems always have quite narrow margins. You haven't even gotten onto the effects and issues of the wings. Shuttle managed a payload fraction of less than 1.5% of the whole Shuttle stack, BFS's goal is much higher.
Reverting back to LiAl will seriously cut the payload. We know the payload loss due to the TPS mass and additional stiffening killed the idea of recovering the F9 US so if that's too heavy BFS payload will go down a lot.

Basically SX wouldn't be planning new TPS and new structural materials and new engine cycles and new engine fuels if what worked for F9 or even FH would get the job done. They innovate when necessary.

The fact that BFS has so much innovation says it's all needed to fly the mission. Some might under perform and you end up with a lower payload but you don't need much to seriously compromise a design. It's an open question how much you could sacrifice before the design is no longer viable. 1 tonne? 10tonnes? 50tonnes?

Remember that NASA did a "Weight scrub" on Shuttle to lose about 15% of its mass because (IIRC) the SSME was 3secs below target Isp and the SRBs were 2secs below target Isp.
MCT ITS BFR SS. The worlds first Methane fueled FFSC engined CFRP SS structure A380 sized aerospaceplane tail sitter capable of Earth & Mars atmospheric flight.First flight to Mars by end of 2022 TBC. T&C apply. Trust nothing. Run your own #s "Extraordinary claims require extraordinary proof" R. Simberg."Competitve" means cheaper ¬cheap SCramjet proposed 1956. First +ve thrust 2004. US R&D spend to date > $10Bn. #deployed designs. Zero.

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1307 on: 02/22/2018 11:23 pm »
Many of his 'aspirational' targets are for things that wouldn't even be on the horizon (or happen at all) if it wasn't for his companies. That's why I'm a bit baffled when people dismiss him entirely for the delays.
The worst go from 'This will never happen, how can you take him seriously?' to 'Yeah, it happened, but it took 4 years instead of 2, how can you take him seriously?'
Some people like to only see the negative side I guess, even when the positive prevails...
What they are failing to take into account is that SX is not publicly quoted and he is (AFAIK) its majority stockholder by far. That allows them to pivot when new data invalidates old plans.

That also means he does not have to manage the expectations of the vast army of Wall Street analysts, pundits and pension funds who can send a stock price soaring or plummeting on a daily basis, or return a constantly rising dividend to stop owners selling out and the stock price falling. 

Such people also grossly underestimate the focus of his determination to do this, one way or another and his ability to see the way diverse developments build (or can be made to build) on each other.
MCT ITS BFR SS. The worlds first Methane fueled FFSC engined CFRP SS structure A380 sized aerospaceplane tail sitter capable of Earth & Mars atmospheric flight.First flight to Mars by end of 2022 TBC. T&C apply. Trust nothing. Run your own #s "Extraordinary claims require extraordinary proof" R. Simberg."Competitve" means cheaper ¬cheap SCramjet proposed 1956. First +ve thrust 2004. US R&D spend to date > $10Bn. #deployed designs. Zero.

Offline speedevil

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1308 on: 02/23/2018 12:11 am »
Graphite structure:
Swap to Al/Li, for either stage, or end up at double the target structural mass
Raptor:
If you get no better thrust than has already been announced, and it explodes occasionally.
Tanks:
End up needing to line the tanks.
Heatshield:
Ends up only capable to 5km/s.

None of these alone, or even some combinations happening kill the project, and it is still an enormous advance in $/kg, and $/launch.BFS's goal is much higher.
Reverting back to LiAl will seriously cut the payload. We know the payload loss due to the TPS mass and additional stiffening killed the idea of recovering the F9 US so if that's too heavy BFS payload will go down a lot.

Basically SX wouldn't be planning new TPS and new structural materials and new engine cycles and new engine fuels if what worked for F9 or even FH would get the job done. They innovate when necessary.

The fact that BFS has so much innovation says it's all needed to fly the mission. Some might under perform and you end up with a lower payload but you don't need much to seriously compromise a design. It's an open question how much you could sacrifice before the design is no longer viable. 1 tonne? 10tonnes? 50tonnes?

That depends how you define 'the mission'.

If you narrowly define it as the IAC 2017 presentation to Mars, yes, it pretty much all has to work.

There are various 'failures' that might lead to a hundred, not a hundred and fifty tons on Mars, or ten, not five launches per BFS to Mars.

For example, if the ship is fifty tons overweight, or it needs fifty tons extra fuel to brake because the heatshield isn't up to spec, it's not a mission failure. (you may need three ships, not two to get to Mars with the same payload, or refueling in a higher energy orbit)

If the heatshield works, but is not up to earth entry from solar orbit, it may be needed to catch it after a light aerobraking pass and refuelling a tanker in HEO to get to it.

If launch is actually cheap, and it can transfer fuel in orbit, a lot of these failures go from 'welp, that can't work', to taking ten, not five launches.

Edit/Lar: Tried to fix the quotes. Might have botched it.
« Last Edit: 02/23/2018 12:39 am by Lar »

Offline hkultala

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1309 on: 02/23/2018 03:20 am »

Reverting back to LiAl will seriously cut the payload. We know the payload loss due to the TPS mass and additional stiffening killed the idea of recovering the F9 US so if that's too heavy BFS payload will go down a lot.

No, we do not know that.

The official explanation to what killed reuse of F9 US is the BFR/BFS that is looming around the corner, concentrating on that and saving the development resources for that.


« Last Edit: 02/23/2018 04:05 am by hkultala »

Online dglow

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1310 on: 02/23/2018 03:30 am »
His core argument is that we've always been able to do Mars, even with the launchers we had (or could derive from what we had) readily available. Simple vehicles driven by more launches off smaller rockets... vs giant rockets flying very, very complex vehicles.
I think you have that backwards. Watching his presentation suggests more "viable (big) rockets regularly launching a minimal architecture with the least possible number of special units."

I hear you and take your point. Mine was more longitudinal: proposing Saturn V could take us to Mars when conventional wisdom said we needed Nova. Or that we could move faster via a Shuttle derivation than waiting for something newer and bigger like NLS.

Quote
The key single point of failure seemed to be the 100Kw nuclear reactor. Today Kilopwer promises much more "granular" units at 10Kw each (so no single point of failure) with a real shot at being built and flying.
My money is on the first BFR ISRU plant using nuclear, possibly supplemented by solar. Elon will only publicly admit to solar for the foreseeable future, simply to avoid the public shitstorm education that we'll all endure before the first reactors are launched.

Quote
TBH with the description of the BFR/BFS, initial flights using at least 2 BFS's at a time (with a goal to grow the fleet over time) and the possibility of Kilopower reactors going with them, I'd think Zubrin was feeling that Musk and SX are seeing things more his way. Big rockets with a few big payloads used repeatedly.  I could see him still being concerned about prioritizing short journey times over a fail-safe free return trajectory, but that could change before first flight.
Yes

Quote
I think what causes problems is Zubrins willingness to admit that that in fact NASA cannot have everything.". NASA has a finite budget and if you want to actually send people out (rather than talking about sending someone out) you have to make hard (and sometimes very hard) choices.  :(   
That is a very uncomfortable message for a lot of people to hear.
Indeed. Zubrin is every bit the hot-blooded, wild-eyed visionary. Makes Elon look like a diplomat by comparison.
« Last Edit: 02/23/2018 03:32 am by dglow »

Offline speedevil

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1311 on: 02/23/2018 04:24 am »
Remember that NASA did a "Weight scrub" on Shuttle to lose about 15% of its mass because (IIRC) the SSME was 3secs below target Isp and the SRBs were 2secs below target Isp.

A more worked through example with underperforming engines.

Taking a BFR with the nominal published liftoff mass of 4400 tons, and assuming that is a 150 ton payload, leads to a BFR with a fuelled mass of some 3045 tons, and assuming the same dry mass ratio as BFS leads to 255 tons dry.
700m/s of landing delta-v comes to 314 tons including landing fuel.

At staging, the stack weight is this + 1355 (150 ton payload, 85t structure, 20t landing fuel)
So, 4400->1669, with ISP of 330 for SL raptors gives 3134m/s.

Second stage comes out to 255t in LEO.
1355 -> 255, with ISP of 375 = 6138m/s.

Or 9272m/s.

In order to 'only' have a 50 ton payload to the same delta-v, the second stage vacuum raptors can be replaced with sea-level merlins, with a little margin.

Use vacuum merlins, and it's up at 120 tons.

Replacing the booster raptors with merlins drops the delta-v from 3134 to 2678, 456m/s.

Redoing the sums comes out with the thing still launching 59 tons (79 if you don't need to land) (with merlins throughout)

The most important part here is that much, or most of what's going to Mars is not unitary payloads.
Even if the raptors underperform so they are no better than flying merlins, Mars is not much affected, as you can launch 79 tons of payload to orbit in one lump.

I think it's going to be quite hard to find a design where you don't have 71 tons of movable stores, tank contents, ... in the nominal 150 ton payload.
Yes, it's more annoying, and you now need to launch about 18 tankers, not 6 to fill BFS, and you do need an extra two flights to launch cargo, and you are going to have to man (or robot) handle it over.

And yes, if you do not fix this by the time you get crew on Mars, it about doubles your ISRU needs as you need two BFS to get back to earth (one retanks the other in orbit and relands).

Without the ability to on-orbit refuel, and in this case, transship cargo, any tiny mass growth balloons into inevitable loss of capability really fast.

With it, and without a specific unitary payload that absolutely must be launched, there is flexibility for really quite large underperformance.


This doesn't of course work at all if the 12 extra launches cost $100M each.

In addition to any built in margin on the above structures, the 20t of landing fuel lets 20t of weight growth happen, or 20s underperformance of the s2 engines, or 300m/s lower booster speed, 200t of booster weight growth, with no penalty at all to the nominal unitary launch performance.

You just need to refuel.

Almost anything else other than the ability to be able to refuel, and rapidly turn around the vehicle can be forgiven.

(I am not above suggesting kerosene is a suitable propellant for BFR, especially to BEO, it would raise all sorts of issues, just a raptor underperforming that badly)
« Last Edit: 02/23/2018 04:47 am by speedevil »

Offline envy887

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1312 on: 02/23/2018 04:45 am »
Remember that NASA did a "Weight scrub" on Shuttle to lose about 15% of its mass because (IIRC) the SSME was 3secs below target Isp and the SRBs were 2secs below target Isp.

A more worked through example with underperforming engines.

Taking a BFR with the nominal published liftoff mass of 4400 tons, and assuming that is a 150 ton payload, leads to a BFR with a fuelled mass of some 3045 tons, and assuming the same dry mass ratio as BFS leads to 255 tons dry.
700m/s of landing delta-v comes to 314 tons including landing fuel.

At staging, the stack weight is this + 1355 (150 ton payload, 85t structure, 20t landing fuel)
So, 4400->1669, with ISP of 330 for SL raptors gives 3134m/s.

Second stage comes out to 255t in LEO.
1355 -> 255, with ISP of 375 = 6138m/s.

Or 9272m/s.

In order to 'only' have a 50 ton payload to the same delta-v, the second stage vacuum raptors can be replaced with sea-level merlins, with a little margin.

Use vacuum merlins, and it's up at 120 tons.

Replacing the booster raptors with merlins drops the delta-v from 3134 to 2678, 456m/s.

Redoing the sums comes out with the thing still launching 59 tons (79 if you don't need to land)

The most important part here is that much, or most of what's going to Mars is not unitary payloads.
Even if the raptors underperform so they are no better than flying merlins, Mars is not much affected, as you can launch 79 tons of payload to orbit in one lump.

I think it's going to be quite hard to find a design where you don't have 71 tons of movable stores, tank contents, ... in the nominal 150 ton payload.
Yes, it's more annoying, and you now need to launch about 18 tankers, not 6 to fill BFS, and you do need an extra two flights to launch cargo, and you are going to have to man (or robot) handle it over.

And yes, if you do not fix this by the time you get crew on Mars, it about doubles your ISRU needs as you need two BFS to get back to earth (one retanks the other in orbit and relands).

Without the ability to on-orbit refuel, and in this case, transship cargo, any tiny mass growth balloons into inevitable loss of capability really fast.

With it, and without a specific unitary payload that absolutely must be launched, there is flexibility for really quite large underperformance.


This doesn't of course work at all if the 12 extra launches cost $100M each.

In addition to any built in margin on the above structures, the 20t of landing fuel lets 20t of weight growth happen, or 20s underperformance of the s2 engines, or 300m/s lower booster speed, with no penalty at all to the nominal unitary launch performance.

You just need to refuel.

Almost anything else other than the ability to be able to refuel, and rapidly turn around the vehicle can be forgiven.

(I am not above suggesting kerosene is a suitable propellant for BFR, especially to BEO, it would raise all sorts of issues, just a raptor underperforming that badly)

You are assuming the tanker has the same dry mass and payload to orbit as the crew ship? That's unnecessarily conservative.

The booster does need more than 700 m/s to land, but it also won't have the same dry mass fraction as the ship. The F9 booster is already almost 40% better mass fraction than the BFS.

I really like this type of sensitivity analysis. It's very useful to find the weak points in the design. A spreadsheet that calculates everything for a given under performance would be neat.

Offline speedevil

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1313 on: 02/23/2018 04:55 am »
You are assuming the tanker has the same dry mass and payload to orbit as the crew ship? That's unnecessarily conservative.

The booster does need more than 700 m/s to land, but it also won't have the same dry mass fraction as the ship. The F9 booster is already almost 40% better mass fraction than the BFS.

I really like this type of sensitivity analysis. It's very useful to find the weak points in the design. A spreadsheet that calculates everything for a given under performance would be neat.

I was mostly basing the booster landing on estimates of F9 landing delta-v, and using the IAC mars as conservative.
I think the mars 700m/s or so delta-v is likely overkill on earth for the top stage.
Why do you think the booster is likely to need more delta-v?
Part of the reason for not assuming lower masses for parts is to allow for it to be rugged enough for lots of launches.

The (poorly titled) "BFS, how bad can it be" https://forum.nasaspaceflight.com/index.php?topic=45000 could really do with a spreadsheet - I'm not awake enough to have a stab at it at the moment.

I'm pretty sure it comes out to the above outline though - if you can get it to reliably and rapidly be reused without explosions, a very great amount can be forgiven in the other technical aspects that might be 'failure' with a vehicle designed to launch specific payloads.

Perversely, the very lack of a clue about what to put on Mars actually helps.
If you're not very deep down the Martian equipment rabbit-hole when it becomes clear what the actual maximum mass you can launch in one lump is, then it becomes much less of an issue than if you then have the rocket people and the payload people screaming at each other, and them both screaming at the finance people if it's less than planned.


« Last Edit: 02/23/2018 05:04 am by speedevil »

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1314 on: 02/23/2018 07:08 am »
I hear you and take your point. Mine was more longitudinal: proposing Saturn V could take us to Mars when conventional wisdom said we needed Nova. Or that we could move faster via a Shuttle derivation than waiting for something newer and bigger like NLS.
On that basis Saturn V was the lightweight vehicle. He seems a pragmatist. What's available now is way better than something that's (maybe) going to be available in 10 years time. Or not.  Which sounds a pretty smart approach if you actually want to go out there, rather than keep talking about it.  :(
Quote from: dglow
My money is on the first BFR ISRU plant using nuclear, possibly supplemented by solar. Elon will only publicly admit to solar for the foreseeable future, simply to avoid the public shitstorm education that we'll all endure before the first reactors are launched.
Musk is also a pragmatist. He knows SX can't develop a reactor on their own but if a design were to become available (at a reasonable cost to SX, with manageable safety requirements) I'm sure they would incorporate it into their planning. On the Kilopower thread it seems SX are watching the test programme closely and are on good terms with the team.

If it happens I doubt it will be only one however. ISRU is pretty power hungry.  :) Solar is available now. It's strengths and weaknesses are known and can be dealt with. It's the official baseline (insofar as SX ISRU has an official baseline).
However by first Mars launch new options may have become available.  A lot can happen in 4 years, if the Kilopower team can move things along.

Quote from: dglow
Quote
I think what causes problems is Zubrins willingness to admit that that in fact NASA cannot have everything.". NASA has a finite budget and if you want to actually send people out (rather than talking about sending someone out) you have to make hard (and sometimes very hard) choices.  :(   
That is a very uncomfortable message for a lot of people to hear.
Indeed. Zubrin is every bit the hot-blooded, wild-eyed visionary. Makes Elon look like a diplomat by comparison.
I think Zubrin can be quite charming, but I think he does not suffer fools gladly and he'd like people to be a bit more up front. If you want to go to the Moon, say so. Likewise if you want to go Mars....
MCT ITS BFR SS. The worlds first Methane fueled FFSC engined CFRP SS structure A380 sized aerospaceplane tail sitter capable of Earth & Mars atmospheric flight.First flight to Mars by end of 2022 TBC. T&C apply. Trust nothing. Run your own #s "Extraordinary claims require extraordinary proof" R. Simberg."Competitve" means cheaper ¬cheap SCramjet proposed 1956. First +ve thrust 2004. US R&D spend to date > $10Bn. #deployed designs. Zero.

Offline envy887

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1315 on: 02/23/2018 01:51 pm »
You are assuming the tanker has the same dry mass and payload to orbit as the crew ship? That's unnecessarily conservative.

The booster does need more than 700 m/s to land, but it also won't have the same dry mass fraction as the ship. The F9 booster is already almost 40% better mass fraction than the BFS.

I really like this type of sensitivity analysis. It's very useful to find the weak points in the design. A spreadsheet that calculates everything for a given under performance would be neat.

I was mostly basing the booster landing on estimates of F9 landing delta-v, and using the IAC mars as conservative.
I think the mars 700m/s or so delta-v is likely overkill on earth for the top stage.
Why do you think the booster is likely to need more delta-v?
Part of the reason for not assuming lower masses for parts is to allow for it to be rugged enough for lots of launches.

The (poorly titled) "BFS, how bad can it be" https://forum.nasaspaceflight.com/index.php?topic=45000 could really do with a spreadsheet - I'm not awake enough to have a stab at it at the moment.

I'm pretty sure it comes out to the above outline though - if you can get it to reliably and rapidly be reused without explosions, a very great amount can be forgiven in the other technical aspects that might be 'failure' with a vehicle designed to launch specific payloads.

Perversely, the very lack of a clue about what to put on Mars actually helps.
If you're not very deep down the Martian equipment rabbit-hole when it becomes clear what the actual maximum mass you can launch in one lump is, then it becomes much less of an issue than if you then have the rocket people and the payload people screaming at each other, and them both screaming at the finance people if it's less than planned.

The upper stage could probably land with as little as 500 m/s on Earth, though IMO it will be close to 1,000 m/s on Mars.

But the booster needs ~3000 m/s for RTLS (roughly 1800 for boostback, 500 for entry, 700 for landing). Though it could probably be adapted for downrange landing with less than 1,500 m/s. This is obviously already feasible with Merlin and Al-Li tanks, since F9 does it. Raptor and CRFP should be a significant improvement.

Offline speedevil

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1316 on: 02/23/2018 02:55 pm »
But the booster needs ~3000 m/s for RTLS (roughly 1800 for boostback, 500 for entry, 700 for landing). Though it could probably be adapted for downrange landing with less than 1,500 m/s. This is obviously already feasible with Merlin and Al-Li tanks, since F9 does it. Raptor and CRFP should be a significant improvement.

Argh - brain fade.

I guess this is somewhat (totally?) cancelled as though the landing booster needs 2300m/s or so more delta-v than I was calculating for, the initial delta-v I have calculated is similarly wrong, as what I actually calculated was for a booster which will crash with an empty tank after inadequate burns, and the nominal performance requires less delta-v.

And yes, another trade that could be made to recover from underperforming that may not affect Mars much is to recover downrange, and fly back after retanking.
« Last Edit: 02/23/2018 02:58 pm by speedevil »

Offline jpo234

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1317 on: 02/25/2018 09:40 pm »
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 ChrML

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1318 on: 02/27/2018 01:03 am »
No doubts the concept will work. The shuttle was expensive due to the large solid boosters and the huge refurb job needed on the shuttle itself. BFS is tank and go.

Fuel is essentially free, even when this vehicle needs lots of it. Especially if you can in long term produce it with solar energy at no environmental harm.

The huge cost here is the time the vehicle is in use. Sattelite launches and ISS visits will take short time and cost little.

Mars trips will cost more because, like airliners, the ship is useless for anything else floating in interplanetary space. Binding up the ship for 2 years is maybe 10% of its life. Assuming the ship costs $400M to build, it's at least 40M$ cost just for renting the ship. Still cheap compared to old ways, but you cannot go much cheaper than 1M$ per person assuming 100 people.

Offline Lar

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1319 on: 02/27/2018 02:06 am »
That suggests that long term the ships won't cost 400MM USD to build.
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

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