Author Topic: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)  (Read 76737 times)

Offline AncientU

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #20 on: 07/04/2018 09:03 pm »

*Note: For calibration, the BFS is approximately the size and double wet mass of the SLS core stage with approaching double the propulsive thrust.  This is a B-I-G F'n Spaceship.
https://www.nasa.gov/sites/default/files/thumbnails/image/core_stage_infographic_reboot.jpg
In all the discussions of refueling I haven't seen an estimate of how may BFS tankers it takes to refuel an empty BFS in LEO.  Is it 150 tons of fuel per tanker? How many does it take?

Sorry for the error above; corrected in original post.  (Should be 130% the wet mass of SLS core stage, not double.)  The IAC 2017 presentation mentioned 4-5 tankers, I believe... something around 200-250t per tanker.
Discussed starting at about 27:20:
https://forum.nasaspaceflight.com/index.php?topic=43839.msg1727785#msg1727785
« Last Edit: 07/04/2018 10:03 pm by AncientU »
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Offline speedevil

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #21 on: 07/04/2018 10:37 pm »
In all the discussions of refueling I haven't seen an estimate of how may BFS tankers it takes to refuel an empty BFS in LEO.  Is it 150 tons of fuel per tanker? How many does it take?

Sorry for the error above; corrected in original post.  (Should be 130% the wet mass of SLS core stage, not double.)  The IAC 2017 presentation mentioned 4-5 tankers, I believe... something around 200-250t per tanker.

It is possible they won't make tanker variants for some considerable time.
See this thread on tankers.
In short - if the BFS is actually readily reusable, and the tanker costs around the same to make, but its only benefit is to save 20% on launch cost, you need to have quite a lot of launches to justify it.

Offline AncientU

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #22 on: 07/05/2018 12:50 am »
In all the discussions of refueling I haven't seen an estimate of how may BFS tankers it takes to refuel an empty BFS in LEO.  Is it 150 tons of fuel per tanker? How many does it take?

Sorry for the error above; corrected in original post.  (Should be 130% the wet mass of SLS core stage, not double.)  The IAC 2017 presentation mentioned 4-5 tankers, I believe... something around 200-250t per tanker.

It is possible they won't make tanker variants for some considerable time.
See this thread on tankers.
In short - if the BFS is actually readily reusable, and the tanker costs around the same to make, but its only benefit is to save 20% on launch cost, you need to have quite a lot of launches to justify it.

Whether BFS is refueled with another cargo BFS or a full-up tanker version changes little.  BFS will not be filled by FH or F9 as posited above.  The 2017 IAC presentation discussed to the Moon and back with a single HEO fuel transfer, so Lunar capability is certainly within easy reach of BFR/BFS.

On your point of having tanker versions... there will likely be tankers in time to support the second Mars visit where four BFSs (two cargo, two crew) are planned.  Main advantage is that tankers will fly much more often than any other version and will be the least expensive BFS configuration -- just a flying tank.  If you are going to wear something out, might as well be the cheapest version.  The 20% or whatever more fuel per trip also minimizes wear and tear -- which will become a consideration as launch frequency climbs.

EM stated that the tanker version would look strange (discussed on another thread).  Since it will only do Earth entries from low orbital velocities, and always be coming back very light(fluffy), it could easily get away with less thermal protection or have other mods that optimize it further over the standard cargo BFS.
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Offline OneSpeed

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #23 on: 07/05/2018 06:49 am »
In all the discussions of refueling I haven't seen an estimate of how may BFS tankers it takes to refuel an empty BFS in LEO.  Is it 150 tons of fuel per tanker? How many does it take?
...  The IAC 2017 presentation mentioned 4-5 tankers, I believe... something around 200-250t per tanker...

Taking an 85mT, 250 bar cargo variant with 1100mT of propellant, and no other payload, my model gets 160mT of propellant to LEO, plus about 40mT of landing fuel. The slight improvement over the published 150mT payload is due to reduced gravity losses because of the GLOW reduction from 4400mT to 4250mT.

1100/160 = 6.875 refuelling missions, so around 7.

Simply changing to 300 bar engines gives 244mT of fuel to LEO, perhaps this is where the 4-5 missions figure comes from?

Offline Archibald

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #24 on: 07/05/2018 07:41 am »
A crude way of doing the math: BFR carries BFS from zero to 2 km/s of delta-v.
 Which mean BFS can "do" 7 km/s with the tanks full - to loft itself into LEO with a very large payload (150 mt).

From LEO, if entirely refueled: LEO to Moon surface is 3 km/s + 2.5 km/s = 5.5 km/s, one way. Two way would be 11 km/s, fortunately, there is "free" aerobraking on return.

So BFS can, with a complete LEO refueling, fly to the Moon with some fuel margin plus a pretty large payload.

The return trip is a mix of aerobraking and propulsive braking.

Of course the more one cut into the payload, the least refuelings needed. For example, if the payload is cut from 150 mt to 20 mt or less, no refueling are needed (AFAIK).

As I said, those are crude calculations, just to give a broad example of the system capabilities.
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Offline speedevil

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #25 on: 07/05/2018 09:54 am »
<snip my comment>
On your point of having tanker versions... there will likely be tankers in time to support the second Mars visit where four BFSs (two cargo, two crew) are planned.  Main advantage is that tankers will fly much more often than any other version and will be the least expensive BFS configuration -- just a flying tank.  If you are going to wear something out, might as well be the cheapest version.  The 20% or whatever more fuel per trip also minimizes wear and tear -- which will become a consideration as launch frequency climbs.

'Least expensive' only matters if you're flying a them enough that you beat out the cost differential in launching, and you have no reason to fly your base model more.
Tankers in 2022-24 imply either:
Terrible cost of the base model,  that reusability diddn't work well, or that things are going ridiculously well, and putting 100 flights on a tanker in 2 years, along with other flights is something that is reasonable.
Or as a fourth option that the tanker does something the base model cannot - 1000 tons of fuel in LMO, for example.

(ridiculously well =  >>500 flights of all vehicles between 2022 and 2024).

Even that 500 flights has caveats - if you are actively trying to start a passenger service, and trying to beat down the costs on that, actually just using those vehicles for everything may be worth it.

Tankers flying 'more often than the main version' either implies very low flight rate, or that well over a sixth of launches are to BEO.



Offline nacnud

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #26 on: 07/05/2018 10:10 am »
There might be issues with even getting near the surface. Exhaust gas velocities are well above orbital speeds so a landing anywhere will contaminate the entire moon with fast dust particles. There was a great post by a post-Apollo researcher recently about an instrument returned by an Apollo mission. They were told to look for a white box but when they found it it was brown. Years later they found out that millions of dust particles had lodged themselves into the paint after flying around the moon for a bit.

I just wanted to make a point about sending dust into orbit by landing a BFR on the Moon. You can't send anything into (non hyperbolic) orbit with a single short duration impulse, such trajectories would be sub orbital i.e. they would fall back to the moons surface at some point equal to or less than one orbit.

Not saying dust and debris wouldn't be a problem, I just wanted to point that out.
« Last Edit: 07/05/2018 10:43 am by nacnud »

Online Robotbeat

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #27 on: 07/05/2018 01:21 pm »
I missed this about 12 days ago. I’m sure it was posted on a thread SOMEwhere, but can’t find it.
New internal view of BFS with huge windows.

Violin concerto in microgravity before departure to Mars.

https://www.instagram.com/p/BkVgkN-gH9A/?taken-by=elonmusk

I think considerations like large windows are important for getting lots of people interested in making this crazy trip.

Without culture and adventure, few would want to do this.
« Last Edit: 07/05/2018 01:23 pm by Robotbeat »
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Offline speedevil

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #28 on: 07/05/2018 02:51 pm »
New internal view of BFS with huge windows.
I can't make the scale work for a BFS, unless perhaps she's three feet tall.
Even if you make the whole top half of the upper BFS glass.

Offline nacnud

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #29 on: 07/05/2018 02:57 pm »
New internal view of BFS with huge windows.
I can't make the scale work for a BFS, unless perhaps she's three feet tall.
Even if you make the whole top half of the upper BFS glass.

I can't work out how she'd remain in the correct orientation to the audience, is the dress fitted with a RCS?

Offline marsbase

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #30 on: 07/05/2018 03:11 pm »
Uh, I don't think there will be real windows.  Too heavy and a safety risk.  The view outside will be projected onto a thin film screen.  And of course the violinist could be a superimposed projection as well.  CGI or real.  The point is to distract the colonists from the existential danger of the propellant refueling going on under their feet.  Or above their feet.  Oh, you know what I mean. :)

Offline jpo234

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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 Nomadd

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #32 on: 07/05/2018 03:58 pm »
Uh, I don't think there will be real windows.  Too heavy and a safety risk.  The view outside will be projected onto a thin film screen.  And of course the violinist could be a superimposed projection as well.  CGI or real.  The point is to distract the colonists from the existential danger of the propellant refueling going on under their feet.  Or above their feet.  Oh, you know what I mean. :)
Or they could just stay home and watch the mission on tv. There are still people who don't equate the digital world with the real thing. Don't underestimate the importance of looking out a window.
« Last Edit: 07/06/2018 01:20 am by Nomadd »
Those who danced were thought to be quite insane by those who couldn't hear the music.

Online Robotbeat

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #33 on: 07/06/2018 12:16 am »
Also, we're closer to ITS than we are to a display that can display large, crisp images from basically any possible angle like that without very large optical artifacts. I say, let there be huge windows. The risk is only really high during the short period in LEO, anyway. Allows it to function as a greenhouse when on Mars, too. :)
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Online Robotbeat

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #34 on: 07/06/2018 12:18 am »
New internal view of BFS with huge windows.
I can't make the scale work for a BFS, unless perhaps she's three feet tall.
Even if you make the whole top half of the upper BFS glass.

I can't work out how she'd remain in the correct orientation to the audience, is the dress fitted with a RCS?
Training and timed pushes. See how Hadfield floats about with similar control in this video:


(That video is very similar to this image, when you think about it... Still have big windows, still have a stringed instrument, still floating around, still in LEO...  Just smaller scale.)

As far as overall scale, probably have to assume this was taken with a wide field of view lens and perhaps on the larger ITS.
« Last Edit: 07/06/2018 12:22 am by Robotbeat »
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Offline woods170

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #35 on: 07/06/2018 09:14 am »
Uh, I don't think there will be real windows.  Too heavy and a safety risk. 

I guess you've never heard of transparent aluminium?

And no, I don't mean the stuff Scotty mentioned on Star Trek IV.

See here: https://en.wikipedia.org/wiki/Aluminium_oxynitride
And here: https://hackaday.com/2018/04/03/whats-the-deal-with-transparent-aluminum/

Actual windows on BFS, made from Aluminium_oxynitride, could very well work and would be lighter than windows made from fused silica (aka glass).

Offline marsbase

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #36 on: 07/06/2018 12:47 pm »
Uh, I don't think there will be real windows.  Too heavy and a safety risk. 
I guess you've never heard of transparent aluminium?
I've never actually seen it.  Hope it's strong enough for the BFS. But honestly, I think SpaceX wants a simple, low risk design.  If someone is going to live on Mars (most likely in an underground tunnel) I suspect actual windows are not a concern.    Musk wants the general public to be able to imagine themselves traveling to/living on Mars.  Helps with political support.  But in the early years it will be a pioneering life. Like the novel "Wool" by Hugh Howey... but horizontal rather than vertical.

Offline speedevil

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #37 on: 07/06/2018 01:16 pm »
Uh, I don't think there will be real windows.  Too heavy and a safety risk.
By 'cant work out' - I don't mean that windows are impractical. (that is a separate argument).

I mean I don't think the scene fits inside a BFS at all, it is >>9m in diameter, and may either be a lot of artistic licence not paying any attention to actual dimensions, or more excitingly, indicative of 'ITS2016', BFS-H, or perhaps the first hint of in-orbit assembled infrastructure.

Offline edzieba

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #38 on: 07/06/2018 01:53 pm »
Uh, I don't think there will be real windows.  Too heavy and a safety risk. 

I guess you've never heard of transparent aluminium?

And no, I don't mean the stuff Scotty mentioned on Star Trek IV.

See here: https://en.wikipedia.org/wiki/Aluminium_oxynitride
And here: https://hackaday.com/2018/04/03/whats-the-deal-with-transparent-aluminum/

Actual windows on BFS, made from Aluminium_oxynitride, could very well work and would be lighter than windows made from fused silica (aka glass).
'Transparent Aluminium' has been available for close to 4 decades, and has failed to be used in any big windows for spacecraft (e.g. the ISS' Cupola). Unlike it's sci-fi namesake, it does not behave like bulk Aluminium (or Al alloys) when formed into large sheets, being a ceramic after all. Great for small non-loadbearing bulletproof windows, not so great for huge load bearing windows.

Offline Archibald

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 4 (Post Speech)
« Reply #39 on: 07/06/2018 05:52 pm »
I forgot a detail (I told you it was crude thing) With 1.5 km/s left, standing on the Moon surface the BFS... won't go anywhere. Indeed in order to escape the Moon, it's 2.5 km/s. Past Moon escape, aerobraking can do the job.

So I was a little wrong for the return trip... then again, it is possible to cut into the payload landed on the lunar surface, until 2.5 km/s are left in the tanks after the landing. This way, BFS can escape from the Moon, and aerobrake back into LEO.

I remember Steve Pietrobon did excellent calculations that showed the system might be even better with some refueling in LLO. Which makes sense.

One should note that, even with zero refueling, payload is somewhere between 5 mt to 20 mt to Moon surface. That's still way, way better than any Lunar Module derivatives as studied during AAP. LM truck, for a start, was barely 3 mt.
« Last Edit: 07/06/2018 05:55 pm by Archibald »
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