Author Topic: Elon Musk Reddit AMA on BFR  (Read 66170 times)

Offline KelvinZero

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Re: Elon Musk Reddit AMA on BFR
« Reply #100 on: 10/16/2017 12:06 am »
In fact, the "ISRU dev is pretty far along" comment is possibly the most important single piece of info from the AMA. If ISRU can't be done and done reliably, SpaceX's entire strategy is dead in the water.

I badly want to see what exactly is meant by "pretty far", I'd love to see some additional info on their progress :D Things will certainly start to get interesting if they have had considerable success, as NASA is years away from their first real test of ISRU, to be included on the Mars 2020 rover.
I would like to see a thread started on this, by someone with the enthusiasm to keep updating the OP with the actual information we have from SpaceX (which would not make a very large post at this point)

Offline vaporcobra

Re: Elon Musk Reddit AMA on BFR
« Reply #101 on: 10/16/2017 12:56 am »
In fact, the "ISRU dev is pretty far along" comment is possibly the most important single piece of info from the AMA. If ISRU can't be done and done reliably, SpaceX's entire strategy is dead in the water.

I badly want to see what exactly is meant by "pretty far", I'd love to see some additional info on their progress :D Things will certainly start to get interesting if they have had considerable success, as NASA is years away from their first real test of ISRU, to be included on the Mars 2020 rover.
I would like to see a thread started on this, by someone with the enthusiasm to keep updating the OP with the actual information we have from SpaceX (which would not make a very large post at this point)

Teeeeempting. I think I'll write a feature article discussing SpaceX's public ISRU comments, combined with some summary of relevant planetary science. The links/quotes I find would make for a good thread foundation. Even a chance that there's already an ISRU thread somewhere around here :)

Edit: Upon further examination, there are a ton of ISRU threads that were productive for about a month and then just died out. Definitely worth creating a SpaceX-focused thread.
« Last Edit: 10/16/2017 01:14 am by vaporcobra »

Offline rsdavis9

Re: Elon Musk Reddit AMA on BFR
« Reply #102 on: 10/16/2017 12:59 am »
I think they plan to leave some of the first BFS's to mars on the surface as part of the station.

That seems a waste. It's a pretty expensive piece of kit - why not refuel it and have it return to Earth for reuse?
First dwellings with everything they need. Maybe they will get modded too much or too old beyond reflight back. Really just the first few.
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.

Online aero

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Re: Elon Musk Reddit AMA on BFR
« Reply #103 on: 10/16/2017 01:58 am »
I think they plan to leave some of the first BFS's to mars on the surface as part of the station.

That seems a waste. It's a pretty expensive piece of kit - why not refuel it and have it return to Earth for reuse?
First dwellings with everything they need. Maybe they will get modded too much or too old beyond reflight back. Really just the first few.

Take a brand new BFS (ship A) to Mars, in 2022 or 2024. That's what, 150 days travel then it sits while being unloaded, habitats established and proven, after that, it returns to earth. That's what,  another 150 days. The ship is now how old? At least a year old assuming establishing habitats is a priority, It could be much older.

How far will SpaceX advance the design of the currently new BFS's while ship A is making this round trip? Or another way of looking at it is, "How useful is a year or more old Falcon 9 these days?" Or, "Has SpaceX ever built a rocket that didn't undergo major evolutionary changes over the span of a year's time?"

In particular, we are addressing the first BFS's out of the box, not a mature, stable design as planned to exist by 2026.
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Offline Ludus

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Re: Elon Musk Reddit AMA on BFR
« Reply #104 on: 10/16/2017 02:07 am »
I think they plan to leave some of the first BFS's to mars on the surface as part of the station.

That seems a waste. It's a pretty expensive piece of kit - why not refuel it and have it return to Earth for reuse?
First dwellings with everything they need. Maybe they will get modded too much or too old beyond reflight back. Really just the first few.

Take a brand new BFS (ship A) to Mars, in 2022 or 2024. That's what, 150 days travel then it sits while being unloaded, habitats established and proven, after that, it returns to earth. That's what,  another 150 days. The ship is now how old? At least a year old assuming establishing habitats is a priority, It could be much older.

How far will SpaceX advance the design of the currently new BFS's while ship A is making this round trip? Or another way of looking at it is, "How useful is a year or more old Falcon 9 these days?" Or, "Has SpaceX ever built a rocket that didn't undergo major evolutionary changes over the span of a year's time?"

In particular, we are addressing the first BFS's out of the box, not a mature, stable design as planned to exist by 2026.

There’s a cheaper easier approach which is just build one kind of BFS, the Cargo/Tanker and no “Heart of Gold” style passenger ships. Put Habs in the Cargo holds if you want to take passengers. Fake the windows. Once on Mars you can leave the Habs where they’re more useful and return the ships without wasting propellant hauling back stuff you don’t need to carry.
« Last Edit: 10/16/2017 02:08 am by Ludus »

Offline mnelson

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Re: Elon Musk Reddit AMA on BFR
« Reply #105 on: 10/16/2017 02:07 am »
Q: Will the BFS tanker's payload section be empty, or include extra propellant tanks?

A (Elon): At first, the tanker will just be a ship with no payload. Down the road, we will build a dedicated tanker that will have an extremely high full to empty mass ratio (warning: it will look kinda weird).

Any ideas why the dedicated tanker would "look kinda weird?"
My theory is it's because the density of the fuel is low enough that they can strap on nearly double tanks to take up the mass payload capacity.

See Airbus Belluga if you want to see what he's talking about.

Hmmm, that doesn't make sense to me. Wouldn't the density be higher than most payloads so the tanker would be *smaller* than the BFS? 250MT of propellants would be 54MT fuel and 196MT of LOX. If cylindrical tanks with a diameter of 9m are used then the fuel tank would only need to be 2m tall. The LOX tank 2.7m tall. Why not just replace the whole cargo area with a simple nosecone and stretch the existing tanks by 4.7m? It could be named "stubby" for short.

Edit: fixed bad calculations. Same conclusion though: the right name would be "stubby." :-)
« Last Edit: 10/16/2017 03:44 am by mnelson »

Offline Ludus

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Re: Elon Musk Reddit AMA on BFR
« Reply #106 on: 10/16/2017 02:23 am »
Q: Will the BFS tanker's payload section be empty, or include extra propellant tanks?

A (Elon): At first, the tanker will just be a ship with no payload. Down the road, we will build a dedicated tanker that will have an extremely high full to empty mass ratio (warning: it will look kinda weird).

Any ideas why the dedicated tanker would "look kinda weird?"
My theory is it's because the density of the fuel is low enough that they can strap on nearly double tanks to take up the mass payload capacity.

See Airbus Belluga if you want to see what he's talking about.

Hmmm, that doesn't make sense to me. Wouldn't the density be higher than most payloads so the tanker would be *smaller* than the BFS? 250MT of propellants would be 54MT fuel and 196MT of LOX. If cylindrical tanks with a diameter of 9m are used then the fuel tank would only need to be 56cm tall. The LOX tank 3.5m tall. Why not just replace the whole cargo area with a simple nosecone and stretch the existing tanks by 4m? It could be named "stubby" for short.

Yep.Propellant is actually denser than any normal cargo, which is why a Tanker looks just like a dedicated cargo ship with a bit bigger tanks. It’s the same with Air Force Tankers which have fuel tanks in the lower part of the fuselage as well as the wings but are basically empty cargo planes otherwise because the maximum load of fuel they can take off with leaves the entire cargo area of the plane empty.

A simple dedicated Tanker version of BFS would just change the OML to leave out the empty cargo hold in the forward Area so it would be like a shortened stubby BFS. I’m probably missing something important because Elon did seem to say a dedicated Tanker was eventually worth doing and in this model not much is gained. I suppose if you are flying them thousands of times a year any gain in efficiency is important so stubby Tanker BFSs are worth it. Until then it’s cool that you really just have to build one kind of BFS to do everything.

Offline rakaydos

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Re: Elon Musk Reddit AMA on BFR
« Reply #107 on: 10/16/2017 02:46 am »
Q: Will the BFS tanker's payload section be empty, or include extra propellant tanks?

A (Elon): At first, the tanker will just be a ship with no payload. Down the road, we will build a dedicated tanker that will have an extremely high full to empty mass ratio (warning: it will look kinda weird).

Any ideas why the dedicated tanker would "look kinda weird?"
My theory is it's because the density of the fuel is low enough that they can strap on nearly double tanks to take up the mass payload capacity.

See Airbus Belluga if you want to see what he's talking about.

Hmmm, that doesn't make sense to me. Wouldn't the density be higher than most payloads so the tanker would be *smaller* than the BFS? 250MT of propellants would be 54MT fuel and 196MT of LOX. If cylindrical tanks with a diameter of 9m are used then the fuel tank would only need to be 56cm tall. The LOX tank 3.5m tall. Why not just replace the whole cargo area with a simple nosecone and stretch the existing tanks by 4m? It could be named "stubby" for short.

Yep.Propellant is actually denser than any normal cargo, which is why a Tanker looks just like a dedicated cargo ship with a bit bigger tanks. It’s the same with Air Force Tankers which have fuel tanks in the lower part of the fuselage as well as the wings but are basically empty cargo planes otherwise because the maximum load of fuel they can take off with leaves the entire cargo area of the plane empty.

A simple dedicated Tanker version of BFS would just change the OML to leave out the empty cargo hold in the forward Area so it would be like a shortened stubby BFS. I’m probably missing something important because Elon did seem to say a dedicated Tanker was eventually worth doing and in this model not much is gained. I suppose if you are flying them thousands of times a year any gain in efficiency is important so stubby Tanker BFSs are worth it. Until then it’s cool that you really just have to build one kind of BFS to do everything.
You're missing that he didnt say "Simple dedicated tanker version." What he said was "a dedicated tanker that will have an extremely high full to empty mass ratio (warning: it will look kinda weird)."

Throw out the current design entirely, optimize entirely for wet to dry mass ratio... I'm thinking something vaguely spherical.
Of course, a clean sheet tanker design is expensive and not something needed right away. so, "eventually", even in elon-time.

Offline vaporcobra

Re: Elon Musk Reddit AMA on BFR
« Reply #108 on: 10/16/2017 02:54 am »
You're missing that he didnt say "Simple dedicated tanker version." What he said was "a dedicated tanker that will have an extremely high full to empty mass ratio (warning: it will look kinda weird)."

Throw out the current design entirely, optimize entirely for wet to dry mass ratio... I'm thinking something vaguely spherical.
Of course, a clean sheet tanker design is expensive and not something needed right away. so, "eventually", even in elon-time.

Not quite so simple, as it will necessarily be an extremely reusable orbital-class lifting body optimized for a crazy mass ratio. The need for aerodynamic functionality and hypersonic speeds in atmosphere ups the complexity a tad and is a ceiling for just how weird the tanker could look, hence "kinda weird" :)

Offline moreno7798

Re: Elon Musk Reddit AMA on BFR
« Reply #109 on: 10/16/2017 03:25 am »
Yeah, there's still no clear information about the ISRU plan, as noted in the last few posts. It's simply unclear as to whether the first mission will be robotic ISRU, or if ISRU will have to wait for the first crewed mission and that they'll take the risk of a one way trip. That seems unreasonable to me, but the scant amount of information leaves the possibility open that that's the plan: send robots to find the water, but send people to set up the Sabatier reactors and solar panels. :o The second option is feasible if they send extra fuel once as a bootstrap operation, but that's total supposition. Further supposition rests on the hint that Gwynne dropped about nuclear in SpaceX's future, and the potential that solar won't scale past the beginnings of the project... How I wish we had answers!

SpaceX has hinted at even larger ships in the future that would dwarf BFS. Solar panels would just not be sufficient for the larger ships (think of cruise ships in space!). I think that's where nuclear is going to come in.
The only humans that make no mistakes are the ones that do nothing. The only mistakes that are failures are the ones where nothing is learned.

Offline moreno7798

Re: Elon Musk Reddit AMA on BFR
« Reply #110 on: 10/16/2017 03:33 am »
Do we actually know whether they are bringing H2, in whatever form, on the first flights? Or are they going straight for water extraction on Mars, and if so how will they do that?
Also, how will they transfer propellants between vehicles on Mars- and fundamental to this is another question, how close to each other can they land? Or will the whole ISRU plant be mobile itself, and load directly into the BFS?
We KNOW they aren't bringing hydrogen. We KNOW they're going straight to ISRU. This has been the clear plan since the beginning.

In fact, the "ISRU dev is pretty far along" comment is possibly the most important single piece of info from the AMA. If ISRU can't be done and done reliably, SpaceX's entire strategy is dead in the water.

I badly want to see what exactly is meant by "pretty far", I'd love to see some additional info on their progress :D Things will certainly start to get interesting if they have had considerable success, as NASA is years away from their first real test of ISRU, to be included on the Mars 2020 rover.

IAC 2018. There is a method to his madness.
The only humans that make no mistakes are the ones that do nothing. The only mistakes that are failures are the ones where nothing is learned.

Offline vaporcobra

Re: Elon Musk Reddit AMA on BFR
« Reply #111 on: 10/16/2017 03:48 am »
After some preliminary research, I've come to the conclusion that a tanker optimized aggressively for mass ratio would try to increase its fineness ratio and get rid of the graduated nose, replacing it with a nose cone that is as close as possible to being a common bulkhead for the LOX tank.

There are a whole lot of variables, still, like preserving enough space at the rear to fit the necessary propulsion and maintaining the ability to mate with ships of the larger diameter linkages. Not a ton of room for crazy optimization.

Sources below.

Offline Peter.Colin

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Re: Elon Musk Reddit AMA on BFR
« Reply #112 on: 10/16/2017 11:10 am »
The weird tanker might be a big almost empty tanker.

More like the nose cone and second stage of a Falcon 9 (12-15 meter wide)
Without heat shield
Without landing legs
Without deep space engines

That has an “extremely high full to empty mass ratio”.
« Last Edit: 10/16/2017 11:19 am by Peter.Colin »

Online rockets4life97

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Re: Elon Musk Reddit AMA on BFR
« Reply #113 on: 10/16/2017 11:20 am »
The weird tanker might be a big almost empty tanker.

More like the nose cone of a Falcon 9 (12-15 meter wide)
Without heat shield
Without landing legs
Without deep space engines

That has an “extremely high full to empty mass ratio”.

The whole point is re-use. Did I miss something or do you think a tanker without a heat shield and landing legs could return and land?

Offline Peter.Colin

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Re: Elon Musk Reddit AMA on BFR
« Reply #114 on: 10/16/2017 11:34 am »
The weird tanker might be a big almost empty tanker.

More like the nose cone of a Falcon 9 (12-15 meter wide)
Without heat shield
Without landing legs
Without deep space engines

That has an “extremely high full to empty mass ratio”.


The whole point is re-use. Did I miss something or do you think a tanker without a heat shield and landing legs could return and land?

Not in in one piece.

It’s a depot-tanker which remains in orbit.
That’s the only thing that makes sense, that would have an extremely full to empty mass ratio.
The reduction in mass from launching it empty and stripped can be used to make it much bigger.

You could use the largest part of the 26 months between departures to launch the depot-tankers, and filling them up with regular tankers.
And than launch the spaceships, that are filled up by the depot-tankers.
Maybe 10 launches are needed to fill up a depot-tanker, could someone calculate this?

The Mars depot-tanker would need a heat shield.
But no landing legs.


« Last Edit: 10/16/2017 11:53 am by Peter.Colin »

Offline hkultala

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Re: Elon Musk Reddit AMA on BFR
« Reply #115 on: 10/16/2017 11:42 am »
The weird tanker might be a big almost empty tanker.

More like the nose cone of a Falcon 9 (12-15 meter wide)
Without heat shield
Without landing legs
Without deep space engines

That has an “extremely high full to empty mass ratio”.


The whole point is re-use. Did I miss something or do you think a tanker without a heat shield and landing legs could return and land?

Not in in one piece.

It’s a depot-tanker which remains in orbit.

That’s the only thing that makes sense, that would have an extremely full to empty mass ratio.
The reduction in mass from launching it empty and stripped can be used to make it much bigger.


No, your "depot tanker which remains in orbit" does not make any sense.

What is needed is to have multiple tanker launches to fill the tanks of the outbound craft.

To have these multiple tanker launches economically the tanker needs to return to earth and be launched again.

Offline GORDAP

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Re: Elon Musk Reddit AMA on BFR
« Reply #116 on: 10/16/2017 12:30 pm »
The weird tanker might be a big almost empty tanker.

More like the nose cone of a Falcon 9 (12-15 meter wide)
Without heat shield
Without landing legs
Without deep space engines

That has an “extremely high full to empty mass ratio”.


The whole point is re-use. Did I miss something or do you think a tanker without a heat shield and landing legs could return and land?

Not in in one piece.

It’s a depot-tanker which remains in orbit.
That’s the only thing that makes sense, that would have an extremely full to empty mass ratio.
The reduction in mass from launching it empty and stripped can be used to make it much bigger.

You could use the largest part of the 26 months between departures to launch the depot-tankers, and filling them up with regular tankers.
And than launch the spaceships, that are filled up by the depot-tankers.
Maybe 10 launches are needed to fill up a depot-tanker, could someone calculate this?

The Mars depot-tanker would need a heat shield.
But no landing legs.




It sounds like what you are describing is an orbital fuel depot.  Which ultimately may make sense, but is completely separate from the tankers that would supply it.  It just confuses things to call it a "depot-tanker".

And what is a "Mars depot-tanker"?  How could it possibly need a heat shield but not landing legs (unless it's doing an ocean splash down I suppose)?

Offline jpo234

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Re: Elon Musk Reddit AMA on BFR
« Reply #117 on: 10/16/2017 12:39 pm »
And what is a "Mars depot-tanker"?  How could it possibly need a heat shield but not landing legs (unless it's doing an ocean splash down I suppose)?

Not that I think that this makes sense, but if it made sense, than it would probably have to dip into the atmosphere to slow down to reach a stable orbit. Hence the need for a heat shield without landing legs.
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 Peter.Colin

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Re: Elon Musk Reddit AMA on BFR
« Reply #118 on: 10/16/2017 12:55 pm »
It’s semantics, is the tanker the thing that tanks fuel into the ship?
Or is the tanker the thing that tanks fuel into the thing that tanks fuel into the ship?

There is no ocean on Mars yet, JPO234 is correct :)
« Last Edit: 10/16/2017 01:04 pm by Peter.Colin »

Offline nacnud

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Re: Elon Musk Reddit AMA on BFR
« Reply #119 on: 10/16/2017 01:05 pm »
The tanker Musk was referring to was a modified BFS dedicated to refueling, not an on orbit depot. There is no need for such a vehicle yet as the standard BFS can perform the role.

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