Author Topic: Elon Musk Reddit AMA on BFR  (Read 47090 times)

Online nacnud

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Re: Elon Musk Reddit AMA on BFR
« Reply #80 on: 10/15/2017 11:43 AM »
Wow. All this new information (there will be a BFS Grasshopper! They're well on the way with ISRU tech! Tanker version is going to look totally different!) and all we can talk about is the number of legs??

I'm sure everything will get mulled over in time. :D

Offline philw1776

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Re: Elon Musk Reddit AMA on BFR
« Reply #81 on: 10/15/2017 01:30 PM »
Really disappointed that we didn't learn if they plan to use 39A for BFR.  >:(

That was my question, of course it ended up way down at the bottom under the stack of __Rockot__'s questions.

Unbelievable that they allowed unlimited questions per person.

I don't know how that happened.  I tried to post a 2nd question and got a message saying I needed to wait 8 minutes.
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline guckyfan

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Re: Elon Musk Reddit AMA on BFR
« Reply #82 on: 10/15/2017 01:32 PM »
Really disappointed that we didn't learn if they plan to use 39A for BFR.  >:(

That was my question, of course it ended up way down at the bottom under the stack of __Rockot__'s questions.

Unbelievable that they allowed unlimited questions per person.

I don't know how that happened.  I tried to post a 2nd question and got a message saying I needed to wait 8 minutes.

He was accepted by the mods as representative of r/spacex and given priority.

Offline Formica

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Re: Elon Musk Reddit AMA on BFR
« Reply #83 on: 10/15/2017 08:35 PM »
I think the most interesting comment from Musk was the part about them working on ISRU.
People in this forum like to focus on flight technology and comments like the number of landing legs or how to fit cargo containers, which are rather mundane engineering problems. However, ISRU and ECLSS are the two major long poles in SpaceX's Mars plan with low TRLs. They are not easy to solve, there is very little prior art, and until now, we had no evidence that SpaceX was actually working on them.

I agree wholeheartedly, and I really want to know more about SpaceX's ISRU plans. The AMA answer is the most we've heard on the topic, yet it's an absolute lynchpin of the whole endeavour! No ISRU, no BFR, CH4 is central to the design, etc etc. Hopefully this is the beginning of more information becoming public on the topic.
I'm just a space fan, please correct me if I'm wrong!

Offline rsdavis9

Re: Elon Musk Reddit AMA on BFR
« Reply #84 on: 10/15/2017 08:44 PM »
WRT legs. If you have active legs you can adjust for rough terrain. 3 works too.
Questions not asked:
1. What active adjustment of COG will be employed. Depending on entry cargo the COG may need to adjusted. It may even be true that in different flight regimes the COG could be adjusted. Imagine if the deltawings/flaps are near the end of their compensation position, the COG could be adjusted by moving propellants from tank to tank.
2. Isn't it true that for the first crew return from mars you could just fly extra ships with enough fuel so one could return. In that way you wouldn't need ISRU setup by robots. You could have humans/robots assembling and operating the mining etc stuff.
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 nacnud

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Re: Elon Musk Reddit AMA on BFR
« Reply #85 on: 10/15/2017 08:45 PM »
Well given the hints so far it's going to be solar powered and need to arrive on two BFS. Over the course of 18 months it will need to produce enough LOX and CH4 to refuel at least two BFS.

Have I missed anything?

Offline rsdavis9

Re: Elon Musk Reddit AMA on BFR
« Reply #86 on: 10/15/2017 08:47 PM »
I think they plan to leave some of the first BFS's to mars on the surface as part of the station.
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 nacnud

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Re: Elon Musk Reddit AMA on BFR
« Reply #87 on: 10/15/2017 09:00 PM »
Isn't it true that for the first crew return from mars you could just fly extra ships with enough fuel so one could return. In that way you wouldn't need ISRU setup by robots. You could have humans/robots assembling and operating the mining etc stuff.

Well you should know before the manned launch whether the initial robotic ISRU has produced enough propellent for return. Then you have the options of either taking all the fuel needed to return, just what is needed to land or some mix in between.

Offline Kaputnik

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Re: Elon Musk Reddit AMA on BFR
« Reply #88 on: 10/15/2017 09:37 PM »
Well given the hints so far it's going to be solar powered and need to arrive on two BFS. Over the course of 18 months it will need to produce enough LOX and CH4 to refuel at least two BFS.

Have I missed anything?

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?
Waiting for joy and raptor

Offline vaporcobra

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Re: Elon Musk Reddit AMA on BFR
« Reply #89 on: 10/15/2017 09:43 PM »
Comprehensive collation of all of Musk's AMA answers and questions, organized and sorted by general category. Includes the silly stuff ;)
http://www.teslarati.com/elon-musks-spacex-ama-mars-bfr-bfs-spaceship/


Living on Mars

Q: Obviously there will be an extreme amount of care put into what is sent on the first missions, and the obvious answer of “Solar Panels” and “Fuel Production Equipment” is included, but what else?

A (Elon): Our goal is get you there and ensure the basic infrastructure for propellant production and survival is in place. A rough analogy is that we are trying to build the equivalent of the transcontinental railway. A vast amount of industry will need to be built on Mars by many other companies and millions of people.


Q: Does your Mars city feature permanently anchored BFS spaceships?

A (Elon): Wouldn't read too much into that illustration


Q: Have any candidate landing sites for the Mars base been identified?

A (Elon): Landing site needs to be low altitude to maximize aero braking, be close to ice for propellant production and not have giant boulders. Closer to the equator is better too for solar power production and not freezing your ass off.


Q: Who will design and build the ISRU system for the propellant depot, and how far along is it?

A (Elon): SpaceX. Design is pretty far along. It's a key part of the whole system.


Spaceship (BFS)

Q: Will the BFS landing propellants have to be actively cooled on the long trip to Mars?

A (Elon): The main tanks will be vented to vacuum, the outside of the ship is well insulated (primarily for reentry heating) and the nose of the ship will be pointed mostly towards the sun, so very little heat is expected to reach the header tanks. That said, the propellant can be cooled either with a small amount of evaporation. Down the road, we might add a cryocooler.

A (Elon): exactly (while methane could be kept in its liquid form solely through high pressure storage, the pressures required are immense and would require tanks that would be far too heavy for a rocket's second stage.


Q: Will the BFS heat shield be mounted on the skin, or embedded?

A (Elon): The heat shield plates will be mounted directly to the primary tank wall. That's the most mass efficient way to go. Don't want to build a box in box.


Q: Can the BFS delta wings and heat shield be removed for deep space missions?

A (Elon): Wouldn't call what BFS has a delta wing. It is quite small (and light) relative to the rest of the vehicle and is never actually used to generate lift in the way that an aircraft wing is used.

Its true purpose is to "balance out" the ship, ensuring that it doesn't enter engines first from orbit (that would be really bad), and provide pitch and yaw control during reentry.


Q: Why is the 2017 BFS spaceship largely cylindrical?

A (Elon): Best mass ratio is achieved by not building a box in a box. The propellant tanks need to be cylindrical to be remotely mass efficient and they have to carry ascent load, so lowest mass solution is just to mount the heat shield plates directly to the tank wall.

                                               
Q: How does the BFS achieve vertical stabilization, without a tail?

A (Elon): Tails are lame

A (Elon): +1 (The space shuttle's vertical stabilizer was completely useless for most of the reentry profile, as it was in complete aerodynamic shadow. I think it's clear a craft doesn't need one for reentry, only for subsonic gliding, which BFS doesn't really do.)


Q: Why was the number of BFS landing legs increased from 3 to 4?

A (Elon): Because 4

A (Elon): Improves stability in rough terrain


Q: How is the radiation shielding in the ITS?

A (Elon): Ambient radiation damage is not significant for our transit times. Just need a solar storm shelter, which is a small part of the ship. Buzz Aldrin is 87.


Q: Why was the location and shape of the BFS header/landing tanks changed?

A (Elon): The aspiration by the change was to avoid/minimize plumbing hell, but we don't super love the current header tank/plumbing design. Further refinement is likely.


BFS Tanker

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).


Q: Will the BFS tanker ships (have to) do a hoverslam landing?

A (Elon): Landing will not be a hoverslam, depending on what you mean by the "slam" part. Thrust to weight of 1.3 will feel quite gentle. The tanker will only feel the 0.3 part, as gravity cancels out the 1. Launch is also around 1.3 T/W, so it will look pretty much like a launch in reverse....


Development schedule

Q: With the first two cargo missions scheduled to land on Mars in 2022, what kind of development progress can we expect to see from SpaceX in the next 5 or so years leading up to the maiden flight?

Will we see BFS hops or smaller test vehicles similar to Grasshopper/F9R-Dev? Facilities being built? Propellant plant testing? etc. etc.

A (Elon): A lot. Yes, yes, and yes.

A (Elon): Will be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance. Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep space Raptor engines.

Next step will be doing orbital velocity Ship flights, which will need all of the above. Worth noting that BFS is capable of reaching orbit by itself with low payload, but having the BF Booster increases payload by more than an order of magnitude. Earth is the wrong planet for single stage to orbit. No problemo on Mars.


Raptor and rocket propulsion

Q: Why was Raptor thrust reduced from ~300 tons-force to ~170 tons-force?

A (Elon): We chickened out. The engine thrust dropped roughly in proportion to the vehicle mass reduction from the first IAC talk. In order to be able to land the BF Ship with an engine failure at the worst possible moment, you have to have multiple engines. The difficulty of deep throttling an engine increases in a non-linear way, so 2:1 is fairly easy, but a deep 5:1 is very hard. Granularity is also a big factor. If you just have two engines that do everything, the engine complexity is much higher and, if one fails, you've lost half your power. Btw, we modified the BFS design since IAC to add a third medium area ratio Raptor engine partly for that reason (lose only 1/3 thrust in engine out) and allow landings with higher payload mass for the Earth to Earth transport function.


Q: Will the BFR autogenous pressurization system be heat exchanger based?

A (Elon): We plan to use the Incendio spell from Harry Potter

A (Elon): But, yes and probably


Q: Will the BFS methalox control thrusters be derived from Raptor or from SuperDraco engines?

A (Elon): The control thrusters will be closer in design to the Raptor main chamber than SuperDraco and will be pressure-fed to enable lowest possible impulse bit (no turbopump spin delay).


Q: Could you update us on the status of scaling up the Raptor prototype to the final size?

A (Elon): Thrust scaling is the easy part. Very simple to scale the dev Raptor to 170 tons.

The flight engine design is much lighter and tighter, and is extremely focused on reliability. The objective is to meet or exceed passenger airline levels of safety. If our engine is even close to a jet engine in reliability, has a flak shield to protect against a rapid unscheduled disassembly and we have more engines than the typical two of most airliners, then exceeding airline safety should be possible.

That will be especially important for point to point journeys on Earth. The advantage of getting somewhere in 30 mins by rocket instead of 15 hours by plane will be negatively affected if "but also, you might die" is on the ticket.                                     


Q: Can BFS vacuum-Raptors be fired at sea level pressure?

A: The "vacuum" or high area ratio Raptors can operate at full thrust at sea level. Not recommended.
f l o w  s e p a r a t i o n  i n t e n s i f i e s

Mars communications

Q: Does SpaceX have any interest in putting more satellites in orbit around Mars (or even rockets) for internet/communications before we get feet on the ground? Or are the current 5-6 active ones we have there sufficient?

A (Elon): Yes


Q: Also will there be some form of an internet or communications link with Earth? Is SpaceX going to be in charge of putting this in or are you contracting some other companies?

A (Elon): If anyone wants to build a high bandwidth comm link to Mars, please do.


Q: The concept of an internet connection on Mars is kinda awesome. You could theoretically make an internet protocol that would mirror a subset of the internet near Mars. A user would need to queue up the parts of the internet they wanted available and the servers would sync the relevant data.

A (Elon): Nerd

A (Elon): But, yes, it would make sense to strip the headers out and do a UDP-style feed with extreme compression and a CRC check to confirm the packet is good, then do a batch resend of the CRC-failed packets. Something like that. Earth to Mars is over 22 light-minutes at max distance.

A (Elon): 3 light-minutes at closest distance. So you could Snapchat, I suppose. If that's a thing in the future.


Boring!

Q: Boring question about Mars:

A (Elon): More boring!


Miscellaneous silliness

Q: This is one bizarre AMA so far...

A (Elon): Just wait...

Q: i feel like thats a threat. "just wait. it will get way more bizarre than that. let me finish my whiskey"

A (Elon): How did you know? I am actually drinking whiskey right now. Really.

...No comment...
« Last Edit: 10/16/2017 12:49 AM by vaporcobra »

Online nacnud

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Re: Elon Musk Reddit AMA on BFR
« Reply #90 on: 10/15/2017 09:46 PM »
Q: Have any candidate landing sites for the Mars base been identified?

A (Elon): Landing site needs to be low altitude to maximize aero braking, be close to ice for propellant production and not have giant boulders. Closer to the equator is better too for solar power production and not freezing your ass off.

---------------------

Whether that means they plan to not bring hydrogen on the first trip I don't know!

Offline Formica

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Re: Elon Musk Reddit AMA on BFR
« Reply #91 on: 10/15/2017 10:21 PM »
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!
« Last Edit: 10/15/2017 10:28 PM by Formica »
I'm just a space fan, please correct me if I'm wrong!

Offline Robotbeat

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Re: Elon Musk Reddit AMA on BFR
« Reply #92 on: 10/15/2017 10:23 PM »
Isn't it true that for the first crew return from mars you could just fly extra ships with enough fuel so one could return. In that way you wouldn't need ISRU setup by robots. You could have humans/robots assembling and operating the mining etc stuff.

Well you should know before the manned launch whether the initial robotic ISRU has produced enough propellent for return. Then you have the options of either taking all the fuel needed to return, just what is needed to land or some mix in between.
Manned launch? Are you implying the first crewed launch will be all women? Guess that makes sense as women generally weigh less, require less food and oxygen, and have fewer problems with eyesight in microgravity. Although as a man, I hope they allow men not too long afterward.
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Offline Robotbeat

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Re: Elon Musk Reddit AMA on BFR
« Reply #93 on: 10/15/2017 10:25 PM »
Q: Have any candidate landing sites for the Mars base been identified?

A (Elon): Landing site needs to be low altitude to maximize aero braking, be close to ice for propellant production and not have giant boulders. Closer to the equator is better too for solar power production and not freezing your ass off.

---------------------

Whether that means they plan to not bring hydrogen on the first trip I don't know!
There's literally nothing SpaceX has said that would suggest them bringing hydrogen. Not. Gonna. Happen.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Robotbeat

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Re: Elon Musk Reddit AMA on BFR
« Reply #94 on: 10/15/2017 10:27 PM »
Well given the hints so far it's going to be solar powered and need to arrive on two BFS. Over the course of 18 months it will need to produce enough LOX and CH4 to refuel at least two BFS.

Have I missed anything?

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.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline vaporcobra

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Re: Elon Musk Reddit AMA on BFR
« Reply #95 on: 10/15/2017 10:55 PM »
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.

Offline mnelson

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #96 on: 10/15/2017 11:24 PM »
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?"

Offline Geron

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Re: Elon Musk Reddit AMA on BFR
« Reply #97 on: 10/15/2017 11:34 PM »
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.

Offline CuddlyRocket

Re: Elon Musk Reddit AMA on BFR
« Reply #98 on: 10/15/2017 11:38 PM »
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?

Offline Jcc

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Re: Elon Musk Reddit AMA on BFR
« Reply #99 on: 10/15/2017 11:53 PM »
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?

Use it for spare parts?

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