Author Topic: Elon Musk Reddit AMA on BFR  (Read 47187 times)

Offline speedevil

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Elon Musk Reddit AMA on BFR
« on: 10/14/2017 06:09 PM »
Around 1:55 from the date of this post.

https://twitter.com/elonmusk/status/919262509227323392
Quote

BFR AMA on r/space in 2 hours
« Last Edit: 10/14/2017 08:59 PM by Chris Bergin »

Offline gongora

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« Last Edit: 10/15/2017 12:01 AM by gongora »

Offline gongora

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #2 on: 10/14/2017 06:32 PM »
[Reserved for transcript of Q&A]

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.


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: 10/15/2017 10:00 PM by gongora »

Offline gongora

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #3 on: 10/14/2017 07:35 PM »
The AMA thread is live for posting questions, he hasn't started answering yet:
https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/

Online nacnud

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #4 on: 10/14/2017 07:50 PM »
Should we try voting on questions we like? I've not been on reddit much but the number of people posting is huge. Is there an easy way to follow Elons replies?

This is my question:

https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dod9rqk/
« Last Edit: 10/14/2017 07:51 PM by nacnud »

Offline Robotbeat

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #5 on: 10/14/2017 07:59 PM »
I think we should upvote good questions. Especially people here. The signal to noise ratio is pretty terrible there right now, and many questions ask things which have already been answered. Here's my question:

https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dod9lw5/
Quote
Robotbeat 22 minutes ago
What is the status of the sweet Mars mining droids needed to fuel up BFR? Has SpaceX built any prototypes, yet? What about other necessary Mars surface infrastructure equipment, like regolith water extractors, electrolysis units, Sabatier reactors, cryocoolers, etc?

I encourage everyone to post question links here so we can upvote good ones.
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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #6 on: 10/14/2017 08:13 PM »
There are ~3000 comments there. My expectations aren't high given that

--- Tony

Offline Michael Baylor

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #7 on: 10/14/2017 08:14 PM »
If you are having trouble following Elon's replies, just use this link. They will start showing up once he begins.

https://www.reddit.com/user/ElonMusk/comments/

Offline Michael Baylor

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #8 on: 10/14/2017 08:18 PM »
The questions near the top so far are actually pretty good! :)

Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #9 on: 10/14/2017 08:19 PM »
The questions near the top so far are actually pretty good! :)

Many of them are from one person who has been posting the top questions from r/spacex, so cheers to them!

Offline Michael Baylor

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #10 on: 10/14/2017 08:29 PM »
AMA has begun.
« Last Edit: 10/14/2017 08:31 PM by Next Spaceflight »

Online calapine

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #11 on: 10/14/2017 08:35 PM »

Online nacnud

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #12 on: 10/14/2017 08:35 PM »
'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.'-EM

Offline Michael Baylor

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #13 on: 10/14/2017 08:38 PM »
I hope and think he's joking.  ::)

Edit: He was.
« Last Edit: 10/14/2017 08:42 PM by Next Spaceflight »

Online calapine

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #14 on: 10/14/2017 08:44 PM »

Offline intrepidpursuit

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #15 on: 10/14/2017 08:48 PM »
Elon likes talking about nerdy things and possibilities. I hope he gets around to some more of the vehicle based questions.

Offline Orbiter

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #16 on: 10/14/2017 08:52 PM »
Q: Will the BFS landing propellants have to be actively cooled on the long trip to Mars?

A: 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.
Attended space missions: STS-114, STS-124, STS-128, STS-135, Atlas V "Curiosity", Delta IV Heavy NROL-15, Atlas V MUOS-2, Delta IV Heavy NROL-37, Falcon 9 CRS-9, Falcon 9 JCSAT-16, Atlas V GOES-R, Falcon 9 SES-11, Falcon Heavy Demo, Falcon 9 Es'hail-2.

Offline vaporcobra

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #17 on: 10/14/2017 08:54 PM »
'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.'-EM

This is very interesting. So, I take this to mean that a third sea-levelish Raptor has been added to the center cluster. Also suggests that SpaceX is relatively serious about P2P Earth transport.

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #18 on: 10/14/2017 08:57 PM »

Online nacnud

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #19 on: 10/14/2017 09:03 PM »
'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.'-EM

This is very interesting. So, I take this to mean that a third sea-levelish Raptor has been added to the center cluster. Also suggests that SpaceX is relatively serious about P2P Earth transport.

Yes I think they mean another sea level raptor. Increasing the re-entered mass would be useful for aborts too, you'll get your payload back if something happens to the booster on ascent.

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