Author Topic: Elon Musk Reddit AMA on BFR  (Read 47377 times)

Offline speedevil

  • Senior Member
  • *****
  • Posts: 3191
  • Fife
  • Liked: 1631
  • Likes Given: 1958
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

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 4023
  • US
  • Liked: 3455
  • Likes Given: 1994
« Last Edit: 10/15/2017 12:01 AM by gongora »

Offline gongora

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 4023
  • US
  • Liked: 3455
  • Likes Given: 1994
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

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 4023
  • US
  • Liked: 3455
  • Likes Given: 1994
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/

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
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

  • Senior Member
  • *****
  • Posts: 28616
  • Minnesota
  • Liked: 8585
  • Likes Given: 5600
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.
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

Online jebbo

  • Full Member
  • ****
  • Posts: 652
  • Cambridge, UK
  • Liked: 275
  • Likes Given: 260
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

Online Michael Baylor

  • Full Member
  • ****
  • Posts: 542
  • Liked: 1056
  • Likes Given: 569
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/

Online Michael Baylor

  • Full Member
  • ****
  • Posts: 542
  • Liked: 1056
  • Likes Given: 569
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!

Online Michael Baylor

  • Full Member
  • ****
  • Posts: 542
  • Liked: 1056
  • Likes Given: 569
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

  • Full Member
  • **
  • Posts: 217
  • Linz, Austria
  • Liked: 177
  • Likes Given: 129
Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #11 on: 10/14/2017 08:35 PM »

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
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

Online Michael Baylor

  • Full Member
  • ****
  • Posts: 542
  • Liked: 1056
  • Likes Given: 569
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

  • Full Member
  • **
  • Posts: 217
  • Linz, Austria
  • Liked: 177
  • Likes Given: 129
Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #14 on: 10/14/2017 08:44 PM »

Offline intrepidpursuit

  • Full Member
  • ****
  • Posts: 624
  • Orlando, FL
  • Liked: 410
  • Likes Given: 243
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

  • Senior Member
  • *****
  • Posts: 2237
  • Florida
  • Liked: 482
  • Likes Given: 934
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

  • Full Member
  • ****
  • Posts: 1183
  • Tacoma, WA
  • Liked: 2222
  • Likes Given: 2488
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.

Online calapine

  • Full Member
  • **
  • Posts: 217
  • Linz, Austria
  • Liked: 177
  • Likes Given: 129
Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #18 on: 10/14/2017 08:57 PM »

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
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.

Offline FutureSpaceTourist

  • Senior Member
  • *****
  • Posts: 6865
  • UK
    • Plan 28
  • Liked: 6773
  • Likes Given: 2113
Re: Elon Musk Reddit AMA on BFR
« Reply #20 on: 10/14/2017 09:05 PM »
Bit of info on BFS development steps

Edit to add: more follow-up detail in 2nd attachment
« Last Edit: 10/14/2017 09:17 PM by FutureSpaceTourist »

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
Re: Elon Musk Reddit AMA on BFR
« Reply #21 on: 10/14/2017 09:08 PM »
*Speculation*

Perhaps this is the plan for Boca Chita now that flights from McGregor are a no no.

Online Michael Baylor

  • Full Member
  • ****
  • Posts: 542
  • Liked: 1056
  • Likes Given: 569
Re: Elon Musk Reddit AMA on BFR
« Reply #22 on: 10/14/2017 09:10 PM »
*Speculation*

Perhaps this is the plan for Boca Chita now that flights from McGregor are a no no.
Only other possibility is the Cape from a modified launch site. Doubtful that they could make that work though at a reasonable cost. This is going to need a serious flame trench, even though it will be possibly be less thrust.
« Last Edit: 10/14/2017 09:12 PM by Next Spaceflight »

Offline vaporcobra

  • Full Member
  • ****
  • Posts: 1183
  • Tacoma, WA
  • Liked: 2222
  • Likes Given: 2488
Re: Elon Musk Reddit AMA on BFR
« Reply #23 on: 10/14/2017 09:14 PM »
*Speculation*

Perhaps this is the plan for Boca Chita now that flights from McGregor are a no no.
Only other possibility is the Cape from a modified launch site. Doubtful that they could make that work though at a reasonable cost. This is going to need a serious flame trench, even though it will be possibly be less thrust.

FWIW, LC-39A was overengineering for Nova from the start. It can handle BFR and even ITS-level thrust. The question is if SpaceX could realistically modify 39A to support Falcon and BFR while still satisfying their Commercial Crew obligations.

Online calapine

  • Full Member
  • **
  • Posts: 217
  • Linz, Austria
  • Liked: 177
  • Likes Given: 129
Re: Elon Musk Reddit AMA on BFR
« Reply #24 on: 10/14/2017 09:15 PM »

Online Michael Baylor

  • Full Member
  • ****
  • Posts: 542
  • Liked: 1056
  • Likes Given: 569
Re: Elon Musk Reddit AMA on BFR
« Reply #25 on: 10/14/2017 09:16 PM »
FWIW, LC-39A was overengineering for Nova from the start. It can handle BFR and even ITS-level thrust. The question is if SpaceX could realistically modify 39A to support Falcon and BFR while still satisfying their Commercial Crew obligations.
I am aware of that. I just logically ruled it out since the current timeline is before 2022. I suppose I maybe should not have ruled that out though. I just don't see them getting Pad 39A ready in time. It would take some serious modifications to an active launch site. But anything is possible!

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
Re: Elon Musk Reddit AMA on BFR
« Reply #26 on: 10/14/2017 09:25 PM »
The development plan for BFS seems like what the follow on to the DC-X should have been.

Online calapine

  • Full Member
  • **
  • Posts: 217
  • Linz, Austria
  • Liked: 177
  • Likes Given: 129
Re: Elon Musk Reddit AMA on BFR
« Reply #27 on: 10/14/2017 09:26 PM »
Next one...
Edit: fixed image
« Last Edit: 10/14/2017 09:28 PM by calapine »

Offline FutureSpaceTourist

  • Senior Member
  • *****
  • Posts: 6865
  • UK
    • Plan 28
  • Liked: 6773
  • Likes Given: 2113
Re: Elon Musk Reddit AMA on BFR
« Reply #28 on: 10/14/2017 09:30 PM »
Tanker design
« Last Edit: 10/14/2017 09:31 PM by FutureSpaceTourist »

Offline FutureSpaceTourist

  • Senior Member
  • *****
  • Posts: 6865
  • UK
    • Plan 28
  • Liked: 6773
  • Likes Given: 2113
Re: Elon Musk Reddit AMA on BFR
« Reply #29 on: 10/14/2017 09:33 PM »
BFS heat shield mounting

Offline FutureSpaceTourist

  • Senior Member
  • *****
  • Posts: 6865
  • UK
    • Plan 28
  • Liked: 6773
  • Likes Given: 2113
Re: Elon Musk Reddit AMA on BFR
« Reply #30 on: 10/14/2017 09:40 PM »
BFS thrusters

Offline FutureSpaceTourist

  • Senior Member
  • *****
  • Posts: 6865
  • UK
    • Plan 28
  • Liked: 6773
  • Likes Given: 2113
Re: Elon Musk Reddit AMA on BFR
« Reply #31 on: 10/14/2017 09:44 PM »
BFR heat exchangers

Online calapine

  • Full Member
  • **
  • Posts: 217
  • Linz, Austria
  • Liked: 177
  • Likes Given: 129
Re: Elon Musk Reddit AMA on BFR
« Reply #32 on: 10/14/2017 09:48 PM »
Raptor and 3D printing

Offline FutureSpaceTourist

  • Senior Member
  • *****
  • Posts: 6865
  • UK
    • Plan 28
  • Liked: 6773
  • Likes Given: 2113
Re: Elon Musk Reddit AMA on BFR
« Reply #33 on: 10/14/2017 09:55 PM »
BFS wings

Offline FutureSpaceTourist

  • Senior Member
  • *****
  • Posts: 6865
  • UK
    • Plan 28
  • Liked: 6773
  • Likes Given: 2113
Re: Elon Musk Reddit AMA on BFR
« Reply #34 on: 10/14/2017 09:57 PM »
BFS cylindrical shape

Offline FutureSpaceTourist

  • Senior Member
  • *****
  • Posts: 6865
  • UK
    • Plan 28
  • Liked: 6773
  • Likes Given: 2113
Re: Elon Musk Reddit AMA on BFR
« Reply #35 on: 10/14/2017 10:01 PM »
Why no BFS tail

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
Re: Elon Musk Reddit AMA on BFR
« Reply #36 on: 10/14/2017 10:04 PM »
Very interesting about the wings, if you watch the Mars descent video from the IAC presentation you can see the BFR entry is nothing like a shuttle or any other lifting body.

Offline FutureSpaceTourist

  • Senior Member
  • *****
  • Posts: 6865
  • UK
    • Plan 28
  • Liked: 6773
  • Likes Given: 2113
Re: Elon Musk Reddit AMA on BFR
« Reply #37 on: 10/14/2017 10:10 PM »
4k rocket video (and with that I’m out of time, not sure how much more Elon is going to do)
« Last Edit: 10/14/2017 10:11 PM by FutureSpaceTourist »

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
Re: Elon Musk Reddit AMA on BFR
« Reply #38 on: 10/14/2017 10:15 PM »
Why was the number of BFS landing legs increased from 3 to 4?

The BFS/2016 design used three landing legs, while the new BFS/2017 design uses four.
What is the motivation behind this change?

Elon Musk-Because 4

Elon Musk-Improves stability in rough terrain

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
Re: Elon Musk Reddit AMA on BFR
« Reply #39 on: 10/14/2017 10:17 PM »
This is an odd answer, three legs are more stable than four. I'll be back with links.

Edit: Can't find the link I was after (NASA study for lander legs, Mars Phoenix if I recall correctly) but basically three points define a plane so the lander won't rock. Where it is unlikely that four points on a rough surface form a plane, so it will wobble. Suspension can correct for this.
« Last Edit: 10/14/2017 10:24 PM by nacnud »

Offline Craig_VG

  • Full Member
  • **
  • Posts: 204
  • Liked: 637
  • Likes Given: 425
Re: Elon Musk Reddit AMA on BFR
« Reply #40 on: 10/14/2017 10:20 PM »

Offline mlindner

  • Software Engineer
  • Senior Member
  • *****
  • Posts: 2090
  • Space Capitalist
  • Silicon Valley, CA -- previously in Ann Arbor, MI
  • Liked: 748
  • Likes Given: 290
Re: Elon Musk Reddit AMA on BFR
« Reply #41 on: 10/14/2017 10:31 PM »
Looks like the AMA is being a bit ruined by a user named __Rocket__. I've seen him a lot on /r/spacex and often makes bad leaps of logic in his posts and theorycrafts even far beyond what we do in this forum.
Internal combustion engine in space. It's just a Bad Idea.TM - Robotbeat

Offline Star One

  • Senior Member
  • *****
  • Posts: 9816
  • UK
  • Liked: 1892
  • Likes Given: 183
Re: Elon Musk Reddit AMA on BFR
« Reply #42 on: 10/14/2017 10:35 PM »
Looks like the AMA is being a bit ruined by a user named __Rocket__. I've seen him a lot on /r/spacex and often makes bad leaps of logic in his posts and theorycrafts even far beyond what we do in this forum.

That Space X Reddit in general has a bad rep from what I’ve read elsewhere on Reddit.

Offline livingjw

  • Full Member
  • ***
  • Posts: 384
  • Liked: 431
  • Likes Given: 181
Re: Elon Musk Reddit AMA on BFR
« Reply #43 on: 10/14/2017 10:36 PM »
This is an odd answer, three legs are more stable than four. I'll be back with links.

Edit: Can't find the link I was after (NASA study for lander legs, Mars Phoenix if I recall correctly) but basically three points define a plane so the lander won't rock. Where it is unlikely that four points on a rough surface form a plane, so it will wobble. Suspension can correct for this.

I would expect the legs to be self leveling.
John

Offline mlindner

  • Software Engineer
  • Senior Member
  • *****
  • Posts: 2090
  • Space Capitalist
  • Silicon Valley, CA -- previously in Ann Arbor, MI
  • Liked: 748
  • Likes Given: 290
Re: Elon Musk Reddit AMA on BFR
« Reply #44 on: 10/14/2017 10:39 PM »
Looks like the AMA is being a bit ruined by a user named __Rocket__. I've seen him a lot on /r/spacex and often makes bad leaps of logic in his posts and theorycrafts even far beyond what we do in this forum.

That Space X Reddit in general has a bad rep from what I’ve read elsewhere on Reddit.

To be clear, he's not well liked there either. SpaceX subreddit is one of the few places on reddit with actual quality content. Quoting Chris.

Quote
Hold the r/space mods hostage until they move it to r/spacex

Or riot. 😉

Internal combustion engine in space. It's just a Bad Idea.TM - Robotbeat

Offline mlindner

  • Software Engineer
  • Senior Member
  • *****
  • Posts: 2090
  • Space Capitalist
  • Silicon Valley, CA -- previously in Ann Arbor, MI
  • Liked: 748
  • Likes Given: 290
Re: Elon Musk Reddit AMA on BFR
« Reply #45 on: 10/14/2017 10:44 PM »
Looks like the AMA is being a bit ruined by a user named __Rocket__. I've seen him a lot on /r/spacex and often makes bad leaps of logic in his posts and theorycrafts even far beyond what we do in this forum.

__Rocket__ just posted questions that were in past days selected on SpaceX subreddit (when it wasn't sure which subreddit will host this QA session) - not sure if that was agreed ahead of time, or not, but in principle, I don't see a problem with reposting those questions in this QA.

Many people are saying that, but if you look, none of the questions he asked is actually from the SpaceX subreddit. At best some of the questions informed the question that he completely rewrote to his own question. So this is false. There was a prep thread, and several of the questions from it were asked by other people, but not him.
« Last Edit: 10/14/2017 10:44 PM by mlindner »
Internal combustion engine in space. It's just a Bad Idea.TM - Robotbeat

Offline yg1968

  • Senior Member
  • *****
  • Posts: 9718
  • Liked: 1425
  • Likes Given: 882
Re: Elon Musk Reddit AMA on BFR
« Reply #46 on: 10/14/2017 10:44 PM »
Looks like the AMA is being a bit ruined by a user named __Rocket__. I've seen him a lot on /r/spacex and often makes bad leaps of logic in his posts and theorycrafts even far beyond what we do in this forum.

__Rocket__ just posted questions that were in past days selected on SpaceX subreddit (when it wasn't sure which subreddit will host this QA session)

He said that they were his own questions:

https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodcpgk/?context=3

Offline mlindner

  • Software Engineer
  • Senior Member
  • *****
  • Posts: 2090
  • Space Capitalist
  • Silicon Valley, CA -- previously in Ann Arbor, MI
  • Liked: 748
  • Likes Given: 290
Re: Elon Musk Reddit AMA on BFR
« Reply #47 on: 10/14/2017 10:56 PM »
AMA is over.

Quote from: Elon Musk
Thanks for tuning in to the AMA. Great questions nk!!
Internal combustion engine in space. It's just a Bad Idea.TM - Robotbeat

Online Michael Baylor

  • Full Member
  • ****
  • Posts: 542
  • Liked: 1056
  • Likes Given: 569
Re: Elon Musk Reddit AMA on BFR
« Reply #48 on: 10/14/2017 10:57 PM »
That was nearly three hours of Elon's time. Very generous of him considering how busy he is.
« Last Edit: 10/14/2017 10:57 PM by Next Spaceflight »

Offline Chris Bergin

Re: Elon Musk Reddit AMA on BFR
« Reply #49 on: 10/14/2017 11:02 PM »
Looks like the AMA is being a bit ruined by a user named __Rocket__. I've seen him a lot on /r/spacex and often makes bad leaps of logic in his posts and theorycrafts even far beyond what we do in this forum.

That Space X Reddit in general has a bad rep from what I’ve read elsewhere on Reddit.

To be clear, he's not well liked there either. SpaceX subreddit is one of the few places on reddit with actual quality content. Quoting Chris.

Quote
Hold the r/space mods hostage until they move it to r/spacex

Or riot. 😉



I don't know about Reddit politics, per Star One's reference to what parts of Reddit think of other parts of Reddit, but the SpaceX Reddit is a *VASTLY* improved sub. It should have been on there.

Re: Elon Musk Reddit AMA on BFR
« Reply #50 on: 10/14/2017 11:05 PM »
Looks like the AMA is being a bit ruined by a user named __Rocket__. I've seen him a lot on /r/spacex and often makes bad leaps of logic in his posts and theorycrafts even far beyond what we do in this forum.

That Space X Reddit in general has a bad rep from what I’ve read elsewhere on Reddit.

To be clear, he's not well liked there either. SpaceX subreddit is one of the few places on reddit with actual quality content. Quoting Chris.

Quote
Hold the r/space mods hostage until they move it to r/spacex

Or riot. 😉



I don't know about Reddit politics, per Star One's reference to what parts of Reddit think of other parts of Reddit, but the SpaceX Reddit is a *VASTLY* improved sub. It should have been on there.

Agreed. The mods over there do a good job and I'm also not sure why Elon didn't do the AMA there. 

Online Ronsmytheiii

  • Moderator
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 22510
  • Liked: 839
  • Likes Given: 300
Re: Elon Musk Reddit AMA on BFR
« Reply #51 on: 10/14/2017 11:08 PM »
Really wish Elon would do another interview with NSF (NSF interviewed him back in the Falcon 1 days in 2006, has to mean something!)

https://www.nasaspaceflight.com/2006/07/elon-muskspacex-interview-part-1/

Edit: With NSF excellent community and moderators, preselecting forum questions for Elon to answer would be much more efficient and effective than any AMA.....
« Last Edit: 10/14/2017 11:10 PM by Ronsmytheiii »
And this is a good reminder that just because one of your fellow space enthusiasts occasionally voices doubts about the SpaceX schedule announcements or is cautious about believing SpaceX has licked a problem before actually seeing proof that's true, it doesn't mean they hate SpaceX.

Online Michael Baylor

  • Full Member
  • ****
  • Posts: 542
  • Liked: 1056
  • Likes Given: 569
Re: Elon Musk Reddit AMA on BFR
« Reply #52 on: 10/14/2017 11:12 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.
« Last Edit: 10/14/2017 11:13 PM by Next Spaceflight »

Offline Geron

  • Full Member
  • **
  • Posts: 217
  • Liked: 55
  • Likes Given: 6
Re: Elon Musk Reddit AMA on BFR
« Reply #53 on: 10/14/2017 11:13 PM »
Thank you Elon for doing the AMA.

My favorite Q and A with Elon was when Helodriver got him nailed down at the SpaceX Dragon 2 revealing. The question quality was superb, the answers flowed freely. It was amazing. 3 Cheers for Helodriver!!

Offline Geron

  • Full Member
  • **
  • Posts: 217
  • Liked: 55
  • Likes Given: 6
Re: Elon Musk Reddit AMA on BFR
« Reply #54 on: 10/14/2017 11:16 PM »
Really wish Elon would do another interview with NSF (NSF interviewed him back in the Falcon 1 days in 2006, has to mean something!)

https://www.nasaspaceflight.com/2006/07/elon-muskspacex-interview-part-1/

Edit: With NSF excellent community and moderators, preselecting forum questions for Elon to answer would be much more efficient and effective than any AMA.....

I think this is why Elon did it in Space and not SpaceX. Last time the mods removed any questions not asked by mods/voted up. I can understand from a mod perspective its nice if only moderators ask questions, but it kind of goes against the spirit of an AMA if no one actually there can participate in the AMA, if its all just arranged ahead of time.

Offline Mongo62

  • Full Member
  • ****
  • Posts: 977
  • Liked: 619
  • Likes Given: 138
Re: Elon Musk Reddit AMA on BFR
« Reply #55 on: 10/14/2017 11:16 PM »
Looks like the AMA is being a bit ruined by a user named __Rocket__. I've seen him a lot on /r/spacex and often makes bad leaps of logic in his posts and theorycrafts even far beyond what we do in this forum.

That Space X Reddit in general has a bad rep from what I’ve read elsewhere on Reddit.

To be clear, he's not well liked there either. SpaceX subreddit is one of the few places on reddit with actual quality content. Quoting Chris.

Quote
Hold the r/space mods hostage until they move it to r/spacex

Or riot. 😉



I don't know about Reddit politics, per Star One's reference to what parts of Reddit think of other parts of Reddit, but the SpaceX Reddit is a *VASTLY* improved sub. It should have been on there.

Agreed. The r/spacex subreddit is surprisingly high quality -- not up to NSF levels, of course, but still good -- but the r/space subreddit is considerably lower in quality. I seem to recall that an earlier AMA by another high-level SpaceX employee ended up being held on r/AMA, and it was basically ruined by the low-quality questions asked. The r/spacex people were furious, given that they had thought the AMA would be held on their subreddit, and had assembled a list of questions that had been high quality, but were left unasked by the r/AMA people. This time was not as bad, but the r/spacex questions would have been better (including many great questions by helodriver).

Re: Elon Musk Reddit AMA on BFR
« Reply #56 on: 10/14/2017 11:19 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 limited myself to about three questions cause I felt anything beyond that would be equivalent to spamming. Oh well. At least a few good questions got answered  :-\
« Last Edit: 10/14/2017 11:20 PM by tvg98 »

Offline RedLineTrain

  • Full Member
  • ****
  • Posts: 757
  • Liked: 467
  • Likes Given: 522
Re: Elon Musk Reddit AMA on BFR
« Reply #57 on: 10/14/2017 11:25 PM »
I don't think the Reddit format is conducive to getting a lot of high quality questions asked in a reasonable amount of time.

Offline Lars-J

  • Senior Member
  • *****
  • Posts: 4440
  • California
  • Liked: 3977
  • Likes Given: 2402
Re: Elon Musk Reddit AMA on BFR
« Reply #58 on: 10/14/2017 11:33 PM »
I don't think the Reddit format is conducive to getting a lot of high quality questions asked in a reasonable amount of time.

Perhaps, but we still learned a lot. An imperfect format, but beggars can’t be choosers, as the saying goes.

Offline Aussie_Space_Nut

  • Full Member
  • **
  • Posts: 211
  • South Australia
  • Liked: 72
  • Likes Given: 228
Re: Elon Musk Reddit AMA on BFR
« Reply #59 on: 10/14/2017 11:36 PM »
This is an odd answer, three legs are more stable than four. I'll be back with links.

Edit: Can't find the link I was after (NASA study for lander legs, Mars Phoenix if I recall correctly) but basically three points define a plane so the lander won't rock. Where it is unlikely that four points on a rough surface form a plane, so it will wobble. Suspension can correct for this.

It is simply geometry.

If you draw 3 points evenly spaced around a circle and then connect each point with a line it creates a triangle.

If you draw 4 points evenly spaced around a circle and then connect each point with a line it creates a square.

Now measure the distance from any side to the centre.

You will now see why 4 legs are preferred. The 3 legged version side distance to centre is much smaller than the 4 legged version.

In other words, the 3 legged version may not 'rock' but it will tip over way easier than the 4 legged version.

Offline biosehnsucht

  • Full Member
  • ***
  • Posts: 308
  • Liked: 107
  • Likes Given: 209
Re: Elon Musk Reddit AMA on BFR
« Reply #60 on: 10/14/2017 11:39 PM »

Offline Chris Bergin

Re: Elon Musk Reddit AMA on BFR
« Reply #61 on: 10/14/2017 11:45 PM »
Really wish Elon would do another interview with NSF (NSF interviewed him back in the Falcon 1 days in 2006, has to mean something!)

https://www.nasaspaceflight.com/2006/07/elon-muskspacex-interview-part-1/

Edit: With NSF excellent community and moderators, preselecting forum questions for Elon to answer would be much more efficient and effective than any AMA.....

Wow, look how messy that formatting is. Several CMS' ago! ;D

Braddock's full set transcripts from his Q&As with Elon are in L2, here for what it's worth:
https://forum.nasaspaceflight.com/index.php?topic=3533.0

That was back in the day Elon e-mailed ME asking if we wanted to interview him! (Seriously!) ;D Helo's excellent Dragon 2 Q&A was great as it wasn't pre-planned. Elon simply gravitated towards him as he was asking the right questions.

And NSF got a nice shoutout from Elon at that time:


Now if Elon did a Q&A here, it'd kill our servers. Even the new servers. Reddit is a massive company, so they can cope. We're not and we wouldn't.

We're improving the site (see signature) starting with the news site and then the forum and then the servers (again), so there's potential for Q&A's like this one:
https://forum.nasaspaceflight.com/index.php?topic=37295.0

But yeah, a live AMA style event....only commercial sites like Reddit or Facebook could handle that sort of traffic from a real time event.

ANYWAY! Let's get focused on what he actually said.

Offline Aussie_Space_Nut

  • Full Member
  • **
  • Posts: 211
  • South Australia
  • Liked: 72
  • Likes Given: 228
Re: Elon Musk Reddit AMA on BFR
« Reply #62 on: 10/14/2017 11:47 PM »
Perhaps NASASpaceFlight should publicly ask Elon 1 question per week?

We could all make suggestions etc on a dedicated thread then pick the best and just ask him.

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
Re: Elon Musk Reddit AMA on BFR
« Reply #63 on: 10/14/2017 11:47 PM »
This is an odd answer, three legs are more stable than four. I'll be back with links.

Edit: Can't find the link I was after (NASA study for lander legs, Mars Phoenix if I recall correctly) but basically three points define a plane so the lander won't rock. Where it is unlikely that four points on a rough surface form a plane, so it will wobble. Suspension can correct for this.

It is simply geometry.
If you draw 3 points evenly spaced around a circle and then connect each point with a line it creates a triangle.
If you draw 4 points evenly spaced around a circle and then connect each point with a line it creates a square.
Now measure the distance from any side to the centre.

You will now see why 4 legs are preferred. The 3 legged version side distance to centre is much smaller than the 4 legged version.

In other words, the 3 legged version may not 'rock' but it will tip over way easier than the 4 legged version.

But that is not all the story, there is the mass of the legs, the ability of the lander to pick a landing site, etc. It's not a simple 2D geometric trade. As another data point if Falcon 9 had 3 legs we wouldn't have ended up with the magic walking booster dancing around the ASDS, it would have been stable.

Offline science_business

  • Member
  • Posts: 10
  • Liked: 5
  • Likes Given: 4
Re: Elon Musk Reddit AMA on BFR
« Reply #64 on: 10/14/2017 11:49 PM »
I would have loved to know what an economy ticket for earth p2p would go for on BFS. Competitive with current airline business class?

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
Re: Elon Musk Reddit AMA on BFR
« Reply #65 on: 10/14/2017 11:51 PM »
For long haul it would be competitive with current economy tickets!  :o

Instagram:-
elonmusk Fly to most places on Earth in under 30 mins and anywhere in under 60. Cost per seat should be about the same as full fare economy in an aircraft. Forgot to mention that.

https://www.instagram.com/p/BZnVfWxgdLe/
« Last Edit: 10/14/2017 11:58 PM by nacnud »

Offline Aussie_Space_Nut

  • Full Member
  • **
  • Posts: 211
  • South Australia
  • Liked: 72
  • Likes Given: 228
Re: Elon Musk Reddit AMA on BFR
« Reply #66 on: 10/14/2017 11:58 PM »
This is an odd answer, three legs are more stable than four. I'll be back with links.

Edit: Can't find the link I was after (NASA study for lander legs, Mars Phoenix if I recall correctly) but basically three points define a plane so the lander won't rock. Where it is unlikely that four points on a rough surface form a plane, so it will wobble. Suspension can correct for this.

It is simply geometry.
If you draw 3 points evenly spaced around a circle and then connect each point with a line it creates a triangle.
If you draw 4 points evenly spaced around a circle and then connect each point with a line it creates a square.
Now measure the distance from any side to the centre.

You will now see why 4 legs are preferred. The 3 legged version side distance to centre is much smaller than the 4 legged version.

In other words, the 3 legged version may not 'rock' but it will tip over way easier than the 4 legged version.

But that is not all the story, there is the mass of the legs, the ability of the lander to pick a landing site, etc. It's not a simple 2D geometric trade. As another data point if Falcon 9 had 3 legs we wouldn't have ended up with the magic walking booster dancing around the ASDS, it would have been stable.

I agree.

But.

I'm sure SpaceX have done their sums and determined that they can build a 4 legged system more efficiently than a 3 legged system.

In other words, given that they must have some minimum 'tip over' figure, 4 smaller legs is more efficient than 3 larger legs.

Offline Lars-J

  • Senior Member
  • *****
  • Posts: 4440
  • California
  • Liked: 3977
  • Likes Given: 2402
Elon Musk Reddit AMA on BFR
« Reply #67 on: 10/15/2017 12:05 AM »
There has been plenty of discussion in the past here about the optimal number of legs, so let’s not re-fight that battle. Lots of pros and cons were discussed, just search for it.

Once they abandoned trilateral symmetry, it is IMO natural that they reverted to 4 legs.
« Last Edit: 10/15/2017 12:05 AM by Lars-J »

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
Re: Elon Musk Reddit AMA on BFR
« Reply #68 on: 10/15/2017 12:09 AM »
I have been hunting for the documentary that went into the number of legs, I believe it was shot at JPL. I can't find it and I hate discussions that are just opinion with no data to back them up! :) It will be interesting to see how the designs evolve from here.
« Last Edit: 10/15/2017 12:22 AM by nacnud »

Offline Aussie_Space_Nut

  • Full Member
  • **
  • Posts: 211
  • South Australia
  • Liked: 72
  • Likes Given: 228
Re: Elon Musk Reddit AMA on BFR
« Reply #69 on: 10/15/2017 12:21 AM »
Yes the old 3 legs / 4 legs debate is an old one, not just limited to landers either.

My experience came from a general engineering point of view. (I'm not an Engineer, I'm a CNC Machinist/CAD Drafty.)

3 legs is always more stable so long as you can make them far enough apart.

However if you can't for some reason then 4 legs is the way to go.

Offline garcianc

  • Member
  • Posts: 63
  • washington, dc
  • Liked: 79
  • Likes Given: 15
Re: Elon Musk Reddit AMA on BFR
« Reply #70 on: 10/15/2017 03:10 AM »
Sorry for continuing the 3 leg vs 4 leg sidebar. Mods, feel free to move this post.

Hopefully, the attached paper helps. It discusses, among other things, exactly what Aussie_Space_Nut mentions, that a 3 leg landing gear needs to have a wider diameter than a 4 leg landing gear.

Offline su27k

  • Full Member
  • ****
  • Posts: 1207
  • Liked: 937
  • Likes Given: 81
Re: Elon Musk Reddit AMA on BFR
« Reply #71 on: 10/15/2017 03:34 AM »
Looks like the AMA is being a bit ruined by a user named __Rocket__. I've seen him a lot on /r/spacex and often makes bad leaps of logic in his posts and theorycrafts even far beyond what we do in this forum.

Does it really matter who asks the questions as long as the right questions get asked? His questions are not bad, there're one or two that are a bit out there but most are what I'd like to know.

Offline su27k

  • Full Member
  • ****
  • Posts: 1207
  • Liked: 937
  • Likes Given: 81
Re: Elon Musk Reddit AMA on BFR
« Reply #72 on: 10/15/2017 03:42 AM »
Really wish Elon would do another interview with NSF (NSF interviewed him back in the Falcon 1 days in 2006, has to mean something!)

https://www.nasaspaceflight.com/2006/07/elon-muskspacex-interview-part-1/

Edit: With NSF excellent community and moderators, preselecting forum questions for Elon to answer would be much more efficient and effective than any AMA.....

I think this is why Elon did it in Space and not SpaceX. Last time the mods removed any questions not asked by mods/voted up. I can understand from a mod perspective its nice if only moderators ask questions, but it kind of goes against the spirit of an AMA if no one actually there can participate in the AMA, if its all just arranged ahead of time.

I don't think that's the case, last /r/spacex AMA went pretty well. I suspect Elon chose /r/space because it's way more popular than /r/spacex. Right now the AMA thread has 63k upvotes, 11k comments and it's at #6 on the reddit homepage.

Offline meekGee

  • Senior Member
  • *****
  • Posts: 8796
  • N. California
  • Liked: 4988
  • Likes Given: 904
Re: Elon Musk Reddit AMA on BFR
« Reply #73 on: 10/15/2017 05:23 AM »
This is an odd answer, three legs are more stable than four. I'll be back with links.

Edit: Can't find the link I was after (NASA study for lander legs, Mars Phoenix if I recall correctly) but basically three points define a plane so the lander won't rock. Where it is unlikely that four points on a rough surface form a plane, so it will wobble. Suspension can correct for this.

3 legs indeed do, but have a higher chance of the c.g. falling outside the triangle.
The square is a larger base, and the leg that's left in the air (if out of range) - you don't want it anyway.
ABCD - Always Be Counting Down

Online Cheapchips

  • Full Member
  • **
  • Posts: 234
  • UK
  • Liked: 136
  • Likes Given: 264
Re: Elon Musk Reddit AMA on BFR
« Reply #74 on: 10/15/2017 05:49 AM »

This (IG) noble prize winning piece of research may be invaluable in preventing wobbliness of a four legged rocket.

https://www.improbable.com/2010/06/30/on-the-wobbliness-of-tables/

Offline Hauerg

  • Full Member
  • ****
  • Posts: 757
  • Berndorf, Austria
  • Liked: 361
  • Likes Given: 1147
Re: Elon Musk Reddit AMA on BFR
« Reply #75 on: 10/15/2017 06:12 AM »
This is an odd answer, three legs are more stable than four. I'll be back with links.

Edit: Can't find the link I was after (NASA study for lander legs, Mars Phoenix if I recall correctly) but basically three points define a plane so the lander won't rock. Where it is unlikely that four points on a rough surface form a plane, so it will wobble. Suspension can correct for this.

3 legs indeed do, but have a higher chance of the c.g. falling outside the triangle.
The square is a larger base, and the leg that's left in the air (if out of range) - you don't want it anyway.

But with 4 there is a higher chance for one of the legs failing than w 3.
And then the ship is doomed in both cases.
So 3 would be the safer way, as long as you cannot shift CG instantly.

Offline Nibb31

  • Full Member
  • **
  • Posts: 263
  • France
  • Liked: 154
  • Likes Given: 8
Re: Elon Musk Reddit AMA on BFR
« Reply #76 on: 10/15/2017 08:45 AM »
There have been plenty of studies about the number of landing legs, including Apollo, where 4 legs were adequate. The number of legs depends on the diameter of the base circle, the CoG of the ship, and the weight of the system. If you use 3 legs instead of 4, then the circle needs to be wider, and the legs longer and heavier. The more legs you use, the shorter they can be to cover the same circle, however there are diminishing returns and weight penalties. Apollo studies determined that 4 was the optimal number for their application.

Offline KelvinZero

  • Senior Member
  • *****
  • Posts: 3878
  • Liked: 633
  • Likes Given: 164
Re: Elon Musk Reddit AMA on BFR
« Reply #77 on: 10/15/2017 09:55 AM »
I wonder if the landing software could be clever enough to detect one failed leg, and tilt the ship a bit to keep the COM within the remaining legs.

That is one trick that would not work well starting with three legs :)

Offline Kaputnik

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2831
  • Liked: 475
  • Likes Given: 452
Re: Elon Musk Reddit AMA on BFR
« Reply #78 on: 10/15/2017 11:30 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??
Waiting for joy and raptor

Offline Nibb31

  • Full Member
  • **
  • Posts: 263
  • France
  • Liked: 154
  • Likes Given: 8
Re: Elon Musk Reddit AMA on BFR
« Reply #79 on: 10/15/2017 11:32 AM »
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.
« Last Edit: 10/15/2017 11:34 AM by Nibb31 »

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
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

  • Full Member
  • ****
  • Posts: 1110
  • Seacoast NH
  • Liked: 762
  • Likes Given: 325
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.”

Online guckyfan

  • Senior Member
  • *****
  • Posts: 6793
  • Germany
  • Liked: 1809
  • Likes Given: 1844
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

  • Rocket Boi
  • Member
  • Posts: 70
  • Richmond, CA
  • Liked: 61
  • Likes Given: 434
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.

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
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.

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
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

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2831
  • Liked: 475
  • Likes Given: 452
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

  • Full Member
  • ****
  • Posts: 1183
  • Tacoma, WA
  • Liked: 2222
  • Likes Given: 2488
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 »

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
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

  • Rocket Boi
  • Member
  • Posts: 70
  • Richmond, CA
  • Liked: 61
  • Likes Given: 434
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

  • Senior Member
  • *****
  • Posts: 28616
  • Minnesota
  • Liked: 8585
  • Likes Given: 5600
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.
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

  • Senior Member
  • *****
  • Posts: 28616
  • Minnesota
  • Liked: 8585
  • Likes Given: 5600
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

  • Senior Member
  • *****
  • Posts: 28616
  • Minnesota
  • Liked: 8585
  • Likes Given: 5600
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

  • Full Member
  • ****
  • Posts: 1183
  • Tacoma, WA
  • Liked: 2222
  • Likes Given: 2488
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

  • Member
  • Posts: 44
  • Salt Lake City, Utah
  • Liked: 68
  • Likes Given: 165
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

  • Full Member
  • **
  • Posts: 217
  • Liked: 55
  • Likes Given: 6
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

  • Full Member
  • ****
  • Posts: 895
  • Liked: 210
  • Likes Given: 108
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?

Offline KelvinZero

  • Senior Member
  • *****
  • Posts: 3878
  • Liked: 633
  • Likes Given: 164
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

  • Full Member
  • ****
  • Posts: 1183
  • Tacoma, WA
  • Liked: 2222
  • Likes Given: 2488
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.

Offline aero

  • Senior Member
  • *****
  • Posts: 3005
  • 92129
  • Liked: 801
  • Likes Given: 285
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.
Retired, working interesting problems

Offline Ludus

  • Full Member
  • ****
  • Posts: 1170
  • Liked: 563
  • Likes Given: 283
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

  • Member
  • Posts: 44
  • Salt Lake City, Utah
  • Liked: 68
  • Likes Given: 165
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

  • Full Member
  • ****
  • Posts: 1170
  • Liked: 563
  • Likes Given: 283
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

  • Full Member
  • ****
  • Posts: 572
  • Liked: 234
  • Likes Given: 7
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

  • Full Member
  • ****
  • Posts: 1183
  • Tacoma, WA
  • Liked: 2222
  • Likes Given: 2488
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.

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.

Offline vaporcobra

  • Full Member
  • ****
  • Posts: 1183
  • Tacoma, WA
  • Liked: 2222
  • Likes Given: 2488
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

  • Full Member
  • *
  • Posts: 182
  • Belgium
  • Liked: 26
  • Likes Given: 44
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 »

Offline rockets4life97

  • Full Member
  • ****
  • Posts: 567
  • Liked: 272
  • Likes Given: 236
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

  • Full Member
  • *
  • Posts: 182
  • Belgium
  • Liked: 26
  • Likes Given: 44
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

  • Full Member
  • ****
  • Posts: 884
  • Liked: 360
  • Likes Given: 329
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.

Online GORDAP

  • Full Member
  • *
  • Posts: 177
  • St. Petersburg, FL
  • Liked: 74
  • Likes Given: 29
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

  • Full Member
  • ****
  • Posts: 1157
  • Liked: 1063
  • Likes Given: 167
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

  • Full Member
  • *
  • Posts: 182
  • Belgium
  • Liked: 26
  • Likes Given: 44
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

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
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.

Offline rockets4life97

  • Full Member
  • ****
  • Posts: 567
  • Liked: 272
  • Likes Given: 236
Re: Elon Musk Reddit AMA on BFR
« Reply #120 on: 10/16/2017 01:15 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.

Right. The value of a modified BFS as a tanker would be to reduce the number of re-fueling flights needed to refill a cargo or crew BFS to full. If 5 current refueling flights are planned, then the new tanker would need to have the ability to get 1.25 times the fuel to orbit.

Offline Explorer

  • Member
  • Posts: 25
  • Third rock from the sun
  • Liked: 10
  • Likes Given: 1
Re: Elon Musk Reddit AMA on BFR
« Reply #121 on: 10/16/2017 01:29 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?

They will use them as base camp for a while, but send them back when they are not needed anymore. One BFS probably will be left as a monument. Could be used as city hall, museum or whatever.
Send orbiters to Uranus and Neptune, dammit.

Offline envy887

  • Senior Member
  • *****
  • Posts: 4829
  • Liked: 2716
  • Likes Given: 1446
Re: Elon Musk Reddit AMA on BFR
« Reply #122 on: 10/16/2017 01:57 PM »
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.

Can use an ogive tank for the nose cone: see the Space Shuttle External Tank LOX tank. The ET had crazy dry mass fractions.

Online GORDAP

  • Full Member
  • *
  • Posts: 177
  • St. Petersburg, FL
  • Liked: 74
  • Likes Given: 29
Re: Elon Musk Reddit AMA on BFR
« Reply #123 on: 10/16/2017 02:29 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 :)

Err, I guess it is semantics, but everyone here has for decades referred to something in orbit that holds fuel as an 'orbital depot', and the craft that launches from a surface to supply such a depot (or another ship directly) as a 'tanker'.  So I think it is less confusing for all concerned if we stick with common terminology.

If you're also proposing sending one of these depots to Mars orbit (after doing aerobraking in the Mars atmosphere), I think that is not ever going to happen, for the following reasons:  A) One has to use ISRU produced fuel to get off of Mars surface anyway, so just use it to return to Earth - the BFS is sized to make this trip without requiring Mars orbit refueling.  And B), once you have ISRU production on Mars surface, it is much easier to supply a Mars orbiting depot with fuel from Mars surface than from Earth's surface (if a need for such a depot ever materialized).

Offline corneliussulla

  • Full Member
  • **
  • Posts: 211
  • Liked: 63
  • Likes Given: 19
Re: Elon Musk Reddit AMA on BFR
« Reply #124 on: 10/16/2017 02:32 PM »
As the only cargo for a tanker is in the fuel tanks u would think the overall volume for the tanker will be less than the BFS cargo/passenger variants. The reduction in weight with a smaller structure can be extra propellant. I suppose u could imagine a future when (given the low cost of taking stuff to LEO) SpaceX build a much larger tank in space which has a continuous flow of tankers travelling up and filling it. Then BFS's going to the moon, cis lunar space, Lagrange points, mars and potentially contracted to asteroid mining companies could fill up without having to hang around in orbit waiting for 4/5 tankers. If such a thing could hold 5 BFS refills worth a tanker delivery each day would enable 5 BFS beyond LEO every 20 days, assuming dedicated tankers held 25% more fuel than dual purpose tankers.

Maybe a worthwhile thing to ensure rockets with human cargo not loitering in LEO waiting for tank fill.


Offline John Alan

  • Full Member
  • ****
  • Posts: 781
  • Central IL - USA - Earth
  • Liked: 473
  • Likes Given: 1908
Re: Elon Musk Reddit AMA on BFR
« Reply #125 on: 10/16/2017 02:39 PM »
The problem with dedicated orbital depots is... what orbit do you put it in?... ???

With using just normal BFS tankers... (1100 tonnes prop tanks, no payload)
The thought is, you look at planning and determine a full load is needed in this orbit, on this date...
5 tanker flights later... there is a full tanker in that orbit... on that date...
Work backwards from point and time of need... and it's there...

Only if many can agree on an orbit, would a permanent on orbit depot make any sense... 

Now a version of BFS tanker that can be flown with a partial load... then filled well beyond 1100 tonnes in orbit... then tops up an outgoing (to Moon, Mars, etc) BFS brim full with enough prop left over to get back to the ground..
Now that makes sense long term... once there is such flights happening...which fits what EM is implying here...
 ;)
« Last Edit: 10/16/2017 02:52 PM by John Alan »

Offline rsdavis9

Re: Elon Musk Reddit AMA on BFR
« Reply #126 on: 10/16/2017 03:11 PM »
The problem with dedicated orbital depots is... what orbit do you put it in?... ???

With using just normal BFS tankers... (1100 tonnes prop tanks, no payload)
The thought is, you look at planning and determine a full load is needed in this orbit, on this date...
5 tanker flights later... there is a full tanker in that orbit... on that date...
Work backwards from point and time of need... and it's there...

Only if many can agree on an orbit, would a permanent on orbit depot make any sense... 

Now a version of BFS tanker that can be flown with a partial load... then filled well beyond 1100 tonnes in orbit... then tops up an outgoing (to Moon, Mars, etc) BFS brim full with enough prop left over to get back to the ground..
Now that makes sense long term... once there is such flights happening...which fits what EM is implying here...
 ;)

Also I saw Elon said that for a lunar landing and return with no lunar surface refueling would require refueling in an elliptical transfer orbit. Basically it takes more delta-v to go to the moon and back versus going to mars surface. So 2 different orbits for missions that we know they will be wanting to do in the near future.
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.

Offline philw1776

  • Full Member
  • ****
  • Posts: 1110
  • Seacoast NH
  • Liked: 762
  • Likes Given: 325
Re: Elon Musk Reddit AMA on BFR
« Reply #127 on: 10/16/2017 03:22 PM »
A future low dry mass tanker could simply be a 2nd stage with only 2 SL Raptors and a very short, nearly nil "cargo" volume.  Extra propellant tanks pretty much fit in the nose volume.  Or simply stretch the std propellant tanks by just a couple meters and have zero cargo volume/length. 

So the BFS Custom Tanker would be very short & stubby and "look weird".

Ironic that the shorter BFR tanker would deliver more mass to LEO than the "standard" taller config.
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline John Alan

  • Full Member
  • ****
  • Posts: 781
  • Central IL - USA - Earth
  • Liked: 473
  • Likes Given: 1908
Re: Elon Musk Reddit AMA on BFR
« Reply #128 on: 10/16/2017 06:07 PM »
The problem with dedicated orbital depots is... what orbit do you put it in?... ???

With using just normal BFS tankers... (1100 tonnes prop tanks, no payload)
The thought is, you look at planning and determine a full load is needed in this orbit, on this date...
5 tanker flights later... there is a full tanker in that orbit... on that date...
Work backwards from point and time of need... and it's there...

Only if many can agree on an orbit, would a permanent on orbit depot make any sense... 

Now a version of BFS tanker that can be flown with a partial load... then filled well beyond 1100 tonnes in orbit... then tops up an outgoing (to Moon, Mars, etc) BFS brim full with enough prop left over to get back to the ground..
Now that makes sense long term... once there is such flights happening...which fits what EM is implying here...
 ;)

Also I saw Elon said that for a lunar landing and return with no lunar surface refueling would require refueling in an elliptical transfer orbit. Basically it takes more delta-v to go to the moon and back versus going to mars surface. So 2 different orbits for missions that we know they will be wanting to do in the near future.
A tanker that can be overstuff in LEO to deliver 1100 spare tonnes of prop to a high elliptical orbit waiting ship and then get back to the ground on the leftover prop perhaps would be handy...
I saw somewhere here on NSF that someone calculated just lengthening the prop tank to the end of the cylindrical (outer mold line [1]) nets 1350 tonnes prop on board...
I'm still on the "one tank" group here that thinks adding a separate tank is heavier and more complicated then just make it fit in the OML but launch on partial tanks...
EM comment of "looks different" does puzzle me however...  ??? 

[1] defined OML as outer mold line for reference

Offline Peter.Colin

  • Full Member
  • *
  • Posts: 182
  • Belgium
  • Liked: 26
  • Likes Given: 44
Re: Elon Musk Reddit AMA on BFR
« Reply #129 on: 10/16/2017 06:26 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 :)

Err, I guess it is semantics, but everyone here has for decades referred to something in orbit that holds fuel as an 'orbital depot', and the craft that launches from a surface to supply such a depot (or another ship directly) as a 'tanker'.  So I think it is less confusing for all concerned if we stick with common terminology.

If you're also proposing sending one of these depots to Mars orbit (after doing aerobraking in the Mars atmosphere), I think that is not ever going to happen, for the following reasons:  A) One has to use ISRU produced fuel to get off of Mars surface anyway, so just use it to return to Earth - the BFS is sized to make this trip without requiring Mars orbit refueling.  And B), once you have ISRU production on Mars surface, it is much easier to supply a Mars orbiting depot with fuel from Mars surface than from Earth's surface (if a need for such a depot ever materialized).


Suppose it’s 2028 and only 2 cargo ships are on the surface of Mars, the rest returned back to Earth.
There are 10 ships heading to Mars in 2030, and 9 of them want to fly back immediately after they unloaded their cargo so they are back on Earth 3 months later. They can only do that if this fuel gets stored in something other than the 2 cargo ships.

When the two cargo ships are full they could offload the ISRU fuel in the larger weird looking Mars orbital depot-tanker, and go back to the surface of Mars, to get more ISRU fuel.

When the ten ships land nine of them can immediately be filled up with 20% fuel out of the fully fueled cargo ships, to just make it into orbit.
There they all can be fueled up fully with one orbital depot-tanker.

Storing fuel in a large depot-tank in orbit has a few advantages:
1) The fuel to get fuel into orbit doesn’t need to be storred, its burned to get the fuel into orbit.
2) The large tanks don’t need to land, so that they can be bigger.
3) More cargo can be brought back.
4) Ships can fly back faster because they can be topped off in orbit.

It could be weird looking because it could be larger in diameter than the BFR that initially launches it.




« Last Edit: 10/16/2017 06:38 PM by Peter.Colin »

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2383
  • Liked: 601
  • Likes Given: 238
Re: Elon Musk Reddit AMA on BFR
« Reply #130 on: 10/16/2017 06:36 PM »
Thats a very specific set of circumstances, not really in the vain of what was said in the AMA, or in the IAC presentation.

Offline DOCinCT

Re: Elon Musk Reddit AMA on BFR
« Reply #131 on: 10/16/2017 06:43 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.
Back in 2011, Precision Combustion, Inc (a local CT company) and NASA built and evaluated a small, highly efficient catalyst based Sabatier reformer.  Smaller than a coffee can, it produced enough O2 for at least 4 people.  There were strong indications that the reactor could operate under vacuum conditions.
Additionally, NASA has had a demonstration unit on the ISS producing water from excess C02 and H20.
Whether or not SpaceX is using PCI tech we don't, and may never, know, but regardless, the technology exists, and works.

Online guckyfan

  • Senior Member
  • *****
  • Posts: 6793
  • Germany
  • Liked: 1809
  • Likes Given: 1844
Re: Elon Musk Reddit AMA on BFR
« Reply #132 on: 10/16/2017 07:33 PM »
We have one more little detail. Elon mentioned water ice as a requirement for a landing site. This probably means getting water out of minerals is out.

The remark "ISRU dev is pretty far along" hopefully includes development of the mining equipment which IMO was always the long pole. I never doubted the other components.

Offline vaporcobra

  • Full Member
  • ****
  • Posts: 1183
  • Tacoma, WA
  • Liked: 2222
  • Likes Given: 2488
Re: Elon Musk Reddit AMA on BFR
« Reply #133 on: 10/16/2017 09:25 PM »
We have one more little detail. Elon mentioned water ice as a requirement for a landing site. This probably means getting water out of minerals is out.

The remark "ISRU dev is pretty far along" hopefully includes development of the mining equipment which IMO was always the long pole. I never doubted the other components.

Yep. Actually gathering the raw material is undoubtedly the longest pole for ISRU on Mars or the Moon. The obvious solution is to gather the inputs from the atmosphere, but that would not be applicable on the Moon or basically anywhere else in the solar system. I would love to see some fancy SpaceX robotics with Tesla/Solar City synergy :)

Offline mnelson

  • Member
  • Posts: 44
  • Salt Lake City, Utah
  • Liked: 68
  • Likes Given: 165
Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #134 on: 10/17/2017 12:46 AM »
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....

Does the statement above mean that the tanker will be incapable of hovering? My understanding is that F9 is incapable of hovering, i.e. T/W is always >1. So the key is to reach a velocity of 0 right at the landing surface. Is he saying it will be the same procedure for the tanker? And how come there was'n a similar discussion about BFS. Is it expected to be able to hover.

Offline philw1776

  • Full Member
  • ****
  • Posts: 1110
  • Seacoast NH
  • Liked: 762
  • Likes Given: 325
Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #135 on: 10/17/2017 01:22 AM »
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....

Does the statement above mean that the tanker will be incapable of hovering? My understanding is that F9 is incapable of hovering, i.e. T/W is always >1. So the key is to reach a velocity of 0 right at the landing surface. Is he saying it will be the same procedure for the tanker? And how come there was'n a similar discussion about BFS. Is it expected to be able to hover.

174 ton thrust SL landing engine.  BFS mass 85 tons.  Add in propellant.
Now if a Raptor is speced to throttle down to 20%, yes it could hover.
Why it would waste propellant like that, I don't know.
« Last Edit: 10/17/2017 01:23 AM by philw1776 »
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline vaporcobra

  • Full Member
  • ****
  • Posts: 1183
  • Tacoma, WA
  • Liked: 2222
  • Likes Given: 2488
Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #136 on: 10/17/2017 01:31 AM »
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....

Does the statement above mean that the tanker will be incapable of hovering? My understanding is that F9 is incapable of hovering, i.e. T/W is always >1. So the key is to reach a velocity of 0 right at the landing surface. Is he saying it will be the same procedure for the tanker? And how come there was'n a similar discussion about BFS. Is it expected to be able to hover.

Hmmmmm. Just realized we don't know if the tanker will also be updated to have three SL Raptors. I'd expect the answer to be yes if the main purpose is increased reliability and granularity.

With deep throttle capabilities of 20%, three Raptors rated for 1700kN (380 klbf), and a dry mass no more than 85t (187393 lb), the minimum thrust with three engines firing is 228 klbf. With two, it's 152 klbf. So a three engined BFS definitely could hover briefly with 5-10% of fuel retained for landing, especially with a dedicated a considerably lighter tanker version.

Hovering begs the question, however. It's a horrible idea if you value efficiency, gravity losses for a hovering rocket are comically large. On Mars, that changes and I'd expect hovering to be used whenever possible, but absolutely not on Earth.
« Last Edit: 10/17/2017 01:33 AM by vaporcobra »

Offline biosehnsucht

  • Full Member
  • ***
  • Posts: 308
  • Liked: 107
  • Likes Given: 209
Re: Elon Musk Reddit AMA on BFR
« Reply #137 on: 10/17/2017 05:10 AM »
They might almost-hover for P2P flights to make nice soft landings. But not actually hover, just make the landings as soft and smooth as they can versus say, a high energy ASDS Falcon 9 landing :D

Offline rsdavis9

Re: Elon Musk Reddit AMA on BFR
« Reply #138 on: 10/17/2017 11:24 AM »
Whats interesting is that if they switch to one engine to land then having the 3 engines in a row might make more sense so that one engine is dead center in the middle. That way the one engine does not need to gimbal off center.
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.

Offline Peter.Colin

  • Full Member
  • *
  • Posts: 182
  • Belgium
  • Liked: 26
  • Likes Given: 44
Re: Elon Musk Reddit AMA on BFR
« Reply #139 on: 10/17/2017 01:41 PM »
Whats interesting is that if they switch to one engine to land then having the 3 engines in a row might make more sense so that one engine is dead center in the middle. That way the one engine does not need to gimbal off center.

We never seen the center cluster in a line, but it does make sense.

Offline hkultala

  • Full Member
  • ****
  • Posts: 884
  • Liked: 360
  • Likes Given: 329
Re: Elon Musk Reddit AMA on BFR
« Reply #140 on: 10/17/2017 01:53 PM »
Whats interesting is that if they switch to one engine to land then having the 3 engines in a row might make more sense so that one engine is dead center in the middle. That way the one engine does not need to gimbal off center.

No, it would just make the engine-out redundancy much harder, because the 2 other engines would be much further away from the centerline.

Offline hkultala

  • Full Member
  • ****
  • Posts: 884
  • Liked: 360
  • Likes Given: 329
Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #141 on: 10/17/2017 01:59 PM »
Hmmmmm. Just realized we don't know if the tanker will also be updated to have three SL Raptors. I'd expect the answer to be yes if the main purpose is increased reliability and granularity.

With deep throttle capabilities of 20%, three Raptors rated for 1700kN (380 klbf), and a dry mass no more than 85t (187393 lb),

Dedicated tanker version will be lighter than the cargo and crew versions. Something like 70 tonnes maybe.

Quote
the minimum thrust with three engines firing is 228 klbf. With two, it's 152 klbf. So a three engined BFS definitely could hover briefly with 5-10% of fuel retained for landing, especially with a dedicated a considerably lighter tanker version.

No, the hovering is only interesting just before touchdown, not high in the air.

And just before touchdown it does not anymore have 5-10% of fuel left. It only has like 1% margin left.

So the numbers become like 70 tonnes vehicle weight, and 10 tonnes fuel weight, total ~ 80 tonnes.
Two engines at 20% is 68 tonnes, three engines at 20% is 102 tonnes.

Offline meekGee

  • Senior Member
  • *****
  • Posts: 8796
  • N. California
  • Liked: 4988
  • Likes Given: 904
Re: Elon Musk Reddit AMA on BFR
« Reply #142 on: 10/17/2017 03:53 PM »
They're going to have to have a very full-proof mechanism against fueling mistakes...  Like aforementioned landing tanks.

When an airliner runs out of fuel, it's one thing...  But if you're 100 gallons short on a VTVL landing, engine redundancy is not going to be helpful...

Since atmospheric free fall is "the great equalizer", it erases any fuel consumption deviations that occurred prior to reentry. But the hoverslam happens after that, and so the landing tanks need to be 100% full before reentry, and used only for the hoverslam.


ABCD - Always Be Counting Down

Offline speedevil

  • Senior Member
  • *****
  • Posts: 3191
  • Fife
  • Liked: 1631
  • Likes Given: 1958
Re: Elon Musk Reddit AMA on BFR
« Reply #143 on: 10/17/2017 04:01 PM »
When an airliner runs out of fuel, it's one thing...  But if you're 100 gallons short on a VTVL landing, engine redundancy is not going to be helpful...

Which is why every landing area needs a ball-pit next to it for low fuel entries.
(just kidding) (probably)

Offline Peter.Colin

  • Full Member
  • *
  • Posts: 182
  • Belgium
  • Liked: 26
  • Likes Given: 44
Re: Elon Musk Reddit AMA on BFR
« Reply #144 on: 10/17/2017 05:28 PM »
Whats interesting is that if they switch to one engine to land then having the 3 engines in a row might make more sense so that one engine is dead center in the middle. That way the one engine does not need to gimbal off center.

No, it would just make the engine-out redundancy much harder, because the 2 other engines would be much further away from the centerline.

If the middle one fails it doesn’t matter if they are further away from the centerline both fire at 50% throttle and the force is balanced out.
If one of the outer ones fails you have one in the centerline at 100% throttle, so perfect balance.
In a triangular confuguration the force is never balanced out when one engine fails.


« Last Edit: 10/17/2017 05:36 PM by Peter.Colin »

Offline leetdan

  • Full Member
  • **
  • Posts: 211
  • Space Coast
  • Liked: 139
  • Likes Given: 70
Re: Elon Musk Reddit AMA on BFR
« Reply #145 on: 10/17/2017 06:07 PM »
If the 'hover-slam' is a solvable control problem with a single engine, it isn't much of a stretch to envision a 'hover-tip' maneuver with multiple engines firing off-center.  The stack is tilted during descent to prevent XY acceleration, with Z velocity and now also deviation from vertical reaching 0 precisely at touchdown.  It's obviously harder, with additional trades needed WRT landing gear, but not unsolvable.
« Last Edit: 10/17/2017 06:08 PM by leetdan »

Offline Lar

  • Fan boy at large
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 10799
  • Saw Gemini live on TV
  • A large LEGO storage facility ... in Michigan
  • Liked: 7722
  • Likes Given: 5593
Re: Elon Musk Reddit AMA on BFR
« Reply #146 on: 10/17/2017 06:57 PM »
If the 'hover-slam' is a solvable control problem with a single engine, it isn't much of a stretch to envision a 'hover-tip' maneuver with multiple engines firing off-center.  The stack is tilted during descent to prevent XY acceleration, with Z velocity and now also deviation from vertical reaching 0 precisely at touchdown.  It's obviously harder, with additional trades needed WRT landing gear, but not unsolvable.

Coslne losses suggest it might use more fuel than is absolutely necessary though, no?
"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 acsawdey

  • Full Member
  • ****
  • Posts: 402
  • Minnesota
  • Liked: 244
  • Likes Given: 479
Re: Elon Musk Reddit AMA on BFR
« Reply #147 on: 10/17/2017 08:15 PM »
If the 'hover-slam' is a solvable control problem with a single engine, it isn't much of a stretch to envision a 'hover-tip' maneuver with multiple engines firing off-center.  The stack is tilted during descent to prevent XY acceleration, with Z velocity and now also deviation from vertical reaching 0 precisely at touchdown.  It's obviously harder, with additional trades needed WRT landing gear, but not unsolvable.

Coslne losses suggest it might use more fuel than is absolutely necessary though, no?

The gimbal angle can't be all that big and cos(12 degrees) is 0.978 so the losses are a few percent or less.

Offline Lar

  • Fan boy at large
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 10799
  • Saw Gemini live on TV
  • A large LEGO storage facility ... in Michigan
  • Liked: 7722
  • Likes Given: 5593
Re: Elon Musk Reddit AMA on BFR
« Reply #148 on: 10/17/2017 08:28 PM »
If the 'hover-slam' is a solvable control problem with a single engine, it isn't much of a stretch to envision a 'hover-tip' maneuver with multiple engines firing off-center.  The stack is tilted during descent to prevent XY acceleration, with Z velocity and now also deviation from vertical reaching 0 precisely at touchdown.  It's obviously harder, with additional trades needed WRT landing gear, but not unsolvable.

Coslne losses suggest it might use more fuel than is absolutely necessary though, no?

The gimbal angle can't be all that big and cos(12 degrees) is 0.978 so the losses are a few percent or less.


If you're landing on fumes, it does matter. but yeah.
"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 cppetrie

  • Full Member
  • ****
  • Posts: 625
  • Liked: 363
  • Likes Given: 3
Re: Elon Musk Reddit AMA on BFR
« Reply #149 on: 10/18/2017 01:34 AM »
I got the impression that under nominal conditions all three landing engines will be firing. If there is an engine out the remaining two would throttle up to compensate vs only one running normally and another starting up in an engine out event. Startup would take way longer than just throttling up, which is very problematic when a failure occurs at the worst possible point in the landing. So with normal operation of all three engines running a triangular arrangement is symmetrical in all directions. Under engine out, the remaining two only need enough gimble to compensate for the failed engine. Worst case you provide enough gimble to compensate for two engines out. Since you can’t predict which engine will go out you want all three engines as close to center as possible, which the triangular arrangement also provides. Triangular is also the most centrally compact arrangement. My guess is a triangular arrangement for the landing engines.

Caveat: I’m only an arm-chair rocket engineer so I might have a flaw in my logic. If so, please feel free to kindly point it out. Thx.

Offline mgeagon

  • Member
  • Posts: 88
  • Hong Kong
  • Liked: 69
  • Likes Given: 1
Re: Elon Musk Reddit AMA on BFR
« Reply #150 on: 10/18/2017 05:30 AM »
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.

I believe taking a look at the productuon runs of large transport category aircraft is instructive. The first few planes out of the factory are largely hand built, with tooling modified on the go and quality control standards yet to be developed. These aircraft are then added to the certification regimen and put through flight envelope testing. After a type certificate is issued by the FAA or other governmental agency, the test aircraft are refurbished and delivered to the launch customers.

These initial articles are typically overweight and are susceptible to long-term chronic maintenance, yet they continue to fly for decades. When major upgrades or airworthiness directives are issued, all aircraft of the same type are upgraded to the new standard.

It appears the business case for the BFR is predicated upon 1000 times reuse, meaning the development costs are spread between the number of ships built X the amount of flights they make. Counter to this would be a single flight to Mars and then becoming a museum piece in SITU. It would make much more fiscal sense to send the ship back to Earth for reuse. Even major upgrades to the engines or avionics would be far cheaper than scrapping the entire rocket. At least that is clearly true with atmospheric vehicles.

Offline aero

  • Senior Member
  • *****
  • Posts: 3005
  • 92129
  • Liked: 801
  • Likes Given: 285
Re: Elon Musk Reddit AMA on BFR
« Reply #151 on: 10/18/2017 06:51 AM »
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.

I believe taking a look at the productuon runs of large transport category aircraft is instructive. The first few planes out of the factory are largely hand built, with tooling modified on the go and quality control standards yet to be developed. These aircraft are then added to the certification regimen and put through flight envelope testing. After a type certificate is issued by the FAA or other governmental agency, the test aircraft are refurbished and delivered to the launch customers.

These initial articles are typically overweight and are susceptible to long-term chronic maintenance, yet they continue to fly for decades. When major upgrades or airworthiness directives are issued, all aircraft of the same type are upgraded to the new standard.

It appears the business case for the BFR is predicated upon 1000 times reuse, meaning the development costs are spread between the number of ships built X the amount of flights they make. Counter to this would be a single flight to Mars and then becoming a museum piece in SITU. It would make much more fiscal sense to send the ship back to Earth for reuse. Even major upgrades to the engines or avionics would be far cheaper than scrapping the entire rocket. At least that is clearly true with atmospheric vehicles.

Your point is well taken. A significant difference between the initial aircraft articles and the initial BFS is that the weight penalty is much greater for the rocket than for the aircraft. Engineers developing aircraft and the SpaceX engineers both will do their best to eliminate excess weight, but overweight articles do happen in both cases.

It may be that Elon intends to deliberately over spec the initial ships for reliability (because of schedule pressure) then remove the excess based on flight data and tear down inspections. If that were the case then perhaps leaving the first two ships on Mars wouldn't be such a loss - that is - if the maintenance records, flight, and inspection data were gathered before the ship's departure for Mars.

Rocket developers have not had the luxury of using this WWII develop method but with the throw weight of the BFR/BFS, it is a possibility in this instance. I guess time will tell.
Retired, working interesting problems

Offline rsdavis9

Re: Elon Musk Reddit AMA on BFR
« Reply #152 on: 10/18/2017 11:23 AM »
I got the impression that under nominal conditions all three landing engines will be firing. If there is an engine out the remaining two would throttle up to compensate vs only one running normally and another starting up in an engine out event. Startup would take way longer than just throttling up, which is very problematic when a failure occurs at the worst possible point in the landing. So with normal operation of all three engines running a triangular arrangement is symmetrical in all directions. Under engine out, the remaining two only need enough gimble to compensate for the failed engine. Worst case you provide enough gimble to compensate for two engines out. Since you can’t predict which engine will go out you want all three engines as close to center as possible, which the triangular arrangement also provides. Triangular is also the most centrally compact arrangement. My guess is a triangular arrangement for the landing engines.

Caveat: I’m only an arm-chair rocket engineer so I might have a flaw in my logic. If so, please feel free to kindly point it out. Thx.

could be that the engine reliability is mostly on startup. Once going the engines will be very likely to continue to work. I agree 3 at first is the best. But on the final landing approach it might be desirable to shut 2 down and throttle up the center.

So the question is what is easier to change throttle on?
1. 3 engines at low throttle setting.
2. 1 engine at high throttle setting.

I would guess 3 engines because the turbines are spinning slower and easier to change speed at a slower spin speed.
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.

Offline oldAtlas_Eguy

  • Senior Member
  • *****
  • Posts: 3535
  • Florida
  • Liked: 1945
  • Likes Given: 237
Re: Elon Musk Reddit AMA on BFR
« Reply #153 on: 10/18/2017 10:00 PM »
If the 'hover-slam' is a solvable control problem with a single engine, it isn't much of a stretch to envision a 'hover-tip' maneuver with multiple engines firing off-center.  The stack is tilted during descent to prevent XY acceleration, with Z velocity and now also deviation from vertical reaching 0 precisely at touchdown.  It's obviously harder, with additional trades needed WRT landing gear, but not unsolvable.

Coslne losses suggest it might use more fuel than is absolutely necessary though, no?


The gimbal angle can't be all that big and cos(12 degrees) is 0.978 so the losses are a few percent or less.


If you're landing on fumes, it does matter. but yeah.
A fallacy.

Other than for maneuvering X-Y the angle of thrust is center-lined on the CG. So there is no cosine losses on engine out unless for some reason the engines are gimbaling not in unison but gimbling in opposition.

Offline aero

  • Senior Member
  • *****
  • Posts: 3005
  • 92129
  • Liked: 801
  • Likes Given: 285
Re: Elon Musk Reddit AMA on BFR
« Reply #154 on: 10/19/2017 03:59 AM »
If the 'hover-slam' is a solvable control problem with a single engine, it isn't much of a stretch to envision a 'hover-tip' maneuver with multiple engines firing off-center.  The stack is tilted during descent to prevent XY acceleration, with Z velocity and now also deviation from vertical reaching 0 precisely at touchdown.  It's obviously harder, with additional trades needed WRT landing gear, but not unsolvable.

Coslne losses suggest it might use more fuel than is absolutely necessary though, no?


The gimbal angle can't be all that big and cos(12 degrees) is 0.978 so the losses are a few percent or less.


If you're landing on fumes, it does matter. but yeah.
A fallacy.

Other than for maneuvering X-Y the angle of thrust is center-lined on the CG. So there is no cosine losses on engine out unless for some reason the engines are gimbaling not in unison but gimbling in opposition.

It seems hardly worth the trouble but controlling the engines to gimbal in opposition would give an extra reduction in minimum thrust. The engines throttle down to 20% then if gimbaled in opposition by 12 degrees gives the vertical force of 19.56% of thrust. Does anyone know a number for the common maximum gimbal angle of rocket engines? Or is there even such a number outside of SpaceX? One half of one percent reduction in vertical force is likely less than vertical acceleration reduction due to the added mass needed to strengthen the engine gimbal supports.
Retired, working interesting problems

Offline Lars-J

  • Senior Member
  • *****
  • Posts: 4440
  • California
  • Liked: 3977
  • Likes Given: 2402
Re: Elon Musk Reddit AMA on BFR
« Reply #155 on: 10/19/2017 04:51 AM »
It seems hardly worth the trouble but controlling the engines to gimbal in opposition would give an extra reduction in minimum thrust. The engines throttle down to 20% then if gimbaled in opposition by 12 degrees gives the vertical force of 19.56% of thrust. Does anyone know a number for the common maximum gimbal angle of rocket engines? Or is there even such a number outside of SpaceX? One half of one percent reduction in vertical force is likely less than vertical acceleration reduction due to the added mass needed to strengthen the engine gimbal supports.

So a whole 0.44% gain? That should make it clear for you why they are NOT doing it. If you are running that close to the margin where that makes all the difference, you will not have a reliable system.
« Last Edit: 10/19/2017 05:19 AM by Lars-J »

Offline hkultala

  • Full Member
  • ****
  • Posts: 884
  • Liked: 360
  • Likes Given: 329
Re: Elon Musk Reddit AMA on BFR
« Reply #156 on: 10/19/2017 01:12 PM »
It seems hardly worth the trouble but controlling the engines to gimbal in opposition would give an extra reduction in minimum thrust. The engines throttle down to 20% then if gimbaled in opposition by 12 degrees gives the vertical force of 19.56% of thrust. Does anyone know a number for the common maximum gimbal angle of rocket engines? Or is there even such a number outside of SpaceX? One half of one percent reduction in vertical force is likely less than vertical acceleration reduction due to the added mass needed to strengthen the engine gimbal supports.

So a whole 0.44% gain? That should make it clear for you why they are NOT doing it. If you are running that close to the margin where that makes all the difference, you will not have a reliable system.

No, not 0.44% difference but 2.2% difference and 0.44 percentage point difference.

Offline meekGee

  • Senior Member
  • *****
  • Posts: 8796
  • N. California
  • Liked: 4988
  • Likes Given: 904
Re: Elon Musk Reddit AMA on BFR
« Reply #157 on: 10/19/2017 03:41 PM »
It seems hardly worth the trouble but controlling the engines to gimbal in opposition would give an extra reduction in minimum thrust. The engines throttle down to 20% then if gimbaled in opposition by 12 degrees gives the vertical force of 19.56% of thrust. Does anyone know a number for the common maximum gimbal angle of rocket engines? Or is there even such a number outside of SpaceX? One half of one percent reduction in vertical force is likely less than vertical acceleration reduction due to the added mass needed to strengthen the engine gimbal supports.

So a whole 0.44% gain? That should make it clear for you why they are NOT doing it. If you are running that close to the margin where that makes all the difference, you will not have a reliable system.

No, not 0.44% difference but 2.2% difference and 0.44 percentage point difference.
The down side of such an arrangement is that an engine-out event is instantaneously catastrophic. (Well, probably, but much more so then when all engines are pointing through the c.g.)
ABCD - Always Be Counting Down

Offline intrepidpursuit

  • Full Member
  • ****
  • Posts: 624
  • Orlando, FL
  • Liked: 410
  • Likes Given: 243
Re: Elon Musk Reddit AMA on BFR
« Reply #158 on: 10/19/2017 07:37 PM »
I don't think the landing engines will gimbal separately but as a cluster like was suggested for the BFR IAC 2016. With 3 engines in a triangle I would cant them all slightly so they are all thrusting through CG so that a single engine failure would not require an immediate adjustment for stability. Then in a failure situation it just has to adjust to kill horizontal velocity, which should be minimal.

Offline rsdavis9

Re: Elon Musk Reddit AMA on BFR
« Reply #159 on: 10/19/2017 08:05 PM »
They have to be independently TVC so that you can get roll as well as left right top bottom.

EDIT: of course cold gas thrusters could do the roll correction.
« Last Edit: 10/19/2017 08:06 PM by rsdavis9 »
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.

Offline intrepidpursuit

  • Full Member
  • ****
  • Posts: 624
  • Orlando, FL
  • Liked: 410
  • Likes Given: 243
Re: Elon Musk Reddit AMA on BFR
« Reply #160 on: 10/19/2017 08:08 PM »
They have to be independently TVC so that you can get roll as well as left right top bottom.

EDIT: of course cold gas thrusters could do the roll correction.

Falcon 9 doesn't have roll correction via TVC on landing. Seems like the split flaps and the pressure fed methane thrusters would be sufficient for roll control.

Offline MP99

Re: Elon Musk Reddit AMA on BFR
« Reply #161 on: 11/05/2017 06:02 PM »
I don't think this is likely, but...

A BFR heavy (3x BFR cores) could launch a heavier tanker which fills much more of the volume of BFS. I don't see this as an FH style where the boosters separate, but where the three cores are permanently and rigidly connected, so would land together and be ready for rapid recycling.

This assumes that a heavier BFS would not suffer excessive gravity losses after staging (or might be upgraded with more vac Raptors). It also assumes that SpaceX get comfortable with FH, and ultimately decide that it's not so hard to replicate after all.

Alternatively, early 12m boosters might start out launching only 9m tankers until it has the flight history to prove it safe for launching more valuable cargos.

Cheers, Martin

Sent from my Nexus 6 using Tapatalk


Offline Peter.Colin

  • Full Member
  • *
  • Posts: 182
  • Belgium
  • Liked: 26
  • Likes Given: 44
Re: Elon Musk Reddit AMA on BFR
« Reply #162 on: 11/05/2017 06:46 PM »
A single 9 meter booster could also launch a 12 meter tanker with a dry weight of 235 tons to LEO.
« Last Edit: 11/05/2017 06:51 PM by Peter.Colin »

Offline hkultala

  • Full Member
  • ****
  • Posts: 884
  • Liked: 360
  • Likes Given: 329
Re: Elon Musk Reddit AMA on BFR
« Reply #163 on: 11/05/2017 06:55 PM »
A single 9 meter booster could also launch a 12 meter tanker with a dry weight of 235 tons to LEO.

There is no need nor a point for a 12-meter 235 tonne tanker, and it makes absolutely no sense at all.
« Last Edit: 11/05/2017 10:40 PM by hkultala »

Offline drzerg

  • Member
  • Posts: 82
  • Kyiv
  • Liked: 15
  • Likes Given: 11
Re: Elon Musk Reddit AMA on BFR
« Reply #164 on: 11/05/2017 10:02 PM »
one tanker could be specificly designed for long term storage on orbit, more like depo then tanker. regular tankers are fast and light and have only role to delivering fuel from point to point.

Tags: