BFR AMA on r/space in 2 hours
A Reddit user made a convenient summary of Q&A : https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodhawf/
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 MarsQ: 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 illustrationQ: 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 lameA (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 4A (Elon): Improves stability in rough terrainQ: 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 TankerQ: 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 scheduleQ: 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 propulsionQ: 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 PotterA (Elon): But, yes and probablyQ: 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 communicationsQ: 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): YesQ: 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): NerdA (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 sillinessQ: 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...
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?
The questions near the top so far are actually pretty good!
'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
Quote from: nacnud 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.'-EMThis 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.
*Speculation*Perhaps this is the plan for Boca Chita now that flights from McGregor are a no no.
Quote from: nacnud 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.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.
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.
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.
Quote from: mlindner on 10/14/2017 10:31 pmLooks 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.
Hold the r/space mods hostage until they move it to r/spacex Or riot. 😉
Quote from: mlindner on 10/14/2017 10:31 pmLooks 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.
Quote from: mlindner on 10/14/2017 10:31 pmLooks 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)
Thanks for tuning in to the AMA. Great questions nk!!
Quote from: Star One on 10/14/2017 10:35 pmQuote from: mlindner on 10/14/2017 10:31 pmLooks 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.QuoteHold the r/space mods hostage until they move it to r/spacex Or riot. 😉
Quote from: mlindner on 10/14/2017 10:39 pmQuote from: Star One on 10/14/2017 10:35 pmQuote from: mlindner on 10/14/2017 10:31 pmLooks 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.QuoteHold 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.
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.....
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 think the Reddit format is conducive to getting a lot of high quality questions asked in a reasonable amount of time.
Quote from: nacnud on 10/14/2017 10:17 pmThis 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.
Quote from: Aussie_Space_Nut on 10/14/2017 11:36 pmQuote from: nacnud on 10/14/2017 10:17 pmThis 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.
Quote from: Ronsmytheiii on 10/14/2017 11:08 pmReally 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.
Quote from: nacnud on 10/14/2017 10:17 pmThis 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.
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??
Quote from: Next Spaceflight on 10/14/2017 11:12 pmReally 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.
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.
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 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?
Quote from: rsdavis9 on 10/15/2017 08:44 pmIsn'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.
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!
Quote from: nacnud on 10/15/2017 08:45 pmWell 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?
Quote from: Kaputnik on 10/15/2017 09:37 pmDo 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.
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?
Quote from: vaporcobra on 10/15/2017 09:43 pmQ: 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'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).
I think they plan to leave some of the first BFS's to mars on the surface as part of the station.
Quote from: rsdavis9 on 10/15/2017 08:47 pmI 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?
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 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.
Quote from: vaporcobra on 10/15/2017 10:55 pmIn 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 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)
Quote from: CuddlyRocket on 10/15/2017 11:38 pmQuote from: rsdavis9 on 10/15/2017 08:47 pmI 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.
Quote from: rsdavis9 on 10/16/2017 12:59 amQuote from: CuddlyRocket on 10/15/2017 11:38 pmQuote from: rsdavis9 on 10/15/2017 08:47 pmI 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.
Quote from: mnelson on 10/15/2017 11:24 pmQuote from: vaporcobra on 10/15/2017 09:43 pmQ: 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.
Quote from: vaporcobra on 10/15/2017 09:43 pmQ: 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?"
Quote from: Geron on 10/15/2017 11:34 pmQuote from: mnelson on 10/15/2017 11:24 pmQuote from: vaporcobra on 10/15/2017 09:43 pmQ: 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.
Quote from: mnelson on 10/16/2017 02:07 amQuote from: Geron on 10/15/2017 11:34 pmQuote from: mnelson on 10/15/2017 11:24 pmQuote from: vaporcobra on 10/15/2017 09:43 pmQ: 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.
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. 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!
Quote from: Robotbeat on 10/15/2017 10:27 pmQuote from: Kaputnik on 10/15/2017 09:37 pmDo 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 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.
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 enginesThat has an “extremely high full to empty mass ratio”.
Quote from: Peter.Colin on 10/16/2017 11:10 amThe 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 enginesThat 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?
Quote from: rockets4life97 on 10/16/2017 11:20 amQuote from: Peter.Colin on 10/16/2017 11:10 amThe 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 enginesThat 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.
Quote from: rockets4life97 on 10/16/2017 11:20 amQuote from: Peter.Colin on 10/16/2017 11:10 amThe 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 enginesThat 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.
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)?
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.
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.
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
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...
Quote from: John Alan on 10/16/2017 02:39 pmThe 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.
Quote from: Peter.Colin on 10/16/2017 12:55 pmIt’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).
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.
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....
Quote from: vaporcobra on 10/15/2017 09:43 pmQ: 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.
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.
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.
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...
Quote from: rsdavis9 on 10/17/2017 11:24 amWhats 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 '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.
Quote from: leetdan on 10/17/2017 06:07 pmIf 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?
Quote from: Lar on 10/17/2017 06:57 pmQuote from: leetdan on 10/17/2017 06:07 pmIf 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.
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.
Quote from: aero on 10/16/2017 01:58 amTake 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.
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.
Quote from: acsawdey on 10/17/2017 08:15 pmQuote from: Lar on 10/17/2017 06:57 pmQuote from: leetdan on 10/17/2017 06:07 pmIf 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.
Quote from: Lar on 10/17/2017 08:28 pmQuote from: acsawdey on 10/17/2017 08:15 pmQuote from: Lar on 10/17/2017 06:57 pmQuote from: leetdan on 10/17/2017 06:07 pmIf 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.
Quote from: aero on 10/19/2017 03:59 amIt 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.
Quote from: Lars-J on 10/19/2017 04:51 amQuote from: aero on 10/19/2017 03:59 amIt 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.
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.
A single 9 meter booster could also launch a 12 meter tanker with a dry weight of 235 tons to LEO.