BFS tanker variant – I would be interested in people’s views of what this might look like.
This is inconsistent with the IAC2017 presentation showing similar OML though.
Quote from: speedevil on 03/24/2018 12:35 pmThis is inconsistent with the IAC2017 presentation showing similar OML though.I saw that quote from Musk and I believe that the tanker shown in the 2017 presentation is essentially the cargo version of BFS without cargo bay doors. Musk however indicated on Reddit that there will be a dedicated tanker version that will look "kind of weird". So what ways are there to cut down the outer mold line without losing aerodynamic properties for launch and re- entry? Maybe they will also move (or remove) the wings?
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).
Just a simple question. We know that at least five tankers will be needed to fill the tanks of a single BFS. So every single tanker will have to dock back-to-back with the BFS, rince, repeat. I wonder instead about a true, permanent propellant depot with 2200 mt or 3300 mt of methalox, enough to refuel 2 or 3 BFS with a single docking. Kind of space gas station, really. I suppose Musk thought about it but don't want permanent orbital infrastructures with their maintenance cost...
Quote from: Archibald on 03/26/2018 05:57 amJust a simple question. We know that at least five tankers will be needed to fill the tanks of a single BFS. So every single tanker will have to dock back-to-back with the BFS, rince, repeat. I wonder instead about a true, permanent propellant depot with 2200 mt or 3300 mt of methalox, enough to refuel 2 or 3 BFS with a single docking. Kind of space gas station, really. I suppose Musk thought about it but don't want permanent orbital infrastructures with their maintenance cost...I'm not sure what the advantage of that would be. The only difference is slightly looser scheduling regarding resupply launches as compared to direct tanking for significantly higher cost. If BFR is really meant to fly every few days, I doubt they'd gain much by being able to store fuel beforehand.
A small fleet of 2 crew and 4 cargo ships would require 30+ tanker flights. At a launch cadence of 1 per day some of the ships would have to hang around in LEO for a month. A disruption in tanker flights, e.g. by a hurricane, anywhere in that month could lead to the optimum TMI window being missed. Boil-off from the fueled ships in LEO would also be an issue.
Quote from: MikeAtkinson on 03/26/2018 11:59 amA small fleet of 2 crew and 4 cargo ships would require 30+ tanker flights. At a launch cadence of 1 per day some of the ships would have to hang around in LEO for a month. A disruption in tanker flights, e.g. by a hurricane, anywhere in that month could lead to the optimum TMI window being missed. Boil-off from the fueled ships in LEO would also be an issue.It is interesting to note that you can have BFR take off and climb at 100MPH, for most of the ascent through harsh weather, and still get a notable payload to orbit. This is just fine for a fuel launch, if you happen to be constrained somewhat by weather.
No, lightning strikes would still be a problem.
Are you certain? The 2016 ITS showed differences between the tanker and the ship. The tanker had significantly lower dry mass and significantly greater propellant mass than the ship.At first, sure, the tanker and the ship might be the same thing.
The purpose of the winglets is adapt the aero profile across a variety of payload amounts and atmospheric conditions. Given that the tanker will always land under the same conditions (essentially empty and on earth) the winglets may not be needed. Or they may be fixed or different shaped.
Quote from: cppetrie on 03/27/2018 06:10 pmThe purpose of the winglets is adapt the aero profile across a variety of payload amounts and atmospheric conditions. Given that the tanker will always land under the same conditions (essentially empty and on earth) the winglets may not be needed. Or they may be fixed or different shaped. I assume there will be 1 mold line, one set of tooling and 1 way to do things. Right up to the point they decide if it's Cargo, Crew or Tanker.Tanker should be the easiest of all of them.Not expecting to see anything soon. Lots of development to be done. I'll entertain myself with the epic accomplishments of the Falcon family until then.
Quote from: wannamoonbase on 03/27/2018 06:31 pmQuote from: cppetrie on 03/27/2018 06:10 pmThe purpose of the winglets is adapt the aero profile across a variety of payload amounts and atmospheric conditions. Given that the tanker will always land under the same conditions (essentially empty and on earth) the winglets may not be needed. Or they may be fixed or different shaped. I assume there will be 1 mold line, one set of tooling and 1 way to do things. Right up to the point they decide if it's Cargo, Crew or Tanker.Tanker should be the easiest of all of them.Not expecting to see anything soon. Lots of development to be done. I'll entertain myself with the epic accomplishments of the Falcon family until then.We’ve already been told by Elon that the dedicated tanker will look “weird”. To me that means the dedicated tanker will probably not share the same OML as the ship and cargo versions. But the dedicated tanker will come (much) later. The early tankers will just be cargo versions with no cargo and so of course will have the same OML.
Just speculating here...I like the idea mentioned of improving payload to orbit by having outward gimbaled sea level engines outboard on the second stage, to improve liftoff TWR. Possibly with some aerospike-like effects as the plume interacts with the body, if you have enough balls to design it that way. (note: this very dangerous if not designed correctly)I'm thinking a rounded diamond or arrowhead shape, outboard SL engines in the "wings" for landing, Vac engines in the circular interface with Stage 1, (up to 7 R-vac with no gimbal, but probably not that many) and the rest of the internal volume is fuel. Exact size is iterated based on how much fuel the rocket can lift to a reference orbit.It would be very fluffy on reentry, with engine mass to the rear, and more reentry surface than a normal BFR. I would think that this would make up for any extra mass for a larger design by saving landing fuel from a softer landing.
Quote from: rakaydos on 03/29/2018 08:35 pmJust speculating here...I like the idea mentioned of improving payload to orbit by having outward gimbaled sea level engines outboard on the second stage, to improve liftoff TWR. Possibly with some aerospike-like effects as the plume interacts with the body, if you have enough balls to design it that way. (note: this very dangerous if not designed correctly)I'm thinking a rounded diamond or arrowhead shape, outboard SL engines in the "wings" for landing, Vac engines in the circular interface with Stage 1, (up to 7 R-vac with no gimbal, but probably not that many) and the rest of the internal volume is fuel. Exact size is iterated based on how much fuel the rocket can lift to a reference orbit.It would be very fluffy on reentry, with engine mass to the rear, and more reentry surface than a normal BFR. I would think that this would make up for any extra mass for a larger design by saving landing fuel from a softer landing.The engines in the wings would not be thrusting through the centre of mass. Gimballing them through the centre would induce a backwards translation. I suppose if you're coming in from orbit, this might actually help do the flip upright and reduce forward velocity?
I'm thinking a rounded diamond or arrowhead shape, outboard SL engines in the "wings" for landing, Vac engines in the circular interface with Stage 1, (up to 7 R-vac with no gimbal, but probably not that many) and the rest of the internal volume is fuel. Exact size is iterated based on how much fuel the rocket can lift to a reference orbit.Quote from: rakaydos on 04/02/2018 05:41 pmThrusting throught the center of mass is done to avoid a rotation, which is nessisary for asymetric thrust. With symmetric thrusters, however, a tractor arrangement (skycrane?) should be reasonablePerhaps I'm misunderstanding you - are you suggesting 4 winglets instead of 2, with an engine in each wing (therefore symmetric thrust) or are you saying the wings should be moved to the centre of the cylinder so that you can have symmetric thust without a second set of wings? Otherwise I'm not following.
Thrusting throught the center of mass is done to avoid a rotation, which is nessisary for asymetric thrust. With symmetric thrusters, however, a tractor arrangement (skycrane?) should be reasonable
Just speculating here...I like the idea mentioned of improving payload to orbit by having outward gimbaled sea level engines outboard on the second stage, to improve liftoff TWR. Possibly with some aerospike-like effects as the plume interacts with the body, if you have enough balls to design it that way. (note: this very dangerous if not designed correctly)
Quote from: rakaydos on 03/29/2018 08:35 pmI'm thinking a rounded diamond or arrowhead shape, outboard SL engines in the "wings" for landing, Vac engines in the circular interface with Stage 1, (up to 7 R-vac with no gimbal, but probably not that many) and the rest of the internal volume is fuel. Exact size is iterated based on how much fuel the rocket can lift to a reference orbit.Quote from: rakaydos on 04/02/2018 05:41 pmThrusting throught the center of mass is done to avoid a rotation, which is nessisary for asymetric thrust. With symmetric thrusters, however, a tractor arrangement (skycrane?) should be reasonablePerhaps I'm misunderstanding you - are you suggesting 4 winglets instead of 2, with an engine in each wing (therefore symmetric thrust) or are you saying the wings should be moved to the centre of the cylinder so that you can have symmetric thust without a second set of wings? Otherwise I'm not following.
I'm thinking a rounded diamond or arrowhead shape, outboard SL engines in the "wings" for landing, Vac engines in the circular interface with Stage 1, (up to 7 R-vac with no gimbal, but probably not that many) and the rest of the internal volume is fuel. Exact size is iterated based on how much fuel the rocket can lift to a reference orbit.
Quote from: rakaydos on 03/29/2018 08:35 pmJust speculating here...I like the idea mentioned of improving payload to orbit by having outward gimbaled sea level engines outboard on the second stage, to improve liftoff TWR. Possibly with some aerospike-like effects as the plume interacts with the body, if you have enough balls to design it that way. (note: this very dangerous if not designed correctly)Makes absolutely no sense.Lifting weight of these engines and their tanks all the way to the orbit.Heat shielding the first stage tanks from the plume of these enginesAll the stability/control trouble of engines whose thrust vector is very far from the center of the gravity.Better to have more engines in the first stage instead - and oh, it already contains 31 of those.
As far as heat shielding... a properly expanded bell apparetly makes the exaust not so hot. Something about turning heat into movement.
Apologies for the multiple posts with messed up quotes. I'm not quite sure how that happened.
Quote from: rakaydos on 04/03/2018 03:57 pmAs far as heat shielding... a properly expanded bell apparetly makes the exaust not so hot. Something about turning heat into movement.Only with respect to the moving exhaust.If you place a thermometer moving along with the exhaust, it may read around 100C.If you place one sticking into the exhaust stationary, it will read about the same temperature as the combustion chamber.Plume interaction with structure can heat it a lot.Plus, if there is residual air, that gets heated lots by turbulence and impact from the jet.
I'm not sure about a 2,000 ton fuel depot, but the idea of what could take advantage of a replenishable, available BFS tank rigged for long term use has to be going through a lot of minds. Or single use tankers rigged for non SpaceX users. NASA or commercial, it would enable entirely new architectures.
Quote from: Nomadd on 04/03/2018 11:40 pm I'm not sure about a 2,000 ton fuel depot, but the idea of what could take advantage of a replenishable, available BFS tank rigged for long term use has to be going through a lot of minds. Or single use tankers rigged for non SpaceX users. NASA or commercial, it would enable entirely new architectures.SpaceX is basically going to define the Industry Standard Cryomethane refueling interface. I hope they are planning around future designs being backward compatable.
And then no one else will use it, even though it's clearly better. Instead, they'll develop much larger interfaces that cost more and can't transfer fuel as quickly...
Quote from: octavo on 04/03/2018 07:12 amQuote from: rakaydos on 03/29/2018 08:35 pmI'm thinking a rounded diamond or arrowhead shape, outboard SL engines in the "wings" for landing, Vac engines in the circular interface with Stage 1, (up to 7 R-vac with no gimbal, but probably not that many) and the rest of the internal volume is fuel. Exact size is iterated based on how much fuel the rocket can lift to a reference orbit.Quote from: rakaydos on 04/02/2018 05:41 pmThrusting throught the center of mass is done to avoid a rotation, which is nessisary for asymetric thrust. With symmetric thrusters, however, a tractor arrangement (skycrane?) should be reasonablePerhaps I'm misunderstanding you - are you suggesting 4 winglets instead of 2, with an engine in each wing (therefore symmetric thrust) or are you saying the wings should be moved to the centre of the cylinder so that you can have symmetric thust without a second set of wings? Otherwise I'm not following.Moving the wings to parallel the axis of thrust- sorry I thought that was imlied by putting things in them. I was unclear.Quote from: hkultala on 04/03/2018 07:33 amQuote from: rakaydos on 03/29/2018 08:35 pmJust speculating here...I like the idea mentioned of improving payload to orbit by having outward gimbaled sea level engines outboard on the second stage, to improve liftoff TWR. Possibly with some aerospike-like effects as the plume interacts with the body, if you have enough balls to design it that way. (note: this very dangerous if not designed correctly)Makes absolutely no sense.Lifting weight of these engines and their tanks all the way to the orbit.Heat shielding the first stage tanks from the plume of these enginesAll the stability/control trouble of engines whose thrust vector is very far from the center of the gravity.Better to have more engines in the first stage instead - and oh, it already contains 31 of those.In case you didnt notice, the upper stage already includes sea level engines for landing purposes. Moving them to the wings to be used on ascent doesnt actually add mass. Tanks do, but we're adding tanks because it's a tanker.As far as heat shielding... a properly expanded bell apparetly makes the exaust not so hot. Something about turning heat into movement.
Quote from: rakaydos on 04/03/2018 03:57 pmQuote from: octavo on 04/03/2018 07:12 amQuote from: rakaydos on 03/29/2018 08:35 pmI'm thinking a rounded diamond or arrowhead shape, outboard SL engines in the "wings" for landing, Vac engines in the circular interface with Stage 1, (up to 7 R-vac with no gimbal, but probably not that many) and the rest of the internal volume is fuel. Exact size is iterated based on how much fuel the rocket can lift to a reference orbit.Quote from: rakaydos on 04/02/2018 05:41 pmThrusting throught the center of mass is done to avoid a rotation, which is nessisary for asymetric thrust. With symmetric thrusters, however, a tractor arrangement (skycrane?) should be reasonablePerhaps I'm misunderstanding you - are you suggesting 4 winglets instead of 2, with an engine in each wing (therefore symmetric thrust) or are you saying the wings should be moved to the centre of the cylinder so that you can have symmetric thust without a second set of wings? Otherwise I'm not following.Moving the wings to parallel the axis of thrust- sorry I thought that was imlied by putting things in them. I was unclear.Quote from: hkultala on 04/03/2018 07:33 amQuote from: rakaydos on 03/29/2018 08:35 pmJust speculating here...I like the idea mentioned of improving payload to orbit by having outward gimbaled sea level engines outboard on the second stage, to improve liftoff TWR. Possibly with some aerospike-like effects as the plume interacts with the body, if you have enough balls to design it that way. (note: this very dangerous if not designed correctly)Makes absolutely no sense.Lifting weight of these engines and their tanks all the way to the orbit.Heat shielding the first stage tanks from the plume of these enginesAll the stability/control trouble of engines whose thrust vector is very far from the center of the gravity.Better to have more engines in the first stage instead - and oh, it already contains 31 of those.In case you didnt notice, the upper stage already includes sea level engines for landing purposes. Moving them to the wings to be used on ascent doesnt actually add mass. Tanks do, but we're adding tanks because it's a tanker.As far as heat shielding... a properly expanded bell apparetly makes the exaust not so hot. Something about turning heat into movement.Moving them to the wings would mean they could not be used for landing.So, makes no sense.
My $0.02We've heard from Musk that the initial tanker will be a BFS cargo, and logic dictates it'll likely be gutted to educe mass & increasd margins. We also heard from Musk that a dedicated Tanker will look “kind of weird”. Starting from those items, I checked the below linked NASA patent PDF documenting potential aeroshell shapes,https://patents.google.com/patent/US8725470B1/en#14888 is circular in cross section and with more parallel sides than BFS's foreward section. Given this additional forward tanks seem simpler. Perhaps it could also form the basis of an improved Chomper satellite dispenser - one without the tapered payload bay & door. Perhaps larger winglets, though I didn't depict them. OK, pick it apart
Since for the tanker doesn't have boil-off issues, the landing tanks can be taken out of the main tanks and placed anywhere on the ship, helping with control of c.g. for reentry.
Quote from: meekGee on 06/23/2018 10:29 pmSince for the tanker doesn't have boil-off issues, the landing tanks can be taken out of the main tanks and placed anywhere on the ship, helping with control of c.g. for reentry. Possible, but it would have to bring major advantages. I think they will keep the propulsion part of BFS as similar as possible, just tank stretches.. Moving the landing tanks elsewhere would be a major redesign.
Remember that a lot of the standard ITS payload area is empty (living quarters). A dedicated tanker version will be full of liquid. I'm assuming that will take up a lot less room for the same weight of fuel.
Quote from: tea monster on 06/24/2018 10:55 amRemember that a lot of the standard ITS payload area is empty (living quarters). A dedicated tanker version will be full of liquid. I'm assuming that will take up a lot less room for the same weight of fuel.Assuming for the moment that BFS/tanker cost $150M, and launch does in fact cost (to spacex) $5M, and the tanker gets 20% more for that launch, this all ends up as tankers being quite a way in the future.Assuming you place no benefit at all on having spare BFS, a tanker will pay for itself in around 150 launches - around 30 launches to the Moon/Mars.So, there is no real point until we have many, many thousands of tons of stuff on Mars or Moon.Other side benefits as well as having spare BFS may be significant - more testing per BFS 'free', higher production volume, ...Of course, if for some reason reuse is very, very expensive - these numbers all change, in a way that might make the tanker come sooner.
QuoteMoving them to the wings would mean they could not be used for landing.So, makes no sense.Why not? Center of thrust is still aligned with center of mass, it just needs the same pitch-up maneuver the regular BFR needs.
Moving them to the wings would mean they could not be used for landing.So, makes no sense.
Quote from: rakaydos on 06/23/2018 02:39 pmQuoteMoving them to the wings would mean they could not be used for landing.So, makes no sense.Why not? Center of thrust is still aligned with center of mass, it just needs the same pitch-up maneuver the regular BFR needs.Aligning center of mass with thrust vector of wing mounted engines would mean that1) the engines would be greatly canted outwards.This would also mean that 1A) some considerable cosine losses would occur.1B) single-engine failure would affect the direction of vector greatly and causing big problems. 2) the engines would be greatly canted downwards.This would mean that the landing position would be far from vertical.Practically: It does not work.
Quote from: hkultala on 06/24/2018 07:11 pmQuote from: rakaydos on 06/23/2018 02:39 pmQuoteMoving them to the wings would mean they could not be used for landing.So, makes no sense.Why not? Center of thrust is still aligned with center of mass, it just needs the same pitch-up maneuver the regular BFR needs.Aligning center of mass with thrust vector of wing mounted engines would mean that1) the engines would be greatly canted outwards.This would also mean that 1A) some considerable cosine losses would occur.1B) single-engine failure would affect the direction of vector greatly and causing big problems. 2) the engines would be greatly canted downwards.This would mean that the landing position would be far from vertical.Practically: It does not work.Single engine out is indeed a weakness.I do not feel it would be instantly fatal. While a BFT is much larger than a Dragon II, they can handle engine out just fine with outboard engines.
Also, I'm not sure I understand your 2). if the engines are firing vertically, why would the landing position NOT be vertical?
Quote from: meekGee on 06/24/2018 06:04 pmAssuming you place no benefit at all on having spare BFS, a tanker will pay for itself in around 150 launches - around 30 launches to the Moon/Mars.Well that's true if the tanker is "extra". But if you need a certain (large) number of tanking launches, you need the vehicles for that. They can be either BFSs, or tankers. Either way you have to build them. And if he tankers carry more, then so much the better.You'll never be at the point where you are left with a tanker that you wish was a BFS, because you could have launched it to Mars.
Assuming you place no benefit at all on having spare BFS, a tanker will pay for itself in around 150 launches - around 30 launches to the Moon/Mars.
SpaceX has always red-lined some hardware while leaving enormous margins in other places. They have a one size fits all rocket so most of their payloads are under maximum capacity. And despite this excess they apply all the gains to every rocket, every rocket gets the cooled fuelling and the titanium gridfins even though some of them wont need them. They have (correctly IMHO) made the conclusion that you want as little variation as possible because the greater will serve in place of the lesser.For this reason, I think a tanker version is going to be a very low priority. Suppose a non-tanker can lift 140 tons of fuel while a tanker can lift 165. That 25 tons might seem like a lot but it's only saving perhaps one reusable launch and only on missions leaving earth orbit. Saving one launch here or there isn't nearly as important as getting the launch rate up.
Quote from: johnfwhitesell on 06/25/2018 03:24 pmSpaceX has always red-lined some hardware while leaving enormous margins in other places. They have a one size fits all rocket so most of their payloads are under maximum capacity. And despite this excess they apply all the gains to every rocket, every rocket gets the cooled fuelling and the titanium gridfins even though some of them wont need them. They have (correctly IMHO) made the conclusion that you want as little variation as possible because the greater will serve in place of the lesser.For this reason, I think a tanker version is going to be a very low priority. Suppose a non-tanker can lift 140 tons of fuel while a tanker can lift 165. That 25 tons might seem like a lot but it's only saving perhaps one reusable launch and only on missions leaving earth orbit. Saving one launch here or there isn't nearly as important as getting the launch rate up.Even if the numbers are correct, it's still so much cheaper to make a ship with no windows, airlocks, decks, cabins, ECLS...
I have doubts about if the windows will materialise.
Since tankers are required anyway, why not build a couple of tankers first, and get basic reliability demonstrated with them?
Quote from: johnfwhitesell on 06/25/2018 03:24 pmSpaceX has always red-lined some hardware while leaving enormous margins in other places. They have a one size fits all rocket so most of their payloads are under maximum capacity. And despite this excess they apply all the gains to every rocket, every rocket gets the cooled fuelling and the titanium gridfins even though some of them wont need them. They have (correctly IMHO) made the conclusion that you want as little variation as possible because the greater will serve in place of the lesser.For this reason, I think a tanker version is going to be a very low priority. Suppose a non-tanker can lift 140 tons of fuel while a tanker can lift 165. That 25 tons might seem like a lot but it's only saving perhaps one reusable launch and only on missions leaving earth orbit. Saving one launch here or there isn't nearly as important as getting the launch rate up.Even if the numbers are correct, it's still so much cheaper to make a ship with no windows, airlocks, decks, cabins, ECLS...Think about a cargo 747..Except also other advantages such as a shorter barrel.Since tankers are required anyway, why not build a couple of tankers first, and get basic reliability demonstrated with them?And if one of them exposes a fatal flaw, at least the images are of a clearly ubmannable ship crashing, not a passenger ship.-----ABCD: Always Be Counting Down
Quote from: meekGee on 06/26/2018 03:20 amQuote from: johnfwhitesell on 06/25/2018 03:24 pmSpaceX has always red-lined some hardware while leaving enormous margins in other places. They have a one size fits all rocket so most of their payloads are under maximum capacity. And despite this excess they apply all the gains to every rocket, every rocket gets the cooled fuelling and the titanium gridfins even though some of them wont need them. They have (correctly IMHO) made the conclusion that you want as little variation as possible because the greater will serve in place of the lesser.For this reason, I think a tanker version is going to be a very low priority. Suppose a non-tanker can lift 140 tons of fuel while a tanker can lift 165. That 25 tons might seem like a lot but it's only saving perhaps one reusable launch and only on missions leaving earth orbit. Saving one launch here or there isn't nearly as important as getting the launch rate up.Even if the numbers are correct, it's still so much cheaper to make a ship with no windows, airlocks, decks, cabins, ECLS...Think about a cargo 747..Except also other advantages such as a shorter barrel.Since tankers are required anyway, why not build a couple of tankers first, and get basic reliability demonstrated with them?And if one of them exposes a fatal flaw, at least the images are of a clearly ubmannable ship crashing, not a passenger ship.-----ABCD: Always Be Counting DownDedicated tankers will carry nothing except fuel. Nobody is buying fuel in orbit today, so how is that supposed to make money?SpaceX will start with the unmanned cargo version which can deliver satellites to LEO and GTO and can also function as a less efficient tanker. That avoids the complications of crew, while actually earning revenue.