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.