The question that jumps to mind for me is: are there other missions in planning that require refuelling in space? If it's only Mars missions then having propellant sit around in space for a year or so waiting for launch windows just seems like a waste of infrastructure and an opportunity for gradual wastage of propellant. In that case it would be better to have a BFS tanker that takes more fuel per launch to reduce the number of launches needed for each round of the colonisation fleet.
That's likely it. Elon likes his rockets to be tall and slender, so he might describe rocket that is not tall and not slender as "looking weird."
I'm fully on board for the 'Stumpy the Weird Tanker' being the most likely thing. How blunt can you go with the nose cone on a 9m vehicle?
This is with the same shaped nose as the manned/cargo version.
It might look weird because the BFR is 9m in diameter, and the tanker variant could be larger, more bulbous. Say 12m (ITS) or even 15m.
Quote from: tea monster on 10/16/2017 06:02 pmThis is with the same shaped nose as the manned/cargo version.Wouldn't it still need the delta for reentry?
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.
The wing moves the aerodynamic center aft to counter the long nose. John
There is not going to be any spherical tanks or wider stages for the tanker version. The payload for the BFR is 150 tonnes, the propellant load for the second stage is 1100 tonnes. All they need to do is stretch the tanks by 10% and lop off the payload section. They may not even need to stretch the tanks. It's going to look a little weird because the tanker version is going to be half the length of the normal BFS.
I don't think you need separate tanks in the nose section. The payload would be propellant within tanks that also hold the propellant for ascent. You'd just need tanks for ascent fuel including payload fuel and the header tanks holding the landing fuel.
From the Reddit AMA:"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)."...
Quote from: nacnud on 10/17/2017 01:14 amI don't think you need separate tanks in the nose section. The payload would be propellant within tanks that also hold the propellant for ascent. You'd just need tanks for ascent fuel including payload fuel and the header tanks holding the landing fuel.While what you write is true, is it the most cost-effective to build? SpaceX is already building one very large tank and building another, even larger tank adds to the cost of the system. It may not double the cost of tankage for the fleet but it would certainly add to the development and testing costs, maybe even tooling costs. Your proposal is for additional very large tanks, while I think it would be more cost-effective to build additional small tanks for the payload.The density of liquid methane is 0.421 metric tons per cubic meter and the density of liquid oxygen is 1.142 metric tons per cubic meter. Unfortunately, I don't know the methane/oxygen ratio for the raptor engines and so can't calculate the size of tanks needed to contain 150 tons of propellant. I can say that I doubt the cargo propellant tank sizes will stay constant for very long, however.I also wonder if that 150-tons is short tons, long tons or metric tons. Worst case, if it were metric tons and all liquid methane then the cargo propellant tank size would be the equivalent of a spherical tank 8.8 meters in diameter. Even that fits within the form line of the nose section. Or its close anyway.Separating the tanker's propellant from its cargo is a much simpler approach to getting the job done while allowing for future growth of engine thrust, hence cargo capacity, and engine propellant capacity.JMO
So, the egg has nearly the same surface area as a sphere, but is more aerodynamic, especially with a booster under it. The engine arrangement could be the same as the crew ship, but 3 of the 200 bar SL Raptors might be more appropriate for landing. But most importantly, when combined with the booster, it would look kinda weird.
Shockingly there will be no wings on the tanker since it slways lands on the same planet and empy, thus eliminating the quote reas for having wings.
Quote from: hkultala on 10/16/2017 11:58 amFrom the Reddit AMA:"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)."...Down the road, there will be new factories near the launch sites, so SpaceX may not be limited to 9mØ structures.The crew ship GLOW is 1335mT, so assuming a very light tanker (no box within a box), it might carry say 1,300mT of fuel.Assuming 1:3.8 fuel mixture ratio: (0.421 + (3.8 * 1.142)) / 4.8 = 0.992 mT / m^3. So, 1300mT requires roughly 1300m^3 of volume:Taking an ideal spherical pressure vessel first, For the CH4, 1300 / 4.8 = 270m^3 or an 8mØ sphere.For both propellants, 1300m^3 requires a 13.54mØ sphere.Surface area = 575.95 m2.Taking a 12mØ ovoid (egg) next:For 1,300m^3 the base radius (a) = 6, upper height h1 = 12.4, lower height h2 = 4.8 Height (h) = h1 + h2 = 17.2, Width (w) = 12.Surface area = 594.45 m^2.Taking a 9mØ hemispherically capped cylinder last:For 1,300m^3 the overall dimensions are 9m x 23.5m.Surface area = 790.27 m^2, much more than the other two.So, the egg has nearly the same surface area as a sphere, but is more aerodynamic, especially with a booster under it. The engine arrangement could be the same as the crew ship, but 3 of the 200 bar SL Raptors might be more appropriate for landing. But most importantly, when combined with the booster, it would look kinda weird.
Quote from: livingjw on 10/16/2017 11:27 pmThe wing moves the aerodynamic center aft to counter the long nose. JohnHi, to clarify, you mean for the case when the nose is empty, right? If you could rely on 50 tons in the nose that would reduce the need for more drag at the back, but you want a design that can handle a variety of masses for payload, up and down.All things being equal though, what does balance this tanker then? It has all the engines at the back, and the landing propellant too if following the recent layout. The front is just an empty balloon. if not wings, what can they move except the landing propellant towards the nose?https://en.wikipedia.org/wiki/Intermediate_eXperimental_Vehicle .. this actually does have flaps hanging down past the back btw, though I guess that wasn't what you were referring to.
One reason it won't happen is all the tooling involved that is totally unrelated to the BFS/BFR designs.
How cheap does the $/Kg cost to orbit have to be that the cost of fuel becomes a significant % of the launch cost? I had assumed that the end goal of any dedicated tanker variant is to make less trips to fill a BFS, and would therefore be bigger. I was thinking "weird" meant beluga plane weird.
Are there any advantages to building tanker variants that specialise in either LOX or liquid methane transport? Obviously a LOX tanker will still need to carry methane - and vice versa - for take-off and landing, but you'd otherwise only need to stretch one of the two tanks. As the LOX is 80% of the mass of methalox propellant, a tanker optimised for LOX would probably look quite different from one optimised for methane.
I know the title of this thread is tanker variant but given Musk's descriptions, I would not be surprised if the tanker was the first to fly. Given how it would be stripped down, lowest cost, less to lose in case of mishap, I'd try to fly the tanker first, and then make the full BFS the variant of that.
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 the tanker will be spherical.
Yeah it took me a while to work that out , just adjust the cg by moving the header tanks
Except that he explicitly stated the opposite - initial tanker flights will simply be BFS without cargo.QuoteElon: 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).
Quote from: mikelepage on 10/18/2017 02:48 pmHow cheap does the $/Kg cost to orbit have to be that the cost of fuel becomes a significant % of the launch cost? I had assumed that the end goal of any dedicated tanker variant is to make less trips to fill a BFS, and would therefore be bigger. I was thinking "weird" meant beluga plane weird.More massive (payload mass) is not the same as bigger, because prop is 5 to 10 times denser than just about any other payload.A dedicated tanker could be about 60% the length of the cargo/crew BFS, yet deliver 30% more payload mass. Any more length and it will start needing more engines to keep ascent T/W reasonable.
Quote from: envy887 on 10/18/2017 04:16 pmQuote from: mikelepage on 10/18/2017 02:48 pmHow cheap does the $/Kg cost to orbit have to be that the cost of fuel becomes a significant % of the launch cost? I had assumed that the end goal of any dedicated tanker variant is to make less trips to fill a BFS, and would therefore be bigger. I was thinking "weird" meant beluga plane weird.More massive (payload mass) is not the same as bigger, because prop is 5 to 10 times denser than just about any other payload.A dedicated tanker could be about 60% the length of the cargo/crew BFS, yet deliver 30% more payload mass. Any more length and it will start needing more engines to keep ascent T/W reasonable.Ah okay thanks. Does that mean that with the current booster/dedicated tanker you would only expect to see a small reduction in the number of tanker flights (from 5 to 4)?Second question: The lunar mission plan proposed refilling BFS in a higher elliptical orbit, (perhaps GTO at 1.9km/s above LEO), which means a lot more than 5 tanker flights... I get more like ~17 tanker flights using Elon's presentation chart (I read it as ~45 ton payload to 1.9km/s above LEO). In that case a dedicated tanker with 30% payload improvement might reduce the number of flights to 13 (right?). That's still a lot of flights to get 150 ton to the lunar surface.Last question(s): Ideally speaking, wouldn't you eventually want to get to one BFS tanker flight for each BFS ship flight? I know the size of the booster is a hard limit on how big the tanker can get, but to flip the question around, if you were to keep the aeroshell of the BFS constant and fill the entire thing with prop, how much prop would that be? and how much bigger would the booster need to be to cope with it? Are we talking something the original 12m BFR could do, or is this getting into ridiculous triple stick launch scenarios? Because I can imagine there will eventually be separate dedicated factories for crew/tanker variants of BFS, plus a dedicated factory for making those boosters.
Only one tanker in high elliptical orbit. It's far more efficient to fuel up one tanker in LEO with lots of other tankers, then fly that one tanker to high elliptical orbit than to just send a bunch of tankers directly to the elliptical orbit. This makes a huge difference.
Quote from: Robotbeat on 10/19/2017 03:05 amOnly one tanker in high elliptical orbit. It's far more efficient to fuel up one tanker in LEO with lots of other tankers, then fly that one tanker to high elliptical orbit than to just send a bunch of tankers directly to the elliptical orbit. This makes a huge difference.Uh right, my mistake. So for example, you'd refill your high elliptical (HE) tanker to full in LEO (5 tanker flights), send it to HE orbit for 1.9km/s or so, then launch your BFS to LEO with 150 ton payload/crew, refill it in LEO only enough to get itself to HE orbit AND to compensate for what the HE tanker lost getting to HE orbit (so 2-3 tanker flights).
Quote from: Robotbeat on 10/19/2017 03:05 amOnly one tanker in high elliptical orbit. It's far more efficient to fuel up one tanker in LEO with lots of other tankers, then fly that one tanker to high elliptical orbit than to just send a bunch of tankers directly to the elliptical orbit. This makes a huge difference.Uh right, my mistake. So for example, you'd refill your high elliptical (HE) tanker to full in LEO (5 tanker flights), send it to HE orbit for 1.9km/s or so, then launch your BFS to LEO with 150 ton payload/crew, refill it in LEO only enough to get itself to HE orbit AND to compensate for what the HE tanker lost getting to HE orbit (so 2-3 tanker flights).Then you can refill the crew/payload BFS to full in HE orbit in a single refill operation, and be on your way to the moon without passing through the Van Allen belts too many times? The HE tanker can aerobrake back to LEO in its own time.With a 30% improvement in tanker prop payload from dedicated tanker, you might reduce your lunar mission profile from ~8 (5+3) to ~6 (4+2) tanker flights to LEO.
Quote from: mikelepage on 10/19/2017 04:17 amQuote from: Robotbeat on 10/19/2017 03:05 amOnly one tanker in high elliptical orbit. It's far more efficient to fuel up one tanker in LEO with lots of other tankers, then fly that one tanker to high elliptical orbit than to just send a bunch of tankers directly to the elliptical orbit. This makes a huge difference.Uh right, my mistake. So for example, you'd refill your high elliptical (HE) tanker to full in LEO (5 tanker flights), send it to HE orbit for 1.9km/s or so, then launch your BFS to LEO with 150 ton payload/crew, refill it in LEO only enough to get itself to HE orbit AND to compensate for what the HE tanker lost getting to HE orbit (so 2-3 tanker flights).Then you can refill the crew/payload BFS to full in HE orbit in a single refill operation, and be on your way to the moon without passing through the Van Allen belts too many times? The HE tanker can aerobrake back to LEO in its own time.With a 30% improvement in tanker prop payload from dedicated tanker, you might reduce your lunar mission profile from ~8 (5+3) to ~6 (4+2) tanker flights to LEO.A dedicated HE tanker could be filled with more than 5 tanker flights. More like 20 tanker flights.The mass of a dedicated HE tanker can be much bigger than that of the regular tanker.This is a quite essential, and the main reason it will look kinda weird on top of a 9 meter BFR.If the dry mass of a ship is 85 tons and it can carry 150 tons of fuel to LEO.A dedicated HE tanker can roughly have a dry mass of 235 tons, and will burn up all it’s fuel to get to orbit.A dedicated HE tanker with a mass of 235 tons will be at least 4 times the volume of a regular ship.
Quote from: Peter.Colin on 10/19/2017 08:31 pmQuote from: mikelepage on 10/19/2017 04:17 amQuote from: Robotbeat on 10/19/2017 03:05 amOnly one tanker in high elliptical orbit. It's far more efficient to fuel up one tanker in LEO with lots of other tankers, then fly that one tanker to high elliptical orbit than to just send a bunch of tankers directly to the elliptical orbit. This makes a huge difference.Uh right, my mistake. So for example, you'd refill your high elliptical (HE) tanker to full in LEO (5 tanker flights), send it to HE orbit for 1.9km/s or so, then launch your BFS to LEO with 150 ton payload/crew, refill it in LEO only enough to get itself to HE orbit AND to compensate for what the HE tanker lost getting to HE orbit (so 2-3 tanker flights).Then you can refill the crew/payload BFS to full in HE orbit in a single refill operation, and be on your way to the moon without passing through the Van Allen belts too many times? The HE tanker can aerobrake back to LEO in its own time.With a 30% improvement in tanker prop payload from dedicated tanker, you might reduce your lunar mission profile from ~8 (5+3) to ~6 (4+2) tanker flights to LEO.A dedicated HE tanker could be filled with more than 5 tanker flights. More like 20 tanker flights.The mass of a dedicated HE tanker can be much bigger than that of the regular tanker.This is a quite essential, and the main reason it will look kinda weird on top of a 9 meter BFR.If the dry mass of a ship is 85 tons and it can carry 150 tons of fuel to LEO.A dedicated HE tanker can roughly have a dry mass of 235 tons, and will burn up all it’s fuel to get to orbit.A dedicated HE tanker with a mass of 235 tons will be at least 4 times the volume of a regular ship.It can't fly to orbit with ANY fuel, so now you are describing a depot not a tanker. You are talking about at least dozens of refueling trips with a tanker to fill this depot, and now they still don't have a mass optimized tanker. What does this gain us? They can already fly to mars and land. If they have extra fuel it will just have to be vented into space.I think this is venturing well beyond SpaceX potential plans and into your own.
Quote from: intrepidpursuit on 10/19/2017 09:01 pmQuote from: Peter.Colin on 10/19/2017 08:31 pmQuote from: mikelepage on 10/19/2017 04:17 amQuote from: Robotbeat on 10/19/2017 03:05 amOnly one tanker in high elliptical orbit. It's far more efficient to fuel up one tanker in LEO with lots of other tankers, then fly that one tanker to high elliptical orbit than to just send a bunch of tankers directly to the elliptical orbit. This makes a huge difference.Uh right, my mistake. So for example, you'd refill your high elliptical (HE) tanker to full in LEO (5 tanker flights), send it to HE orbit for 1.9km/s or so, then launch your BFS to LEO with 150 ton payload/crew, refill it in LEO only enough to get itself to HE orbit AND to compensate for what the HE tanker lost getting to HE orbit (so 2-3 tanker flights).Then you can refill the crew/payload BFS to full in HE orbit in a single refill operation, and be on your way to the moon without passing through the Van Allen belts too many times? The HE tanker can aerobrake back to LEO in its own time.With a 30% improvement in tanker prop payload from dedicated tanker, you might reduce your lunar mission profile from ~8 (5+3) to ~6 (4+2) tanker flights to LEO.A dedicated HE tanker could be filled with more than 5 tanker flights. More like 20 tanker flights.The mass of a dedicated HE tanker can be much bigger than that of the regular tanker.This is a quite essential, and the main reason it will look kinda weird on top of a 9 meter BFR.If the dry mass of a ship is 85 tons and it can carry 150 tons of fuel to LEO.A dedicated HE tanker can roughly have a dry mass of 235 tons, and will burn up all it’s fuel to get to orbit.A dedicated HE tanker with a mass of 235 tons will be at least 4 times the volume of a regular ship.It can't fly to orbit with ANY fuel, so now you are describing a depot not a tanker. You are talking about at least dozens of refueling trips with a tanker to fill this depot, and now they still don't have a mass optimized tanker. What does this gain us? They can already fly to mars and land. If they have extra fuel it will just have to be vented into space.I think this is venturing well beyond SpaceX potential plans and into your own.A empty/full mass optimized tanker that stores 20 tankers in LEO is just enough to fill up the 4 ships planned for 2024.I don’t think that’s venturing well beyond SpaceX plans.
So you mean to say the HE-tanker is a depot also?
Quote from: Peter.Colin on 10/19/2017 09:13 pmSo you mean to say the HE-tanker is a depot also?What you are describing is not ALSO a depot; it is ONLY a depot. It is not a tanker at all. And SpaceX has made no mention thus far of using a depot. Just like your 350/1 T/W Raptor, you make this stuff up out of the ether then ascribe it to SpaceX. Please quit projecting.
Quote from: TomH on 10/19/2017 09:34 pmQuote from: Peter.Colin on 10/19/2017 09:13 pmSo you mean to say the HE-tanker is a depot also?What you are describing is not ALSO a depot; it is ONLY a depot. It is not a tanker at all. And SpaceX has made no mention thus far of using a depot. Just like your 350/1 T/W Raptor, you make this stuff up out of the ether then ascribe it to SpaceX. Please quit projecting.What’s the definition of a Depot? And if my ethereal space tanker can travel to mars but not land is it still a depot?I could bet you $50 the “kinda weird” tanker Elon Musk is talking about is bigger in volume than a regular spaceship, because it’s intended to stay in space
A empty/full mass optimized tanker that stores 20 tankers in LEO is just enough to fill up the 4 ships planned for 2024.
If the dry mass of a ship is 85 tons and it can carry 150 tons of fuel to LEO.A dedicated HE tanker can roughly have a dry mass of 235 tons, and will burn up all it’s fuel to get to LEO.In LEO it can be filled up by 20 tanker flights.A dedicated HE tanker with a mass of 235 tons will be at least 4 times the volume of a regular ship.
Quote from: Peter.Colin on 10/19/2017 08:31 pmIf the dry mass of a ship is 85 tons and it can carry 150 tons of fuel to LEO.A dedicated HE tanker can roughly have a dry mass of 235 tons, and will burn up all it’s fuel to get to LEO.In LEO it can be filled up by 20 tanker flights.A dedicated HE tanker with a mass of 235 tons will be at least 4 times the volume of a regular ship.With 235t dry mass your tanker would be about 140m long. This results in a 200m stack, too long to be stable enough to fly. Same reason why Falcon9 can't get a longer fairing.A bigger diameter could make this beast possible, but would require a second production line. Quite unlikely.A tanker optimized for maximum payload is way more reasonable. If it can reduce the number of launches needed to refuel BFS headed for Mars or Moon costs will drop. BFR/BFS architecture is all about reducing costs.
Quote from: CuddlyRocket on 10/18/2017 05:50 pmAre there any advantages to building tanker variants that specialise in either LOX or liquid methane transport? Obviously a LOX tanker will still need to carry methane - and vice versa - for take-off and landing, but you'd otherwise only need to stretch one of the two tanks. As the LOX is 80% of the mass of methalox propellant, a tanker optimised for LOX would probably look quite different from one optimised for methane.Not really. Any user of the propellant would need both, so now you always need to fly twice even if one flight might be sufficient for a small final top-off. Plus you have to design, test, and build two different types of tankers instead of just one. And as you say, the tanker will have to carry both for its own needs. So - No, I see absolutely no benefit.
Quote from: mikelepage on 10/19/2017 02:58 am(snip)but to flip the question around, if you were to keep the aeroshell of the BFS constant and fill the entire thing with prop, how much prop would that be? (snip)Only one tanker in high elliptical orbit. It's far more efficient to fuel up one tanker in LEO with lots of other tankers, then fly that one tanker to high elliptical orbit than to just send a bunch of tankers directly to the elliptical orbit. This makes a huge difference.
(snip)but to flip the question around, if you were to keep the aeroshell of the BFS constant and fill the entire thing with prop, how much prop would that be? (snip)
More massive (payload mass) is not the same as bigger, because prop is 5 to 10 times denser than just about any other payload.A dedicated tanker could be about 60% the length of the cargo/crew BFS, yet deliver 30% more payload mass. Any more length and it will start needing more engines to keep ascent T/W reasonable.
Quote from: ZachF on 10/17/2017 04:50 pmI think the tanker will be spherical.I would encourage you to do some reading about the baseball pitch known as a knuckleball. There is also a knuckleball serve in volleyball. Watch some slo-mo YouTube vids of those balls in flight. The same complex aerodynamics cause musket balls to be highly inaccurate in contrast to bullets fired from rifled barrels.The aerodynamic flow would be highly unstable. Max Q and Max Drag would also be immense negatives.
Quote from: OneSpeed on 10/17/2017 09:56 amSo, the egg has nearly the same surface area as a sphere, but is more aerodynamic, especially with a booster under it. The engine arrangement could be the same as the crew ship, but 3 of the 200 bar SL Raptors might be more appropriate for landing. But most importantly, when combined with the booster, it would look kinda weird.I can't get over the large amount of extra tooling required and the complete reworking of the design rather than adding 10% to the barrel sections of the tanks and lopping off the nose of the BFS.Then again the tanker isn't required at all for the current Mars plan so maybe for a second generation BFR.
QuoteWill the BFS landing propellants have to be actively cooled on the long trip to Mars?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.
Will the BFS landing propellants have to be actively cooled on the long trip to Mars?
Quote from: Explorer on 10/19/2017 10:28 pmQuote from: Peter.Colin on 10/19/2017 08:31 pmIf the dry mass of a ship is 85 tons and it can carry 150 tons of fuel to LEO.A dedicated HE tanker can roughly have a dry mass of 235 tons, and will burn up all it’s fuel to get to LEO.In LEO it can be filled up by 20 tanker flights.A dedicated HE tanker with a mass of 235 tons will be at least 4 times the volume of a regular ship.With 235t dry mass your tanker would be about 140m long. This results in a 200m stack, too long to be stable enough to fly. Same reason why Falcon9 can't get a longer fairing.A bigger diameter could make this beast possible, but would require a second production line. Quite unlikely.A tanker optimized for maximum payload is way more reasonable. If it can reduce the number of launches needed to refuel BFS headed for Mars or Moon costs will drop. BFR/BFS architecture is all about reducing costs.Any opportunity for inflatables here, like Bigelow habs but for fuel instead? (Space bladders) Might help keep the dimensions smaller without sacrificing volume.
I don't think this is likely, but...A BFR heavy (3x BFR cores) could launch a heavier tanker which fills much more of the volume of BFS. I don't see this as an FH style where the boosters separate, but where the three cores are permanently and rigidly connected, so would land together and be ready for rapid recycling. This assumes that a heavier BFS would not suffer excessive gravity losses after staging (or might be upgraded with more vac Raptors). It also assumes that SpaceX get comfortable with FH, and ultimately decide that it's not so hard to replicate after all. Alternatively, early 12m boosters might start out launching only 9m tankers until it has the flight history to prove it safe for launching more valuable cargos.Cheers, Martin
Does SpaceX have the Raptor design using supercooled propellant? Because that won't work very well for refueling operations. Works only on the launch pad where cyro-coolers are available.
Quote from: aero on 11/05/2017 09:57 pmDoes SpaceX have the Raptor design using supercooled propellant? Because that won't work very well for refueling operations. Works only on the launch pad where cyro-coolers are available.My recollection is that Raptor is being designed for subcooler propellant.However, I agree, when not working from (earth) ground operations that subcooling is going to take an already very complicated storage problem much harder.
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.
Quote from: wannamoonbase on 11/05/2017 10:03 pmQuote from: aero on 11/05/2017 09:57 pmDoes SpaceX have the Raptor design using supercooled propellant? Because that won't work very well for refueling operations. Works only on the launch pad where cyro-coolers are available.My recollection is that Raptor is being designed for subcooler propellant.However, I agree, when not working from (earth) ground operations that subcooling is going to take an already very complicated storage problem much harder.From the Reddit AMA. Sounds like the header tanks + main tanks form a big dewar.https://www.reddit.com/r/spacex/comments/76fg3f/elon_musk_ama_questions_and_answers_xpost_from/QuoteThe 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.
I am under the impression that venting only cools the liquid to the boiling point at the pressure of the liquid. Am I wrong, or will the tank pressure be maintained at a level where the boiling of the propellants is considered "subcooled?"
Quote from: docmordrid on 11/05/2017 10:24 pmQuote from: wannamoonbase on 11/05/2017 10:03 pmQuote from: aero on 11/05/2017 09:57 pmDoes SpaceX have the Raptor design using supercooled propellant? Because that won't work very well for refueling operations. Works only on the launch pad where cyro-coolers are available.My recollection is that Raptor is being designed for subcooler propellant.However, I agree, when not working from (earth) ground operations that subcooling is going to take an already very complicated storage problem much harder.From the Reddit AMA. Sounds like the header tanks + main tanks form a big dewar.https://www.reddit.com/r/spacex/comments/76fg3f/elon_musk_ama_questions_and_answers_xpost_from/QuoteThe 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.I am under the impression that venting only cools the liquid to the boiling point at the pressure of the liquid. Am I wrong, or will the tank pressure be maintained at a level where the boiling of the propellants is considered "subcooled?"
Quote from: Explorer on 10/19/2017 10:28 pmQuote from: Peter.Colin on 10/19/2017 08:31 pmIf the dry mass of a ship is 85 tons and it can carry 150 tons of fuel to LEO.A dedicated HE tanker can roughly have a dry mass of 235 tons, and will burn up all it’s fuel to get to LEO.In LEO it can be filled up by 20 tanker flights.A dedicated HE tanker with a mass of 235 tons will be at least 4 times the volume of a regular ship.
Quote from: Peter.Colin on 10/19/2017 08:31 pmIf the dry mass of a ship is 85 tons and it can carry 150 tons of fuel to LEO.A dedicated HE tanker can roughly have a dry mass of 235 tons, and will burn up all it’s fuel to get to LEO.In LEO it can be filled up by 20 tanker flights.A dedicated HE tanker with a mass of 235 tons will be at least 4 times the volume of a regular ship.
QuoteWith 235t dry mass your tanker would be about 140m long. This results in a 200m stack, too long to be stable enough to fly. Same reason why Falcon9 can't get a longer fairing.A bigger diameter could make this beast possible, but would require a second production line. Quite unlikely.A tanker optimized for maximum payload is way more reasonable. If it can reduce the number of launches needed to refuel BFS headed for Mars or Moon costs will drop. BFR/BFS architecture is all about reducing costs.Yes it’s all about reducing costs.
With 235t dry mass your tanker would be about 140m long. This results in a 200m stack, too long to be stable enough to fly. Same reason why Falcon9 can't get a longer fairing.A bigger diameter could make this beast possible, but would require a second production line. Quite unlikely.A tanker optimized for maximum payload is way more reasonable. If it can reduce the number of launches needed to refuel BFS headed for Mars or Moon costs will drop. BFR/BFS architecture is all about reducing costs.
The argument made above by Robotbeat about first transfering all 150 ton LEO tankers to one fully tanked High Eleptic tanker is about reducing the amounts of BFR launches needed.Or better stated reducing the total amount of fuel burned to get fuel into an HE orbit.
Creating a bigger Space only Tanker that could be launched by the 9m BFR, would further reduce the amount of fuel needed to burn to get fuel into an HE-Orbit.
A bigger 12 meter diameter tank has already been made without a second production line, so it’s not unthinkable that they could eventually make one bigger diameter Space-tanker without a new production line. With a 9 meter engine base, that could look kinda strange.
There’s no reason to extend the propellant tanks to fill the BFS volume for ordinary use launching propellant into orbit, since it couldn’t launch that much mass if they were filled. So Tanker and Cargo are the same BFS.OTOH if you built a variant of the BFS like that, with fully extended propellant tanks and no cargo volume, it could launch no problem with the tanks partially filled. It couldn’t carry as much propellant into orbit because of the extra mass from the extended tanks but it would still a carry significant amount.This variant might have a use though as a fuel depot/deep space Booster.It would hold significantly more propellant than a normal BFS, it just couldn’t launch with it. On orbit it could dock with regular BFS Tanker missions and fill up.The advantage of being a ready made propellant depot is Tankers could be launched at any convenient time to load the depot and missions could load propellant in one shot from the depot rather than in 4 or 5 separate events. Since the Depot is a standard BFS other than the tanks, nothing changes about the propellant transfer.The other function is as a deep space booster. Using a BFS this way has been discussed on NSF. The extra tank space just adds delta V. Any BFS could do this. One with extended tanks could just do it even better. With a full propellant load a Booster would enable BFS missions to the asteroids and outer planet moons that are impossible otherwise as well as landing more massive cargos on Mars.As a Booster it would come back for reuse. It could serve either role interchangeably, remaining in orbit.
Question. Speaking of customizing a tanker for on orbit and deep space missions, would this customized tanker need to carry the mass of all of the engines? That is, once on orbit, would any significant mission capability be added by removing, say, six of the raptors? Or even replacing them with a low thrust, long burning engines? I know this thought is for the far future but then, that is what we think about here on NSF.
Quote from: Ludus on 11/27/2017 06:26 amThere’s no reason to extend the propellant tanks to fill the BFS volume for ordinary use launching propellant into orbit, since it couldn’t launch that much mass if they were filled. So Tanker and Cargo are the same BFS.OTOH if you built a variant of the BFS like that, with fully extended propellant tanks and no cargo volume, it could launch no problem with the tanks partially filled. It couldn’t carry as much propellant into orbit because of the extra mass from the extended tanks but it would still a carry significant amount.This variant might have a use though as a fuel depot/deep space Booster.It would hold significantly more propellant than a normal BFS, it just couldn’t launch with it. On orbit it could dock with regular BFS Tanker missions and fill up.The advantage of being a ready made propellant depot is Tankers could be launched at any convenient time to load the depot and missions could load propellant in one shot from the depot rather than in 4 or 5 separate events. Since the Depot is a standard BFS other than the tanks, nothing changes about the propellant transfer.The other function is as a deep space booster. Using a BFS this way has been discussed on NSF. The extra tank space just adds delta V. Any BFS could do this. One with extended tanks could just do it even better. With a full propellant load a Booster would enable BFS missions to the asteroids and outer planet moons that are impossible otherwise as well as landing more massive cargos on Mars.As a Booster it would come back for reuse. It could serve either role interchangeably, remaining in orbit.Question. Speaking of customizing a tanker for on orbit and deep space missions, would this customized tanker need to carry the mass of all of the engines? That is, once on orbit, would any significant mission capability be added by removing, say, six of the raptors?
Or even replacing them with a low thrust, long burning engines? I know this thought is for the far future but then, that is what we think about here on NSF.
Quote from: Ludus on 11/27/2017 06:26 amThere’s no reason to extend the propellant tanks to fill the BFS volume for ordinary use launching propellant into orbit, since it couldn’t launch that much mass if they were filled. So Tanker and Cargo are the same BFS.OTOH if you built a variant of the BFS like that, with fully extended propellant tanks and no cargo volume, it could launch no problem with the tanks partially filled. It couldn’t carry as much propellant into orbit because of the extra mass from the extended tanks but it would still a carry significant amount.This variant might have a use though as a fuel depot/deep space Booster.It would hold significantly more propellant than a normal BFS, it just couldn’t launch with it. On orbit it could dock with regular BFS Tanker missions and fill up.The advantage of being a ready made propellant depot is Tankers could be launched at any convenient time to load the depot and missions could load propellant in one shot from the depot rather than in 4 or 5 separate events. Since the Depot is a standard BFS other than the tanks, nothing changes about the propellant transfer.The other function is as a deep space booster. Using a BFS this way has been discussed on NSF. The extra tank space just adds delta V. Any BFS could do this. One with extended tanks could just do it even better. With a full propellant load a Booster would enable BFS missions to the asteroids and outer planet moons that are impossible otherwise as well as landing more massive cargos on Mars.As a Booster it would come back for reuse. It could serve either role interchangeably, remaining in orbit.Question. Speaking of customizing a tanker for on orbit and deep space missions, would this customized tanker need to carry the mass of all of the engines? That is, once on orbit, would any significant mission capability be added by removing, say, six of the raptors? Or even replacing them with a low thrust, long burning engines? I know this thought is for the far future but then, that is what we think about here on NSF.
Quote from: aero on 11/27/2017 03:54 pmQuestion. Speaking of customizing a tanker for on orbit and deep space missions, would this customized tanker need to carry the mass of all of the engines? That is, once on orbit, would any significant mission capability be added by removing, say, six of the raptors?No. The engines weight only few tonnes, but removing them would worsen gravity losses considerably.
Question. Speaking of customizing a tanker for on orbit and deep space missions, would this customized tanker need to carry the mass of all of the engines? That is, once on orbit, would any significant mission capability be added by removing, say, six of the raptors?
Quote from: hkultala on 11/27/2017 06:05 pmQuote from: aero on 11/27/2017 03:54 pmQuestion. Speaking of customizing a tanker for on orbit and deep space missions, would this customized tanker need to carry the mass of all of the engines? That is, once on orbit, would any significant mission capability be added by removing, say, six of the raptors?No. The engines weight only few tonnes, but removing them would worsen gravity losses considerably.I believe this was an on orbit modification, and this vessel then would never return to a gravity well but instead stay in orbit. So gravity losses might not apply, although maybe I am confused?
Quote from: Lar on 11/29/2017 02:49 amQuote from: hkultala on 11/27/2017 06:05 pmQuote from: aero on 11/27/2017 03:54 pmQuestion. Speaking of customizing a tanker for on orbit and deep space missions, would this customized tanker need to carry the mass of all of the engines? That is, once on orbit, would any significant mission capability be added by removing, say, six of the raptors?No. The engines weight only few tonnes, but removing them would worsen gravity losses considerably.I believe this was an on orbit modification, and this vessel then would never return to a gravity well but instead stay in orbit. So gravity losses might not apply, although maybe I am confused?If I’m following this, the variant I was talking about was a BFS built on earth with propellant tanks completely filling its volume rather than the large cargo space left over. It could launch only with those extended tanks partially filled. It might however have utility as a fuel depot/deep space Booster. My notion was this would be an otherwise standard BFS with bigger tanks. I took the question about removing engines to be asking could it launch successfully and operate as an in space Booster if you left some engines off considering it wouldn’t land on earth again. I’m not sure what’s meant by gravity losses unless that’s assuming all the engines would be removed? I guess “propellant depot” sort of suggests it might have no engines but that’s not compatible with using it as a Booster too. It’s really more about function than any modification to the BFS. Assigning a BFS to the specialized function of collecting propellant from the Tanker flights so it can load it in one go to a mission. The same BFS could also dock nose to tail with a mission BFS and act as a Booster. A BFS that happened to be modified so it’s entire OML was occupied by propellant tanks would work better in this function. Unlike a Tanker it would remain in space rather than landing and repeatedly launching. Maybe it could get along without some of the ordinary BFS hardware.