Author Topic: SpaceX tanker variant  (Read 38346 times)

Offline hkultala

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SpaceX tanker variant
« on: 10/16/2017 11:58 am »
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).
"

So, first just using same (cargo) BFS, with empty tanks it will have plenty of fuel left when it reacher orbit.


But the dedicated tanker:

The "cargo space" of current BFR is has much higher area than tanks needed for 150 tonnes of propellant.

No need for cargo hatch etc might save some weight, so the optimized tanker might be able to actually carry slightly more propellant than 150 tonnes. Let's say 160 tonnes.

So, the "trivial design" would be to just increase the tanks lengths so that it can carry 160 tonnes more propellant.
This would mean much shorter craft than the ordinary BFS.

What about the fins? Are they needed for the tanker?

The tanker would always be practically empty when landing, and it would always land on earth. Might not need the fins.

3 landing engines also not needed. Only 1 would be enough for the T/W, but maybe still have two for redundancy?

Not having the fins and third engine might save more mass. Maybe could carry even 170 tonnes of fuel?


Offline Peter.Colin

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Re: SpaceX tanker variant
« Reply #1 on: 10/16/2017 12:05 pm »
The new “kinda weird” tanker might be a big almost empty tanker.
More like the nose cone of a Falcon 9 (12-15 meter wide)
Without heat shield
Without landing legs
Without deep space engines
Without fins

It would be basically 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 cargo ships.
And than launch the spaceships, that are filled up by the depot-tankers.
Maybe 10 launches are needed to fill up a depot-tanker, could someone calculate this?

The Mars depot-tanker would need a heat shield.
But no landing legs.
« Last Edit: 10/16/2017 12:10 pm by Peter.Colin »

Offline Athrithalix

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Re: SpaceX tanker variant
« Reply #2 on: 10/16/2017 12:11 pm »
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.

Offline Peter.Colin

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Re: SpaceX tanker variant
« Reply #3 on: 10/16/2017 12:23 pm »
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.

It depends on how many Mars missions will there be in the future?
The gradual wastage of fuel is negleable if the heat isolated nosecone is pointed to the sun at all times.
Like will be done when traveling to Mars.
You could even reduce it to zero if there is a cryocondensor built in, that is planned for future ships.

« Last Edit: 10/16/2017 12:24 pm by Peter.Colin »

Offline corneliussulla

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Re: SpaceX tanker variant
« Reply #4 on: 10/16/2017 03:03 pm »
Remember BFS going to a lot more places than Mars, cis lunar space, lunar surface, potential lagrangepoint missions, possible contracted to asteroid miners. In fact as Mars only in place once every 2 Years BFS probably gets used a lot more to other destinations at first.

Offline philw1776

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Re: SpaceX tanker variant
« Reply #5 on: 10/16/2017 03:04 pm »
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.

Not necessarily in planning however that's interpreted but Lunar surface missions require re-fueling as would very high payload to HEO and beyond Earth orbital and cis-lunar missions.
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Offline Cheapchips

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Re: SpaceX tanker variant
« Reply #6 on: 10/16/2017 03:21 pm »
Did Musk mean that it literally looks weird or that it would seem weird?

What would fall into either interpretation?

I'd have thought that's it's a given that it's reusable and derived from the regular BFS.  That's cheapest and every landing counts towards crew rating /passenger rating.

Offline philw1776

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Re: SpaceX tanker variant
« Reply #7 on: 10/16/2017 03:23 pm »
A future low dry mass tanker could simply be a 2nd stage with only 2 SL Raptors and a very short, nearly nil "cargo" volume.  Extra propellant tanks pretty much fit in the nose volume.  Or simply stretch the std propellant tanks by just a couple meters and have zero cargo volume/length. 

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

Ironic that the shorter BFR tanker would deliver more mass to LEO than the "standard" taller config.
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Offline aero

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Re: SpaceX tanker variant
« Reply #8 on: 10/16/2017 03:32 pm »
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."
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Offline Lars-J

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Re: SpaceX tanker variant
« Reply #9 on: 10/16/2017 03:56 pm »
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."

Yep, that's what I expect... An optimized tanker would be a shorter and more squat looking BFS.

Some here seem to think that it would been a larger diameter BFS or something even weirder. No. Propellant is HEAVY. There is a practical limit to what the BFR booster can lift. If anything it will be smaller. And certainly not expendable.

Offline Cheapchips

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Re: SpaceX tanker variant
« Reply #10 on: 10/16/2017 04:10 pm »
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?


Offline envy887

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Re: SpaceX tanker variant
« Reply #11 on: 10/16/2017 05:48 pm »
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?

No need to blunt it, the same ogive shape as the cargo ship is fine.

Offline tea monster

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Re: SpaceX tanker variant
« Reply #12 on: 10/16/2017 06:02 pm »
This is with the same shaped nose as the manned/cargo version.

Offline Peter.Colin

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Re: SpaceX tanker variant
« Reply #13 on: 10/16/2017 06:51 pm »
This is with the same shaped nose as the manned/cargo version.

It doesn’t really qualify, “warning: it will look kinda weird”
« Last Edit: 10/16/2017 06:53 pm by Peter.Colin »

Offline nacnud

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Re: SpaceX tanker variant
« Reply #14 on: 10/16/2017 07:07 pm »
I think you are reading far too much into that one comment, bear in mind that the whole comment was.

'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).' - Elon

Offline philw1776

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Re: SpaceX tanker variant
« Reply #15 on: 10/16/2017 07:11 pm »
This is with the same shaped nose as the manned/cargo version.


Shorter yet.  Nose atop just a few meters of expanded tankage.
Zero crew or cargo space.
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Offline GalacticIntruder

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Re: SpaceX tanker variant
« Reply #16 on: 10/16/2017 07:34 pm »
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. 
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Offline meekGee

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Re: SpaceX tanker variant
« Reply #17 on: 10/16/2017 07:41 pm »
I'm going with "spherically short".

On a different question, Musk referred to cylindrical tanks as having "even acceptable" mass ratios, still with some disdain..

I'm thinking it'll almost look like a flying beach ball.

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Offline Lar

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Re: SpaceX tanker variant
« Reply #18 on: 10/16/2017 07:47 pm »
This is with the same shaped nose as the manned/cargo version.

Wouldn't it still need the delta for reentry?
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Offline nacnud

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Re: SpaceX tanker variant
« Reply #19 on: 10/16/2017 07:56 pm »
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.
« Last Edit: 10/16/2017 07:58 pm by nacnud »

Offline hkultala

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Re: SpaceX tanker variant
« Reply #20 on: 10/16/2017 07:56 pm »
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.

There is no need for the tanker to be larger. Exactly the opposite, it's supposed to be much smaller.

Liquid methane + LOX is much more dense than average cargo, so it needs less space, smaller vehicle.

Offline ZachF

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Re: SpaceX tanker variant
« Reply #21 on: 10/16/2017 07:56 pm »
This is with the same shaped nose as the manned/cargo version.


Probably something more like this (I think it would still the need wings though, right?)
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Offline nacnud

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Re: SpaceX tanker variant
« Reply #22 on: 10/16/2017 08:00 pm »
Yep probably need the wings. I go with an ogive nose and halve the length compared to the normal ship.
« Last Edit: 10/16/2017 08:00 pm by nacnud »

Offline hkultala

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Re: SpaceX tanker variant
« Reply #23 on: 10/16/2017 08:02 pm »
This is with the same shaped nose as the manned/cargo version.

Wouldn't it still need the delta for reentry?

Not necessary.

1) Lacking the big (typically mostly empty) cargo space in front means center of pressure is much closer to the end of the vehicle, meaning less drag-inducing surfaces needed to tail to get center of pressure behind enough.

2) I't always re-entering earth, not other planets

3) It's always re-entering as empty, never with cargo

2+3) It's always re-entering at similar conditions. No need for the flaps to trim to different situations.


One possibility is that the tanker has fins, but much smaller than the normal version.

Offline tea monster

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Re: SpaceX tanker variant
« Reply #24 on: 10/16/2017 08:02 pm »


Shorter.



Weirder, with a ballistic nose instead of an aerodynamic one

Offline nacnud

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Re: SpaceX tanker variant
« Reply #25 on: 10/16/2017 08:04 pm »
Aren't the wings there to counter the mass of the engines and to stop it entering tail first?

Offline guckyfan

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Re: SpaceX tanker variant
« Reply #26 on: 10/16/2017 08:08 pm »
The version where the top is just the tank dome is the most weight and volume efficient. Plus it fits the weird comment. Will it need an additional aerodynamic cap?

Offline nacnud

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Re: SpaceX tanker variant
« Reply #27 on: 10/16/2017 08:14 pm »
Well given that the only other two flying fuel tanks* (that I know of) that nearly made it to orbit both had ogive noses I'm going to go with: Yes an ogive nose is worth the weight penalty.


*Shuttle and Buran tanks.

Offline KelvinZero

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Re: SpaceX tanker variant
« Reply #28 on: 10/16/2017 10:01 pm »
Maybe it needs more wing. That could be what makes it look weird.

Even weirder thought:
Instead of wider wings, what if they stretched back further, like droopy ears? This could be the most efficient way to balance the drag on both sides of the COM. Also it would increase the drag less on the way up.

I don't know how such a thing would land. Maybe the droopy flaps can fold, or maybe a specialised cradle.

Offline livingjw

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Re: SpaceX tanker variant
« Reply #29 on: 10/16/2017 11:27 pm »
The wing moves the aerodynamic center aft to counter the long nose. When you shorten the nose and lighten the reentry weight, you will need less wing, but you will still need the flaps for pitch and roll. I think they would clip off the outboard portion of the wing. That will make it look somewhat like the ESA XVI. As stated above, the nose would probably be shaped similar to the shuttle external tank.  Would be an ugly little critter, but hard working like a switching engine.

John
« Last Edit: 10/17/2017 11:37 am by livingjw »

Offline tea monster

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Re: SpaceX tanker variant
« Reply #30 on: 10/17/2017 12:20 am »
I thought I read somewhere in the Reddit Q/A that the tanker wouldn't have winglets. I can't find it in the transcript so I can't verify it.

On the topic of engines, this quote is interesting.:
Quote
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.

Source: https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/

Offline KelvinZero

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Re: SpaceX tanker variant
« Reply #31 on: 10/17/2017 12:50 am »
The wing moves the aerodynamic center aft to counter the long nose.
John
Hi, 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.

Offline aero

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Re: SpaceX tanker variant
« Reply #32 on: 10/17/2017 01:00 am »
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.

So how far is it from the engine exit nozzles to the top of the propellant tank? Call that number 1100 length units, then add 150 units of length to see that the length of the tanker to the top of the payload is 1250 units. Now slap on the nose of choice and sketch it. You will see that the tanker is much shorter than the BFS. In fact, I speculate that the 150 tons of propellant cargo would easily fit within tanks in the nose, so the total length of the tanker might be just those 1100 length units plus the nose section. Adding tanks within the nose section would seem to me to be the right choice as it makes maximum use of commonality between the tanker and the BFS. Of course, you may need to keep the equipment bay.
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Offline nacnud

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Re: SpaceX tanker variant
« Reply #33 on: 10/17/2017 01:14 am »
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.

Offline aero

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Re: SpaceX tanker variant
« Reply #34 on: 10/17/2017 02:48 am »
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.

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

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Offline meekGee

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Re: SpaceX tanker variant
« Reply #35 on: 10/17/2017 07:02 am »
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 didn't mean the tanker will be spherical.  I meant that in order to achieve the best mass fraction, the tanks should be as near spherical as possible.

Of course the engine section will add length, and there will be some aerodynamic consideration.

No wings, since it doesn't have to fly anywhere other than Earth.

Maybe they'll stick the two tanks inside each other  :)

But short-and-stubby, with emphasis on reducing tank mass.
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Online OneSpeed

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Re: SpaceX tanker variant
« Reply #36 on: 10/17/2017 09:56 am »
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).
"
...

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 * 0.417 = 541.5m^3 or a 10mØ 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.
« Last Edit: 10/17/2017 06:19 pm by OneSpeed »

Offline nacnud

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Re: SpaceX tanker variant
« Reply #37 on: 10/17/2017 10:37 am »
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.

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



Maybe this can help

Offline nacnud

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Re: SpaceX tanker variant
« Reply #38 on: 10/17/2017 10:49 am »
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.

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.

Offline Hauerg

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Re: SpaceX tanker variant
« Reply #39 on: 10/17/2017 10:52 am »
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.

Offline nacnud

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Re: SpaceX tanker variant
« Reply #40 on: 10/17/2017 11:27 am »
 Yeah it took me a while to work that out :-[, just adjust the cg by moving the header tanks  ;D

Online OneSpeed

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Re: SpaceX tanker variant
« Reply #41 on: 10/17/2017 11:30 am »
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.

I didn't render any wings, but that doesn't mean that control authority is not still an issue. Vernier thrusters may not be sufficient. Control surfaces like split body flaps, with or without wings may still be necessary.

Offline tea monster

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Re: SpaceX tanker variant
« Reply #42 on: 10/17/2017 02:32 pm »

Offline Lar

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Re: SpaceX tanker variant
« Reply #43 on: 10/17/2017 04:12 pm »
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.

Always is a very long time. I can see (in the far future, after I'm dead) tankers leaving from other planetary surfaces, and reentering there as well.
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline Lars-J

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Re: SpaceX tanker variant
« Reply #44 on: 10/17/2017 04:38 pm »
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.

No... The wings/fins will likely have to be LARGER. This is why the fins are needed, to provide balance. An empty tanker will be extremely tail heavy, and will need larger control services to maintain the proper AoA during re-entry.

Offline ZachF

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Re: SpaceX tanker variant
« Reply #45 on: 10/17/2017 04:50 pm »
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).
"
...

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.

You're on to the same train of thought I was last night, when doing some doodles.

I think the tanker will be spherical.

Moreover, I think they will wait until Raptor 'full thrust' is ready. Increasing the pressure from 250 bar to 300 bar would increase thrust by the same amount (20%), and allow the BFR stack to lift 5,200 tonnes rather than 4,400 tonnes. This would allow the second stage to increase in size by about 800 tonnes to 2,100 tonnes.

It would be able to lift >250 tonnes of fuel to orbit, refueling a BFS in four trips. It would re-enter face first then flip over to land. There would be no wings, save perhaps winglets around the edge for stability since the CoG will be near the bottom/back.

If the tanker itself was refuelled, it would also be able to fully refill a 1,100 tonne BFS tank in high EEO.

A spherical tanker would be able to have a crazy good mass ratio, The old 2016 ITS tanker has a 3.6% mass ratio, and this could be even better. Look at how compact it is, even with a 2,000+ tonne fuel tank.

It also perfectly fits Elon's description of funny/unexpected. It looks like a Mechwarrior dropship, or the old handheld nuke launcher from the cold war... and other things.
artist, so take opinions expressed above with a well-rendered grain of salt...
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Offline John Alan

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Re: SpaceX tanker variant
« Reply #46 on: 10/17/2017 05:15 pm »
Round nosed bullets do not like breaking the sound barrier...
Known fact for about 100 years...

All I got to say, on the above tanker idea...  ???

Offline acsawdey

Re: SpaceX tanker variant
« Reply #47 on: 10/17/2017 05:19 pm »
Looks cool but no way will it re-enter that way, it would be very unstable with most of the mass in the back like that.


Offline Peter.Colin

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Re: SpaceX tanker variant
« Reply #48 on: 10/17/2017 07:06 pm »
This definitely qualifies for the kinda weird warning  :)
But it looks plausible.

How many engines does this tanker have?

How big could an empty spherical tank be (as orbital depot tanker) or would that need to be elongated to remain practical?
« Last Edit: 10/17/2017 07:11 pm by Peter.Colin »

Offline matthewkantar

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Re: SpaceX tanker variant
« Reply #49 on: 10/17/2017 08:21 pm »
I love the idea of a spherical CH4 tank inside a spherical LOX tank with the CH4 tank near the bottom of the bigger tank. The header tanks could be in the a stalk that connects the inside tank to the outside tank. The plumbing is much shorter as well. The design is best from a heat exchange and mass fraction viewpoint.

One reason it won't happen is all the tooling involved that is totally unrelated to the BFS/BFR designs. The aero dynamics are daunting as well.

Matthew
« Last Edit: 10/17/2017 08:27 pm by matthewkantar »

Offline livingjw

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Re: SpaceX tanker variant
« Reply #50 on: 10/17/2017 09:15 pm »
The wing moves the aerodynamic center aft to counter the long nose.
John
Hi, 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.

Aerodynamic Center, AC, is independent of the CG. Adding a wing to a body moves the AC towards the wing. The bigger the wing the closer the vehicle AC is to its wing AC. This vehicle's needs to be about neutral stability on reentry, so the AC and CG need to be very near each other. Split controls at the rear give you control authority needed to control pitch and roll. RCS thrusters for yaw. It has been suggested that the landing tanks can be moved forward if it is necessary to move the reentry CG forward if needed to lineup the CG with the AC.

John

Offline mikelepage

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Re: SpaceX tanker variant
« Reply #51 on: 10/18/2017 02:48 pm »
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.


Offline Cheapchips

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Re: SpaceX tanker variant
« Reply #52 on: 10/18/2017 04:10 pm »

One reason it won't happen is all the tooling involved that is totally unrelated to the BFS/BFR designs.

They could use it to test tooling for the BFR successor?  On paper at least, in the second part of the decade they need to have BFR2 ready to compete with New Armstrong.

It's also not like they haven't built a 12m tank already, even if it's not part of their normal factory pipeline. I do wonder if someone at SpaceX took that test tank shooting into the air as inspiration. :)

I'm not expecting a tanker in active use before Mars mission 2.  I don't think any type of tanker, BRS derived or more exotic, saves you money before mission 3?

(I think it's fair to assume each mission will grow the number of ships)

Offline envy887

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Re: SpaceX tanker variant
« Reply #53 on: 10/18/2017 04:16 pm »
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.


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.

Offline CuddlyRocket

Re: SpaceX tanker variant
« Reply #54 on: 10/18/2017 05:50 pm »
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.

Offline Lars-J

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Re: SpaceX tanker variant
« Reply #55 on: 10/18/2017 07:44 pm »
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.

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.
« Last Edit: 10/18/2017 07:45 pm by Lars-J »

Offline Norm38

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Re: SpaceX tanker variant
« Reply #56 on: 10/18/2017 08:34 pm »
There's a lot more widely accessible oxygen on the moon bound up in rocks than either carbon or hydrogen.  So in a far future where lunar LOX was supplying an orbital fuel depot, there may be a need to truck only methane. But that's a long ways off.

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.

Offline Lars-J

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Re: SpaceX tanker variant
« Reply #57 on: 10/18/2017 09:00 pm »
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.

Except that he explicitly stated the opposite - initial tanker flights will simply be BFS without cargo.
Quote
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).

Offline TomH

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Re: SpaceX tanker variant
« Reply #58 on: 10/18/2017 11:42 pm »
I 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.

Offline KelvinZero

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Re: SpaceX tanker variant
« Reply #59 on: 10/19/2017 12:18 am »
Yeah it took me a while to work that out :-[, just adjust the cg by moving the header tanks  ;D
This could be another reason to move the header tanks in the general design as well. They must have thought about it really hard but there was an elon comment somewhere that they were not perfectly happy with this.
"..avoid/minimize plumbing hell, but we don't super love the current header tank/plumbing design,”  “further refinement [are] likely.”

As a layman, I don't like the extra shared surface area between oxygen and methane tanks, and cramming everything into the lower tank like that looks like it would make tweeks or stretches suddenly very fiddly.

Offline Norm38

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Re: SpaceX tanker variant
« Reply #60 on: 10/19/2017 02:17 am »
Except that he explicitly stated the opposite - initial tanker flights will simply be BFS without cargo.
Quote
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).

Yeah I wasn't that clear. I didn't mean the final super tanker, but the first one. The BFS with everything stripped out. That's what I'd fly first.
« Last Edit: 10/19/2017 04:18 pm by Norm38 »

Offline mikelepage

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Re: SpaceX tanker variant
« Reply #61 on: 10/19/2017 02:58 am »
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.


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.

Offline Robotbeat

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Re: SpaceX tanker variant
« Reply #62 on: 10/19/2017 03:05 am »
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.


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.
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Offline mikelepage

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Re: SpaceX tanker variant
« Reply #63 on: 10/19/2017 04:17 am »
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.

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.

Offline speedevil

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Re: SpaceX tanker variant
« Reply #64 on: 10/19/2017 11:59 am »
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.

I wonder if this might mean an orbital tanker which can't land is useful.
A shell, perhaps close to the mold line of BFS, one vacuum raptor to boost it with minimal fuel to get to orbit.

But, this only has a point if no maintenance would be needed, and perhaps has no point if inclination changes is often desired.

One tiny engine on a very light BFS sized thing which can carry a couple of thousand tons of fuel would look 'weird'.
Perhaps even add a fold-out planetary-radiation shield for extra weirdness.

I think 'look weird' includes things that look weird from the perspective of someone experienced in the industry, and not only things that look like the crazy frog perched on BFR.

Now I'm imagining stupid stuff like packing a massive inflatable radiation cooled sun-earth shielded tank that runs at 5PSI inside BFS.
I think you can almost get a base frame with the standard fuel transfer ports (if those are the four lines in the image of BFS) out of the stock airlock of BFS, with minimal folding. Zip-tie tank onto it, and that's gonna look weird.
« Last Edit: 10/19/2017 12:41 pm by speedevil »

Offline hkultala

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Re: SpaceX tanker variant
« Reply #65 on: 10/19/2017 01:04 pm »
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.

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).

Leaving LEO with anything less than full tanks makes no sense.
The fuel has to be lifted to HEO anyway. It's most efficiently done when there is no wasted tank capasity between LEo and HEO.

Offline intrepidpursuit

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Re: SpaceX tanker variant
« Reply #66 on: 10/19/2017 08:06 pm »
The tanker will be a workhorse ship that would fly much more often than any other variant. Bulkheads and engines are the heaviest things in a rocket, so optimizing for fuel fraction would most likely mean shifting the common bulkhead up and then putting an oberth nose that replaces the upper bulkhead of the upper tank. The latter is supported by Elon's insistant avoidance of building a "box within a box" which is what an aerodynamic nose cone would be. It shouldn't be hard to figure out how much more fuel the booster could push and adjust the tanks accordingly.

Stretching a fuel tank seems a pretty easy thing for SpaceX to do since it has happened several times. Keeping the outer diameter the same would be the key to keeping it buildable with most of the same tooling.

The thrust structure and base will likely be identical to the other variants. The vacuum raptors are most needed when functioning as a second stage so with a maximum weight payload those aren't going anywhere. They could use 2 engines instead of 3 for landing to sacrifice redundancy for weight, but similarity probably saves more than the cost of the engine and these need to be reliable too since they will likely get an order of magnitude more flights than the other ships.

I don't see a tanker ever needing to land on any other body, so the wing structures may be optimized for earth EDL. As others have pointed out, this one will be much more tail heavy than the others so I don't see the wing going away and might even get bigger, though perhaps the wing is more optimized for mars than we are realizing and there is another shortcut they could use for earth that is lighter.

Offline Peter.Colin

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Re: SpaceX tanker variant
« Reply #67 on: 10/19/2017 08:31 pm »
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.

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 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.
« Last Edit: 10/19/2017 09:01 pm by Peter.Colin »

Offline intrepidpursuit

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Re: SpaceX tanker variant
« Reply #68 on: 10/19/2017 09:01 pm »
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.

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.
« Last Edit: 10/19/2017 09:03 pm by intrepidpursuit »

Offline Peter.Colin

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Re: SpaceX tanker variant
« Reply #69 on: 10/19/2017 09:13 pm »
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.

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?
« Last Edit: 10/19/2017 09:18 pm by Peter.Colin »

Offline ZachF

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Re: SpaceX tanker variant
« Reply #70 on: 10/19/2017 09:15 pm »
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.



SpaceX claimed $162/tonne for methalox fuel in their slides last year, and it carries about 4000 tonnes of fuel, so

4000 x 162 = $648,000

$648,000 / 150,000kg = $4.32/hg

Pretty cheap.

fuel would only need to be about 1/10th of the cost to get down to around $40/kg.
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Offline intrepidpursuit

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Re: SpaceX tanker variant
« Reply #71 on: 10/19/2017 09:16 pm »
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.

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.

It is not a tanker. The definition of tanker is something that transports fuel, your concept would not be capable of transporting fuel except in space.

They won't want to put the fuel up too far in advance or too much of it would boil off. They could accomplish the exact same with with a ship sized tanker for every ship that is departing and not have to build a totally different airframe. In fact, that is precisely what they said they were going to do. So yes, your depot idea is your own.

Offline Peter.Colin

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Re: SpaceX tanker variant
« Reply #72 on: 10/19/2017 09:31 pm »
“It is not a tanker. The definition of tanker is something that transports fuel, your concept would not be capable of transporting fuel except in space.”

Hmmm...

A depot is something which holds fuel but doesn’t transport it.

A space-tanker with broken engines, becomes a depot.
« Last Edit: 10/19/2017 09:35 pm by Peter.Colin »

Offline TomH

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Re: SpaceX tanker variant
« Reply #73 on: 10/19/2017 09:34 pm »
So 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.
« Last Edit: 10/19/2017 09:36 pm by TomH »

Offline Peter.Colin

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Re: SpaceX tanker variant
« Reply #74 on: 10/19/2017 10:00 pm »
So 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 ;)

Offline TomH

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Re: SpaceX tanker variant
« Reply #75 on: 10/19/2017 10:19 pm »
So 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 ;)

There you go again, pretending to have said something different. Here's what you said earlier:

A empty/full mass optimized tanker that stores 20 tankers in LEO is just enough to fill up the 4 ships planned for 2024.

What you described is a depot. That is a huge depository in orbit where propellant is deposited. On Earth, such a thing would sit in a fixed location, but obviously, to remain in space, it must move, therefore its position is adjustable. Nevertheless, the thing remains in space. Tankers are smaller and transport prop from a planet's surface either to a spacecraft or to a depot.

A regular spaceship. Please define the exact dimensions of a regular spaceship. NSF is not a bookie. It is a place for serious discussion. You previously offered to bet anyone $50 that the T/W of Raptor would be announced at 350/1 or greater. When DJPledger countered your reasoning, you claimed that he had taken you up on your bet, when in fact he had done no such thing. Musk has since stated that Raptor thrust will not be as high as anticipated. Did you send DJPledger the $50 which by your logic you owe him? Your petulant childish behavior needs to stop. Quit making up outlandish ideas. Quit daring people to make bets with you. It is immature and inappropriate. Act like an adult.

Offline Explorer

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Re: SpaceX tanker variant
« Reply #76 on: 10/19/2017 10:28 pm »
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.

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.
Send orbiters to Uranus and Neptune, dammit.

Offline Peter.Colin

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Re: SpaceX tanker variant
« Reply #77 on: 10/20/2017 12:53 am »
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.

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.

Yes it’s 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.

@TomH: This $50 bet with John hasn’t been settled yet, the T/W ratio of a Raptor engine is not known yet.
« Last Edit: 10/20/2017 02:51 am by Peter.Colin »

Offline nacnud

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Re: SpaceX tanker variant
« Reply #78 on: 10/20/2017 01:30 am »
That is not what spacex has proposed. In their plan.

Fuel is cheap
Flights of vehicles are cheap.
Marginal costs of flights are cheap.
Reuse is cheap.
Vehicles are expensive.

The savings you propose are on are all on the cheap side of the equation, saving flights and fuel. You solution is to spend on the expensive side with dedicated vehicles.

It doesn't make sense in the short to medium term.

Offline CuddlyRocket

Re: SpaceX tanker variant
« Reply #79 on: 10/20/2017 04:26 am »
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.

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.

You'd have to design etc three types of tankers - LOX only, methane only and LOX+methane. If one flight is all that's needed you would use the last of these. But normally, you'd need multiple flights so some mixture of the three types might be most efficient.

Initially, when there's relatively few tanker flights, it makes sense to develop the LOX+methane variant first. Only when there's a relatively large number of tanker flights would you consider developing the other two variants. Naturally, this would cost money and I assume the appropriate financial trades would be considered to see if it's economically worthwhile. But first you'd want to know that theoretically the existence of such tanker variants would enable more efficient refuelling (if it doesn't, then there's no point!).

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Re: SpaceX tanker variant
« Reply #80 on: 10/20/2017 12:44 pm »
(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.

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.

Emphasis on one of my original questions and combining previous comments...

Suppose you have a dedicated HEO tanker with same engine config, and the exact same aeroshell as the normal BFS, but is nothing but tanks.  You can launch and land as normal, with the caveat that the tanks are only x% full at launch (so it is the same launch mass as a regular BFS).

Once in LEO that is filled 100% by other tankers and that is transported to HEO.  What problem is there with this (other than that it wouldn't look weird)?

Offline ZachF

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Re: SpaceX tanker variant
« Reply #81 on: 10/20/2017 01:55 pm »
I 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.

Perhaps. If it's not spherical I would not be surprised for it to be very short and squat, like some of the other variants I drew. The closer it is to a sphere, the better the mass ratio will be.
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Offline ZachF

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Re: SpaceX tanker variant
« Reply #82 on: 10/20/2017 01:59 pm »
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.

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.


I think they will build a new tanker variant to take advantage of the greater thrust of future Raptor upgrades. The dedicated tanker might not fly until like 3-6 years after BFS cargo/tanker1.0 and Spaceship.

If Raptor is upgraded to 300 bar, thrust would increase by a similar amount, and the stack could grow to ~5,200 tonnes from 4,400 tonnes, allowing a much larger tanker. Second stage could grow from ~1,335 tonnes to ~2,150 tonnes.

Using an empty cargo BFS to refill would take about 6 flights to fully refill a BFS. The upgraded 2,100 tonne US could probably refill a BFS in 4 flights. It would also be able to refill in LEO, boost up to high EEO, and still have enough left over to full top off a BFS.
« Last Edit: 10/20/2017 02:03 pm by ZachF »
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Offline DreamyPickle

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Re: SpaceX tanker variant
« Reply #83 on: 10/20/2017 05:50 pm »
If you define a depot as stationary long-term storage (many months) then SpaceX hasn't shown much interest. A very interesting snippet from the AMA is this:

Quote
Quote
Will 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.

So the plan seems to rely on just keeping the header tanks cool for the trip to Mars. It's a surprisingly minimalist approach and obviously wouldn't work for a depot.

SpaceX has also shown no plans for on-orbit reusability. What they're doing instead is using a single stage from Earth orbit to Mars and back to Earth surface, to the desperation of some Mars Direct fans. I think this is deliberate because it allows inspecting used hardware on the ground. They're even planning to "recover but not reuse" the second stage!

Offline wes_wilson

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Re: SpaceX tanker variant
« Reply #84 on: 10/21/2017 12:36 am »
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.

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.
@SpaceX "When can I buy my ticket to Mars?"

Offline yokem55

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Re: SpaceX tanker variant
« Reply #85 on: 10/21/2017 01:24 am »
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.

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.
See this thread here:
http://forum.nasaspaceflight.com/index.php?topic=39966.0

TL,DR - Flexible, stretchable material that can handle cyrogenic fluids under flight pressures and loads are hard.

Online MP99

Re: SpaceX tanker variant
« Reply #86 on: 11/05/2017 06:37 pm »
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 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

A heavy tanker (tanks expanded to fill the BFS OML?) could perhaps launch with a predominantly lox cargo. A second heavy tanker(s) could then launch with another heavy lox load to top it up.

Standard tankers could then launch with less lox and an expanded methane load, to provide a large 3.8:1 prop load.

Once a tanker with expanded tanks is topped up, it would be ideal to send to HEO to refuel a BFS.

Cheers, Martin

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Offline aero

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Re: SpaceX tanker variant
« Reply #87 on: 11/05/2017 09:57 pm »
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.
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Offline wannamoonbase

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Re: SpaceX tanker variant
« Reply #88 on: 11/05/2017 10:03 pm »
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.

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.
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Offline docmordrid

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Re: SpaceX tanker variant
« Reply #89 on: 11/05/2017 10:24 pm »
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.

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/

Quote
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.
« Last Edit: 11/05/2017 10:26 pm by docmordrid »
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Offline aero

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Re: SpaceX tanker variant
« Reply #90 on: 11/05/2017 11:38 pm »
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.

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/

Quote
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.

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?"
« Last Edit: 11/05/2017 11:39 pm by aero »
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Offline guckyfan

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Re: SpaceX tanker variant
« Reply #91 on: 11/05/2017 11:46 pm »
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?"

I think it was at the AMA. Elon was asked how they would subcool the propellant for landing. He mentioned they would initially vent some to vacuum. They may later use active cooling.

Offline Robotbeat

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Re: SpaceX tanker variant
« Reply #92 on: 11/06/2017 12:10 am »
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.

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/

Quote
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.

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?"
If you vent to vacuum, you'll be cooling it all the way to its triple point, which is usually pretty close to the (1atm) freezing point. So it would be far subcooled (compared to standard pressure boiling point).
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Offline hkultala

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Re: SpaceX tanker variant
« Reply #93 on: 11/07/2017 10:41 am »
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.

Compared to other cargo, propellant is not volume limited. It's mass-limited. There is very little need for "more volume". How hard is this to understand?

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Quote

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.

Yes it’s all about reducing costs.

... and you would be only INCREASING costs.

Quote
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.

No, it would not reduce the total amount of fuel burned to get to HE orbit. It would INCREASE it because the craft mass going to HEO would increase by about 75 tonnes. Which practically means at least one MORE launch from earth.


Lets say our HE orbit is LEO + 3km/s.

Normal 9m tanker (lets say empty weight about 80 tonnes, as the tanker can be simpler and lighter than the cargo version, can lift 155 tonnes of propellant from earth to LEO) can carry about 430 tonnes of fuel from LEO to this HEO.

So, actually only TWO of these tankers need to go to HEO.
This is less structural mass than your mega-tanker.
Calculations below:


Mission/tanking plan, with normal-sized tankers, when we want to have a mission ship(85 tonnes ship with 150 tons of cargo), full of fuel on HEO that is LEO + 3 km/s. isp of 370 second used in the calculations.


Tanker 1 goes from earth to LEO. Arrives LEO with 155 tonnes of fuel.
Tanker  flights 2-7 go from earth to LEO and refuel tanker 1
Tanker 1 now has 1085 tonnes of fuel. For 3km/s delta-v it burns 655 tonnes fuel to get to HEO. It has 430 tonnes of fuel left on HEO.


Tanker 8 goes from earth to LEO. Arrives LEO with 155 tonnes of fuel.
Tanker  flights 9-14 go from earth to LEO and refuel tanker 8
Tanker 8 now has 1085 tonnes of fuel. For 3km/s delta-v it burns 655 tonnes fuel to get to HEO. It has 430 tonnes of fuel left on HEO.

Mission ship goes from earth to LEO. Arrives LEO with 0 tonnes of fuel.
Tanker flights 15-21 go from earth to LEO and refuel mission ship.
Misson ship now has 930 tonnes of fuel. For 3km delta-v it burns 655 tonnes to get to HEO. It has 275 tonnes of fuel left.

Tankers 1 and 8 transfer their fuel to mission ship. It now has full tanks.

Only TWO tankers needed to get to the HEO. Total tanker structural mass to HEO 160 tonnes.

Thes two together weight much less that your hyphothetical and expensive super-tanker that would be very problematic for launching etc and would require developing a totally new craft.




Quote
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.

No, it could not. There is more to payload than mass.

BRF launch facilities could not handle your mega-tanker.

And it would INCREASE the amount of mass going to HE-orbit, not decrease it.

So it would just increase costs, and require much more expensive infrastructure, for negative benefit.

Quote
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.

Creating a single and simple prototype tank very expensively is totally different thing than mass-producing many rockets relatively cheaply.


And you are solving the totally wrong problem, and by that making the real problems much worse.


Empty tank space is about the ONLY thing of abundance in space. You get it free with every launch, when you spent ~85% of your tanks space to reach orbit.

And you propose wasting rockets sending more of it into space, for negative benefit.



And this thread was meant to be for serious speculation about SpaceX BFR tanker, not about third party "fantasy ship designs".
« Last Edit: 11/07/2017 12:32 pm by hkultala »

Offline Ludus

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Re: SpaceX tanker variant
« Reply #94 on: 11/27/2017 06:26 am »
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.

« Last Edit: 11/27/2017 07:04 am by Ludus »

Offline aero

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Re: SpaceX tanker variant
« Reply #95 on: 11/27/2017 03:54 pm »
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.
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Offline speedevil

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Re: SpaceX tanker variant
« Reply #96 on: 11/27/2017 05:47 pm »
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.

If fuel gets cheap enough in orbit, sure.
The three SL raptors are quite enough to lift a tanker empty to orbit, perhaps even fewer than three.
If the mission is going from a high earth eliptical orbit and delivering fuel into an energetic mars insertion before barely getting back to HEO, you don't really need more than one engine for anything but redundancy.

If this means working on the tanker in orbit to remove engines, or assembling tankers in orbit, or simply that there is no point in doing this as engines and carbon shells are 'almost free' - we can't reasonably extrapolate.

In many cases, you will at least want the tanker to have some heat protection, if for nothing else than aerobraking, even if it is never intended to land.

Offline hkultala

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Re: SpaceX tanker variant
« Reply #97 on: 11/27/2017 06:05 pm »
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?

No. The engines weight only few tonnes, but removing them would worsen gravity losses considerably.

Quote
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.

Effect would be the same, and there are no such engines.

Offline Ludus

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Re: SpaceX tanker variant
« Reply #98 on: 11/28/2017 01:47 am »
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.

It obviously needs the vacuum raptors. I don’t know if it needs all the others if it’s only launched once through earth’s atmosphere and otherwise stays in space and never re enters or lands. It probably doesn’t need legs, a heat shield or other aero features. As a deep space Booster it needs whatever is decided as hardware to enable docking the nose to the regular tail to tail propellant docking points. In general, I don’t think it gains very much performance by subtracting gear but there’s also no reason to have legs if it’s never going to land. Add features that help it keep propellant longer.

This seems like a pretty practical variant that adds a lot of new capabilities without too much reengineering.
« Last Edit: 11/28/2017 04:27 am by Ludus »

Offline Lar

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Re: SpaceX tanker variant
« Reply #99 on: 11/29/2017 02:49 am »

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?

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?
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Offline biosehnsucht

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Re: SpaceX tanker variant
« Reply #100 on: 11/29/2017 07:37 am »
If it wasn't coming back you could probably not install the sea level raptors used for landing in the first place. If that drops needed thrust to make orbit with full tanks somehow, just decrease fuel level until it makes it to orbit and top it off with regular tankers.

Probably not worth the effort until there are a LOT of flights going on.

Would a SL-less BFS have enough power to land and take off from the Moon at least? You could do limited servicing there.

Offline philw1776

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Re: SpaceX tanker variant
« Reply #101 on: 11/29/2017 12:59 pm »
Yes.  Plenty of thrust for lunar landing & takeoff.  Throttled down.
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Offline guckyfan

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Re: SpaceX tanker variant
« Reply #102 on: 11/29/2017 03:04 pm »
With SL engines close to the center landing will be easier, especially dealing with engine out.

Offline Ludus

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Re: SpaceX tanker variant
« Reply #103 on: 11/29/2017 03:25 pm »

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?

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.

Offline aero

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Re: SpaceX tanker variant
« Reply #104 on: 11/29/2017 04:35 pm »

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?

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.

My question had to do with removing some engines after reaching orbit. I doubt the ship could safely reach orbit with some engines removed but that is a different question. As for landing legs, those could be removed before the launch of course. There may be other massive items that could be removed before launch or after reaching orbit. What they are and how massive they are is still a different question. The point of my questions is to what extent could the mission capabilities as you gave them be increased by removing unneeded mass either before or after the modified BFS reaches orbit. Perhaps the "thrust structure" could be replaced with a less massive structure if engines are removed?
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