Author Topic: What will the BFS tanker look like?  (Read 15325 times)

Offline Slarty1080

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What will the BFS tanker look like?
« on: 03/24/2018 11:42 AM »
BFS tanker variant – I would be interested in people’s views of what this might look like. We know that it will be optimised for maximum fuel transfer, will be reusable, be based on the BFS design and according to Elon will look “kind of weird”. So what options are there?

From a weight saving angle it should resemble an elongated sphere with a diameter of 9 meters. But it will also need to re-enter the atmosphere and land so will need the BFS engine layout and heat shield. But how much of the rest of the BFS structure will it use? Will it just be a cut down BFS shell? Or a standard BFS shell with empty space in it? What might count as kind of weird?
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Offline speedevil

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Re: What will the BFS tanker look like?
« Reply #1 on: 03/24/2018 12:35 PM »
BFS tanker variant – I would be interested in people’s views of what this might look like.

I would not be surprised if it is not considerably delayed.
BFS in the minimum Mars capable form - no windows, passenger door, ... is quite capable of lifting fuel, and at best you're going to get another 20-30 tons fuel from a tanker, due to lowering structural weight.

This means a tanker flies 10-20% less, for bulk fuel deliveries - while still being a large slice as expensive to construct as a 'normal' BFS.

A spare BFS may be operationally more valuable at this point.

Addressing the 'it's gonna look wierd' comment.
In principle, thermal protection may be moderately easier in some ways with a 15m tanker, that launches mostly dry, and has great ability to act as a depot.

This is inconsistent with the IAC2017 presentation showing similar OML though.

Offline Archibald

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Re: What will the BFS tanker look like?
« Reply #2 on: 03/24/2018 03:18 PM »
Interestingly enough, while the KC-135 was Boeing main aerial tanker, there were also a lot of converted civilian 707 tankers, less efficient but cheaper to operate. Same dilema here !

On top of my head, bar USAF of course, in fact few other air forces ever bought KC-135s: perhaps France and Turkey. Everybody else, from Israel to Canada to Australia, got converted 707s.
« Last Edit: 03/25/2018 09:34 AM by Archibald »
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Offline Elmar Moelzer

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Re: What will the BFS tanker look like?
« Reply #3 on: 03/25/2018 03:01 PM »
This is inconsistent with the IAC2017 presentation showing similar OML though.
I saw that quote from Musk and I believe that the tanker shown in the 2017 presentation is essentially the cargo version of BFS without cargo bay doors. Musk however indicated on Reddit that there will be a dedicated tanker version that will look "kind of weird". So what ways are there to cut down the outer mold line without losing aerodynamic properties for launch and re- entry? Maybe they will also move (or remove) the wings?

Offline speedevil

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Re: What will the BFS tanker look like?
« Reply #4 on: 03/25/2018 03:44 PM »
This is inconsistent with the IAC2017 presentation showing similar OML though.
I saw that quote from Musk and I believe that the tanker shown in the 2017 presentation is essentially the cargo version of BFS without cargo bay doors. Musk however indicated on Reddit that there will be a dedicated tanker version that will look "kind of weird". So what ways are there to cut down the outer mold line without losing aerodynamic properties for launch and re- entry? Maybe they will also move (or remove) the wings?

I guess there are several kinds of weird.
It looks like a chicken. (it is clear to the general public that is confused if Voyager is a real science show it looks weird).
It looks very different to IAC2017 BFS.
It looks very odd to people who understand rockets at first glance.

The last one might almost be 'lol - we did nothing other than remove the windows', from the perspective of the third.
Those who have 'it must be as light as possible' baked in as a mantra, when easy reusability trumps it so far as to make lightness irrelevant mostly.
But that's a bit of a cop-out.

Significantly oversized/diameter tanks, compared to BFS.
I think this is possibly the most plausible.
Once experience is gained by operating BFR, the stresses on the interface during normal launch will be well known, and allow for reasonable safe extrapolation to rather more draggy and heavy payloads.
This is not going to look very weird to someone in the general public.

The only sort-of-sane 'really weird' suggestion I can think of that makes some degree of sense would be a very large tank, capable of storing 4000 tons or so of propellant, which normally lifts off with 2000ish tons, with the normal BFS raptors supplemented by four raptors on the outside diameter, several degrees out so as to parallel stage with S1 at liftoff to get the launch acceleration reasonable.

This would look weird and sort-of-work, and be a really, really awesome moon lander basis, as the jets from the top would not impinge on the legs.

However, as above, I'm not sure we're seeing tankers soon - unless they are cutdown BFS.

Offline Elmar Moelzer

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Re: What will the BFS tanker look like?
« Reply #5 on: 03/25/2018 08:52 PM »
The exact quote by Musk is:
Quote
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 Archibald

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Re: What will the BFS tanker look like?
« Reply #6 on: 03/26/2018 05:57 AM »
Just a simple question. We know that at least five tankers will be needed to fill the tanks of a single BFS. So every single tanker will have to dock back-to-back with the BFS, rince, repeat.
I wonder instead about a true, permanent  propellant depot with 2200 mt or 3300 mt of methalox, enough to refuel 2 or 3 BFS with a single docking. Kind of space gas station, really.
I suppose Musk thought about it but don't want permanent orbital infrastructures with their maintenance cost...
« Last Edit: 03/26/2018 05:59 AM by Archibald »
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Offline niwax

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Re: What will the BFS tanker look like?
« Reply #7 on: 03/26/2018 08:08 AM »
Just a simple question. We know that at least five tankers will be needed to fill the tanks of a single BFS. So every single tanker will have to dock back-to-back with the BFS, rince, repeat.
I wonder instead about a true, permanent  propellant depot with 2200 mt or 3300 mt of methalox, enough to refuel 2 or 3 BFS with a single docking. Kind of space gas station, really.
I suppose Musk thought about it but don't want permanent orbital infrastructures with their maintenance cost...

I'm not sure what the advantage of that would be. The only difference is slightly looser scheduling regarding resupply launches as compared to direct tanking for significantly higher cost. If BFR is really meant to fly every few days, I doubt they'd gain much by being able to store fuel beforehand.
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Offline MikeAtkinson

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Re: What will the BFS tanker look like?
« Reply #8 on: 03/26/2018 11:59 AM »
Just a simple question. We know that at least five tankers will be needed to fill the tanks of a single BFS. So every single tanker will have to dock back-to-back with the BFS, rince, repeat.
I wonder instead about a true, permanent  propellant depot with 2200 mt or 3300 mt of methalox, enough to refuel 2 or 3 BFS with a single docking. Kind of space gas station, really.
I suppose Musk thought about it but don't want permanent orbital infrastructures with their maintenance cost...

I'm not sure what the advantage of that would be. The only difference is slightly looser scheduling regarding resupply launches as compared to direct tanking for significantly higher cost. If BFR is really meant to fly every few days, I doubt they'd gain much by being able to store fuel beforehand.

For Mars a "fleet" is supposed to leave Earth orbit at the same time (where at the same time might be weeks for cargo, but instantaneous for crew). Being able to decouple launching the fleet in a few days (and then refueling them almost immediately) from launching the tankers over 6 months is a major advantage.

A small fleet of 2 crew and 4 cargo ships would require 30+ tanker flights. At a launch cadence of 1 per day some of the ships would have to hang around in LEO for a month. A disruption in tanker flights, e.g. by a hurricane, anywhere in that month could lead to the optimum TMI window being missed. Boil-off from the fueled ships in LEO would also be an issue.

This issue only gets worse as the size of the fleet grows.

Some form of propellant depot(s) has major advantages, whether those advantages are enough to cause SpaceX to use a propellant depot for early synods is debatable. I tend to think not, but only SpaceX have all the information.

Offline speedevil

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Re: What will the BFS tanker look like?
« Reply #9 on: 03/26/2018 03:19 PM »
A small fleet of 2 crew and 4 cargo ships would require 30+ tanker flights. At a launch cadence of 1 per day some of the ships would have to hang around in LEO for a month. A disruption in tanker flights, e.g. by a hurricane, anywhere in that month could lead to the optimum TMI window being missed. Boil-off from the fueled ships in LEO would also be an issue.

It is interesting to note that you can have BFR take off and climb at 100MPH, for most of the ascent through harsh weather, and still get a notable payload to orbit. This is just fine for a fuel launch, if you happen to be constrained somewhat by weather.

You also need several tankers, for redundancy reasons.

Remember, this is 2024, not 2022.
Starlink is launched, and SpaceX at the very least have the additional revenue from operations and launch of a full constellation.
In 2024, it's 5-6 years since BFS first flew.
They will have a very reasonable idea of operations.

If you don't care much about weight, suitable tanks are off the shelf.
https://www.alibaba.com/product-detail/ASME-or-GB-Standard-Liquid-Oxygen_60681993254.html
Contains 50 tons of propellant, boils off at 0.1%/day, weighs 13 tons.
You can in three BFS launches for about $3M buy the tanks for a propellant depot to retank one BFS.

(this is just an example - you may well want to use a larger tank)

« Last Edit: 03/26/2018 03:34 PM by speedevil »

Offline JamesH65

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Re: What will the BFS tanker look like?
« Reply #10 on: 03/26/2018 03:30 PM »
In answer to the original question.

How about a large spherical 2nd stage sitting on a BFB? Would definitely look odd, but best space efficiency. Slow launch, ramp up speed as you get out of the atmosphere. Fuel is cheap so you can afford to do that as long as the BFB can lift it. Can be tank in tank if you want duel fuel, or launch two, one for each fuel.

Online RonM

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Re: What will the BFS tanker look like?
« Reply #11 on: 03/26/2018 04:16 PM »
A small fleet of 2 crew and 4 cargo ships would require 30+ tanker flights. At a launch cadence of 1 per day some of the ships would have to hang around in LEO for a month. A disruption in tanker flights, e.g. by a hurricane, anywhere in that month could lead to the optimum TMI window being missed. Boil-off from the fueled ships in LEO would also be an issue.

It is interesting to note that you can have BFR take off and climb at 100MPH, for most of the ascent through harsh weather, and still get a notable payload to orbit. This is just fine for a fuel launch, if you happen to be constrained somewhat by weather.

No, lightning strikes would still be a problem.

Offline speedevil

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Re: What will the BFS tanker look like?
« Reply #12 on: 03/26/2018 04:50 PM »
No, lightning strikes would still be a problem.

For some weather, yes, this will not help at all.

Offline Ludus

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Re: What will the BFS tanker look like?
« Reply #13 on: 03/27/2018 02:05 AM »
The Tanker and Cargo ship are the same ship. As a Tanker it just flies with an empty cargo bay. This isn’t much different from AF tankers for the same reasons. If the OML was filled with topped off propellant tanks it couldn’t take off. This isn’t a bug it’s a feature. It lets SpaceX concentrate on a single simple design that performs all required missions.

Obviously SX did some analysis for a tweaked dedicated Tanker that looks weird. I’m doubtful there’s enough performance advantage to justify building it for a long time.

The Cargo/Tanker really could perform the human passengers to Mars mission too and might be more efficient if less cool. Just put an appropriately sized hab module in the cargo bay. On Mars, you can leave the entire module where it’s more useful and send the ship back empty instead of hauling back all that extra mass as with a dedicated passenger ship. No windows is about the only drawback.

« Last Edit: 03/27/2018 02:09 AM by Ludus »

Offline Robotbeat

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Re: What will the BFS tanker look like?
« Reply #14 on: 03/27/2018 02:10 AM »
Are you certain? The 2016 ITS showed differences between the tanker and the ship. The tanker had significantly lower dry mass and significantly greater propellant mass than the ship.

At first, sure, the tanker and the ship might be the same thing.
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Offline Ludus

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Re: What will the BFS tanker look like?
« Reply #15 on: 03/27/2018 02:24 AM »
Are you certain? The 2016 ITS showed differences between the tanker and the ship. The tanker had significantly lower dry mass and significantly greater propellant mass than the ship.

At first, sure, the tanker and the ship might be the same thing.

I have no special knowledge. The 2016 presentation emphasized the Ship as a Buck Rodgers passenger liner. 2017 got around to more emphasis on the Cargo variant that just leaves that whole forward Ship volume empty. I’m inclined to think they got much more practical. It’s been a long time since I did the numbers on it but I became convinced at the time that the volume of full propellant tanks couldn’t be increased very much and that suggested there wasn’t much point in building Cargo and Tanker with different specs.

There was a discussion of building a Deep Space Booster variant that could dock nose to tail. THAT version, that SX has said nothing about, might launch with Tanks that filled the whole OML but only partially filled. Then it could take more Tanker loads and be a better Booster.

Offline tea monster

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Re: What will the BFS tanker look like?
« Reply #16 on: 03/27/2018 05:25 PM »
Assuming that it will be "Kind of weird" in a way that disposes of winglets and has a more conical nose, it may look like this:



These were from another thread where it was posited that the tanker wouldn't have winglets like the main ship. Personally, I think it will.

Online cppetrie

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Re: What will the BFS tanker look like?
« Reply #17 on: 03/27/2018 06:10 PM »
The purpose of the winglets is adapt the aero profile across a variety of payload amounts and atmospheric conditions. Given that the tanker will always land under the same conditions (essentially empty and on earth) the winglets may not be needed.  Or they may be fixed or different shaped.

Offline wannamoonbase

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Re: What will the BFS tanker look like?
« Reply #18 on: 03/27/2018 06:31 PM »
The purpose of the winglets is adapt the aero profile across a variety of payload amounts and atmospheric conditions. Given that the tanker will always land under the same conditions (essentially empty and on earth) the winglets may not be needed.  Or they may be fixed or different shaped.

I assume there will be 1 mold line, one set of tooling and 1 way to do things.  Right up to the point they decide if it's Cargo, Crew or Tanker.

Tanker should be the easiest of all of them.

Not expecting to see anything soon.  Lots of development to be done. 

I'll entertain myself with the epic accomplishments of the Falcon family until then.
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Online cppetrie

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Re: What will the BFS tanker look like?
« Reply #19 on: 03/27/2018 07:47 PM »
The purpose of the winglets is adapt the aero profile across a variety of payload amounts and atmospheric conditions. Given that the tanker will always land under the same conditions (essentially empty and on earth) the winglets may not be needed.  Or they may be fixed or different shaped.

I assume there will be 1 mold line, one set of tooling and 1 way to do things.  Right up to the point they decide if it's Cargo, Crew or Tanker.

Tanker should be the easiest of all of them.

Not expecting to see anything soon.  Lots of development to be done. 

I'll entertain myself with the epic accomplishments of the Falcon family until then.
We’ve already been told by Elon that the dedicated tanker will look “weird”. To me that means the dedicated tanker will probably not share the same OML as the ship and cargo versions. But the dedicated tanker will come (much) later. The early tankers will just be cargo versions with no cargo and so of course will have the same OML.

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