Shifting subtopic gears here a minute away from FOD in the prop tanks...

Meanwhile, I've found a solution for the bounded problem (150 t LEO/20 t GTO for reuse, 250 t LEO for expendable version). The solution requires that second stage dry mass be roughly 45 tonnes, much less than the 85 tonnes mentioned in the presentation. With PMF ~ 0.96 for both stages, the numbers work out if something like 6-7% propellant fraction is assumed to be required for RTLS, landing, etc. I have S1 at 3278 t/131 t GLOW/Dry and S2 at 1122 t/45 t.

- Ed Kyle

Using Ed's cargo solution numbers...

Where do we end up on the Tanker version?

How many tonnes of off loadable prop to LEO... the 220 tonnes (1/5 full) hinted in the 2017 presentation?...

And are the tanks likely a stretched 1250 tonnes prop volume as some have opinion'd?...Or something else?

With no need to support a payload in front of them... could the tanks be a lighter, less beefy version?

It's also thought the nose section is as light as possible with no openings beyond maybe a maintenance access hatch...

I have been looking at Ed's solution and pondering it's various nuances...

Trying to Wrap my head around what SpX is doing here...

One key thing that blew me away...

(on edit... I'm thinking the 1.25:1 take off thrust to weight working on Gravity Losses is key in this working)

250 tonnes payload (expendable) only has 850 tonnes of prop sloshing around in BFS's 1100 tonnes capacity tanks...

And between the BFR and BFS both running to empty...

It gains 9200+m/s (LEO) velocity...

...295 tonnes...

250 tonnes payload (plus the 45 tonnes empty BFS)...

...going around and around in orbit...

And the prop tanks were not full...

Profound... and here is why I think so...

As the Raptor matures and chamber pressure inches toward the 300 bar goal.

They can instantly take advantage to add more delta/V to the stack...

As take off thrust from those 31 Raptors increases... add more prop to the BFS...

Someday... 250 tonnes RTLS may be possible... (wild guess not confirmed)

The other concept that I think helps understand it, is this example...

Lets say the Payload is a 250 tonnes water tank...

You drain 15 tonnes of water out prelaunch and put 15 tons more prop in the BFS...

You get to LEO with the 235 tonnes of payload... 9200+m/s and SECO it...

But now you have 15 tonnes of prop left in the BFS tanks...

You can eject the payload and recover the BFS (just over 1000m/s delta/v avalible + aerobraking)

So ironically... You have to expend stage 1... but can save stage 2...LMAO...

Now then... Continuing down the payload to prop weight redistribution path...

In between the 235 and 150 tonnes payload range...

Still adding more and more Prop to BFS to keep GLOW at 4400 tons spec...

You end up with enough delta/v margin at SECO, to add a MECO and start doing ASDS style BFR recoveries...

In other words... LOAN spare delta/v from the BFS back to the BFR...

At first the landings are "HOT" and very ballistic...

But with less and less payload... they get cooler and easier on the hardware with bigger delta/V loans...

At 150 tonnes payload...

You have enough delta/v to add a flip/boost back burn and start landing back on the launchpad..

And ironically... the BFS still only has 950 tonnes in the tanks at launch and still could take another 150 if the Raptor 300 bar upgrade pans out...

From 150 tonnes down to 0 tonnes payload..

If needed be... Adding more prop to BFS = more delta/v to do plane or orbit changes after reaching LEO...

You are gaining delta/v, because GLOW is now falling below 4400 tonnes...

Now to the tanker...

I'm thinking... but I'm not sure on this part...

IF the payload is reduced to zero...

And then you go a bit further on a tanker BFS by cutting weight to a bare minimum on the airframe...

I mean after all, in the big picture sense...

There is no need to have a load path thru the tank to hold a payload...

A flying gas tank could be well under 45 tonnes empty weight...

Every tonnes saved in EW helps in what is needed on the landing burn also...

It's implied by the 2017 slides and presentation...

...that something like 217 tonnes of prop can be hauled up on each load...

217x5 loads would be 1085 tonnes of prop...

...plus the 15 tonnes assumed on board that could have got it home...

Equals full tanks... 1100 tons...

But it's still not clear to me how they got from 150 tons of spare prop to a 217 or so number...

There is a 67 tonnes gap there that needs explained somehow...

Anyway... My thoughts on BFS/BFR and how as a system it may work...

(On edit... now back to the ongoing FOD discussion)