Falcon Heavy propellant crossfeed of LOX only

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I get an increase in payload of ~5% using gravity feed tank to tank, 2 extra bulkheads in the boosters (mass not accounted for), LOX tank at bottom of main core (which may need some extra plumbing – mass not accounted for). Not really worth it?  It seems throttling back the core engines to 70% reduces crossfeed advantage, but it seems like throttling back incurs some loss I haven't accounted for?

It looks like this - stacks represent core and boosters tank placement.  red is RP-1, blue is LOX, green is crossfeed LOX.





Main core mass

LOX: 243t (always metric tonne)

RP-1: 161t

Total mass: 404t


Booster mass

LOX crossfeed: 85t

LOX: 242t

RP-1: 95t

Total mass: 422t


Stage1 xfeed

Propellant mass: 910t

Total mass: 1,468t

Isp: 301s

Delta-v: 2,855m/s


Stage2 xfeed

Propellant mass: 338t

Total mass: 530t

Isp: 311s

Delta-v: 3,097m/s


Stage 3 xfeed

propellant mass: 108t

total mass: 178t

Isp: 342s

Delta-v: 3,119m/s


Crossfeed payload mass: 66.1t




Propellant mass: 1,123t

Total mass: 1,465t

isp: 301s

Delta-v: 4,296m/s



Propellant mass: 125t

Total mass: 313t

Isp: 311s

Delta-v: 1,550m/s


Stage 3

Propellant mass: 108t

Total mass: 174t

Isp: 342s

Delta-v: 3,224m/s


Total delta-V for baseline: 9,070m/s

FH as I modelled it payload: 62.6t

Payload improvement: 5.61%



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To keep the RP-1 tank at the bottom, adding a baffle to the LOX tank and holding a portion in reserve for post-separation allows LOX to flow into the lower section from the boosters and post separation from the top section of the LOX tank.  The extra baffles would improve the rocket structure as well.  The same baffle trick can be used for RP-1 if pumping is an issue, like this:


The cores (middle) and boosters (sides) at 3 different times - launch at top, midway to separation at middle, at separation at bottom.  Green is LOX propellant crossfeed, yellow is RP-1 crossfeed, blue is LOX and red is RP-1.  There are baffles between green and blue and between red and yellow which allow gravity crossfeed with some extra plumbing and valves.


Then the 15% extra delta-v expected from crossfeed can cover the loss for reuse. 


The BFR plan calls for transfer of cryogenic propellant by micro acceleration, so LOX transfer under large acceleration calls for similar plumbing work/knowhow.

Starting with cryogenic LOX crossfeed only might minimize risk of failure, compared to transferring RP-1 or liquid methane (for BFR).


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Instead of using pipes in tanks, I inverted the extra baffles and gave them a slight slope.  A sealed box would give access to the valves and ports.  Two ports below the inverted baffle on either side to receive LOX (blue), via a check valve, blocked when on the boosters.  A single valve for the core across the baffle so it acts as a single tank once separated, this port is also permanently closed or blocked on the boosters.  A xfeed valve on the boosters just above the inverted baffle and on the low side connects to the ports below the baffle on the core.  Similar idea for RP-1 (red).

Here is an animated gif of the propellant cross-feed (Xfeed) time is compressed 10x, proportions are to a 40m rocket.  Burn time was set to 150s for the middle once separated and results in 100s burn time with all 27 engines on (9 per fuselage), burning through 300s worth of fuel.  The connecting hoses are missing, as is the rest of the rocket, but I think it's pretty clear what's happening?

I have no idea if this idea is innovative, common or stupid.  I haven't found anything similar on the net yet.



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A Falcon Super Heavy version of full propellant cross-feed normally arranged in a cross, not a row as I depicted for illustration.  Propellant transfers into a tank instead of trying to match pressure in direct feed to engines.  The tank could be carbon-fibre and as small as allows buffering and gas elimination etc...  Making it span the width of the rocket allows direct access from the sides.  The smaller tank means the whole pressure of the rocket head must be borne, but is less than 100 psi, so manageable.  What's neat about doing it this way is that the outer rockets don't need much modification except a port with a valve, no extra dome.

So, what I called a baffle is actually called a dome (an oblate spheroid shape?)  Another animation:


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