Falcon Heavy Cross-Feed

[Robotbeat]

Would it be possible for each booster to feed 4 engines each and the core only one engine?

Anything is "possible."

So most likely not practical?

That depends on detailed trade studies. The only such analysis we have is that they are leaning toward the core doing 3 engines (I believe).

[cuddihy]

Can anyone explain why FH s0 is antipated to be 12/3/12 rather than 13/1/13 or 13.5/0/13.5?

Is it too much head loss in the cross feed piping or concerns about flow instabilities if the side boosters are cross connected?

[Robotbeat]

Can anyone explain why FH s0 is antipated to be 12/3/12 rather than 13/1/13 or 13.5/0/13.5?

Is it too much head loss in the cross feed piping or concerns about flow instabilities if the side boosters are cross connected?

I think the plan is that after the boosters stage off, only the engines fed by the core will stay lit (that way they don't have to switch propellant supplies). If you have too few engines burning at that point you have very high gravity losses, which is bad (obviously). Also, engine-out capability may still be desired.

[cuddihy]

Can anyone explain why FH s0 is antipated to be 12/3/12 rather than 13/1/13 or 13.5/0/13.5?

The Dec 2012 date is for hardware delivered to VAFB. Most likely for hardware comparability tests. When the FH gets launched depends on the readiness of the pad at SLC-4E and how the Cassiope flight goes.

Oops. Was talking about engine feed schemes, not dates. 12 right booster/3 core /12 left booster. Etc

[RDoc]

I think the plan is that after the boosters stage off, only the engines fed by the core will stay lit (that way they don't have to switch propellant supplies). If you have too few engines burning at that point you have very high gravity losses, which is bad (obviously). Also, engine-out capability may still be desired.

What is the major risk factor in doing crossfeed?

It would seem, on the face of it, to be valves failing to open and/or close, but that sounds like something that could be heavily ground tested. While I have no doubt that cryogenic valves have their own idiosyncrasies, it doesn't seem like a high risk item, but I have zero experience in the area.

If valve reliability is high, then wouldn't it be relatively safe to use 6 valves (2 fuel and 2 LOX for the outboard tanks, and another 2 for the core tanks)? Then all 9 core engines could be used. I'm assuming there already are valves in the outboard boosters to shut off the fuel and LOX, but perhaps there need to be 2 more each at the booster couplings?

[grakenverb]

How many Angels can dance on the head of a pin?

42

Sorry, the correct answer is 41, you are forgetting that this is a leap year.

[RocketmanUS]

Would the boosters separate around 127 seconds after lift off? ( That is for 12 engines feeding on a booster ).

Would only three engine on the core burning give it enough thrust for that velocity and altitude after booster separation?

[dcporter]

Oops. Was talking about engine feed schemes, not dates. 12 right booster/3 core /12 left booster. Etc

13/1/13 tipped me off. Even on a leap year.

I think the plan is that after the boosters stage off, only the engines fed by the core will stay lit (that way they don't have to switch propellant supplies). If you have too few engines burning at that point you have very high gravity losses, which is bad (obviously). Also, engine-out capability may still be desired.

Whoa FH is gonna run on three engines after booster sep? Any chance you remember where you heard that?

[charliem]

I have my own theory of how cross-feed works, made from the little info we have.

Spacex's website says that FH's mass and thrust at lift off is 1,400 mt and 3,800,000 lbf (1,724 mtf) (22 Merlin-1D at full thrust is the absolute minimum to elevate 1,400 mt).

It also says: "Propellant cross-feeding leaves the center core still carrying the majority of its propellant after the side boosters separate".

Can't find where I saw it but I remember to have read about a 90% propellant remaining in the core tanks right after boosters cut off.

In the video looks like all 27 engines are started at the same time, and we know that a Merlin-1D can only be throttled down to a 70% of its nominal thrust.

Easy to calculate that even with 3 engines at 70% more than a 10% of the core fuel would be gone when the booster tanks reach the empty mark.

So, I think it could be like this: Feed valves are set as Jim depicted. Before lift-off the core tanks valves are closed and all 9 core engines are fed from both booster tanks.

This mode lasts until the booster tanks are near 10%, when the core tanks valves are switch to open, and the valves that communicate core turbopumps and booster tanks are closed.

After that core and boosters keep burning exclusively its own fuel until the booster tanks are depleted, triggering separation.

P.E. And no, three engines are not enough to keep FH in flight after separation.

[jimvela]

That depends on detailed trade studies. The only such analysis we have is that they are leaning toward the core doing 3 engines (I believe).

I have no knowledge of SpaceX' plans, but it also depends on how simple they want to make a side-fed design (note I didn't say cross-fed, and neither should any of you when referring to the present concept).

The easiest would seem to be to use three mostly identical cores, with the outer cores each driving two engines in the inboard core, and shutting down those engines at separation.  Lets call that 11/5/11.  I think this also preserves the most likely engine-out capability through the flight profile.  It may not provide the best outcome as the center core may be overpowered and under-fueled after booster separation.

If they want to stretch the outer cores (and possibly SHRINK the center core), then something like 12/3/12 might make sense- again depending on the trades.  Without the stretch/shrink, the center core may end up under-powered and over-fueled after separation.   I can't help picturing this as looking Titan-esq with big (liquid) side boosters, and a big honking fairing not far above the booster nose cones.  I'd find that pretty satisfying considering the VAFB pad heritage. 

Either of the side-fed options might benefit from being able to drop those side engines away with the host core, but that added complexity may not be worth the cost/reliability trades given the lighter Merlins coming online.

The next BIG leap is to actually do cross-feed, in which case you want to draw as little fuel fromthe center core as possible while burning the outer cores, so that at separation the launcher is as close to fully fueled and fully powered as possible, maximizing the benefit of having boosters in the first place.

Cross-fed is a much tougher problem to solve, not just for plumbing and valve concerns, but also because you have to separate, and designing separable plumbing that is both reliable while mated and reliable in separation is a non-trivial task compared to just feeding some engines in the other core that you shut down when you sever the feeds.

Oh, to be a fly on the walls in Hawthorne... 

[jimvela]

I have my own theory of how cross-feed works, made from the little info we have.

That's a pretty good theory!

[SpacexULA]

Whoa FH is gonna run on three engines after booster sep? Any chance you remember where you heard that?

No 18 Engines at start 3 engines on left side of core feed off left strap-on.  3 engines on right side of core feed off Right Strap on. (So each of the strap-on are actually feeding 12 engines instead of 9, and core is only feeding 3)

After low fuel on a strap-on Staging happens.  Now all 9 core engines are feeding off a mostly full Center Core (only fed 3 engines for as long as it takes 12 engines to suck down a stage).

Does that make since?

[jimvela]

You are making harder than it is.
See heritage Atlas.
No additional turbopumps. Just valves and large interconnect ducts.  Isolate the core tanks from launch until right before staging. Then open core valves and then isolate boosters via valves in interconnect ducts.  Shutdown both boosters when one shows dry.

Isn't the first hard part of heritage Atlas that the valves below the center core are subject to all manner of bad things?  (e.g. boiling cryo/vapor lock, some oddity where you get surges in the RP supply line when you first open it, etc?)

High-speed liquid turbopumps don't like to ingest gasses or to have wildly varying inlet fuel conditions.

Isn't the second hard part of heritage Atlas getting the severable part of the feeds from each core to reliably sever/separate cleanly?

I can see an education in my near-term future 

[jimvela]

Whoa FH is gonna run on three engines after booster sep? Any chance you remember where you heard that?

No 18 Engines at start 3 engines on left side of core feed off left strap-on.  3 engines on right side of core feed off Right Strap on. (So each of the strap-on are actually feeding 12 engines instead of 9, and core is only feeding 3)

After low fuel on a strap-on Staging happens.  Now all 9 core engines are feeding off a mostly full Center Core (only fed 3 engines for as long as it takes 12 engines to suck down a stage).

Does that make since?

It would seem that if they do cross-feed at all, it's an all-or-nothing proposition as the other Jim points out. 

Partial cross-feed is MORE complicated, and has LESS benefit!

[RocketmanUS]

You are making harder than it is.
See heritage Atlas.
No additional turbopumps. Just valves and large interconnect ducts.  Isolate the core tanks from launch until right before staging. Then open core valves and then isolate boosters via valves in interconnect ducts.  Shutdown both boosters when one shows dry.

So need to control the flow of propellant from each booster to core to keep a balance from each. If so can that be done with variable flow valves?

[pippin]

So need to control the flow of propellant from each booster to core to keep a balance from each. If so can that be done with variable flow valves?

What for? Again: don't make it more complicated than it is, these are communicating vessels:
http://en.wikipedia.org/wiki/Communicating_vessels

flow-control valves would actually only make things worse.

Sometimes physics can be so simple

[Jim]

So need to control the flow of propellant from each booster to core to keep a balance from each. If so can that be done with variable flow valves?

What for? Again: don't make it more complicated than it is, these are communicating vessels:
http://en.wikipedia.org/wiki/Communicating_vessels

flow-control valves would actually only make things worse.

Sometimes physics can be so simple

Only if the pressure in the boosters are kept equal

[phred]

So, considerng the additional hardware required (which sounds fairly minimal to my civilian eyes), what would be a realistic performance enhancement estimate for the FH?

[RDoc]

So need to control the flow of propellant from each booster to core to keep a balance from each. If so can that be done with variable flow valves?

There's already flow control to keep the engines running at proper thrust and mixture.

For lots of reasons, one tank is going to run out first, or get down to the level needed for stage recovery if that's planned.

[RDoc]

I have no knowledge of SpaceX' plans, but it also depends on how simple they want to make a side-fed design (note I didn't say cross-fed, and neither should any of you when referring to the present concept).

The easiest would seem to be to use three mostly identical cores, with the outer cores each driving two engines in the inboard core, and shutting down those engines at separation.  Lets call that 11/5/11.  I think this also preserves the most likely engine-out capability through the flight profile.  It may not provide the best outcome as the center core may be overpowered and under-fueled after booster separation.

Does that work well?

With 5 engines running isn't there barely enough thrust to lift the core if it were pointing straight up? If it's pitched over at all, doesn't it require at least 6 or 7 engines to avoid losing altitude and/or speed? Plus, would there be any engine out capability at all for some time after staging?

I don't have any way to calculate take off trajectory trade offs, but wouldn't it be best to keep both climbing and accelerating?