Falcon Heavy Cross-Feed

[modemeagle]

The simplest setup is like Jim's diagram, 2 pipes from each booster terminating into a single manifold for each propellant and fed down to the core stage's engines.  They could shut down an upper valve (above manifold/octopus) and keep the central core pressurized, but not using propellants if full cross feed is used.  They could regulate the flow rate from each booster to make sure the booster's drain at the same rate for balance.  They could also skip the Core valve (one more thing to fail closed) and just pressurize the boosters higher than the core to keep the flow essentially zero.

Last year I sat down and wrote out how I would do cross feed and its similar to the above and diagram.

[Idiomatic]

The simplest setup is like Jim's diagram, 2 pipes from each booster terminating into a single manifold for each propellant and fed down to the core stage's engines.  They could shut down an upper valve (above manifold/octopus) and keep the central core pressurized, but not using propellants if full cross feed is used.  They could regulate the flow rate from each booster to make sure the booster's drain at the same rate for balance.  They could also skip the Core valve (one more thing to fail closed) and just pressurize the boosters higher than the core to keep the flow essentially zero.

Last year I sat down and wrote out how I would do cross feed and its similar to the above and diagram.

I do still worry about what that will do to pressures. Wonder what mechanism they will use to reduce the shock of switching streams.

[modemeagle]

The simplest setup is like Jim's diagram, 2 pipes from each booster terminating into a single manifold for each propellant and fed down to the core stage's engines.  They could shut down an upper valve (above manifold/octopus) and keep the central core pressurized, but not using propellants if full cross feed is used.  They could regulate the flow rate from each booster to make sure the booster's drain at the same rate for balance.  They could also skip the Core valve (one more thing to fail closed) and just pressurize the boosters higher than the core to keep the flow essentially zero.

Last year I sat down and wrote out how I would do cross feed and its similar to the above and diagram.

I do still worry about what that will do to pressures. Wonder what mechanism they will use to reduce the shock of switching streams.

1.  If there is a valve on the core above the manifold then open it before cutting the cross feed, then close the cross feed valves over a few second period to limit any shock.
2.  If using pressure differential then increase the pressure of the core over a few seconds to stop the cross flow.  Then close the booster valves over a few seconds to limit any shocks.

[Jim]

I do still worry about what that will do to pressures. Wonder what mechanism they will use to reduce the shock of switching streams.

Not a big deal, see heritage Atlas or any launch vehicle.   They all have valves that close to shut down engines.

[dwightlooi]

I do still worry about what that will do to pressures. Wonder what mechanism they will use to reduce the shock of switching streams.

Not a big deal, see heritage Atlas or any launch vehicle.   They all have valves that close to shut down engines.

Forget the Atlas... every single engine on the Falcon 9 has valves that shut down the engine. That's how they achieve MECO!

[Jim]

I do still worry about what that will do to pressures. Wonder what mechanism they will use to reduce the shock of switching streams.

Not a big deal, see heritage Atlas or any launch vehicle.   They all have valves that close to shut down engines.

Forget the Atlas... every single engine on the Falcon 9 has valves that shut down the engine. That's how they achieve MECO!

Didn't I say "or any launch vehicle"
What do you mean forget the Atlas?  It is the best example.  Shutting down some engines while others keep burning

[Idiomatic]

I do still worry about what that will do to pressures. Wonder what mechanism they will use to reduce the shock of switching streams.

Not a big deal, see heritage Atlas or any launch vehicle.   They all have valves that close to shut down engines.

Forget the Atlas... every single engine on the Falcon 9 has valves that shut down the engine. That's how they achieve MECO!

Didn't I say "or any launch vehicle"
What do you mean forget the Atlas?  It is the best example.  Shutting down some engines while others keep burning

FH crossfeed is a bit more difficult. As I understand it:

You have 3 cores running, fed by squid thing which are fed by the booster tanks. This is fine, pressure is steady though lower I would assume than the F9 1.1 since they get 2/3 of a tank each core.

When the boosters are low, then
- the central tank is opened into squid thing resulting in a sudden boost in pressure on the center or all cores
- the crossfeed shuts off. resulting in a drop in pressure on the main core and a rise in the side cores
- the side cores shut off, if this happens before crossfeed shutoff then it causes a pressure boost on the main core. it it happens after then... nothing ... so that's probably the way to do it.


So... What mechanism is in place to handle the change in pressure? Can the funny octopus things handle it? Will the valves have to handle it? Can the engines handle it? Or something else.

[cordor]

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?

spacex has long track record of valve problems.

[kevin-rf]

spacex has long track record of valve problems.

Huh? I thought they had a long history of having the software constraints set to tightly causing last minute aborts.

[Lobo]

Not sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core.  I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?

or not?
And if not, why not?

[meekGee]

spacex has long track record of valve problems.

Huh? I thought they had a long history of having the software constraints set to tightly causing last minute aborts.

Well, at least they're now being given the credit of "long track record"...
One could even say that they had valve problems on 1/3 of their F9 launches.

[krytek]

You can do that, but that's probably not as efficient. Crossfeed is essentially like adding a another stage since the core is almost full at separation.

[modemeagle]

Not sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core.  I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?

or not?
And if not, why not?

Comparison using my simulator.  The only change is shut down cross feed and activate throttle down.  Even the initial pitch is the same.

EDIT:  Updated sheet to make MECO with ~300 m/s residual prop.
Edit again:  Updated with data for cross feed between 1 and 8 engines.

[notsorandom]

Not sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core.  I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?

or not?
And if not, why not?

Comparison using my simulator.  The only change is shut down cross feed and activate throttle down.  Even the initial pitch is the same.

I am not seeing much of a difference at all. Am I looking at it incorrectly?

[modemeagle]

Not sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core.  I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?

or not?
And if not, why not?

Comparison using my simulator.  The only change is shut down cross feed and activate throttle down.  Even the initial pitch is the same.

I am not seeing much of a difference at all. Am I looking at it incorrectly?

I don't see much of a difference either, but my simulator does not model the change is ISP as the engine is throttled down to 70%.  Maybe later I will try to integrate that into the model.

[MP99]

Not sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core.  I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?

or not?
And if not, why not?

Well, you do start off with the same prop in the three first stage tanks, but if you throttle down the core, you reduce the thrust when the vehicle is heaviest. That increases gravity losses (but does assume the vehicles' structure can take all the thrust).

Also, if speculation is correct that the outriggers feed twelve engines each (their own, plus 3x core engines), then the outriggers will drain 4x faster than the core. At outrigger burnout, the core should have at least 75% of it's prop load remaining.

If you were to do the same via throttling the core, you'd need to throttle down to 25% (and lose a lot of T/W). M1Ds seem to throttle only down to 70%.

cheers, Martin

[aero]

Not sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core.  I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?

or not?
And if not, why not?

Well, you do start off with the same prop in the three first stage tanks, but if you throttle down the core, you reduce the thrust when the vehicle is heaviest. That increases gravity losses (but does assume the vehicles' structure can take all the thrust).

Also, if speculation is correct that the outriggers feed twelve engines each (their own, plus 3x core engines), then the outriggers will drain 4x faster than the core. At outrigger burnout, the core should have at least 75% of it's prop load remaining.

If you were to do the same via throttling the core, you'd need to throttle down to 25% (and lose a lot of T/W). M1Ds seem to throttle only down to 70%.

cheers, Martin

IMO, using crossfeed benefits by a big reduction in gravity loss. Gravity loss is t*g*((re+H)^2)/re^2 in units of velocity. H is altitude. For Low Earth Orbits, H is small compared to re (radius of Earth) so Gravity loss is almost equal to g * t. (In the vertical direction.) F = m*a, so in terms of force, gravity loss becomes (mass-mdot)*g.

Yes, you could design your rocket many different ways, but the objective is to reach orbital altitude quickly and to get rid of mass quickly. Using crossfeed, the rocket burns the fuel from the booster tanks more quickly, so that the boosters can be jettisoned sooner. Then the core rocket engines do not need to carry that mass further. That is, both the fuel mass and the tank/engine mass are expended at the earliest possible time.

[Lobo]

Not sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core.  I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?

or not?
And if not, why not?

Well, you do start off with the same prop in the three first stage tanks, but if you throttle down the core, you reduce the thrust when the vehicle is heaviest. That increases gravity losses (but does assume the vehicles' structure can take all the thrust).

Also, if speculation is correct that the outriggers feed twelve engines each (their own, plus 3x core engines), then the outriggers will drain 4x faster than the core. At outrigger burnout, the core should have at least 75% of it's prop load remaining.

If you were to do the same via throttling the core, you'd need to throttle down to 25% (and lose a lot of T/W). M1Ds seem to throttle only down to 70%.

cheers, Martin

Which leads me to my 2nd question, would it be cheaper/easier to develop  M1d with that deep of throttling?  Or the extra complexity of crossfeed?

I don't know, just curious. 

[vigleik]

Not sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core.  I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?

or not?
And if not, why not?

Comparison using my simulator.  The only change is shut down cross feed and activate throttle down.  Even the initial pitch is the same.

I am not seeing much of a difference at all. Am I looking at it incorrectly?

I agree that the difference is smaller than expected, but it's there. The residual fuel for all three stages is less in the throttle down scenario. My BOE calculation shows this to correspond to approximately a 25m/s difference in delta V, or a 500kg difference in payload, if you want to keep the residual fuel constant. (I don't have exact numbers so this is a rough estimate only.)

[Lobo]

Not sure if this question has already been answered, as I've not read all the pages on this thread, but I was wondering the reasons for doing crossfeed, rather than simply throttling down the central core during ascent to preserve it's fuel, and then after booster separationthere is still fuel left in the core.  I guess this would lead to higher staging, as the boosters aren't drainging their propellant into the central core, but wouldn't it accomplish roughly the same thing?

or not?
And if not, why not?

Comparison using my simulator.  The only change is shut down cross feed and activate throttle down.  Even the initial pitch is the same.

I am not seeing much of a difference at all. Am I looking at it incorrectly?

I agree that the difference is smaller than expected, but it's there. The residual fuel for all three stages is less in the throttle down scenario. My BOE calculation shows this to correspond to approximately a 25m/s difference in delta V, or a 500kg difference in payload, if you want to keep the residual fuel constant. (I don't have exact numbers so this is a rough estimate only.)

Is that assuming just M1d's 70% planned throttling?

Hmmm...if that's true, 1/2mt doesnt seem like a lot of lost payload on a 50mt-ish LV.  52.5mt instead of 53mt? 
To save the extra cost of designing and building a crossfeed system?

Dunno...doesn't seem like a bad trade if that's even close to accurate...