Drain fuel evenly across a rocket stage.

[Frozen_Heart]

This is something that has been a minor irritant to me since I got KSP but with the KSP fairing bug has been made even more important. Fuel should drain evenly from a rocket stage rather than from the top down. This would reduce rockets becoming unbalanced and flipping on the way up. Real life rockets have a separate oxidiser and fuel tank and so drain much more evenly than KSP tanks.

All the weight being concentrated at the bottom of the rocket with all the drag coming from beyond the rockets nose has pretty much stopped me using rockets in 1.0.5. (I like falcon 9 like designs so adding huge fins isn't an option for me)

[EladDv]

i agree- it'd make it much more realistic and easier to control with new aero

[pandaman]

Yes, as an easy way to more accurately simulate what happens IRL that does seem to make sense.  As well as helping reduce the 'sudden flip syndrome'.

[sebi.zzr]

This is already possible in version 1.0.5.All it needs to be done is some *.cfg tweaking.But,since it will take long time to edit every engine,you can try this little MM patch to see if fits your needs.

@PART[*]:HAS[@MODULE[ModuleEngines*]]:Final
{    
@MODULE[ModuleEngines*]
{
@PROPELLANT[LiquidFuel]
 {
 %resourceFlowMode = STAGE_STACK_FLOW_BALANCE
 }
@PROPELLANT[Oxidizer]
 {
 %resourceFlowMode = STAGE_STACK_FLOW_BALANCE
 }
}
}

 

Edited January 26, 2016 by sebi.zzr

[Temstar]

In real life fuel drains from the top down as well, assuming we're talking multi-stage rocket. This change would mess with asparagus staging and other such designs that rely on fuel flow logic.

[pandaman]

30 minutes ago, Temstar said:

In real life fuel drains from the top down as well, assuming we're talking multi-stage rocket. This change would mess with asparagus staging and other such designs that rely on fuel flow logic.

If the object is to simulate that (IRL) LF and Ox are in separate tanks, one above the other,  then even if they are in the same 'outer' tank casing then the shift in CoM from top to bottom would be slower than for a single large container due to there still being a part filled tank near to the top.  Would it be possible and/or practical to simulate this by not shifting a tank's CoM so much as it empties?

Edited January 26, 2016 by pandaman

[Mr. Speed]

23 minutes ago, Temstar said:

In real life fuel drains from the top down as well, assuming we're talking multi-stage rocket. This change would mess with asparagus staging and other such designs that rely on fuel flow logic.

So how about a toggle for fuel flow behavior in the VAB, in the tweakables. Just select an engine and choose between "normal drain/drain evenly".

[pandaman]

26 minutes ago, Mr. Speed said:

So how about a toggle for fuel flow behavior in the VAB, in the tweakables. Just select an engine and choose between "normal drain/drain evenly".

That could work 

I guess it would need to be per stage rather than by engine though.

[royying]

IRL, as the compress liquify fuel drain, the remain fuel should expand to fill up the space, does it affect how the CoM shift?

[Temstar]

11 minutes ago, royying said:

IRL, as the compress liquify fuel drain, the remain fuel should expand to fill up the space, does it affect how the CoM shift?

No, fuel tanks are kept at constant pressure by pumping in inert glasses like helium as the tanks are emptied. The CoM change is only because liquid (which is heavy) are being removed from the bottom of the tank and replaced by gas which is much less dense.

[Frozen_Heart]

13 hours ago, Temstar said:

In real life fuel drains from the top down as well, assuming we're talking multi-stage rocket. This change would mess with asparagus staging and other such designs that rely on fuel flow logic.

Real life rockets have a separate oxidiser tank and fuel tank. One will be at the top of the rocket stage so the CoM doesn't move down in the same way it does in KSP.

Maybe you should be able to choose which tanks or stages drain first? That way asparagus could still be set up.

[Kaos]

Setting these values for every rocket or stage would help, yes. I would prefer if one could set this not only on construction, but also in flight, also for redistributing fuel in the tanks. Right now unless one completely fills or empties the tanks, uneven filling is unavoidable when refueling in flight. Hence, in refueled rockets one easily gets imbalanced propulsion.

[Blaarkies]

40 minutes ago, Kaos said:

Setting these values for every rocket or stage would help, yes. I would prefer if one could set this not only on construction, but also in flight, also for redistributing fuel in the tanks. Right now unless one completely fills or empties the tanks, uneven filling is unavoidable when refueling in flight. Hence, in refueled rockets one easily gets imbalanced propulsion.

Do you mean uneven ratio between LF en OX in a tank?(after full burn, theres always some LF left...or OX left)
Or do you mean like filling spaceplane fueltanks, resulting in you left wing having slightly more fuel?

I hate both, but they can be avoided with a small "measurement cup" fuel tank.

[Kaos]

11 minutes ago, Blaarkies said:

Do you mean uneven ratio between LF en OX in a tank?(after full burn, theres always some LF left...or OX left)
Or do you mean like filling spaceplane fueltanks, resulting in you left wing having slightly more fuel?

I hate both, but they can be avoided with a small "measurement cup" fuel tank.

I meant the spaceplane like case, but for rockets. Center of mass not equal center of force is in both cases annoying. But the other is problematic as well, yes.

I know the measurement cup trick, but that is ugly, time consuming and easily forgotten. Just recently I decided to simply burn quarter of my fuel until everything fits in one tank, because it was a fully equipped ISRU lander with then still enough fuel for landing and this was the most simple solution. Further I had forgotten to include a measurement tank. And if burning through fuel just to get rid of it is the most simple and time efficient solution and has no drawbacks, some changes of the program might be useful

[KerikBalm]

6 hours ago, Frozen_Heart said:

Real life rockets have a separate oxidiser tank and fuel tank. One will be at the top of the rocket stage so the CoM doesn't move down in the same way it does in KSP.

Maybe you should be able to choose which tanks or stages drain first? That way asparagus could still be set up.

It doens't move down *in the same way* but it does move down. Each tank empties from its top, so the CoM of each tank shifts down as it is emptied, but the net CoM shift is less for most of the duration (when tanks are completely empty, it makes no difference).

In a multistage rocket, IRL and in KSP, CoM shifts up as the bottom stage empties, but the mass of the top upper stages stays the same.

In modular kerbal rockets, I don't see why we can't treat a stack of two tanks as two actual tanks, and drain from both evenly - that would even work IRL. Indeed we can even manually disable flow from the top tanks so that the bottom tank empties, and then open the top tank just before the bottom runs out... CoM shifts up as fuel burns.

This works for single stacks... but it can mess you up trying to rapidly click on multiple tanks for multiple stacks.

Allowing us to toggle fuel flow from tanks via action groups would be very much appreciated... maybe it merits its own suggestion.

I have made, and have seen others make, designs that rely on proper timing of enabling and disabling tank flows to remain stable throughout flight (while staying stock, TAC fuel balancer would help I guess)

[Mr. Speed]

17 hours ago, KerikBalm said:

This works for single stacks... but it can mess you up trying to rapidly click on multiple tanks for multiple stacks.

Allowing us to toggle fuel flow from tanks via action groups would be very much appreciated... maybe it merits its own suggestion.

I have made, and have seen others make, designs that rely on proper timing of enabling and disabling tank flows to remain stable throughout flight (while staying stock, TAC fuel balancer would help I guess)

Fuel tank toggle in action groups would make me very happy. It's actually kind of weird it has never been implemented before, seems like such a useful and logical thing to have.