Stop drawing fuel across stages?

[martincmartin]

I try not to use decouplers when I have an engine on the other side, as starting the engine will burn off the earlier stage.  However, if I have 2 liquid stages, the bottom stage draws fuel from the top tanks, so I have to transfer fuel back to the top tanks before burning the higher engine.  Is there a way to stop that, so that the bottom engine only draws from the bottom tanks?

To be concrete, I have a rocket like this:

Command Capsule

Tank 2

Engine 2 (e.g. terrier/poodle)

Tank 1

Engine 1 (e.g. swivel/skipper)

No stack separator in between.

 

[bonyetty]

Right click the top tank and left click the green arrow next to the liquid and oxidizer bars to lock the fuel in the tank. 

Running a fuel line from the bottom tank to the top tank may also achieve what you desire but I have not tested this. Should work in theory though. 

Edited March 31, 2016 by bonyetty
Thought of another method

[Plusck]

1 hour ago, bonyetty said:

Right click the top tank and left click the green arrow next to the liquid and oxidizer bars to lock the fuel in the tank. 

Running a fuel line from the bottom tank to the top tank may also achieve what you desire but I have not tested this. Should work in theory though. 

Yes to the first part.

No to the second part due to the way fuel is sourced: the engine looks for fuel in the first part it is connected to, puts it on a list then looks for more fuel. If it encounters a tank it already has on its list, it'll ignore it. So the fuel line here will do nothing since the source (the bottom tank) is already on the list.

[FullMetalMachinist]

2 hours ago, martincmartin said:

I try not to use decouplers when I have an engine on the other side, as starting the engine will burn off the earlier stage.  

This doesn't answer your actual question (sorry about thatu, feel free to ignore this) but I would suggest you reconsider your philosophy on this. 

Not only will using a stack decoupler completely get rid of the issue you're having, it's also literally the thing they're made for. I mean, I get that you're trying to save weight and/or part count, but scrapping an essential function of a rocket is probably not the best place to start. 

[martincmartin]

On 3/31/2016 at 6:07 AM, bonyetty said:

Right click the top tank and left click the green arrow next to the liquid and oxidizer bars to lock the fuel in the tank. 

Thanks, that's perfect!  The only down side is that I switch stages in the middle of a circularization burn when getting into Kerbin orbit, and this slows down the staging by several seconds, as I have to switch from map mode back to ship mode, right click, toggle the settings, then switch back.  But it's not so bad, I can just start my circularization burn a little early.  Thanks!

[Snark]

On 3/31/2016 at 3:01 AM, martincmartin said:

Command Capsule

Tank 2

Engine 2 (e.g. terrier/poodle)

Tank 1

Engine 1 (e.g. swivel/skipper)

No stack separator in between.

 

I'm still kind of confused.  Why is there no decoupler?  How do you get rid of the bottom stage?

The simple answer is "put a decoupler below Engine 2", that solves the problem nicely.  I get from your description that you have some reason you don't want to do that, but I'm not quite getting what that reason is.  Can you explain a bit more?

[Zargg]

Are you literally exploding Tank 1 using Engine 2 exhaust heat?

I mean....I guess that works...but most people avoid exploding their craft on purpose lol.

And I wouldn't be surprised if the deltaV loss from using the second stage with empty first stage still attached is greater than if you just have a decoupler to get rid of it.

[FullMetalMachinist]

15 minutes ago, Snark said:

I'm still kind of confused.  Why is there no decoupler?  How do you get rid of the bottom stage?

I'm pretty sure Zargg is right. 

7 minutes ago, Zargg said:

Are you literally exploding Tank 1 using Engine 2 exhaust heat?

Which is exactly why I suggested using decouplers when I first replied. 

Edited April 1, 2016 by FullMetalMachinist

[KamenRiderzero1]

Not using a decoupler is like building an erector (meccano) set model without using nuts, just sticking the screws in and hoping it stays together.

[Plusck]

The technique figures in a couple of Scott Manley videos, I think. I specifically remember one video where the aim was to fully explore Minmus on the first mission to unlock most of the science tree. At the time of the video (it was long, long ago) mounting several fleas in line and staging the next one shortly before the previous one ran out was surprisingly effective.

I tried this in 1.0.4 and it doesn't work that well. The exhaust while heating the tank has zero thrust (I think in the early days, exhaust cone collision with your own ship didn't entirely - or maybe at all - cancel out thrust) so for the Flea, you lose a significant percentage of each stage burning off the one below. So much so that you need a huge pile of Fleas to get anywhere, and you quickly reach the VAB and launch pad limits.

So I'd chalk this one down to it being so Kerbal that even the Kerbals don't do it anymore.

Edited April 1, 2016 by Plusck

[martincmartin]

3 hours ago, Zargg said:

Are you literally exploding Tank 1 using Engine 2 exhaust heat?

Yep.  I got the technique from maccollo in the challenge I posted: 

 

My main goal is to save $.  A stack decoupler costs 400, as much as a Hammer and almost as much as an FL-T400.

3 hours ago, Zargg said:

And I wouldn't be surprised if the deltaV loss from using the second stage with empty first stage still attached is greater than if you just have a decoupler to get rid of it.

Why is that?  It burns off in a fraction of a second.  I highly suggest you try it, it's a lot of fun.

Confuses the hell out of Kerbal Engineer Redux though, it doesn't realize the lower stage will disappear when the upper stage starts, so it thinks my stage 2 mass includes the empty stage 1, and thus tells me a much lower delta V.  But in VAB, I can put in a decoupler just to measure the delta-v, then take it out again before I launch.

[Pecan]

Does it also count as a SSTO if you blow half of it up instead of staging? *hehe*