Can I use different fuel tanks in different stages?

[Panichio]

[ATTACH=CONFIG]31203[/ATTACH]

So is it possible to only use the fuel from the fuel tanks labelled '1' then decouple them and then start using the fuel of the fuel tanks labelled '2'? Or would that not help at all? Because I don't know if it would be more efficient to then have just two fuel tanks to be lighter but I know I would still have to lift that weight that I'm not using, so would it balance out in the end, have a negative or positive effect?

[Atimed]

By my understanding its not you can use from all 4 stacks by simply connecting it with fuel lines.. But not independently..

If you scenario would be possible id would help, 1t is a small burden but burden non the less..

[zarniw00p]

You could add fuel lines from the "1" tanks to the "2" tanks and then from the "2" tanks to the middle tank. This will result in the middle engine first using up the fuel in the "1" tanks, then when they're empty the fuel in the "2" tanks and finally the middle tank fuel.

You may also be interested in this thread:

http://kerbalspaceprogram.com/forum/showthread.php/9299-Asparagus-Stalk-Booster-(Longer-Thrust-from-Efficient-Fuel-Lines)?highlight=asparagus+stalk

[Panichio]

You could add fuel lines from the "1" tanks to the "2" tanks and then from the "2" tanks to the middle tank. This will result in the middle engine first using up the fuel in the "1" tanks, then when they're empty the fuel in the "2" tanks and finally the middle tank fuel.

You may also be interested in this thread:

http://kerbalspaceprogram.com/forum/showthread.php/9299-Asparagus-Stalk-Booster-(Longer-Thrust-from-Efficient-Fuel-Lines)?highlight=asparagus+stalk

Oh nice, thanks

[Ziff]

So is it possible to only use the fuel from the fuel tanks labelled '1' then decouple them and then start using the fuel of the fuel tanks labelled '2'? Or would that not help at all? Because I don't know if it would be more efficient to then have just two fuel tanks to be lighter but I know I would still have to lift that weight that I'm not using, so would it balance out in the end, have a negative or positive effect?

[ATTACH=CONFIG]31203[/ATTACH]

You can, but it gets a little tricky. You can easily disable the fuel flow to a tank by right clicking on it and selecting disable. Then you can enable it again when you need it. However, when you add a part with symmetry in the VAB, right clicking on the part in flight will activate all those parts that were applied with symmetry. So in your case, likely all 4 tanks would disable.

So to get around this, when you build your rocket if you build those drop tanks with the 2x symmetry, and place the 2 sets you need, then you can then disable the 2 sets of tanks separately.

From there you would just right click and disable the tanks you didn't want, then right before or after stage separation just right click them and enable them again.

As for it being beneficial, well I think it would be slightly beneficial for 2 reasons. 1, you get rid of some empty mass sooner. 2, you will lose the aerodynamic drag created by those tanks earlier in the flight. Which the lower you are when this occurs, the more beneficial it will be overall.

Edited August 13, 2012 by Ziff

[Ydoow]

Ziff is right, and the suggested diagram for fuel lines from Tanks 1 to Tanks 2 to the Engines is also correct to get what you desire.

As Ziff said, I also believe it will be beneficial.

The amount of fuel you'll be using will be the same if you used 4 tanks or 2 tanks at the same time.

However using 2 tanks would allow you to jettison them, eliminating their Dry Mass (mass of tank without fuel) which is very beneficial.

This raises a question to me as to how radical kerbal you could take this design.

A while back I did try building outward instead of upward.

The advantage is what we told you, ridding yourself of dry mass sooner.

Downsides are:

Collision with the Tower

Extra mass of Radial Decouplars add weight and slim the benefit of jettisoning dry mass.

Lots of spacebar.

Theoretically, you'd want to use lots of really small tanks assuming the Radial Decouplars add no mass, and the Volume of Fuel Held:Tank Dry Mass stayed the same (which it does, I believe, between the full and half liquid engine tanks)

[Ziff]

A while back I did try building outward instead of upward.

The advantage is what we told you, ridding yourself of dry mass sooner.

Downsides are:

Collision with the Tower

Extra mass of Radial Decouplars add weight and slim the benefit of jettisoning dry mass.

Lots of spacebar.

Theoretically, you'd want to use lots of really small tanks assuming the Radial Decouplars add no mass, and the Volume of Fuel Held:Tank Dry Mass stayed the same (which it does, I believe, between the full and half liquid engine tanks)

Actually, all of my new designs now use radial/lateral stages for the ascent stage. The weight of the small radial decouplers are pretty negligible now. Each one only has a mass of .025. So even on some of my larger creations, the decouplers only total .6.

If you look at one of my medium sized rockets, the

[

, it has 8 boosters, 80 lateral drop tanks with a total of 24 radial decouplers. All fuel lines run inwards, keeping the center tanks full. The weight of those decouplers is easily offset when those tanks are dropped, assuming I have gained enough altitude to maintain my T:W ratio just above 2:1, which I do.

[Ydoow]

Neat! How far can you get with it?

[Ziff]

Well, it can get into a 75km orbit with 4300L of fuel left in the main center tank. But even if you jettison that, the lander has around 2500m/s of delta-v. That's enough for a landing and return. Possibly even a full round trip if you are good with landings.

Actually, if you remove the crew tank and the stupid accidental second set of decoupers I had, it pushes the delta-v up to 2900m/s.

Edited August 14, 2012 by Ziff

[Panichio]

Oh nice, well thanks for the help everyone!