Fuel transfer through decouplers *up* the craft.

[Sharpy]

I was so proud of my probe/lander design.

I'm fairly early into Career mode. Still didn't unlock transfer of resources or the fuel pipe.

I built a really tiny probe meant to land on Mun; The cheap solid boosters, awesome in the new atmosphere (but very hard to steer) will bring its apoaxis near Mun orbit altitude (probably a bit beyond), then the good-ISP engines (Terrier) would pick an encounter, then get me into orbit. Then I discard the big fuel tanks, and with the tiny probe, the two Terriers and two small fuel tanks I'd perform the landing (the last stage, due to very low mass, has like 1200 dV despite the tiny fuel tanks).

The "kinky" thing is the probe with its two tiny tanks and the Terriers is placed upside-down, the two Terriers pointing skywards (and serving as makeshift nose cones), the big fuel tanks supplying it with fuel for getting into Mun orbit hanging down along the central solid booster. Once in space, separated from the solid boosters, it turns around and the two "horns" of long fuel tanks on the Terriers pointing forward not being a problem in void and freefall. In the atmosphere they are tucked alongside the central solid booster and provide occlusion from atmospheric drag to shorter solid boosters placed directly below.

So I performed a test launch into Kerbin orbit. Separation went alright, then I started the LF boosters at full power and they sputtered out a moment later. The two tiny landing stage fuel tanks are depleted, and the big "horns" of long fuel tanks are still full. And I have no way to get the fuel to the engines.

Am I sentenced to attach two more engines on the bottom of the transfer stage or is there a way to get the fuel to the two on top?

Edited June 25, 2015 by Sharpy

[eonmoon]

hmm, not really unless you got fuel lines to connect from the big tanks to your tiny ones, they hold very little compared to the big ones

[tutike2000]

Fuel doesn't flow between parts attached 'radially' (to the 'surface') of another part. You need to use fuel lines to do that.

Fuel by default only flows between through those green nodes.

[Sharpy]

@Tutike: The engines with their fuel tanks are attached in line - all through the "green nodes". Only the tiny fuel tanks with their engines are attached radially to the probe core, but the core doesn't need or provide any fuel.

(the craft's basic profile is three "small" tubes in line; the probe and the huge SRB in the middle, liquid fuel (big&small) + engines as the side tubes. Extra SRBs go under the side tanks, but they are attached radially to the main booster, no connection with the tanks above.) No radial decouplers at all and no need for radial fuel transfer.

[tutike2000]

>Only the tiny fuel tanks with their engines are attached radially to the probe core

I still don't get it. That means you didn't attach via nodes.

We'll need a screenshot/

[rofltehcat]

Mind sharing screenshots?

I don't quite understand the design but I have done things like mounting upper stages the wrong way around or forgot about fuel lines. For automatic fuel routing you'll need fuel lines.

If you want to salvage that craft you'll probably still be able to do manual fuel transfers (alt+rightclick the empty tanks, alt+rightclick a full tank and redirect fuel out of the full tank.

By your design it sounds like the stacks/tanks look like this:

F-F-F Full fuel tanks

E-F-E Empty pods

^---^ Engines

So you'd preferably empty one of the center tanks into the engine pods first, then use that tank to pool fuel from the outer tanks before transfering it to the engine pods to avoid unbalancing the craft. If that isn't possible you'll probably need to turn off the engines between fuel transfers or only fire the engines at low power.

[Reactordrone]

If you have a decoupler in between the small landing tanks and the larger transfer tanks the fuel won't automatically cross over the decouplers, you'll have to use fuel lines.

[Sharpy]

I don't have access to my computer with KSP right now. A rough picture below.

In fact they aren't separated so widely apart, but resting snugly against each other radially, but I expanded them to make it clear where the attachment points are.

[cephalo]

That picture makes no sense to me, but if it's simply a matter of having no fuel in the right tank, you can fix it before separation by right click on one tank, then mod-right-click on the other tank to do a fuel transfer.

[tutike2000]

Stack decouplers break fuel feed. They always have. You need fuel lines from the big LF tanks to the small ones.

Also, those big LF tanks side-mounted won't occlude your SRBs unless you have FAR installed. Stock aero only looks at node attachments.

[rofltehcat]

Yeah, you'll need to do manual fuel transfers into the pods either:

-at low thrust (depends on how many reaction wheels you have)

-shutting off the engines between refills

-switch between fuel tanks: alt-rightclick one of the full large fuel tanks and alt-rightclick both of the small empty fuel tanks, then transfer fuel out of the large tank. After a while deselect everything and repeat it for the other tank. This is so the craft doesn't start flipping around.

[tutike2000]

Still didn't unlock transfer of resources or the fuel pipe.

Ah, I missed this bit.

Sorry, this can't be fixed without either pipes or fuel transfer. You can instead change your design to have 4 Terrier engines. Stick 2 at the end of the large tanks.

[Sharpy]

*sigh* Yep. Two extra Terriers it is.

[Rhomphaia]

Have you unlocked the junior docking port yet? replacing the decouplers between the drop tanks and the lander with those would work

[rofltehcat]

Wait, you need to unlock fuel transfer separately? Is it some space center upgrade? You could leave the craft in orbit and do other missions until you can purchase the upgrade.

[tutike2000]

Wait, you need to unlock fuel transfer separately? Is it some space center upgrade? You could leave the craft in orbit and do other missions until you can purchase the upgrade.

Yeah, it's the 2nd level of the Research facility.

[Sharpy]

No docking port yet. And yes, it's an upgrade of KSC I don't have yet. I have a Mun&Minmus flyby contract scheduled, so I might unlock the fuel transfer soon, although doing rendesvous maneuvers and balancing the load between the two tanks with constant refuelling will be a pain in the neck. (I can pick the two small tanks, the big tank, click "Out" on the tank and it would work... until I need to switch to the other to return the craft to balance. Can't transfer "out from two, into two".)

[nekogod]

You can cheat slightly with the tac fuel balancer mod it allows fuel transfers before you have the relevant facility upgrades

It has a whole host of options for transferring from and to multiple tanks

[tutike2000]

No docking port yet. And yes, it's an upgrade of KSC I don't have yet. I have a Mun&Minmus flyby contract scheduled, so I might unlock the fuel transfer soon, although doing rendesvous maneuvers and balancing the load between the two tanks with constant refuelling will be a pain in the neck. (I can pick the two small tanks, the big tank, click "Out" on the tank and it would work... until I need to switch to the other to return the craft to balance. Can't transfer "out from two, into two".)

Yeah, you'd have to transfer 1-to-1, that means no landing/deorbit until you run out of fuel in the big tanks, Otherwise you end up with a lot of torque mid-burn.