Balancing a Jool ship with four landers

[Alexander_Kerbing]

I had this neat idea to send a ship to Jool with four landers and a probe (for Tylo). I figured I would send the core module, titled Pandu, up into Kerbin orbit first, and send the landers and probe later (cookie if you can guess what I'll call those five items). I had initially planned to attach the landers to the core radially, like this:

Seemed simple enough until I realized that it is likely that my landers will have varying weights and will throw off the CoM causing my ship to spin out of control when I make my burn for Jool. So then I had the idea to make it so I could attach the landers in line with the CoM like this:

Now that seems all well and good until I realized what kind of structural limitations that places on my landers as they need to have an accessible docking port on the top AND the bottom. Also, ideally, they would have Clampotron Sr docking ports so that they might interface with the ones on core ship. I realize my core ship doesn't absolutely need to have Sr. docking ports, but I've been on a kick of using appropriately sized parts and adapters lately.

So how might I attach four landers to this ship? I've also considered stacking them on the front end, but then my beloved Kerbals would have an obstructed view from the cockpit. What's the point in going to Jool if you can't even look at it thru the window?

[Temstar]

Aside from trying to build the landers so they're at least roughly the same mass, one neat trick is build the mothership with more tankage than the mission requires. Then what you do is de-fuel the heaviest lander into the mothership tanks so that its wet mass matches up with the fully fuelled wet mass of the second heaviest lander (which will be docked to the opposing radial docking arm), and like wise the other radial pair. Then once you've reached your destination you can fully refuel the landers from the mothership tanks before releasing them.

This for example is my Jool fleet with 3 landers. The big Tylo lander on the nose (that's another trick, use the nose docking port of your mothership if you have a ship that's radically different in mass to the rest) leaves the mothership during Jool aerocapture and head to Tylo while the mothership and two landers go to Laythe. The smaller lander then go to Bop from Low Laythe Orbit while the medium lander land on Laythe, launch and refuel from the mothership orbiting above and then head to Vall. After that all three landers take off and re-join the mothership at Low Jool Orbit. Pol didn't exist back then so it doesn't get a lander.

On both the trip to Jool and back I would trim the assembled fleet by offloading Laythe/Vall lander fuel into the mothership so that that lander is the same mass as the Bop lander.

One other trick is to break the heavy landers into multiple stages stuck together via docking port (eg a descent stage and a docked lightweight ascent stage, or an orbital tug docking with a lander). When docking you can then break up that lander into component pieces to better distribute that weight around the mothership. At destination undock all the pieces to reassemble the craft before it heads off on its own.

Here is my Gilly lander and its nuclear tug stage reassembling in Low Eve Orbit. Gilly's low gravity means I can have a tiny RCS only lander, but the heavy gravity of Eve and Gilly's erratic orbit means I need a powerful tug stage for it with a lot of delta-V. Since the combined lander+tug is much heavier than the Eve rover on the opposing docking arm what I did is install a docking port on the bottom of the rover and move the Gilly lander to that side, thus balancing things out.

[magnemoe]

Another way is to use drop tanks for balance.

Simply put fuel tanks out on decoplers, might be an idea to put them radial on your ship

Drain tank and disconnect after dropping lander.

[KerbMav]

Completely redesign your ship for the landers to dock in a middle of two stacks of fuel tanks, engines and other components.

Join the stacks with struts, crew modules or anything else you want to bring along and dock you landers - in the line of COM and COT - to these "bridges" between the two "drive sections".

Will require a hell of a launcher or quite some docking in Kerbin orbit.

To save mass and parts you could reuse one crew and equipment module and dock it to the different land- and ascent-modules designed for the different bodies you are visiting.

Suggested mods: Fuel Balancer, Crew Manifest, UbioZur Welding

Edith tells me, that I always wanted to built such a vessel myself, but I seem to procrastinate to much - and have to many things in mind that I also want to build ...

[RoboRay]

You can compensate for imbalanced radial payloads by having the lighter one further out, away from the center of mass. So, big payloads on short arms and small payloads on long arms.

[Sauron]

If you're staging your Tylo lander you could just make the core Pol/Bop/Vall capable (and depending on the configuration of the core also Laythe-capable). This would save a fair amount of mass (though you will still need to carry extra fuel for each mission) because you only need to bring one shrinking lander

[Captain Sierra]

You can use the same lander design on both Bop and Pol. for Vall and Lathe, build a Munar lander that works, add more parachutes than necessary for kerbin re-entry. Then use it on Lathe like you would a Duna lander: un-powered descent to save fuel, re-orbit.

Then dock your Tylo probe inline somewhere.

There, how to do a Jool grand tour with only 2 types of landers (which can be discarded once both of the same type are used.

[Skorpychan]

Identical landers all the way, tbh. That way you don't need to worry about which one goes where.

[KerbMav]

Identical Lander = One Lander

A stack of descent stages if needed.

[BigFatStupidHead]

Use radial engines, and mount your landers in the rear.

Boy, that sounds dirtier than I intended.

[Skorpychan]

Use radial engines, and mount your landers in the rear.

Boy, that sounds dirtier than I intended.

Yeah, but where do you think new landers come from? It's not like they can be assembled and launched or anything...