Balancing uneven payloads

[Warzouz]

On a rocket, you should have your CoM always on the middle of the rocket. Many engines and reactions wheels can compensate for that, but only if SAS is on. Even a small unbalance can wreck ascent and ascending with SAS on is harder than without.

So I feel the urge of balancing everything. That mean redudancy for all non physicless parts not on the central axis and for physicless parts not attached to a central axis part.

So I get a lot of double features (2 mystery goo, 2 antennas, 2 drills, 2 solar panels, 2 RTG, 2 fuel cels, 2 docking rings, 2 radial RCS tanks...)

In the end, that leads to a waiste of fuel for a dead payload mass.

How do you manage that ?

For example, is it possible to create a ship with only ONE drill ?

[Nefrums]

The effect off the imbalance is proportional to the mass of the parts compared to the total mass of the ship, so it is less important to balance low mass parts like antennas etc. Fuel tanks etc are more important to balance.

You could also add more reaction wheels to compensate. It is ofen a good idea to turn gimbal down on your engines to prevent an unbalanced ship from wobbling.

[Mister Dilsby]

How do you manage that ?

For example, is it possible to create a ship with only ONE drill ?

Yes, of course it's possible to do almost anything, it's simply a question of giving up something else to get the thing you want

For example, yes you can create a ship with only one drill--the Hummlebee spaceplane has one mounted on the back, and so I attached radial engine pods to get around the drill mount. To balance the other ones you could try to match them up by mass and put equal mass parts or groups of parts on opposite sides of the ship. You can also maybe balance by partially filling some fuel tanks. Lastly, I have not tried this myself but if the problem is your balance when ascending, you could put the heavier parts on the side that will be the top as you do your gravity turn to make your ship pitch down slower, or on what will be the bottom to make it pitch down faster.

KER (Kerbal Engineer) is really useful when trying to balance things because it will tell you the "thrust angle" that your engine must gimbal to try and make your ship go in a straight line. If this equals zero, then your ship is well balanced around the thrust axis.

Edited August 25, 2015 by Kuzzter

[mhoram]

I use a combination of RCS Build Aid and a medium or small docking port.

First I create the payload and try to build it in a way that the CoM is above a flat surface.

Then I attach the docking port on the flat surface and put an engine at the bottom of the docking port.

Activating RCS build Aid will show me the torque that is created from the engine.

Selecting the Docking port now allows me to move the docking port and engine on the flat surface - and the RCS Build Aid window is automatically updated on every movement I make.

So I can move the docking port to a position where the torque is nearly 0 and attach the docking port there on the flat surface.

Lastly I remove the engine and build my lifter rocket just below the docking port and can be sure that it is nearly rotation-free.

[Sharpy]

For small payloads the redundancy is not a big problem. The extra mass doesn't mean much. Or add a reaction wheel or two and just go with SAS, setting it to prograde after beginning the gravity turn often results in a better gravity turn than without SAS.

For medium payloads: Send two. Two fuel trucks, two rovers, two satellites. A central beam with two Structural Pylons and the two subassemblies attached belly-to-belly.

For large, badly unbalanced, draggy payloads I use SAS and Pinecone Nosecone.

It's expensive and wasteful, but the price is still a small fraction of the price of payloads it serves. And I have yet to find a payload for which this is insufficient.

[FyunchClick]

Also, balancing features like tail fins will help keep it straight.

Even more importantly if you've got a rocket that's less than aerodynamic, hug the sonic wall until you're out of the thickest parts of the atmosphere (keep your speed around 300/350 m/s). I've had plenty of designs that kept going unstable and flipping above those speeds and it turned out there was nothing wrong with the balance, it was just too much speed. Usually around 12-15 km up or so you can safely go full thrust again.

Speaking of aerodynamics, some of your imbalance may come from lopsided aerodynamics rather than mass, put up fairings to get rid of that.

Another reason I found for my rockets rolling out of control is putting up fins in triple symmetry with one fin pointing straight north or south, and then doing a gravity turn (east). The imbalance in lift on the wings meant my rocket kept rolling out of the direction I wanted it to go (and often out of control). Put fins up so they're symmetrical over your east-west axis (or with triple radial symmetry rotate about 30 degrees either way before your gravity turn).

One trick I use is that, as long as the bulk of my fuel is in consumed in a balanced way, its pretty easy to balance payload that's a bit lopsided by putting one or two small radially attachable liquid fuel engines (like a twitch or spider) on the heavy side on the bottom of your rocket, and play with the trust limiter slider till the torque readout on Kerbal Engineer reads just a few tenths of a kNm. It'll free up reaction wheels capacity that may have been fighting at full strength just to keep pointing straight during ascent and restore some reactiveness to your controls (and steadying to your SAS). Mostly for when you're already up in space to prevent being pushed just slightly off course during burns, as those tiny engines don't do much for really big rocket.

Even beter sometimes, some Vernors well away from your CoM, and turning RCS + SAS on during accent helps too, and is more dynamic than the above as it'll push back at the actual torque, not just output a fixed amount on the opposite side as the trick with the radially attached engines, and it doesn't require kerbal engineer, but I think it's wasting a tiny bit more fuel as it's pushing at a right angle instead of straight down, and unless you disable the RCS fuel tanks during ascent, the regular RCS thrusters are consuming that as well on your way up and you find you're dry when you want to dock in orbit.

Edit: d'oh ninja'd

Edited August 25, 2015 by FyunchClick
ninja'd

[Red Iron Crown]

Many engines and reactions wheels can compensate for that, but only if SAS is on.

When SAS is off you can use the trim controls (Alt+WASDQE to set, Alt+X to cancel) to use gimbals, reaction wheels, and control surfaces to compensate for thrust offset.

[FungusForge]

When it comes to balancing rockets without redundancy, you put the heaviest thing (in this case a drill) on one side, and everything else on the other. Worst comes to worse you use the cubic strut thing to put things farther out.

[Warzouz]

I could try to stick outside parts I put on the central axis. For example, I could try to balance the drill with the landing can by pluging it under a lateral tank. Obviously, I 'll have to check with KER...

That would also fix my weak node problem...

I'll look into RCS build aid.

Thx

[Pecan]

1 drill, balanced configuration = put the drill under the centre stack. This implies you have side-stacks, which implies multiple, low-thrust engines. 2 - 4 909s on radially-attached side stacks give a lander a wide base and pretty good lifting power for Mun or similar.

If you're keeping it as small as possible though and have a single stack, which is liable to fall over on landing, make a virtue of necessity - *make* it fall over, onto side-mounted landing legs (if necessary - probably not on Minmus). Your sideways-mounted nose-drill will now be pointing down, ready to mine :-)

[ajburges]

If you have RCS build aid, you can just use the offset gizmo to rebalance thrust and CoM. Thats how I balance asymmetric rings.