Healthy autostrut use

[RocketSquid]

I’ve found the autostruts very helpful, but recently realized that I’m probably using them wrong. My current strategy is usually just to turn rigid attachment on and set autostruts to heaviest part on anything I think will be subjected to a decent amount of force, mostly ignoring the other options for autostrut targets. When is each option best; and when is it best to have no autostruts? How does rigid attachment differ from autostrut: parent?

[Aeroboi]

@RocketSquid Autostruts is always better then no autostruts, so it's always a good idea to use them. The larger the size fuel tanks is like (3.75m) or (mk3) have less rigid attachment connections, so the higher you go beyond 1.25m the more necessary autostruts may become. In case you want to avoid them when not willing to you should use short Mk1 and Mk2 sized fuselages and a short width wingspan with 1 or 2 engines, that could do without autostruts always with minor part rigidity under stress. So beyond that I'd use autostruts always. Below that I'd discard using them. Remember that the amount of attachments nodes, further surface attachments like other fuel tanks and the total width, height and length of the vessel determine the joint strength. So 8 small 1.25m fuel tanks are more flexing then 2 big ones.  Beyond that it is the engine thrust location (usually at the bottom) and aerodynamic stress that further request necessary autostruts use.
Also, the distance from one part connection to another further degrades the rigidity or stiffness. So a part autostrutted to a part that is furthest away will become less potent then a location closer on the vessel.

Rigid attachment is what it says it does. It makes a connection rigid, like glass is. Glass breaks normally when dropped, the same goes with rigid attachment. Part stresses remain the same so if the particular connection is stressed like by impact or through aerodynamic stress on wings then the part is likely to break.
Due to rockets and planes undertaking a lot of stress it is better not to use rigid attachment only if you got a particular good reason.

Out of my mind I would only use rigid attachment  for.

• Certain custom bearings to hold parts in exact places (stock propellers or mechanisms)
• Large ground construction bases using I-Beams and girders to stop flexing
• Sometimes on longer rockets it can be good to stop drag or engine thrust vibrations causing bending and swaying. Imagine a very long rocket that is thin it can make it super stiff so that the drag from the bottom fins or force of the engine gimbal doesn't alter the structure. Just don't try to land that rocket at the bottom hard as rigid attachment will make part attachments brittle and break off more easily.
• On wings. While most people don't it can be used on small to medium large planes when using multiple segmented wing pieces, usually on wide wing aircraft. Don't use it on the wing piece directly attached to the fuel tank as it get's the greatest stress, which is what most people do and then discard the option as wings snap off. The first wing piece is where bending needs to be as it holds the wing so never use rigid attachment there. On the outer wing segments it can make sure wing tips don't overly bend. Because the first segment bends the other segment that is set to rigid attachment doesn't snap off as the first segment absorbs the stress first keeping the snap off point at bay.

For autostruts you have Grandparent which makes that part connected to the part it is node or surface attached to. Heaviest and root part is obvious, however, if using large part count vessels make sure that if you dock to another vessel that has a heavier part the autostrut will connect to there or any of the root parts on any both vessels will change to any of both. Sometimes this will give explosions or problems so quicksave first and if so make sure to set all heaviest parts to any other as you can change autostruts in flight.

Because of this the grandparent is the safest option, if you do a lot of docking using large part count vessels make sure your vessels use mostly this setting. Usually the problem described isn't very common when using less complex vessels.

Grandparent

As described it struts the part to the part it is attached to. In terms of strategy you best use grandparent autostrut on every part within the core stack. A stack is a single long fuselage, in terms of the core stock it means the one in the middle that has the cockpit or command pod.
This stiffens the vessel on every part connection making sure it doesn't bend.

Also use grandparent part on the first part attached radially like a decoupler and also the fuel tank attached to it. The reason for this is that the connection that holds up a entire stack that is radially attached is located at the fuel tank to which it is attached first.
If you have a very long rocket it is best to attach the decoupler and fuel tank (or fuel tank directly) in the middle of the length. This makes neither the top or bottom will flex when fully building the stack further.
Using grandparent on these parts makes sure the Center of Mass of the radial fuel stack is bolted in the center.

I you use either mirror or radial symmetry using grandparent on the lower or top parts will grandparent strut the parts together so i.e. a 6 radial booster setup will have all 6 i.e. nose cones attach to each other so stress will not bend their relative alignments.

Heaviest Part

Using heaviest part seems usually the strongest for me. The reason being is that the heaviest part is usually the largest fuel tank, that fuel tank is usually at the middle. Using heaviest part autostrut means most parts are closer to the heavier part then root or grandparent, and while grandparent won't solve all problems heaviest will universally make the strongest vessel if all people would auto use this setting on every part. The reason why it works best is because the closer the autostrut connection is, the stronger that connection holds up, the same goes for using the strut part connector. Dragging a heavy part further out will bend more under stress when using any autostrut or strut part connector.

In practice "Heaviest part" it is best used on parts in the radial stacks like boosters and extra fuel stacks for larger space planes. This involves most of the fuel tanks, cones and engines other then the one directly attached to the core stack which should be set to grandparent.
Using it on the bottom will make sure the bottom parts of radial stacks which are usually fuel tanks and engines stay in symmetry to the center stack as this will stop bending them under stress.

Root Part

Root part is best used on parts that are closest to it that are not in the center stack at which the root part is located other then those directly attached to it. In that case using root is similar to using grandparent, which is the reason why  
On a regular rocket root part is usually a command pod on top. Using root on the top cones on any radial stacks or any fuel tank below there will stiffen those parts in aligment with the root and center stack.

Also, using a very small middle sections like on a 0.625 or 1.25m probe core that could be in the center of a stratolauncher type aircraft then the very large fuselages on the side would be very flexing unless every part of those fuel tanks were strutted to root instead of heaviest.

In other words, if the root part is not a very heavy part that takes in a lot of stress from the heavier parts then it is best to set root autostrut on the heavier parts.
In case of a space plane, the heaviest part is usually a fuel tank in the middle. To that fuel tank are wings, engines and landing gear attached that take in impact force from landing. When radial outer parts selected to heaviest the wing, landing and weight stresses of those outer parts will be strengthened more.

In relation to the above, that center fuel tank could be the root part, but usually it isn't. Imagine that you have any vessel, rover, space plane, aircraft or rocket to which most of those stresses are located at or near the root part, like in the example of a stratolauncher vessel. In that case strut every part to root that you would normally strut to heaviest.

 

Edited May 8, 2019 by Aeroboi

[RocketSquid]

@Aeroboi so I should use grandparent if it’s supposed to dock with something, heaviest otherwise, and use rigid attachment only when I want to avoid bending at all costs?

[Serenity]

9 hours ago, RocketSquid said:

I’ve found the autostruts very helpful, but recently realized that I’m probably using them wrong. My current strategy is usually just to turn rigid attachment on and set autostruts to heaviest part on anything I think will be subjected to a decent amount of force, mostly ignoring the other options for autostrut targets. When is each option best; and when is it best to have no autostruts? How does rigid attachment differ from autostrut: parent?

Be very careful when using HEAVIEST part.When you dock with something that has a heavier part you might call the kraken the moment you undock!

Also RIgid Attachment will frequently cause your craft to rip apart!(Landing, Docking/Aero forces etc, smashing in stuff accidentally)

I recommend using Grandparent and Root if you have a really tall rocket to launch!After it hits orbit make sure you switch the heaviest or root autostrut back to grandparent just in case!

Hitting Alt+F12 and selecting the Visualize Autostrut is a very effective way to make sure you have a good ''tree'' of grandparent autostruts so the whole thing is strong.

 

Edited May 8, 2019 by Boyster

[Aeroboi]

5 hours ago, RocketSquid said:

@Aeroboi so I should use grandparent if it’s supposed to dock with something, heaviest otherwise, and use rigid attachment only when I want to avoid bending at all costs?

In all cases the answer is yes, however, if a vessel and space station aren't very complex in terms of part count then docking them shouldn't be a problem in most cases. That's why I recommended to quicksave and if necessary change autostruts before docking.