Autostrut options--what difference do they make?

[Klapaucius]

Can someone explain why and under what circumstances they would choose one of the various autostrut options (heaviest part, root part, or grandfather part) over another?  What implications are there in the KSP physics model?

[Superfluous J]

I use "Root part" a lot on things like fairing bases, because frequently the root part (your capsule) is on top and that makes it so that the vessel is very secure to the base. You can also put them on a couple parts in your main stack to keep the ship a bit more rigid while rising through the atmosphere.

I use "Grandparent" on ALL docking ports so whenever you dock, the ships latch on with an extra strut from that docking port to whatever the other port is attached to. I also use them on ALL boosters attached with radial decouplers, so they get an extra thing attaching them to the center stack.

I use "Heaviest part" infrequently, but it works well for asteroids as that will always be the heaviest part and it's frequently attached otherwise quite poorly to your vessel.

The implications are about the same as if you used a regular strut, except there is one less part, and nothing to add mass or drag to your vessel. However, you don't get to pick HOW they attach and it's always from the center of one part to the center of another part. If you can live with that restriction (and in 99% of cases you can) the autostruts are ALWAYS the way to go.

Don't overdo them though as too many struts (of any kind) can confuse the engine and cause uncontrolled wobbling that can tear a ship apart.

Edited October 8, 2018 by 5thHorseman

[bewing]

If you have ever used KJR, then "Grandparent" is basically the same as what KJR does automatically. It is the safest kind of autostrut -- the least likely to make your ship destroy itself through stress buildup. If you want to stiffen up an entire section of your craft, then you (obviously) need to use it on at least every other part in your subassembly, so you typically need a lot of grandparent autostruts when you use them.

Root and Heaviest work for quickly making long-distance struts. Such as from the tip of a wing back to your fuselage. This can add a lot of strength and stiffness using very few autostruts. If you think you may end up in a situation where you have to turn off all the autostruts on your craft, then having very few of them is extremely helpful. If one section of a space station is going all wobbly, then putting a Root or Heaviest autostrut from that section back to some more sturdy part of your station can stop the wobblies.

However, Root and Heaviest autostruts are also very dangerous -- especially for stations and bases. When you dock two vessels together, the root part and/or heaviest part of the combined craft may suddenly change. This will cause all existing Root and Heaviest autostruts to forcibly disconnect and then reconnect to the new correct part, one by one. This process can involve large forces, and can easily be violent enough to destroy a craft. So it may be necessary to manually disable a bunch of autostruts before doing a docking. (Note that landing legs and wheels automatically use "Heaviest" autostruts, always.)

As 5th Horseman said -- especially for Root and Heaviest autostruts, be careful not to overuse them because they can easily store stress in a craft that will destroy the craft. My rule of thumb is that 5 is usually safe enough.

 

[fourfa]

Great posts. Just to break it down even more simply - root and heaviest can change in flight and when docking, and have been the cause of many space station kraken strikes. Grandfather never changes, so it’s safer but perhaps not quite as powerful. 

[Superfluous J]

Grandfather can change. Upon docking one ship will lose its root part and then many of its parts will flip their parent/child orientation as the other ship's root part becomes their root part.

[fourfa]

oh good point.  That's probably happened to me but never had a disaster yet...