Auto Struts and Rigid Attachments

[MisterKerman]

KSP has never been the most forthcoming with it's gameplay quirks and auto strutting/rigid attachments until recently have eluded me as a necessity.

From what I understand auto struts simply act as a direct invisible strut that clips through every other part and snaps to either the root, parent, grandparent, or highest mass part. There seems to be no disadvantage from what I can tell.

I wish I knew about Rigid Attachments however. I have a space station with a bunch of goofy stuff hanging off of it right now and it's crazy wobbly.

The way they work from what I can tell is rigid attachments just fuse two parts together when you attach them during assembly.

The disadvantage appears to be loss of flexibility between the attached parts which could result in destruction of said parts when stress is put on them, such as landing a lander when the legs are designed to negate the impact of contact with the surface you're landing on.

What are some simple guidlines to follow when considering using these features to design craft? I'm hesitant to make atmospheric lift utilizing aircraft wings rigid due to the stress they encounter, or landing gear, but I'm not very knowledgeable about it. It seems like they're both probably utilized most effectively in certain situations even though they're both used for increasing structural integrity in some way and I feel like for new players it would be nice to have it all in one place for a quick reference; or at the very least for my own curiosity if you guys would indulge me.

[Geonovast]

Are you willing to use mods?  Because KJR is absolute magic, and you don't need to turn it on on every. frakin. part. like you do with autostruts or rigid attachment.

[bewing]

Rigid attachment was invented because the devs thought that wiggly composite airplane wings were unreasonable. So you can rigidize them, and maybe it helps and maybe it doesn't. Maybe rigid wings can RUD during high-G maneuvers. I never tested it, and I don't know if anyone else has, either. I do know that it makes joints somewhat more fragile -- because you can get away with some really crazy bending without breaking for non-rigid joints.

Autostruts were invented to help with noodly-rocket syndrome, and to prevent wheels from splaying under load. They are invisible, free, and zero drag. They can reach a long way -- and that is useful sometimes.

Grandparent autostruts are safe and effective.

With the other varieties of autostrut, you need to be very careful of Autostrut Reattachment Disease. If you have an autostrut to the Heaviest part (usually a fuel tank), and you use up the fuel to the point where it's no longer the heaviest part -- the autostrut has to disconnect from the old heaviest part and attach to the new heaviest part. This process is very forceful, and can be violent enough to destroy a craft or spacestation. The safe thing to do with these other types of autostruts is to use very few of them ... just enough to make your craft stiff. Like, try limiting yourself to 5 per craft.

 

[MisterKerman]

1 hour ago, bewing said:

...

Grandparent autostruts are safe and effective.

With the other varieties of autostrut, you need to be very careful of Autostrut Reattachment Disease. If you have an autostrut to the Heaviest part (usually a fuel tank), and you use up the fuel to the point where it's no longer the heaviest part -- the autostrut has to disconnect from the old heaviest part and attach to the new heaviest part. This process is very forceful, and can be violent enough to destroy a craft or spacestation. The safe thing to do with these other types of autostruts is to use very few of them ... just enough to make your craft stiff. Like, try limiting yourself to 5 per craft.

 

I've used a lot of them for a recent refueling craft... ×m×

What about docking? I've noticed craft names changing due to docking and undocking. Would auto-strutting have a reaction to that? I'm legitimately worried now.

Edited February 6, 2019 by MisterKerman

[bewing]

Quote

What about docking?

The root part has 50% odds of changing during docking. So any part on either craft with autostrut that's set for the Root part has a 50% chance of suffering from ARD. (Either one ship's root will change, or the other one will -- so if you have Root autostruts on both craft then the odds jump to 100%.) It is wise to turn off all Root autostruts before docking.

Similarly with Heaviest autostruts. If the new Heaviest part is on the other craft during docking, then the Heaviest autostruts will detach/reattach. So similarly, turn all those off during docking too. The problem with that is wheels and landing legs. They always have Heaviest autostruts turned on, and you can't turn it off. Note that this is true even if you don't have autostruts "turned on" in your settings. Landing legs and wheels always have autostruts.

So for craft with wheels or landing legs, either 1) don't ever dock them, 2) make sure the "heaviest" part on your docking craft remains the heaviest part, even after docking (by adding fuel to it, because it's usually a fuel tank), or 3) make sure that the two craft being docked are smallish, because Autostrut Reattachment Disease hits big ships really hard, but is mostly not noticeable for small ships. And if you absolutely must dock a ship with legs/wheels to a big station, then quicksave first and cross your fingers.

 

 

Edited February 6, 2019 by bewing

[MisterKerman]

I haven't noticed any jarring physics anomalies during or after docking. I hope everything is okay, but I had a full Jumbo64 on board my STTO the last time I docked which consisted of parts that I believe were heavily autostrutted to the heaviest part when I built it. I'll have to look into this but I might be deorbiting that station just to be safe. I wanted to start fresh anyways... I guess...

I have a save I made just before making my SSTO and upgrading my runway so I'll load that one up and use the funds to send new station modules up in the SSTO instead. I think the save file for the craft will still remain. I desperately need a more efficient lander and transfer booster to cut time spent delivering fuel, so it seems perhaps the time was right to run into catastrophy.

[bewing]

Autostrut problems during docking will either destroy your ship within seconds, or you are completely safe. There are no long-term consequences.

[Aeroboi]

@MisterKerman

Generally its best to grandparent autostrut all the spacestation modules. Or other modules for any other vessel like a mothership that is constructed in orbit.

Usually the cargo that consists of any of your spacestation modules is on the top of your rocket, mostly encased in a fairing.

They are generally well connected especially when they're faired so the cargo usually doesn't require heaviest or root part autostrut for rigidity anyway. The lifter itself however which could be quite large has especial need for strutting to heaviest or root part.
A Spacestation itself also is under microgravity, or almost all lack thereof (in i.e. solar orbit) so it isn't under strain. Only when you interact with it by docking is there a physics interaction. IME having grandparent autostrut on all the modules is sufficient.

You also have to plan ahead. If you want to make a ginormous spacestation but your center modules are connected with the 1.25m docking port then the center connection points are weak so you might want to plan ahead and only use the sr docking port if your intention is to make a much larger space station.

Remember that you can change root in the editor. It is best for the root part to be in the middle of your rocket. This is usually one of the center fuel tanks.

I personally never use rigid attachment. The only occasion where I used it was for a large contructional tug to haul heavy cargo on the surface made from I beams. The rigid attachment makes sure that any attachments are 100% rigid and cannot be bent. If I hadn't used the rigid attachment the construction would wiggle on the joints and the vesel would steer sideways, worse even clip inside and break apart.

Furthermore it's perfectly safe to use heaviest part on your spacestation. Especially if it's wide and sticking out with a lot of modules a center heaviest part connection point could be ideal for optimal rigidity.
However, you will have to plan ahead. If your intention is to dock vessels to that station that have a heaviest part that is heavier then the space station consists off then it will change and physics may rip it apart.

If your intention is to never dock anything heavier then a regular MK3 cargo spaceplane or 2.5m rocket ship module a heavier kerbodyne fuel tank on your station could be a good option. All you have to make sure is that you never dock a part that is heavier or as heavy as that heavier part, or you will have to re-strut all the heaviest parts back to grandparent if you do so.

Remember that these physics bugs don't always surface, usually when the spacestation is part heavy or is over extended in height, width or length.

 

[MisterKerman]

Well I had only used autostrutting for my single SSTO cargo shuttle and docked with a wobbly space station. Everything seemed to be okay. I didn't realize that autostruts reoriented themselves to the new heaviest part at any point so uhhh... thanks for more unpleasant secret surprises KSP.

The space station is just a cylinder. It's not very long either. This is how it looks when it has deorbited it's spent fuel payload. I included a small transfer/slosh tank under the hitchhiker storage containers for that purpose, and will just use the transfer booster as a fuel tank in the future to reduce weight and part count.

My only question left is what exactly the behavior of autostrutting parent or grandparent parts is in regards to docking 2 separate craft together, such as space station modules. Is there any way to ensure added rigidity between the modules once constructed in space? Or once a parent or grandparent strut-to-location is assigned it never reorients? (Heaviest part is obviously the main culprit for that, but if I make a space station in pieces it would be sort of cool if there was some non-cheaty way to ensure rigid connections as the station will never be put under stress.)

My station is very small so it's an irrelevant question right now but this information is legitimately hard to find for current versions of the game from a source you can guarantee is credible. I'd prefer to not stay in the dark because they're actually a really useful game mechanic that should be taken into consideration no matter what sort of craft you're engineering, and I plan to use them lots in the furture.

[bewing]

There is no such thing as a parent autostrut. Because of the tree structure of craft, the grandparent part always stays the same. Once you are docked, you intentionally create "root" or "heaviest" autostruts across the docking port to stabilize the station to create rigidity. It should also work to autostrut your docking port to grandparent if you initially build your craft with a detachable root part above the docking port.

 

[MisterKerman]

9 minutes ago, bewing said:

There is no such thing as a parent autostrut. Because of the tree structure of craft, the grandparent part always stays the same. Once you are docked, you intentionally create "root" or "heaviest" autostruts across the docking port to stabilize the station to create rigidity. It should also work to autostrut your docking port to grandparent if you initially build your craft with a detachable root part above the docking port.

 

Wait... can you change the autostrut arrangement/type after you've launched your craft? I'm not sure I understood what you mean by "intentionally create root or heaviest autostruts"

I've only recently activated advanced tweakables and travel for work a lot so I'm sort of legitimately too busy to play the game and experiment. I appreciate your willingness to discussing theory instead of me having to figure it out on my own.

[bewing]

3 hours ago, MisterKerman said:

can you change the autostrut arrangement/type after you've launched your craft?

Yes, that's the other major benefit of autostruts. You can turn them on and off, and change their type in flight.