Identical craft, different aerodynamics

[QF9E]

I ran into something weird a couple of days ago: my Eve lander worked perfectly when I tested it separately: it ascended into Eve orbit from its landing in the Explodium Sea. However, the same craft could not complete its ascent during the actual mission.

I've done some testing, and my conclusion is that it matters which component is the root component. If I make the fairing the root component, the center core generates a lot more drag compared to when I make one of the fuel tanks inside the fairing the root component. See pictures below for these differences:

This is with the fairing as root element. Note the very large drag spike of the center core, and that boosters are empty at this point.

This is the same craft, but with the upper stage fuel tank (inside the fairing) as root element, at about the same airspeed. Note the much smaller central drag spike, and the fuel remaining in the boosters.

Does this sound familiar to you? Is this a known bug, or if not, is it worth adding to the bug list? Could this be the result of a mod I am running? This is a complete list of mods of my game, according to CKAN:

• Click through blocker
• Docking port alignment indicator
• HyperEdit
• Kerbal Alarm Clock
• KER
• Precise Maneuver
• Trajectories
• Transfer window planner
• TriggerAu flags
• Warp Everywhere

Nothing in this list jumps out at me saying "this mod messes with the aerodynamics while flying", but I could of course be wrong.

 

 

 

 

 

[Jodo42]

Can confirm this is an issue in KSP 1.8.1, either with the base game or one of the DLCs, don't have time to check right now. Also not sure if this is already known or not. Test rocket:

Test results below. 5 flights, all at 4x physwarp from before liftoff, no SAS

Choosing the Oscar-B and Thumper as root parts had, within the margin of error, identical results. Choosing the fairing as the base part had radically different effects; the vehicle deviated from a straight-up trajectory below 3km and peaked below 6km before inverting while still firing and splashing down hard. Apoapsis was recorded through the map screen for the booster and fuel tank tests; fairing test apo came from F3. The fairing-root flights also had a much higher degree of unpredictability regarding their trajectory, although whether that's because the game is treating it differently or just because the numbers are smaller is not clear.

[Foxster]

Also confirmed. 

Making a fairing root increased the atmospheric drag of a test craft at 5km from 1.61m/s to 2.66m/s (according to MJ) and the craft achieved a lower altitude before using all the fuel.

https://imgur.com/EyNl9zp

Obvious workaround is to not make a fairing the root part - not sure why you'd want to in the first place. 

Edited November 11, 2019 by Foxster

[Streetwind]

A bug is still a bug, even if it has a workaround  

Please check if an issue exists on the bug tracker (https://bugs.kerbalspaceprogram.com/). If not, you can open one. @Jodo42's test case is very useful to include (ideally, attach the vessel file).

[Pds314]

NEVER make a fairing the root part. I have found that doing so with a boat removes the fairing's buoyancy. Which is ashame because it means the only way to autostrut it is to make it the heaviest part. If I had to guess I would say root part fairings are similar to nonexistent fairings.

Edited November 12, 2019 by Pds314

[Thygrrr]

Wow that explains a lot of problems I have had with payloads that I saved with their fairings.

 

[QF9E]

Good to know that others are seeing the same. I can confirm it is not the DLCs, as I don't have them. 

In my case the problem arose in my "Douglas Adams" mission to Eve. As I only had 42 parts to play with I coupled my command module to my lander (which had a command seat inside a fairing) via one of the fairing nodes. And since the capsule in my command module was the root node for the entire ship, after decoupling the fairing became the root node of the lander, which then failed on ascent because of this issue. 

My hunch is that if the fairing is the root, the aero system reacts as if you don't have a fairing at all, just the flat base plate.

[HansAcker]

26 minutes ago, QF9E said:

My hunch is that if the fairing is the root, the aero system reacts as if you don't have a fairing at all, just the flat base plate.

That appears to be the case if I understand this old report right: https://bugs.kerbalspaceprogram.com/issues/5987

I learned not to place fairings as root parts early on as it comes up every now and then in the forum.

[MechBFP]

Oh wow, this bug has been around forever. I would just open a new ticket and reference the old one. They are NEVER going to bother looking at a ticket from such an old version.