I'm out of answers... Re-entry question

[GabeTeuton]

I would describe myself as an experienced KSP player, probably not even close to a professional, but usually i can pull my own weight around if i may put it in those words...

however i can't seem to fully understand what is going on... the thing is i started a new career mode not long ago, i have the last mile to go before unlocking the entire tech tree, so at this point i thought it would be a good idea (a little late maybe...) to start using pre-designed first stages based on weight to be placed in orbit, and might aswell go on and try to recover those stages once they are done lifting the thingy...

i started with a 5 tons weight and designed a rocket that was able to put those 5 tons easily into orbit and spare around 1k delta v depending on my ascend profile (yeah i know 1k delta v to spare is waaaay too much, does not matter to the question at hand)... that works fine... but whenever i try to reenter the spent booster and land somewhat close to ksc, at some point usually around 20k mts (or so) the rocket losses all of its "stability" and flips trying to face the air "transversally", doesn't matter if i'm entering prograde or retrograde, with airbrakes on or off... and i can't figure out why, i suspect the CoM has something to do with it but i'm not entirely sure... (i tried moving the fuel around, sitting naturally balanced, all on airbrakes side, all on engine side and it's still the same behaviour)

i would assume the brakes should be farthest away from the "front" facing part, so i designed it that way for entering retrograde (engines forward), despite that and despite if i'm reentering forward or backward, at some point it wants to face the air "horizontally" and then inmidiately breaks appart due to aero forces... which makes sense, even if it didn't break i would still find it annoying...

I'm using FAR, i know i could try and analize all of its data windows, but what's actually painful is that i can't find "intuition" on what's going on, if i throw a tube parabolically (with one side weighted let's say) how does the tube "likes" to fall, weight forward, or backward? and why does the rocket wants to face "long side" the air stream, how can that happen? i can't rationally undestand that...

^^^^^Fuel towards the bottom^^^^^^

 

 

 

 

^^^^^^^^fuel towards top^^^^^^

 

 

 

 

 

^^^^^^empty or balanced fuel^^^^^^

[Vanamonde]

The design looks sensible enough. Weight at rear and drag up front should make it act like a dart. Is the SAS setting getting confused, perhaps? 

[18Watt]

Looks to me like it should work.  I would think the airbrakes when deployed would be enough to keep it pointed the correct way.  However, maybe the drag from the fins at the engine end is too much.  Perhaps try it without the fins near the engine, at least to see how it handles re-entry.

If removing the lower fins makes it stable on re-entry, then at least you'll know what you're up against.

[GabeTeuton]

9 hours ago, Vanamonde said:

The design looks sensible enough. Weight at rear and drag up front should make it act like a dart. Is the SAS setting getting confused, perhaps? 

i tried disabling SAS as my game senses tell me it should keep whatever profile it had when entering the atmo, i also tried keeping it on when i discovered it wants to face the stream the wrong way, i also tried different reentry profiles, very agressive, or half around the glob on the >70k:<40k alt, and either way at some point in the thicker atmo below bit tries to face the stream the wrong way...

 

4 hours ago, 18Watt said:

Looks to me like it should work.  I would think the airbrakes when deployed would be enough to keep it pointed the correct way.  However, maybe the drag from the fins at the engine end is too much.  Perhaps try it without the fins near the engine, at least to see how it handles re-entry.

If removing the lower fins makes it stable on re-entry, then at least you'll know what you're up against.

just tried it seems a little bit more stable, but if for any reason i close the airbrakes, everything goes to hell again

i wonder... the writing PERSHING is made up of flags in 2-way simmetry, could those be breaking everything?

last attempt i managed to made a safe landing 2.2 km from ksc, however it depended a lot on me having fuel to burn at around 20k to bring my actual impact point to that mark (being over 200km away otherwise) so trajectories for some reason is not "detecting" or accounting for the brakes to be deployed or not, the prediction does not change based on that

 

edit: thanks for the help so far!

Edited August 11, 2022 by GabeTeuton
forgot to be thankful to people trying to help, my bad

[king of nowhere]

2 hours ago, GabeTeuton said:

i wonder... the writing PERSHING is made up of flags in 2-way simmetry, could those be breaking everything?

it could be that they are generating additional drag.

you can activate drag visualization in different ways, from the alt-f12 menu-physics-aerodynamics. check if those parts are making drag. or just try to remove them and see what happens. worth a try

[GabeTeuton]

26 minutes ago, king of nowhere said:

it could be that they are generating additional drag.

you can activate drag visualization in different ways, from the alt-f12 menu-physics-aerodynamics. check if those parts are making drag. or just try to remove them and see what happens. worth a try

yeah i made a test with the flags on the vessel and drag visualization active, and they had a tiny arrow, but an arrow none the less on each of them, and then removed them all and tried the re entry again without SAS on, position the rocket in a retrograde profile before entering the atmo and it stayed there while the airbrakes where open until the end... then tried it again trying to compensate for the effect they make on impact point, and managed to land a second time, i need some time to do some more testing but i believe the flags were to blame... i downloaded conformal decals to try and "decorate" the rockets that way... maybe those won't generate drag... seriously the only reason i'm trying to set up this kind of RP (having predefined first stages for different payloads) is to have those named and decorated accordingly, if i can't i might aswell abandon this RP u.u.

i'll post any news as soon as i can for closure!

[king of nowhere]

Just now, GabeTeuton said:

yeah i made a test with the flags on the vessel and drag visualization active, and they had a tiny arrow, but an arrow none the less on each of them, and then removed them all and tried the re entry again without SAS on, position the rocket in a retrograde profile before entering the atmo and it stayed there while the airbrakes where open until the end... then tried it again trying to compensate for the effect they make on impact point, and managed to land a second time, i need some time to do some more testing but i believe the flags were to blame... i downloaded conformal decals to try and "decorate" the rockets that way... maybe those won't generate drag... seriously the only reason i'm trying to set up this kind of RP (having predefined first stages for different payloads) is to have those named and decorated accordingly, if i can't i might aswell abandon this RP u.u.

i'll post any news as soon as i can for closure!

sometimes the game is bugged. i once had a perfectly sound laythe lander that failed because it was made by two docked parts, and the game was modeling it as if both docking ports - that were docked with each other and perfectly in line with the stack - were fully exposed to the air flow,, thus tripling the drag my poor rocket was experiencing.

But you could use the bug to your advantage! if those decorations are causing drag, you could try to move them on top of the rocket, so that they act as airbrakes. you could even skip the actual airbrakes at that point.

[GabeTeuton]

8 hours ago, GabeTeuton said:

yeah i made a test with the flags on the vessel and drag visualization active, and they had a tiny arrow, but an arrow none the less on each of them, and then removed them all and tried the re entry again without SAS on, position the rocket in a retrograde profile before entering the atmo and it stayed there while the airbrakes where open until the end... then tried it again trying to compensate for the effect they make on impact point, and managed to land a second time, i need some time to do some more testing but i believe the flags were to blame... i downloaded conformal decals to try and "decorate" the rockets that way... maybe those won't generate drag... seriously the only reason i'm trying to set up this kind of RP (having predefined first stages for different payloads) is to have those named and decorated accordingly, if i can't i might aswell abandon this RP u.u.

i'll post any news as soon as i can for closure!

well i believe i have enough evidence to conclude the flags were the problem...

8km away from ksc, needed SAS because 2 of the 4 winglets broke during launch... besides that, not a single issue... (this is the 20t variant...), tested with the 5t variant and the same result, no issues... (after changing the flags with decals from the amazing mod...)

 

thanks guys, @king of nowhere, @18Watt and @Vanamonde for the support/suggestions, and the least i can do is also thank the guys from Conformal Decals for making the mod that helped me solve this problem by (first) not causing it

 

Edited August 11, 2022 by GabeTeuton
grammar