Problem with FAR Mun rocket

[Drethon]

I've been playing for about three months now and everything I send up with stock KSP runs without a hitch, so I decided to start playing with mods. I'm trying to create a somewhat basic science Mun lander but I'm having stability problems.

I'm trying to launch this:

My initial design was this rocket:

But during my gravity turn, also about the time the rocket broke the speed of sound, the rocket would tip over and start tumbling.

So I tried this(initially without the fins):

Obviously I don't know as much as I thought as this rocket had similar issues. By adding the fins I was able to successfully get the rocket into orbit and to the Mun but the rocket will only go straight up until about 20k altitude. This makes the gravity turn rather less efficient.

I suspect something is missing in my knowledge of FAR or Kerbal rocketry in general. I've searched on the topic but failed miserably at finding answers so I'm coming here for assistance. Thanks in advance!

Edited March 23, 2014 by Drethon

[Hodo]

You need to have some kind of control surface with those fins to help it turn or better gimbles on the rockets.

With FAR I usually start my gravity turn as soon as it hits 100m/s, and then I throttle back to keep it from going supersonic till I get over 10km altitude. If you are using MechJeb, I suggest setting your accent guidence to 30-35% and start the gravity turn at 1km altitude.

[lumpman2]

Realistic aerodynamics don't really allow for much building outward. If you add more and more radial tanks, the total surface area of the rocket that's exposed to air (basically all of the nose cones and the capsule) increases, whereas if you build upward, only the capsule and maybe a few other things are exposed to the air, greatly reducing drag. You might want to try Procedural Fairings, which creates a disposable shell around the top of your rocket to protect it from drag. Also, with FAR, flat surfaces are terrible and will almost never work in atmosphere. I'm not sure if you're doing the gravity turn right; you should do a veeeery gradual turn, following the prograde marker otherwise your rocket will flip out from the drag.

[Drethon]

Without the fins if I turned earlier and slower the rocket flipped backward instead of continuing the turn and started tumbling. However if I leave the fins on and turn at 100ms (I was typically turning as late as 10km or as early as 5km) and then follow the prograde down until the rocket is at 45 degrees it seems pretty stable. I bet the control surfaces or gimbals would have helped but I think I'l leave them off to work with the physics rather than trying to fight it.

I know building out doesn't work well with FAR but I'm trying to strike a balance between not being too wide but still having a wide lander for stability. Seeing how far I can take things.

Thanks for the input from both of you!

[Starstrider42]

lumpman2's got it right. You're getting problems with excessive drag on your lander. A fairing can help with that. Just make sure to edit the fairing higher up on the staging list, as the default value will separate the fairing while drag's still a big problem -- I've gotten flipovers from something as simple as a dish antenna while still 30 km up.

[LethalDose]

I had the exact same problem when I started using FAR earlier this year. I think what causes the tumble is your drag vector, thrust vector, and gets to far out of alignment with the direction of travel.

For example, if you accelerate to 300 m/s at 10 km traveling straight up, then the drag vector is straight down (I assume , and being counter acted by thrust vector, straight up.

Now, same situation, and you suddenly turn over into a 45 degree gravity turn, you're still traveling straight up (you don't suddenly change direction), you tumble. Your drag is still going straight down (opposing the direction of travel), but the thrust is no longer in line with the drag, so the drag produces torque and rotates the nose towards the ground.

And when that happens, you're gonna have a bad day.

What I have found works well is to turn gradually in the atmosphere, and increase control methods to counter-act the drag-torque that wasn't there in stock KSP. Control methods include fins (ones with control surfaces are better), SAS (okay for this situation), thrust vectored engines (better), and I find RCS (debateable). Personally, I find RCS to be really effective when you put it way up high on your stack, which is far away from the CoM in a launch vehicle, so it works as a lever arm to keep you pointed in the right direction. It's not an efficient use of RCS though.

In my experience, the best way to handle this FAR drag-tumble is to avoid making any sudden orientation changes while in the low atmosphere (the light blue part of the pressure indicator below the altimeter). With FAR, I nudge the heading off vertical about 5 degrees right off the pad, then I slowly nudge it over as much as the rocket tells me I can. If I see funny things happening, or RCS staying on while the heading is steady, I leave it alone until higher up.

Keep at it, and you'll get used to FAR's model. It's more challenging than stock, and learning how to finesse your rockets into orbit what makes it fun.

[Taki117]

You should also balance that Goo Can on the side of your rocket. It's causing uneven drag which could contribute to the instabilities you describe.

[Drethon]

Oh, the goo can was only unbalanced in this version. I had one on each side in other versions that did the same thing, just messed up trying to pull everything off to try with less drag. That didn't help much either. Seems to be an overall rocket thing.

[Streetwind]

Read what LethalDose wrote, it's important with FAR. If you turn your nose too far away from the prograde marker while in an atmosphere, your craft will flip and tumble. Doesn't matter if it's an aircraft or a rocket, doesn't matter if you're pitching or yawing. The more drag you are experiencing (i.e. going fast at low altitudes), the worse this problem becomes. People who are used to the general "fly straight up to 10k then make a full 45° turn" will instantly fall prey to this.

The fins and winglets counter this effect by making the rocket want to stay flying straight ahead on its own (provided you mount them at the bottom). This increases the leeway you have for turning away from the prograde marker. In some situations where you would flip normally, the fins will pull the rear back in line and thus keep the rocket going straight. They can only do so much though - if you turn far enough that you're offering the broad side of your rocket to the onrushing air, you will not go to space today.

Having a lot of drag at the top of your rocket - for example having a wide lander not covered by a fairing - does the exact opposite: it makes your rocket want to turn away from the prograde marker on its own.

Edited March 24, 2014 by Streetwind