Rocket Design Question

[kananesgi]

First off, I don't know if this is the right place for this, but it makes sense. I'm at work and using my phone, so browsing is rather difficult.

I have a number of mods installed, but I think the only ones affecting this issue are SpaceY, MRS, and FAR.

I'm trying to build a small lifter to carry a rover on a sub-orbital flight to the far side of Kerbin. The mission is to fulfill a survey contract and test some ideas for a planned Mun rover mission.

I've built a rover and placed it atop a launch vehicle consisting of 1 stock 800 FLO tank and 1 400 tank, with the Swivel type engine under them. A couple of structural fuselage pieces sit atop that, with the rover mounted inside a small procedural fairing. There is also a small reaction wheel and a second small drone core inside the fairing. The fairing expands to about 2.5m to incase the rover. All other components are the standard small size (1.25m?). Some B9 procedural full-flying stabilizers finish it off.

Apologies for the rather vague description. As I said, I'm at work, and will be til the wee hours of the morning. When I get home, I can make a better description and maybe some pictures. Just wanted to get the ball rolling before I get home.

Anyhow, the rocket is highly unstable. At first, I had some small SRBs from SpaceY on it, but the moment it reached 200m/s,it would pitch over and explode. I thought perhaps too much power, so I reduced the number, then eliminated the SRBs completely. It still goes out of control at 200. It also does so if I try to maneuver away from straight up at all.

If I control the throttle on the swivel and keep the velocity below 180 during the ascent, I can keep it flying straight up till it's out of the thick of the atmosphere. Then I can maneuver, but that's incredibly hard on fuel and the engine wants to overheat.

I'm thinking this is caused by drag and something like area ruling on the fairing, but I haven't a clue how to fix it. Any ideas?

[Vanamonde]

Rocket Builders is more for people to ask others to build ships for them, so your thread has been moved to Gameplay Questions.

[Empiro]

Your intuition about it being drag is likely correct. I'd also check to see how the center of lift compares to the center mass (you want CoL to be under your CoM). The quickest change would be to increase the size of the winglets in the back.

[JTpopcorn]

bigger winglets further down should increase stability. Also, if the nose is really top heavy, your rocket could "fall over" if speed is to low. Remeber, you need to pitch really slowly, but with FAR thats been standard for a while.

[sdj64]

Your rocket probably wants to flip because it is too lightweight and draggy at the top and too heavy at the bottom (sort of like an arrow or a dart flying backwards). If you eliminate the "couple of structural fuselages" (what were those for anyway?) you can probably reduce the problem because then your payload would be closer to the center of mass and therefore there would be less torque. Also try locking the FL-T400 tank until you are done with the larger tank and then unlock it so you can use it.

[cantab]

The fairing expands to about 2.5m to incase the rover. All other components are the standard small size (1.25m?).

That's asking for trouble. Your fairing has four times the cross-sectional area of the rocket below it resulting in massive drag, and it's also relatively light.

The best solution is to make the fairing not much wider than the rocket beneath it, which means going to a 2.5m rocket. The next best solution is whopping great big fins on the bottom of your rocket. They don't need to move and it's probably better if they're fixed, but like the fins on a dart they'll keep the rocket flying straight. Since you're using FAR remember to right click them and take the strength down, the default is for a fighter/acrobatic level of robustness.