I did not go to space today (aerodynamics problems)

[Clear Air Turbulence]

I'd appreciate some critique of my rocket

I haven't had problems with somersaulting rockets for a while. But this one can't decide where to point its pointy end. Now it's the sky, now it is the ground...

The payload is fairly light (just a scanner probe really, I want to go mining). I put a fairing on like they told me to. I have those RV winglets, and reaction wheel for a bit of attitude control.

The moment I go above 350 m/s it starts veering all over the show like a shopping trolley with a wonky wheel. I have tried several times, trying with low gimbal limits and with high. What am I doing wrong?

any help will be appreciated!

Edited November 14, 2015 by Clear Air Turbulence

[stormdot5]

It looks to me that th epoint behind the payload where the rocket diameter goes from 1.25m to 2.5m is a weak point. This allows the fairing to go off center and develop drag on the tip of the rocket, whice at transonic speeds is enough to overcome the stabilising fins at the bottom. The Thrust to weight ratio of the mainsail is good to get the rocket off the pad but it will get too fast too low in the stmosphere to remain stable. Suggestions:

1. Strut between the main rocket and payload to prevent wobble

2. Reduce throttle when you are close to 300m/s and then gradually increase speed once you are above 10km. Once you get to 25km ish, you should be able to go full throttle again.

I would also suggest switching to a 2 stage rocket. Not for any more dV, though that would likely result. A second stage would be much smaller and would mean your SAS and RCS systems could more easily rotate and adjust the rocket. You could even make that stage recoverable it you wanted.

[Clear Air Turbulence]

Thanks. Getting rid of that weak point (I just put in a bigger fairing) improved matters somewhat. But now it just takes longer before we go somersaulting. I get up to 570 m/s at about 25 km

I wonder whether it could be centre of weight problems, with the rocket becoming more unstable as the fuel in that big stage gets used up?

- - - Updated - - -

OK, staging sorted it. I did think that rocket was too big. One stage to get me to 20 km, then a short stubby stage with winglets got me into orbit easily. Now for a 20 minute ion engine burn to get to minmus. Thanks!

Edited November 14, 2015 by Clear Air Turbulence

[Snark]

I see three problems.

First is the weak point that the previous poster pointed out.

Second is that having that weak point is also a big aero drag up near the front of the rocket, which will contribute to aerodynamic instability. Make your rocket a smooth cylinder, especially up near the front.

Third is that you're going to have CoM problems, for two reasons. First, your payload is light, as you said. Any time you have a big, light payload in front of a heavy rocket, that's a recipe for trouble. The other problem is that you have a vertical stack of fuel tanks. That's almost always bad, because your engine will drain the tanks from the top down, which causes your CoM to move sharply downward as you go faster, which is the exact opposite of what you want.

It's always better to have one tall tank than a stack of smaller ones, because the tall tank's CoM stays put as it drains. I'm guessing you don't have the big orange tank researched yet?

One way to mitigate the top-down draining problem for tank stacks: go to the top tank in the VAB and disable its resources. That will prevent it from draining at all, and will help to hold your CoM forward. Then as your other tanks get near empty (and you're a lot higher in the atmosphere where aero stability doesn't matter as much), you can renable it to get at the fuel.

[Greenfire32]

Try disabling the engine gimbals. You don't really need the gimbals inside the atmosphere anyway, only in space does that REALLY start working for you.

[Alshain]

Don't disable the gimbal, disable the pitch, yaw, and roll on the fins, or swap them out for ones that are not control surfaces.

You might consider using a 2 stage stack, you will need far less fuel that way and in fact you can get rid of the mainsail and use a skipper/poodle a lot cheaper. Just make sure your skipper stage can get you up to ~30km. This will also make the rocket shorter and easier to fly. EDIT: oops, didn't read far enough, guess you figured that out already.

Edited November 14, 2015 by Alshain

[Jovus]

The basic problem is probably that your center of pressure (or center of lift in KSP) is above your center of mass. You can both push your center of mass higher up the rocket, and push your center of lift lower down the rocket.

First, for the center of mass: you've mentioned that your payload is small. Rockets fly best when the mass is concentrated (as much as possible) at the nose. Paradoxically, a heavier payload would fly truer.

Second, for the center of pressure: there are a lot of suggestions here. First, you want to minimize drag on the front of the rocket. This involves, as other posters have said, getting rid of that unsightly divot where you drop to 1.25 up at the top. If you have the Procedural Fairings mod, use a structural fairing. Otherwise, I'm afraid your choice is limited to either redesigning the rocket to do without that upper stage, or using a 2.5m engine.

In addition, you want the fairing to be as small as possible while still not causing a disconnect in diameter between itself and the rocket. If you can, shrink the fairing. Further, a teardrop shape is less drag than any other, whereas you appear to have something more akin to a bullet shape. (Disclaimer: I fly FAR, and don't know if stock KSP properly models coefficient of drag according to such shapes.) Finally, you can employ bigger fins: try sticking the small delta or even regular delta on the bottom with a control surface, rather than the AV-R8s you have.

As an end note, you can also get around this problem by keeping SAS on, having sufficient control authority, and being careful not to deviate too much from the prograde vector. This is how most orbital lifters actually do things, since fins are extra mass.

[Clear Air Turbulence]

All these are helpful suggestions.

I am running up against a technical limit at the moment - my 2.5 metre fairing is simply not big enough to cover the whole payload. Is that normal?

Yes, I guess that most of the problem is related to centre of mass problems related to that long body.Breaking it up into smaller stages seems to have made a big difference. I am also going to have to build a small transfer stage for the journey out to Minmus. Going all the way there with an ion engine is just too tiresome.

[eddiew]

You shouldn't need to break it into stages; if you can pump fuel around, just keep the front tank full during flight and you'll likely be fine. Rockets work much better when they drain the rearmost tank first, but alas... that ain't KSP's mechanic

[BoilingOil]

That little thing under the fairing in the picture, what is it? If it's a stack separator or similar device, I'd put it *inside* the fairing, at the bottom. That way, when it is activated, they are both separated from the payload.

At the same time, it would remove the drag-inciting bottle-neck in the design. That might already be helpful, though perhaps not as much as other suggestions that have been made.

In fact, if it's a reaction-wheel, I'd probably *still* hide it inside the fairing.

Edited November 14, 2015 by BoilingOil

[Clear Air Turbulence]

@boiling oil that's zacktly what i did. Though I find that I can only put so much in the fairing... then the fairing can't be extended to cover it.

@eddiew, I like my stubby, high-twr second stage!

[Harry Rhodan]

my 2.5 metre fairing is simply not big enough to cover the whole payload. Is that normal?

Considering that you are not using the 2.5m but the 1.25m fairing I guess it's normal.

[BoilingOil]

@boilingoil that's zacktly what i did. Though I find that I can only put so much in the fairing... then the fairing can't be extended to cover it.

True, there is a limit, but your design had not reached that yet

- - - Updated - - -

Considering that you are not using the 2.5m but the 1.25m fairing I guess it's normal.

Actually, they said that *after* having mentioned that they replaced the fairing with a larger one. I guess that's the 2.5 mtr version

[Clear Air Turbulence]

Yep, I was having trouble with the 2.5. Somehow it sometimes just does not want to extend long enough. And then sometimes it does. It looks as if the changing colours on the text that flashes up has something to do with it.