Rocket turns over after first stage

[Cairol]

I'm trying to assemble a space station in Kerbin orbit.

First I brought a 2.5 t power module into orbit with no problems at all.

Now I try to bring a 5.7 t science lab into orbit.

But when I jettison the SRBs of the first stage, the rocket immediately turns over. It's too head heavy after the SRBs are gone.

How can I reduce the head heavyness without going overboard with my fuel tanks?

I have already more than enough delta V to get the payload into its designated orbit.

Edited June 1, 2015 by Cairol
Answered

[Wanderfound]

No, it isn't "head heavy". It's head draggy.

Mass at the front of your rocket is good; it promotes stability. You want your centre of mass as high as possible.

Drag at the front of your rocket is bad; it promotes instability. You want your centre of drag as low as possible.

So: skinnier, more streamlined payload if possible. Much bigger fins at the base of the rocket as well. And gimbal is your friend; strapping a quartet of radial engines around the base of your core rocket would provide a lot of control authority. It's what the Vernier engines (e.g. Thuds) are designed for.

[cybersol]

Head heavy is actually good, but head draggy is bad. It's really hard to see what your rocket looks like after the SRBs leave from the pic you posted, plus you are not showing staging. Can you post one where they and the launch stabilizers are removed or dropped?

Meanwhile, here are some general tips:

Spinning out of control essentially comes down to a contest between drag and control. When the drag overcomes your control authority, you go for a spin. You can decrease drag by staying closer to prograde and building a more aerodynamic rocket. You can increase control using engine gimbals, stabilizer wheels, and control surfaces.

Also note that drag increases significantly right around mach 1 (340 m/s). So you should stay pointed as prograde as possible at speeds around 290-400 m/s.

On a rocket control fins should be as low and outside as possible. If you are using control fins on more than one stage, the CoL needs to stay below the CoM at all times, even as the tanks empty.

Edit: some ninja action!

[mbartelsm]

the problem is not the weight of the head per se, but the amount of drag it generates, imagine shooting an arrow with the fins at the head, it will just turn around. My suggestion would be using fuel tanks with a bigger radius and shorter length, if you don't have those available yet then work with a more horizontal rocket (shorter tanks, more asparagus stages)

Been ninja'd

[Motokid600]

Hm. There's some very strange symmetry your using with that craft. Why are the SRBs not symmetric? And then there's something strange with your fins at the rear. They too don't appear to be symmetric as well. I see one sticking out the right side, but not the other. So that can't help.

I'm not sure if KER is showing sea level thrust in that readout, but 2.0 is too much for the first stage. Try 1.6 at the max.

And another issue is your rocket isn't heavy up top enough. You say "too head heavy" well that's exactly what rockets need to be. You want your thrust to be as far away from the CoM as possible. And I can see that your CoM is already really low. Are those srbs doing all the work of the first stage with a mid air firing of the second? If so your CoM probably isn't moving much and the rocket is too light up top to be controllable.

What's in that fairing? CoM should be much higher with a payload that size. So for starters fix your symmetry. Then work on what some of the others have said. Your rocket does have a good bit of drag up top, but honestly that is doable with what you got.

Edited May 31, 2015 by Motokid600

[Cairol]

Hm. There's some very strange symmetry your using with that craft. Why are the SRBs not symmetric? And then there's something strange with your fins at the rear. They too don't appear to be symmetric as well. I see one sticking out the right side, but not the other. So that can't help.

The SRBs and also the fins are actually symmetric. Not per SRB but as a whole. Those 4 SRBs create a centered CoM in the core stage. I jettison them all at once. So I don't loose the centered CoM.

Same for the fins. Each SRB has only one, but each one has a different orientation. So all 4 again give a symmetrical air control flow. (at least I hope this last sentence makes sense...)

In the fairing holds a mobile processing lab, a schience Jr module and some smaller parts. Mass of the payload is 5.7 t total.

My control systems are:

4 delta wings with attached elvons at the core stage.

4 controlled fins at the outer LFB boosters (1 in each direction)

4 controlled fins at the SRB boosters (1 in each direction)

2x SAS between the center tank and the orbiter stage.

4x RCS thruster in the orbiter stage

The outer LFBs are asparagus-style connected to the center tank.

Staging is:

1. Detach stability enhancer and fire all 9 boosters. (I go 60% for the LFBs and 100% for the SRBs in this stage)

2. When SRBs are burned out (@ about 12 km), they are jettisoned and the 5 LFBs are set to 100% power.

3. When outer LFBs are burned out, I jettison them. The core stage is remaining.

Rest is in-orbit action.

The current behaviour is this:

Start with active SAS -> all good

At about 230 m/s the body begins to wobble a bit. SAS can compensate. (If I use the MechJeb autopilot instead of manual flight, the body begins to wobble like mad at this speed. IRL it would rip apart.)

When I jettison the SRBs at about 12 km, the whole thing flips. After one flip, I can get it under control and steer it to orbit. But this flip is still a total no go...

Rest of the flight is without problems.

OK. So I actually need to get that CoM up and maybe change the layout of my booster stages to get that thing stable.

I've probably seen every video of Scott Manley twice. But it seems like rocket science is in fact erm....rocket science.

Edited May 31, 2015 by Cairol

[Geredis]

Mokotoid - looking at it, it seems that it's a 4-4 rocket - 4 outer boosters (the 'missing' fourth can be seen vaguely on the far side through the structural supports - the tail fin and engine nozzle gives it away), 4 secondary engines up on the radially on the main column that seems to only exist to support the payload node and a stack of control modules like SAS or probe cores that seem to serve primarily as anchors for the radial inner ring of rockets.

Looking though at the way his fins are aligned I think that might be a big part of the problem as well. They're at these odd angles on some of the boosters it looks like. Simply having htem stick 'straight' on them along the center line of each stack might help as well.

[Motokid600]

Hm. Well if you continue to have issues Cairol maybe post more pics with different views.

[Norcalplanner]

Might also want to make your fairing pointier. Some of the other posts in the past week showed that fairing shape at the tip has a big effect on drag.

[cybersol]

From your description, which says more than that weird angled picture, you need more control or symmetry on your 4 LFO stage. As overkill, I would try 4x2=8 AV-R8 in a very symmetrical arrangement (i.e. each 45* off the axis from the central core). If you post the craft file somewhere and its stock, I would also check if I can help more.

[match]

SRB's are creating angular torque due to the offset connection point (think of the decoupler as the fulcrum of a lever and visualize where the force goes). The fins are able to maintain control authority until staging relieves both the torque and the opposing force. Equilibrium is disturbed and flip ensues. Once back in plane, equilibrium is retored.

~Just a guess, but who knows?

Also... "head heavy" and "head draggy"? I have never heard those terms before. Gotta be correct, both posted within moments of each other. Must be industry jargon! I gotta keep up with these new terms...

[Cairol]

One more note: I installed FAR yesterday without knowing it would also effect my rockets. I thought it's made for airplanes only.

So this is another factor why my rocket is so susceptible.

I only realized this today while watching a tutorial video for FAR...

FAR shows alot of drag at the fairing on the top. And when I remove my SRBs, my center of lift rises dramaticly in direction of my CoM.

So that totally explains why my rocket flips immediately after jettisoning the SRB stage.

This nifty little FAR tool that shows drag as a curve projected on the model is really a fine thing.

And what I also saw: When removing the fairing completely, the drag on the tip reduces a lot. That's a bit odd, isn't it?

Because my payload is not even close to beeing aerodynamic.