Rocket Flipping During Gravity Turn

[KomradeKerman]

Hello there.  I've been designing a LKO tourist vehicle, and my latest version has begun to hit some strange snags.  Despite the COL being below the COM at all times, this rocket tends to flip over whenever I begin my gravity turn.  Even the slightest nudge east causes it to spin.  Any suggestions?

[Numerlor]

Where is the CoM?

[Harry Rhodan]

16 minutes ago, KomradeKerman said:

COL being below the COM

Here's the problem: The CoL indicator will only show you the lift produced by wings and winglets. It is not showing you the body lift or the Center of Pressure of your rocket and that enormous fairing. Using the CoL indicator for rockets is basically useless.

Your payload is probably to large and light and using a slim and tall second stage will regularly just give the aerodynamic forces a longer lever to work against you while not adding much mass to raise the CoM.

[AeroGav]

I'd install CorrectCoL  and RCS build aid, I think this is another case of where the stock CoL indicator gets it wrong.

That fairing is the same diameter as the first stage,  and it is longer,  but it is mostly air.    CoM is going to be way down low, with the SRBs and the core stage.    Like I say, that fairing has same diameter, and more length than the core stage.   Admittedly you don't have the additional area of the boosters,  but i suspect the CoM is way closer to the boosters/core stage than the fairing, so the fairing got a longer lever arm to torque you with.

TL:DR - get CorrectCoL,   and it will make you realise you need to put fins on it.

[Gordon Fecyk]

That's an odd design. I concur with Gav and Harry in that the rocket is probably too bottom-heavy for this.

What's inside that fairing? Perhaps it's something that can be launched with a smaller launch vehicle or a different one. Just getting tourists to LKO in a single command pod plus probe can be as simple as this:

...using this:

 

 

[KomradeKerman]

3 hours ago, AeroGav said:

I'd install CorrectCoL  and RCS build aid, I think this is another case of where the stock CoL indicator gets it wrong.

That fairing is the same diameter as the first stage,  and it is longer,  but it is mostly air.    CoM is going to be way down low, with the SRBs and the core stage.    Like I say, that fairing has same diameter, and more length than the core stage.   Admittedly you don't have the additional area of the boosters,  but i suspect the CoM is way closer to the boosters/core stage than the fairing, so the fairing got a longer lever arm to torque you with.

TL:DR - get CorrectCoL,   and it will make you realise you need to put fins on it.

Well, I've installed CorrectCoL, and it does not seem that the fairing really does anything to change the center of lift or aerodynamic stability.  I have followed this up with flight tests without the fairing, and I am encountering the same problem.

2 hours ago, Gordon Fecyk said:

That's an odd design. I concur with Gav and Harry in that the rocket is probably too bottom-heavy for this.

What's inside that fairing? Perhaps it's something that can be launched with a smaller launch vehicle or a different one. Just getting tourists to LKO in a single command pod plus probe can be as simple as this:

...using this:

[snip]

This is inside the fairing.  It's meant to carry six tourists and a pilot.

[StrandedonEarth]

When in doubt, slap on some fins, as low as they'll go. If at first that doesn't work, use bigger ones.

Unrelated: I think those airbrakes should be facing the other direction...?

[Harry Rhodan]

Again: Don't look at the CoL indicator. Your main problem is that your CoM is extremely low while balancing a long payload and a thin second stage that weigh next to nothing on top of it. Attacking forces on the top of your rocket have an extremely long lever to work against you while the fins and the engine have an extremely short lever to fight against it.

My solution would be to make the payload shorter and to use a shorter but heavier 2.5m second stage to keep (a) the lever for the bad forces shorter and (b) to raise the CoM, the fulcrum of the lever a bit to make it even shorter for the bad forces and longer for the good ones.

And you are using tweakscale: As far as I can remember the part mass will rise with a factor of 2³=8. Depending on which engine you chose that means your engine alone will weigh ten to twelve tons. That is more than your whole second stage! A normal 2.5m Skipper only weighs 3t. That will not only pull your CoM even lower but will also make your rocket less fuel efficient at the same time.

[Gordon Fecyk]

This being a 1.25m craft, you don't need 2.5m parts to launch it. That, plus the fins that everyone's reminding you to use, not only seem to keep the thing under control on launch, but I think it'll cost less too. You don't even need that fairing.

Here's this craft's file. The decouplers have fuel crossfeed enabled so this does asparagus-like staging without separate fuel lines, but keep an eye on the booster tanks to know when to stage them away. You should reach orbit with your seventh ascent engine intact and with fuel remaining, saving those Spiders as a last resort for manoeuvres.

I stashed the OKTO in this service bay, along with batteries and an extra reaction wheel. The air brakes are the only things that will help with control on descent, but mind the deflection amount during re-entry or they'll melt.

[qzgy]

4 hours ago, KomradeKerman said:

This is inside the fairing.  It's meant to carry six tourists and a pilot.

Why do you have that in the fairing in the first place? I think it would be ok outside the fairing, with the heatshield on a decoupler. Also, whats the probe core for? You already have a pilot.

[The Aziz]

Not sure how cost-efficient you want it to be, but you can also use Mk2 pod + Hitchhiker module. Same capacity, much shorter, a bit wider.

[Urses]

7 hours ago, Gordon Fecyk said:

The air brakes are the only things that will help with control on descent, but mind the deflection amount during re-entry or they'll melt.

One question. I think the airbrakes are installed as he will brake in nose forward position?

1 hour ago, The Aziz said:

Not sure how cost-efficient you want it to be, but you can also use Mk2 pod + Hitchhiker module. Same capacity, much shorter, a bit wider.

2nd Question. Would it not make the mass of the returnveicle higher and lower the TWR of upperstage?

Funny Kabooms 

Urses

[The Aziz]

12 minutes ago, Urses said:

Would it not make the mass of the returnveicle higher and lower the TWR of upperstage?

Probably, but you can do anything with Poodle. Plus 1/4 the size of orange tank should be enough for local trips.

And I think this configuration should make it easier to stabilize at re-entry.

[Urses]

8 minutes ago, The Aziz said:

And I think this configuration should make it easier to stabilize at re-entry.

I Think it would Upgrade the ascend too.

Bring CoM higher up. He need more tweaking this way was my though. 

Funny Kabooms 

Urses

[Aegolius13]

7 hours ago, The Aziz said:

Not sure how cost-efficient you want it to be, but you can also use Mk2 pod + Hitchhiker module. Same capacity, much shorter, a bit wider.

For 5 kerbal ships, I like to put a Mk 1 passenger cabin on top of a Mk 1-2 command pod.  It stays very stable during reentry thanks to the low center of mass, and makes use of the command pod's better heat and crash resistance.  I haven't tried two passenger cabins for a 7-kerbal ship, but I imagine it would work similarly.  

[Snark]

17 hours ago, StrandedonEarth said:

I think those airbrakes should be facing the other direction...?

8 hours ago, Urses said:

I think the airbrakes are installed as he will brake in nose forward position?

Note that it doesn't matter which way airbrakes are installed-- they're equally effective at braking regardless of which way up they are.

 

[StrandedonEarth]

6 minutes ago, Snark said:

Note that it doesn't matter which way airbrakes are installed-- they're equally effective at braking regardless of which way up they are.

 

Yeah, it just looks wrong aesthetically. In RL if they're backwards the airflow would be trying to rip them off, but KSP probably doesn't model that

 

Edited April 2, 2017 by StrandedonEarth

[Kryxal]

Airbrakes can be set up either way, but IRL that way is usually for ones you're not meant to retract again.  With that craft, I'd worry about coming in too fast or losing the airbrakes to overheating.  Do you have enough torque and battery capacity to generate some body lift once in a safe range?

[Spricigo]

Based in the OP's craft and purpose , I designed this

There is room for improvement (deltaV in orbit its just a bit over the needed to deorbit, last stage TWR is pathetic, no docking port). But it brings eight kerbals to orbit and back for about 10k funds and not a single touch at WASD keys. Actually with a few Smart parts attached it can do the whole trip with 4 control imputs, lazyness at the finest.