Why is this fighting me?

[Perotis]

I've been trying to get a scanner lauched and every time it starts a gravity turn, it either flips or breaks apart.  I've tried many different configurations to launch this turd into orbit.  But even this simple, straight forward, design just breaks apart as soon as the turn starts.  I DON'T UNDERSTAND!

Edited March 14, 2016 by Perotis

[Findthepin1]

I don't know. 

EDIT: Try adding struts. 

Edited March 14, 2016 by Findthepin1

[Xavier513]

That is a big engine! Ascending too fast, perhaps.

[lobe]

I found that long, skinny rockets tend to be more stable, (probably due to the greater arm from the center of pressure and mass) and keeping the crosshairs on the navball inside the green prograde circle usually helps.

[Perotis]

One of my first attempts was an orange tank with a mainsail first stage and then a half tank with a poodle.  It was nice and long, but still didn't survive the turn.   

 

Im using mech jeb ascent guidance.  I've tried adjusting the hight and the angle of the turn.  I don't seem to be getting any progress towards figuring this out.  

[Mastikator]

Start doing the gravity turn instantly. I mean RIGHT after you launch. A VERY slight turn, a degree or two at most and then just maintain prograde.

Actually that rocket probably has a huge TWR so you should be at 10 degree right at the start. That way you never go off prograde but still maintain a strong and efficient gravity turn.

[maceemiller]

Not sure if youve got one already but if not try adding a reaction wheel. I had an issue like this quite a while ago and it worked for me.

 

Edited March 14, 2016 by maceemiller

[Mait3rz]

24 minutes ago, Perotis said:

One of my first attempts was an orange tank with a mainsail first stage and then a half tank with a poodle.  It was nice and long, but still didn't survive the turn.   

 

Im using mech jeb ascent guidance.  I've tried adjusting the hight and the angle of the turn.  I don't seem to be getting any progress towards figuring this out.  

I had this same issue last night, I'd just finished launhing 2 interplanetary ships, a station to dock with them and also a lander, launched and docked them all in orbit no problem. Then I went to build a scanner probe to dock to the ships and send to Duna. I had the same issues it seems to be very prone to flipping out. In the end I used an orange tank main sail, with X 4 grey 1.25 m drop tanks, fuel lines and asperagus staging. Along with 4 delta deluxe winglets, had to use RCS to help stabilise it. Also had to start the gravity turn quite late.

You may also want to try removing the fairing all together, on my design I did keep it however in the past I've had quite a few issues with fairings actually causing the craft to flip. Correct me if I'm wrong but I believe there is some sort of bug with the way fairings calculate drag?

Edited March 14, 2016 by Mait3rz

[FullMetalMachinist]

51 minutes ago, Perotis said:

I've been trying to get a scanner lauched and every time it starts a gravity turn, it either flips or breaks apart.  I've tried many different configurations to launch this turd into orbit.  But even this simple, straight forward, design just breaks apart as soon as the turn starts.  I DON'T UNDERSTAND!

 

It's likely that you're running into a combination of two bad things.

The first is that you have a light, bulky payload on top of a big fuel tank/engine combo.  That means that your CoL is likely in front of CoM.  A stable rocket has the opposite. You want your CoM as far forward as possible (like a dart or arrow).  One thing I would suggest is to get rid of all the 2.5m parts all together. That scanner sat should fit just fine on top of a long 1.25m rocket. You'll probably need two stages. One with a Swivel engine and a couple of the longest 1.25m fuel tanks, and then one stage with a terrier engine and one medium 1.25m fuel tank.  That should get you really close to orbit, and if it doesn't, then add more fuel, and possibly a couple SRB's to the first stage.

The second problem is that fairing. Unless you already know about this and have the fix, you're probably experiencing a nasty bug with the fairings. What happens is that the lift and drag forces that the fairings create are applied way out in front of the physical part. Which means those forces have a huge lever arm to wrench your nose around backwards.  The fix for this is Claw's Stock Bug Fix Modules.  This puts the lift/drag forces back where they should be on the base of the fairing. 

[oglommi]

Add more weight to the top of the rocket. Or lock the fuel/oxidizer in the top tank and unlock it as the bottom tank is emptied.

Edited March 14, 2016 by oglommi

[Deutherius]

8 minutes ago, FullMetalMachinist said:

It's likely that you're running into a combination of two bad things.

The first is that you have a light, bulky payload on top of a big fuel tank/engine combo.  That means that your CoL is likely in front of CoM.  A stable rocket has the opposite. You want your CoM as far forward as possible (like a dart or arrow).  One thing I would suggest is to get rid of all the 2.5m parts all together. That scanner sat should fit just fine on top of a long 1.25m rocket. You'll probably need two stages. One with a Swivel engine and a couple of the longest 1.25m fuel tanks, and then one stage with a terrier engine and one medium 1.25m fuel tank.  That should get you really close to orbit, and if it doesn't, then add more fuel, and possibly a couple SRB's to the first stage.

The second problem is that fairing. Unless you already know about this and have the fix, you're probably experiencing a nasty bug with the fairings. What happens is that the lift and drag forces that the fairings create are applied way out in front of the physical part. Which means those forces have a huge lever arm to wrench your nose around backwards.  The fix for this is Claw's Stock Bug Fix Modules.  This puts the lift/drag forces back where they should be on the base of the fairing. 

This. You can verify the second problem by enabling aerodynamic forces overlay (F12).

31 minutes ago, Mastikator said:

Start doing the gravity turn instantly. I mean RIGHT after you launch. A VERY slight turn, a degree or two at most and then just maintain prograde.

Actually that rocket probably has a huge TWR so you should be at 10 degree right at the start. That way you never go off prograde but still maintain a strong and efficient gravity turn.

Also this. If your rocket breaks apart right at the start of a gravity turn, then you are turning too late, too much and too sudden. With such a powerful engine and light load it might be beneficial to turn on fine control (capslock) to avoid oversteer.

[Nich]

More like 15 or 20

[T.A.P.O.R.]

3 hours ago, Perotis said:

I've been trying to get a scanner lauched and every time it starts a gravity turn, it either flips or breaks apart.  I've tried many different configurations to launch this turd into orbit.  But even this simple, straight forward, design just breaks apart as soon as the turn starts.  I DON'T UNDERSTAND!

Just forget the fairings.

for some reason unbeknownst to me they don't quite have  joints with the rigidity of the rest of the rocket parts.

this will cause your payload to flollop about and tip you over.

some people can use them but they don't work for me.

Edited March 14, 2016 by T.A.P.O.R.

[Mjarf]

I put several Vernor thrusters on my lifting rockets to force it to go straight, this combined with careful maneuvering and several strategically placed struts make for a nice and stable rocket. I've lifted similar payloads on larger lifting rockets with no issues, might be because it has 3 Mammoth engines giving it plenty steering torque. I also set the gimbal limit to 30% to make it turn more gently, reaction wheels haven't been necessary with this configuration. I have noticed the fairing bug, and my rockets were more stable without fairings even with relatively un-aerodynamic payloads.

Edited March 14, 2016 by Mjarf

[AlamoVampire]

Well, first, stop using the stock "fairings" as they are not all that friendly. Look into a mod called Procedural Fairings. They are far easier to use and far more forgiving. Also, try adjusting mechjebs ascent profile. The default one is a bit ugly. Lastly, while that tank/engine combo is good for a booster stage, its not suited as a primary lifter. The other suggestions are good too. Try a combo plate of our suggestions some combo will fix you up nice  

[Kerbin vonKerbal]

The stock fairings are bugged. Next time you try launching it, turn on your aerodynamic forces display (F12) and you'll see the drag for the fairing actually starts above the top of it. 

[Warzouz]

Yes I think this is it : fairing bug + probably too high TWR. This satellite doesn't need such a big rocket. You did a rocket SSTO which is useful when you want to recover it. Also, the adapter is not needed and use de real decoupler not a stack separator. This will reduce weight of the fairing if you don't want to remove it.

A lighter launcher will also cost you less. a nice TWR is from  1.3 to 1.7. Add a engine and fuel under you satellite. Then add a first stage helped with boosters if needed.

As for fairings, use the fix or don't use fairing or use it backward (but I don't know how you can fix it).

[Wemb]

Yup - what the others have said - try flying it without the fairing - I bet it's a lot more stable.  That and your launcher is colossally overpowered, so if you are going to fly, it don't fly it at anything like 100% thrust.

 

Wemb

[sal_vager]

Moved to Gameplay Questions.

As your fuel is consumed the balance point, or center of mass (CoM) of your craft moves downwards.

But aero forces act on the whole length of the rocket, the center of lift (Col) would be a bit below half way up the rocket due to the fins at the base providing a larger aero surface

Once the CoM has moved far enough below the CoL that the fins can't provide enough force to maintain stability the rocket will try to fly backwards.

You need more mass in the nose, this can be done by using an upper stage with its own tank and engine so the top of the rocket acts like the head of an arrow, keeping the CoM in front of the CoL.

[AlextheBodacious]

Try removing the adapter, it's a weak link.

[Warzouz]

48 minutes ago, AlextheBodacious said:

Try removing the adapter, it's a weak link.

But as the mass on top of the adapter is small, that doesn't matter, I think. The adapter is useless, use weight and make the fairing bigger and heavier. As the fairing is bigger, the fairing bug is stronger.

[AlextheBodacious]

On 3/14/2016 at 6:40 AM, Warzouz said:

But as the mass on top of the adapter is small, that doesn't matter, I think. The adapter is useless, use weight and make the fairing bigger and heavier. As the fairing is bigger, the fairing bug is stronger.

Top heavy rockets are nothing but death.

[Tex_NL]

On 14-3-2016 at 7:35 AM, FullMetalMachinist said:

The second problem is that fairing. Unless you already know about this and have the fix, you're probably experiencing a nasty bug with the fairings. What happens is that the lift and drag forces that the fairings create are applied way out in front of the physical part. Which means those forces have a huge lever arm to wrench your nose around backwards.  The fix for this is Claw's Stock Bug Fix Modules.  This puts the lift/drag forces back where they should be on the base of the fairing. 

This is the ONLY correct answer (others have said the same but FullMetalMachinist was first) in this entire list of answers. Get the bug fixed and it WILL fly correct.

On 14-3-2016 at 11:16 AM, sal_vager said:

Moved to Gameplay Questions.

As your fuel is consumed the balance point, or center of mass (CoM) of your craft moves downwards.

But aero forces act on the whole length of the rocket, the center of lift (Col) would be a bit below half way up the rocket due to the fins at the base providing a larger aero surface

Once the CoM has moved far enough below the CoL that the fins can't provide enough force to maintain stability the rocket will try to fly backwards.

You need more mass in the nose, this can be done by using an upper stage with its own tank and engine so the top of the rocket acts like the head of an arrow, keeping the CoM in front of the CoL.

How is it fragging possible even SQUAD staff is incapable of providing the correct answer? If any it should be SQUAD that knows about this stupid bug.

[ForScience6686]

Still all the fairing bashing.  They still work fine for me.  in fact, a fairing just  protected an 80t payload to orbit for me.  If the payload moves, more struts.  If you're flipping, stop getting off prograde and add fins.  Forcing the gravity turn mid flight is not a good idea, use thrust to make adjustments.

[Plusck]

4 hours ago, ForScience6686 said:

Still all the fairing bashing.  They still work fine for me.  in fact, a fairing just  protected an 80t payload to orbit for me.  If the payload moves, more struts.  If you're flipping, stop getting off prograde and add fins.  Forcing the gravity turn mid flight is not a good idea, use thrust to make adjustments.

Yes, I agree.

Also, I noticed that people are talking about "surviving the [gravity] turn" and MechJeb ascent guidance.

It has been said many many times, but could perhaps be repeated again here:

The "gravity turn" is by definition, and must in reality always be, a gentle and gradual affair that cannot go against the airflow. The fact that old versions of KSP allowed sharp turns is regrettable. I don't have MechJeb but it sounds like MechJeb will try to force a sharp turn if you tell it to.

A sharp turn against the airflow will destroy rockets IRL and also (now) in KSP. While you're in the atmosphere, prograde is the direction of the airflow. Try suddenly turning 20° against the airflow at mach 1 in anything other than a fighter jet (and even then...) or air-to-air missile and see what happens.

Fairings are problematic because they are big empty shells. Whether they are "bugged" or not is actually far less important than the essential point about not turning a big, long empty shell at an angle to the airflow.

Edited March 19, 2016 by Plusck