Why my Rocket is unstable

[Pawelk198604]

I want make Minmus flyby to get money from contract and gather some of spicy scientific data:)

[Kaos]

The flight does not look unstable. So I do not understand your problem and cannot answer the question.

[GoSlash27]

Pawelk,

I don't understand what you mean by 'unstable'. It looked okay to me.

Best,

-Slashy

[Pawelk198604]

Pawelk,

I don't understand what you mean by 'unstable'. It looked okay to me.

Best,

-Slashy

It was stable because i must constantly use SWAD :-) It was actually my 10th launch of that rocked design all previous 9 was humbled and lost control on some part of flight :-(

This time i succeed fortunately

[softweir]

Show us your rocket. Also, what mods are you using?

[katateochi]

Not 100% sure what you're asking because it looks like it flies ok. Only things about it's design I would note are that you have a goo container on just one side of the probe core at the top, that may cause the craft to pull to one side a bit while under thrust. But a goo container isn't much weight, but still I would balance that with either another goo container or something else of the same weight. I'd also make the fairing fit tighter around the payload, but that's more an aesthetic choice than one that will affect it's flight (may have some effect).

From a flight point of view, I'd suggest you want to be pitched further down sooner on. Rough guide I follow is when the Ap is at 20km be pitched to 40 degrees, when Ap is at 40km be pitched to 20 degrees, and by the time the Ap is at 60-70km be pitched right right over to ~0-5 degrees (rough guide, different craft need different approaches).

edit

(also, best to post these kinda Qs to Gameplay Questions and Tutorials)

Edited June 21, 2015 by katateochi

[Pawelk198604]

Show us your rocket. Also, what mods are you using?

https://drive.google.com/file/d/0Bx2uGwZ4w808NE5lY3MyUm5JZlk/view?usp=sharing

No mods only Kerbal Engineer

[RandomRyan]

looks OK to me. only your gravity turn is a bit off

Try getting to 45 degree mark at 10k

(PS: try adding wings.(cause' it helps!)And try to make the fairing look more realistic)

also, press f12 to overlay the aerodynamics.(the fairing makes a lot of drag

[Juicy]

It was stable because i must constantly use SWAD :-) It was actually my 10th launch of that rocked design all previous 9 was humbled and lost control on some part of flight :-(

This time i succeed fortunately

Did you try launching it without any fairings but with a symetrical load instead? Most of the time, I find fairings more hassle than they're worth.

[*Aqua*]

What made my rockets instable:

- atmospheric forces

- atmospheric forces

- weak joints

- CoM

- CoM

- poor piloting ability

What made my rockets stable:

- adding wings

- streamlining the craft

- more struts

- changing the size of the stages (so it doesn't become too top-heavy)

- center the CoM by adding counterweights or something similar

- concentration!

[Pawelk198604]

Thanks

Edited June 21, 2015 by Pawelk198604

[Pawelk198604]

Mission to Mimnmus

[BagelRabbit]

I always link my little tutorial to rocket aerodynamics when people discuss instability, though there are numerous other great tutorials out there...

...but to make a long story short:

In KSP 1.0+, fairings are extremely draggy, thus raising the so-called 'center of pressure' to the point where the rocket becomes quite unstable. Adding fins on the bottom will help the rocket maintain better stability throughout the ascent, at the cost of slightly higher overall drag. But if you can't get your rocket to space without that little bit of extra drag, there's no point in flying it anyway.

By the way, congratulations on your successful Minmus voyage! Glad you finally managed to wrangle the rocket into working

-Upsilon

[Pawelk198604]

I always link my little tutorial to rocket aerodynamics when people discuss instability, though there are numerous other great tutorials out there...

...but to make a long story short:

In KSP 1.0+, fairings are extremely draggy, thus raising the so-called 'center of pressure' to the point where the rocket becomes quite unstable. Adding fins on the bottom will help the rocket maintain better stability throughout the ascent, at the cost of slightly higher overall drag. But if you can't get your rocket to space without that little bit of extra drag, there's no point in flying it anyway.

By the way, congratulations on your successful Minmus voyage! Glad you finally managed to wrangle the rocket into working

-Upsilon

Thanks

It was actually the continuation of my unsuccessful launch

I think that if did not have to fight with my rocket in initial phase of flight i wold can do even Mimnus orbit instead fly by

[DeMatt]

I want make Minmus flyby to get money from contract and gather some of spicy scientific data:)

Lemme see...

• No fins. A set of 4 Delta-Deluxe Winglets on your main stack will help you SO much.
  
• Weird two-part fairing. Try making it three parts, or better yet, moving the fairing base up to the top of the Materials Bay so you only include the under-1.25m parts.
  
• Underfueled boosters. Try reducing the side stacks from 4 to 2, and then doubling the fuel in the side stacks. Or more, and add fuel lines running inwards so the center engine draws from the side stacks. Or switching 'em out for solids.
  
• Underthrottle. You reduced throttle because the side boosters were too powerful, then didn't increase it when you staged them off.
  
• Steep launch. This is more of an effect and not a cause, solve the above problems and you should be better able to handle the atmospheric flight.
  
• Late node planning. You kept making "fussy" burns during the coast from burnout to circularization, when you should've been planning the circularization burn instead.
  

That's my commentary on that design and launch.

[Arkalius]

One thing to note is that you have several fuel tanks stacked up on that bottom stage. They will drain top to bottom which causes your CoM to shift to the bottom of the rocket, which makes it more unstable (the drag at the top applies more torque). This CoM shifting would happen even with a single large fuel tank, but it's more pronounced when you use several smaller ones. You can try and disable the fuel feed from the top ones and enable them as needed in order to resolve this (or even have the CoM shift upward, increasing stability). You can also just do manual crossfeeds.

[Pawelk198604]

Lemme see...

• No fins. A set of 4 Delta-Deluxe Winglets on your main stack will help you SO much.
  
• Weird two-part fairing. Try making it three parts, or better yet, moving the fairing base up to the top of the Materials Bay so you only include the under-1.25m parts.
  
• Underfueled boosters. Try reducing the side stacks from 4 to 2, and then doubling the fuel in the side stacks. Or more, and add fuel lines running inwards so the center engine draws from the side stacks. Or switching 'em out for solids.
  
• Underthrottle. You reduced throttle because the side boosters were too powerful, then didn't increase it when you staged them off.
  
• Steep launch. This is more of an effect and not a cause, solve the above problems and you should be better able to handle the atmospheric flight.
  
• Late node planning. You kept making "fussy" burns during the coast from burnout to circularization, when you should've been planning the circularization burn instead.
  

That's my commentary on that design and launch.

One thing to note is that you have several fuel tanks stacked up on that bottom stage. They will drain top to bottom which causes your CoM to shift to the bottom of the rocket, which makes it more unstable (the drag at the top applies more torque). This CoM shifting would happen even with a single large fuel tank, but it's more pronounced when you use several smaller ones. You can try and disable the fuel feed from the top ones and enable them as needed in order to resolve this (or even have the CoM shift upward, increasing stability). You can also just do manual crossfeeds.

Thanks and what do you think about my Cheap Sat launcher? (In the end it was not as cheap as i wanted because i loss two SRBs that i planned to recover)

I wanted to make something like Delta rocket with SRB

Edited June 22, 2015 by Pawelk198604

[ColKlonk]

I find a tapered fuselage (pic below), works well in keeping the front pointed in the right direction, and eliminates the need for wings

That 'bulbous' fairing up top is going to be problematic wrt to drag... try streamline it with the rest of the rocket.

Also the more thrust you push.. the faster the ship destabilises. Slow down the ascent as necessary - there's no rush to get there

Edited June 22, 2015 by ColKlonk

[Harry Rhodan]

You are missing some of the basics:

- Press c to enable angle snap in the VAB.

- Use a TWR around 1.6 at launch. Not 5, not 2.5 and definitely not 1. If you're seeing the shock effects around your rockets you are going to fast . You can tweak down the thrust of the SRBs or throttle back your main engine but that will cost you efficiency.

- Try to turn a bit more smoothly and keep your heading inside the prograde marker until you are out of the thickest parts of the atmosphere. If you find yourself going to steep, start your turn earlier.

[eddiew]

If the problem appears mid way through the flight, it may relate to as the fuel draining. When you stack small tanks, the top one drains first, then the second, then third, with the result that your CoM moves backwards and can get very close to your fins. Manually pumping fuel from back to front during ascent can help

You also have a huge weight of rockets and no fins. If you can keep the TWR up, a pair of taller booster rockets would serve you better than a quartet of short ones.

[cephalo]

Like others have said, it looks like a pretty good launch. The reason why you had to fight the whole way is because the air resistance becomes a brick wall near the speed of sound. That means that even a small angle off prograde is going to slap the tip of your rocket sideways. The rocket is never aerodynamic travelling sideways, so you want to point prograde during that critical time when air resistance is great. Also you don't want to hit the speed of sound too low, as a thick atmosphere compounds the problem.