Why does this design tip over at 10km?

[sormi]

I'm really confused about the aerodynamics of the launch stage. This design is based on my earlier spacecrafts, which retain full control perfectly. It really seems like adding just a little bit weight to the craft messes up everything. I recently watched this youtube video after getting frustrated with this thing. My design follows the line towards horizon perfectly, stays in line perfectly, but everything goes to ruin once I get over 10km and the thing just flips over.

 

http://steamcommunity.com/sharedfiles/filedetails/?id=588095675

http://steamcommunity.com/sharedfiles/filedetails/?id=588095758

http://steamcommunity.com/sharedfiles/filedetails/?id=588095812

http://steamcommunity.com/sharedfiles/filedetails/?id=588095887

http://steamcommunity.com/sharedfiles/filedetails/?id=588095949

http://steamcommunity.com/sharedfiles/filedetails/?id=588096005

http://steamcommunity.com/sharedfiles/filedetails/?id=588096087

http://steamcommunity.com/sharedfiles/filedetails/?id=588096161

http://steamcommunity.com/sharedfiles/filedetails/?id=588096242

 

I'm trying to launch 2 small probes into the orbit to try out the docking port. Launching 2 probes to the orbit really shouldn't be this hard :/

Edited December 30, 2015 by sormi

[Snark]

A few things.

• Take those four AV-R8 winglets and move them all the way down to the bottom, as low as they can go.  Right now they're in the middle of the rocket, which means they're doing practically nothing to help you stabilize (might even be making the problem worse).
• Streamline your front end, you need a nosecone on there.  Suggestion:  Move your probe core inside a 1.25m service bay, then you can put a nosecone on top.  You want to reduce drag on the front end of your rocket as much as possible.
• What does your flight profile look like? The video shows a not-very-good ascent profile that involves suddenly cranking over the orientation of the rocket, which is not only inefficient but also likely to cause flipping.  You want your rocket to stick very close to prograde the whole way up-- a gentle gradual gravity turn, not a sudden rotation like the video shows.  Having all those multiple screenshots in the VAB isn't super helpful, they're all basically the same-- what would be useful would be some shots of the rocket in flight, in particular what happens right around when it flips.

A description of how you're flying it would be useful.  Specifically:

• How high are you when you pass 300 m/s?
• How fast are you going when it flips?
• Do you make any sudden rotation maneuvers?

[Daripuff]

Not nearly enough fin down low, and too much up high. Replace those "basic" fins with full size ones.

Especially if it flips when you decouple the side boosters, with the fact the only fins you have on the core stack are mounted so high.

So, remove those fins in the middle, and install them on the base

[sormi]

Thanks for the help. The 10km really seems the most difficult part of every take off I've taken in the past aswell, and the more complicated the aircrafts go, the higher hurdle it seems to be.

I'll explain my logic behind this. I put control surface wings to top because I figured they would stabilize the craft if it starts overturning towards horizon, essentially softening the turns. I already did very very subtle turns though, so I guess those proved useless in the end. I don't have a cone at the tip of the ship because there is a docking port there and I'd like to keep it that way as long as possible.

So yeah I figured I should forget pretty much everything I saw in that video and stopped worrying about the angle too much. Just focusing all my energy to keep the thing in stable turn, turning as slow as it'll take as long as the craft keeps properly aligned.

I guess the most simplistic way once again prevailed  and I should not have gone overboard with the control surfaces. Now on to add one more stage for suborbit-orbit transfer and I guess I'm good to go to!

Thanks again.

Spoiler

 

 

Edited December 30, 2015 by sormi

[Snark]

49 minutes ago, sormi said:

I'll explain my logic behind this. I put control surface wings to top because I figured they would stabilize the craft if it starts overturning towards horizon, essentially softening the turns.

Control surfaces that are at (or very close to) the CoM won't help you.  They have no lever arm to work with.  It's like trying to move the steering wheel of a car by pushing on the hub of the wheel instead of the rim.

In terms of promoting aero stability:  you want your control surfaces (or passive fins) as far behind the CoM as possible.

It can also be useful sometimes to have active control surfaces as far in front of the CoM as possible, but those are a bit risky because they become less and less useful as your AoA increases, and when your AoA passes the max deflection angle of the control surfaces, they become a liability rather than an asset.

Anyway, congratulations for solving the problem!  Happy flying. 

 

[monstah]

1 hour ago, sormi said:

 I don't have a cone at the tip of the ship because there is a docking port there and I'd like to keep it that way as long as possible.

Docking ports act as a weaker decoupler, don't you know? They dont stage, but you can attach parts such as a nose cone inline with them, and in flight right click the docking port and choose "Decouple".

Edited December 30, 2015 by monstah

[sormi]

 

16 minutes ago, monstah said:

Docking ports act as a weaker decoupler, don't you know? They dont stage, but you can attach parts such as a nose cone inline with them, and in flight right click the docking port and choose "Decouple".

Oh, no I didn't. I was skimming through the wiki and found this:

Quote

There are six docking ports available for different sizes and docking directions. In order to dock two spacecrafts, both must be equipped with docking ports of equal size. When constructing a spacecraft, another part which is not a docking port can be placed on it. That part can then be undocked, but not redocked. This is a common mistake for beginners!

So I figured they don't act as decouplers so that you can't dock anything to it after you've decoupled something from it. I guess I misunderstood that part.

 

1 hour ago, Snark said:

Control surfaces that are at (or very close to) the CoM won't help you.  They have no lever arm to work with.  It's like trying to move the steering wheel of a car by pushing on the hub of the wheel instead of the rim.

In terms of promoting aero stability:  you want your control surfaces (or passive fins) as far behind the CoM as possible.

It can also be useful sometimes to have active control surfaces as far in front of the CoM as possible, but those are a bit risky because they become less and less useful as your AoA increases, and when your AoA passes the max deflection angle of the control surfaces, they become a liability rather than an asset.

Anyway, congratulations for solving the problem!  Happy flying. 

Thanks again for the help! Bought the game three days ago and I've been doing so many experiments and test configurations lately. I think I was starting to confuse myself and fail to find any logic behind this

Edited December 30, 2015 by sormi

[monstah]

42 minutes ago, sormi said:

So I figured they don't act as decouplers so that you can't dock anything to it after you've decoupled something from it. I guess I misunderstood that part.

Yeah. That is written so as to make you avoid a mistake, only to lead you into making a different one. Nice!

What it means is that you can't recouple with the nosecone, but you can still dock. If you check the action group menu for a docking port in the editor, you'll see it has both "decouple" and "undock". That's the difference: Decouple ejects what was structurally attached there in the editor; Undock only ejects another docking port that's docked to it.

An Apollo-like ship, for example, would have the LES attached to the docking port on the CSM, and the docking module on the LM attached to the engine on the CSM. Works like a charm.

[Plusck]

On 30/12/2015 at 6:01 PM, sormi said:

I'll explain my logic behind this. I put control surface wings to top because I figured they would stabilize the craft if it starts overturning towards horizon, essentially softening the turns. I already did very very subtle turns though, so I guess those proved useless in the end. ...

 

Understandable logic, until you try to imagine the airflow around your rocket. If you are pointing any direction other than prograde, air is slamming sideways into your control surfaces. If they are at the bottom of your craft, that will tend to push it back in line with the front (like an arrow's feathers). If they are at the top, then they are pushing it away from your direction of travel. The fact that they are control surfaces means that they can effectively avoid this effect up to the limit of their control angle, but as soon as you exceed that angle they become effectively counter-productive.

This is also why canards only became common on fighter aircraft after computers became good enough to manage them, and why they have such an enormous control range. Humans aren't really able to deal with canards at high AoA, and flipping your aircraft during a combat turn is not the best survival strategy...

[Snark]

1 hour ago, Plusck said:

Understandable logic, until you try to imagine the airflow around your rocket. If you are pointing any direction other than prograde, air is slamming sideways into your control surfaces. If they are at the bottom of your craft, that will tend to push it back in line with the front (like an arrow's feathers). If they are at the top, then they are pushing it away from your direction of travel. The fact that they are control surfaces means that they can effectively avoid this effect up to the limit of their control angle, but as soon as you exceed that angle they become effectively counter-productive.

This is also why canards only became common on fighter aircraft after computers became good enough to manage them, and why they have such an enormous control range. Humans aren't really able to deal with canards at high AoA, and flipping your aircraft during a combat turn is not the best survival strategy...

Excellent explanation!

Or, put another way, "arrows fly better if you put the feathers on the back end." 

[Superfluous J]

Two things.

One: in the image you posted where you were 20km up, you should probably be far more tilted over to the horizon than you are by that point. My personal goal is to be at about 45 degrees over at about 10km (or a little less!) and sideways by 30km (or less). Actually I go by my rising Apoapsis, because that tends to have an in-built leeway to different TWR in a rocket. I try to be (along the Eastern 90 degree line going down) 40 degrees (more horizontal than Vertical) by the time my Ap is 20km, and horizontal when my Ap gets to about 50 or 60km. You will see mach effects, and probably fire. Those are okay

Two:

On 12/30/2015 at 1:59 PM, sormi said:

Thanks again for the help! Bought the game three days ago and I've been doing so many experiments and test configurations lately. I think I was starting to confuse myself and fail to find any logic behind this

This is awesome. Your rockets look better after 3 days than mine did after 3 weeks. Granted, the atmosphere I learned it was far different from yours but still. Well done!

[sormi]

Lol this topic is still going. Thanks for the helpful tips. I think this topic can be buried though as far as I'm conserned. So much has happened since making this I'm quite baffled myself.

I landed to Mun, left my Kerbal there and then sent a rescue party with a scientist to Mun to fetch her back. Unfortunately I ran out of fuel at Kerbin orbit and had to rescue the rescue party using jetpacks

 

Love this game.

 

[Kryxal]

Just for the "nose cone on docking port" issue, I like to use an inverted decoupler to give it a good push, preferably while coasting to your orbital insertion maneuver.  It gets it right out of the way.