Solid engine flipping

[IRobot]

Hello Guys,

I´ve got a problem with my rockets. No clue why, but the tend to flip over after rolling a little bit. btw. I´m a beginner

A time ago I didn´t have this problem, but with the time my rockets kept getting bigger and the problems started.

That´s my rocket at the moment: https://www.dropbox.com/s/lnuockbi32q4nxs/Q%26AKSP.png?dl=0

Could you please help me prevent this or improve my rockets?

Thank you

IRobot

[Snark]

Hello, and welcome to the forums! 

Moving to Gameplay Questions, since this is more about "how do I play the game" than "the game appears to be working incorrectly."

The problem is that your rocket's not aerodynamically stable.  I see you've got a good start on it, by putting fins on the back, but your CoM is too low, so the fins aren't getting a good lever arm to work with.  If you can rearrange your rocket so that the heavier parts are up at the top and the lighter parts at the bottom, that would help.

It also looks like you're using 3-fold symmetry; that's been known to be a little flaky with stability sometimes.  Any chance you could go to 4-fold?

Try moving the SRBs as far down as you can (i.e. mount the radial decouplers as low as possible on the central core, then mount the SRBs as low as possible on the decouplers), then add fins to the SRBs (also as low as you can possibly get away with); that could help.

Out of curiosity, what do you have inside that service bay?

[bewing]

Or perhaps the service bay is just something soft for the SciJr to land on top of?

An additional tip -- minimizing drag and preventing flipping are sometimes opposing goals. If a rocket really wants to flip, sometimes you can prevent it by increasing the drag at the back end. Which you might be able to do by taking the nosecones off the Hammers.

 

[IRobot]

@Snark Wow thank you very much! Besides from moving the solids down can i do anything else to raise the CoM?    Ok i will try to change to the 4-fold system. Is there also a reason why the rocket spins in one direction? I use the service bay, because the experiment storage unti didn´t seem very aerodynamic to me.

@bewing Also thanks to you!! You got the reason for the service bay xD 

I understand what you mean, i just decided to put these on, because without them the rockets started turning even faster.

 

You both were a great help!! Thank you very much!!

[Nicias]

Why do you have two decouplers? Why do you have chutes on the materials bay?

[Snark]

57 minutes ago, IRobot said:

can i do anything else to raise the CoM?

Basically, "avoid having anything big and lightweight near the front of the rocket."  Put your heaviest, densest components on top.  (You've got a good start with the command pod.)

If you've got a liquid-fuel rocket that's draining a vertical stack of multiple tanks, by default it's draining them all evenly.  I'd suggest turning on "advanced tweakables" in game options, so that when you right-click on a fuel tank, it will show you its "fuel priority".  That's a number that tells you which tanks will be drained first (higher number = drained sooner).  Then click the little + or - buttons to make sure that the tank on the bottom has a higher priority than the one on the top.  That will cause the rocket to drain the bottom tank first, which will move your CoM upwards and help with your aerodynamic stability.  (Of course, that only helps with the liquid-fuel-burning stage of your ascent.  If it's flipping when you're still just burning SRBs, that won't help you.)

Have you unlocked enough of the tech tree to get the "Structural Fuselage"?  That's a cheap, super-lightweight 1.25m component.  You could put one of those on the bottom of your central stack (under a stack decoupler), then attach your SRBs there.  (You could even do away with the radial couplers and just attach the SRBs directly to the fuselage, so that will save you some part count).

Because it's so lightweight, it will have the effect of raising the heavier part of your central core higher up on the rocket, thus helping with stability.  Just a thought.  If you do that, you'd probably want to have big fins on the SRBs but small ones on the central stack above the fuselage, since those central-stack fins will be high up and may be slightly working against stability until you jettison the bottom part.

[IRobot]

@Nicias The first decoupler was there, in order to get the pod and the science stuff into the orbit and after this i have to seperate them, so i can avoid that the atmosphere burns my parachute. The other chutes are there to bring the data back to Kerbin, so i don´t have to send it.

@Snark Unfortunately not. Does it work like the girder segment?   When i add the fuselage, doesn´t get the nose also a longer lever arm? and as a reult of this more power against the fins? 

Btw. have you an idea how i could get more science points, as long as i´m not able to get into the orbit?.

 

[Snark]

57 minutes ago, IRobot said:

Does it work like the girder segment?

Main thing is that it's a 1.25m cylinder, so it fits nicely in-line with your main stack, not adding any drag.  The girder segment would be draggy and eat into your performance, not to mention being heavier.

57 minutes ago, IRobot said:

When i add the fuselage, doesn´t get the nose also a longer lever arm? and as a reult of this more power against the fins?

No, I think you're misunderstanding how aerodynamic stability works.

You want the CoM to be at one end, and the center of dynamic pressure to be behind that.

The structural fuselage is extremely lightweight-- so lightweight it might as well not be there.

So, by adding it to the back end of your rocket, you're making it so that the fins are farther behind the CoM.  You're moving the top end of the rocket upwards, with the CoM, to move it farther away from where the fins are.  By moving the CoM farther away from the fins, you're giving the fins a bigger lever arm to work with.

One concern could be if you have too much of your total ship mass in those SRBs.  If that's where most of your mass lives, you're going to have problems, because those are moving down, too.  Do you actually need three SRBs?  Could you get by with just two?

57 minutes ago, IRobot said:

Btw. have you an idea how i could get more science points, as long as i´m not able to get into the orbit?.

There's a lot of science to be had.

First, make sure that every science instrument has collected science from every situation you can get at.  For example:

• on the ground
• in the air ("flying")
• in the upper atmosphere
• in space near Kerbin

Note that you don't have to be in orbit to get the "space near Kerbin" science.  You only need to be out of the atmosphere, i.e. above 70 km altitude.  It's a whole lot easier to just shoot straight up until you're above 70 km, than it is to accelerate to orbit.

Make sure you've done the above for every science experiment available.  This includes the Science Jr., the goo unit, the thermometer, the barometer (if you have it), and crew report.  Also, EVA report whenever you're landed or splashed.  Note that if you're standing on the surface, you can jump, and quickly take an EVA report before you touch the ground again.  That will count as "while flying over Kerbin" which gives you another science result.

Note that you have multiple biomes available.  "On the launchpad" is one.  "Kerbin's Shores" is another (basically, anywhere near the KSC that's not actually inside KSC).  "Kerbin's Water" is another.  If you launch slightly westwards, you have grasslands, highlands, and mountains within reasonable distance.  So do all of the above for every biome you can get to.

If you get really desperate, you can go on tours around KSC, since each building (runway, launchpad, VAB, tracking station, etc.) is its own mini-biome with its own science available.

When you do have enough science to unlock a node, you should prioritize the tech nodes that unlock new science instruments, since that opens the door to more science points.  For example, if you don't have the barometer yet, that should be your next top priority.

At some point, when you have the cash, update the Astronaut Complex.  That will allow you to go EVA when you're not on the surface, which means you can get more science:  from the upper atmosphere, and from space.  Once you finally do achieve orbit, being able to EVA is a science bonanza, because "EVA report from space over Kerbin's <biome>" works for all the various biomes you pass over during your orbit, so the first time you orbit you'll be able to collect quite a few EVA reports.

Aside from getting science instruments, make sure you unlock the Terrier as soon as you can.  It's a fantastic upper-stage engine and will make it a lot easier for you to get to orbit.  It's possible to orbit without the Terrier, but considerably easier with it. 

[paulprogart]

Curious about one thing that hasn't been mentioned yet:  What about switching the fins for the AV-R8 winglets?  Is that overkill?

(Assuming they're unlocked of course, but I think they're just the next level up.)

[bewing]

1 hour ago, paulprogart said:

Curious about one thing that hasn't been mentioned yet:  What about switching the fins for the AV-R8 winglets?  Is that overkill?

(Assuming they're unlocked of course, but I think they're just the next level up.)

Generally, that sort of thing works nicely. Control surfaces have a bit more drag than fixed winglets, which helps stability -- and being steerable allows the SAS system to dynamically stabilize the rocket well beyond the range where passive stability would fail.

 

[Vanamonde]

In addition to what others have said, those fins and solid engines do not move to exert any force when you are applying steering commands. On that ship, only the inherent torque of the crew capsule is actively contributing to steering commands. 

[DerekL1963]

1 hour ago, Vanamonde said:

In addition to what others have said, those fins and solid engines do not move to exert any force when you are applying steering commands. On that ship, only the inherent torque of the crew capsule is actively contributing to steering commands. 

Which can be a problem in-and-of itself, even if the rest of the vehicle is well designed from a stability POV.

[Kryxal]

You said they flip, but you didn't say WHEN they flip, or if you have SAS available.  If you have prograde-hold, the capsule should have lots of torque for at least the start of it all.

[IRobot]

@Snark Wow thank you a lot!!

@Vanamonde I understand, but i haven´t unlocked soch that much so far.

@Kryxal SAS is activated. First they just turn a little bit, but at about 4000-5000m it flips.

[Vanamonde]

IRobot, what I mean is, once it starts to deviate, that ship doesn't have much control authority to return to the intended heading and hold it. Perhaps replace the solid engines with gimbaled liquid fuels? 

[Superfluous J]

That's a lot of fire with very little cracker. By the time you're 4000-5000 meters up, I'm surprised you're flipping over and not exploding

You should start your gravity turn at about 50-100m/s, and slowly gradually turn it eastward as you rise, so that you're already 45 degrees over when you get to about 10km up. By the time you're 5000 meters up you should be tilted maybe 70 degrees, not just starting your turn.

This whole time you should be aiming close to prograde on Surface mode (the default) and not venturing too far from it.

And this is all in addition to the great adivce above, mostly about the COM.

But really you'd be better off redesigning the craft to not have quite so much thrust in atmosphere. Have you tried a single BACC with 4 controllable fins at the bottom for steering? Or maybe just 2 Hammers and some more fuel for the liquid rocket?

[Kryxal]

That craft looks pretty odd ... just what is the engine that kicks in when you stage off the SRBs?  It really looks like you don't have very much fuel for the engines, so I don't see the point of staging to the second engine.  Also, how fast are you going when you flip, have you dropped the boosters, and are you pointed at all off prograde?