Why Does My FLIPing Rocket Always Flip Over! (Easy Picture Explanation)

[EtherDragon]

Why Does My FLIPing Rocket Always Flip Over!

AKA: Forces at Work During Launch and Ascent and How to Deal with Them.

Part 1:

A Stable Rocket

First let's take a look at a stable multi-stage rocket, like the one in this picture:

Notice that the Center of Mass is closer to the bottom than the top; that's normal. Also, notice that the fins are below the center of mass which helps with stability.

Take a look at what happens when the rocket is moving through the air:

As you can see, the air is coming straight on. Air flow induces some drag at the nose, and some drag on the fins. Now, what happens when you try to tip this rocket over? Take a look:

As the rocket tries to rotate around its center of mass the fins expose more surface area to the air-flow. This increases the drag on the fins, which is stronger than the drag on the nose. The result is the tail-drag pulls the tail back in line. The rocket is stable and will point its nose into the wind.

Part 2:

An Unstable Rocket

Now, consider the following rocket:

Notice that the drag is mostly on the nose. So long as the air-flow is in line with the center of mass, this isn't an issue. However, if you tip the rocket over, even just a little you end up in this situation:

There is more drag on top of the center of mass than below it! Tip over far enough and the torque from this nose-heavy drag flips your rocket around the center of mass because the drag near the nose has more leverage than the drag near the tail. You lose control, pull out your hair, and swear that KSP is the worst game ever!

But it's really just natural, real-world forces at work.

Part 3:

Coping With These Forces

The best solution is to mount tail-fins on your rocket so the drag near the bottom is greater than the drag near the nose. Remember, as the rocket tries to turn sideways, the tail-fins actually end up producing more drag because of the exposed surface area. One issue with this is it's harder to steer your rocket where you want it to go, until you get fins with control surfaces built in.

The other solution is to make sure that your nose is pointed mostly into the wind at all times. Any little bit of tilt induces a little nose-heavy drag; without any control input the rocket wants to flip. But the torque from the nose-heavy drag is small enough for the engine gimbal and/or SAS to overcome so long as you don't tip too far, e.g. less than 5-degrees from your flight path.

Part 4:

Speed Kills!

One really important consideration for dealing with air-flow is, the faster you go, the more air-flow -> the more drag -> and the more torque trying to affect your rocket!

If you use tail-fins this is helpful, to a point, but it does make it harder to steer the rocket for an efficient ascent profile. Without tail-fins the rocket becomes more unstable the faster it goes as the torque on the nose from any minor level of tipping increases. Once you're clear of the atmosphere, or at least high enough where it doesn't have much impact (about 35km) you won't have to worry about it.

Until you reach 35km, though, it's best to keep your rocket at reasonable speeds. Remember speed = air-flow = drag = torque. Result: Less Speed; Less Torque. I recommend you keep your rocket below 500m/s until you reach 35km. I usually fly mine at about 450m/s or so.

- - - Updated - - -

Ademdum

Part 5:

Shifting Center of Mass

As your rocket ascends it expends propellant. Since propellant is used from the top of the rocket first, the center of mass will gradually shift downwards, closer to the tail. This means that any torque on the nose from induced drag will have more leverage as the rocket ascends. This explains why, sometimes, the rocket seems to be under control one moment and more flippy the next, even though your deflection from the air-flow remains the same (say, between 5 and 10 degrees). Add the shifting center of mass to increasing speeds, and you have a rocket that is becoming more and more touchy by the moment.

Hope this tutorial helps! Keep Flying and Stay Shiny!

Edited February 11, 2023 by EtherDragon

[Kerbal007]

This is gold, really appreciate the pictures explaining it all, can I ask what application you used to create them?

[EtherDragon]

This is gold, really appreciate the pictures explaining it all, can I ask what application you used to create them?

Sure! I used paint.net which is free.

[Meithan]

Very clear graphical explanation, good work! Here, have some rep.

I have only one minor observation. I'm not sure about this (somebody correct me if I'm wrong), but I think it's technically the increased lift from the fins (due to the higher angle of attack) what's providing the restoring torque, not the drag. Lift initially increases faster than drag as the angle of attack increases from zero. The effect also comes from the other set of fins (not shown) which are mounted at 90° from the ones you show, but I agree they are harder to depict clearly.

Still, the general conclusion is correct: adding aerodynamic surfaces to the tail makes the rocket more stable. And your tips are sound.

[EtherDragon]

Meithan, you are correct - the fins produce lift. That lift in turn becomes Torque on the tail - much more torque than on the nose.

[problemecium]

That last bit about the shifting center of mass might well be the most important part of your tutorial for some of us. I'll say that as soon as I figured it out (I didn't read this till just now xD) it made a night-and-day difference in my rockets' stability during launches.

[Josefpm]

GREAT! amazing answer. Love it.

Thanks.

[mudkest]

nice guide, will link to it next time someone asks for help.

Also, speed isnt the only thing that kills, staging can too CoM and drag both moving forward

Edited May 3, 2015 by mudkest

[barleyman]

Hmm.. That explains why adding a second big fuel can to the 2nd stage actually made my rocket more stable. COM goes up and the nose has less authority to flip rocket.

[Thorbard]

Useful tutorial, thanks.

I've been using the F12 aero visualisation to help with this little problem with some of my rockets and couldn't quite work out what the different colours are meant to mean. Any ideas?

[Meithan]

I've been using the F12 aero visualisation to help with this little problem with some of my rockets and couldn't quite work out what the different colours are meant to mean. Any ideas?

I've been wondering about that too. I think they are the components of forces along three directions defined by your velocity vector: blue is perpendicular "up" (e.g. lift), red is parallel to velocity (e.g. drag) and yellow is perpendicular to those two, so "sideways" (e.g. your tail vertical fin counteracting yaw). I'm at work so I can't confirm ingame at the moment.

[Francesco]

So how do rocket without fins and aerodynamic surfaces manage to fly?

Just by gimballing the engines, and by "keeping within the prograde marker"?

[Motokid600]

Sticky this if not already.

[Meithan]

So how do rocket without fins and aerodynamic surfaces manage to fly?

Just by gimballing the engines, and by "keeping within the prograde marker"?

Pretty much, yeah. As long as you remain close to aligned with your velocity vector, the aerodynamic forces trying to flip you over are small and easily overcome by the TVC.

[BagelRabbit]

Hello everyone!

A little while back, I made my own tutorial on KSP's new aerodynamic system. It's rather long, but it's probably worth the read. It also covers a lot more information than this little dude (as good as it is).

You can find it here. If you want a bit more knowledge on rocket aerodynamics, be sure to check it out!

(Sorry for the shameless self promotion: this just seems like a logical extension to the thread posted by EtherDragon. Thanks for understanding.)

-Upsilon

[kiwiak]

And i thought that keeping center of mass on bottom of rocket helps with stability, because you know, heavier half of rocket will keep bottom of rocket pointing down and top pointing up.

[Meithan]

And i thought that keeping center of mass on bottom of rocket helps with stability, because you know, heavier half of rocket will keep bottom of rocket pointing down and top pointing up.

That's an example of a very natural notion to have (most of us do) that turns out to be incorrect. It's called the Pendulum Rocket Fallacy. It's so natural that even Robert Goddard, one of the fathers of rocketry, initially worked under this assumption and actually built his first prototypes with the propellant tank -the heaviest part of his rocket- on the bottom.

A simple explanation is to realize that a pendulum is stable with its heavy end at the bottom because it is fixed at the top. A rocket is not, its top is free to move however it likes. Rockets have no inherent stability, which is why you need to add some means for stability control: aerodynamic surfaces for the lower parts of the ascent or, more commonly irl, thrust vectoring on the engines.

[Cirrus]

Thank you for taking the time to make this guide. The pictures are very helpful.

I think now I understand better why I have been flipping.

[KingPhantom]

i'm trying to bring a spaceplane with a rocket in rocketstyle into orbit. shuttle style doesnt work because i have to attach the engine to the additional extratank and it flips over late in the flight.

my rocket style launcher flips early and i dont figure out why... the screenshot shows the moment when shortly when it begins to flip... i think the elevon of the plane is the problem. can i deactivate that for the start? do i have to build bigger wings on the bottom??? it should be able to launch something like that....

image: http://www.omfg.ch/ksp/screenshot83.png

edit:

now i made bigger wings and it looks like it works... but its ugly als hell...

image: http://www.omfg.ch/ksp/screenshot84.png

Edited August 25, 2016 by KingPhantom

[Eric S]

11 hours ago, KingPhantom said:

my rocket style launcher flips early and i dont figure out why... the screenshot shows the moment when shortly when it begins to flip... i think the elevon of the plane is the problem. can i deactivate that for the start?

 

Not a spaceplane expert, but at least part of the problem is that any aerodynamic surfaces ahead of CoM of the craft reduce the stability of the craft, requiring more command authority if the craft is not pointed directly prograde.  Now, the closer to prograde you are, the weaker this is, so if you can do a true gravity turn (aiming prograde for everything except the pitch maneuver), then you'll probably be much more likely to be able to launch this craft without the huge tail fins.  

If you find yourself having to push the nose over closer to the horizon, then you might be able to manage this by doing a more aggressive pitch maneuver or doing it at a lower speed/altitude.  Lowering your TWR is another alternative that might help.  This gives gravity more time to pull the heading of the craft over.

 

 

[wumpus]

While I do like this thread, I have to question why all the drawings only work if prograde is straight up and the rocket has an angle of inclination.  Your rocket can flip out quite well while following the prograde vector (and thus have drag straight backwards, while gravity is some other direction (effectively straight down)).

[Zeiss Ikon]

Rotate the frame of any of the illustrations in any way you like.  Gravity doesn't matter in any way to stability once the ship is off the launch pad.  What does matter is thrust vector, lift and drag and their balance relative to the center of mass.  If you're even a fraction of a degree off prograde (so the airflow is coming from a tiny angle off the body's longitudinal axis), the nose will produce lift and drag that tend to upset the rocket -- and unless there's a restoring force, either from greater fin lift (after accounting for lever arm from CoM) or from another source of torque (vectored thrust, reaction wheels, or RCS), your rocket will tumble.  If the stack flexes enough to see, this will be much worse, because it will also oscillate and may resonate with the speed at which the motors can gimbal, which can even cause a rocket to break up in flight.

If you've flown model rockets and paid attention to their stability requirements, this won't be a mystery to you.  Those are virtually all stabilized by lifting fins, though a few are stabilized by drag from a conical shroud or even just by having the CoM far enough forward that the CoL of the body is behind it (for a plain cylinder, it's generally about 1/3 back from the nose, until angle of attack exceeds roughly 30 degrees, then it's about halfway).  In KSP, we also have gimballed engines (i.e. vectored thrust), as well as reaction wheels and RCS, so we can make rockets that are stable in flight (at least during boost phase, for gimballing) without fins.  NASA doesn't use fins, nor does Space-X, because they add mass and drag, hence costing fuel, but their stability control is a bit better than ours.  They manage to do everything with gimballing during boost, but in KSP many rockets seem to require RCS to keep them headed the right direction.