Sooo, why exactly rockets are flipping?

[beached]

The draggiest end of the rocket will always flip behind the heaviest end. It's not a bug.

Go try and throw a dart backwards and report your results.

Of course without seeing the design you're talking about, you're likely only to get general reponses.

I'll just leave this here: http://imgur.com/a/vw9l3

Even with FAR, as long as your drag was symmetrical and you were flying close to the prograde, rockets would not constantly try to fly themselves backwards, with or without reaction wheel/fin spamming.

Throwing a dart backwards would be an accurate comparison if KSP rockets were built with massive fins on the top, had the majority of their mass at the engine in the bottom rather than the fuel tanks and the payload, and were launched with slingshots instead of several minutes of controlled explosions from rocket engines.

Pancake rockets and gravity turns that don't start until you're out of the thick atmosphere end up being more viable than slender, aerodynamic rockets and realistic gravity turns: the opposite of FAR, and funnily enough, more reminiscent of <1.0 stock.

[0x7be]

Yes, modern rockets can fly without fins, but they have very sophisticated flight control computers that balance thrust, gimbal angles and more. They also have to fight wind, non-uniform fuel burn, etc (you can check out my blog about difficulties that engineers have to fight with in real world http://forum.kerbalspaceprogram.com/entries/3200-Hidden-complexity-of-rockets-Part-1).

As I said if you're smooth you can get by without fins. They simply make your life easier.

Your blog is absolutely fantastic.

Good job!

[TK421d]

keep an eye on the atmosphere also. the air, and obviously its affect on fins, runs out at a very low altitude now, 27-30km.

in 1.0 my game was bugged and my atmosphere meter was in the white by 10k and was what was causing my absurd flipping. I still had a little trouble getting used to 1.0.2 but was nothing like before when i was still trying to fly a rocket with very little airspeed having zero control over it...

[Green Baron]

I am having an enormous problem with this. While my smaller rockets fly just fine using the advice presented, I cannot seem to launch any large payload ( > 90t) in an aeroshell to orbit successfully. They all fall victim to uncontrollable spin in the high transonic range.

Fins? Got 'em. Reaction wheels? Got 3. TWR? I have acceleration clamped down to 12.5m/s. Gravity turn? Starts at 125 m/s, 3 km, just like it's supposed to.

Nevertheless, my rocket tumbles out of control around 500 m/s no matter what I do at that point. Even if I'm > 30,0000 km, suddenly aerodynamic forces become uncontrollable.

http://i.imgur.com/DFp0dvB.png

Any ideas? This is becoming truly frustrating. I'm already chopping up my payloads into tiny pieces, now I can't even get one piece into orbit!

It never came to me to use plane-wings as winglets. They probably create lift when turning and lift the back of the rocket which can initiate the flip (this is more of a question though). Did you try to fly that thing without them ?

TWR of 3. is very high (my contraptions usually have 1.5 to 1.8 on the launchpad). You'll have to actively control the ascentspeed or aerodramatics will take over.

I can't see any fuelpipes, but if you change the design to an asparagus style first stage you can probably get away with much less mass on the launchpad and save a bundle of struts ....

These hints in addition to what others wrote before.

k

Edited May 16, 2015 by kemde

[Murph]

Bigger rockets should be more stable than small ones (aerodynamic forces have relatively less of an affect compared to small rockets).

Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.

Bigger isn't necessarily more immune to flipping. If it is very tall, with the CoM significantly below the middle, you have a big lever for the force acting above CoM. Bigger also has more surface area for the air to react against. Some bigger rockets will be more stable, some less stable.

More mass does mean more stability, due to inertia, but bigger size means more force from the air. If the mass:force ratio rises, it is more stable; if it drops, it's less stable.

As for the original "why are my rockets flipping?". About 90% chance that it's a mixture of wrong speed for the altitude, or nose too far from prograde (even if prograde is no longer a direction you would like to go…). It could also be lack of control authority (torque wheels, fins, control surfaces, and vectored thrust), or a horrible amount of drag on the upper end.

[DrD]

http://exploration.grc.nasa.gov/education/rocket/rktstab.html

This is the best explanation I found.

Bottom line:

The conditions for a stable rocket are that the center of pressure must be located below the center of gravity.

Edited May 16, 2015 by DrD

[MarvinKitFox]

I am having an enormous problem with this.....

Nevertheless, my rocket tumbles out of control around 500 m/s no matter what I do at that point. Even if I'm > 30,0000 km, suddenly aerodynamic forces become uncontrollable.

Example ship:

That ENORMOUS fairing is just compounding your problem.

Your problem is that your CoM is behind your Center of Drag. Especially when just exceeding local speed-of-sound.

That fairing does reduce total drag, but it presents a huge volume of very low-density near the top of your rocket.

This both pushes your center of drag upwards, and your center of mass downwards.

If you are *at all* off prograde while flying transonic, you will have a huge unbalanced pressure drag on your craft nose.

At this time, the only thing that can save you is if your center of drag is behind your center of mass, this will dampen out any deviation from prograde.

How to achieve this?

*Minimise your fairing size.

*put the heaviest bits as far up as you can.(without compromising staging efficiency, of course)

*route your fuel from bottom tanks first

*fins as far aft as you can. *behind* your engines is even better.

* reduce the impact of the offcenter drag by flying slower.. Lower TWR.

* reduce the impact of the offcenter drag by *less deviation from prograde*. Note this must be prograde as measured by surface, not orbit, as the air moves with surface.

[eataTREE]

That ENORMOUS fairing is just compounding your problem.

Further testing has led me to believe that you are right, sir. The difference between success and failure seems to be that fairing. My 1.02-era Duna/Eve explorer has about the same payload mass as the depicted craft, and it flew to orbit on attempt #1. The only difference is the fairing size.

Flipping the payload around so that it is heavy-end first helped a bit -- but not enough. (The difference between tumbling out of control quickly and tumbling out of control slowly.) I need to make the fairing smaller somehow. Unfortunately, the payload is ridiculously baroque with huge dishes, scanning apparati, and LV-N engine clusters sticking out everywhere. That might have to change.

[Slomo]

I'm starting out on a new career mode. My funds are limited so I need to achieve as many goals as possible. Building a parachute, command module, 1 mini-booster, & 6 mini-boosters around that. Even with staging the boosters so only 2 activate at a time, and spacing their activation so I never go over 300m/s, every time I get over 5k meeters with even 5 degrees angle my rocket flips over. This is a simple rocket and should not be experiencing this problem.

...! This game worked perfectly before 1.0, now I can't even get the very first rocket to work? I admit I'm no real "rocket scientist", but this is ridiculous! Please fix this game, I loved it and was hooked on making my stations & deep space vessels in version 0.92 but now I can't even play it past the first launch! ...!

[Nutter]

So, you effectively made a pancake of boosters and you're trying to toss it with the wide end forwards. The game is fine, it's just your rocket that's a crime unto aerodynamics.

Heck, why do you even need six fleas for your first launch anyhow? If you're that desperate for more boosters!â„¢, just spend a few seconds running around the KSC with a notepad and a goo canister!

Unfortunately, the payload is ridiculously baroque with huge dishes, scanning apparati, and LV-N engine clusters sticking out everywhere.

Well, in case you're not disinclined on modding; Infernal Robotics might help with clustering your kit tighter inside of it. Though, that might require a severe redesign.

But, if you are, there's probably still stuff you can cut. Guessing you could ditch at least a few of the engines, if you already have whole clusters of them. If you have redundant equipment, it might be a good candidate for the chopping block, and, if nothing else works, you could just redesign the whole ruddy thing for orbital assembly. Yes, with enough effort, that includes radially attached engines, but you'll probably need some sort of Frankensteined docking port assembly.

Edited May 16, 2015 by Nutter
Attending a ninja.

[Foxster]

Here's some alternatives for the fin-phobic...

Route the fuel so the bottom tank(s) empty first by isolating the engine with a structural panel and ducting the fuel to the engine via the topmost tank:

Keep all/most of the fuel low:

...and a slightly odd alternative that does work fine:

[cybersol]

Nice one on the last pic -- that would be great for bamboo staging.