[1.0.4] Noob atmosphere issues

[sybeck2k]

Hello everyone,

I'm doing a science gameplay, and I'm stuck at figuring out how to get stable flight in atmosphere. I'm using KSP 1.0.4.

Here is an example rocket:

I've tried to put fins at bottom, use only a liquid engine and throttling down at about 80% to reduce acceleration, but at some point the rocket always looses control and spins...Any help or good guide that can help me out?

Thanks!

[Vanamonde]

There are 3 means of controlling craft in atmosphere. Reaction wheels, engines, and control surfaces. The SRB engines you are using do not have steerable nozzles (called "gimbals"), so that engine is not helping your control the craft. Also, the fins you have chosen do not pivot. That leaves only the inherent reaction wheels of the capsule with which to control the craft. Try experimenting with other parts. But even then, a craft of that general shape just isn't going to fly very well.

Not said to make you feel bad. This is just how one learns to play this game.

[Snark]

So, some tips for that rocket:

First of all, that booster is going to be tremendously overpowered for that little rocket. You'll be going way too fast when you're still low in the thick atmosphere. Use a thrust limiter to reduce the thrust and spread it out: in the VAB, right-click on the booster and set the thrust limiter to something lower than its default 100%. Try going down to 50% to start with, and tweak it from there.

Second, I note that you've only put fins on two sides of that bottom rocket-- have you tried making it a foursome, like the liquid stage above it?

Third: Get rid of the little fins on the 2nd stage. I suspect that you don't need them, and they may actually be hurting it if they're in front of the CoM for that 2nd stage (the command pod is only 800ish kg, whereas the engine is like 1.25 tons).

Fourth: What sort of engine is that on the 2nd stage? Reliant, or Swivel? If it's Reliant, switch to Swivel if possible-- the Reliant doesn't have engine gimbal.

[Edit]

So I tried re-creating your craft and played around with it a little bit. I went ahead and used the Reliant engine, which is the "harder" one 'coz it doesn't have gimbal. Here's what I found:

Experiment #1: run it as pictured, with the SRB at full power. Result: it flips out.

Experiment #2: As pictured, but the bottom fins are 4-way instead of 2-wy. Result: it flies just fine, totally stable.

Experiment #3: Get rid of all the fins, reduce the SRB to 50% power. Result: it flies just fine, is stable. And, incidentally, flies a lot higher than experiment #2 (since it's not wasting fuel fighting drag by going too fast too low).

So you've got a fair number of options.

One thing to bear in mind: When you do have fins, you always want them to be as far towards the rear of the rocket as you can possibly manage. For example, you have those little fins on the second stage-- you could actually scootch them down a bit more.

Edited October 13, 2015 by Snark

[Streetwind]

Welcome to the forums!

The "golden rule of the thumb" for rocketry is: heavy bits at the front, draggy bits at the back. Fins are draggy bits, and because you have some at the front, they are destabilizing your vehicle. Snark goes into a little more detail, but for a newcomer, it may potentially help you to have a simple rule to follow.

Rockets are also more stable if they are longer and narrower, provided the weight isn't all concentrated at the bottom (see golden rule, above). If you read these two paragraphs and have the picture of an arrow in your head, you are doing it right

Regarding fins:

- Not all rockets need them, but some do. Launch your vehicle without fins first to check if it remains stable, and revert to the editor to add them if necessary.

- Always mount passive guide fins in three- or four-fold symmetry, unless you know what you are doing. Using less than three does not stabilize all axes of movement.

- Always mount active guide fins ("control surfaces") in four-fold symmetry, unless you know what you are doing. Using three will sometimes screw with your steering, depending on your orientation.

- Only ever mount fins in places other than the bottom of the first stage if it's necessary. I.e. if your rocket flies fine until first stage burnout, and then immediately flips after staging, that's a good reason to mount fins on the second stage too. Besides this, there exist few other reasons.

[sybeck2k]

For the moment...thanks all!!! I'll try tonight with all your great suggestions, and report back