Help w/ out of control rocket

[DucharmeHD]

I'm attempting to make a munar lander mission using realism overhaul. I've got my rocket all suited up & ready to go, but the only thing is trying to control it & get it out of the atmosphere. Some times it will be really stable & then it will get a very slight wobble which will then cascade into a series of unfortunate events. Other times it's just not controllable at all. What do I do to make this more stable, and what should my ascent profile look like? That's my main problem, i have no idea what my ascent profile should look like. I'm just kind of winging it

Edited July 7, 2014 by DucharmeHD

[-root]

With the wobbling, try adding struts in to stiffen it up.

As for the ascent profile, I reckon I read somewhere you need to go higher up (like 40km up) on realism mod to get the gravity turn right. Not sure though, just keep going higher on each iteration if you crash. You'll find the right one. But as a rule, 3km is crazy too low.

Depending on how good the mod is, the atmo gauge at the top of the UI should be able to tell you where you are the in the atmosphere. Wait until you start moving into the darker areas before starting the turn. Start slowly as well. Just do it gradually, 3-5 degrees at a time.

If all else fails, build a new rocket

[AlamoVampire]

one thing, id cut back on the control surfaces, they are all fighting against each other, second, do you have any ASAS on board?

[Jebadiah Kerban]

Ok first of all make sure your center of mass is in the center and center of lift is pointing up strait and center of thrust is pointing down strait

2nd STRUTS!

[Red Iron Crown]

Off-topic, but where did you get those beautiful brushed stainless steel rocket parts?

[Taki117]

Off-topic, but where did you get those beautiful brushed stainless steel rocket parts?

that's one of the Skins for Procedural parts.

On topic: The connection between the base of your fairing and your rocket isn't doing you any favors. and you should move your fins to the bottom of that stage.

[Bobnova]

More fins on the very very bottom of the rocket. I like the small wings with control surfaces, they work the best for me.

Gravity turn on a RSS setup comes much later than stock with FAR.

I usually tilt over 5° or so at 10km, then very very slowly tilt over to 45° at maybe 50km. After that I spend my time watching time to AP and the curve of my trajectory on the map screen.

It's a very touchy business.

[r_rolo1]

1) Radially mounted engines are very prone to make ships go out of control, due to the flexibility of the connections to the central part ( in your case radial decouplers ). More struts/ SAS on are pretty useful to avoid that.

2) If you use wings/winglets to use aerodynamic control, they must be as low as possible ( think in a arrow ... or Saturn V ). Also wings/winglets mounted in radially mounted parts tend to exacerbate the problem outlined in 1) , so, if possible, mount all your aerodynamic control surfaces in the central body of the ship.

[DucharmeHD]

one thing, id cut back on the control surfaces, they are all fighting against each other, second, do you have any ASAS on board?

Since i'm not sure what ASAS is, i'm going to assume no. If it's just SAS, then no to that also, except for my Mk1 Lander can

[Vicidsmart]

1. STRUTS!!!! 2. Add SAS 3. Add RCS

[ferram4]

You should begin your gravity turn at ~100 m/s, and you should aim to be at a pitch angle of 45 degrees when you're going 1000 m/s. Ignore the guy saying to wait until later, doing it his way is going to up your dV requirements by a few km/s and will require your upper stages to have much, much higher TWR to actually achieve orbit.

All of your fins are too high up on the rocket and aren't going to help make it stable. Put them at the bottom.

Your boosters are made up of a lot more parts than you need to; simplify that out and you'll get rid of a bunch of wobble. You also don't have anywhere near enough thrust; you need a TWR around 1.2 or you're just burning fuel to hover there. Try adding some small solids to get you going.

Get rid of that pinched shape going on right below the fairing, that's adding a lot of drag and is making your vehicle really unstable. There's no need for it to do that at all. Use a wider fairing base and don't taper the top of the final stage so much.

[DucharmeHD]

You should begin your gravity turn at ~100 m/s, and you should aim to be at a pitch angle of 45 degrees when you're going 1000 m/s. Ignore the guy saying to wait until later, doing it his way is going to up your dV requirements by a few km/s and will require your upper stages to have much, much higher TWR to actually achieve orbit.

All of your fins are too high up on the rocket and aren't going to help make it stable. Put them at the bottom.

Your boosters are made up of a lot more parts than you need to; simplify that out and you'll get rid of a bunch of wobble. You also don't have anywhere near enough thrust; you need a TWR around 1.2 or you're just burning fuel to hover there. Try adding some small solids to get you going.

Get rid of that pinched shape going on right below the fairing, that's adding a lot of drag and is making your vehicle really unstable. There's no need for it to do that at all. Use a wider fairing base and don't taper the top of the final stage so much.

Every time I try to make a single few degree turn, it goes out of control and turns all the way over.

I added two heavy control winglets down at the very bottom of the first stage, but the center of lift still has the arrow pointing outwards.

I can try just having 2 radial boosters that are much bigger, should I try that?

And how do I get rid of the pinched shape? Should I just make a straight cone, or do I decrease the diameter all together? I would prefer not to do that because I would have to make up for the lost delta V.

Here is what my rocket currently looks like as we speak:

[DucharmeHD]

Ok, now I have another problem. The rocket seems to be much more easy to control (although requires serious manual control to keep it from going crazy).

Now, when I get up to around 47km, at 900m/s, my rocket's first stage seems to just spontaneously blow up. Not completley, but the radial boosters just blow off and start flying every where, my first stage still keeps burning, but it sends the whole rocket into a crazy bend and out on control.

My core engines are slightly overheating, but would that cause the decouplers to blow by any chance?

Edit: found the reason...

Edited July 7, 2014 by DucharmeHD

[ferram4]

The CoL is supposed to point in that direction; it's pointing towards the rocket's top, correct? It's working in the same way that it would in the SPH with a plane, except instead of looking at the vehicle moving horizontally, it's moving vertically. That is not your issue, that looks fine and is intended behavior.

Make the fairing a simple cone. Whatever dV you might lose by adding fairing weight will be offset by the fact that your rocket will be less likely to flip around during launch.

Make the stages out of a single tank, not the many it seems you have now. You're adding unnecessary flexing into your design everywhere with those tanks.

Replacing the 4 boosters with 2 larger ones might help, but last I saw you still don't have enough thrust. I'd instead take the redesigned fewer-part-but-same-size boosters you're going to make and use 6 of them; this will increase your TWR and increase your dV to balance out that added by the fairing, as well as decreasing the dV requirements overall since you won't waste dV just hovering above the pad.

Frankly, the way this looks, your problem might not be that the rocket is unstable, but that it is too stable, combined with the low TWR. Since the rocket doesn't accelerate well, any deviation from pure vertical results in the rocket wanting to pitch over horribly. If that's the case, adding more thrust will fix the issue.

[DucharmeHD]

This is answered now. I'm not sure how to edit the flair....

[AlamoVampire]

This is answered now. I'm not sure how to edit the flair....

go to the first post, edit post: advanced options

[lammatt]

center of thrust is way too far from the center of mass,

[MarvinKitFox]

3 wobbly joints, and un-strutted boosters????

UN STRUTTED BOOSTERS????

Just add one strut to each booster, right at the top or right at the bottom, and your rocket will be 90% better. The strut will force the booster to not twist out of the lign of flight, which is what is eating you.

Putting fins on a booster... ok but iffy. They add even more torque to the poor booster's single mounting point. Its un-strutted mounting point, i may add!

Consider moving your fins to the main body between the boosters, otherwise you will need to cross-strut the boosters to each other both top and bottom.

Once your boosters are stiff & trained, the wobbly-nedd of your main tower should not be a problem. But consider stitching actoss those weak joints.

You can do so wit no visual impact by ataching the sticking struts to cubic octagonals, and rotating them *into* the rocket body. All the benefit, none of the mess.

Launch profile..

You are running FAR, so you have only two options..

1)Start your turning before you even hit 100m/s, so the rocket doesn't mind the aero sideslip. This is more efficient.

or 2) Do 100% of your turning only after reaching 25km, so there *is* no aero forces to shred your rocket. This is way easy, but you waste about 600m/s.

[NecroBones]

Can you take a screenshot inside the VAB, with the buttons in the lower-left corner of the screen turned on? That is, it would be helpful to see where the centers of mass, drag/lift, and thrust are.

The main cause for a rocket wanting to flip upside-down is for the center of drag to be in front of the center of mass. Aerodynamically speaking (even with the stock game's aerodynamics), you will always want your center of drag to be behind the center of mass. My concern is that you have something high in drag, and low in mass, at the front (with those fairings).

The thing to take note here, is that the center of mass will move as the ship flies, because of the fuel getting consumed. Fuel is usually taken from the upper tanks first (within the current stage), so the center of mass will move toward the rear. It's possible to reach a point where, once enough fuel is consumed, the center of mass passes behind the center of drag, and you lose control. Putting fins toward the back, as far back as you can, can help with this.

Another possible cause for a rocket flipping out of control, is to have a symmetry problem. This can be seen if the center of thrust and the center of mass don't form a nice straight line through the middle of the craft. In this case, it doesn't matter which one is in front, as long as a line drawn through the thrust vector will pass through the center of mass.

And Ferram's suggestion about thrust, and slipped off to the side, is a really good point too.

(I didn't read all replies, so forgive me if I'm duplicating discussion)

Edited July 7, 2014 by NecroBones