Serious issues controlling large rocket

[Laughing Man]

Howdy Kerbalers! I've been playing around with 1.0 a bit and loving it so far. I'm having serious issues with one rocket however. I cannot control this thing properly, it bounces around, leans every which way, essentially behaves like a very fragile machine. I've strutted it to ..... I've reduced the gimballing on the mainsail. I've added control surfaces, take control surfaces away, turned SAS on and off, done countless things.

This rocket just will not behave. I can just about get it into orbit if I'm lucky, but only after a shockingly inefficient ascent profile. It is simply uncontrollable no matter what kind of ascent profile I use. I hit f12 to see what was going on, but nothing obvious has jumped out. I've spent 2 whole days on this thing and cannot get it to work. I suspect it might be overbuilt, but I've reduced it as far as I dare. I need an expert to fly it and tell me what's wrong.

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Craft file - http://pastebin.com/9agNw9ch

Please help!

EDIT: It's a little different from the picture, has moar struts for one thing and the top structural thingies aren't there any more (as I suspected they were inducing heavy drag).

Edited May 10, 2015 by Laughing Man

[CalculusWarrior]

Hmm. I'm not quite as familiar with 1.0's aero yet as I was with 0.90's, so I can't immediately tell what's wrong with that ship. I suspect it may be an issue with just how long the thing is, the torque from the engines would give the body a significant bending moment around the CoM.

I'll download the craft file (thanks for including that!) and give it a try, and let you know of my results!

[SRV Ron]

It's controllable and stable, but seriously overpowered which causes aerodynamic issues. See the stats on the stages under Kerbal Engineering and the flight results on the pics posted.

At this stages, parts were failing from overheating

[Jouni]

Your lower stage has way too much power. Everything works fine, once you drop the Mainsail and ignite the LV-T30, but the lower stage has 3x more engine power than it needs. As a result, the rocket flies too fast in the dense low atmosphere, and the aerodynamics become very unforgiving.

[CalculusWarrior]

I've done several tests and can confirm what the guys above are saying, the rocket just goes too fast in the lower atmosphere. I suggest either throttling down during the beginning of flight, or just removing the SRBs altogether; the Mainsail has more than enough power to lift the stack. I personally felt like the ship didn't quite have enough delta-v to comfortably make it to orbit (but you can blame that on the above-mentioned still getting used to 1.0 aero), so I lengthened that core stack, as well as the side asparagus boosters. 'Overbuilt' is practically my middle name, so feel free to disregard this design if you feel comfortable with your flying skills.

Here's the rocket,

A rule of thumb for flying, try to keep the g-meter inside the green area. If you're hitting 10g while your engines are running in the atmosphere, bad things are going to happen.

Which brought Jeb to the Mun! then I realized that this didn't have the delta-v to take off from the Mun, and was likely meant for Minmus.

Edited May 10, 2015 by CalculusWarrior

[MarvinKitFox]

Well OF COURSE your rocket is unstabile, from its viewpoint you are tunneling through a granite mountain.

do not, repeat do NOT ascend so fast that the very air starts to go redhot!

[Laughing Man]

Thanks for the replies everyone! I hadn't even considered it was overpowered, or checked the G meter for that matter. At least my budget will take less of a hit this time round!

[Fishslap]

Howdy Kerbalers! I've been playing around with 1.0 a bit and loving it so far. I'm having serious issues with one rocket however. I cannot control this thing properly, it bounces around, leans every which way, essentially behaves like a very fragile machine. I've strutted it to ..... I've reduced the gimballing on the mainsail. I've added control surfaces, take control surfaces away, turned SAS on and off, done countless things.

This rocket just will not behave. I can just about get it into orbit if I'm lucky, but only after a shockingly inefficient ascent profile. It is simply uncontrollable no matter what kind of ascent profile I use. I hit f12 to see what was going on, but nothing obvious has jumped out. I've spent 2 whole days on this thing and cannot get it to work. I suspect it might be overbuilt, but I've reduced it as far as I dare. I need an expert to fly it and tell me what's wrong.

http://imgur.com/a/wLDXd

Craft file - http://pastebin.com/9agNw9ch

Please help!

EDIT: It's a little different from the picture, has moar struts for one thing and the top structural thingies aren't there any more (as I suspected they were inducing heavy drag).

Best guess is you have to much thrust going through the lower part of the atmosphere. This will then expose any asymmetry in the craft. It`s hard to tell for sure, but it looks to me like some of your struts might twist the lower part of the ship during launch. If you`re too fast then the twisting will strain the joints further up and the entire thing will take on a life of its own. The closest I`ve come to this was when I once used 16 boosters to get something heavy up and it just shot right up to 10k without even responding when I tried keeling over toward my orbit. Then when I tried doing it higher the bottom of my ship looked like someone swinging a club in a giant circle and then the top came off and I crashed. All I did to fix that was to sequence the boosters in two sets of eight and fire the last eight when I dumped the first ones. And then the ascent went perfectly because half my booster thrust went off above 10k where there`s less resistance so you keep more of your speed. In general there shouldn`t bee any thermal effects when you`re going up. Even though you lose speed to that as well, sonic effects are fine. if you don`t get those you`re too slow. But if there are thermal effects you`re going too fast and much of that speed is eaten away by air resistance. It would be more efficient to cut boosters really. If there are thermals you should moderate the thrust of your launch stage asap.

Anyway, what I meant to say was that it`s definitely just a small thing somewhere. Try lowering the thrust of the bottom first and see how that works. And if it`s not that, try going over your struts carefully. It matters whether it`s placed centrally on a fuel tank or on the sides. If it`s attached to the sides it will cause some twisting. A little twisting or a lot of it at slow speed isn`t a problem. But with enough of it and you could get trouble while launching, and especially at high speed.

Edited May 12, 2015 by Fishslap

[Laughing Man]

Best guess is you have to much thrust going through the lower part of the atmosphere. This will then expose any asymmetry in the craft. It`s hard to tell for sure, but it looks to me like some of your struts might twist the lower part of the ship during launch. If you`re too fast then the twisting will strain the joints further up and the entire thing will take on a life of its own. The closest I`ve come to this was when I once used 16 boosters to get something heavy up and it just shot right up to 10k without even responding when I tried keeling over toward my orbit. Then when I tried doing it higher the bottom of my ship looked like someone swinging a club in a giant circle and then the top came off and I crashed. All I did to fix that was to sequence the boosters in two sets of eight and fire the last eight when I dumped the first ones. And then the ascent went perfectly because half my booster thrust went off above 10k where there`s less resistance so you keep more of your speed. In general there shouldn`t bee any thermal effects when you`re going up. Even though you lose speed to that as well, sonic effects are fine. if you don`t get those you`re too slow. But if there are thermal effects you`re going too fast and much of that speed is eaten away by air resistance. It would be more efficient to cut boosters really. If there are thermals you should moderate the thrust of your launch stage asap.

Anyway, what I meant to say was that it`s definitely just a small thing somewhere. Try lowering the thrust of the bottom first and see how that works. And if it`s not that, try going over your struts carefully. It matters whether it`s placed centrally on a fuel tank or on the sides. If it`s attached to the sides it will cause some twisting. A little twisting or a lot of it at slow speed isn`t a problem. But with enough of it and you could get trouble while launching, and especially at high speed.

Thanks for that! All I did was take the solid boosters off and that was it. Flies fine. Had to add a little more fuel to the lander and command mod but otherwise worked fine. The only really annoying thing is I don't have access to larger landing legs yet.

[andruszkow]

Thanks for that! All I did was take the solid boosters off and that was it. Flies fine. Had to add a little more fuel to the lander and command mod but otherwise worked fine. The only really annoying thing is I don't have access to larger landing legs yet.

You could also adjust the boosters thrust limit before launching, if you want to not add extra fuel to other stages.

[cicatrix]

Good TWR at the sea level should be within 1.1-1.3. Anything greater than that is just throwing the fuel out for nothing (not to mention poor maneuverability).