Payload not heavy enough, so my rocket flips. I can't really change my payload, what to do?

[Pingonaut]

I'm playing career mode and I'm not starting out doing manned missions. This mission was meant to just get up into or escape the higher atmosphere to send back data. However, it flips because the payload is so light compared to the middle stage.

I can't really do anything to add weight, and I've winglet. I don't know what to do, is this just not a viable launcher? I really like the payload design. =(

(Also, I'm not doing any gravity turns. SAS is off.)

Edited May 26, 2015 by Pingonaut

[stibbons]

Turn SAS on.

If you haven't already, also thrust limit the BACC booster. Firing that at 100% thrust will be very very overpowered and the extra speed will contribute to the instability. Throttle it back to where it can just get off the ground at launch and you'll probably have a much better time.

[zarakon]

Add more fins/wings to the bottom

[Snark]

A few thoughts:

First, make your front end more aerodynamic. You have a lot of exposed parts, especially those radial bulgy tanks on the adapter-- that drag is gonna get you. Move all that paraphernalia into a 1.25m service bay (it only masses 0.1t), with an appropriate nosecone in front of it. That makes your ship a nice smooth cylinder.

Secondly, you have the "Thumper" SRB driving the thing-- that's way overpowered (by default) for that tiny middle/upper stage, as stibbons has suggested. At the very least, make sure you've put a thrust limiter on that SRB so that it doesn't slam you past Mach1 right away. If you're trying to pick a number, I find a TWR of about 1.5 works pretty well. Look at the craft's total mass on the engineer's report, multiply by 1.5, multiply by 9.8, then divide by 275 (the thrust of a BACC), that'll tell you what to set it at.

I'd also suggest that your mass ratio between that huge BACC booster and your tiny 1-ton 2nd stage is a pretty big gap. Maybe consider downgrading the SRB from a Thumper to a Hammer, and then upping your 2nd stage from 1 ton of LFO to 2 tons. (And of course make sure to re-do the math to figure out how to set the thrust limiter on the SRB.)

[Warzouz]

Small rocket are harder to fly (well, less forgiving) than bigger ones. Your rocket don't have control surfaces (winglets), nor thrust vectoring. And I don't see reaction wheel. So it seems you have nearly no control on you rocket.

You could try adding some additional winglets higher (but not too high or it'll flip even faster). I had some results with that, even I don't use them anymore.

But reducing thrust, as others explained, is he way to go.

[Skorpychan]

I'll break it down for you. That's not a viable launcher.

Use a LFO stack with thrust vectoring, and maybe some basic fins.

Also, why have you got such a big engine on such a tiny craft? An lv-909 would work fine.

[Prepper-Jack]

Have to agree with Skorpychan. Terriers should be the go-to engine for most small loads in 30k+ atmosphere and space. I even use them on my 120 ton mining rigs. They're cheap, light, and very fuel efficient. Use a swivel on the lower stage with a couple good fuel cans, and I'm sure you'll get a better result.

[ftunk70]

BTW, what are those bulbous bulbs you've got there? Nice payload!

[MarvinKitFox]

WAY too much engine, if you runt hat at full throttle you run risk of melting halfway up.

*of course* it want to flip.

[Chaos_Klaus]

I'll break it down for you. That's not a viable launcher.

Use a LFO stack with thrust vectoring, and maybe some basic fins.

Completely disagree. Small probes can be launched with a single SRB as lowest stage, no problem.

With the reduced specific impulse of all the SRBs it got a little harder, as they don't yield that much delta v any more.

Here are my suggestions:

- Why do you bring RCS tanks if you don't have (and need) RCS thrusters? Get rid of those.

- Payload is not very aerodynamic. Infact the way KSP calculates drag it's propably very draggy and makes your craft want to turn around during atmospheric flight. Try putting everything inside a service bay.

- There are no batteries or solar panels on your probe. It will run out of power when you don't fire your engines.

- You need some way to control your craft in space. Add a reaction wheel or you will have absolutely no control when engines are not firing.

- The LV-T45 engine is desingned for use atmospheric use. If you have unlocked the LV-909, use that one for everything above 30km.

- SRBs are great as a cheap lower stage. However, they have no thrust vectoring (gimbal) in KSP. Use fins with actual control surfaces to compensate for that, as those can steer you in atmospheres. Also, set the thrust limiter on the SRB to maybe 20% for your very small payload. The whole thing should not be going faster than 300m/s at 10km. Don't worry if it doesn't take off from the pad very fast.

Seeing that your current design has abolutely no control while the SRB is burning, I'd like to add a link to Scott Manley's tutorial on ascent in KSP 1.0. You need to do a gradual gravity turn in the new aero and he shows how it's done.

https://youtu.be/_q_8TO4Ag0E

That beeing said, one could propably point your rocket slightly east at launch and then don't touch the controls untill the SRB is burned out, but it's not optimal.

Edited May 26, 2015 by Chaos_Klaus

[cybersol]

You have 0.3 command torque and nothing else for control authority in your first stage. That would flip even in 0.90.

Add a small reaction wheel and switch to delta deluxe winglets at the bottom. That should fix it. For the future, see these tips:

Spinning out of control essentially comes down to a contest between drag and control. When the drag overcomes your control authority, you go for a spin. You can decrease drag by staying closer to prograde and building a more aerodynamic rocket. You can increase control using engine gimbals, stabilizer wheels, and control surfaces.

Also note that drag increases significantly right around mach 1 (340 m/s). So you should stay pointed as prograde as possible at speeds around 280-400 m/s.

On a rocket control fins should be as low and outside as possible. If you are using control fins on more than one stage, the CoL needs to stay below the CoM at all times, even as the tanks empty.

[Pingonaut]

Turn SAS on.

If you haven't already, also thrust limit the BACC booster. Firing that at 100% thrust will be very very overpowered and the extra speed will contribute to the instability. Throttle it back to where it can just get off the ground at launch and you'll probably have a much better time.

Tried it. Also added RCS to stop it. Still flipped. Aerodynamic forces too powerful because the weight is imbalanced. I also tried limiting my thrust greatly. No success.

Add more fins/wings to the bottom

Tried it. Didn't help at all.

I'll break it down for you. That's not a viable launcher.

Use a LFO stack with thrust vectoring, and maybe some basic fins.

Also, why have you got such a big engine on such a tiny craft? An lv-909 would work fine.

I'm playing in an alternate career mode. Different tech tree, so that is the only non-solid fuel rocket I have at the moment.

Here are my suggestions:

- Why do you bring RCS tanks if you don't have (and need) RCS thrusters? Get rid of those.

- Payload is not very aerodynamic. Infact the way KSP calculates drag it's propably very draggy and makes your craft want to turn around during atmospheric flight. Try putting everything inside a service bay.

- There are no batteries or solar panels on your probe. It will run out of power when you don't fire your engines.

- You need some way to control your craft in space. Add a reaction wheel or you will have absolutely no control when engines are not firing.

- The LV-T45 engine is desingned for use atmospheric use. If you have unlocked the LV-909, use that one for everything above 30km.

- SRBs are great as a cheap lower stage. However, they have no thrust vectoring (gimbal) in KSP. Use fins with actual control surfaces to compensate for that, as those can steer you in atmospheres. Also, set the thrust limiter on the SRB to maybe 20% for your very small payload. The whole thing should not be going faster than 300m/s at 10km. Don't worry if it doesn't take off from the pad very fast.

- Those RCS tanks are actually for the small RCS thrusters on the bottom of the payload, to get it away from the previous stage.

- I know I should use a service bay, but the design of the payload is really preferable aesthetically, so I decided against it.

- There are batteries on the other side of the payload

- This isn't actually going to space. I'm trying to play very progressively, espcially using this tech tree, so it was really just intended to get as high up as possible

- LV-909 not unlocked yet

- I've been keeping my thrust limited to around 60%, I'll try 20%. Thanks!

I appreciate the suggestions, everyone. I may use fairings, but I can't really make the design more aerodynamic because aesthetically it's where I want it (and unfortunately that's more important to me). I'll take a try at some of these suggestions when I play again.

[Keras]

Just tried this thing out and it will fly straight if you add two empty lfo tanks below the first of the same kind (making the top stage longer makes it more stable). Still spins out after the engine runs out of fuel though due to lack of control.

Edited May 26, 2015 by Keras