Designing a working rocket for long payload

[Aghanim]

I already tried nearly all of rocket staging configuration for this rocket yet it still flips uncontrollably

[sickboy]

it flips? post an unzoomed picture with center of thrust, lift, and mass toggled on

also where is that supposed to go, the Delta-V is pretty low

[Monkeh]

I design like that all the time and it can work with some struts going from your jumbos to the top stage. Once you get out of the atmosphere it's not so bad.

Here's my interplanetary stage delivering my lander and itself to the Joolian system:

Now I've decided cargo bays are the way forward, those and robotic stuff.

[Kasuha]

In some other thread here on forums someone noticed that MechJeb can make rockets flip right after start for some unknown reasons. After he deactivated MechJeb the rocket was flying perfectly.

Other possible reasons for flipping are not enough torque and engines without gimbal.

Another possible reason (but not in your case) are engines mounted above center of mass.

[boomerdog2000]

A few things. That RCS way up on top may be causing you to flip since it is fairly heavy. As stated above if you are using mechjeb's SASS for launch you have to calibrate the roll number so the rocket doesn't try and roll right after take-off.

Suggestion: Have you tried taking out the center orange tank and attaching the main engines to the payload radially?

[Vertibirdo]

I already tried nearly all of rocket staging configuration for this rocket yet it still flips uncontrollably

I see that you are using Interstellar mod. Note that your selected payload is very heavy, and plasma thruster will wobble with any decoupler. Add some strut reinforcements and you'll be okay.

[capi3101]

If you're flipping around, you probably have your center of mass too high. Looks like your booster hasn't got enough delta-V to make orbit, either.

Can we see the rest of your booster?

[BostLabs]

<snip>

I already tried nearly all of rocket staging configuration for this rocket yet it still flips uncontrollably

oh I feel your pain, buddy. The Interstellar mod parts are heavy. Try building your boosters/lower stages around your main ship using trusses and struts. If you take the time and build it correctly your final stage will fly out of the last stage like a blooming flower. Or, careful piloting flying out of a cage.

think wide, not tall.

[Makeone]

That kind of payload could use some fairings as well. You could also flip the payload, the plasma thruster will have a bad thrust anyway, until you plan to upgrade it on launchpad?

The normal things, like turning the craft 45 degrees in vab (one tap at Q when whole ship is selected) so ship 'points' to east upon launch and also, before launch one remembers to right click on the computer core (assuming its the ships command section) and there 'Control from here', it overrides MechJeb.

Good luck on that.

[rhoark]

If you're flipping, center of mass is too low or center of lift is too high. Think of the rocket as a lever and the CoM as the fulcrum. You want the dynamic pressure concentrated on the short arm, not the long arm.

Edited October 10, 2013 by rhoark

[SRV Ron]

Lack of bracing, none shown between the booster and payload, can cause twisting which will promptly cause the rocket to arc over and crash. Also, no cross bracing on booster clusters will result in unwanted twisting causing further control issues. Properly braced and they will fly as intended.

[-Velocity-]

There's a couple methods I use to stabilize unstable rockets-

0) Moar struts

1) big fins near the aft, more canards on the nose.

2) A "control authority", jettisonable, rocket tip. Basically, it's LOTS of RCS nozzles on a detachable nose section, plus some extra canards. I'll do something like, get one of those 2.5 meter, 750 unit RCS tanks, put 36 RCS nozzles on (4X symmetry times 9), and then put about 12 canards on it. This works pretty well for medium sized payloads, or, it did before last patch. This "control authority" rocket tip will be jettisoned once it's no longer needed.

3) Toggle-able rocket engines on action groups 6 through 9. Basically, I can gain control of an unstable rocket by toggling off-axis rocket engines on and off with action groups. I usually make it so 6 yaws my rocket to the right on the navball, 7 yaws my rocket to the left on the navball, 8 yaws my rocket up on the navball, and 9 yaws my rocket down on the navball.

See this screenshot:

See the small rocket engines near the top of the rocket that face outwards? That's their function. They simply burn some of the payload's rocket fuel to correct the rocket's course when it veers off in an unwanted direction. I jettison them once I am in orbit. Also note that I am using one of the "control authority" rocket tips as well. Oh and also, look at the small rocket engines on the sides of the middle section of this rocket. Those are ALSO solely for steering the rocket, and they are ALSO toggled on and off with 6 through 9. Their benefit is that fuel burned in them is not wasted- it still goes towards accelerating the rocket into space, whereas 90 degree rocket engines, while much more effective at steering the rocket (BUT ONLY IF THEY ARE NEAR ONE END OF THE ROCKET), do not contribute to putting the payload into space.

4)Abuse of reaction wheels. On big rockets, I may use upwards of 20 or 30 reaction wheels. Keep in mind that rocket engines provide electrical power, so you'll have the power to use them all.

Frequently, I'll use all four of these tactics to keep a big, unsteady rocket straight; however, by far, 3) is the most powerful and certain way to get control of an unruly rocket. As long as the rocket holds together structurally, you can use 3) to put just about any damn thing into orbit that you want. Unfortunately, 3) can be quite tedious for huge rockets that take 45 minutes to launch (due to the game lagging down under the physics load).

Edited October 10, 2013 by |Velocity|

[Aghanim]

Here is my full rocket design:

[capi3101]

Yeah - the center of mass is definitely too high (or rather it starts out okay, but moves past the point of no return as your fuel tanks drain). You may not need them, but a couple more orange tanks would help matters.

[boomerdog2000]

According to your staging you're not firing the mainsail at launch? you definitely don't want to do that. Then you're just lugging a massively heavy engine halfway into orbit with no use. Run all the engine from launch, using asparagus staging to keep the center stage fueled.

Also moar struts.

[capi3101]

Are you sold on a DSTO design? A six-booster, quad-stage-to-orbit asparagus would help you out here. One of Temstar's Zenith Vs would do the trick. Alternatively, put an orange tank on each stack. For each of the boosters, place a tri-coupler or tri-adapter on the bottom and add a trio of LV-T30s. For the core stack, place an LV-T45, a quad of BZ-52s around it and four more LV-T45s on them. Strut it up real good to your payload and set all the engines to fire on launch.

[Garek]

To me it looks like your rocket could flip over because of drag issues. Especially when using FAR you should put your payload in proper fairings or the rocket will want to fly backwards because an unprotected payload likely has more drag than the launcher. Also, you know that FAR makes Nosecones on your boosters actually benefical?

[Lanzecki]

Another thought.

Ive launched a few landers etc that are upside down on the booster rocker. I usually fit a remote control unit on the lander. Then a remote control on the launch rocket. On the launch pad the controlling remote control is the 1st one you used, IE the lander and that's upside down. . Is the nav ball showing brown ground? That's the problem. Change control to the remote control that's 'facing up', the Nav ball turns blue and Mechjeb will be fine.

I had this problem for hours. Eventually I sussed it.

[asmi]

I see that you've got FAR installed. With payload as un-aerodynamic as yours you either use fairings, or "you ain't going to space today".

Edited October 11, 2013 by asmi

[orangexception]

If you've got FAR, then snag procedural fairings and auto struts will save you. Toss whatever you want under it and push away to orbit. If the launcher sucks at control, then toss some RCS on it. When you hit orbit, then rebalance your payload RCS tanks with TAC.

Stock-ishly/otherwise, build a sled around your payload and it'll work like a charm. This design effectively pulls your payload along instead of pushing, while maintaining a great center of mass, maneuverability, TWR, etc. They're simple to build to boot.