Unmanned rockets tipping over mid-flight

[keyscapeunit]

Hello,

I'm designing some prototypes to orbit the Mun (in advance of my first manned mission), and these include solar panels, the Science Jr, a few Goo Canisters, and a thermometer along with rechargeable batteries. I should note that I am playing Science Mode.

I've successfully sent a couple of manned rockets into orbit with no issue. I have that down pat.

However, it seems that I have issues with unmanned rockets. Somewhere between 10K and 25K (depending on how much boost I give the ship before I disengage boosters and switch to liquid) my rocket tips over and begins spiraling uncontrollably. This happens when I disengage booster rockets and switch to liquid fuel. Obviously this doesn't work well. I initially thought it had something to do with aerodynamics as my initial CoM and (what I think) it translates to after fuel usage seems OK, but I could be wrong.

At any rate, I've tried several rocket models and all have the same issue. Are there any tips out there? Also, in terms of Kerbin orbit for a Mun orbit mission, how high should my Kerbin orbit be or does it matter? I've been trying to get way out there in orbit but maybe that's overkill.

[Claw]

It might be that your "usual" kerballed designs have enough torque to compensate for your build style. Many of the pods have a fairly high torque rating, whereas the probe cores have very low (or none at all). Sounds like after staging off your boosters, there's enough drag up front that your rocket doesn't have the control authority to keep the rocket pointed the right way.

Try adding some fins or a bit of torque. Also, having several science bits up top (which is hard to avoid) also adds to the drag at the tip of the rocket. So if this particular rocket has more science bits than your usual rocket, that might be adding to the issue.

Also, welcome to the forums! :D

Cheers,
~Claw

[S1gmoid]

Well, also probe cores are very un-aerodynamic. I wouldn't dream of launching one without a fairing. (You probably need to install the Procedural Fairings mod to get usable fairings early in the game.)

[keyscapeunit]

Thanks y'all! Would it be typical to add two fins bilaterally or four? Also, how are these unmanned probes supposed to get into orbit without mods?

[LN400]

As mentioned above, without fairings (stock or mods) or at least a nose cone of sorts to streamline the nose of that rocket, together with fins at the bottom, launching will be a proper challenge. If you haven't unlocked the stock fairings yet, you probably have unlocked at least one nose cone. Strap that and a few fins on. See if that doesn't sort it out.

As for number of fins. Depends. I usually go for 3 or 4, partially determined by number of boosters, partially out of sheer need, but occationally I find I need more fins, 6 or 8 to get a proper behaviour from the rocket.

EDIT: One final thought. If you haven't already, put the science stuff in a service bay. This is especially true for the goo cannisters. Having them sticking out will give you more drag up front where you don't want more drag. Edited November 16, 2015 by LN400
Adding to post

[Streetwind]

[quote name='keyscapeunit']Thanks y'all! Would it be typical to add two fins bilaterally or four? Also, how are these unmanned probes supposed to get into orbit without mods?[/QUOTE]

For passive guide fins, three are enough if they are in symmetry. With just two, you'll stabilize only one lateral axis out of two.

For active control surfaces, you want four in symmetry. While three is enought to stabilize both axes, trying to steer with three linked control surfaces makes your rocket go directions you didn't intend it to go (trying to pitch induces yaw forces etc).

[keyscapeunit]

Thanks everyone. I unfortunately don't have the science bay yet, and nose cones don't go on the stayputnik.

However, I was able to get this to work by making the rocket smaller and faster with some solid fuel boosters and fins at the base. It will still spin (slowly) like an artillery shell once I drop boosters, but I learned to let the boosters carry it over 12K, ditch them when they flame out, and then keep the engines off until the rocket starts to stabilize (the engines will finalize the stabilization). At that point I was able to get into orbit and made it to the Mun for a couple of orbits and back on my first attempt. Wow, getting to the Mun is easy once you get into Kerbin orbit!

[Claw]

[quote name='keyscapeunit']Wow, getting to the Mun is easy once you get into Kerbin orbit![/QUOTE]

Haha. Everyone has their challenge areas. I still remember smashing into the Mun repeatedly trying to figure out how to land there. :P

Congrats!
-Claw

[keyscapeunit]

:) I have no problem with manned rockets, but this stayputnik troubled me for over a week. I spent countless hours just trying to get it into orbit before figuring it out... next up, Kerbals to the Mun! Thanks again everyone.

[Pecan]

[quote name='keyscapeunit']...Wow, getting to the Mun is easy once you get into Kerbin orbit![/QUOTE]

The quote is "Once you get to earth orbit, you're halfway to anywhere in the solar system" [URL="http://www.quote-wise.com/quotes/robert-a-heinlein/once-you-get-to-earth-orbit-youre-halfway-to"]Robert Heinlein[/URL]. Manoeuvres in space have their own considerations but, yes, getting to orbit reliably is quite an achievement in its own right.

[Kerbart]

No SAS on a stayputnik. That might have a big impact.