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Can't Fly Straight?


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So I recently was able to fly straight, get into orbit, set an orbit, and return my little Kerbal home.  I was able to then do some research, and I've now got the ability to add a nice, little science capsule onto my ship.  So I add that to the basic design I put forth in another thread (linked below)...and now I can't even fly straight.  About 10 seconds after lift-off, the ship starts wobbling all over the place, then does a bunch of loops, and then crashes into the ground.  I've tried all of the following:

  • SAS turned on
  • SAS turned off
  • Full throttle
  • No throttle
  • Added winglets
  • Removed winglets
  • More engines
  • Less engines

I've tried staging during lift-off, but I've got a whole host of Kerbals that have paid the price for this as the engines will explode upon staging.

What am I doing wrong?  I've tried placing the winglets below where center of mass should be (2, 3, 4, 6 - nothing works), as well as adding winglets above center of mass as that will change once I stage.  But nothing is working here.  Why is my ship pretending as if it's a roller coaster?

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A picture of the craft would help, is it terribly bendy? That can be a problem before you get struts.

Some general rocket advice though:

A good rocket looks like an arrow, mass at the tip, long sturdy shaft, fins at the rear for stability.

It may be an over simplification of the problem, but in general you want your COL as low as you can get it on a rocket.

You may also just need to be more gentle with your gravity turn, can you describe your ascent to us? If you are going too fast in the dense atmo that can be an issue as well when trying to turn.

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24 minutes ago, Rocket In My Pocket said:

A picture of the craft would help, is it terribly bendy? That can be a problem before you get struts.

Some general rocket advice though:

A good rocket looks like an arrow, mass at the tip, long sturdy shaft, fins at the rear for stability.

It may be an over simplification of the problem, but in general you want your COL as low as you can get it on a rocket.

You may also just need to be more gentle with your gravity turn, can you describe your ascent to us? If you are going too fast in the dense atmo that can be an issue as well when trying to turn.

I'll see what I can do to post a picture of the craft, but it's essentially an arrow.  And the looping happens whether I attempt to turn or not.  I can turn on SAS and then not touch my keyboard, and the rocket starts turning all over the place.

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SAS set on stability can actually cause issues if your rocket's design requires it to fly perfectly prograde.

Do you have any probe cores, extra pods, or docking ports on the rocket?  Is your navball completely blue when sitting on the launchpad?

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14 minutes ago, Geonovast said:

SAS set on stability can actually cause issues if your rocket's design requires it to fly perfectly prograde.

Do you have any probe cores, extra pods, or docking ports on the rocket?  Is your navball completely blue when sitting on the launchpad?

Well, considering how early in the game I am, I only get one option for SAS.  And the navball is completely blue when sitting on the launchpad.  No probe cores, no extra pods other than the science pod, no docking ports.  It's a freaking arrow with some engines and winglets.

I just test launched this again, and after several times of sitting there and doing nothing, it went straight.  On the way back to Kerbin, I had it at retrograde so it would enter the atmosphere heat shield first.  And at 60,000 km, it somehow decided that the parachute was top-heavy and turned into a bullet, parachute first, heading for the ground.  My whole guess on this is that I'm somehow not accounting for the extra weight added by the Science module here (the original design to get this into orbit was everything you see EXCEPT the science module).

ciHKtrT.jpg

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2 minutes ago, Popestar said:

My whole guess on this is that I'm somehow not accounting for the extra weight added by the Science module here

Not necessarily that you're not accounting for the extra weight, but having the module there is moving your CoM.  The pod's a lot heavier, so it's pushing your CoM up.  Nothing to do with the parachute.

In the VAB, if you remove everything except what's going to be re-entering, you can click the small round button immediately to the right of your price indicator to see where your Center Of Mass is.  You need that as close to the heat shield as possible.  If it's too high, it will flip.

As for launch, it seems like it should be fairly stable.  Which LFO engine is in that shroud?  The Swivel or Reliant?

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14 hours ago, Popestar said:

....  On the way back to Kerbin, I had it at retrograde so it would enter the atmosphere heat shield first.  And at 60,000 km, it somehow decided that the parachute was top-heavy and turned into a bullet, parachute first, heading for the ground.  My whole guess on this is that I'm somehow not accounting for the extra weight added by the Science module here (the original design to get this into orbit was everything you see EXCEPT the science module).

I'm a bit confused by the pic, because I can't see a decoupler between the Flea and the science module.

I've tried recreating your craft and the only way I can get it to look like the pic is by NOT having a decoupler, removing the Shroud from the heatshield, enabling staging for heatshield jettison, and offsetting the Flea upwards.

If that is actually what you've done, that stage 1 is actually you getting rid of the heatshield altogether. You'll find it very hard to maintain a retrograde approach on re-entry because (1) if the flea is still attached, it's very light and will act perfectly as draggy feathers to your pointed pod, and (2) once you jettison the HS, your science module is very light and will do exactly the same thing. Plus your science module will probably blow up from excess heat.

It is quite possible to return from orbit with a heatshield, science module and pod, but it's tricky without retrograde hold. The slightest deviation from retrograde once drag kicks in (and remember this is surface retrograde, not orbit retrograde, so click on the navball to get it in the right mode) will be almost impossible to recover from. Frankly, the simpler solution is to get your pilots to do one full orbit each, so they gain the skill to use retrograde hold, before attaching the science module to your craft.

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2 hours ago, Plusck said:

I'm a bit confused by the pic, because I can't see a decoupler between the Flea and the science module.

I've tried recreating your craft and the only way I can get it to look like the pic is by NOT having a decoupler, removing the Shroud from the heatshield, enabling staging for heatshield jettison, and offsetting the Flea upwards.

If that is actually what you've done, that stage 1 is actually you getting rid of the heatshield altogether. You'll find it very hard to maintain a retrograde approach on re-entry because (1) if the flea is still attached, it's very light and will act perfectly as draggy feathers to your pointed pod, and (2) once you jettison the HS, your science module is very light and will do exactly the same thing. Plus your science module will probably blow up from excess heat.

It is quite possible to return from orbit with a heatshield, science module and pod, but it's tricky without retrograde hold. The slightest deviation from retrograde once drag kicks in (and remember this is surface retrograde, not orbit retrograde, so click on the navball to get it in the right mode) will be almost impossible to recover from. Frankly, the simpler solution is to get your pilots to do one full orbit each, so they gain the skill to use retrograde hold, before attaching the science module to your craft.

I could swear I have a TD-12 in there between the Flea and the heat shield, but if I missed it then that's a big one on me.  Which is one of the great things about this game - the literal infinite possibilities for putting stuff together and trying to get it to work.

I was going to try adding some radiant heat shields to the science lab, which would not only protect it from the heat upon re-entry, but also provide a bit of weight to keep the ship in retrograde as it re-enters the Kerbin atmosphere.  However, I think I'm going to take your advice and first get Jebediah into orbit for a couple of circles around Kerbin so he can gain the retrograde hold skill before trying to launch the science module again.  I keep forgetting that there is experience to be gained in this game for our pilots, engineers, and science junkies!  But first...I have to let Jeb respawn.  Poor guy died on re-entry last time I launched.  As you correctly predicted, both the science module AND the command module exploded from overheating.  Oh well - back to the digital drawing board!

And before I forget - thanks to everyone for all the help!

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Do you have EVA in space unlocked yet?

If so, just grab up the science, and leave the science bits in space, then de-orbit the command pod by itself. You won't even need a heatshield.

If you don't, I'd prioritize it. Once you have that you'll pretty much never need to re-enter with anything more than a Command Pod again. (With the exception of rescue/part retrieval missions.)

Edited by Rocket In My Pocket
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6 hours ago, Popestar said:

I was going to try adding some radiant heat shields to the science lab,

I wouldn't bother with that. Radiators are good for dealing with heat that builds up gradually and can be dissipated over a long time.

I haven't tried any spaceplanes with the latest couple of versions of KSP. In earlier versions radiators could be useful to stop Mk 2 cockpits from blowing up, since they slowed the heat spike just enough to survive reentry heating. However, for your case I'm not sure it would do any good, certainly a lot less good than a functioning heatshield and good reentry.

And while we're at it, you definitely don't need to weigh yourself down with all of the ablative on the default heatshield. 20% is perfectly enough to survive reentry from anywhere within the Kerbin system. However, if you reduce the ablative you reduce weight, so again think carefully about weight distribution and CoM while you have a science module attached to your craft.
As @Rocket In My Pocket clearly said, going on EVA, taking the science and then just dumping the science module is by far the easiest solution.
Which again shows how varied the possible solutions to your issue are! :D

6 hours ago, Popestar said:

And before I forget - thanks to everyone for all the help!

No need for thanks, I suspect most of the people who respond to such questions live for this ;)

Edited by Plusck
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So I went into orbit without a science module in the hopes of gaining experience with my Kerbal pilot, which would then grant additional maneuvers and coolness....only to find out that in the Science sandbox, which I'm playing, all Kerbs are maxed out on XP already.  The only thing to really do in the science sandbox is to gain science and do research and try out all kinds of different builds for rockets.

My big problem?  Trying to bring unnecessary equipment back to the ground when all I care about is the science points anyhow.  Time to put a TD-12 between the command module and the science lab.  If it turns into space junk, so be it!

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4 minutes ago, Popestar said:

If it turns into space junk, so be it!

It shouldn't.   You'll need to have completed your re-entry burn before jettisoning that section of the craft, so it will also re-enter and burn up. 

 

And what's with using a SRB as your upper stage?  Betcha you'd fly a lot better if you swapped out your 2nd & 3rd stage engines...

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