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WillWam

What can I put on my rocket to shift the center of mass away from the center of thrust?

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I've been trying to launch my rocket to my space station, only to have my rocket to start spinning uncontrollably in Kerbin's lower atmosphere.  I know it does this because my center of mass is to close to my center of thrust, but what can I add to my rocket to make it more... top heavy. Are there weights? I'd prefer to avoid mods at this moment. Thanks for your help in advance.

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Ok, this is kind of an XY problem, sorta. 

To answer your question directly...  You can add fuel tanks or batteries to the top of your rocket.  That will shift the COM upwards.  

But that's not the real problem.    The real problem is your are not asking the right question.    Center of thrust really doesn't play that big of an issue with a launcher, unless you have some really funky designs (if so, pics please). 

The right question is, How do I get my COM above the center of drag? (paraphrased)

So, you have two options.   You can shift your center of drag downwards, or your COM upwards. 

To shift your CoD (which isn't displayed in the game), you need to add draggy bits (Fins) to the bottom of the rocket, or make the upper stages more streamlined. 

To shift your CoM, you need to add mass higher up on the craft.   One method I use is to set my fuel flow priority so that the lower of the main tanks empty first, leaving the higher up ones still full.   

Another option is to add a fairing to your payload or upper stages.  Not only will this help stream line your upper stage (if built correctly), but it also adds mass, so it works by shifting both your CoD and CoM

Others are going to come along now and add their very valid two cents, including some links to good threads on this topic, and how I'm technically wrong in using CoD.

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3 hours ago, Gargamel said:

Others are going to come along now and add their very valid two cents, including some links to good threads on this topic, and how I'm technically wrong in using CoD.

Technically right or wrong doesn't really matter (to me) gameplaywise, as long as it works! And this works!

Easiest way without really altering the design to much is to place fins at the bottom of your craft. If a launcher still flips/spins I add more fins. If it still spins I set the fuel flow as Gargamel describes. If it still spins after that, I bin the design and start over with another design.

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11 hours ago, bewing said:

If tweaking your design doesn't work in the end, there is one other option. A rocket is significantly more stable flying straight up. So if you forget about your gravity turn until you are above 36km or so, you may be able to keep your final design. It won't be as efficient to get into orbit, but sometimes efficiency loses out to just making something work.

 

If you have launch clamps, you can also angle the craft slightly in the VAB so that it starts the gravity turn before even launched, so to speak. :)

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2 things. Ore tanks. Kerbals on ladders.

Ignore me im just kidding.

Jokes aside, give us a screenshot of your rocket. This will help others diagnose the problem.

Edited by Xd the great

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If tweaking your design doesn't work in the end, there is one other option. A rocket is significantly more stable flying straight up. So if you forget about your gravity turn until you are above 36km or so, you may be able to keep your final design. It won't be as efficient to get into orbit, but sometimes efficiency loses out to just making something work.

 

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I like modifying the fuel priority of the tanks on the stage causing you problems. Set the priority on the bottom tanks to highest and top tanks to lowest, and as you burn fuel your rocket will become MORE stable, as the COM shifts upward.

Edited by 5thHorseman

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Fins at the rear are preferable to useless ballast mass at the front for a couple of reasons...

  1. Fins are lighter, so you are carrying less deadweight to orbit and wasting dV
  2. Some fins are steerable, which helps with getting a good gravity turn. 

There are downsides though: Fins add  mass and, particularly, more drag. So if you design the rocket to minimise or avoid them then it can be more efficient but perhaps not enough to matter much unless the craft is large. Then a combination of keeping mass high with fuel priority, using engines with good gimballing, and some fins will probably be efficient.   

Edited by Foxster

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Good point about the gimbal, that can make or break a marginal design.  I wonder just which engine the OP is using...

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