Dry Center of Mass

[Scarecrow71]

So, I've been having a problem with my Drespollo rocket when staging in that boosters and empty fuel tanks aren't being ejected far enough away from the main ship when activating the radial decouplers.  I've done a bit of searching on this. And the common answer im seeing is to place the boosters so that their dray center of mass is just a hair below the middle of the decoupler.

So what is the problem?  Well, I can't seem to figure out how to display the dry CoM on a given fuel tank.  There is no option in the VAB or SPH to show this on a given part, only on the ship as a whole.  And at that, it shows current CoM; to show dry CoM on the ship, I have to drain the tanks.

How does one display the dry CoM on a tank?  I'd like to get this sorted so I can eject boosters properly without them crashing into the rest of my ship and blowing it up.

[antipro]

are the Sepratrons not enough?
about to show booster dry com , can't you just attach a light root probe core on it?

Edited November 6, 2021 by antipro

[Admiral Fluffy]

Try comparing the dry COM to the wet COM.

I’ve never tried that, but it seems like it would work.

[18Watt]

I don’t know an easy way to do that in stock KSP.  What I do is look at the COM full, and then drain the fuel tanks one by one in the SPH, and see what happens to the COM.

With a lot of fuel tanks, that is tedious.

Im sure there are mods which can help with that, but in stock KSP the only way I know of is to drain all the tanks in the SPH.

As a design consideration, I try to place fuel tanks as close to the COM as I can.  I also try to use bigger tanks, instead of lots of smaller tanks, so it’s easier to drain the fuel in the SPH if needed.

In many KSP plane designs, the engines are usually the heaviest part, and are often mounted on the back of the plane.  This causes a huge COM shift to the rear after using fuel.  What can happen is the COM shifts too far back, and the plane becomes unstable.

Ideally you also want the engines to be mounted near the center of the plane too, but that is often wildly impractical.

[Vanamonde]

You can use the move tool to slide the boosters down on the decouplers. 

[Scarecrow71]

2 hours ago, Vanamonde said:

You can use the move tool to slide the boosters down on the decouplers. 

Yeah, I've done that.  And ther still doesn't seem to be enough force on them during ejection.  When I stage, the boosters merely fall off, sliding straight down instead of being pushed away from the craft.

[Vanamonde]

Oh, mine often work that way. Doesn't matter as long as they don't hit the ship. 

[Scarecrow71]

5 hours ago, Vanamonde said:

Oh, mine often work that way. Doesn't matter as long as they don't hit the ship. 

The big issue is that I stage after I've started turning, and the boosters always fall into he ship.  So I'm looking for a way to prevent that.

Edited November 8, 2021 by Scarecrow71

[18Watt]

7 hours ago, Scarecrow71 said:

I stage after I've started turning

I recommend not turning while dropping boosters.  At all.

Also, I throttle way down, sometimes to near zero thrust while staging boosters.  If their relative velocity is low when they bump into the rocket, usually no damage occurs.

Instead of dropping boosters while turning:

• Reconfigure your rocket so you don't need to do that, or
• Keep the spent boosters until you are done turning.

[Scarecrow71]

6 hours ago, 18Watt said:

I recommend not turning while dropping boosters.  At all.

Also, I throttle way down, sometimes to near zero thrust while staging boosters.  If their relative velocity is low when they bump into the rocket, usually no damage occurs.

Instead of dropping boosters while turning:

• Reconfigure your rocket so you don't need to do that, or
• Keep the spent boosters until you are done turning.

Not sure I was articulate enough, but how do I do anything if I have to fly straight up all the time?  I have to turn at some point, and I will have to eject boosters.  I've never had this problem until trying to do Kerpollo.

[Zhetaan]

14 hours ago, Scarecrow71 said:

The big issue is that I stage after I've started turning, and the boosters always fall into he ship.  So I'm looking for a way to prevent that.

How many boosters?  I try to avoid this when I can, but I have taken a rocket with a four-booster radial stack and thrust-limited two of them so that I could stage them in pairs.  The trick, then, is to execute a roll program so that the pairs of boosters decouple to the sides, rather than above and below the main rocket body.

If you're still having a problem getting the boosters away from the vessel, then you have a few possible solutions.  One is to use a Seperatron.  My preferred configuration is to use one on the outside side of the nose cone, upside down and essentially pointed so that it pushes the nose of the booster down and away relative to the rest of the rocket.  I keep the decoupler near the nose, as well (fuel lines work a lot like struts and hold the bottom steady), but I don't know what the limitations are for a Kerpollo, so you'll need to see for yourself whether that fits in the rules.

Another solution is a passive one; use aerodynamics to pull the spent booster away from the rocket.  You can do this with nothing more than a basic fin on the outside of it; once you decouple, the drag from the fin will pull the booster away.  You could possibly see a similar result by using a radial drogue parachute, but you'd need to be low and slow enough that it would at least semi-deploy.

Another solution--admittedly, a bad one--is to carry the empty tank with you until you're far enough out of the atmosphere that you won't need to worry about collision when you do decouple it.  If you run out of booster right before you start coasting to apoapsis (assuming that your program has you doing that), then you'll lose a bit to drag but otherwise won't spend too much in letting the tank ride until you're nearly ready to thrust again and put some distance between it and the main rocket.

[18Watt]

5 hours ago, Scarecrow71 said:

Not sure I was articulate enough, but how do I do anything if I have to fly straight up all the time?

 Not turning does not mean you have to fly straight up.  It just means you are not turning.  As in moving the nose of the ship away from the pro-grade vector.  You could be flying horizontally, in a straight line.  Or at a 45 degree angle.  Or straight down.

If you have basic SAS modes, engage Pro-Grade hold, and leave it there while you stage your boosters.  Don’t try to alter your flight path away from the Pro-Grade vector (what some would call ‘turning’) while the boosters are being staged.