Space Plane Epiphany! Center of Mass issues possibly solved

[ArdeaHerodias]

Today I created a space plane with an open center section using the awesome B9 mod (I used basically trusses that left an open middle separating the front section from the back on the sides of the plane). As I was flying I noticed my fuel tanks were drawing unevenly (I had one fore and one aft). We know that the program preferentially draws from the tank furthest from the center of mass. This means as long as you don't have a fuel tank directly on the center of mass the program will automatically maintain the starting center of mass.

Perhaps I'm over excited but my initial testing seems to show this should be the case.

[Vanamonde]

I'm not clear about what you're describing. Could you perhaps post some screenshots of your ship?

(And by the way, welcome to the forum , and I am moving your thread to the how-to sub-forum, where it will get a better reception.)

[jsfalconero]

Structural fuselage at the center of mass? I might have to try this.

I thought it drew from the forward most tank first though.

[ArdeaHerodias]

Here is the pic of my ship, there is a liquid/ox tank both in front and behind the center of mass.

hooray! i figured out the imaging thing ha

Edited November 6, 2013 by ArdeaHerodias
image issue

[Vanamonde]

Sorry, I don't use Steam. Can someone nice tell ArdeaHerodias how to post a screenshot through Steam?

[annallia]

Structural fuselage at the center of mass? I might have to try this.

I thought it drew from the forward most tank first though.

I thought so as well... I guess its time to make a top heavy rocket to test this.

Edit- I don't use steam either, though it should work the same way it would from photobucket. At least it works in the previews...

[ArdeaHerodias]

Looks as though its a coincidence of my design. Since the engines are positioned right on the center of mass.

[ArdeaHerodias]

It's strange, I cannot replicate this with a simpler design.

The engines are furthest from the forward tank, but for some reason it draws from the rear tank more

[Geschosskopf]

Another option that I find helpful is the mod RCS Build Aid. The newest version of this now shows CoM markers for both full and empty fuel tanks. It's possible to juggle them around so the CoM doesn't move as you burn fuel. Brilliant

[ArdeaHerodias]

Ok, I think I have figured it out. It has to due with part numbers from the tanks. Not sure why it would choose to drain fuel unevenly like that though. Shouldn't it just draw from one fully, then the next?

[AlamoVampire]

Sorry, I don't use Steam. Can someone nice tell ArdeaHerodias how to post a screenshot through Steam?

probably the same as posting screen shots from the non steam variant: F1 to capture the picture, then go to any free picture hosting site. this is in response to vanamondes request for ArdeaHerodias

[JumpsterG]

I haven't used the B9 pack, but I betcha some engines are drawing from different tanks. I notice 10 active engines, so my guess is you have 6 drawing from one and 4 drawing from the other. The rear might only have access to the tank directly in front of it due to lack of crossfeed or something.

On the runway: Put the brakes on, set throttle up just half notch, and right-click activate one engine at a time and see where the fuel is drawing from.

You could try running fuel lines from both tanks to a single part with crossfeed and see if it draws evenly then.

[Zephram Kerman]

How weird! Please post here what you find out. I'm pretty sure JumpsterG is correct though; it seems like a fuel routing thing.

I've always avoided the problem by positioning all fuel tanks to match the Center of Mass. But if this effect can be tamed, it would allow more interesting arrangements.

[Hodo]

TAC fuel balancer is the mod you need.

[Goobest]

I haven't used the B9 pack, but I betcha some engines are drawing from different tanks. I notice 10 active engines, so my guess is you have 6 drawing from one and 4 drawing from the other. The rear might only have access to the tank directly in front of it due to lack of crossfeed or something.

On the runway: Put the brakes on, set throttle up just half notch, and right-click activate one engine at a time and see where the fuel is drawing from.

You could try running fuel lines from both tanks to a single part with crossfeed and see if it draws evenly then.

Yah, it's either 6/4 or 8/2. Tank distance is the determining factor, yes?

[Shrike42]

When designing planes, you can try putting all your fuel tanks in the same plane (all in a row), so no matter which one drains first, the CoM doesn't move. It can be a bit awkward to look at, but hard to argue with the results.

[JumpsterG]

Yah, it's either 6/4 or 8/2. Tank distance is the determining factor, yes?

It seems to be. The rear tank is draining faster, so I'm thinking the more numerous engines on the wings are pulling from there, while the rear engines pull from the front tank.

To maintain the design, it'd probably need to resort to using fuel lines. If both tanks have lines going to one common part that all engines have access to, they should drain evenly, but this needs verification via testing. Alternatively, it could be brute forced by leading one line to every engine, 5 fed from the front tank, 5 fed from the rear, but that could look ugly.