An idea to deal with in-flight shifting CoM in spaceplanes

[GoSlash27]

Nice! this is exactly the sort of scale I was talking about. I was messing around with spaceplanes more myself yesterday and came up with a very similar design: use the cargo bay to push to cockpit (and thereby also the dry CoM) forward. I just need to figure out what to put into the cargo bay now. Rovers, science equipment and sattelites are good contenders.

Cirocco,

The best thing to put in the cargo bay in that case would be "nothing". The CoM needs to be centered in the fuel tanks to keep it from shifting during flight, and adding/ removing payload in an unbalanced configuration will aggravate that.

If you have a payload bay at one end of a ship that you've added for balance, you should either leave it empty or leave it full.

If you want useful cargo bays, their weight moment needs to balance around the CoM both when they're full and when they're empty.

Best,

-Slashy

[Wanderfound]

Counterpoint: you can get away with off-centre cargo bays if you use them for things that land with the craft (SAS units, batteries, Goo pods, PB-NUK's, landers or rovers that return to the cargo bay, etc) and things that are so lightweight as to be trivially disruptive (e.g. your basic ultralight satellite: probe core, solar panel, antenna, small monoprop tank and not much else).

I often use passenger cabins to drag CoM forwards or back, though, if there's some reason I can't do it by shifting lateral tanks/engines. Slashy's right about the need to put cargo bays near CoM if you want to transport serious mass.

[Cirocco]

Counterpoint: you can get away with off-centre cargo bays if you use them for things that land with the craft (SAS units, batteries, Goo pods, PB-NUK's, landers or rovers that return to the cargo bay, etc) and things that are so lightweight as to be trivially disruptive (e.g. your basic ultralight satellite: probe core, solar panel, antenna, small monoprop tank and not much else).

http://i1378.photobucket.com/albums/ah120/craigmotbey/Kerbal/Beta/Kerbodyne%20Showroom/Stratos%20N/screenshot1023_zpsf92b0f45.jpg

I often use passenger cabins to drag CoM forwards or back, though, if there's some reason I can't do it by shifting lateral tanks/engines. Slashy's right about the need to put cargo bays near CoM if you want to transport serious mass.

Yup, this is what I was thinking: light stuff that wouldn't later the CoM in a very meaningful way. And if it does, I can just mess around with the placement of my 4 nuke engines, those things are so heavy I should easily be able to tweak with them.

Though now that I think about it: passenger segments would also be an excellent idea.

[numerobis]

The vertically stacked engine design seems like a good idea - you can use thrust limiting to create torque when you need it. Unlike with angling you get full thrust any time you don't need torque.

And ultimately maybe the best option is to just use TAC Fuel Balancer or Goodspeed.

Nice. And doing that on a mk2 bicoupler at the back means two things:

1. It's a rudder that's centered on the center of mass. If you have any slip, its drag will increase and its lift will point in the direction to reduce slip -- but it won't induce any roll.

2. It's a fuel tank with substantially less drag than other parts: just 0.1 drag unless you have slip (and most planes have little slip). This as opposed to most parts that have 0.2 drag, or other Mk2 parts mounted the normal way that will have 0.15 to 0.2 drag if you maintain a 30-degree angle of attack, as one does when trying to reach orbit.

[Wanderfound]

Nice. And doing that on a mk2 bicoupler at the back means two things:

1. It's a rudder that's centered on the center of mass. If you have any slip, its drag will increase and its lift will point in the direction to reduce slip -- but it won't induce any roll.

2. It's a fuel tank with substantially less drag than other parts: just 0.1 drag unless you have slip (and most planes have little slip). This as opposed to most parts that have 0.2 drag, or other Mk2 parts mounted the normal way that will have 0.15 to 0.2 drag if you maintain a 30-degree angle of attack, as one does when trying to reach orbit.

The vertical fuselage Lightning knockoff is remarkably stable under extreme duress:

Quick, too:

[Louella]

is it possible to make it drain all fuel tanks simultaneously ?

or do you always have to manually shift fuel around to balance the aircraft ?

[Cirocco]

is it possible to make it drain all fuel tanks simultaneously ?

or do you always have to manually shift fuel around to balance the aircraft ?

You certainly don't have to constantly shift fuel manually, I never do it. You can direct which tanks drain in what order through proper application of fuel ducts, however I'm afraid I'm no expert on fuel flow mechanics in KSP.

The thing I alway tend to do and which has served me very well is to interconnect all my tanks with two-way fuel flow. To clarify:

say I have a plane with 1 center tank and 2 side tanks (one on each side) like this:

tank1 ---- tank2 ---- tank3

then I would use fuel lines to connect tank1 to tank2 and tank3 to tank2, but also put fuel lines connecting tank 2 to tank 1 and tank 2 to tank 3. Doing this, I've never had any problems with draining tanks, assuming that the dry CoM and the wet CoM don't lie too much apart. (another possible configuration using slightly less fuel lines is connecting tank1 to tank2, tank2 to tank3 and tank3 to tank1 again).

Mind you, all this assumes that your dry CoM and wet CoM don't lie far apart. If they do, you need to either adjust the design or take additional measures to balance the plane throughout the flight.

and of course there's always the fuelbalancer mod.

Edited January 16, 2015 by Cirocco

[Janos1986]

The vertical fuselage Lightning knockoff is remarkably stable under extreme duress:

Quick, too:

Aren't the wings a little too stubby? Just saying.

[Wanderfound]

Aren't the wings a little too stubby? Just saying.

It's not intended to be a perfect Lightning replica (I was just messing about with vertical fuselages and low-set wings to see what would work), and it ain't lacking for lift:

More wing would just weigh it down and slow the roll rate.

[Janos1986]

NP, I was concerned about lift, but if it does its job then it's fine.

[KerikBalm]

Well, What I normally do, is have rocket engines at the back, cockpit in the front, and jets mounted in the middle along the center of lift.

That design barely works.. I had to quick load a lot because it wasn't very stable, and I don't even recall how much fuel I was reentering with.

I think I could slide the jet engine pods forward, and split the forward rocket fuel tank into two smaller ones, one in front of the cargo bah, one behind.

I've also made designs where the jets are farther forward, but the aesthetics aren't as nice. In NEAR, the designs work quite fine, but I'm having a lot of trouble in FAR with stability at high mach, and I think it may be due to the forward placed intakes, but I really don't want to put intakes behind the engines... it works in game, but feels cheaty to me.

Edited January 16, 2015 by KerikBalm

[InterCity]

The lightning designs seems intriguing to me. What a pity that it's atmospheric only...

BTW, have you tried it with FAR? If yes, how did it behave?

I've came up with a quite interesting wings, double delta with uneven trailing edge and FAR seems to like it. My hull design and balance... not so much.

Once I tweak it, I'll show the entire design.

[7499275]

Someone posted back a few pages ago the Buran had a engine under the dispoable tank and one under the shuttle itself. It adjusted throttle to maintain balance, we could handle most space places and rockets in KSP like this... We just don't have a whole team filling a command center to control all these small aspects.

Like one watching fuel, one watching each engine. etc...

[marach]

The lightning designs seems intriguing to me. What a pity that it's atmospheric only...

BTW, have you tried it with FAR? If yes, how did it behave?

I've built a copy and it handles pretty well in FAR, heck stick a couple of Rapiers on it and instant SSTO (though I recommend thrust limiting one of the engines if you want it to fly level with both on)

[rynak]

I tend to do something similiar than the OP, for planes where the CoM shifts a lot.... especially for transporters that are designed to carry stuff piggyback. Basically, what i do is have one set of engines on a hinge via robotics.... then use the hing to emulate minor VTOL ability. The engines don't have to be overly strong compared to the main ones (so, no actual need for outright hover capability).

EDIT: This also helps a lot with getting very heavy loads off the runway.

[Sufficient Anonymity]

When building SSTOs, I tend to start with the cargo bay (or crew tanks) as the root part, then build outwards from there, ensuring the fuel tank setup is the same fore and aft. Means that the CoM stays in pretty much the same place however full the tanks are or what payload is present. Whack on a Skylon-esque wing and engine arrangement, and you'll stay balanced pretty much whatever you do (though TAC fuel balancer is nice to ease fuel pumping woes).

[GetOutAndPush]

Rocket balance is easy, spaceplane balance can be annoying.

SSTO VTOL dropship balance is a serious pain.

74 tons (fueled for Orbit takeoff weight) of "You Know, You Should Really Just Put Wings On This And Go To Bed".

You can't really say this "flies" without using air quotes. It can take off, achieve stable orbit with a bit of cargo, de-orbit and land successfully (I play with FAR). It also putters around in atmosphere quite nicely and can cruise at supersonic speeds. After some tweaking I intend to take it on a joyride to visit various other planets.

I find that once you start getting in this weight range that Firespitter's VTOL steering and hover functions cease working entirely (you will just fall out of the sky) and the B9 compressed-air RCS adds weight faster than it adds effective attitude control.

If you want to fly this, it's just you, Jeb, manual engine tilt, throttle, air brakes, and 2 gigantic reaction wheels cranked to 200 and powered by a bunch of RTG's. ActionGroupsExtended is pretty much mandatory.

You can actually get more mass into space using the S2 fuselages, 4 normal sized VTOL jet engines, and two rockets, and the hover commands will actually work - but then you miss out on flying a ridiculous ship.

Balance has to be fairly exact both loaded and unloaded. Minor discrepancies in the 2 main LFO tanks when you're not fully loaded are fine, but you can't use the Monoprop tank in the command pod. I build everything around the central cargo bay so that I can add cargo without having to rebalance the whole ship, then use symmetrical tanks around that for takeoff. Fore-aft fuel balance is irrelevant in space so you can add more fuel after orbiting. The tail is hollow for interplanetary fuel loads.

I find the easiest way to fine tune that last bit during building and updating is to tweak the mass/strength ratio on the control surfaces. RCS Build Aid is great, but you can get it close enough watching the way the CoM and CoT spheres clip through each other.

And yes, I'm going to put a ladder on it at some point.

[SAI Peregrinus]

For VTOLs, TCA is a great help. It controls the thrust limiters on each engine independently to maintain stability. It's a lot like the systems used in the Saturn or Falcon 9 rockets to provide engine-out capability and, in the case of Falcon 9, vertical landing stability.

[GetOutAndPush]

Both that and Firespitter should work great with rockets, but the large jet engines are problematic because of throttle response.