VTOL Fuel balancing.

[princessjane]

I have been trying to use the new rapier engines to build a SSTO VTOL. My largest hurdle has been fuel balancing. Because fuel drains from the first tank first it throws weight distribution out the window. Currently I am trying to separate the tanks and run a fuel line to each engine so all fuel tanks drain at the same rate, the issue here is the lack of parts being labeled fuel cross feed so I am having some issues with this as well. Anyone have any success in balancing large amounts of fuel through the entire burn?

Edited December 24, 2013 by princessjane

[Murph]

You'll need something like TAC Fuel Balancer for that. If it's set to "Balance All", that will probably stop the CoM moving around horizontally.

[Tank Buddy]

If you have vertically oriented fuel tanks and manage to make the fuel be consumed evenly you could be able to do so in stock.

[Traches]

You could put a decoupler or some other non-fuel-crossfeeding part between the engine and the last tank, and then run a line from the furthest forward tank to the engine. That should (I think?) cause fuel to drain from the aft first, keeping your CG forward and and therefore stable.

Or you could manage your fuel manually, turning off tanks and transferring it around in flight.

Edit: I looked right past the VTOL bit! (derp.)

You could still use cleverness with fuel lines and decouplers to control your fuel burn. Manually transferring it around might work too. The RCS build aid mod would be very helpful here.

[Smidge204]

A screenshot of the craft in question would help. It's hard to imagine a scenario where this happens unless the entire craft itself is not symmetrical.

=Smidge=

[princessjane]

A screenshot of the craft in question would help. It's hard to imagine a scenario where this happens unless the entire craft itself is not symmetrical.

=Smidge=

Any VTOL using more than a single fuel tank will have this problem and has nothing to do with symmetry. It's a ship not a rocket, as fuel drains from the front tank the center of weight shifts to the rear causing the craft to automatically pull up when hovering. I have 5 or 6 different designs atm and every single one this is a problem.

[princessjane]

If you have vertically oriented fuel tanks and manage to make the fuel be consumed evenly you could be able to do so in stock.

Turns it into a rocket and not a flyable aircraft.

[princessjane]

You'll need something like TAC Fuel Balancer for that. If it's set to "Balance All", that will probably stop the CoM moving around horizontally.

I will look into that. I was hoping for a vanilla option but I think I am at the final staw.

[Markus Reese]

Vanilla option. the following is what I did for all my VTOL prior to .23 With the thrust settings, this could get off kilter but here is how I used to do it.

Essentially I always VTOL with four engines. Front and rear. All engines the same ones. Then, I have the front and rear use isolated fuel supply. This allows mass to drain equally from both ends. I would use Kerbin to adjust engine placements until liftoff and landing was able to maintain orientation. Because they use balanced fuel, this usually did not change as you lost fuel mass. So therefore trick one, plan engine position to allow them to run the same thrust settings.

Second thing is thrust curves and adjustment on the fly. Can thrust percent be set while moving? I did this using mod parts, but not sure if can be done with vanilla. I had a rocket with a single tail engine that I used to overcome a slight full thrust variance. I had a long tail on the rocket as a counter balance and put an engine on the tail of it. The landing and takeoff could be done with 75% power. By increasing throttle, it would tail up, by decreasing, I would tail down. This allowed me to control pitch with just the throttle.

http://i529.photobucket.com/albums/dd332/KwKiller/Kerbal%20Space%20Command/screenshot87.png for an example I guess. Earlier version of KSP. Now you can place radial engines wherever and disabling fuel crossflow can prevent one set of engines from drawing from another. When in horizontal flight, might need to configure with central mass fuel tanks only?

Edited December 24, 2013 by Markus Reese

[princessjane]

Vanilla option. the following is what I did for all my VTOL prior to .23 With the thrust settings, this could get off kilter but here is how I used to do it.

Essentially I always VTOL with four engines. Front and rear. All engines the same ones. Then, I have the front and rear use isolated fuel supply. This allows mass to drain equally from both ends. I would use Kerbin to adjust engine placements until liftoff and landing was able to maintain orientation. Because they use balanced fuel, this usually did not change as you lost fuel mass. So therefore trick one, plan engine position to allow them to run the same thrust settings.

Second thing is thrust curves and adjustment on the fly. Can thrust percent be set while moving? I did this using mod parts, but not sure if can be done with vanilla. I had a rocket with a single tail engine that I used to overcome a slight full thrust variance. I had a long tail on the rocket as a counter balance and put an engine on the tail of it. The landing and takeoff could be done with 75% power. By increasing throttle, it would tail up, by decreasing, I would tail down. This allowed me to control pitch with just the throttle.

As for the first part I am VTOLing with 6 engines, basically at the points of a rocket with 6 symmetry turned on. They all have their own small tank. This alone does not give me enough fuel to then also be able to sustain hover or reach orbit (where I would then refuel). The issues start when the center of the craft, typically several tanks in a row making the center of the ship start to drain.

The second part may work although I was hoping for something that did not require adjustments on the fly as flying a VTOL with any kind of precision requires plenty of real time adjustments as it is.

[Zephram Kerman]

The simplest way to deal with this (and for regular space planes, too) is to put all your fuel tanks at the CoM. They can be in a row side-to-side. That way, as the fuel tank(s) drain, the CoM does not shift at all. No compensation needed. I also like to put VTOL engine(s) along the CoM as well.

Here's one example, from 0.19, I think:

Basically, I try to put anything variable along the pitch axis, such as: fuel tanks, kethane buffer, VTOL engine. CoL and main wheels go close to there as well. It's a very popular place.

[Smidge204]

Any VTOL using more than a single fuel tank will have this problem and has nothing to do with symmetry. It's a ship not a rocket, as fuel drains from the front tank the center of weight shifts to the rear causing the craft to automatically pull up when hovering. I have 5 or 6 different designs atm and every single one this is a problem.

Symmetry includes symmetrical fuel usage, not just vehicle assembly symmetry.

Something to try: Individual fuel tanks drain "Bottom to Top" and rotating them can change the way COM shifts as fuel is used. You might be able to exploit this to keep everything balanced. This VTOL craft remains perfectly balanced; note how the two tanks are mirrored, and the fuel's center of mass for each tank creeps towards the center as it's used up:

That's what I'm talking about when I say symmetry...

=Smidge=

[Markus Reese]

Good example of the splitting your fuel drainage. ^^^^^^

That type of split could be key to making the vanilla stack tanks work. If you put a no crossflow break in the middle of your stack and have an even number of tanks, then the stack should drain from middle outwards preserving your weight balance alot more!

[princessjane]

Thanks to Zephram I split the tanks with my command pod and have a balanced 6 tank VTOL. Anyone want to help get it SSTO? I'm using rapier engines for both directions of travel.

Also I discovered if you run lines to a fuel crossfeed part attached to engines it will drain equally from all attached tanks. That is how I balanced the forward thrusting engines fuel supply.

Edited December 24, 2013 by princessjane

[Markus Reese]

Great find on the using lines! That is one tip I will do in all my planes from now on!!!

[barrenwaste]

Ok, I have some thoughts. First, why are you using rockets in atmo? Even if you don't use anything but rocket fuel tanks you should be using jet engines for propulsion until you reach to high an altitude for your airhogged platform. The jet engines will use far less fuel, this will mean you will likely not have to worry about fuel balancing until reentry. Jet engines can also lift far more per engine then all but the largest rocket engines, meaning you'll need less of them which will reduce balance issues even further. Also, instead of using multiple small tanks I suggest you use a single or even two large tanks. If all fuel comes from one tank then you have no balance issues to deal with at all. With two tanks, each one with it's own jet engine, you'll have an equal burn and not have balance issues either.

[princessjane]

Ok, I have some thoughts. First, why are you using rockets in atmo? Even if you don't use anything but rocket fuel tanks you should be using jet engines for propulsion until you reach to high an altitude for your airhogged platform. The jet engines will use far less fuel, this will mean you will likely not have to worry about fuel balancing until reentry. Jet engines can also lift far more per engine then all but the largest rocket engines, meaning you'll need less of them which will reduce balance issues even further. Also, instead of using multiple small tanks I suggest you use a single or even two large tanks. If all fuel comes from one tank then you have no balance issues to deal with at all. With two tanks, each one with it's own jet engine, you'll have an equal burn and not have balance issues either.

Rapier engines are both jet and rocket engines. That is specifically the reason I am using them. This craft theoretically should be able to land anywhere in the solar system given it is able to refuel.