Need help with a VTOL aircraft.

[Starwhip]

Hi. I've tried to make a VTOL spaceplane before, but cannot seem to get the thing to balance correctly. How many engines do I need? Where do I put them? Where should the center of mass be? I have created aircraft capable of flying, and quite well, but any tips would be great.

[Scarecrow88]

For a stable VTOL, the centre of thrust of your vertical engines needs to be totally in line with your centre of mass. i.e. if your CoT is behind your CoM then the craft will drift forward on takeoff. The further out of line they are, the faster the rate of drift.

As for number of engines, as long as your thrust to weight ratio is greater than 1, you will lift off. The more above 1 your TWR is, the quicker you will go up.

Edited November 4, 2013 by Scarecrow88

[Col_Jessep]

For anything VTOL I would suggest to look at a couple of videos from Cupcakes: https://www.youtube.com/user/CupcakesLanders

I think you should probably start with VTOLs without wings. They work just as fine and you have a lot less headaches.

If you absolutely MUST have wings I think your vector of thrust should go right through your center of mass. (Replace the engines for forward thrust with parts of roughly equal mass and size in the editor to see the pure up-thrust vector, then fine-tune with useful but light parts like RTGs/batteries/science equipment until you have a stable caft.)

[Starwhip]

Okay, those help, but here's my main problem: The ship balances perfectly for, oh, 20 seconds or so. Then the engines have drained enough fuel to upset the balance. So how do you counteract that? Can you even do so?

[Traches]

The RCS build aid mod can help-- it shows both your full and empty CoM, so with some clever design you can put them on top of each other. It'll also let you balance your engines more precisely.

[Col_Jessep]

Yes, you can. You have to design your whole craft around your center tank masswise. Start with the main tank, add cockpit and forward-thrusting engines and other parts until your center of mass is EXACTLY in the middle of the main tank. You can now add more tanks to the side and connect them with fuel lines if you want. Important is to have an uneven number of tanks. Let me show you an example how you can draw fuel in a way that keeps your center of mass in the middle of your fuel tanks:

This is what usually happens if you drain your tanks and what moves your center of mass around:

You can avoid this by connecting a fuel line from the center tank to the engine. The decoupler is not necessary as I found out later.

As you can see the engine now drains the fuel from the tanks that are farthest away from the fuel line connection first. If you are very careful you can balance it so that your center of mass does not move.

Edited November 4, 2013 by Col_Jessep

[annallia]

Okay, those help, but here's my main problem: The ship balances perfectly for, oh, 20 seconds or so. Then the engines have drained enough fuel to upset the balance. So how do you counteract that? Can you even do so?

In addtion to what others have said above this another option would be the fuel balance mod (not sure if updated). You can use it to constantly transfer fuel to all of your tanks keeping them all about the same so it won't get out of balance.

[Traches]

In addtion to what others have said above this another option would be the fuel balance mod (not sure if updated). You can use it to constantly transfer fuel to all of your tanks keeping them all about the same so it won't get out of balance.

TAC fuel balancer, yeah. I don't know if it's updated, but I had problems with engines stuttering when I used the "keep balanced" option.

[700NitroXpress]

You want to balance the thrust of the rear of the plane to the thrust of the front of the plane so that you get equal thrust distribution. It's a very careful balancing act between the engines and where your lift is too. You'll need more engines wherever there is the most mass, usually the rear. I also angled my engines in the front a little to help push the plane forward slightly so it doesn't flip over.

Here's my version,

Edited November 4, 2013 by 700NitroXpress

[Hodo]

For a VTOL, they are surprisingly easy. The hardest part is getting the thrust for the VTOL setup correctly. I use FAR, and B9 amoungst other plugins so I have access to more parts, but I have built stock VTOLs.

Here is a simple trick.

Build the craft complete, turn on CoM and CoT.

put a light weight object over the CoM marker on the vessel. Best thing is a light, they weigh next to nothing and change none of the distribution.

Then remove your rear mounted engines. Notice how your CoT is now pointing straight down. Line this line up with the CoM place holder, you know that light you placed, so your CoT should go straight through your CoM.

After that you remove that light, place your rear engines back on the craft. You now have a VTOL engine setup for your craft.

I suggest figuring out how much your craft weighs so you can get your correct thrust to weight for take off. VTOLs you will want greater than 1:1 ratio. Ideally you want a 1.5:1 ratio.

Make sure you have everything on your craft before you start setting your VTOL engines, otherwise you will be going back to the assembly building and moving stuff again and again.

If you find out that it is a little off balance you can always add ballast tanks, small RCS tanks, are perfect.

One add-on I can not suggest enough is the Kerbal Engineering Redux plugin. For nothing more than the building information it provides. You will know exactly how much dV you will have, and how much thrust to weight ratio you will have for each stage.

[bsalis]

Well this is something I have a lot of experience at!

How many engines do I need?

Enough vertical lifting engines to give you a >1 TWR.

Where do I put them?

Vertical lift engines must must have net thrust vector be directly under the CoM, even as fuel drains. You also cannot place them too much above or below the horizontal thrust vector, otherwise you will get a pitching moment.

Where should the center of mass be?

Best to pick your CoM placement first, then work around that. Dry mass must balance dry mass and wet mass must balance wet mass. Generally this means symmetry, with CoM in the middle of the craft.

Anyhows, some examples of mine...

Lets start with a simple 2 jet VTOL aircraft - the Wasp. One vertical and one horizontal jet.

Here in the SPH, I have pulled away some wings to show the underside. The vertical lift jet is attached to a big plate (which is obscured by the top wings). This puts the engine along the centerline, and bang on the CoM. Dry mass at front (fore) balances dry mass at the back (aft), and note that the neck is fuselage, not a tank. There's four jet tanks there, symmetric around the engine. A small plate separates the stacks, and I have routed all the fuel by pipes to have them drain together.

This is the Butterfly, a VTOL SSTO (see K-prize here)

Here is the SPH view of the underside, with some wings removed. Reaction wheels chosen to be where the CoM should be. Note the symmetry of all the tanks, and that the vertical lift jets are each side of the CoM. They are also under the wings, so do not shift the CoM up or down. Again dry mass fore balances aft, with four small rockomax rockets attached top and bottom help the balance... top-tip here: slide surface mount dry mass fore-aft to fine tune the CoM. (keen viewers may notice that the centerline rocket tanks will not drain symmetrically, but they are only used in ascent to orbit and are empty when you get back, do do not unbalance the craft).

This is the Valkyrie, a three seater VTOL SSTO (see K-prize here)

Here is the SPH view of the underside, with some wings removed. Big jet tank chosen to be where the CoM should be. Once again, tank layout is symmetric. This has three vertical lift jets, one fore, two aft. This is a more difficult setup and not recommended, but can work if you are persistent enough.

Hope this helps!

[Starwhip]

Thanks! Going to attempt later today.

[bsalis]

Thanks! Going to attempt later today.

... and how did you go?