Help with VTOL SSTO

[Gamer217]

I need help designing a personnel transport VTOL SSTO using the B9 Aerospace pack. Here are are pics of the vehicle, note that direction of thrust isn't entirely accurate due to me using tilting VTOL engine to lift the craft up.

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Front:

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When I try to tack off the vehicle rises slow but after a while while I let it rise enough to maneuver I lose control of the vehicle. Any help is appreciated.

[Gauss H2K]

I know that VTOLs in KSP are a bit tricky since the game doesn't like control surfaces going backwards, so on take off try gaining a bit of forward speed. Also I when I make VTOL's I add an equivalent weight of the main engines in the spots where the engines will be, so I can get an idea for CoT balance.

[bsalis]

How are you losing control? Pitching?

You want ample torque to control it in a hover. Avoid moving backwards because the control surfaces will start to work against the torque.

Also the CoL will generally be where the CoD is. So if that is far from the CoM, if you ascend or descent to quickly, you will get pitching from the drag.

[Gamer217]

The craft suddenly pitches forward after it gets about a foot off the ground. Also would using the Ferram Aerospace Research Mod be recommended?

EDIT: And is this possiblly a bit overkill to build a VTOL SSTO for getting 6 passengers into space (not including pilot and copilot) loosely following the design of the Valkyrie from Avatar?

Edited August 6, 2013 by Gamer217

[Gamer217]

Ok so I got a revised model of this SSTO, it can actually get off the ground big milestone for this thing considering yesterday's results. after it flys for a while (quite stable flight and fuel efficient after VTOLs are shut off surprisingly ) the craft back flips at roughly 20,000 , also it loses intake air quickly past 10,000 but that just mean more intakes so no big deal. But with the flipping is that caused by weight shifting in the fuel tanks? And if so how should I distribute the tanks on the craft. Right now there are 4 LFO tanks between the last cabin and the tail. New pics follow

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[bsalis]

Backflips can happen from the CoM drifting to be behind the CoL. The CoT being lower than the CoM can cause pitching up too, that may be kept under control from the control surfaces until you get to thin air... where they will not. I suspect it's just CoM drift though.

If you're building a VTOL plane that you intend to take off and land vertically then you need to make sure the CoM stays in the same place... at least for when you are needing to use the vertical lift.

[Gamer217]

So should I move the fuel tanks closer towards the nose? I plan on sliding the SABREs more forward along the wing and maybe adding some more intakes on the front to increase intake air at higher altitudes. So would these changes help?

[Gauss H2K]

I would try to get your fuel tanks on or near the exact CoM, so that when the fuel drains you don't need to worry too much about CoM changing. You should also avoid having your CoT ahead of your CoM, as that also leads to instabilities (talking about your SABREs), also adding more air intakes turn your vessel into an air hog that can get to near orbital flights on jet engines alone.