21

I tried building a "pad" that would allow my Kerbals to fly around at moderate altitudes, hovering in the air; something like a quadcopter, just with jet engines.

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I made sure the center of thrust is above the center of mass, that way the thing should be self-stabilizing. Well, it isn't. It flips over a few seconds after take-off.

I thought the center of mass may be still too high. I strapped the huge Roxkomax X200-64 fuel tank on the bottom to weigh it down, and still - it managed to take off despite the very heavy load, and promptly flipped over.

What should I do for a hovering contraption so that it can hover unattended without flipping over?

  • 2
    "An aircraft with a centre of lift behind the centre of mass will fly poorly. An aircraft with a centre of mass behind the centre of lift will fly exactly once." Just because the centre of lift is behind the centre of mass doesn't mean the aircraft is aerodynamically stable. Also, your engines probably gimbal in the wrong direction in response to inputs. – MBraedley May 25 '15 at 1:19
24

You've managed to stumble upon the pendulum rocket fallacy. Don't worry, you're in good company; Robert Goddard, the inventor of the liquid-fueled rocket, famously made the same mistake.

Here's a nice video by Scott Manley demonstrating the issue:

Scott's video also suggests one potential solution; if you can mount your engines on flexible supports, in such a way that they'll tend to preferentially flex towards the ground, you may be able to get your vehicle to stay upright. It may be somewhat difficult to plan a configuration that will work, though.

Alternatively, just turn on the SAS and make sure your vehicle has some control mechanism (gimbaled engines, RCS thrusters or reaction wheels; you probably don't want aerodynamic surfaces if you're trying to hover) for it to steer with. Note that older KSP versions had a bug where gimbaled engines mounted above the center of mass would steer the wrong way. That should be fixed in KSP 1.0, however.

  • I would still test the engine gimballing to make sure that they are indeed moving in the correct direction. – MBraedley May 25 '15 at 1:20
  • @MBraedley: These are the standard Jet engines. They don't gimball at all. – SF. May 26 '15 at 12:39
  • They do gimbal. You see that if you look at the expanded info card. – MBraedley May 26 '15 at 21:21

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