Somewhat off-topic: Alastair Reynolds' "Revelation Space" features an aircraft (NOT a spacecraft) which does not violate fundamental laws of physics, but probably takes the cake for "excessive unobtainium overengineering when much easier solutions would work for far less energy". The plane is aerodynamic (delta wing), and its entire underside is covered in miniature jet nozzles (I assume intakes are on top) which heat the air to plasma temeperature and expel it downward and backward -- or sideways if needed. Energy source is unexplained. Nozzles stand up to near-sun-surface temperatures. Looking up at a low-flying plane can cause permanent eye damage.

[Edited]: Now that I think of it, if you DO have materials that can stand up to photosphere temperatures, and a power source of appropriate energy density, then it is not a bad design. Very few moving parts, and if some nozzles fail, there is only slight loss of performance. The fact that you can not look at it without welders' goggles is a problem though.

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