Some submersible aircraft proposals have involved using jet engines in a dual role, both propelling the vehicle in the air using conventional combustion and providing thrust underwater by being spun via an electric motor; some researchers have proposed using turboshaft engines to get the best efficiency and performance in both air and underwater environments. To prevent salt water from prematurely entering the engines when the aircraft is not submerged, the engines could be mounted on the craft's dorsal surface and to the rear. However, one issue is that because jet engines run at several hundred degrees when in air, they could not immediately transition underwater, as being exposed to seawater would subject them to extreme temperature change which would damage them, requiring the aircraft wait for several hours on the surface to cool its engines to submerge, thus any such configuration would require a novel cooling system in order to make a faster transition.