Hybrid Dirigible

I've never seen a blimp or dirigible that used the waste heat of the thrust engines. Water-cooled aircraft engines aren't too hard to come by, and it should be a fairly simple matter of placing the radiator inside the lift gas bags.

As heat inside the gas bag goes up, either pressure or volume must increase; the volume will increase to the limit of the lift ballonet's elasticity, then pressure increases until a failsafe switch operates a compressor to scavenge excess lifting gas from the ballonet to a high-pressure cylinder for later use. In this way, the lifting gas' density is minimized, and its lift thus maximized, making the best possible use of expensive helium (or some other gas that expands a suitable amount when heated).

The temperature increase can't be all that great, and will have an absolute upper limit equal to the maximum operating temperature of the engine. If material tolerances will allow it, further gains can be had by also running the exhaust through an air-to-air heat exchanger inside the ballonets. At this point we're talking about pretty serious temperature increases, raising the temperature inside the ballonets to something like what is experienced inside a hot air balloon. The radiator will continue to function well if the system is engineered to appropriate ratios of heat transfer through the skin of the gas bag yielding an appropriate ambient operating temperature for the radiator. If the temperature should rise too high, then an external radiator is switched into the loop and heating is provided by the exhaust only. If a hot air balloon can work as well as it does using just hot air, imagine then the increase in performance when expanding an LTA gas such as helium or a nonflammable helium-hydrogen mix.

In a funny way, this brings the engine into the job of directly providing lift, and obviates my blue-sky fantasy of solar-electric powered dirigibles.