The gas turbine Engine is universally used in military aircraft because it is simple, lightweight, and reliable. However, the small engine (500 hp category) has a specific fuel consumption (SFC) generally above 0.6 (pounds of fuel per brake horsepower hour). Future improvements will decrease the 500 shaft horsepower engine to an SFC of 0.43. However, the adiabatic diesel has demonstrated an SFC of only 0.285. This is a reduction of 52% over existing engines and, compared to future engines, a savings of 41% is possible, so why stay with the gas turbine engine? Well, other requirements must also be satisfied. For example, how heavy can the engine be? A computation using the Blackhawk helicopter as an example shows the allowable specific weight of the engine to be 0.76 lb/hp. This computation is based on allowing the engine to increase in weight to offset fuel, tankage, and support weight saved such that the take-off gross weight of the aircraft is not changed. This is felt to be attainable and examples of paper designs available in the literature are briefly discussed.This preliminary study indicates that the Army helicopter fleet fuel consumption could be reduced by more than 40% if it could be retrofitted with adiabatic compound diesel engines. Furthermore, the logistics problem would not only be reduced by 40%, but since most Army trucks and tanks use diesel fuel the supply system would also be greatly simplified.