7 Future developments The propulsive efficiency can be further boost- ed only with a higher bypass ratio. A major step forward is the further development of the GTF into the second-generation GTF. At the same time, alternative solutions are being in- vestigated, such as the counter rotating inte- grated shrouded propfan, or Crisp for short. In this derivative of the geared turbofan, two counter-rotating fan rotors are arranged one behind the other. The shroud is intended to reduce noise emissions. The technical foundations of this concept had been laid already back in the mid-1980s, when also its general feasibility had been demon- strated. The low fuel prices at the time, how- ever, prevented the concept from going into production. Intercooled recuperated engine In the quest for higher efficiencies advanced thermodynamic cycles are also being investi- gated. Among others, the recuperated propfan appears to be a promising concept which helps further enhance the thermal efficiency of en- gines. It is designed to take the last hurdle on the route to 30 percent carbon dioxide reduc- tion. This concept, too, bases on the geared turbofan with a high-speed low-pressure tur- bine. The reduction gearbox used in the geared turbofan en- gine decouples the fan from the low-pressure turbine. The counter-rotating shrouded propfan has been extensively tested back in the 1980s. The efficiency of an engine can be optimized by the use of downstream recuperators. In addition, it features an intercooler between the compressors and a recuperator in the exhaust gas stream. Intercooling and recuper- ating energy from the exhaust gas stream markedly increase the engine’s thermal effi- ciency. Considering that intercooler and recuperator involve weight and cost penalties, the integra- tion of these components poses new techno- logical challenges for the overall system.