Pallabazzer 



performances. A turbofan whose thrust can be augmented by water injection 

 has been recently presented [5,6], but the results were too concise to achieve 

 an exact idea of the characteristics. 



A propulsive ejector driven by compressed air was projected some years 

 ago [7,8], without later being developed. Apparently the interruption of the 

 project was due to the actual performances of the propulsor. 



As will be outlined later, while the energetic efficiency of the ejector can 

 be sufficiently high, being increased by the velocity, the thrust performances 

 can be very poor, showing strong collapse at increasing speed. It will appear 

 that cold air expressly compressed for powering an ejector is not the best 

 generator for this purpose; besides, new ejector designs can raise the thrust 

 performances. 



The reasons for studying the ejector as a means of propelling a body in 

 or on water, in spite of the aforementioned limitations, depend on the funda- 

 mental simplicity of such a propulsive device whose merits are weight, main- 

 tenance, failure and noise, no mechanical transmission, no water deadweight 

 on board, no inlet clog risk, brief internal duct length and reduction of internal 

 losses with regard to water-jet ducts, and availability of underwater instead of 

 surface jet. The gain of weight by itself is a sufficient merit of a two-phase 

 propulsor compared to a water-jet system. 



For example, if we figure substituting the propulsor of the Boeing water- 

 jet hydrofoil PG-H(2) with an ejective device, the gain of weight is of 1.78 tons, 

 which means 53% of the power apparatus (turbine + pumps + water) and 3.5% of 

 the net displacement (50 tons). 



These reasons are sufficiently valid to require an exhaustive analysis of 

 the ejector as a device for two-phase propulsion. This analysis has to be de- 

 veloped in two directions: (a) powerplant investigation of the gas generator 

 and its connection with the ejector, with the purpose of identifying the configu- 

 rations which allow the best propulsive performances; this analysis can be 

 initially developed by means of some idealizations about the two-phase exchange 

 phenomena; (b) a theoretical- experimental investigation of propulsive ejectors, 

 which allows identification of the most correct analytical idealization of the ex- 

 change and the optimum geometrical configurations of the ejector with regard 

 to propulsive performances. 



At the Istituto di Macchine, Politecnico di Milano, the first investigation 

 has been widely developed, and its results are presented here, while the second 

 investigation is just beginning at a two-phase tunnel which has been recently 

 realized. 



TWO-PHASE JET PROPULSION: STATUS OF ART 

 AND BASES OF THE PRESENT STUDY 



The working principle of an ejector is well known: In a propulsive device 

 (Fig. 1), a secondary water stream, which arrives at the propulsor chamber 



1106 



