Development of a Cavitating Pintle for a Throttleable Hybrid Rocket Motor

Hybrid rocket motors have several potential advantages respect to current used propulsion systems (i.e. solids and liquids) like simplicity, safety, reliability, environmental friendliness, lower cost. A particular positive feature of hybrid rockets is the possibility to control the thrust level operating only on the oxidizer mass flow. Thanks to this it is possible to develop a relatively simple propulsion system that is throttleable on demand without the complex mixture ratio control and related hardware of a liquid system. In the past University of Padua has developed a lab-scale hybrid rocket motor that can be throttled at few different discrete levels with the use of parallel feedlines. To give the possibility of having a continuous throttling capability a new mass flow control has been developed recently. The mass flow control make use of a cavitating pintle. The cavitating pintle acts as a cavitating venturi in order to choke the mass flow and make it independent of downstream pressure. The pintle is used to change the venturi throat area and consequently varying the oxidizer mass flow keeping a constant upstream pressure. The paper presents the design of the cavitating pintle and the experimental campaign composed by cold tests followed by hot fire tests of the lab scale hybrid rocket.