Development and characterisation of a dual bi-directional vortex cooling approach to lower the cost of a small liquid bi-propellant rocket engine

Low cost access to space, particularly for small satellites has been identified as one of the major needs of the 2015-2030 UK Space Innovation & Growth Strategy. Chemical propulsion, specifically liquid bi-propellant rocket engines, is the major cost driver for large and small launch vehicles, and has not historically been a strength of the UK. Analysis of the liquid bi-propellant rocket engine highlights that the cooling subsystem has historically been one of the main contributors to overall system costs. The majority of current liquid bi-propellant engines cooling approaches increasing engine complexity, may increase engine mass, reduce performance and require the use of exotic materials to manage high thermal loads. Therefore, the focus of this research is to develop a small low cost liquid rocket engine using a new thermal management approach called Dual Bi-Directional Vortex (DBDV) cooling to enable the use of an inexpensive lightweight metallic alloy for chamber construction, in conjunction with Additive Layer Manufacturing to further reduce costs. DBDV cooling involves producing a coaxial bi-directional vortex flow field using oxidiser injection at the nozzle end of the engine, which then flows towards the fuel injection in a spiral fashion along the chamber wall. This flow field and the resulting pressure gradients confine combustion to the centre vortex or 'hot core' and significantly reduce conduction and convection of heat to the inner chamber sidewall, negating the need for chamber cooling and the attendant disadvantages. The dual vortices also provide intense mixing of propellants which have the potential to increase combustion efficiency and engine performance. Three techniques were used to investigate and develop a small DBDV liquid rocket engine. First CFD studies were used to develop an oxidiser injector design by modelling the complex vortex flows. Second the final design was additive layer manufactured and subjected to cold flow water testing with fast visual data capture to deduce the general characteristics of the dual vortex flow. Finally, experimental hot fire testing compare an uncooled liquid rocket engine, designated the 20N conventional, to an aluminium chamber and ALM stainless steel/bronze vortex liquid rocket engine to evaluate the cooling benefit. Hot fire testing used gaseous oxygen and gaseous propane as the propellants and required significant modifications and upgrades to the existing rocket test facility at Kingston University. Multiple engines were built and instrumented with thermocouples at various chamber sidewall depths and locations to measure the temperature ride during the firing. Mass flow rate, thrust and pressure were also measured to evaluate performance. Engine hot fire testing reported a 77.5% chamber sidewall temperature reduction when using DBDV cooling approach compared to the uncooled engine, which also suffered thermal failure after 7.5 seconds. Average vortex engine chamber inner sidewall temperatures were 125 ̊C verses a predicted combustion flame temperature of 2731 ̊C, which permitted the steady state use of an uncooled aluminium alloy chamber and an additive manufactured stainless steel/bronze oxidiser injector. It was found the tangential oxidiser injector produced a reliable and symmetrical dual vortex flow field which functioned over a wide range of operating parameters. It was also found that increasing the fuel injection mass flow rate for a fixed oxidiser flow rate destabilised the flow field resulting in increased sidewall temperatures. Perspex chambers were also tested, but failed due to transient heating upon ignition leading to unwanted chamber combustion similar to a hybrid rocket engine. In summary, this research demonstrated that the DBDV cooling approach is effective at drastically reducing chamber sidewall temperatures enabling the use of a low-cost material for engine construction whilst maintaining engine performance.