A non-toxic nitrous oxide fuel blend (NOFB) monopropellant with a high adiabatic flame temperature reaching and probably exceeding 3,450 K and a very high thermal decomposition limit (>390 °C) is under development. To design an optimal rocket engine that can handle the high adiabatic temperature during continuous rocket thruster operations, a regeneratively cooled rocket engine is desirable, but the regenerative jacket temperatures must remain well below the monopropellant’s thermal decomposition limit. In fact, the entire engine during operation should ideally remain well below the thermal ignition limit so that heat soak-back cannot potentially decompose the monopropellant following an engine restart.
The fluid and heat transfer theory for regenerative cooling of a rocket combustion chamber with a microfluidic porous media coolant jacket is presented. This model is used to design a regeneratively cooled rocket or other high-temperature engine cooling jacket that could use a gas phase monopropellant as a coolant, a liquid phase monopropellant as a coolant, or some combination of both. This jacket design should be relatively insensitive to liquid boiling in a jacket. Cooling jackets comprising impermeable inner and outer walls, and microfluidic porous media channels, are disclosed. Also disclosed are microfluidic porous media coolant jackets with additional structures incorporated in the microfluidic porous media to augment fluid mixing and heat transport.
The thermally insulating properties of gas phase monopropellant require large thermal gradients to drive heat into this type of coolant. These large thermal gradients drive up the inside jacket wall temperatures to which the monopropellant coolant is exposed. By volume-mixing low-thermal-conductivity gases with a higher-thermal-conductivity medium (e.g. a microfluidic porous media metal structure), the effective thermal conductivity of the combined solid/fluid medium can be greatly increased (close to that of metal). Care must be taken in designing this microfluidic porous media jacket to ensure the local gas speed through the micropores is less than sonic velocity, and the mass flux of coolant and microfluidic porous media geometry is sufficient for adequately cooling the engine while simultaneously not producing excessive pressure drop. The microfluidic porous media fluid theory and model were developed for conforming the microfluidic porous media structure to a rocket engine nozzle contour in order to effectively cool the rocket engine, minimize pressure drop, and ensure the jacket temperatures remain below the monopropellant thermal decomposition temperatures.
Advanced fluid mixing concepts also were incorporated to lower the temperature gradient across the jacket to more effectively utilize propellant as a coolant. Prior art has typically used open-channel passages for cooling. While initial attempts have been made to incorporate microfluidic porous media structures into high-heat-flux environments with simple geometries, or by using such structures in rocket engines for liquid propellants, this is believed to be the first case of actually generating a design tool and method for designing a contoured rocket nozzle with a microfluidic porous media jacket specifically designed to handle worst-case gas cooling scenarios and successfully demonstrating this design in practice using a worst-case gas phase coolant.
The proposed microfluidic porous media jacket design approach should be relatively insensitive to the formation of gases inside coolant jackets, and thus may help alleviate or solve the problem of film-boiling failures in high-heat-flux environments.
This work was done by Greg Mungas and David Fisher of Firestar Engineering, Jack Fryer of Micro Cooling Concepts, and Adam London of Ventions for Johnson Space Center. For further information, contact the JSC Technology Transfer Office at (281) 483-3809. MSC-24754-1