Be-4 Preburner As Described by Jeff Bezos

Pre-burner Injector Model Credit: Blue Origin

Pre-burner Injector Model
Credit: Blue Origin

A bit of rocket science courtesy of Jeff Bezos:
In the BE-4 preburner, a very small portion of the engine’s liquefied natural gas (LNG) fuel mixes and burns with all of the engine’s liquid oxygen to produce hot gaseous oxygen, which is used to drive the turbine and spin the turbopumps. Oxygen and LNG burn stoichiometrically above 6,000 degrees Fahrenheit, and temperatures of about 3,000 degrees Fahrenheit or more are needed to reliably ignite and sustain the reaction. No practical turbine materials would survive at that temperature, especially in a reusable application. To resolve this, the BE-4 preburner mixes unburned oxygen into the burned gas stream to dilute the combustion gases and reduce the overall temperature to about 700 degrees Fahrenheit. If this mixing process isn’t meticulously designed, hot spots can persist in the stream and limit turbine life.

To design the preburner to provide uniform temperature, we use 3-D Computational Fluid Dynamics (CFD) to model the LNG and liquid oxygen combustion process. CFD predicts fluid behavior by solving the Navier-Stokes equations to describe how the velocity, pressure, temperature, and density of a moving fluid relate. CFD of reacting flows, especially those that also involve a phase change, is much, much harder because it must also solve chemistry along with state equations. Combusting CFD has only become practical with recent advances in chemical physics models and computing power.

To date, we’ve completed several million core hours of CFD modeling of BE-4 combustion processes. Modeling of the preburner shows good mixing and temperature uniformity upstream of the turbine. The combustion and temperature data we’ve gathered in our subscale testing correlate with our CFD predictions and show that our preburner sizing and injector element design meet design requirements. The ability to do combusting CFD simulations doesn’t eliminate the need for rigorous testing, but it will significantly shorten the test-fail-fix loop on the test stand. We’ll keep you updated.

Gradatim Ferociter!

Jeff Bezos

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