Watching the injector spray

An injector has one job: break the propellants into fine droplets and mix them so they burn quickly and evenly inside the chamber. Whether it does that well cannot be seen from the drawing, so before E2 is fired, the injector is flow-tested with water.

Making the spray visible

For these tests the injector is mounted in front of a bright, diffuse backlight and photographed while water flows through it at a set pressure. The backlight turns the liquid dark against a light background, which makes the shape of the spray easy to see. Close to the injector face the water leaves as a thin, continuous conical sheet. A short distance downstream the sheet becomes unstable, tears into ligaments and finally into droplets, which spread out into the wide cone visible in the photo.

What we look for

From these images the team checks the spray angle, how symmetric the cone is around the injector axis, and how far from the face the sheet breaks up. An uneven cone or a sheet that stays intact for too long points to a manufacturing problem or a design that will mix poorly in the engine. Together with the measured flow rate and pressure drop, the pictures tell us whether the injector behaves as designed before it ever sees nitrous oxide and ethanol.

Next steps

The results go into the setup for the first hot-fire campaign of E2 at our test site in Aldenhoven, where the injector will run with real propellants for the first time.