Anyone who watched the launch of United Launch Alliance’s Delta IV Heavy rocket on Saturday was treated to an up-close view of the liftoff. This vantage point, showing the three-core rocket taking off beneath blue skies, offered a distinct view of a fireball engulfing the rocket during launch.

This can be rather distracting if you’ve never seen it before—uhh, is that rocket about to blow up?—but in reality it’s a byproduct of the RS-68 rocket engines that power each of the three cores of the Delta IV Heavy launch vehicle.

Developed during the 1990s by Rocketdyne, the expendable RS-68 engine was designed to be less expensive and more powerful than the Space Shuttle’s reusable RS-25 main engines. Like the Shuttle’s engines, the RS-68 engine runs on a cryogenic fuel mix of liquid hydrogen and liquid oxygen.