The test, conducted at the Fred Hayes Test Stand at NASA’s Stennis Space Center, was the third of the year and is part of an ongoing series of hot fire certifications. This was also the third test since a retrofit nozzle was fitted to the RS-25 engine just before the February 8 fire. The rebuilt engines, supplied by SLS engine contractor Aerojet Rocketdyne, will be used on future Artemis lunar missions. Beginning with Artemis V, NASA continues its mission to discover the secrets of the universe for the benefit of all.

A mounted field camera provides a close-up view as NASA conducts a hot fire test of the RS-25 at the Fred Haise Test Stand at NASA’s Stennis Space Center south of Mississippi on March 8, 2023. Image credit: NASA/Stennis

“Initial indications are that the nozzle is performing well,” said Mike Lauer, deputy program manager for the RS-25 engine at Aerojet Rocketdyne. “Although we changed the production methods, we did not want to radically change the performance of the nozzle. We are very pleased that the nozzle performs very well in line with our previous history.”

During testing, operators ran the RS-25 engine for the planned 520 seconds and dissipated up to 113% of power. During lunar missions, the engines run for about 500 seconds and reach a power level of 111% to help the SLS lift the Orion spacecraft and other payloads into orbit. With added seconds and increased power levels during ground testing, engineers can learn more about engine performance and provide a margin of safety during operation.

The nozzle, or bell-shaped part, on the bottom of the RS-25 engine helps create thrust by expelling the engine’s exhaust gases at top speed to propel the SLS into space. It is one of many upgraded components of the redesigned engine and is also critical for generating thrust and ensuring efficiency by using the least amount of propellant or propellant gas to assist the rocket in space. The nozzle is one of the reasons the RS-25 is one of the most economical rocket engines in the world.

Throughout the storied history of the RS-25 engine, many components have undergone changes or upgrades. One component that didn’t change much was the nozzle. However, the team focused on redesigning every part of the nozzle for the redesigned RS-25 engines. This included the use of precision machining to improve the production of the more than 1,000 tubes that make up the nozzle wall. These tubes are critical for the supercooled flow of liquid hydrogen to keep the nozzle cool. The team also uses 3D printing to make individual parts of the nozzle.

The nozzle upgrade includes a new type and level of insulation; this is a critical factor because RS-25 engines are exposed to more heat during SLS launches than during previous space shuttle missions. The SLS is equipped with four RS-25 engines instead of the three main engines used for the Space Shuttle, and the RS-25 nozzle is located closer to the SLS rocket thrusters than the Space Shuttle.

The purpose of the upgrade is to simplify the nozzle manufacturing process, make it more cost-effective, and maintain the same level of performance as previous RS-25 engines. Although new manufacturing techniques were introduced into the production of the tip, its shape, length and diameter remained the same. Sixteen main engines remain from the shuttle program. All modified and tested at NASA Stennis for use on the first four Artemis missions.