NASA engineers at Stennis Space Center near Bay St. Louis, Miss., took a major step Dec. 11 towards hotfire testing of RS-25 rocket engines that will help power the new Space Launch System (SLS) on missions to deep-space destinations. A-1 Test Stand operators at Stennis completed a chill test of developmental engine No. 0525, clearing the way for hotfire testing to begin in 2015. A chill test is a full-dress rehearsal for hotfire testing. During the test super-cold rocket propellants are flowed through stand and engine piping to ensure there are no problems with delivery. RS-25 engines are fueled by liquid hydrogen and liquid oxygen, which flow at temperatures reaching less than -400 degrees Fahrenheit. During the chill test, engineers monitor such things as temperatures, pressures, and flow rates of the propellants. They also seek to verify the steps of the chill procedure and accurately gauge the amount of time needed to chill the pumps and engine for hotfire tests. Such data is studied to make any needed adjustments and to prepare the hotfire test sequence and requirements. A similar test was conducted earlier in the year without an engine installed. Liquid nitrogen was flowed through newly installed piping as a preliminary test of its design. Modification of the A-1 Test Stand for RS-25 testing has been under way for more than a year. Various stand components and configurations had to be changed to enable RS-25 testing. RS-25 engines are remaining space shuttle main engines, which have been modified for the SLS vehicle. A configuration of four RS-25 engines will power the SLS core stage. In addition to conducting development and acceptance testing of individual RS-25 engines on the A-1 stand, NASA engineers are making preparations to test the SLS core stage configuration with four RS-25 engines on the B-2 Test Stand at Stennis.