The XR4A3 was a fully operational 1,780-newton, pressure-fed, regeneratively cooled, liquid oxygen and alcohol engine. Four such engines were built and, combined, had been run 558 times for over 6,434 seconds. These engines also were flown on the EZ-Rocket, a modified Long-EZ aircraft fitted with two of the engines. The EZ-Rocket completed 15 successful flight tests since July 2001, including two flights at the Experimental Aircraft Association's AirVenture 2002 air show in Oshkosh, Wisconsin, in July 2002.