The XLR11, company designation RMI 6000C4, was the first liquid-propellant rocket engine developed in the United States for use in aircraft.

It was designed and built by Reaction Motors Inc., and used ethyl alcohol and liquid oxygen as propellants to generate a maximum thrust of 6,000 lbf (27 kN).

On October 14, 1947, the X-1 became the first aircraft to fly faster than the speed of sound (Mach 1).

The XLR11-RM-5 was also used in the X-1A and X-1B, and as a booster engine in the U.S. Navy's D-558-2 Douglas Skyrocket turbojet (where it was designated the XLR8-RM-5).

The XLR11-RM-13 was also used in the Dryden lifting bodies, and as a booster engine in the Republic XF-91 Thunderceptor turbojet.