XCOR has developed the 3N22 for use as the thrusters, or reaction control system on our Lynx spacecraft. The 3N22 is a 40 lbf thruster, and its fuels are non-toxic. Unlike most thrusters throughout the history of spacecraft design, the 3N22 uses a safer, less costly proprietary bi-propellant compared to current baseline systems using propellants such as hydrazine and nitrogen tetraoxide, both of which are highly toxic. The capabilities of the 3N22 system include significantly better performance than systems that use hydrazine monopropellant and similar performance to NTO/MMH. Further, due to regenerative cooling, the 3N22 is significantly easier to integrate into uninsulated mounting configurations. Specific impulse and storage density are also comparable between 3N22 performance and traditional toxic thrusters.


XCOR Aerospace has designed, built, and tested a new 56 lbf rocket engine. The engine, designated XR-3E17, is a direct descendent of the company’s XR-2P1 “Tea Cart” engine developed in 2000. Although it weighs just half as much as its predecessor, the new engine has nearly four times the thrust of the 15 lbf original. This regeneratively cooled engine, developed using private investor funding, is made of copper with a lightweight aluminum cooling jacket.

This spark-ignited engine is capable of an unlimited number of starts and restarts. It is fueled by a self-pressurizing propellant mixture of ethane and nitrous oxide.

The XR-3E17 engine has a number of commercial and military applications including use as reaction control system.


In 2005 XCOR built and tested the XR-3M9, a 50 lbf, self-pressurized LOX/methane engine with regenerative cooling and specially designed injector. The design, fabrication, and testing of this engine was initially funded through private investment capital. Further engine tests were conducted as part of a Phase I SBIR contract under the Air Force Research Lab’s (AFRL’s) Propulsion Directorate at Edwards Air Force Base, and as part of development for an advanced regeneratively cooled LOX/methane engine for space applications.

XCOR built the 3M9 to be the reaction control system for its next generation vehicle. The AFRL SBIR Phase 1 contract allowed XCOR to validate the design models and test the engine for performance characteristics. In turn, this work enabled XCOR to predict performance of larger engines with safer, environmentally-friendly propellants and reliable, responsive operations for low cost launch vehicles, satellite maneuvering stages, and commercial sounding rockets.

This engine has four distinct advantages: 

  • It uses non-toxic LOX/methane propellants with inherently high specific impulse.
  • It has a new injector design that is inherently low cost to manufacture.
  • It is optimized to operate with self pressurizing propellants, which eliminate the need for either pumps or a propellant tank pressurization system. Additionally, the engine can operate with subcooled propellants.
  • Use of LOX/methane means that the main engines, orbital maneuvering system (OMS), and reaction control system (RCS) can all use the same propellants, which will simplify the overall system and reduce weight.

LOX/methane rocket engines show promise to provide higher performance necessary for manned moon missions. NASA, in its Exploration Systems Architecture Study (ESAS) of how to return to the Moon, recognized the importance of LOX/methane:

" ground test has shown the combustion performance to be suitable for use as a propellant. LOX/LCH4 is a clean-burning propellant combination. LOX does have an extensive history as a fluid on spacecraft and as propellant on propulsion stages; however, the on-orbit operational experience for LOX systems is limited. The safe handling and safe system design aspect of LOX are well understood. LOX/methane does offer higher Isp performance compared to state-of-the-art storables (i.e.,hypergols), without the volume increase that is common with LOX/liquid hydrogen systems, which results in an overall lower vehicle mass as compared to [hypergolic] propulsion systems. A LOX/liquid methane system uses less power, on the order of 1,000 watts less power, than comparable [hypergolic] propulsion systems, thereby significantly reducing the mass of the spacecraft power system(s)." ESAS, Section 4, p. 115.

In addition to higher performance, LOX/methane engines have long term storability necessary for manned Moon or Mars missions, LOX and methane can be obtained from the Martian soil and atmosphere, and LOX can come from the lunar soil. Also, its non-toxic nature significantly lowers operations costs, enhances crew safety, and is safer for the environment.

This engine resulted from a National Reconnaissance Office (NRO) contract to develop a long-life, multi-start 50 lbf maneuvering rocket based on non-toxic propellants. The approach that we selected used regenerative cooling with nitrous oxide and isopropyl alcohol propellants. For this engine, XCOR designed, built, and tested a dual-cooled chamber with high temperature tolerance that employed both fuel and oxidizer as coolant.

This design demonstrated significantly enhanced heat transfer to the oxidizer – where roughly half the total waste heat is transferred to the oxidizer – and demonstrated sufficient heat transfer for long-term engine operation. With this project, we achieved a prototype, regeneratively cooled 50 lbf engine with unlimited lifetime and gained experience in stability and chamber design lessons that we have applied to our other engine developments.

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