Status: Prototype Development

The XR-8H21 is a regeneratively cooled Liquid Oxygen / Liquid Hydrogen rocket engine that generates approximately 25,000 lb of thrust in vacuum. It is the largest thrust class engine XCOR has developed to date, and is intended for use on EELV (Evolved Expendable Launch Vehicles) upper stages as a low-cost alternative to legacy engines. It incorporates many advancements that were pioneered on the 5H25 engine, such as positive displacement piston pumps and a lightweight aluminum nozzle. The method of operation and construction of the 8H21 engine allows for significant cost reductions compared to other liquid hydrogen rocket engines, offering equal performance at a superior value. In the increasingly competitive launch market, the 8H21 is designed to offer a critical advantage to EELV operators.   

The 8H21 is currently undergoing design of the first generation of prototype hardware using lessons learned through the 5H25 test program.

8H21 Specifications:

Chamber pressure:         800 psi (55.16 bar)

Thrust:                               24k lbf (106.98 kN)

O:F                                      5.5 nominal; 4.0-7.5 achievable in-flight range.

Isp (vacuum):                    460 sec

Engine Mass:                     498 lbm (226 kg) dry without TVC actuators

Dimensions:                       89” Length, 56” diam. at engine exit.

Expansion ratio:                150

Throttling:                          75-100%

XCOR 5H25 First Close Cycle


Status: Retired

The XR-5H25 is a regeneratively cooled Liquid Oxygen / Liquid Hydrogen rocket engine that generates approximately 2,500 lb of thrust at Mojave altitude. It is XCOR’s first rocket engine developed to run with Liquid Hydrogen fuel, marking a significant step in the advancement of XCOR’s rocket technologies.  The 5H25 was designed to operate similarly to the 5K18 engine used on the Lynx, utilizing the advantages of positive displacement piston pumps and a novel closed-loop engine cycle. It was a workhorse development engine, funded by United Launch Alliance and purpose built to mature critical technologies that fed into the design of the full-scale 8H21 engine.

The 5H25 was in active service from 2013 through the end of 2015. Testing began with pressure-fed runs on the engine core assembly, followed by the integration of piston pumps and concluding with full closed-loop testing. The data collected from the 5H25 program was immensely valuable in the design of the 8H21, and many critical insights were applied to the design of the first generation 8H21 prototype.

5H25 Specifications:

Chamber Pressure:     450 psia, measured at injector exit

O:F                                  5.5, variable

Thrust (e=10, Mojave ambient):      2454 lbf (10920N)

Isp (e=10, Mojave ambient):             394 sec

XCOR 5H25 first hotfire
XCOR 5H25 test site at Mojave Air & Space Port
XCOR 5H25 hotfire


Status: Testing

The XCOR XR-5K18 is the LOX-Kerosene main rocket engine which will power the XCOR Lynx to the edge of space. The first iteration of the 5K18 was test fired on December 15th, 2008 at XCOR’s facilities in Mojave, California.

XCOR 5K18 engine

Like all of XCOR’s engines, the XR-5K18 has demonstrated the ability to safely and reliably stop and restart using XCOR’s proprietary spark torch ignition system.  Its regenerative cooling system allows it to run for indefinite periods without maintenance and disassembly.

Our 5K18 engine produces between 2900 lbf thrust while burning a mixture of liquid oxygen and kerosene.

It has design and performance characteristics similar to a previous XCOR engine, the 4K14, which had a full test and flight history as the X-Racer engine. This made the 4K14 an ideal model on which to base the Lynx engine.

The primary differences between the 5K18 and 4K14 design are:

The 5K18’s chamber pressure and thrust are approximately twice the power of the 4K14.  The 5K18 has a nozzle extension to optimize specific impulse and thrust at higher altitudes

While both engine chambers are regeneratively cooled by kerosene, the 5K18 has additional coolant loops for the nozzle extension.

XCOR XR-4K14 (X-Racer Engine)

Status: Retired

XR-4K14 Engine on the test stand
4K14 Engine in the X-Racer


Status: Program Complete

Based upon prior XCOR intellectual property, XCOR and Alliant Techsystems Inc. (ATK) Tactical, Propulsion and Controls Division, GASL Operations was awarded a NASA contract to design, build, and test a 33 kN (7,500 lbf) LOX/methane pressure-fed engine. This engine was subsequently designated the XR-5M15.

In total, the XR-5M15 demonstrated a series of forty three hot fire tests at our Mojave facilities. Applications for the 5M15 include returning manned space vehicles from Lunar orbit and Mars to the Earth, and performing in-space maneuvering.

The approach was a rapid design, build and test of a workhorse main engine based closely on relevant design features from our previous engine programs. The 5M15 program built upon XCOR’s existing 3M9 LOX/methane engine, as well as our 1,800 lbf 4K5, 10,000 lbf 5M12 design, and 400 lbf 4A3 EZ-Rocket engines.

This main engine development uses several features from the 3M9, including the same augmented torch electrical ignition system and the propellant combination.

Relevant features of the 4K5 engine include the igniter geometry, the torch spark igniter, and the fuel cooling configuration. While the 5M15 uses many of the same 4K5 component designs, we changed the fuel, lowered the chamber pressure, and increased the size.

The XCOR 5M12 engine study was the closest configuration of the main engine. It was developed for a DARPA program through to layout design, but not built. The DARPA contract enabled XCOR to develop the methane cooling analysis tools, which have also been used with the 3M9 engine runs.

The 5M15 engine has many relevant features that are shared with XCOR’s FAA licensed manned vehicles, the EZ-Rocket and the X-Racer.

The regeneratively cooled “workhorse” engine served several purposes. It validated the key engine design elements, including the regen-cooled chamber/throat assembly, the stability and performance of the injector, and the reliability of ignition. It incorporated a number of design features for safety and reliability, critical for human-rated application that were demonstrated on previous XCOR engine designs. Finally, it is a modular design, which facilitated rapid test of new components during development, and enabled modification for future NASA Exploration applications.


Status: Retired

In June of 2000, after we had concluded tests of our proprietary oxygen/alcohol igniter, we began work on a 160 lbf liquid oxygen and isopropanol engine, which we ran in October of the same year. Following successful tests of this engine, known as the XR-3A2, in November 2000 we started to design and build the regeneratively cooled 400 lbf LOX/alcohol engines that we eventually integrated into a Long-EZ airplane, which became our EZ-Rocket aircraft.

400 lbf LOx/alcohol XR-3A2
EZ-Rocket in flight

The XR-4A3 engines went through multiple acceptance tests before the first flight of the EZ-Rocket on July 21, 2001.  Early flights on engines 101 and 102 quickly lead to design improvements, and serial numbers 103 and 104 powered 20 of the 26 flights of the EZ-Rocket.

The flight test program passed its first milestone by flying with both engines for an engine run time of 96 seconds, and a total flight time of five minutes and twenty seconds. Later flights achieved burn times of up to 2.5 minutes, altitudes to 11,500 feet, and the point-to-point distance record for a rocket-powered vehicle.

Specific impulse of this engine is higher than either LOX/alcohol engines in the Bell X-1 and the Redstone.

The fact that two 4A3 engines powered an aircraft safely, reliably, and routinely multiple times proved the viability of civilian rocket-powered aviation. Routine operations must be the primary criterion for rocket engine development. The XCOR approach is to build safer and more reliable rocket engines first, then progress to the higher performance needed for orbital launch vehicles.