XCOR Three-Cylinder Rocket Propellant Crankshaft Piston Pump

Fuel and oxidizer pumps are the primary components of many rocket engines. In 2002, XCOR demonstrated its first breadboard prototype for a reciprocating piston pump. The development took fewer than four weeks from initial design to demonstration of pumping pressures and flow rate appropriate to XCOR's 400 lbf XR-4A3 EZ-Rocket engines.

The first pump XCOR built was the result of a design study on upper stage engines that demonstrated pump-fed engines had substantial benefits. We realized, however, that affordable flight-weight pumps, which combined the flow rate, pressure, and closed thermodynamic cycle required for XCOR's engines did not exist.  So we developed a suitable piston pump--the breadboard piston pump--and thermodynamic cycle concept , and applied for a DARPA Small Business Technology Transfer (STTR) contract to test it.

Breadboard Free Piston Pump

We were awarded this DARPA STTR Phase I contract to test the breadboard motor and pump assembly, and used the data to calibrate customized analytical models of the pump behavior. We also completed initial design work for a 2,000 lbf thrust rocket engine piston pump, identified the key technology requirements for future work, and carried out design work on both crankshaft and free-piston configurations.

A bridge contract from DARPA and matching private investment enabled us to continue to focus on the free piston assembly.  At this time we recommended the final design configuration of a free piston machine to be developed during Phase II. While working on this bridge task, a new check valve approach was developed that provided considerable pump speed improvements. As a result, we incorporated the new valve into the pump design.

In 2003, XCOR received a DARPA STTR Phase II contract to design, build, and test a small, lightweight, and responsive liquid oxygen (LOX) pump with integrated (pump and motor) assembly. At the same time, we also produced a fuel pump and motor assembly that demonstrated flight-type flow rates and pressures.

Both motor and pump sections were designed as piston machines, instead of the more traditional turbo-centrifugal hardware.

Intermediate Phase II Fuel Pump

The Phase II LOX piston pump was designed and tested to handle repeated stop-start cycles in the propulsion system. We also qualified custom-designed cryogenic seals for the pump piston and rod, and built a check valve test apparatus. In conjunction with our academic partner, we measured check valve performance, purchased and qualified the reciprocating seals for LOX, and built and tested a sleeve-valve actuated drive motor.

XCOR also contributed its own resources to concurrently develop the fuel pump, which allowed us to accelerate LOX pump development by learning lessons about the integrated pump/motor assembly. Both pumps were designed to be used with our 1,800 lbf, XR-4K5 LOX/kerosene engine.

A Phase II contract extension allowed us to conduct preliminary design of an integrated two stage, air-launched concept using this piston pump along with our engine and composite LOX tank technology.  This included substantial preliminary design work on LOX/methane engines and initial coupon tests of XCOR’s LOX tank materials.

The Single Cylinder Crankshaft Pump: Precursor to the Three Cylinder

After the DARPA contracts were completed, we devoted internal resources to developing a more robust pump configuration, one designed for reliability and long life in commercial applications. This work culminated in the single cylinder crankshaft pump. During the early test program, this pump accumulated over four hours of run time.

That convinced us the crankshaft design was fundamentally sound and led to the design, build, and test of the three cylinder pump (photo above), which, after accumulating over five hours of run time in the test program, was integrated into the 1,500 lb thrust XR-4K14 engine and flown on the Rocket Racer.

This is a single acting motor and pump. The components are all rated to run at 6,000 rpm, but the 4K14 engine does not come near to reaching that potential. Instead the pump speed is less than 900 rpm.  Therefore, this pump can be turned faster to supply propellants to a much larger rocket engine.

These latest piston pumps are powered with engine heat, which, when used with XCOR's patent pending thermodynamic cycle, can achieve an Isp as high as the staged combustion cycle, which is the most efficient cycle for LOX/hydrocarbon engines and used on the best engines now.

3 Cylinder Pump Providing Fuel to the XR-4K14 1,500 lb-thrust LOX/kerosene engine in flight configuration.

 XCOR’s proprietary piston pumps can pump like  turbopumps but are able to stop and start quickly.  They are also less expensive. A benefit of a piston-style design is that the pump is capable of pumping more fuel at a higher operational speed. Using this innovative pump design, drive gas to operate the pumps can be delivered by any of the three classical methods: staged combustion, gas generator, or expander. We have chosen a fourth, however, which is a proprietary thermodynamic cycle that is most similar to the expander. XCOR has patented this cycle, which has the advantage of not lowering the engine specific impulse, as a gas generator would.

While our pumps are designed for LOX and kerosene propellants, they can be easily adapted to pump any other commonly used propellants.

For pump videos and photos, please visit the piston pump media gallery.