stethoscope 2

The propulsion system on Lynx is supported by a steel truss that attaches to the rear firewall of the fuselage. Composite blast shields surround each engine.


In front of the engines, the truss supports all the valves, pumps, and plumbing needed to control and deliver fuel and oxidizer to the engines. In these shots, engineer Jeremy Voigt checks for small control line leaks using a stethoscope.

XCOR Lynx: Engines 101 — What 5K18 really means, and a bit on pumps and regenerative cooling


This shot of the Lynx 5K18 engine truss and Lynx main fuselage was captured during a hotfire outside XCOR’s bunker at the Mojave Air and Space Port in early 2013. This particular engine test was one of the world’s very first fully piston pump-driven rocket engine firings.

Welcome back!

As discussed last week, Mondays are dedicated to the Lynx propulsion system. This includes Lynx main engines, engine nozzles, reaction control systems, and related technologies such as XCOR rocket piston pumps, valves and piping.

Let’s start with the Lynx main engines, how we name them and a bit about how 5K18 pumps and regenerative cooling work.

How we name our engines

As just mentioned, Lynx main engines include four XR-5K18s. The engine naming and numbering scheme we use follows a specific naming convention, like most things in aerospace or automotive.

The “XR” stands for “XCOR Rocket”.

The “5” stands for the thrust class of this rocket engine.

The “K” in this case is for the fuel, here it’s kerosene.

The “18” is the 18th overall engine design since XCOR was founded. For example, our X-Racer was powered by the XR-4K14 engine, a fourth-generation engine powered by kerosene and liquid oxygen, and the 14th overall engine design we had initiated at the time.

Fully pump-fed engines

Sometimes when testing our engines we use highly-pressurized inert gas, such as helium, to force the oxidizer and fuel into the engine for combustion. Or our engines may be powered by pumps that move oxidizer and fuel from low pressure tanks to high pressure before feeding them into the engine for combustion. And sometimes we do half and half: half pressure-fed and half pump-fed.

In operations, Lynx 5K18 engines will be fully pump-fed.

Regenerative cooling

The 5K18 engine uses “regenerative cooling” to prevent it from melting during the extreme temperatures of rocket combustion. Tiny capillaries inside the wall of each engine (and engine nozzle) have cooling liquid that flows through them to maintain appropriate temperatures. In the 5K18 we use kerosene fuel to cool the engine just prior to it being injected into the engine for combustion.

This is not a new idea. In fact, it has been used on most liquid rocket engines since the early days by pioneers such as Wernher von Braun (Germany, 1930s) and Reaction Motors (US, 1940s).

Over the next few Mondays you will experience a set of behind-the-scenes photos and see some of what it takes to build, prepare and test these engines. We will also highlight the unique features of this propulsion system that make it one of the most advanced and reliable rocket propulsion systems in existence today.

Tomorrow, it’s all about the cockpit!

Stay tuned.

Today’s news: One more step

We are excited to announce the most recent major milestone in our Lynx engine program, truly a part of aviation and space history: a 67-second fully pump-fed firing of our XR-5K18 rocket engine (and the first firing of a full piston pump-powered rocket engine in history).

For details, do read the release. Here we will just say that we believe this is leading toward a new era of fully reusable and reliable spacecraft.

XCOR CEO Jeff Greason inspects the Lynx main engine after a hotfire test while Chief Test Engineer Doug Jones looks on.

As the Lynx engine program continues to evolve we anticipate that the net result will be a dramatic reduction in per-flight costs and turnaround time, and that it will lead to a serious increase in affordable and routine spaceflight.

And with pumps as powerful as turbines but as reliable as automotive engines, today’s news on the 5K18 program is one more step toward the goal of true reusability and reliability.

We hope you join us.

Shock diamonds are visible during a test of the Lynx main engine. The diamonds are an interaction between supersonic gasses escaping from the engine and the pressure of earth’s atmosphere.

Jeremy Voigt prepares to test fire the Lynx main engine at XCOR’s test site on the Mojave Air and Space Port in Mojave, CA