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.