Dealing with extremely high temperatures in rockets

The exhaust stream of a rocket engine is an incredibly extreme environment. At velocities several times the speed of sound, temperatures in excess of 2000 °C in a gas stream loaded with small abrasive aluminium oxide particles, this is a severely hostile environment for any commonly used aerospace material.

For this reason, in the past DARE has generally used heat resistant nozzles made out of graphite. This material can withstand the high temperatures found in rocket engines and is soft enough to be readily machined with conventional manufacturing methods. The major downside of these nozzles is that they are on the heavy side. The current graphite nozzle is almost 15% of the Stratos III empty weight, which means that saving weight would result in a significantly lighter rocket, meaning that we can go a lot higher.

With this in mind, the Stratos III propulsion team started investigating more lightweight options, eventually resulting in the Stratos III R&D nozzle pictured below. At just 8 kilograms, this nozzle is almost half the mass of the original and is produced out of grade 23 titanium by 3D Systems Leuven using additive manufacturing. The powder used for this part was supplied by Oerlikon Metco, a company that specializes in amongst others additive manufacturing and coatings.

Even with a material that has excellent high temperature properties, like this titanium alloy, firing the engine with this nozzle as is would result in it melting after just several seconds. In cooperation with Oerlikon Metco it was decided to apply a so-called ‘YSZ Thermal Barrier Coating’, to prevent this from happening. This is a refractory ceramic coating comprised of zirconium and yttrium oxides, which is often used in extremely high temperature environments, like fighter jet afterburners (which are also commonly made out of titanium alloys) and gas turbines. Coatings like this one can withstand temperatures in excess of 2500 °C!

A full scale nozzle test piece employing this coating was first produced and successfully tested in DHX-400 tests 7 & 8. These tests were performed to validate our heating model, and to ascertain if the coating would hold up in the extremely high velocity & abrasive exhaust stream.  They showed promising results, with no superficial damage to the coating and the titanium staying within allowable boundaries to move further with the eventual full scale nozzle.

The nozzle pictured below (note the white thermal barrier coating on the inside) is a full scale nozzle of the DHX-400 ‘Nimbus’ engine, which has allowed us to learn more about additive manufacturing and metal coating techniques for future Stratos missions.

The Titanium Nozzle.

Project Stratos would like to thank Oerlikon Metco for their support in the development and production of this nozzle.

by Dion van Strydonck