Engineers test actively cooled CMC Panels for rocket and scramjet engines

  • Published
  • By Mindy Cooper
  • Mindy Cooper, AFRL Materials and Manufacturing Directorate
WRIGHT-PATTERSON AIR FORCE BASE, Ohio --  Researchers have successfully completed environmental testing on actively cooled ceramic matrix composite panels in both rocket and scramjet rigs, proving that the materials have the durability necessary for these extreme environments.

The ceramic matrix composite (CMC) testing, directed by Air Force Research Laboratory's Materials and Manufacturing Directorate here under an Integrated High Payoff Rocket Propulsion Technology contract with Teledyne Scientific Company of Thousand Oaks, Calif., went extremely well in both designs, according to Steve Steel, materials research engineer who managed the program.

"RX (Materiels & Manufacturing Directorate) initiated this IHPRPT contract with Teledyne in 2002 to develop CMCs as light weight alternatives to the nickel-based alloys from which rocket engine and scramjet components are fabricated," explained Steel, "Since then, the effort has achieved many innovations in CMC fabrication and has steadily advanced the technology readiness level of these materials for rocket and scramjet propulsion applications."

At the heart of Teledyne's innovative composites are the three-dimensional woven fiber preforms. Preforms are the fiber reinforcement backbones that lend strength and toughness to the ceramic composite. The fibers used by Teledyne were carbon or silicon carbide. The fiber preforms were infiltrated with a matrix material, in this case silicon carbide, to form the completed composite.

According to Steel, Teledyne designed the fiber preforms so that cooling channels are integral to the fiber architecture right off the loom. The preform geometry is tailored to the application, for example tapered tubes for the rocket nozzle verses square channels for the scramjet combustor. Fiber placement is optimized for coolant pressure containment, e.g., twice as many fibers in the circumferential direction of the tubes as in the longitudinal direction to match the stress state for a pressurized cylinder. These specially designed and optimized preforms lead to extremely lightweight structures whose "open woven" passages permit active cooling while maintaining structural integrity.

Other innovations that were developed under this research effort included an attachment method so that adjacent composite panels could be joined as segments to assemble full nozzles and combustors, in conjunction with Pratt & Whitney Rocketdyne. Also, a method for forming smooth composite surfaces was developed. This reduces hot spots caused by turbulent flow over rough surfaces, and it also decreases the composites' permeability, which is a requirement for fuel containment.

"The research effort culminated with several important demonstrations," Steel said.

The first demonstration took place at NASA's Glenn Research Center. Actively cooled carbon-fiber-reinforced silicon carbide (C/SiC) and silicon-carbide-fiber-reinforced silicon carbide panels were tested in the Cell 22 rocket rig, which is used to evaluate advanced rocket engine materials in an oxygen/hydrogen combustion environment. The test panels, which measured three inches wide by ten inches long, were placed in the rig so that the high temperature combustion gases (which reach 5800 degrees Fahrenheit) impinged directly onto the panels, with aerothermal conditions and heat flux equivalent to those experienced at the upper end of a typical boost engine nozzle. The test conditions were similar to those encountered aboard the shuttle orbiter's main engines. Panels survived multiple firings of various durations, with cumulative exposure of 40 minutes and individual cycles lasting up to eight minutes.

This is the longest oxygen/hydrogen rocket combustion exposure ever achieved for such actively cooled materials and is equivalent to five to ten launch cycles for a boost rocket engine, depending on flight profile.

The second test series took place in the AFRL Propulsion Directorate's scramjet rig. An actively cooled C/SiC panel was installed in the combustor wall of the rig and was subjected to 20 engine runs lasting about one minute each. The panel looked pristine after the testing was completed.

"Results from this testing were extremely positive, which further demonstrated the CMCs' suitability for the extreme environments of engine combustors and nozzles," Steel said.

Now that the durability of the materials and robustness of the cooled structures have been demonstrated at subscale in flat-panel configurations, and fabrication processes have been demonstrated for other shapes such as bell nozzles, the next steps to advance the technology will be to increase the size and complexity of test articles to be more representative of actual components.