CraterMaker Special Combustion Chamber
Manufacturing and testing of PSPL's CraterMaker Special's ablative combustion chamber.
CraterMaker Special (CMS) is PSPL's third generation liquid rocket and uses an ablative MethaLOX Coaxial swirl/shear combustion chamber and injector. It is targeting its maiden flight during April 2025. I had only joined the team once CMS had been entirely designed and am only responsible for the manufacturing and testing of the engine, but will briefly discuss the design of the engine.
CMS's Engine uses a LOX centered coaxial injector with swirled fuel outer posts with 16 injector elements, film cooling, and a Copper-Inconel bimetallic faceplate. The chamber was designed using the Rao nozzle method with an exit pressure of ~11 psi. The chamber inner wall is Scilica-Phenolic ablative that was obtained through a sponsorship. The ablative is followed by a volcanic silicone sealant, which is overwrapped in carbon fiber for increased strength.
Project Background
PSPL's fluid system lead besides CMS during Full vehicle testing (without flight structures) in ZL3
Manufacturing:
My work started on manufacturing the chamber mandrel. I created the CAM and was able to machine the sides with 4 operations across the CNC Lathe and 3-axis mill. Followed by tapping of the main bore and 10 M5 mounting holes on each side.
Chamber and nozzle mandrel sides after machining
Once the mandrel was ready, the ablative wrap was the next step. This process requires a minimum of 3 members as the material needs to be cut into strips, heated, and applied to the mandrel contour respectively. Due to the phenolics outgassing of formaldehyde during the heating and bonding process we had to work in a dedicated composites room in the Purdue Technology Center.
Ablative post wrapping on the mandrel
After the ablative wrap is complete it is vacuum bagged and sent to a privately sponsored furnace to cure over the course of several hours. After it is cured the ablative is hand post-machined to chamber inner dimensions. Then a flange is manufactured and bonded to the ablative. The chamber exterior is then coated in a volcanic silicone sealant to prevent leakage across the interfacing materials.
Then using a in-house fiber winder, pre-impregnated carbon fiber is wrapped over the silicone coated ablative chamber.
After the chamber carbon wrapped chamber is vacuum bagged once again and sent over to be cured one final time.
Carbon chamber post curing
Post curing the carbon is post-machined to encase the chamber dimensions, as the wrapping requires additional length on the nozzle end to complete symmetric passes. Similar to the previous post-machining there will be harmful particulates released during the cutting of the material so it is usually done outdoors in an secured area (ZL5 exterior) with proper PPE. After this is done the chamber is complete and can be integrated to the injector for testing.
Completion of the carbon post-machining
Testing:
Testing the chamber is unique to the other pressure tests required for pressure vessel components. Normally Hydro-proofing (testing pressure vessels to their MAWP using incompressible substances such as water) is favorable because of hydraulic fluids incompressibility posing a significantly lower potential energy failure than its compressible counter part. Unfortunately, the phenolic liner will saturate and absorb water so pneumatic-proofing was required.
The chamber was fitted with the injector and the fuel inlet and PT port were plugged. A rubber lined chamber plug was also made to plug the rough (slightly uneven) inner surface of the chamber nozzle. The ox line was than connected to our fill system and was supplied with incrementally higher pressure nitrogen. The chamber was inspected after every depress for damages and was snooped for leakage (if high leak rates were observed).
Additional Media about the project