What is Saguaro
Saguaro is the Experimental Rocket Propulsion Laboratory’s composite engine program that expands our manufacturing capability in tooling, layup, and cure, seal control, and repeatable assembly while proving a reliable composite ablative chamber. It is developed in parallel with our regeneratively cooled engine and uses a shared injector with common interfaces to optimize cost efficiency in the development phases of this engine. The engine body is intentionally modular and scalable, allowing injector count, throat size, and overwrap sequence to grow with thrust class so Saguaro can mature into whatever engine ERPL needs it to be based on mission goals. The aim is to cut mass and turnaround time while preserving test safe operations.
Parameters
Propelants Kerosene/LOX
Mixture Ratio 1.5
Chamber Temp 2493K
Thrust 750lbf
Chamber Pressure 300psi
Specific Impulse 233s
Burn Time ≥15s
Composites Silica/Carbon Fiber
Ablative Liner
Saguaro’s ablative liner was transitioned from a silica-phenolic wet layup to a silica-epoxy system to meet campus safety rules and improve build quality. We wrap 1-inch silica tape wetted with high-temperature epoxy under constant tension. The liner is set at roughly 0.5-inch thickness to add a margin for accumulating test data. Because silica epoxy will not hold to our margins at the peak heat flux at the throat during a full 15-second run, we integrated a COTS phenolic throat insert from Boedeker. Early validation uses short five-second burns with surface thermocouples to measure regression and ablation, then we tune thickness and layup as duration increases.
My Role in Saguaro
I am the lead for Saguaro development and engineered both the engine body pieces and the shared coaxial swirl injector. My designs have made it through PDR, closing in on CDR in the next month. I presented Saguaro, as part of the DRACO Evolution Project, to the Embry-Riddle Philanthropy Council as one of 3 finalists from a pool of 170 project submissions to help fund the development of the injector and engine body. Next steps for this engine are to push our composite coupon testing forward and have manufacturing take place post our liquid flight vehicle launch. In the case we need to refocus our engine design on the flight vehicle, Saguaro will be configured to mount onto that existing injector hardware and chamber dimensions.
Carbon Overwrap
The overwrap posed a distinct structural challenge: carrying chamber pressure over a constantly changing ablative core while managing throat heat flux and thermal gradients. To control stress distribution, we lay up with a ±45° laminate and a 0° starter ply. At the throat, there is a layer of silica-epoxy over the COTS phenolic, which adds another layer of thermal protection and simplifies the manufacturing process.
Injector Interface
The injector interface allows the adaptation of the composite engine into the injector. The interface uses a flange, as well as a sealing feature at the base to seal the regen exit holes off. The flange piece takes the axial forces of the engine and prevents the composite from blowing off.
Coaxial Injector
A coaxial swirl injector was chosen for its efficient mixing and atomization, and because ERPL has previous experience with swirls. To design the injector, I started by identifying my constraints, such as the sealing design for the regen holes, and how to manage options for either filling fuel through the tregen holes or the manifold plate. After I had a general idea of my design, I wrote code to analyze the swirl geometry and export the fluid’s conditions through the element and exit. I optimized for a dP of ~75 psi in both the fuel and LOX chambers, as well as a target RN(recces number) of 1.66. Having that slightly higher dP —25% of chamber pressure — allows us to throttle down the injector through our pressure-fed system instead of changing annulus size, as in an actuating pintle.
Element Design
The elements are threaded together, and the outer fuel swirls into the base plate. This decision was made to prevent the plate from deforming into the chamber, compromising the seals not only for the elements but also for our central torch tube. The elements are open-type co-swirl in design to facilitate manufacturing and to preserve the recirculation zone at the injector exit.