Castelion was founded to restore America’s ability to field advanced weapons at speed and scale. Inspired by the engineering urgency of the WWII and Apollo eras, we combine rapid iteration, vertical integration, and scalable manufacturing to deliver affordable, high-performance strike systems. Our mission: rebuild industrial agility to strengthen national security and outpace evolving threats. About the Role: We are seeking a Senior Mechanical Design Engineer to design and deliver attitude control system (ACS) hardware for Castelion's long-range hypersonic strike vehicles. This role is critical to enabling reliable propellant storage, flow control, and thrust generation for precise vehicle control. You will work hands-on with multidisciplinary engineering teams to design, analyze, build, and validate ACS tanks, valves, thrusters, and integrated assemblies in a fast-paced, hardware-driven environment. Requirements Bachelor's, Master's, or PhD in Mechanical Engineering or related field 5+ years of experience in mechanical design for aerospace or high-performance fluid/propulsion systems Strong background in pressure vessel design, fluid systems, and high-pressure component integration Experience designing tanks, valves, thrusters, or similar propulsion or ACS hardware Familiarity with sealing methods, leak detection, and cleanliness requirements for high-reliability fluid systems Proficiency in CAD and analysis tools (e.g., NX, SolidWorks, ANSYS, or equivalent) Experience with structural analysis, thermal analysis, and tolerance stack-up analysis Familiarity with high-temperature materials and operation in extreme environments Experience with hardware build, test, and troubleshooting in a lab or flight hardware environment Strong problem-solving mindset, thriving in a fast-paced, hardware-rich, high flight cadence environment Benefits Employee Equity: Meaningful equity for all full-time employees - share value has increased 150x in three years Personal Time Off: Four (4) weeks of paid time off to unplug, reset, and come back sharp—velocity matters, burnout doesn't Company Paid Holidays: Ten (10) company paid holidays Medical: We offer plans at 100% employee covered and 80% of dependent coverage Dental & Vision: We cover 90% employees and 80% for dependents to ensure you and your family have access to quality preventive and ongoing care FSA/HSA: To support long-term health and financial resilience, we offer FSA and HSA options that allow employees to set aside pre-tax dollars for eligible medical and dependent care expenses Retirement: 401k plan offered to support long term financial growth along side high-impact, high-velocity work Paid Parental Leave: We support growing families with meaningful time to bond and recover. Birthing parents are eligible for up to 10 weeks of paid leave, while non-birthing parents may take up to 4 weeks of paid leave. This time can be used for pre-natal care, pregnancy, childbirth, and related medical needs—ensuring every parent has the support they need during this important life moment.8-weeks of paid parental leave for all parents. Additional paid pregnancy leave for CA employees Meals & Catering: Frequently catered lunches and team meals, plus company-covered meals when extra time or effort is required Snacks & Beverages: Fully stocked kitchen with unlimited snacks and drinks to keep you fueled throughout the day Performance & Fitness: $100 monthly fitness stipend to support peak performance and health, both inside and outside of work EV Charging: Onsite electric vehicle charging available for employees