Senior Mechanical Design Engineer (ACS Systems)

Why Castelion, Why Now Castelion is moving incredibly fast to develop and deliver advanced defense systems at a time when execution matters more than ever. We believe focus and technical excellence are decisive advantages - and we are building a team that can deliver real capability, not just concepts. This is a rare opportunity to join at an early stage, where your work will directly shape critical systems, influence major technical decisions, and have immediate, real-world impact. Senior Mechanical Design Engineer (ACS Systems) 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.

Responsibilities Design and develop ACS hardware, including propellant tanks, valves, thrusters, and associated fluid system assemblies Lead the architecture and detailed design of pressure vessels, fluid lines, mounts, and integrated ACS packaging within the vehicle Perform structural, thermal, and fluid analyses to ensure performance and reliability under extreme pressure, temperature, and dynamic environments Work closely with propulsion, GNC, avionics, and structures teams to ensure seamless integration of ACS systems with vehicle architecture Conduct tolerance analysis, sealing strategy development, and leak-tightness assessments for high-pressure systems Support prototyping, assembly, and hardware bring-up, including troubleshooting and root cause analysis of system performance issues Define and execute qualification and environmental test plans, including pressure, vibration, thermal, and shock testing Optimize designs for mass, reliability, manufacturability, and ease of integration Mentor junior engineers and contribute to best practices in mechanical and fluid system design Partner closely with program leadership to make technical trades aligned with mission success Basic

Qualifications 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 Other Duties Please note this job description is not designed to cover or contain a comprehensive listing of activities, duties or

responsibilities that are required of the employee for this job. Duties,

responsibilities and activities may change at any time with or without notice.

Benefits And

Perks All full-time employees are granted meaningful long-term equity, sharing in the company's significant growth trajectory. We offer four (4) weeks of paid time off, ten (10) company-paid holidays, and comprehensive health

benefits - including 100% employee-covered medical and strong dependent coverage, along with dental and vision plans. We also provide paid parental leave to support growing families, a $100 monthly fitness stipend to promote health and performance, and onsite EV charging for convenience. In addition, employees enjoy

perks like catered meals, company-covered food during high-demand periods, and a fully stocked kitchen to stay fueled throughout the day. While this position does not currently require a U.S government-issued security clearance, it may in the near future. Therefore, the ability to obtain and maintain a U.S. government–issued security clearance is required. U.S. citizenship is required, as only U.S. citizens are eligible for a security clearance. Affirmative Action/EEO Statement Employment with Castelion is governed on the basis of competence and

qualifications and will not be influenced in any manner by race, color, religion, gender, national origin/ethnicity, veteran status, disability status, age, sexual orientation, gender identity, marital status, mental or physical disability or any other legally protected status.