Flight Software Engineer Intern

About Castelion

Castelion is bringing a new approach to defense development and production: one that focuses on short, iterative design cycles, rapid testing in development, and modern commercial manufacturing strategies for production at scale. We're designing, building, and testing next generation long range strike weapons systems to give America and its Allies a definitive edge and deter future conflicts.

Flight Software Engineer Intern - Summer 2026

Castelion is seeking driven and talented Flight Software Engineer Interns for Summer 2026. You'll help design, build, and test the real-time embedded software that powers our next-generation missile systems, ensuring they operate reliably under extreme conditions. You'll work alongside experienced flight software, GNC, systems, and hardware engineers to design, implement, test, and debug critical flight code. You wont be writing throwaway code; your contributions will directly support flight tests, hardware-in-the-loop simulation, and real mission deployments.

Key Responsibilities

• Design, develop, and test real-time embedded flight software for missile systems
• Collaborate with GNC engineers and hardware teams to integrate flight software with guidance, navigation, and control systems
• Support simulation and hardware-in-the-loop (HIL) testing to validate performance
• Debug and analyze software and hardware interfaces during bring-up and test campaigns
• Participate in design reviews, code reviews, and configuration management
• Document your work clearly and share learnings with the team to strengthen future designs

Basic Qualifications

• Currently pursuing a BS or MS in Computer Engineering, Electrical Engineering, Computer Science, Aerospace Engineering, or a related technical field
• Rising junior, senior, or graduate student at the time of the internship
• Strong programming skills in C/C++ for embedded systems
• Familiarity with real-time operating systems (RTOS) or bare-metal development
• Understanding of control systems fundamentals and how they interact with flight software
• Experience with debugging tools and working with hardware interfaces
• U.S. Person status required (due to ITAR regulations)

Preferred Skills and Experience

• Familiarity with simulation environments and HIL test setups
• Exposure to FPGA-based systems and embedded software optimization
• Familiarity with missile systems, defense technology, or aerospace components
• Strong interest in working in a fast-paced, high-ownership startup environment

You'll leave with

• Hands-on experience developing and testing flight software that flies
• Real-world exposure to simulation and hardware-in-the-loop testing
• Mentorship from engineers solving some of the hardest aerospace challenges
• A chance to help shape the future of defense manufacturing

Leadership Qualities

• Bias to Action and Creative Problem Solving.
  Desire and experience questioning assumptions in ways that lead to break through ideas that are ultimately implemented. Successfully bring in applicable processes/concepts/materials from other industries to achieve efficiency gains. The ability to personally resolve minor issues in development without requiring significant support.
• High Commitment, High Initiative.
  A successful candidate will have a genuine passion for Castelion's mission and consistently look for ways to contribute to the company's technical goals and prevent hardware blockers. Ability to work in a fast paced, autonomously driven, and demanding atmosphere. Strong sense of accountability and integrity.
• Clear Communicator.
  Proactively communicates blockers. Trusted in previous roles to be voice of company with regulators, suppliers, gate keepers and customers. Capable of tactfully managing relationships with stakeholders to achieve company-desired outcomes without compromising relationships. Emails, IMs and verbal interactions are logical, drive clarity, and detailed enough to eliminate ambiguity.

ITAR Requirements

• To conform to U.S. Government export regulations, the applicant must be a (i) U.S. citizen or national, (ii) U.S. lawful, permanent resident (green card holder), (iii) Refugee under 8 U.S.C. § 1157, or (iv) Asylee under 8 U.S.C. § 1158, or be eligible to obtain the required authorizations from the U.S. Department of State.

Why Castelion

At Castelion, you'll be part of a mission-driven company where every project matters. Our interns don't just observe they contribute directly to critical hardware and software that supports national security. This is a unique opportunity to work at the cutting edge of defense and aerospace engineering, while building the skills and experience to grow into a future leader in the field.

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.

This internship is paid at an hourly rate of $30 per hour. Compensation may vary based on experience and education level. For non-local interns, housing or relocation stipends may be available to help offset temporary living expenses.