Simulation Software Engineer

Location: London Travel: Travel required several times per year (UK and international), sometimes at short notice and for up to 1 week Security: Must be eligible to obtain and maintain UK security clearance to at least SC level Hadean is an award-winning software company building software that enables real-time decision support, simulation, mission rehearsal and digital environments for the defence industry. Hadean is a deep-tech company building cutting-edge distributed computing technology that powers scalable, secure, and interoperable digital environments. Our platform enables real-time simulation and training, mission rehearsal, command & control and digital twin capabilities—transforming the way defence, government agencies and enterprises plan, train, and make decisions. Hadean’s solutions have helped organisations including the UK MoD, US DoW, BAE, Leidos, and more, and we work with these customers at the forefront of defence innovation, collaborating with global partners to deliver next-generation capabilities that unlock operational advantage. The Role: As a Simulation Software Engineer in our populAI team, you will own substantial parts of the simulation that makes continent-scale defence training feel like the real world - the behaviour and interactions of pedestrians, traffic, shipping, aircraft, satellites and critical national infrastructure that populate a scenario. This is the core of populAI: the difference between an exercise that feels like a map overlay and one that reacts to a trainee's decisions with the emergent chaos of actual people and systems. You'll shape how a class of simulated actor decides and acts, how the simulation stays coherent when partitioned across a compute cluster, and how new LLM-backed capabilities - natural language control, richer actor generation, simulated social media - are brought into the engine. Your work will ship into customer exercises, live demonstrators and integration hackathons, where the loop between building something and watching users bend it to a real problem is measured in weeks, not quarters. You'll work across a C++ simulation engine, a custom distribution framework, and a TypeScript/React front-end used by Simulation Operators, inside a cross-functional team of Software, QA and DevOps Engineers led by an Engineering Manager. The team is made up of seasoned simulation engineers looking for a peer willing to bring energy, thought leadership and a product engineering mindset. If you enjoy moving between specialisms, raising the bar for the people around you, and shipping real things for users who depend on them, this is a role where your voice will carry a long way. Key Responsibilities: Design and implement behaviour and decision-making logic for simulated entities - pedestrians, vehicles, shipping, aircraft - in our C++ simulation engine. Integrate third-party simulation libraries and algorithms, maintaining enough intuition for their internals to combine them into a coherent system. Shape how simulation state and behaviour are partitioned across our distribution framework, designing for IPC, bandwidth and fidelity trade-offs up front rather than retrofitting them. Contribute across the web/sim boundary, extending REST services and the TypeScript/React front-end when features reach into them. Write design docs and run design reviews in your area, helping the team converge on technical direction quickly. Review peers' designs and code with care, and collaborate with Product and defence stakeholders to turn customer problems into feature shapes. Mentor teammates through pairing, code review and knowledge sharing - raising the team's capability in simulation craft, product-engineering practice, or defence knowledge. Deploy and support populAI on-site at customer exercises, integration hackathons and industry events - working issues live alongside users, and folding what you learn back into the product. Skills, Knowledge and Experience: 4+ years of software engine