Full Stack Software Engineer

Location: London Travel: Travel required several times per year (UK and international), sometimes at short notice and for up to 7 days 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 Full Stack Software Engineer in our populAI team, you will own how Simulation Operators actually bring a continent-scale simulation to life - the screens where they author a scenario, issue commands across thousands of simulated entities, compose simulated social media, and script the large set pieces that make a training exercise land. This is the surface where our engine meets its users, and the difference between a good UI and a great one is the difference between a training exercise that feels like a game and one that feels like the real world. Your work will ship into live demonstrators, integration hackathons and customer exercises, where the loop between building something and watching users bend it to a real problem is measured in weeks, not quarters. You'll lead on the web side of populAI - the TypeScript/React front-end and the Node.js services behind it - while sitting inside a team of seasoned simulation engineers writing the C++ engine that powers it. As a polyglot, you'll be trusted to drop into the C++ simulation when it matters; equally, you'll be the team's centre of gravity for modern web engineering, raising the bar on what's possible in the UI and helping teammates sharpen those skills. If you enjoy moving between specialisms, thinking hard about how complex information is made clear, and shipping real things for users who depend on them, this is a role where your voice will carry a long way. Key Responsibilities: Lead the development of the populAI web application - TypeScript/React front-end, Node.js services and REST APIs - used by SimOps operators to configure, control and monitor the simulation. Work with UX designers to design the interfaces operators use to issue complex commands across thousands of simulated entities and surface the current state of a running simulation clearly under pressure. Extend the social media authoring and monitoring toolset that drives simulated public sentiment and content in response to in-simulation events. Build the macro and scripting tooling operators use to design large scripted set pieces across a simulated scenario. Contribute to the C++ simulation engine when features cross the web/sim boundary, working alongside simulation specialists. Collaborate with Product and defence stakeholders to turn customer problems into effective operator experiences. Mentor and upskill teammates in modern React, TypeScript and Node.js practices, raising the web engineering bar across the team. Support populAI deployments on-site at customer exercises, hackathons and industry events, and bring what you learn back into the product. Skills, Knowledge and Experience: 4+ years of software engineering experience building complex web applications with React, TypeScript and Node.js. Track record of delivering non-trivial UI - real-time data, dense information surfaces, or visualisations where clarity is the product. Experience