The Challenges of Multi-Restart Engine Design

Designing engines that can fire repeatedly in the vacuum of space demands a comprehensive rethinking of propulsion systems. Unlike single-use boosters that perform one burn and are discarded, multi-restart engines must operate reliably under extreme thermal cycling, maintain structural integrity across dozens or even hundreds of starts, and manage propellant dynamics in microgravity. Each restart introduces unique stresses that single-burn engines never experience.

Thermal Management Across Multiple Cycles

The most fundamental challenge is dissipating heat generated during each burn. In space, there is no atmosphere to carry away heat; only radiation and conduction can remove it. After the first ignition, engine components such as the combustion chamber, nozzle, and injector face reach temperatures exceeding 3000°C. When the engine shuts down, residual heat must be managed to prevent damage to seals, valves, and adjacent spacecraft structures. Successive restarts can cause heat soak-back into propellant lines, vaporizing fuel before it reaches the chamber and causing ignition failures. Advanced thermal modeling and active cooling systems, such as regenerative cooling channels or film cooling, are essential to keep temperatures within safe bounds.

Component Durability Under Repeated Stress

Every restart imposes mechanical and thermal shock on critical components. Turbopumps spin up from zero to tens of thousands of revolutions per minute in seconds, subjecting bearings and blades to extreme acceleration forces. Combustion chamber walls undergo rapid thermal expansion and contraction, leading to low-cycle fatigue. Seals, particularly those in valves and turbopump inlets, must remain leak‑tight across hundreds of thermal cycles. Materials that excel at high temperature—like Inconel, niobium alloys, and ceramic matrix composites—extend component life, but designers must also account for creep, oxidation, and embrittlement. The industry has moved toward additive manufacturing to produce complex internal cooling channels that improve durability.

Propellant Management in Microgravity

In a weightless environment, propellant does not settle at the bottom of a tank. Without positive village control, gas bubbles can infiltrate the fuel line, leading to cavitation in the turbopump or incomplete combustion. Multi‑restart engines require robust propellant management devices—such as diaphragms, bellows, or surface tension vanes—to ensure that only liquid propellant reaches the engine. Some designs use small settling thrusters or the engine’s own low‑thrust preburner to settle the propellant before a main burn. This complexity increases with the number of restarts, as the propellant’s vapor pressure and temperature change over the mission.

Ignition Reliability

A single failed ignition can doom an entire mission. In multi‑restart engines, the ignition system must work consistently after hours or days of idle time in deep space. Hypergolic propellants, which ignite upon contact, eliminate the need for spark igniters and are highly reliable, but they are often toxic and require special handling. Spark igniters—commonly used with liquid hydrogen and oxygen—must be free of contamination and provide a consistent spark across many cycles. Some engines now use laser ignition or “torch” igniters that burn a small amount of propellant to generate a pilot flame. Redundant ignition systems and health monitoring sensors further improve reliability.

Technological Solutions That Enable Multiple Restarts

Engineers have developed a suite of technologies to overcome the challenges above. These solutions blend proven materials science with advanced control logic and manufacturing techniques.

Storable Propellant Systems

Hypergolic propellants—such as monomethylhydrazine (MMH) with nitrogen tetroxide—have been the workhorse for multi‑restart engines since the Apollo era. They need no external ignition source, can be stored for years, and tolerate wide temperature swings. Modern versions, like the propellant used in SpaceX’s Draco thrusters, offer restart capability with minimal system complexity. However, the push toward greener alternatives has led to research in storable, non‑toxic hypergols, such as AF‑M315E used on NASA’s Green Propellant Infusion Mission.

Advanced Materials for Combustion Chambers

High‑temperature alloys and ceramic‑matrix composites (CMCs) now allow chambers to withstand thousands of thermal cycles. For example, NASA’s RS‑25 engine (Space Shuttle Main Engine) used a copper‑alloy liner with milled channels for regenerative cooling, enabling over 100 starts between major overhauls. More recently, 3D‑printed copper alloys have allowed designers to create complex internal geometries that improve cooling efficiency and reduce weight. Inconel 718 and Haynes 230 are common for hot‑gas manifolds and preburners.

Improved Ignition Systems

Modern engines employ dual‑redundant igniters with health monitoring. The RL10 engine, used on the Centaur upper stage, uses a spark igniter that has been refined over decades to achieve near‑perfect reliability for multiple restarts. For electric pump‑fed engines or those using methane and oxygen, glow‑plug igniters and plasma torches are emerging. SpaceX’s Raptor engine uses a torch igniter as part of its full‑flow staged combustion cycle, allowing multiple restarts even after long coast phases.

Active Thermal Control

Thermal management systems now incorporate heat‑switch radiators, phase‑change materials, and active cryocoolers to maintain engine temperatures within narrow bands between burns. The engine’s own fuel can be routed through cooling channels during a coast phase, using a small pump to circulate cryogenic propellant and absorb residual heat. For larger engines, separate coolant loops with non‑reactive fluids, such as a water‑glycol mixture, are used to keep avionics and valves from overheating.

Applications in Modern Missions

Multi‑restart engines are no longer a luxury; they are a necessity for the most ambitious space missions planned today.

Upper Stage Maneuvers and Orbit Insertions

Launch vehicle upper stages rely on multiple engine restarts to deliver payloads to precise orbits. The United Launch Alliance Centaur upper stage, powered by one or two RL10 engines, can perform up to four restarts over a six‑hour mission. This capability is critical for direct‑to‑GEO insertion, multi‑payload deployment, and deep‑space missions. The SpaceX Falcon 9’s second stage uses a single Merlin 1D Vacuum engine capable of multiple restarts, enabling complex satellite deployments and launches to Mars.

Human Lunar and Mars Missions

NASA’s Orion spacecraft uses the European Service Module with an Orbital Maneuvering System engine derived from the Space Shuttle’s OMS engine. This engine can restart multiple times to perform trans‑lunar injection, course corrections, and lunar orbit insertion. Future crewed lunar landers, such as the Human Landing System (HLS), will require engines that can start and stop repeatedly during descent and ascent, often in dusty environments. Blue Origin’s BE‑7 engine, designed for HLS, promises over 100 starts using non‑toxic propellants.

Deep‑Space Exploration

Interplanetary probes like the Psyche mission and the Europa Clipper use Hall‑effect thrusters for primary propulsion, but chemical engines still handle critical burns. The Europa Clipper’s main engine, based on the RS‑72, can restart multiple times for orbit insertion around Jupiter and subsequent flybys. Similarly, the ESA’s JUICE mission uses a bipropellant engine that will perform several restarts during its eight‑year journey to the Jovian system.

Satellite Servicing and Debris Removal

In‑space servicing missions, like NASA’s OSAM‑1 or private initiatives, require engines that can make many precise burns to rendezvous with client satellites, perform inspections, and then de‑orbit. The engines must operate with high reliability after long periods of inactivity and in the harsh radiation environment of geostationary orbit. Companies such as Northrop Grumman’s Mission Extension Vehicle have demonstrated dozens of engine restarts using hypergolic thrusters.

Future Developments in Multi‑Restart Propulsion

The demand for even more complex mission sequences is driving innovation in several areas.

Electric Pump‑Fed Engines

By replacing turbopumps with battery‑powered electric motors, engine designers can dramatically simplify the restart process. Electric pumps can be started and stopped almost instantly, without the need for complex spin‑up sequences or preburner ignition. This architecture is being explored by companies like Rocket Lab with their Rutherford engine and others for future upper stages.

Additive Manufacturing for Integrated Designs

3D printing allows entire combustion chambers, injectors, and nozzles to be built as single parts, eliminating braze joints and welds that are common failure points. The ability to create intricate regenerative cooling channels and embedded sensors will enable engines that can withstand more restarts with less degradation. NASA has already tested a 3D‑printed copper engine that completed 30 starts in a ground test campaign.

In‑Space Refueling and Long‑Duration Staging

Future missions, such as those to Mars, will require multiple restarts over months or years. In‑space refueling, where propellant is transferred between tankers and spacecraft, demands engines that can start after long coast periods. Cryogenic propellant management—keeping methane or hydrogen at supercooled temperatures—is a key challenge. Engines designed for the Moon-to-Mars architecture, like SpaceX’s Raptor, are being developed to support dozens of restarts, including after multiple refueling events.

Reusable Launch Vehicles and Rapid Restart

Reusability increases the number of restarts an engine must handle over its lifetime. The Falcon 9 first stage performs a landing burn after stage separation, while the second stage may restart several times during a single mission. The next generation of reusable vehicles, like Starship, will require engines that can start reliably hundreds of times without maintenance. This pushes materials and control systems to their limits.

Conclusion

Designing engines capable of multiple restarts has evolved from a niche requirement to a core capability for modern and future space missions. Overcoming thermal, material, and ignition challenges has required decades of incremental innovation. Today’s multi‑restart engines, using storable hypergols, advanced alloys, and robust ignition systems, enable complex orbital insertions, human lunar landings, and deep‑space exploration. As the industry moves toward additive manufacturing, electric pump cycles, and in‑space refueling, the number of starts an engine can achieve will continue to grow. The result will be a new generation of spacecraft capable of executing the most ambitious mission sequences ever imagined.

External Links:
- NASA
- SpaceX
- United Launch Alliance
- European Space Agency