NASA and General Atomics Engineers Propose a Dual-Mode Nuclear Rocket That Could Halve Mars Transit Time
NASA engineer Kurt Polzin and General Atomics engineer Robert Schleicher have proposed a bimodal nuclear spacecraft design that could shorten a round trip to Mars by up to two years. The design, called the synchronal bimodal nuclear rocket (S-BNR), was detailed in a paper presented at the American Institute of Aeronautics and Astronautics and described in IEEE Spectrum.
The S-BNR combines two propulsion technologies into a single reactor core. The first is nuclear thermal propulsion (NTP), where a nuclear reactor heats a propellant directly — think of it as a nuclear-powered version of heating water to make steam. This produces high thrust, useful for quick maneuvers like leaving Earth orbit or arriving at Mars. The second is nuclear electric propulsion (NEP), where the reactor generates electricity to power ion or Hall-effect thrusters. Ion thrusters accelerate charged particles to very high speeds, which makes them extremely fuel-efficient but low in raw push. NEP is better suited for long, steady cruising between planets.
The key innovation is that the same reactor can switch between these two modes. NTP handles the high-thrust departure and arrival burns; NEP takes over for the efficient cruise phase in between. That matters because each technology has a weakness on its own. Pure NTP is powerful but only about twice as fuel-efficient as a conventional chemical rocket, capping out around 900 seconds of specific impulse (a measure of how efficiently a rocket uses propellant). Pure NEP is far more fuel-efficient but produces too little thrust to quickly leave Earth's gravity or brake into Mars orbit. Running both from one reactor avoids the weight penalty of carrying two separate reactor systems.
The electric mode also solves a practical problem. A nuclear thermal rocket only produces thrust while it is actively heating propellant, which means it generates no electricity during coast phases. But crewed missions need continuous power for life support, communications, and thermal management. The S-BNR's electric mode fills that gap without a separate power source adding dead weight during high-thrust maneuvers.
NASA's shortest existing plan for a crewed Mars round trip requires 620 days in space plus 30 days on the Martian surface. The S-BNR design aims to bring total crew transit time down to 335 days or less. That reduction matters because prolonged exposure to deep-space radiation and microgravity is one of the principal obstacles to crewed Mars missions. Halving the transit duration does not eliminate the risk, but it materially compresses the window during which crews are exposed.
The technical lineage runs deep. Researchers recognized as early as 1946 that nuclear reactors could serve as efficient thermal rocket engines. In the 1960s, the U.S. NERVA and Rover programs conducted ground tests that demonstrated much of the underlying technology. Despite that progress, the United States has launched only one nuclear reactor into space: the prototype SNAP-10A, orbited in 1965. NTP and NEP have since been studied extensively but never flown as integrated crewed propulsion systems.
The broader context is that NASA has cycled through several propulsion architectures for Mars missions over the past two decades, including chemical-aerocapture and solar-electric variants, none of which has progressed to flight hardware. The S-BNR proposal remains a paper design; no flight reactor has been built or tested under this configuration, and the U.S. has not orbited a nuclear reactor in over 60 years. Testing a space-rated nuclear propulsion system would require not just engineering maturation but also regulatory pathways for launching fissile material, which have historically been politically and procedurally fraught.
In my view, the S-BNR is best understood as a serious architectural argument rather than a near-term hardware program. What makes it notable is not the novelty of nuclear propulsion itself, which has been on the drawing board for eight decades, but the specific integration of thermal and electric modes from a single core. That integration directly targets the two variables that matter most for crewed Mars transit: total trip duration and sustained electrical power. If the design advances to ground-based prototype testing, the key engineering question will be whether a single reactor core can reliably switch between thermal and electric modes without degrading either function over the operational lifetime a Mars mission demands.
The 335-day transit target is aspirational and contingent on design parameters that no hardware has yet validated. But the direction of travel in nuclear propulsion concepts has been consistent: each successive proposal tightens the integration and reduces the number of separate systems a Mars vehicle must carry. The S-BNR continues that trajectory. Whether it becomes the architecture that finally sends humans to Mars, or another well-engineered concept that never leaves the ground, depends on funding, testing, and policy decisions that have not yet been made.
