Bimodal Nuclear Rocket Design Could Cut Mars Round Trip to Under 335 Days
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 (IEEE Spectrum).
The S-BNR combines nuclear thermal propulsion (NTP) and nuclear electric propulsion (NEP) into a single bimodal core capable of operating in either a thermal mode or an electric mode. In thermal mode, the reactor heats a propellant directly to produce high thrust, suited for rapid orbital maneuvers. In electric mode, the reactor generates electrical power to drive an ion or Hall-effect thruster system, delivering high specific impulse at lower thrust for efficient sustained acceleration over long cruise phases. Crucially, the electric mode also generates electricity for life support and other onboard systems, addressing a persistent engineering constraint for deep-space crewed missions where solar power density falls off with the inverse square of distance from the Sun.
NASA's shortest existing blueprint for a crewed Mars round trip requires 620 days in space plus 30 days on the Martian surface. The S-BNR design aims to reduce total crew transit time to 335 days or less. That reduction matters because prolonged exposure to deep-space radiation and microgravity remains 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 here runs deep. Researchers recognized as early as 1946 that nuclear reactors could serve as extremely efficient thermal rocket engines. In the 1960s, the U.S. NERVA and Rover programs conducted open-air ground tests that demonstrated much of the underlying technology for nuclear thermal rockets. 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.
Worth flagging is what the bimodal architecture specifically solves that either mode alone does not. Pure NTP delivers high thrust but tops out at specific impulses roughly double that of chemical rockets, around 900 seconds, which still imposes long transit times for Mars-class missions. Pure NEP achieves much higher specific impulse but produces low thrust, lengthening the departure and arrival burns and complicating trajectory design. By running the same reactor core in both modes, the S-BNR could use NTP for high-thrust maneuvers near Earth and Mars and NEP for efficient cruise in between, without the mass penalty of two separate reactor systems.
The architecture also addresses a practical constraint that has limited earlier NTP-only concepts. A nuclear thermal rocket produces thrust only while the reactor is actively heating propellant, meaning it generates no onboard electricity during coast phases. Crewed missions need continuous power for life support, communications, and thermal management. The S-BNR's electric mode fills that gap without requiring a separate power source, which would add dead mass during the high-thrust phases.
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 this author's 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 attacks 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.
