Thin Film Nuclear Propulsion

NASA NIAC funded the Thin-Film Nuclear Engine Rocket (TFINER). By May 2025, it advanced to Phase II, focusing on maturing the technology, mission designs, small-scale thruster experiments, and isotope production pathways in collaboration with Northwestern University, Yale University, Los Alamos National Laboratory, and NASA Marshall Space Flight Center (MSFC) for hybrid architectures.

A single stage design is very simple and can generate velocity changes of ~100 km/s using a few kilograms of fuel and potentially more than 150 km/s for more advanced architectures. The concept uses thin layers of energetic radioisotopes to directly generate thrust. The emission direction of its natural decay products is biased by a substrate to accelerate the spacecraft.

Thrust doesn’t diminish with distance from the Sun. It is simpler than fission-fragment or nuclear thermal rockets, relying on passive decay rather than reactors.

How It Works

TFINER builds on a 1970s idea by Wolfgang Moeckel for thrust sheets where thin films coated with radioactive isotopes use the momentum of alpha decay particles to produce thrust.

The system consists of large, thin sheets (e.g., ~250 m² for baseline designs) made of a three-layer structure. There is an active radioisotope fuel layer. This is about ~10-22 microns thick, containing isotopes that undergo alpha decay.

There is a retention film. A thin layer to prevent fuel atoms from escaping.

There is a substrate or absorberlayers which is about ~35-50 microns thick (e.g., beryllium). This captures forward-directed decay particles, creating an asymmetric momentum transfer that propels the spacecraft backward.

Alpha particles are emitted at speeds up to 5% of the speed of light (~15,000 km/s).

Configuring the sheets or using tethers (up to 400 neter long) and winches for maneuvers, enabling torque and trajectory adjustments.

Key differentiators of the concepts are:

• Cascading isotope decay chains (Thorium cycle) increases performance by ~500%

• Multiple ‘stages’ (materials) increases delta-V and lifetime without reducing thrust

• Thrust sheet reconfiguration enables active thrust vectoring and spacecraft maneuvers

• Substrate thermo-electrics can generate excess electrical power (e.g. ~50 kW @ eff=1%)

• A substrate beta emitter can be used for charge neutralization or to induce a voltage bias that preferentially directs exhaust emissions and/or to exploit the outbound solar wind

Leveraging 30kg of radioisotope (comparable to that launched on previous missions) spread over ~250 m^2 of area would provide more than 150 km/sec of delta-V to a 30 kg payload.

The TFINER baseline uses 400 meter tethers to connect the payload module. The sheet’s power comes from Thorium-228 decay (alpha decay) — the half-life is 1.9 years. We get a ‘decay cascade chain’ that produces daughter products – four additional alpha emissions result with half-lives ranging from 300 ns to 3 days. The uni-directional thrust is produced thanks to the beryllium absorber (~35-microns thick) that coats one side of the thin film to capture emissions moving forward. Effective thrust is thus channeled out the back.

The propulsion system enables a rendezvous with intriguing interstellar objects such as ‘Oumuamua that are on hyperbolic orbits through our solar system. A particular advantage is the ability to maneuver in deep space to find objects with uncertainty in their location. The same capabilities also enable a fast trip to the solar gravitational focus to image multiple potentially habitable exoplanets. Both types of missions require propulsion outside the solar system that is an order of magnitude beyond the performance of existing technology. The phase 2 effort will continue to mature TFINER and the mission design. The program will work towards small scale thruster experiments in the near term. In parallel, isotope production paths that can also be leveraged for other space exploration and medical applications will be pursued. Finally, advanced architectures such as an Oberth solar dive maneuver and hybrid approaches that leverage solar sails near the Sun, will be explored to enhance mission performance.