Sometimes all it takes to spawn a new idea is a tiny smudge in a telescopic image. What counts, of course, is just what that smudge implies. In the case of the object called ‘Oumuamua, the implication was interstellar, for whatever it was, this smudge was clearly on a hyperbolic orbit, meaning it was just passing through our Solar System. Jim Bickford wanted to see the departing visitor up close, and that was part of the inspiration for a novel propulsion concept.

Now moving into a Phase II study funded by NASA’s Innovative Advanced Concepts office (NIAC), the idea is dubbed Thin-Film Nuclear Engine Rocket (TFINER). Not the world’s most pronounceable acronym, but if the idea works out, that will hardly matter. Working at the Charles Stark Draper Laboratory, a non-profit research and development company in Cambridge MA, Bickford is known to long-time Centauri Dreams readers for his work on naturally occurring antimatter capture in planetary magnetic fields. See Antimatter Acquisition: Harvesting in Space for more on this.

Image: Draper Laboratory’s Jim Bickford, taking a deep space propulsion concept to the next level. Credit: Charles Stark Draper Laboratory.

Harvesting naturally occurring antimatter in space offers some hope of easing one of the biggest problems of such propulsion strategies, namely the difficulty in producing enough antimatter to fuel an engine. With the Thin Film Nuclear Engine Rocket, Bickford again tries to change the game. The notion is to use energetic radioisotopes in thin layers, allowing their natural decay products to propel a spacecraft. The proper substrate, Bickford believes, can control the emission direction, and the sail-like system packs a punch: Velocity changes on the order of 100 kilometers per second using mere kilograms of fuel.

I began this piece talking about ‘Oumuamua, but that’s just for starters. Because if we can create a reliable propulsion system capable of such tantalizing speed. we can start thinking about mission targets as distant as the solar gravitational focus, where extreme magnifications become possible. Because the lensing effect for practical purposes begins at 550 AU and continues with a focal line to infinity, we are looking at a long journey. Bear in mind that Voyager 1, our most distant working spacecraft, has taken almost half a century to reach, as of now, 167 AU. To image more than one planet at the solar lens, we’ll also need a high degree of maneuverability to shift to multiple exoplanetary systems.

Image: This is Figure 3-1 from the Phase 1 report. Caption: Concept for the thin film thrust sheet engine. Alpha particles are selectively emitted in one direction at approximately 5% of the speed of light. Credit: NASA/James Bickford.

So we’re looking at a highly desirable technology if TFINER can be made to work, one that could offer imaging of exoplanets, outer planet probes, and encounters with future interstellar interlopers. Bickford’s Phase 1 work will be extended in the new study to refine the mission design, which will include thruster experiments as well as what the Phase II summary refers to as ‘isotope production paths’ while also considering opportunities for hybrid missions that could include the Oberth ‘solar dive’ maneuver. More on that last item soon, because Bickford will be presenting a paper on the prospect this summer.

Image: Artist concept highlighting the novel approach proposed by the 2025 NIAC awarded selection of the TFINER concept. This is the baseline TFINER configuration used in the system analysis. Credit: NASA/James Bickford.

But let’s drop back to the Phase I study. I’ve been perusing Bickford’s final report. Developing out of Wolfgang Moeckel’s work at NASA Lewis in the 1970s, the TFINER design uses thin sheets of radioactive elements. The solution leverages exhaust velocities for alpha particle decays that can exceed 5 percent of the speed of light. You’ll recall my comment on space sails in the previous post, where we looked at the advantage of inflatable components to make sails more scalable. TFINER is more scalable still, with changes to the amount of fuel and sheet area being key variables.

Let’s begin with a ~10-micron thick Thorium-228 radioisotope film, with each sheet containing three layers, integrating the active radioisotope fuel layer in the middle. Let me quote from the Phase I report on this:

It must be relatively thin to avoid substantial energy losses as the alpha particles exit the sheet. A thin retention film is placed over this to ensure that the residual atoms do not boil off from the structure. Finally, a substrate is added to selectively absorb alpha emission in the forward direction. Since decay processes have no directionality, the substrate absorber produces the differential mass flow and resulting thrust by restricting alpha particle trajectories to one direction.

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.

Note as well the tethers in the illustration, necessary to protect the sensor array and communications component to minimize the radiation dose. Manipulation of the tethers can control trajectory on single-stage missions to deep space targets. Reconfiguring the thrust sheet is what makes the design maneuverable, allowing thrust vectoring, even as longer half-life isotopes can be deployed in the ‘staged’ concept to achieve maximum velocities for extended missions.

Image: This is Figure 7-8 from the report. Caption: Example thrust sheet rotation using tether control. Credit: NASA/James Bickford.

From the Phase I report:

The payload module is connected to the thrust sheet by long tethers. A winch on the payload module can individually pull-in or let-out line to manipulate the sail angle relative to the payload. The thrust sheet angle will rotate the mean thrust vector and operate much like trimming the sail of a boat. Of course, in this case, the sail (sheet) pressure comes from the nuclear exhaust rather than the wind

It’s hard to imagine the degree of maneuverability here being replicated in any existing propulsion strategy, a big payoff if we can learn how to control it:

This approach allows the thrust vector to be rotated relative to the center of mass and velocity vector to steer the spacecraft’s main propulsion system. However, this is likely to require very complex controls especially if the payload orientation also needs to be modified. The maneuvers would all be slow given the long lines lengths and separations involved.

Spacecraft pointing and control is an area as critical as the propulsion system. The Phase I report goes into the above options as well as thrust vectoring through sheets configured as panels that could be folded and adjusted. The report also notes that thermo-electrics within the substrate may be used to generate electrical power, although a radioisotope thermal generator integrated with the payload may prove a better solution. The report offers a good roadmap for the design refinement of TFINER coming in Phase II.

Image: TFINER imaged as in the Phase I study using a panel configuration. Credit: NASA/James Bickford.

The baseline TFINER concept considered in the report deploys 30 kg of Thorium-228 in a sheet area measuring over 250 square meters, a configuration capable of providing a delta-v of 150 km/s to a 30 kg payload. Bickford’s emphasis on maneuverability is well taken. A mission to the solar gravitational focus could take advantage of this capability by aligning with not just one but multiple targets through continuing propulsive maneuvers. Isotopes with a longer half-life (Bickford has studied Actinium-227, but other isotopes are possible) can provide for ‘staged’ combined architectures allowing still longer mission timeframes. A high-flux particle accelerator is assumed as the best production pathway to create the necessary isotopes.

Clearly we’re in the early stages of TFINER, but what an exciting concept. To return to ‘Oumuamua, the report notes that a mission to study it “…is not possible without the ability to slow down and perform a search along the trajectory since the uncertainty bubble in its trajectory is larger than the range of any sensors that would work during a flyby. The isotope fuel can be chosen to optimize for higher accelerations early in the mission or longer half-life options for extended missions.” As the Phase II report lays out the development path, questions of fuel production and substrate optimization will be fully explored.

I asked Dr. Bickford about how the Phase II study will proceed. In an email on Wednesday, he pointed to continuing analysis of the thrust sheet, fuel production and spacecraft design, which should involve potential mission architectures. But he passed along several other points of interest:

• Northwestern and Yale Universities have joined the team to operate a ~1 cm² scale thruster demonstration to validate the force models and better understand the sheet’s electrical charging behavior.
• Draper Laboratory has expanded its work with Los Alamos National Laboratory to explore novel production approaches including new particle accelerators and fuel production architectures.
• ”We’ve added NASA MSFC as consultants to explore hybrid mission architectures which exploit solar pressure during the close solar flyby of an Oberth maneuver.”

Concepts like TFINER push the envelope in the kind of ways that pay off not only in a bank of new technical knowledge but novel technologies that will bear on how we explore the Solar System and eventually go beyond it. I’m reminded of Steve Howe and Gerald Jackson’s antimatter sail concept, which produces fission by allowing antimatter, stored probably as antihydrogen, to interact with a sheet of U-238 coated with carbon (see Antimatter and the Sail). TFINER uses no antimatter, but in both cases we have what looks like a sail surface being reinvented to offer missions that could put exotic targets within reach.

The other reason the antimatter sail comes to mind is that Jim Bickford is the man who reminded us how much naturally occurring antimatter may be available for harvest in the Solar System. The Howe/Jackson concept could work with milligrams of antimatter, which is conceivably available trapped in planetary magnetic fields, including that of Earth. In earlier work, Bickford has calculated that about a kilogram of antiprotons enter our Solar System every second, and any planet with a strong magnetic field is fair game for collection.

We hammer away at propulsion issues hoping for the breakthrough that will get us to the solar gravitational lens and the outer planets with much shorter mission timelines than available today. The thought of catching an interstellar interloper like ‘Oumuamua adds spice to the TFINER concept as the work continues. I look with great interest in the direction Bickford is taking with the Oberth maneuver, which we’ll be discussing further this summer.