Thin Film Nuclear Rocket With Californium for Fast Gravitational Lensing Missions

Jim Bickford is leading the Thin Film Isotope Nuclear Engine Rocket (TFINER) project to develop a system for propelling a craft through space faster and farther than ever before. This technology has the potential to enable speeds of 0.5% of light speed and near term capability for 0.1% of light speed. This would be ten times faster than chemical rockets.

TFINER is applicable to a wide range of other profound missions that are not possible with existing technology. For example, a mission to the solar gravitational lens (SGL) focus would leverage gravitational lensing to image an extrasolar planet while acting as a precursor interstellar mission. (Turyshev, 2003) The focus is a line that starts at ~550 AU which could be reached in about 25 years using a single stage Th-228 design. In comparison, a conventional spacecraft such as New Horizons to Pluto would take nearly two centuries to reach the gravity focus.

If multiple larger stages (isotopes) and/or an aggressive Oberth maneuver are leveraged, then 250 km/sec exit velocity would reach nearby stars in about 5 thousand years.

Although this report primarily focused on four fuels with half-lives of several years to several decades, there are options for fuels with much shorter and much longer half-lives which have a variety of advantages depending upon the mission. If short half-life materials can be quickly purified, fabricated into sheets, and launched into space, much higher performance levels can be expected. For example, Cf-254 is especially attractive for producing final velocities up to 10X higher than the baseline fuels. Califonium would boost the top speed to over 300 AU per year or about half of a percent of light speed.

A critical capability that the TFINER concept enables is repointing the gravitational lens at different targets over the course of the mission. Existing solar sail concepts require
decades to reach the SGL (Halvajian, 2022) and then are only able to observe a single solar system
because the telescope can only aim at targets that are directly aligned with the vector back to the Sun. In comparison, a TFINER design has a residual ∆V capability which can be used to align with multiple targets. The system could observe one for a several years and then maneuver to a new solar alignment vector. Although each mission would only observe a small fraction of the sky, TFINER would allow repointing rates ~ 1 deg/decade with a substantial impact on mission effectiveness.

Here is the 96 page final report.

TFINER is based on the concept of “thrust sheets,” which were originally proposed by Wolfgang Moeckel in the 1970s. Thrust sheets are thin films coated with radioactive isotopes that leverage nuclear decay to push items through space.

As radioactive isotopes decay, they emit “alpha” particles, some of which can travel as fast as 5 percent of the speed of light.

“This is tremendously fast compared to a conventional chemical rocket and allows us to push payloads at a velocity of almost 100 kilometers a second,” said Bickford, adding that a conventional rocket would need to have a mass greater than the global human population to operate that fast.

TRL

The TFINER concept is currently at a technology readiness level (TRL) of 2 since the concept and
application have both been formulated. Although significant work has been done, a proof-of-concept
or further design and analysis of critical functions needs to be completed to satisfy the criteria for higher TRL levels.

There is a clear path to bringing the concept to between TRL of 3 or 4 in the current phase 2.

The prioritized list of near-term development needs is shown below.
• Isotope Production and Separation Development – the ability to produce the nuclear fuels in the desired quantities and the needed purities in essential to any system design

o Production proof-of-concept – the cross-sections and other aspects of the accelerator-based technique should be proven by producing small quantities of fuel at an existing facility such as the LANSCE proton accelerator.

o Accelerator design – the design of a new accelerator should be started which optimizes fuel production with the ability to generate auxiliary products (e.g. medical isotopes). The design should consider economic feasibility and alternate use-cases to support the development of a new high-fluence US based accelerator.

o Separation and purification – the chemical-based isotope separation system and any additional enrichment should be detailed out and integrated with the logistics of the accelerator operation.

o Launch safety – The concepts for material handling, safety systems, and launch vehicle integration should be matured to identify any programmatic risks.

• Thrust Sheet Proof-of-Concept – The ability to fabricate thrust sheets, generate thrust and the
desired level while retaining its structural integrity should be verified experimentally

o Sheet fabrication demonstration – the thrust sheet fabrication approach can first be verified using isotopes which are chemically equivalent but stable from nuclear decay.

o Thrust model validation – the forces generated by a sheet should be validated by using a readily available alpha emitter fuel with a short half-life.

o Material robustness verification – the ability of the thrust sheets to maintain their structural integrity following very high total radiation doses can be verified by evaluating damage following irradiation of small samples by a particle accelerator or other source which produces the equivalent total lifetime radiation dose.

• Mission Design – The spacecraft design should be matured and optimized
o Mission Development – the mission (including motivation) and integrated spacecraft design should continue to be matured based on the latest results and other activities

o Design Optimization – alternate design and mission approaches, such as leveraging the Oberth maneuver, electrostatics, or new sheet configurations (e.g. helio-gyro) should be developed and evaluated against the baseline.

o Precursor Mission Development – Potential pre-cursor missions that can be used to demonstrate the TFINER technology in space early should be explored in detail.

o Payload Development – Critical cross-cutting technologies such as lightweight telescope optics, science instruments and rad hard electronics should be evaluated for mission integration.

The Thin-Film Nuclear Engine Rocket (TFINER) is an early-stage research project focused on developing a novel approach to space propulsion which enables new missions. A key feature of the technology is the ability to continue to provide significant propulsion after years in deep space.

An important aspect of the proposed concept is that it relies almost entirely on well-known effects or modifications of existing technology. Although there is still substantial risk and arduous engineering challenges ahead, no impassable roadblocks have been identified. The possible exception is the path to isotope production. Although this isn’t a fundamental limitation, there are likely to be programmatic challenges to the nuclear facility build-out for scaling up in the near term. This can be addressed by considering near term missions which leverage less energetic isotopes and/or smaller systems which utilize smaller quantities of materials. In parallel, novel approaches to production should be developed to ensure a fuel supply is available for all desired missions. Safety precautions need to be taken to ensure the safety of personnel and the environment in the event of a launch or other type of accident scenario.

The development path has been laid out and plans made to continue to mature the TFINER technology.
There are still many unanswered questions and unexplored variants, though the hope is that this report highlights the fundamental capability of the technology and baseline concepts for how to realize it in real operational missions of interest.