James Bickford
Charles Stark Draper Laboratory
New and exciting missions, such as rendezvous with passing interstellar objects, or multi-target observation work at the gravitational focus of the Sun, require speeds far exceeding those of conventional rockets. Exotic solar sail approaches may be able to reach the desired distant locations, but cannot perform the required propulsive maneuvers in deep space. Nuclear rockets are large and expensive systems with limited capability to reach the location. In contrast, we propose a thin-film nuclear isotope engine with sufficient capability to search for, rendezvous with, and then return samples from distant and fast-moving interstellar objects.
The same technology allows gravitationally lensed telescopes to be re-aimed so that many high-value targets can be observed with a single mission.
The basic concept is to create a thin sheet of a radioactive isotope and directly use the momentum of its decay products to generate thrust. The baseline design is a thin film of the radioactive isotope Thorium-228, which undergoes alpha decay with a half-life of 1.9 years, about 10 microns thick. The subsequent decay chain cascades to produce daughter products with four additional alpha emissions with half-lives between 300 nanoseconds and 3 days. Thrust is generated when one side of the film is coated with an absorber about 50 microns thick to capture the forward emission. Multiple “stages” consisting of longer half-life isotopes, such as Ac-227, can be combined to maximize speed over extended mission times.
The main differences between these concepts are:
• Cascaded isotope decay chains (Thorium cycle) increase performance by ~500%
• Multiple “stages” (materials) increase Delta-V and lifetime without reducing thrust
• Thrust tab reconfiguration enables active thrust vectoring and spacecraft maneuvering
• Substrate thermoelectrics can generate excess power (e.g. ~50 kW @ eff=1%)
• Substrate beta emitters can be used for charge neutralization or induced voltage bias, preferentially direct exhaust emissions and/or harness outbound solar wind
Utilizing 30 kg of radioisotopes spread over ~250 m^2 (comparable to those launched in previous missions) would provide a delta-V velocity of over 150 km/s for a 30 kg payload. Multiple such systems could be inserted into solar escape orbits using a single conventional launch vehicle, allowing local search and rendezvous operations in the outer solar system. The system is scalable to other payloads and missions. The main advantages are:
• Capable of reaching speeds in excess of 100 km/s with backup capability for rendezvous maneuvers around outer solar system objects
System includes sample return option.
• Simple design based on known physics and well-known materials
• Scalable to smaller payloads (sensors) or larger missions (e.g. telescopes)
• New capability to reach deep space (> 150 AU) very quickly and then continue to actively maneuver (> 100 km/s) over a period of years for faint object searches/rendezvous and/or repositioning of telescopes at the gravitational focus of the Sun.
