It’s not every day we hear of a new space propulsion method. Even rarer to hear of one that actually seems halfway practical. Yet that’s what we have in the case of TFINER, a proposal by [James A. Bickford] we found summarized on Centauri Dreams by [Paul Gilster] .
TFINER stands for Thin-Film Nuclear Engine Rocket Engine, and it’s a hoot. The word “rocket” is in the name, so you know there’s got to be some reaction mass, but this thing looks more like a solar sail. The secret is that the “sail” is the rocket: as the name implies, it hosts a thin film of nuclear materialwhose decay products provide the reaction mass. (In the Phase I study for NASA’s Innovative Advanced Concepts office (NIAC), it’s alpha particles from Thorium-228 or Radium-228.) Alpha particles go pretty quick (about 5% c for these isotopes), so the ISP on this thing is amazing. (1.81 million seconds!)
Now you might be thinking, “nuclear decay is isotropic! The sail will thrust equally in both directions and go nowhere!”– which would be true, if the sail was made of Thorium or Radium. It’s not; the radioisotope is a 9.5 um thin film on a 35 um beryllium back-plane that’s going to absorb any alpha emissions going the wrong way around. 9.5um is thin enough that most of the alphas from the initial isotope and its decay products (lets not forget that most of this decay chain are alpha emitters — 5 in total for both Th and Ra) aimed roughly normal to the surface will make it out.
Since the payload is behind the sail, it’s going to need a touch of shielding or rather long shrouds; the reference design calls for 400 m cables. Playing out or reeling in the cables would allow for some degree of thrust-vectoring, but this thing isn’t going to turn on a dime.
It’s also not going to have oodles of thrust, but the small thrust it does produce is continuous, and will add up to large deltaV over time. After a few years, the thrust is going to fall off (the half-life of Th-228 is 1.91 years, or 5.74 for Ra-228; either way the decay products are too short-lived to matter) but [Bickford]’s paper gives terminal/cruising velocity in either case of ~100 km/s.
Sure, that’s not fast enough to be convenient to measure as a fraction of the speed of light, and maybe it’s not great for a quick trip to Alpha Centauri but that’s plenty fast enough for to reach the furthest reaches of our solar system. For a flyby, anyway: like a solid-fueled rocket, once your burn is done, it’s done. Stopping isn’t really on offer here. The proposal references extra-solar comets like Oumuamua as potential flyby targets. That, and the focus of the Sun’s gravitational lens effect. Said focus is fortunately not a point, but a line, so no worries about a “blink and you miss it” fast-flyby. You can imagine we love both of those ideas.
NASA must have too, since NIAC was interested enough to advance this concept to a Phase II study. As reported at Centauri Dreams, the Phase II study will involve some actual hardware, albeit a ~1 square centimeter demonstrator rather than anything that will fly. We look forward to it. Future work also apparently includes the idea of combining the TFINER concept with an actual solar sail to get maximum possible delta-V from an Oberth-effect sundive. We really look forward to that one.
This articles is written by : Nermeen Nabil Khear Abdelmalak
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