T-FINER, the alpha dog

The starfire / sunbird engines fuse elements to make helium propellant. You know what also emits helium plasma: α-decay. In 2024, quite the year for deep-space rocket ideas, Loura Hall posted how to do it. A yuuge wall poster of its powerpoint can be downloaded (pdf); the project released an update last year.

Thin Film Isotope Nuclear Engine Rocket. But not TFI-Ner[va], oddly, they prefer T-FINER.

This is a sail of (foully) radioactive metal, maybe 36 m². This sail provides its own propulsion, backward. Alpha particles, which are helium-4 ions, blast away up to 0.05c. That calculates to specific-impulse you... you might not even believe if I told you; in the millions (refer to PDF). Mass-ratio would be the mass of the radioactive metal (+ cargo) over the mass of the waste (+ cargo). This ratio would decline by half-life of the element, rather than the rate of burn.

The structure would be to drag the cargo behind. The sail is thin, like 20-50 μm for Thorium-228.

I didn't do the delta-V maths, but NIAC has, and they say they can get 100 km/s. That's a ticket to anywhere in the Solar System. ²²⁸Th (1.9 years) might get us to the 550 AU solar-lens in 25 years. Cf-254 is advertised as 300 AU in one year! (After which, they can maybe flip-and-drag, to 550 AU.) Outside that, this system will stall out so it's not a serious Centauri candidate.

Launching rolls of thin sheets of radioactive sail from Earth is less a problem, I suspect, than making it all in the first place. Alpha-emitters are neutron-deficient and trend heavy. Those meatballs we can spice in space are the opposite. The consortium is talking about skimming reactors or making in cyclotrons (e.g. Ac-227... and Ra-228). That's done on Earth.

Initial thrust remains a problem, unless we just use this to putt around various Earth orbits. One option is to bring layers of this stuff.

Cargo will have to be low-mass; they aim for 8.2 kg. I don't see a comm relay onboard. So I don't know how well it can beam data from Sedna or wherever. This would compete with the sunsail. Because there isn't 'nuff sun out there and, better, the vessel can be steered.

Meanwhile they hope for demo missions. Uranus, with Ra-228, to launch 2034. If someone else sends a full relay to orbit that planet, T-FINER will demand less comms.