'Firefly' Starship to Blaze a Trail to Alpha Centauri?

Robert Freeland wants to launch an interstellar probe — not 100 years from now, but within his lifetime. 

As Icarus Board Member and Deputy Project Leader, Freeland presented at the Tennessee Valley Interstellar Workshop (TVIW) in November, to propose a design for an starship that has all the things he likes: speed, elegance, and a short lead time.

“Firefly” (named so for its bright tail) is almost too pretty. It doesn’t seem right, after decades of tin can projects by NASA, to envision elegance in a practical design.

But let the hard science fans be reassured: the entire morphology of the vessel follows directly from physical constraints. Even the pretty curve of the radiators was chosen to follow the actual heat load from the drive, while minimizing pumping distance and pipe/coolant mass. The design is backed by as much hard science as is available today.

Firefly’s main drive is fueled by deuterium-deuterium (DD) fusion in a Z-pinch reactor. Z-pinch fusion was first explored in the late 1960s, but plasma instabilities relegated it to the dustbin. The idea languished for decades until recent research by Uri Shumlak at the University of Washington brought it back.

Shumlak’s design resolves the plasma instabilities by introducing a strong shear flow of plasma through the pinch area. Simulations and laboratory experiments have shown that this strong shear flow “smooths out” the plasma instabilities that would otherwise occur, resulting in a stable pinch. Lab tests have thus far only been performed with non-fusing plasmas for safety reasons, but the theory and tests strongly indicate that a properly-designed Z-pinch could maintain a stable fusion core.

This is not fringe science — Sandia National Laboratories is doing tests on Z-pinch fusion, and NASA’s Huntsville-based “Charger One” facility is preparing to undertake its own lab tests this year.
Pure deuterium fusion was selected for Firefly because it is available in sea water at about 150 ppm as “heavy water”, and is already commonly extracted in facilities all over the world for service in CANDU type fission reactors. If hydrogen becomes a common fuel source here on Earth, deuterium could readily be extracted even more cheaply as a byproduct of bulk hydrogen production plants.

This contrasts with more exotic fuel choices commonly proposed for fusion starships, particularly deuterium-helium3 (DHe3). The DHe3 reaction is fundamentally more desirable because its first-level reactions are all aneutronic; meaning that they produce only charged particles, with no damaging neutrons. The problem is that He3 is unavailable on Earth except in microscopic quantities, so it would have to be mined from the moon or the gas giant planets. This would require an extensive space program that then pushes an interstellar launch far into the future.

The unfortunate reality of DD fusion, though, is the tremendous flux of damaging high-energy neutron radiation. Even considering beneficial secondary reactions, neutrons account for almost half of the energy released by the reaction.

The Z-pinch drive compounds this problem with a very high flux of x-ray Bremsstrahlung radiation as well, produced as the super-heated electrons in the plasma bang into each other. Owing to its long, thin core, essentially all of the neutrons and x-ray radiation produced in the Z-pinch drive immediately escape the core.