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.
materialwhose whooooooo!
I hope he’ll leave a mention for the late great Robert Forward, who came up with this concept a good while ago:
https://en.wikipedia.org/wiki/Fission_sail
There’s a notably nuttier version of this concept which involves supercharging it with an occasional spritz of antiprotons (which are stored inside solid-state ICs?):
https://www.projectrho.com/public_html/rocket/enginelist2.php#id–Nuclear_Thermal–Fission_Fragment_Type–Antimatter-Driven_Sail
An Oberth-Effect swingby trajectory energy advantage is bigger with the ratio of fuel mass burned during the ascension / flyaway phase (after a shutdown before, during the dive-down phase).
As the TFINER mass loss is miniscule, the Oberth-effect is miniscule, too.
Obviously, since the Oberth effect is all about the potential energy of the mass you’re no longer carrying with you.
If you could dive into a gravity well with a big rock, and then throw that rock behind you fast enough that it would stay in the gravity well, you would gain all the potential energy of that rock and lose its mass as you climb out.
If you’re not losing any mass, or you lose very little, then it doesn’t matter when you accelerate because it’s approximately the same mass going into the gravity well as coming out of it. The potential energy you gain from falling in is lost on the climb up.
The description in the article was in regard to a hybrid mission, meaning this wouldn’t be the only kind of propulsion used – so the Oberth maneuver itself wouldn’t necessarily be for this portion, but possibly for the other propulsion mechanism. And the comment was also mentioning exploiting solar pressure – so you could imagine some weird thing where the sail sheet is either furled or pulled in or something during the infall, there’s a rocket burn near perihelion and the sheet is unfurled or reeled out on the outbound path or something.
“plenty fast enough for to reach the furthest reaches of our solar system.”
Being pedantic here, but technically even highway speeds are plenty fast enough to reach the furthest reaches of our solar system. It’s just the time scale that will be unmanageable.
Well, okay, i realize you need escape velocity from our gravitational well if you are talking about something launched from here on earth. So maybe not Highway speeds. But regardless, the more important data point is how far you can go in a certain amount of time, not just how far you can go.
You kinda want to be out of LEO anyways or the residual atmospheric drag would pull you in.
Beyond that, you can be as slow as you please.
Slow relative to what? If you are in a solar orbit around ~1AU perihelion and you are going slower than about 30 km/s relative to the sun then you are in for a bad time.
It’s a common misconception in orbital mechanics that once you’re out of the atmosphere you can thrust a tiny bit in one direction and you will keep going eternally, and in a few million years you reach Betelgeuse. Not actually how it works.
You accelerate a few meters per second in some direction and you only change the shape of your orbit very slightly. After you come back around the object you’re orbiting, you are in a different spot than your previous orbit, but the second time you come around you’ll still be in the same spot. You don’t go farther than that.
Getting into LEO from the surface is about 8 km/s of change in velocity. But say you’re already in LEO and don’t count that. Earth escape is over 11 km/s, so you still need to change your velocity relative to Earth an additional ~3 kilometers per second to get into solar orbit. So nearly half over again of the total change in velocity from the ground to space.
So now you’re in a solar orbit. You’ve come a long way, but not relative to the sun. You’re still occupying about the same orbit as Earth, which is traveling ~30 km/s around the sun. Solar escape is a bit over 42 km/s. So on top of the 8 km/s to get off the ground and the additional 3-4 km/s to get out of Earth’s gravity well, you now need about 12 km/s to get out of the sun’s gravity well and turn your orbit from an ellipse into an hyperbola. Otherwise you will waste several decades or centuries in an eccentric orbit among the outer planets, only to come back down to 1 AU.
THEN you can say that any additional speed increase is only getting there a bit faster. So it’s nice to have super-efficient propulsion, be glad you inherited a bit of velocity from the Earth itself, and rob velocity from any other planet you pass on your way out like Voyager did.