Sometimes all it takes to spawn a new idea is a tiny smudge in a telescopic image. What counts, of course, is just what that smudge implies. In the case of the object called ‘Oumuamua, the implication was interstellar, for whatever it was, this smudge was clearly on a hyperbolic orbit, meaning it was just passing through our Solar System. Jim Bickford wanted to see the departing visitor up close, and that was part of the inspiration for a novel propulsion concept.
Now moving into a Phase II study funded by NASA’s Innovative Advanced Concepts office (NIAC), the idea is dubbed Thin-Film Nuclear Engine Rocket (TFINER). Not the world’s most pronounceable acronym, but if the idea works out, that will hardly matter. Working at the Charles Stark Draper Laboratory, a non-profit research and development company in Cambridge MA, Bickford is known to long-time Centauri Dreams readers for his work on naturally occurring antimatter capture in planetary magnetic fields. See Antimatter Acquisition: Harvesting in Space for more on this.
Image: Draper Laboratory’s Jim Bickford, taking a deep space propulsion concept to the next level. Credit: Charles Stark Draper Laboratory.
Harvesting naturally occurring antimatter in space offers some hope of easing one of the biggest problems of such propulsion strategies, namely the difficulty in producing enough antimatter to fuel an engine. With the Thin Film Nuclear Engine Rocket, Bickford again tries to change the game. The notion is to use energetic radioisotopes in thin layers, allowing their natural decay products to propel a spacecraft. The proper substrate, Bickford believes, can control the emission direction, and the sail-like system packs a punch: Velocity changes on the order of 100 kilometers per second using mere kilograms of fuel.
I began this piece talking about ‘Oumuamua, but that’s just for starters. Because if we can create a reliable propulsion system capable of such tantalizing speed. we can start thinking about mission targets as distant as the solar gravitational focus, where extreme magnifications become possible. Because the lensing effect for practical purposes begins at 550 AU and continues with a focal line to infinity, we are looking at a long journey. Bear in mind that Voyager 1, our most distant working spacecraft, has taken almost half a century to reach, as of now, 167 AU. To image more than one planet at the solar lens, we’ll also need a high degree of maneuverability to shift to multiple exoplanetary systems.
Image: This is Figure 3-1 from the Phase 1 report. Caption: Concept for the thin film thrust sheet engine. Alpha particles are selectively emitted in one direction at approximately 5% of the speed of light. Credit: NASA/James Bickford.
So we’re looking at a highly desirable technology if TFINER can be made to work, one that could offer imaging of exoplanets, outer planet probes, and encounters with future interstellar interlopers. Bickford’s Phase 1 work will be extended in the new study to refine the mission design, which will include thruster experiments as well as what the Phase II summary refers to as ‘isotope production paths’ while also considering opportunities for hybrid missions that could include the Oberth ‘solar dive’ maneuver. More on that last item soon, because Bickford will be presenting a paper on the prospect this summer.
Image: Artist concept highlighting the novel approach proposed by the 2025 NIAC awarded selection of the TFINER concept. This is the baseline TFINER configuration used in the system analysis. Credit: NASA/James Bickford.
But let’s drop back to the Phase I study. I’ve been perusing Bickford’s final report. Developing out of Wolfgang Moeckel’s work at NASA Lewis in the 1970s, the TFINER design uses thin sheets of radioactive elements. The solution leverages exhaust velocities for alpha particle decays that can exceed 5 percent of the speed of light. You’ll recall my comment on space sails in the previous post, where we looked at the advantage of inflatable components to make sails more scalable. TFINER is more scalable still, with changes to the amount of fuel and sheet area being key variables.
Let’s begin with a ~10-micron thick Thorium-228 radioisotope film, with each sheet containing three layers, integrating the active radioisotope fuel layer in the middle. Let me quote from the Phase I report on this:
It must be relatively thin to avoid substantial energy losses as the alpha particles exit the sheet. A thin retention film is placed over this to ensure that the residual atoms do not boil off from the structure. Finally, a substrate is added to selectively absorb alpha emission in the forward direction. Since decay processes have no directionality, the substrate absorber produces the differential mass flow and resulting thrust by restricting alpha particle trajectories to one direction.
The TFINER baseline uses 400 meter tethers to connect the payload module. The sheet’s power comes from Thorium-228 decay (alpha decay) — the half-life is 1.9 years. We get a ‘decay cascade chain’ that produces daughter products – four additional alpha emissions result with half-lives ranging from 300 ns to 3 days. The uni-directional thrust is produced thanks to the beryllium absorber (~35-microns thick) that coats one side of the thin film to capture emissions moving forward. Effective thrust is thus channeled out the back.
Note as well the tethers in the illustration, necessary to protect the sensor array and communications component to minimize the radiation dose. Manipulation of the tethers can control trajectory on single-stage missions to deep space targets. Reconfiguring the thrust sheet is what makes the design maneuverable, allowing thrust vectoring, even as longer half-life isotopes can be deployed in the ‘staged’ concept to achieve maximum velocities for extended missions.
Image: This is Figure 7-8 from the report. Caption: Example thrust sheet rotation using tether control. Credit: NASA/James Bickford.
From the Phase I report:
The payload module is connected to the thrust sheet by long tethers. A winch on the payload module can individually pull-in or let-out line to manipulate the sail angle relative to the payload. The thrust sheet angle will rotate the mean thrust vector and operate much like trimming the sail of a boat. Of course, in this case, the sail (sheet) pressure comes from the nuclear exhaust rather than the wind
It’s hard to imagine the degree of maneuverability here being replicated in any existing propulsion strategy, a big payoff if we can learn how to control it:
This approach allows the thrust vector to be rotated relative to the center of mass and velocity vector to steer the spacecraft’s main propulsion system. However, this is likely to require very complex controls especially if the payload orientation also needs to be modified. The maneuvers would all be slow given the long lines lengths and separations involved.
Spacecraft pointing and control is an area as critical as the propulsion system. The Phase I report goes into the above options as well as thrust vectoring through sheets configured as panels that could be folded and adjusted. The report also notes that thermo-electrics within the substrate may be used to generate electrical power, although a radioisotope thermal generator integrated with the payload may prove a better solution. The report offers a good roadmap for the design refinement of TFINER coming in Phase II.
Image: TFINER imaged as in the Phase I study using a panel configuration. Credit: NASA/James Bickford.
The baseline TFINER concept considered in the report deploys 30 kg of Thorium-228 in a sheet area measuring over 250 square meters, a configuration capable of providing a delta-v of 150 km/s to a 30 kg payload. Bickford’s emphasis on maneuverability is well taken. A mission to the solar gravitational focus could take advantage of this capability by aligning with not just one but multiple targets through continuing propulsive maneuvers. Isotopes with a longer half-life (Bickford has studied Actinium-227, but other isotopes are possible) can provide for ‘staged’ combined architectures allowing still longer mission timeframes. A high-flux particle accelerator is assumed as the best production pathway to create the necessary isotopes.
Clearly we’re in the early stages of TFINER, but what an exciting concept. To return to ‘Oumuamua, the report notes that a mission to study it “…is not possible without the ability to slow down and perform a search along the trajectory since the uncertainty bubble in its trajectory is larger than the range of any sensors that would work during a flyby. The isotope fuel can be chosen to optimize for higher accelerations early in the mission or longer half-life options for extended missions.” As the Phase II report lays out the development path, questions of fuel production and substrate optimization will be fully explored.
I asked Dr. Bickford about how the Phase II study will proceed. In an email on Wednesday, he pointed to continuing analysis of the thrust sheet, fuel production and spacecraft design, which should involve potential mission architectures. But he passed along several other points of interest:
- Northwestern and Yale Universities have joined the team to operate a ~1 cm2 scale thruster demonstration to validate the force models and better understand the sheet’s electrical charging behavior.
- Draper Laboratory has expanded its work with Los Alamos National Laboratory to explore novel production approaches including new particle accelerators and fuel production architectures.
- ”We’ve added NASA MSFC as consultants to explore hybrid mission architectures which exploit solar pressure during the close solar flyby of an Oberth maneuver.”
Concepts like TFINER push the envelope in the kind of ways that pay off not only in a bank of new technical knowledge but novel technologies that will bear on how we explore the Solar System and eventually go beyond it. I’m reminded of Steve Howe and Gerald Jackson’s antimatter sail concept, which produces fission by allowing antimatter, stored probably as antihydrogen, to interact with a sheet of U-238 coated with carbon (see Antimatter and the Sail). TFINER uses no antimatter, but in both cases we have what looks like a sail surface being reinvented to offer missions that could put exotic targets within reach.
The other reason the antimatter sail comes to mind is that Jim Bickford is the man who reminded us how much naturally occurring antimatter may be available for harvest in the Solar System. The Howe/Jackson concept could work with milligrams of antimatter, which is conceivably available trapped in planetary magnetic fields, including that of Earth. In earlier work, Bickford has calculated that about a kilogram of antiprotons enter our Solar System every second, and any planet with a strong magnetic field is fair game for collection.
We hammer away at propulsion issues hoping for the breakthrough that will get us to the solar gravitational lens and the outer planets with much shorter mission timelines than available today. The thought of catching an interstellar interloper like ‘Oumuamua adds spice to the TFINER concept as the work continues. I look with great interest in the direction Bickford is taking with the Oberth maneuver, which we’ll be discussing further this summer.
1. One of our commenters (Michael Fidler?) has suggested this use of fissionable material for sails for some time.
2. How does this compare in potential performance to the fission fission-fragment rocket? It does seem to be much simpler, which is a plus.
3. I really like the possibility of rotating the sail to decelerate. This was not possible with either solar or beamed sails (with the beam source sunwards of the sail).
4. Manufacture and subsequent deployment may be an issue as it takes time between manufacturing the sail and deployment in space, all the while there is isotope decay. If the sail is folded up for launch, how does this impact the decay rate?
Should such sails be manufactured in space, rather than on Earth?
Is there any reason not to have the sail operate as a solar sail as well? Both photons and alpha emissions slow the sail to fall towards the sun, the photons provide most acceleration as the sail passes perihelion (the alpha particle emission also helps around perihelion, and then the alpha emissions create most of the acceleration as the photons from the sun diminish in intensity.
A 150 km/s velocity is very nice indeed, especially with a 30 kg payload, so a fraction of which is scientific instruments. Presumably, the 250 m^2 sail is further scalable, simply by adding more 50 m^2 panels. As the panels could be rotated with respect to the sail center, the sail could also rotate to retain its structure, possibly even using the old Heliogyro design concept.
IIRC, even Lubin’s beamed sails could only carry up to 1 kg payloads if allowed to accelerate to a far lower velocity for interplanetary propulsion. A very fast sail without needing a very expensive phased laser array to generate a propulsive beam, is very attractive.
150 km/s compares very favorably to less than 25 km/s with a solar sail using a sun diver maneuver. (c.f. “Solar Sails: A Novel Approach to Interplanetary Travel”, Vulpetti, & Matloff.)
This seems very promising. I hope the NIAC Phase II is successful and leads to some space tests of this technology.
Perhaps having the sail folded as a ribbon wound up would do. As it nears the sun it is unwound, it then uses the suns light to open up the chute. After acceleration the radioactive material is sprayed on for a final boost.
Would this system be capable of carrying large payloads within the inner solar system? I’m imagining something akin to our trucking system for transporting supplies, perhaps to Mars. Eventually, as lunar mining becomes more feasible, this entire process could be conducted off Earth.
In the future, we could consider using a solar flyby during a solar flare to generate a cascade of alpha particles or even fission. However, we first need a compelling reason to pursue any of these ideas, as transporting supplies is one of the most fundamental activities humanity has engaged in throughout history.
The Grateful Dead: Keep on truckin’.
http://images.fanpop.com/images/image_uploads/Keep-On-Truckin–the-70s-482814_713_348.jpg
I don’t think so.
For an Oumuamua mission, I want JIMO on Steroids… nuclear thermal hydrogen stage that can limp back over time… the main bus is nuclear electric.
This drills a tiny sample—and that is pasted on the TFINER for sample return.
Initially the truss of the JIMO like truss acts like a railgun to fire a cubesat Earthward, with a TFINER membrane-craft staging off that.
If you can bounce a laser off the bus and have it push against the TFINER—even better…the fragments will act as a brake.
I still suggest SLS, because Starship is a joke.
Rotating the sail, on tethers, would be problematic as the deceleration thrust would collapse the sail towards the payload (absent some other, likely massive, arrangement). Seems like the entire rig would need to be rotated, e.g. thrusting away from the target.
Is there a link to the Phase I and II studies, or other technology paper link?
I’m waiting on permission for that, as it’s on a private server.
I think Alex and myself and others discussed this idea or part of it before on this site. The way I thought about it is the sail is deployed like a parachute. Once deployed a container or bottle of highly radiative material is opened and the material is released out of the bottle by the heat of decay and gets thrown onto the sail where it sticks and decays to provide thrust. The radioactive decay bottle can be used as a source of power as well. I think the concept has merit worth looking into with more detail. I suppose Alex’s idea could help as well, say open the shiny sail near the sun and get pushed out and then open the bottle to coat the sail on the way out, a double whammy.
This is almost “The Expanse” levels of velocity. Pretty exciting and looks technically fairly simple – why hasn’t this been explored before?
Indeed. What I love about it is that it is so simple a concept, yet it awaited someone who could visualize it. I think this is remarkable work.
https://www.centauri-dreams.org/2022/07/26/getting-there-quickly-the-nuclear-option/
Michael on July 27, 2022 at 15:18
Perhaps a nuclear decay sail, the sail deploys and then a container of radioactive material is vapourised and sprayed onto one side of the sail where it sticks and undergoes radioctave decay for propulsion and energy generation.
The first image shows that there is a retention film over the fuel layer to prevent the fuel from escaping the sail before fission has occurred. Assuming this is necessary, it would then be necessary to spray this film on top of the fuel after it has been sprayed onto the sail substrate.
I gather that the manufacture of the sails requires the availability of the nuclear materials chosen, and because of their short 1.2 lives, requires fairly rapid launch after manufacture. There is no way to store complete sail material as one might for solar sails using Mylar or Kapton film as off-the-shelf material as the fuel decays so quickly.
Elsewhere, the author posits space-based manufacture due to the long time to assess safety requirements for a launch, and the need to generate and isolate the isotopes for the sail. As @Michael suggests, space manufacturing may be the best way to go, although that requires its own infrastructure development, perhaps on the Moon.
.
Alex, A ribbon sail can be rolled up like a toilet roll and put on even a small rocket. And the radioactive material could be produced on earth and put in a container, although there is a danger of release in an explosion.
Maybe pronounce it like “Taffy I-NER” propulsion.
That is, like it would rhyme with “Taffy Niner.”
They sometimes refer, particularly in the U.S. Navy, to a task force – abbreviated TF – as “taffy”.
Such as most famously Taffy 1, 2 and especially 3 that punched way, way above their weight in the Battle off Samar as part of the larger Battle of Leyte Gulf.
Sort of like how this propulsion concept would punch way above its weight.
Other than that – short of re-jiggering and subordinating the underlying word choice to produce a more fluid and catchy acronym – there’s perhaps not a whole lot else that one can do with that configuration.
But . . . all that said . . . I guess something like Alpha Emission Propulsion or Alpha Decay Propulsion would be just too . . . easy?
I mean does the name have to represent each and every facet of the overall concept of how you . . . get . . . to using alpha emissions to provide propulsion?
And then maybe use TFINER as sort of a subtitle type name.
I’m sure they’ve given it quite a bit of thought, though, and been through all that, given how far they’ve reached with the concept in the NIAC process.
As Shakespeare might suggest, it’s still a good idea, by any name.
* * * * *
Would the configuration with individual vanes mounted on a truss structure like in the “2022 NIAC Phase 3″ rendering in the image from the paper in this prior piece here perhaps work?
https://www.centauri-dreams.org/2023/04/03/building-smallsat-capabilities-for-deep-space/
That way, you can put the spacecraft bus on the forward end of the truss, ahead of rather than trailing the radioactive emissions. With a light enough truss, there might even be a mass advantage – or at least a near wash – as to not having to deal with protecting the bus (one way or another) against at least propulsion-produced radiation as a design factor.
(And you could similarly place the radioisotope thermal generator (RTG) a distance away on the truss from the rest of the bus to similarly isolate the science and comms packages from craft-produced radiation, thus possibly reducing also the shielding mass necessary at least in space for sundry different radioisotopes that might be used in an RTG. Such as for the Americium-241 RTG being developed by the European Space Agency. The placement of – and thus in-space shielding required for – the radioisotope heater units (RHU’s) might be a different matter, though. Overall, I do agree that using an RTG for bus power most likely will be the best way to go, FWTW. Especially early on where you’re trying to demonstrate feasibility of the propulsion technology itself. Inter alia, trying to get one aspect of a technology all the way home in terms of technology readiness level probably is ambitious enough given the competition in getting there with other technologies also vying for the same limited funding. Something to be said for one new technology – or aspect thereof – per technology demonstration mission. Explore the rest if and after you first get home on the core concept.)
And – back to propulsion – trimming the propulsion surfaces likely then would be more responsive – both in speed of change and also nuanced navigational fine-tuning – than the tether concept.
Particularly if the individual vanes each also can be rotated at their base, over and above being warped or twisted.
Might also be a more robust design in that it might be easier to recover from and work around a mechanical issue relating to the trimming of one vane in isolation as opposed to a mechanical issue with reeling in or letting out one of those long, long tethers.
In a pinch, might be able to even jettison a balky vane as opposed to having to deal with its still active effect on navigating because it’s still emitting alpha particles. A bit harder to do something corresponding to that with a single large panel controlled by those long tethers.
Yeah, I liked that sail vane concept discussed in that prior paper. Looked like it had just so many advantages as a design concept for sail-like propulsion structures.
I agree that the design by L’Garde, Inc. is very innovative and a departure from most previous sail ship designs. As you say, the central truss (keel?) allows the payload to be forward of the sails, protecting it from the particle emissions (or a laser for a beamed sail), and the ability to spin or maneuver is easier as long as the joints remain in working order. The authors had discarded the idea of spinning the craft like the heliogyro design that was once proposed for the 1986 Halley Comet rendezvous, as it would be difficult to manage at the payload end. IMO, this L’Garde design (Lightcraft TDM) solves that problem elegantly.
Thorium 228 has a decay or half life of only a little over on year and the thrust from it’s radioactive decay is too little for a solar sail. Google AI agrees. Furthermore alpha decay is lower energy than fission which is the splitting of Uranium atoms by neutrons which is higher energy and anti matter is even higher energy which is a one hundred percent conversion of matter into energy. Ford 2004, Google AI
Even if we used the decay of Uranium ore for space propulsion, it still would not produce enough thrust for a solar sail. There are more propellant hungry engine designs like a nuclear thermal rocket, etc. Disney’s journey to Mars spacecraft used a nuclear reactor to produce ion power, but I don’t think it would be used for interstellar travel. Nuclear fusion is propellant hungry. Anti matter works, but I like the idea of it being recycled such as in the matter anti matter warp drive which is from Star Trek which is the only way it could work, but is science fiction or is it? If would could convert mass into energy and back again then we could convert kinetic energy into artificial mass and gravity and anti gravity waves. There must be some way to do it efficiently.
@ Geoffrey. Well, Google is wrong.
A simplified argument, using the rocket equation:
delta_V = Vexhaust * ln (Mo/M1)
Thorium 228 decays to Ti 208 or Pb 208, after emitting 5 alpha particles, each with an atomic mass of 4.
Over its first half-life of 1.9 years, each Th 228 will lose the equivalent of 2.5 alphas (of which 1/2 of these will generate thrust.
atomic mass loss = 2.5 * 4 * 0.5 = 5.
Therefore, if the sail were 100% Th 228, the mass ratio after 1.9 years would be 228/223
Taking as given that alphas travel at 5% of c, that is 0.05 x 300,000 km/s = 15,000 km/s (Wikipedia confirms this speed.)
delta V = 15,000 * ln (228/223) ~= 332 km/s.
Obviously, the sail mass is more than just the mass of fuel, so let’s assume the sail = 3x the fuel mass = 684.
delta_V = 15000 * ln (684/679) = 110 km/s
Solar sails can manage 20+ km/s with a sundiver maneuver. I do not see how this performance difference can imply that the radioactive decay-derived thrust would not work and be inferior to a solar sail.
110 km/s is 5x the performance of a solar sail, and the thrust continues. At 3.8 years (2 half lives), deltaV ~= 165 km/s.
The performance is not that good because its first decay product is Ra 228, with a 5.7-year half-life. Nevertheless, it should be quite clear that this approach will work, at least theoretically, with no sundiver maneuver needed.
Google Gemini is simply wrong, assuming your prompt was asking for the correct calculation.
Perhaps this may be a good teaching moment to always “sanity check” LLM/LRM responses. Check with BoE calculations if appropriate, or simply look up answers to ensure the response was at least broadly correct. Then again, a NIAC II grant was hardly going to be awarded if the calculations for even the NIAC I grant were off by at least an order of magnitude, as the technical advisors would have canned the proposal very quickly. [I once saw a video of Lawrence Krauss eviscerating a proposal talk to an audience that he said was a hidden perpetual motion machine. I thought the speaker looked like he wanted to hide, as he had no defense.]
Speaking of which, your suggestion of
If you are implying a 100% recycling, this seems like you are ignoring the law of conservation of energy and, therefore, creating a perpetual motion machine. This would be a red flag for any proposal, even before your idea that energy->mass creates gravity and anti-gravity waves. Do you have a journal reference to support this statement?
Alex and Geoffrey, this relates to what I’m experiencing with AI, following upon our last related discussion and kicking the tires with AI (primarily Grok, which I believe comes with my upgraded X subscription) much more since than I had then.
Current AI is not capable of competently conducting higher level human reasoning.
And it’s a not fully reliable assistant to a human conducting such higher level reasoning, without extensive vetting by the human.
It’s a great souped up and precisely on point search engine for clearly known knowns.
And it – mimics – human discourse and reasoning very . . . deceptively.
But it ain’t actually thinking . . . yet. Not higher level human reasoning anyway.
If and when we reach that vaunted singularity (cue angel choir music), well, we’ll see then.
But, currently, it’s like having a very enthusiastic – and ingratiating, as you’ve noted before, Alex – and also very young and inexperienced research assistant.
The more involved the reasoning, the more vetting – and independent human thought – that is required.
Worst of all, it makes stuff up – what I think people refer to as “hallucinating.”
That’s not a good trait, at all.
I can see it most clearly in my field, law.
In litigation, if you put content in quotes that isn’t actually stated expressly so in the cited material, you get reamed by the other side, you typically lose the argument (unless maybe the judge figures out on their own despite your argument that you’re right, but for the wrong reasons), and the judge in all events never takes anything that you say thereafter at face value – even momentarily for the sake of argument – until they thereafter in chambers pore over each and every thing that you say in a brief or in oral argument.
All the bad – and sometimes true – things that they say about lawyers aside, as a litigator if you don’t have a reputation for candor with the tribunals before which you appear, you’re screwed. Nothing, absolutely nothing, that you say then carries any persuasive force, even initially. Your own reputation puts every single thing that you say thereafter in question. “It’s hard to persuade people when they can’t trust anything that you say.”
And Grok – blithely and repeatedly – puts things in quotes that ain’t in the cited material.
Bad, bad, bad.
Beyond that, it doesn’t have enough experience and insight – even when it’s not doing the above – to be able to see the weaknesses in the legal arguments that it presents as gospel, indeed gratuitously when all I typically actually was asking for was something like a specific statutory cite. (I make my own arguments.)
If I can’t trust it in the field that I do know intimately, I can’t trust it in anything else, especially at higher levels of human reasoning.
I still do use it fairly extensively, but as an exceedingly raw internal resource only.
(Well, I also use it to get something like the best cooking times for the sundry parts of a particular meal cooked in a three-level bamboo steamer, lol. If it hallucinates there, well, then I’ll be the one that’s . . . done.)
I thus wouldn’t dream of copying, pasting and sending out unvetted AI-generated content.
But I’m pretty sure I’m seeing a – ton – of AI-generated content online. It’s getting to where I can tell as I’m reading “oh, that passage clearly was generated by AI, not a human writer.” It’s sort of like the canines being able to sniff out the terminators in the movie – you can just tell the difference. There’s just a manner of processing information and presenting content that is distinctly AI rather than human. The mimicry just falls short at a certain point, even when they may be mimicking a particular writing style.
Oh yes, it is impressive, but also flawed and limited. Unreliably so.
Especially on something like this where a – human – has stepped back and gone “you know, if we just do . . . this . . . we can open up deep space like never before.”
Humans break paradigms and forge truly new ground. Make the theretofore seemingly impossible, possible. Take us to Ultima Thule and beyond.
AI not so much at this point.
Not as anything more than a – still limited – tool that must be used with an extreme measure of care.
Notwithstanding all the hype, especially in the financial markets and on social media.
I stand corrected on that one, Alley Tolley. I forgot the idea that half life means only half. After over two years the same sample is only one quarter and over three years one eighth roughly, but one eighth of very little thrust doesn’t matter which is why not anyone has thought to use the idea to use radioactive decay as a thrust which is too small. I will admit that it is an original idea and I’m an advocate for them. According to Open AI Chat GPT the half life does matter as the thrust would be reduced to fast since thorium 228 has a half life of only a little over a year.
The collisions of atoms with alpha particles also have daughter particles which are gamma rays so there can be a radiation problem.
The solar radiation photon pressure is the power that drives the solar sail which is a much stronger thrust than radioactive decay. Google AI
Chat GPT’s conclusion: the thrust from radioactive decay pressure is too low. A more viable propulsion:
RTG-powered ion drives
Fission fragment rockets
Beamed propulsion (laser sails)
Fusion-based systems (longer-term)
ibid.
Quote by Alex Tolley: “If you are implying a 100% recycling, this seems like you are ignoring the law of conservation of energy and, therefore, creating a perpetual motion machine. This would be a red flag for any proposal, even before your idea that energy->mass creates gravity and anti-gravity waves. Do you have a journal reference to support this statement?”
You are right, it would be a perpetual motion machine and that is exactly what the Alcubierre Warp Drive is, a diametric drive with something added, a free fall geodesic with a non inertial frame with no acceleration due to warped space.
I found a NASA forum about this, where someone linked a ChatGPT calculation that came up with about one micrometer per second squared as the acceleration for one such probe. In one half-life of thorium-228 = 1.97 y, it adds up to 74 meters per second, compared to 15 km/s for Voyager 2. That seems underwhelming for a nuclear fuel, but ChatGPT swore up and down that this was what TFINER does; that it’s only for continuous nudging, not main thrust.
Going back and looking at it myself, Thorium-228 (228.0287397) breaks down to Radium-224 (224.0202104) and Helium-4 (4.002603254). This leaves 0.0059260 g/mol of leftover energy = 532.61 MJ/mol. I get KE = 1/2(228.0228137g/mol)(2161.4 m/s)^2. The 2 km/s is reduced by about half due to fission product geometry, and then there are other inefficiencies considered in the original calculation, and it only went to one half-life, for another square root of two calculation. Still, I’m expecting at least 500 m/s, not 74. Hmm…..
I went back and told ChatGPT that last paragraph, and it said yeah sure, you’re right! (This is why I usually use perplexity for this stuff…)
https://forum.nasaspaceflight.com/index.php?topic=60195.0
https://chatgpt.com/share/68363fc2-dff0-8013-bab9-042e01deadff
Tangentially, perhaps, Paul mentions Oumuamua acting in a hyperbolic orbit; what would an object in such a trajectory being orbiting, exactly?
Closer to the topic at hand, I had no idea that we are able to manufacture films of such thinness.
Good to see you Michael, as always. Something on a hyperbolic orbit doesn’t make a full orbit around the Sun, but rather is moving so fast that its trajectory is changed by the encounter but it moves on past the Sun and re-enters interstellar space. Its velocity, in other words, is too great to be captured by the body it approaches.
There appears no need for a winch only two wheels with the wires looping around them and attaching to the sail ends.
Stability of Solar Sails is an issue with any angle off tangent to the sun delivering a torque. Curved solar sails, unlike flat ones, can offer advantages in terms of shape stability and control of thrust.
Similarly a TFINER Panel off-angle to the intended line of travel would send the craft off-course. Winches and tethers allow correction, but constant realignment would quickly wear the gears out. Would a non-flat geometry allow better course stability? Various shapes could be explored, like pyramidal and saddle shapes, to optimise performance and stability. Curved designs can also be achieved through folding techniques and integration with inflatable structures.
If we could make a much larger sail we could use U235 and tickle it with neutrons to produce the decay products. We have say the U235 as a large number of discs axially and then shot neutrons at the discs, the U235 in the discs should undergo fission to produce the decay products which move out between the discs to coat the large sail.