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SunVoyager: A Fast Fusion Mission Beyond the Heliosphere

1000 AU makes a fine target for our next push past the heliosphere, keeping in mind that good science is to be had all along the way. Thus if we took 100 years to get to 1000 AU (and at Voyager speeds it would be a lot longer than that), we would still be gathering solid data about the Kuiper Belt, the heliosphere itself and its interactions with the interstellar medium, the nature and disposition of interstellar dust, and the plasma environment any future interstellar craft will have to pass through.

We don’t have to get there fast to produce useful results, in other words, but it sure would help. The Thousand Astronomical Unit mission (TAU) was examined by NASA in the 1980s using nuclear electric propulsion technologies, one specification being the need to reach the target distance within 50 years. It’s interesting to me – and Kelvin Long discusses this in a new paper we’ll examine in the next few posts – that a large part of the science case for TAU was stellar parallax, for classical measurements at Earth – Sun distance allow only coarse-grained estimates of stellar distances. We’d like to increase the baseline of our space-based interferometer, and the way to do that is to reach beyond the system.

Gravitational lensing wasn’t on the mind of mission planners in the 1980s, although the concept was being examined as a long-range possibility by von Eshleman at Stanford as early as 1979, with intense follow-up scrutiny by Italian space scientist Claudio Maccone. Today reaching the 550 AU distance where gravitational lensing effects enable observation of exoplanets is much on the mind of Slava Turyshev and team at JPL, whose refined mission concept is aimed at the upcoming heliophysics decadal. We’ve examined this Solar Gravity Lens mission on various occasions in these pages, as well as JHU/APL’s Interstellar Probe design, whose long-range goal is 1000 AU.

What Kelvin Long does in his recently published paper is to examine a deep space probe he calls SunVoyager. Long (Interstellar Research Centre, Stellar Engines Ltd) sees three primary science objectives here, the first being observing the nearest stars and their planets both through transit methods as well as gravitational lensing. A second objective along the way is the flyby of a dwarf planet that has yet to be visited, while the third is possible imaging of interstellar objects like 2I/Borisov and ‘Oumuamua. Driven by fusion, the craft would reach 1000 AU in a scant four years.

Image: The Interstellar Research Centre’s Kelvin Long, here pictured on a visit to JPL.

This is a multi-layered mission, and I note that the concept involves the use of small ‘sub-probes’, evidently deployed along the route of flight, to make flybys of a dwarf planet or an interstellar object of interest, each of these (and ten are included in the mission) to have a maximum mass of 0.5 tons. That’s a lot of mass, about which more in a moment. Secondary objectives involve measurements of the charged particle and dust composition of the interstellar medium, astrometry (presumably in the service of exoplanet study) and, interestingly, SETI, here involving detection of possible power and propulsion emission signatures as opposed to beacons in deep space.

Bur back to those sub-probes, which by now may have rung a bell. Active for decades in the British Interplanetary Society, Long has edited its long-lived journal and is deeply conversant with the Daedalus starship concept that grew out of BIS work in the 1970s. Daedalus was a fusion starship with an initial mass of 54,000 tons using inertial confinement methods to ignite a deuterium/helium-3 mixture. SunVoyager comes nowhere near that size – nor would it travel more than a fraction of the Daedalus journey to Barnard’s Star, but you can see that Long is purposely exploring long-range prospects that may be enabled by our eventual solution of fusion propulsion.

Those fortunate enough to travel in Iceland will know SunVoyager as the name of a sculpture by the sea in central Reykjavik, one that Long describes as “an ode to the sun or a dream boat that represents the promise of undiscovered territory and a dream of hope, progress, and freedom.” As with Daedalus, the concept relies on breakthroughs in inertial confinement fusion (ICF), in this case via optical laser beam, and in an illustration of serendipity, the paper comes out close to the time when the US National Ignition Facility announced its breakthrough in achieving energy breakeven, meaning the experiment produced more energy from fusion than the laser energy used to drive it.

Image: The Sun Voyager (Sólfarið) is a large steel sculpture of a ship, located on the road Sæbraut, by the seaside of central Reykjavík. The work of sculptor Jón Gunnar Árnason, SunVoyager is one of the most visited sights in Iceland’s capitol, where people gather daily to gaze at the sun reflecting in the stainless steel of this remarkable monument. Credit: Guide to Iceland.

Long’s work involves a numerical design tool called HeliosX, described as “a system integrated programming design tool written in Fortran 95 for the purpose of calculating spacecraft mission profile and propulsion performance for inertial confinement fusion driven designs.” As a counterpart to this paper, Long writes up the background and use of HeliosX in the current issue of Acta Astronautica (citation below). The SunVoyager paper contemplates a mission launched decades from now. Long acknowledges the magnitude of the problems that remain to be solved with ICF for this to happen, notwithstanding the encouraging news from the NIF.

…a capsule of fusion fuel, typically hydrogen and helium isotopes, must be compressed to high density and high temperature, and this must be sustained for a minimum period of time. One of the methods to achieve this is by using high-powered laser beams to fire at a capsule in a spherical arrangement of individual beam lines. The lasers will mass ablate the surface of the capsule and through momentum exchange will cause the material to travel inward under spherical compression. This must be done smoothly however, and any significant perturbations from spherical symmetry during the implosion will lead to hydrodynamic instabilities that can reduce the implosion efficiency. Indeed, the interaction of a laser beam with a high-temperature plasma involves much complex physics, and this is the reason why programs on Earth have found it so difficult.

Working through our evolving deep space mission designs is a fascinating exercise, which is why I took the time years ago to painstakingly copy the original Daedalus report from an academic library – I kept the Xerox machine humming in those days. Daedalus, a two-stage vehicle, used electron beams fired at capsules of deuterium and helium-3, the resulting plasma directed by powerful magnetic fields. Long invokes as well NASA’s studies of a concept called Vista, which he has also written about in his book Deep Space Propulsion: A Roadmap to Interstellar Flight (Springer, 2011). This was a design proposal for taking a 100-ton payload to Mars in 50 days using a deuterium and tritium fuel capsule ignited by laser. Long explains:

The capsule design was to utilize an indirect drive method, and so a smoother implosion symmetry may give rise to a higher burn fraction of 0.476. This is where the capsule is contained within a radiation cavity called a Hohlraum and where the lasers heat up the internal surface layer of the cavity to create a radiation bath around the capsule; as opposed to direct laser impingement onto the capsule surface and the associated mass ablation through the direct drive approach.

Image: Few images of the Vista design are available. I’ve swiped this one from a presentation made by C. D. Orth to the NASA Advanced Propulsion Workshop in Fusion Propulsion in 2000, though it dates back all the way to the 1980s. Credit: NASA.

SunVoyager would, the author comments, likely use a similar capsule design, although the paper doesn’t address the details. Vista feeds into Long’s thinking in another way: You’ll notice the unusual shape of the spacecraft in the image above. Coming out of work by Rod Hyde and others in the 1980s, Vista was designed to deal with early ICF propulsion concepts that produced a large neutron and x-ray radiation flux, sufficient to prove lethal to the crew. The conical design was thus an attempt to minimize the exposure of the structure to this flux, with a useful gain in jet efficiency of the thrust chamber. SunVoyager is designed around a similar conical propulsion system. The author proceeds to make predictions for the performance of SunVoyager by using calculations growing out of the Vista design as modeled in the HeliosX software.

In the tradition of Daedalus and Vista, SunVoyager explores ICF propulsion in the context of current understanding of fusion. I want to talk more about this concept next week, noting for now that a fast mission to 1000 AU –SunVoyager would reach that distance in less than four years – would take us into an entirely new level of outer system exploration, although the timing of such a mission remains hostage to our ability to conquer ICF and generate the needed energies to actualize it in comparatively small spacecraft systems. This doesn’t even get into the matter of producing the required fuel, another issue that will parallel those 1970s Daedalus papers and push us to the limits of the possible.

The paper is Long, “Sunvoyager: Interstellar Precursor Probe Mission Concept Driven by Inertial Confinement Fusion Propulsion,” Journal of Spacecraft and Rockets 2 January 2023 (full text). The paper on HeliosX is Long, “Development of the HeliosX Mission Analysis Code for Advanced ICF Space Propulsion,” Acta Astronautica, Vol. 202, Jan. 2023, pp. 157–173 (abstract). See also Hyde, “Laser-fusion rocket for interplanetary propulsion,” International Astronautical Federation conference, Budapest, Hungary, 10 Oct 1983 (abstract).

{ 39 comments… add one }
  • Geoffrey Hillend January 6, 2023, 17:16

    It’s a good idea to make a fusion rocket and send it 1000 AU. The problem is that I don’t think we can easily scale down the size of the NIF. It might be easier to launch the ITER which uses ICF. The pulse lasers in the NIF require several buildings.https://lasers.llnl.gov/about/how-nif-works. There are new table top Chirped Pulse lasers and one can definitely make a small, hot core with those in a small space, but I don’t know if they really could be upgraded to get hot enough to make fusion and match the power of the lasers in the NIF. This might be sometime in the future before we got that efficiency. It’s worth a try. The cost will be expensive and we don’t have that ready now, but only in the future. From what I recall reading, although the ISP of fusion propulsion is good, it still is a fuel hungry engine for propulsion.
    It might be cheaper and take less time to build with some other means of propulsion? It is the speed that matters.

    I have to admit that I still have all my eggs in the basket of a space warp and reaction less drive.

  • jonW January 6, 2023, 17:28

    > the first being observing the nearest stars and their planets both through transit methods as well as gravitational lensing.

    Am I right in thinking that the mission would likely only be able to gravitationally-lens one nearby star system, as there will not be more than one system along exactly the same line of sight which is needed to interpose the sun between the probe’s trajectory and the target system?

    • Paul Gilster January 6, 2023, 20:00

      Makes sense to me, jonW. But perhaps the idea is to shake out grav-lensing technologies as part of the larger mission. Even so, I wondered about that as well. Maybe Kelvin will have further thoughts on it here.

      • Michael January 7, 2023, 12:52

        This gravilens technique no matter where it is pointed will unlock enomous amounts of knowledge.

  • Alex Tolley January 6, 2023, 18:45

    Skimming the 2 papers (HeliosX code development, and SunVoyager) I don’t see any indication of the needed start energy and storage. The calculations appear to assume that the fusion drive can be started easily, somewhat like lighting a match or starting an ICE car engine using a battery and starter motor.

    The NIF experiment uses a vast amount of power to try to ignite the first pellet, which if the energy output is high enough will create a self-sustaining rocket engine. While I can imagine an external kickstart in the solar system, perhaps using beamed energy, one the engine is turned off for example in cruise mode, how can it be restarted without a similar external power source?

    Is there any, even theoretical, mechanism to ignite the fusion pellets with a small amount of energy, even using a different internal power supply (e.g. a nuclear reactor) that will overcome this issue at least for the current state of the art for fusion energy and prospective technologies?

    [BTW, I see Daedalus remains the reference. No mention of any work on the [abandoned?] Icarus project. Was nothing on a fusion drive advanced by the team?]

    • Paul Gilster January 6, 2023, 19:58

      Kelvin does mention the Icarus work in passing. I’ll dig that out in the next post, or else forward you the reference.

      • Alex Tolley January 7, 2023, 14:32

        It is a passing note in the HeliosX paper. There are a number of ICARUS papers in the reference section, but in the text, it is mentioned as a Centauri vs Barnard’s star target. It isn’t clear to me if there is any substantial difference between the 2 projects other than some changes in the masses. Perhaps Kelvin Long can elaborate in the upcoming posts.

    • Brett Bellmore January 7, 2023, 8:06

      I think you’re confusing power and energy here. Power is energy per unit time.

      The NIF experiment used insane power levels only because a very modest amount of energy was delivered in an insanely short time. You could have charged the capacitor bank from a good car battery, if not for all the inefficiencies. 1.8MJ for NIF, vs 4MJ in a car battery.

      The bigger issue here is that you need to get enough gain over the power invested that you can be VERY lossy in collecting output energy and still break even, because you can’t efficiently extract the resulting energy to spark the next reaction, AND use it for propulsion. NIF is nowhere near ‘engineering breakeven’, by many orders of magnitude.

      Project Icarus assumed inertial confinement was much easier than it turned out to be.

      I’d look at something like Helion power for a rocket, not inertial confinement. Which, frankly, mostly gets funded because it’s useful for testing bomb physics.

      • Alex Tolley January 7, 2023, 13:04

        It wasn’t that I was confusing power and energy but that I had no idea that the total energy delivered was so low, less than a car battery.
        It doesn’t seem that the NIF’s power conditioning system is small. Some details of the power conditioning can be found in this document. Table 1 indicates that the facility stores 367 MJ in 216 modules, each storing 1.7 MJ. The document notes that the reliability of the system is only guaranteed for 20,000 pulses. If the starship needs 3 seconds between pulses, that is just 16.6 hours of operation before the power banks and storage need replacing. To reach 1000 AU in 4 years (assuming almost all the time is cruise) requires a velocity of 10^6 m/s, about 0.003 c. To attain that velocity in 16.6 hours requires an acceleration of ~ 1.5g.

        While I have no doubt that the power supply and energy storage technology will have improved by the time this vehicle is ready to fly, it doesn’t seem like the image of a car battery to jump-start the engine is accurate as there must be quite a mass of power conditioning equipment to manage any engine start/restart. There will also need to be a power conversion system to recharge the capacitors if needed, perhaps an MHD system, which will also provide the onboard power.

        Bottom line, you are correct that the total energy needed for that first pulse is low, the hardware to provide the storage for that pulse is not trivial.

      • Geoffrey Hillend January 7, 2023, 17:56

        Not really. It’s a misconception that one can make a bomb with a fusion reactor. One can’t. It’s a completely controlled reaction, but not a runaway since an H bomb uses radiation implosion and a Plutonium spark plug when is fissioned fuses a large amount of Lithium 6 deuteride, but that does not work without an atomic bomb which explodes and creates x rays which creates a large amount of compression which comes from outside the lithium 6 deuteride and compression from the inside, the x rays of the fissioning Plutonium spark plug which is called radiation implosion. With the the ITER, we only fuse at the molecular level, only a few molecules at a time compared to a hydrogen bomb. Every molecule and fusion is counted.

        I do think it is within our technological knowledge we might be able to scale down the power if the NIF. The large amount of power must come from pulse lasers because they have a peak over power which is far greater than normal lasers which don’t use a pulse. I am not the expert in lasers, but the chirped table top lasers do use pulses to get their power. It seems to me it might be possible to scale down the NIF, but that has not been done yet and one would have to design a fusion propulsion engine which was small enough to be launched in a rocket.

    • Jens Fridthjof Knudsen January 7, 2023, 8:11

      This initiative uses plasma toruses and shoot them at each other to obtain fusion conditions, and it looks to me like they are not that far away from building a reactor facility that might work. It uses He3 for fuel, the fusion pulses generate protons with electrical charge that is used to induce electrical current in windings surrounding the reaction chamber. They use capacitors for storing the energy needed to run the pulses. https://www.youtube.com/watch?v=_bDXXWQxK38, https://en.wikipedia.org/wiki/Helion_Energy

  • Antonio January 6, 2023, 19:48

    Why not use a seemingly easier approach like the z-pinch of some of Project Icarus designs?

    • Geoffrey Hillend January 9, 2023, 18:24

      The problem with the Z pinch is the specific impulse ISP is 19400, but a nuclear fusion rocket is 130,000. VASIMR has 3,000 up to 20,000 with a nuclear power less power than the fission reactor of a submarine and can easily be made with today’s technology, but somebody fund it.

      • Antonio January 10, 2023, 12:04

        Not at all. Icarus Firefly (z-pinch) has an isp of 1,020,000 s, Icarus Ghostship (ICF) 540,000 s. Also, of current technologies, ion thrusters have higher isp than VASIMR.

  • Robin Datta January 6, 2023, 21:50

    We can hope that this will be pragmatic and realistic, with the expenditure of so much coin and cognition, and while mederating one’s expectations, for the continuation of the lineages derived from today’s humans. It would be reassuring if the eternal “two decades” away from fusion were to come to a close.

  • Michael January 7, 2023, 3:57

    I still think a micro fission implosion drive has a higher chance of being successful than straight out fusion, the power requirements for the fission implosion is a lot less thats for sure.


  • Kelvin F Long January 7, 2023, 16:58

    Thanks Paul for writing the article and the interest in the paper. Responding to some of the excellent comments from your readers.

    Geoffrey Hillend: ITER doesn’t use ICF, but MCF. It’s heavy due to the magnets. However, I agree its application should be examined for space applications under similar scaling assumptions. Indeed, I have done this for other designs but will talk about that another time. I agree NIF can’t be scaled down easily, but the paper is looking ahead to what may be possible in the latter half of this century, as stated. There are also many other options for generating energy other than optical lasers which can be explored. Whilst we have options, the possibility is there. I agree there are other propulsion options, but this specific paper was looking at what such a mission to 1,000 AU would look like under the ICF assumption.

    JonW: Correct, and I think the Centauri A/B system is the primary candidate. Proxima is ~13,000 AU from the other pair, I don’t know if it would be feasible to image all three. I guess it would depend on the field of view and line of sight position. Claudio Maccone is the expert here. In some ways the science goals are not such a concern for me, but more how do I get the large mass out there through propulsion. The science goals would be driven by others such as decadal surveys. Although I have given indications in the paper of what I think the science goals should be.

    Alex Tolley: If you read the Sunvoyager paper it states the start energy is 3.6 GW since this is fed back via the induction of the main fusion drive and appears in the power cycle. It states this will be a nuclear fission reactor that is 10 tons in mass producing a 3.6 MWe power output, at 0.36 kWe/kg. Nuclear fission reactors are discussed a little in the paragraphs following Table 7. The paper also shows the power cycle which closes the loop on the induction-start drive power.

    Regards no work mentioned on Icarus project. Firstly, this is not the case if you read the HeliosX Acta Astronautica paper there is a large discussion on modelling Daedalus and modelling perturbations of the Daedalus design, in addition to a critical review of some of the issues. To clarify, Project Icarus was never abandoned; it just morphed into a much smaller study which is ongoing with only two design teams now still working. These designs are complex to analyse. Robert Freeland and Michael Lamontagne are still working hard on the Z-pinch version Firefly and making good progress with that. Several papers have appeared in JBIS. There are other designs you may not have heard of which I have been developing called Resolution, Endeavour and Pegasus, where the last two are parallel thrust systems. These are the closest to Daedalus of the all the designs except they use optical lasers and small capsules and with a small pulse frequency. I will agree not much has been reported on those but the design models do exist and I will make an effort over the coming months to post an article on them here at Centauri Dreams.

    Brett Bellmore: good comments overall but it is not the case that Project Icarus assumed ICF to be much easier than it turned out to be. At the outset of the study NIF was coming online around 2009 and I personally had full confidence they would eventually achieve ignition which they did late last year. Although I agree with you there is still a long way to go with this. The main goal of Project Icarus was to ‘redesign’ Daedalus based on the decades long developments, but with the reduced mission requirement of 100 years and to include deceleration. But never did we assume ICF was easy. Neither was the project ever restricted to ICF, hence the teams also examined designs for Ultra dense deuterium and Z-pinch.

    Antonio: Indeed, see the Firefly study, i.e. Plasma Dynamics in Firefly’s Z-Pinch Fusion Engine, JBIS, 71, 8, PP.288-293, 2018.

    Overall, I’m exploring the landscape of possibilities that ICF propulsion can be applied to. You might see some interesting designs appearing down the line.

    Thank you all.

    • Alex Tolley January 8, 2023, 19:02

      The SunVoyager is a big ship, with a wet mass of about twice the mass of the ISS. The pulse frequency of between 1 and 50Hz resulting in 10s to 100s of millions of successful fusion events is going to be quite a challenge. I have to wonder what a misfire failure rate would do to performance – perhaps reducing it 1/(1 – misfire rate)?
      The 10 MT, 3.6 MWe fission reactor avoids the NIF capacitor and power system to fire the lasers. I am not clear whether it is used to start the engine and then no longer used for that purpose, or whether it is supplying the energy needed while the engine is running, and then diverted to the probe’s systems once in cruise mode.

      At least it is only 2% of the wet mass of the Daedalus interstellar probe as the cruise velocity is much lower. It certainly would be a good test platform for the later interstellar craft.

      While a SpaceX FH might offer a bargain basement launch cost to LEO, what might be a realistic project cost of such a craft, and how long from inception to launch? Assuming a program as ambitious as the ISS and a similar funding-to-GDP ratio, what would the partners’ combined GDP need to be to support this compared to today’s GDP?

      In summary, it is a big, bold project. Is it realistic a century from now?

    • Geoffrey Hillend January 9, 2023, 19:07

      Yes the the NIF uses ICF, and the ITER uses MCF and these are not the same. I stand corrected. This brings up some other physical constraints as with the conventional design of a fusion rocket must use both ICF and MCF. ICF uses compression heating and MCF energy confinement. Wikipedia: https://en.wikipedia.org/wiki/Fusion_rocket

      The problem with the NIF does not have constant fusion, but only intermittent since it fuses reflective gold pellets with deuterium and tritium spheres inside them, but the ITER uses a magnetic field form a super conducting magnet and high energy microwaves to fuse the deuterium tritium hydrogen fuel. P. 55, Fusion’s False Dawn, Scientific American. “the NIF was never intended to be a machine that could generate usable energy.” p. 55, ibid. which means one can’t make a fusion power plant out that design.. Also how do you mix in fuel with the lasers which need to contact the pellets, the deuterium and tritium would get in the way of the laser beams and block them unless the fuel is underneath them. At best you might get only a small amount of fusion and a low specific impulse. NASA has a new design, but I suspect it has a low specific impulse like the Z pinch. design base on the implosion of metal foils which sounds more like fission than fusion. One can’t fuse Iron or metal which is proven in large stars because it does not liberate energy, the electromagnetic repulsion or proton repulsion is to strong for fusion, so the iron core gets larger and eventually the extreme gravity collapses it into type 2 supernova, neutron star or black hole. https://www.nasa.gov/directorates/spacetech/niac/2012_Phase_II_fusion_driven_rocket/luminum foil Isn’t the temperature too high to fuse metal, but can only happen in large stars. The temperature to fuse metal, aluminum etc is much higher than fusing hydrogen into helium.

      To get the 130,000 ISP one has to make it big like Daedalus? Intuitively, I assume 130,000 ISP comes from a large fusion magnetic field compression space and a large amount of thrust exiting the magnetic nozzle? I am kind of disillusioned with fusion engines. How do we get the 130,000 ISP?

  • Michael January 7, 2023, 20:12

    Firstlight have an interesting concept which could be applied to a fusion drive. In space it could be easy to produce high velocities for impacts. I love the pressures they get ! I suspect a high energy laser could be used to inject neutrons when the fuel pellet is at maximum density could help in a more efficient burn.


  • Antonio January 8, 2023, 4:04

    Kelvin Long: Thanks for your reply! And I’m delighted to know that Project Icarus is still alive! The website simply evaporated years ago without notice, so I thought it was dead.

  • Kelvin F Long January 9, 2023, 10:48

    Alex Tolley: If you look at the original Project Orion studies they visited the Coca Cola bottle factory to understand how different size ‘units’ could be ejected mechanically. I think the idea is that if one detonation is say 10% under the required performance, then perhaps you load in the next one at 10% higher to balance the thrust profile back out. I see a similar solution to ICF propulsion in that you would have different capsules of slightly different fuel masses, or you could even adjust the firing frequency of the lasers so as to balance out the thrust profile. This would have to be highly instrumented, automated and diagnosed in situ of course.

    The 3.6 GWe nuclear reactor would be used to start the fusion engine, and then the engine operations on induction power mode, as a form of bootstrap. But once the fusion engine shuts down the nuclear reactor can be used to power all of the other systems such as powering the communications antenna and telescopes.

    Yes Sunvoyager is a technological bridge towards more ambitious missions. In Project Icarus we also worked on the concepts Pathfinder (1,000 au) and Starfinder (10,000 and 50,000 au) but these were less well defined. These were published in earlier work and resulted from a workshop meeting in Prague at the IAC in 2010::

    “Project Icarus: Exploring the Interstellar Roadmap using the Icarus Pathfinder and Starfinder Probe Concepts”, JBIS, 65, 7/8, July/August 2012. R. W. Swinney, K. F. Long et al.,

    “Starships of the Future, The Challenge of Interstellar Flight”, Spaceflight Magazine, 53, 4, 2011, K. F. Long, R. Obousy.

    I’m currently working with others to look at running them through my HeliosX code. In the Sunvoyager paper the cost of the mission is estimated in the range $7-10 Billion, but this doesn’t take into account the existence of a solar system economy in the future which may make such a mission cheaper. Yes, I believe its realistic later this century, although this specific concept may not be the one to do it – most likely such a mission would have a much reduced payload mass and also involve augmented expellant fuel (i.e. hydrogen injected into the exhaust stream) for the purpose of increasing the mass flow rate and therefore thrust. I ran some preliminary models for this in the paper but I have done further calculations since, will discuss at a later date.

    Antonio: thank you for the kind words. Project Icarus still exists in limited form.

    Sonam Sharma: without getting distracted from the main discussion, in my opinion elucidation of your three problems (universe, aliens, God) requires a deeper understanding of time and its connection to the mathematically transcendental objects. i.e. the transcendental universe and the transcendental object at the end of time.

  • Antonio January 10, 2023, 5:00

    As a side note, the IAEA has just published the list of the 130 fusion reactors that are currently in operation or in construction in the world:


  • Adam Crowl January 11, 2023, 6:40

    Hi Paul & Kelvin
    Fusion is really hard to do. ICF has hit “Technical Breakeven” but the NIF isn’t optimal for improving. So what’s to be done? VISTA was a great extrapolation into the wild blue, but limited in ambition. Upping it to Starship grade is hard to do – the neutron soaking would be brutal. Personally I think we should try to make use of the biggest fusion reactor in our star system.

    • Michael January 12, 2023, 4:43

      I have to agree Adam, fusion on Earth is hard enough, in space its going to be a lot harder at least mass wise and with the strong neutron emissions issue. I still think a type of fission drive would work with an advantage been not all of the fuel is highly radioactive at the same time and what is used is shot out of the solar system to decay in deep space. The neutron spectrum is also a lot milder than the DT reaction.

  • Bill Osberg January 12, 2023, 11:51

    I don’t understand how DT can be used for a fusion rocket. Most of the energy is released as fast neutrons, which cannot be directed. All the DT fusion reactor designs I have seen use the neutrons in a thermal cycle to heat water to drive a steam turbine to generate electricity. This wastes about half of the energy, not to mention the required mass. Reactions like D3He and p11B release mostly charged particles which can be directed out the back end.

    • Kelvin F Long January 12, 2023, 17:43

      Bill, indeed, I don’t use DT but DHe3 which it is often argued to be aneutronic (but see the argument by Hyde historically).
      But don’t forget you also get helium-4 charged particles released as products in a DT reaction, so that’s its propulsion content.

      • Michael January 14, 2023, 5:15

        The problem with He3-He3 reaction is the very high temperatures and pressures required, billions of degrees. If First light fusion a UK company can get the 10-100 Tpa they claim then fission is a viable way to go. Perhaps someone higher up could ask them if it could be used as a fission/ fusion driver.


        • Kelvin F Long January 14, 2023, 10:49

          Its D-He3 fusion.

          • Michael January 14, 2023, 14:24

            Thanks for the correction, I miss read it. But it still peaks at 2 billion degrees nice if we could get it right though.

      • Michael January 16, 2023, 5:36


        Perhaps you could enquire at Firstlight in Oxford to see if it is a possible spacecraft propulsion idea. Whether fission, fusion or a combo it could be interesting to hear from them. Also the architecture of the design allows them to form lens or neutron channels into the fuel pellet allowing extra compression with lasers and/or neutrons at the right time.


  • Michael January 14, 2023, 6:36
    • Paul Gilster January 14, 2023, 10:01

      Must have been at Starship Congress, which as I recall was in Dallas. A good meeting!

  • Michael January 25, 2023, 11:52

    If it works it has all the hallmarks of been able to be used as a fusion drive.
    Best of luck to the team !


  • Michael January 26, 2023, 7:33

    OT, love that implosion velocity and pressures of machine 4 ! Still a big machine needing reductions in scale if to be used in space.


  • Law Wong January 27, 2023, 10:58

    VISTA isn’t just a cone, but an empty, hollow cone with a hole in it.
    The VISTA ICF vehicle is built around the concept of spin polarization, resulting in neutron radiation being directed along one axis symmetrically. Neutrons from the spin polarized fuel are emitted up and down along the thrust axis, with fewer neutrons going to the sides. They are allowed to escape by building the main elements of the ship – the cargo space, the laser beams etc – in the form of a ring, with the neutrons escaping through the hole in the ring. The ring is then shielded from the rest of the neutron and thermal and gamma radiation with a blade shield, lined with radiators, pointing towards the initiation point – resulting in a hollow cone.
    SunVoyager seems not quite to rely on the design principles of VISTA, instead using a large shadow shield of some sort of absorb neutron radiation.

  • Law Wong January 27, 2023, 11:08

    To follow on to my previous comment on VISTA’s hollow design


    I would also like to make a correction; spin polarization does not seem to have been a critical feature of VISTA (although it could be a fairly useful adjunct); however, the benefits of the hollow conical blade-shielded ring design are grossly similar even when facing an omnidirectional neutron flux from a fusion pellet, the blade shield offering good protection while providing a large area for radiating absorbed energy

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