Kelvin Long’s new paper on the mission concept called Sunvoyager would deploy inertial confinement fusion, described in the last post, to drive a spacecraft to 1000 AU in less than four years. The number pulsates with possibilities: A craft like this would move at 325 AU per year, or roughly 1500 kilometers per second, ninety times the velocity of Voyager 1. This kind of capability, which Long thinks we may achieve late in this century, would open up all kinds of fast science missions to the outer planets, the Kuiper Belt, and even the inner Oort Cloud. And the conquest of inertial confinement methods would open the prospect for later, still faster missions to nearby stars.

Sunvoyager draws on the heritage of the Daedalus starship, that daring design conceived by British Interplanetary Society members in the 1970s, but as we saw last time, inertial confinement fusion (ICF) was likewise examined in a concept called Vista, and one of the pleasures of this kind of research for a scholarly sort like me is digging out the history of ideas, which in the Long paper I can trace through work in JBIS and the IEEE in the 1980s and 90s, where ICF was considered.

Vista itself appeared in the literature in the 1980s, drawing on this earlier and ongoing work, its conical shape a response to the potentially damaging neutron and x-ray flux that ICF produced. Long emulates its form factor in the Sunvoyager design. I should also mention a NASA concept called Discovery II that I hadn’t encountered until now, a spacecraft designed for a mission to the gas giants using a magnetic fusion engine. Both this and an early ICF design by Lawrence Livermore Laboratory’s Rod Hyde and colleagues in the 1970s would use an engine with a mass of 300 tons, a figure which Long selected for the calculations in his Sunvoyager paper as he validated the HeliosX code using Vista as the template: “The current level of accuracy will suffice for making predictions for the expected design performance of the Sunvoyager probe.”

So what do we get as we downselect to achieve the Sunvoyager design? The image below shows the concept.

Image: This is Figure 8 in the paper. Caption: Concept design layout of Sunvoyager spacecraft configuration. Credit: Kelvin Long.

Notice the radiators, a critical part of the design, for we need to find a way to reduce waste heat. Long notes that for Vista, the radiation interaction with the structure was about 3 percent – in other words, the vehicle intercepts about that amount of the neutron and x-ray flux from the fusion reactions. He assumes a higher figure for Sunvoyager, although adding that using a mixture of deuterium and helium-3 as the fuel (Vista used a capsule of deuterium and tritium) would reduce these effects. The design also includes an annular radiation shield within the engine structure.

Long assumes the use of X-band frequencies for communications, transmitting at 8.4 GHz with a power output of 100 W, the signals to be received via the Deep Space Network’s 70-meter dishes. It’s interesting that he does not push for laser methods here, wisely so, I think, given the pointing problems we’ve discussed recently at deep space distances. Pushing data back to Earth from 1000 AU is daunting enough:

The expected data rate at 1000 AU will be 1 kBits?s. Backup medium- and low-gain antennas are also likely to be required. Note that radio signals from a distance of 1000 AU will take around 138 h to reach Earth receiving antennas, and so significant data latency should be expected. The high-gain antenna will be mounted on a rotatable fixing (rather than body mounted) and on a set of rigid extension poles so that it can always be pointed toward Earth, which avoids the need of having to rotate the entire spacecraft such as was performed for the Voyager 2 and New Horizons missions.

The Sunvoyager interstellar precursor probe would be assembled in Earth orbit following multiple launch missions. The author likens building the craft to the construction of the International Space Station, noting on the order of 10 launch vehicles may be needed to get all the parts into the assembly orbit. Booster rockets, perhaps nuclear thermal, would be used to move the vehicle away from Earth at 17 kilometers per second (which happens to be Voyager 1 speed). This reaches twice the mean Earth-Moon distance in a day or so, at which point the fusion engine can be ignited. And here we go with ICF fusion on our way to the outer Solar System:

A capsule is accelerated into the target chamber where the bank of laser beam lines can target it within the open reaction chamber to the point of thermonuclear ignition. A set of externally placed laser-focusing mirrors may be required to ensure a symmetric implosion. The plasma from the detonation will expand into the hemispherical target chamber, with the charge particles then directed by large magnetic fields internal to the chamber. These are then ejected for thrust generation while the next capsule is loaded onto the target ignition point. This occurs 10 times per second, although the hydrodynamic and nuclear phases of the ignition take place on microsecond and nanosecond time scales, respectively, so that in between each ignition there will still be around 10?5 s of time for the loading of the next capsule while the plasma from the previous one is being ejected.

The numbers on the ICF fusion for Sunvoyager are, shall we say, mind-boggling. Consider this: The mission needs 200 million fuel capsules, or 50 million per tank. This is, as the author comments, “no small undertaking,” a thought I can only echo. If we’re looking at constructing and flying a mission like this in, say, 50 years time, we may be able to assume advances in robotic automation and additive manufacturing, but we also have the problem of acquiring the needed fuel. You may recall that the Daedalus starship design was built around the notion of mining the gas giants for helium-3. That, in turn, assumes a Solar System infrastructure sufficient to make such mining feasible.

Image: This is the paper’s Figure 12. Caption: Concept design configuration (side view) of Sunvoyager spacecraft. Credit: Kelvin Long.

I like the sheer daring of concepts like Daedalus and Sunvoyager. Remember that when those frisky BIS engineers put Daedalus together, they worked at a time when it was largely considered impossible to reach another star by any means. Daedalus seemed impossible to build (it still does), but it violated no laws of physics and became a vast engineering problem. The point wasn’t that building it would bankrupt the planet. The point was that if we did decide to build it, nothing in physics would prevent it from working. Assuming, of course, that we did conquer ICF fusion for propulsion.

In other words (and Robert Forward would hammer this home again and again in talks and in papers), interstellar flight was not science fictional dreaming but a matter of reaching the appropriate level of engineering, which one day we might very well do. A mission design like Sunvoyager reminds us that we can stretch our thinking based on what we have today to make wise decisions about how and where we invest in the needed technologies. We gain scientific knowledge in doing this and we also rough out the roadmap that points to still further missions that one day reach another star.

Image: The extraordinary Robert Forward, wearing one of the trademark vests created by his wife Martha. Forward chose this photograph to appear on his own Web site.

So I think Kelvin Long is spot on in his assessment of what he does here:

Additional studies will be required to further develop the design configuration and specification for the Sunvoyager mission proposal so that it can be matured to the point of a credible mission in the coming decades to include a subsystem-level definition. However, the calculations presented in this paper show promise for what may be possible in the future provided that investments into ICF ignition physics are continued and then the applications of this technology pursued with vigor.

I think Bob Forward would have liked this paper. And because I haven’t quoted his famous lines (from JBIS in 1996) in their entirety since 2005, let me do so here. He’s looking into a future when we go from interstellar precursors into actual interstellar crossings to places like Proxima Centauri, and he sees the process:

Travel to the stars will be difficult and expensive. It will take decades of time, gigawatts of power, kilograms of energy and trillions of dollars. Recently, however, some new technologies have emerged and are under development for other purposes, that show promise of providing propulsion systems that will make interstellar travel feasible within the forseeable future — if the world community decides to direct its energies and resources in that direction. Make no mistake — interstellar travel will always be difficult and expensive, but it can no longer be considered impossible.

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).