Some final thoughts on hybrid propulsion will wrap up this series on solar sails, which grew out of ideas I encountered in the new edition of the Matloff, Johnson and Vulpetti book Solar Sails: A Novel Approach to Interplanetary Travel (Copernicus, 2014). The chance to preview the book (publication is slated for later this month) took me in directions I hadn’t anticipated. Solar Sails offers a broad popular treatment of all the sail categories and their history, as you’d expect, but this time through I focused on its four technical chapters on sail theory that helped me review the details.

And because I kept running into the idea of multiple modes of propulsion, my thoughts on avoiding doctrinaire solutions continue to grow. In fact, I’d venture to say that probing into the possibilities of multimodal propulsion may offer a serious opportunity for insights. Centauri Dreams regular Alex Tolley came up with one of these yesterday, asking whether a sail mission to Jupiter space might deploy the planet’s huge magnetic field as an assist. Alex invokes Pekka Janhunen’s ideas about electric sails. Let me quote from the Solar Sails book on what Janhunen has in mind:

Similar to the magsail, this concept uses the solar wind for producing thrust. However, different from the magsail, this sail interacts with the solar plasma via a mesh of long and thin tethers kept at high positive voltage by means of an onboard electron gun. In its baseline configuration, the spacecraft spins and the tethers are tensioned by centrifugal acceleration. It should be possible to control each wire voltage singly, at least to within certain limits.

We get thrust out of this when protons from the solar wind, positively charged, are repelled by the positive voltage of the spacecraft’s tethers, while electrons are captured and ejected — otherwise, their growing numbers would neutralize the voltage in the tether mesh. But Alex also brings to mind Mason Peck’s interesting work at Cornell on miniaturized ‘Sprites,’ tiny chip-like spacecraft that could use the Lorentz force to accelerate in directions perpendicular to the magnetic field. Remember that Jupiter’s magnetic field is 18,000 times stronger than Earth’s, a useful resource if we can tap it even so far as to adjust the orbits of planetary probes.

Alex’s thoughts on the matter deserve to be quoted:

We often think of sail ships as clipper ships – i.e. using large surfaces to capture or direct the wind to move. But modern ships use screws. There have also been numerous wind turbine designs that offer advantages over canvas sails, even if they are not as aesthetic to the eye. (Clipper ships were possibly the most pleasing ship designs ever built). Might we be thinking too much in terms of sails that mimic the romance of traditional sails, rather than designs that might offer better performance, albeit with some aesthetic loss?

Interstellar Arrival

A sense of aesthetics produces pleasing designs but what looks best isn’t always what we need. Back in my flying days some of us used to talk about (and in a few cases actually fly) some of the great aircraft designs of the 1930s and later, and although I never got my hands on the controls of one, a great favorite was the Beech Staggerwing, a gorgeous design with a negative wing stagger, meaning that the lower wing is farther forward than the upper. Designs like this could be sleek and lovely because of the medium they worked in. But spacecraft don’t need wings and streamlined fuselages, and our Voyagers and Cassinis look nothing like the wilder designs of early science fiction because they don’t need to, never encountering a planetary atmosphere.


Image: The Beech Model 17 Staggerwing, first produced in 1932. Credit: Wikimedia Commons.

A beamed lasersail on its way to Alpha Centauri may be anything but a thing of beauty. Once the mission enters its cruise phase, the sail can be safely stowed, and one good use for it would be to shroud the payload to offer additional protection against radiation. We’re always trying to think of ways to get more value out of existing assets, which is what extended missions are all about. Or think about the Benfords’ JPL work that revealed desorption. No one with an eye for design would come up with painting a desorption layer on a sailcraft, but it’s conceivable that desorption, which is the release of CO2, hydrocarbons and hydrogen from within the manufactured sail as it heats up under the beam, could give an added kick to interplanetary sails being pushed by powerful microwave beams.

Mentioning Forward brings me back around to the ‘staged sail’ concept that he worked out for stopping at another star. The sail has three divisions, as shown in the diagram below, which is taken from his paper on a manned mission to Epsilon Eridani. ‘Staging’ the sail means losing first the outer ring, then the middle one, until only the inner ring is left. In sequence, the spacecraft slows down by using laser light beamed from our Solar System, reflected off the now separated outer sail as it approaches the star — the light is directed back at the two remaining sail segments with payload. Ingenious tinkering let Forward use the second sail detachment as the way the crew got home, with laser light boosting the much smaller inner sail by reflection from the middle segment.


Image: Robert Forward’s staged sail concept. What he calls a ‘paralens’ in the diagram is an enormous Fresnel lens in the outer Solar System, made of concentric rings of lightweight, transparent material with free space between the rings. Credit: Robert Forward.

Staged sails are hard to see as anything but a longshot — the success of the mission depends not only upon perfect execution of the staging process but, crucially, upon the laser beam from Earth being able to illuminate the sail segments effectively. Forward was fully aware of the possibilities here, and you can find discussion in places like his novel Rocheworld (Baen, 1990) about how politics on Earth might affect the use of the expensive beam. I for one wouldn’t want to put my life in the hands of a design like this, which depends so crucially upon decisions made far from the spacecraft.

Interestingly, like Mason Peck, Forward had some thoughts on how we might use the Lorentz force as well. Remember that a charged object moving through a magnetic field experiences this force at right angles to its direction of motion and the magnetic field itself. Out of this you get ‘thrustless turning,’ which both Forward and Philip Norem thought could be used for deceleration. Instead of staged sails, you get an electrostatically charged probe — think of Janhunen’s electric sail tethers — on a trajectory that goes well beyond the target star. The spacecraft’s interactions with the galactic magnetic field bend its trajectory so that it approaches the target from behind.

Once it’s inbound to the destination system, a laser beam from Earth can be turned upon it to slow it down for arrival. The idea is anything but aesthetic, just as the Janhunen sail would look like something closer to a porcupine than the silvery lozenge of an early SF starship. It’s also hampered by the fact that mission times, already measured in decades at minimum, are tripled with the use of this maneuver. I should mention that Solar Sails authors Gregory Matloff and Les Johnson have also explored the uses of electrodynamic tethers to supply power to an Alpha Centauri expedition, even if a Norem-style arrival seems too lengthy.

Creative thinking about these matters often springs from putting two or more solutions together to see what can happen. What I’ve always admired about the interstellar community is its ability to re-examine older concepts to look for interesting cross-pollination of ideas. As we move into the era of increasingly tiny components, it’s heartening to think how many designs will be affected by new nanotechnological possibilities. Mason Peck has talked about using Jupiter’s magnetic field to spew thousands of ‘Sprites’ out on interstellar trajectories. What else can we imagine as we look for extended uses of existing tech and ponder where they might lead us?

Forward’s paper on staged sails is “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets 21 (1984), pp. 187-195. The Norem paper is “Interstellar Travel: A Round Trip Propulsion System with Relativistic Capabilities,” AAS 69-388 (June, 1969). Forward’s paper on Lorentz force turning is “Zero-Thrust Velocity Vector Control for Interstellar Probes: Lorentz Force Navigation and Circling,” AIAA Journal 2 (1964), pp. 885-889. Matloff and Johnson discuss electrodynamic tethers in “Applications of the Electrodynamic Tether to Interstellar Travel,” JBIS 58 (June, 2005), pp. 398-402.