There is a natural path through solar sails, which are now flying, toward beam-driven propulsion, and it’s a path Jim Benford has been exploring for the last eighteen years. In my Centauri Dreams book I described how Jim and brother Gregory ran experiments demonstrating that carbon sails could be driven by microwave beams back in the year 2000. We learned that the theory worked — a sail could indeed be propelled by a beam of photons — and moreover, we learned that the configuration of the craft and propulsion system allowed it to be stable.

Now we’re talking about beam-riding, which the Benfords were able to demonstrate in later experiments. For it turns out that the pressure of the beam will keep a concave-shaped sail in tension, and as Jim pointed out in a recent email, the beam also produces a sideways restoring force. His work showed that a beam can also carry angular momentum and communicate it to the sail, allowing controllers to stabilize the structure against yaw and drift. This is as far as our microwave-beaming experiments have taken us so far, but as solar sails become less an experimental than an operational technology, we can move to space-based experimentation.

Image: A near-term sail experiment under microwave beam. Courtesy of James Benford, Microwave Sciences.

Robert Forward’s name always comes up in such discussions. An old friend of Benford’s, Forward developed enormous interstellar mission concepts using beamed propulsion, ideas that physicists like Geoffrey Landis and Robert Frisbee were able to tweak, just as Jim did, to produce smaller systems. Jim went on to take a cost-optimized approach to the issue, understanding that even the most ingenious of starship designs will be driven by economics. His new paper discusses the matter and notes that a design project using his methods called Project Forward will be undertaken by Icarus Interstellar, the group that manages Project Icarus.

Benford’s notions are solid and based on long experience. As he wrote recently:

I feel beam-driven propulsion is more firmly grounded, more thought through and quantified than nuclear propulsion methods at present. We should put more of our effort into beam-driven sails in this era of little funding. The on-going development of solar sails will tell us how to deploy and control sails, so we will keep close links with that community. This will lead to beam-driven experiments and simulations. Let’s get on with it!

Let’s talk for a moment about the experimental work on beam-driven sails, which was enabled by the invention of carbon microtruss material that is both strong and absurdly light. The material from which a sail is made is critical given that a certain fraction of the power the beam provides the sail will be absorbed and must be radiated away. Given that acceleration is strongly temperature-limited, materials with low melt temperatures like aluminum, beryllilum and niobium are ruled out for beam-driven missions, no matter how useful they may be for standard solar sailing, which uses solar photons rather than concentrated beaming to drive the spacecraft.

Carbon mesh materials work admirably for beamed-sail experiments because carbon has no liquid phase and sublimes instead of melting, as Benford explains in his new paper. These materials allow a sail to operate at temperatures up to 3000 C, allowing them to be ‘launched’ in a vacuum chamber here on Earth without burning. The Benfords were able to push ultralight sail materials at several g’s of acceleration, with the sails reaching temperatures in the range of 1725 C from microwave absorption while remaining intact. Bear in mind that various mission concepts call for lower power densities than the scientists used here. Operating on Earth, they needed a powerful push to get the forces needed for liftoff within a gravity well.

Robert Forward’s interstellar concepts were awesome in their scale, but Benford points out that there is a path to be followed before getting to the interstellar stage. From the paper:

It’s important to realize that for large-scale space power beaming to become a reality it must be broadly attractive. This means that it must provide for a real need, make business sense, attract investment, be environmentally benign, be economically attractive and have major energy or aerospace firms support and lobby for it. Therefore, we include missions that could lead to Starwisp missions, from an infrastructure base developed for smaller-scale missions.

Starwisp was another Robert Forward concept that came out of a time when the scientist moved from laser propulsion ideas to microwaves, whose longer wavelength allowed the sail to be little more than a grid — the wavelengths involved are comparable to the human hand, as Benford told me in an interview some years back, whereas lasers operate at minute wavelengths. A microwave sail, in other words, could be far lighter than the sail required for a laser push because the microwaves are stopped by a conducting surface with gaps smaller than a wavelength. From this, Forward came up with the ultralight ‘starwisp’ design.

Imagine a wire mesh about a kilometer in diameter that weighs no more than sixteen grams. You’ll want data return from the spacecraft so Forward included microchips at each mesh intersection. The craft would be so light and insubstantial that it would be invisible to the eye, but it could be accelerated at 115 g’s using a 10 billion watt microwave beam, taking it to a cruising speed of 20 percent of the speed of light within a few days. Forward’s Starwisp paper included his usual love of gigantic objects, including a beaming lens 50,000 kilometers in diameter.

Geoffrey Landis has shown that the wrong materials would cause a Starwisp to be fried by the powerful microwave beam thus generated, which is why people like Benford are looking at entirely new sail materials as they explore closer and more practicable missions. And practicality — a realistic path forward through solar sails to beamed propulsion — is what I want to discuss on Monday, when I’ll run through the mission concepts Jim Benford has looked at from the standpoint of cost-optimization. Because if we’re going to move beamed sailing out of the realm of science fiction, we’ll need missions that are near-term and offer a clear and economical way to deep space.

The paper is Benford, “Starship Sails Propelled by Cost-Optimized Directed Energy.” I’ll post the link when this paper becomes available online.