A ‘Sundiver’ mission may offer the best acceleration we can muster given the current state of our technology. New Horizons is currently moving toward Pluto/Charon at roughly 19 kilometers per second, but back of the envelope calculations can pull out 500 kilometers per second for a solar sail that makes the optimum close approach to our star and then unfurls to full diameter, riding the photon storm outward to the edge of the Solar System and beyond in record time.
But Sundivers are tricky missions even on paper (we have yet to attempt one). Gregory Benford (UC-Irvine), who coined the ‘Sundiver’ term, and brother James (Microwave Sciences) have studied the matter in depth, and bring a unique perspective. They’ve not only theorized about sails and acceleration, but have actually tested the concept in the laboratory. Specifically, they’ve used an intense beam of microwaves to lift a carbon sail vertically in a vacuum chamber, and have studied how to spin and control it.
A Sail Takes Flight
This work took place in 1999 and 2000, and while I wrote the experiments up in my Centauri Dreams book some years back, I hadn’t seen the final report on it (“Wireless Power Transmission for Science Applications” NAS8-99135) until a conversation with James Benford last fall resulted in his passing along a copy. It’s absorbing reading because it’s the kind of essential laboratory work that leads to new thinking in propulsion on a practical level. It’s startling that these significant experiments have received as comparatively little attention in the space community as they have to this point.
The sail material chosen was a carbon fiber ten times thinner than a human hair. A carbon sail — the Benfords used small sails just inches across — has ‘memory,’ able to regain its shape after being rolled or folded. That could make deployment easier in space. And carbon fiber has other benefits that make it stand out as opposed to aluminized mylar sails. A micro-fiber mat like this can handle high temperatures even though it’s lighter than tissue paper. Putting this material close to the Sun poses no risk to the sail’s survivability.
Image: Carbon disk sail lifting off of truncated rectangular waveguide under 10 kW microwave power (four frames, 30 ms interval, first at top). Credit: James and Gregory Benford.
But laboratory work on sail materials is tricky indeed. Trying to get a sail to lift off against the force of gravity poses serious temperature problems, for the beam intensity (the Benfords used a 10 kW, 7 GHz microwave beam in a vacuum chamber) would melt conventional materials. But the Benford’s carbon fiber microtruss reached temperatures above 2000 kelvin from microwave absorption without melting. The concept of microwave beaming to push a spacecraft has been initially validated, and the requisite material tested to ensure it could handle the temperatures involved in a close solar pass.
A Sundiver Concept Emerges
The general shape of the Sundiver mission begins to take on substance. Surely we could use microwave beam technologies to launch a sail into a trajectory that, over time and multiple orbits, would reach the vicinity of 0.1 AU, at which point the sail receives the mighty wallop of solar photons after rotating to face the Sun. It’s an idea with a pedigree in propulsion studies, but one to which the Benford’s laboratory work has added a significant new dimension.
For there seems to be a way to kick in a new form of propulsive ‘burn’ at perihelion to maximize the resultant acceleration. Remarkably, in their experiments both at the Jet Propulsion Laboratory and UC-Irvine, the Benfords found that when they turned their microwave beam on the test sail, it experienced accelerations well beyond what photon pressure alone could account for. Tomorrow I want to look at how this effect could be modified and enhanced, capable of being used both at the initiation of a Sundiver mission and at the critical moment of closest Solar approach.
A sundiver manned interstellar space ark might be launched and might reach velocities commensurate wtih a rocket powered gravity assist. Gravity assists using lone stars such as the Sun are not possible, as opposed to gravity assists using binary or trinary stars, however the use of a rocket burn while doing a gravity assist manuever is a very useful way to gain extra net kinetic energy. Such powered gravity assists can greatly amplify the effective Isp of the rocket fuel.
Now I know that all of this is obvious, but the point that I am trying to make is that nuclear fission reactor powered plasma rockets such as the VASIMR rocket might be ideal for powering such a solardiving space craft in a gravity assist manuever. If the VASIMR rocket’s thrust can be amped up, perhaps interstellar probes and even manned colony star ships could be built which would set sail this century.
It is anyone’s guess how much of a sunlight pressure to sail mass ration we can obtain. If we can produce ultra reflective carbon fiber types of sails, that do not loose their reflectivity under 100 to 1,000 kilowatts/meter of sail area, we might have it made. Perhaps some sort of carbon nanotube based sails that are metalized with extremely reflective and refractive metals or alloys could be used to produce highly reflective and refractive sails that have empty space cells of one square micrometer bounded by 1 nanometer think carbon nanotubes. Such a sail might have mass specific area of (10 EXP 9)(10 EXP 3) square meters per metric ton or 1,000,000 square kilometers per metric ton.
With appopriate sail materials, it is hard to see that solar divers could not reach relativistic velocities. We might be able to achieve accelerations that are so high that some sort of G-force cancellation technology would be required to keep the crew members from being squashed.
The Sundiver (“Dive and Fry”) concept has been talked about quite a bit here before, but it’s nice to get a little technical confirmation that it has merit now.
The cheap way out would be multiple-planet slingshot maneuvers before the final “dive” of course. But I prefer the more direct approach of using a linear accelerator based in a Jovian Lagrange Point to shoot the probe to Jupiter for a slingshot, then on to the Sundiver “fry and dive.”
A one kilo package the size of a soup can with the Benford Sail would work nicely.
I look forward to tomorrow’s blog. I hope that the following apparently contradictory statements can be squared. Will the carbon mesh evaporate away or not?
Regarding the temperature at 0.1C here are my calculations:
T=Ts*(Rs/(D*2))^0.5
Where:
Ts = average sun temperature = 6,000K
Rs = sun radius = 695,000 km
D = distance from the sun
1 AU = 149,600,000 km
T=6,000*(695,000/(15,000,000*2))^.5) = 913K = 640C.
So the max temperature of the mission should be within what has already been seen in their experiments.
John Hunt writes:
As you’ll see tomorrow, the two statements are not contradictory — in fact, the effect you mention turns out to be quite a benefit. The mesh does not melt, but does something far more interesting. More later.
James Essig writes:
In this case, however, the ‘burn’ at perihelion turns out to be quite a different thing — no heavy rocket needed — relying on effects the Benfords have studied in the lab. More tomorrow.