Solar sails have been receiving a lot of attention lately. In part that is due to a series of high profile missions that have successfully proven the concept. It’s also in part due to the high profile Breakthrough Starshot project, which is designing a solar sail powered mission to reach Alpha Centauri. But this versatile third propulsion system isn’t only useful for far flung adventures – it has advantages closer to home as well. A new paper by engineers at UCLA defines what those advantages are, and how we might be able to best utilize them.
The literal driving force behind some solar sail projects are lasers. These concentrated beams of light are perfect to provide a pushing force against a solar sail. However, they are also useful as weapons if scaled up too much, vaporizing anything in its path. As such, one of the main design constraints for solar sail systems is around materials that can withstand a high power laser blast, yet still be light enough to not burden the craft it is attached to with extra weight.
For the missions that graduate student Ho-Ting Tung and Dr. Artur Davoyan of UCLA’s Mechanical Engineering Department envision that weight is miniscule. They expect any sailing spacecraft to weigh less than 100 grams. That 100 grams would include a sail array that measures up to 10 cm square.
With such small masses and large area comes the huge benefit of solar sailing – the maximum speed achievable by this propulsion technology is leaps and bounds faster than the two more traditional technologies – chemical and electrical propulsion. The study focused on two types of orbital maneuvers normally performed by those other propulsion systems – one where the sail moved around in Earth’s orbit, and one where it traveled between planets.
The first system looks at how long it would take to move across the various stages of escape from Earth. As measured by “?v” (i.e. acceleration), the steady increase in acceleration provided by a laser on a solar sail would allow a small spacecraft to get from low Earth orbit to geostationary orbit in under a few minutes, and then up to escape velocity shortly thereafter.
It also has the advantage of being able to accelerate faster than the fastest acceleration ever by a spacecraft – a record currently held by Dawn during its attempt to get to the outer solar system. This new solar sail would reach accelerations in about a half hour of laser time what it took Dawn 5 and a half years to reach using its electric thruster.
Such linear acceleration would dramatically cut down on interplanetary travel times as well – such a solar sail could reach Mars in 20 days (compared to 200 normally), Jupiter in 120 days (5 years for Juno), and Pluto in about 3 years (10 years last time we visited with New Horizons). Drops in travels times means more opportunities for science, but only if the instruments on board can fit into the relatively small, lightweight package the sail can support.
The instruments themselves aren’t the only important part of that package though. Arguably the most important is the design of the sail itself. Its main design constraints take up much of the analysis of the paper. It must be light, strong/flexible, reflective to the laser (so that the laser pushes it rather than being absorbed) and be able to withstand high temperatures.
The last two constraints are paired together, and were really the focus of the paper, as high reflectivity means less need to withstand high temperatures. After selecting a laser that works well in the atmosphere, the team came up with two types of material that could do the trick for a sail – silicon nitride and boron nitride. In both those materials, the extremely high reflectivity combined with a thermal emissivity (how well it dissipates heat) make them ideal candidates to fill the last two constraints.
To get the most out of the material performance, however, they must be formed in a way to take advantage of their material properties. There were two types of structures analyzed in the paper – a “Bragg stack” and a guided mode resonance (GMR) reflector. A Bragg stack is a type of reflector with multiple stacks of material forming a wavelength-specific high quality reflector. A GMR reflector, on the other hand, uses a type of grating or prism to control what wavelengths the structure becomes reflective in. In both cases, the structure is designed so that light not at the specific wavelength of the laser isn’t reflected, causing minimal excess heating to the panel array.
Ultimately, the paper itself is only an attempt to suggest a design for a future mission concept. There isn’t anything concrete planned to adopt any of its suggestions. But, it is a step towards looking at this potentially game changing propulsion technology which is only now starting to get off the ground. If we leverage it correctly, solar sailing could have a fundamental impact on both the science and economics of space exploration.
arXiv – Light-Sail Photonic Design for Fast-Transit Earth Orbital Maneuvering and Interplanetary Flight
UT – A Small Satellite With a Solar Sail Could Catch up With an Interstellar Object
UT – Want the Fastest Solar Sail? Drop it Into the Sun First
UT – NASA is Testing out new Composite Materials for Building Lightweight Solar Sail Supports
Artist concept of a Solar Sail
Credit – Breakthrough Starshot