Use cases for smart materials in space exploration continue to emerge. They are used in everything from deploying antennas on satellites to deforming and reforming rovers. One of the latest ideas is to use it to convert the solar sails, which could serve primarily as a propulsion system for a mission, into a heat shield when the mission reaches its final destination. A new paper by Joseph Ivarson and Davide Guzzetti, both of Auburn’s Department of Aerospace Engineering, and published in Acta Astronautica, describes how the idea might work and lists some possible applications to explore different parts of the solar system.
The concept, which they call “Shape Shifting Sailer” (3S), is simple: a thin sheet of material acts as a shade sail for most of a boat’s journey to its destination. Once it reaches that destination, change the orientation of the foil so that it can act as a heat shield and tugging device for the probe as it enters the target world’s atmosphere or is aerodynamically decelerated into its orbit. This switch could be as simple as a set of shape memory alloy (SMA) hinges that fold the normally flat solar sail into more of a cone or shield shape to provide resistance that would slow the probe, but also deflect some of the heat itself, essentially acting as, admittedly only partially effective, a heat shield.
However, before attempting to build such a system, the engineers did what all good engineers would do: they first modeled the system. In this case, they divided their modeling work into two distinct phases – a “design space” study and a feasibility study.
Fraser is a big fan of solar sails, as he explains in this video.
In engineering parlance, a “design space” has nothing to do with space – it’s a term for trying to capture all the different factors that might affect a particular metric, such as the peak weight temperature that a probe would experience upon entering the Martian atmosphere. By varying these factors in the simulation, engineers can develop a sense of the key design decisions they need to make, particularly around trade-offs such as reducing the sail’s weight or increasing its thermal shielding effectiveness.
In their work, the authors examined case studies for four potential target worlds – Earth, Mars, Titan, Uranus and Neptune. They then turned to a semi-autonomous algorithm, a so-called genetic algorithm, that optimized the trade-off between peak temperature and peak pressure of the sail. They found that these two measures are opposites, as the physical forms that minimize one of these two properties tend to maximize the other. To minimize pressure, it is best to have the shape of a “sheet” – large surface area but very light weight. To minimize temperature, it’s best to have a cannonball shape – very small, thick and dense, which typically implies high “thermal inertia” – how much total heat a material can absorb.
In the next phase of their study, they developed potential flight routes into the atmosphere and orbit of the various worlds studied. They found that the material around the Earth could contribute at least some to the overall thermal load, reducing the peak heating rate by 20-25%, but only if the sail was jettisoned during reentry. Mars was the best scenario for using the 3S idea because, again during a “drop” scenario, it showed that the heating of the incoming probe could be reduced by up to 40%.
LightSail 2 was a good example of a prototype solar sail that could one day bend into a different shape for braking.
Unfortunately, the results for Titan, Uranus and Neptune were not so good. On the gas giants, using the system at all proved impractical because the entry velocities required for air braking in their atmospheres were too high and would burn up any possible material that could possibly be produced in the near future. On Titan, the 3S system would be feasible, but would require about as much mass in the sail as in the payload to be effective. Given that putting mass on Titan is inherently expensive, the idea is unlikely to succeed – or land.
But even if the idea is only applicable to Mars exploration, it will be the focus of many future missions as NASA continues its journey from the Moon to Mars. Given the promising results of the simulations shown in the paper, it might be worth investing some money into developing a prototype of the 3S system to see if there are any flaws in the plan. However, given the current state of funding for space exploration, this may have to wait for some time.
Learn more:
J. Ivarson & D. Guzzetti – Shape-shifting glider: Entry and descent capabilities of self-folding shape-memory membranes
UT – A better way to rotate solar sails
UT – NASA’s next solar sail is about to fly into space
UT – NASA is putting its solar sail through its paces
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