Zero Genie

If we want to get there faster, we need fresh approaches like one that was recently proposed by a team of researchers from McGill University and the Tau Zero Foundation – in a new paper.

They explain how borrowing the idea of dynamic soaring from birds could help us to get around space much faster. 

What is dynamic soaring?

Dynamic soaring is a flying technique perfected by seabirds and mentioned as early as 1513–1515 in the notebooks of Leonardo da Vinci. It allows the birds to exploit differences in wind speed velocities to achieve acceleration, as the scientists write in their .

Birds mastered the process of gaining energy by flying between air masses that move at varying velocities. 

A similar approach can be taken in space, suggest the researchers. A spacecraft could be made to take advantage of the flows of ionized gas in the solar wind or interstellar medium — the material in between the stars.

The goal would be for the craft to accelerate, increasing its speed beyond the speed of the wind. As the paper explains further, the spacecraft would circle “between regions of the heliosphere that have different wind speeds, gaining energy in the process without the use of propellant and only modest onboard power requirements.”

Besides birds, glider enthusiasts also make use of this effect, gaining much more velocity than the original windspeed. The relates the example of recent remote control gliders being able to achieve “remarkable” velocities of over 850 km/hr, which was about ten times the speed of the wind. 

Why dynamic soaring could be effective

One big problem with space travel, as the scientists write, is the amount of time it takes to traverse its immense distances. It can be shown, that a “launch of a Voyager-class spacecraft to α-Centauri with transit times comparable to a human lifetime will not be feasible until the 25th century”.

Other propulsion technologies are being developed that can shorten this time — solar sails, for example. These would propel crafts by taking in photons coming from the Sun.

But this promising approach has limitations since even if you launch such sails near the Sun, close to the energy source, and use the highest temperature materials, you’d still only be able to achieve velocities of about two percent of the speed of light.

And more conventional sails not launched as close to the Sun would achieve no more than 0.5 percent of the speed of light, according to the study.

Analyzing other methods of potentially propelling spacecraft by interacting with the solar wind, the researchers considered magnetic sails, electric sails, and the plasma magnet.

Magnetic sails would employ a superconducting cable to generate an artificial magnetosphere, which would deflect charged particles streaming in the solar wind and cause a reaction force on the cable. 

An electric sail would not use such a superconducting cable but instead, have high voltage wires deflect the charged solar wind particles.

The plasma magnet approach would use a polyphase antenna on the spacecraft to “drive currents in the surrounding medium, creating a magnetic structure that would inflate via self-repulsion until the magnetic pressure is balanced by the dynamic pressure of the impinging solar wind.”

The scientists believe this technique is perhaps the most “encouraging” because it needs just a small, low-powered antenna to interact with a huge volume of solar wind. 

While the other approaches are promising, the authors propose that dynamic soaring sails could achieve even greater speeds, allowing spacecraft to traverse larger distances, like to Jupiter, in months instead of years. Before being able to use the dynamic soaring in the turbulent zones of the interstellar space, however, the craft would first need to be propelled up to speed by another energy source. The scientists think a direction plasma wave antenna could serve that purpose.  

Interesting Engineering (IE) reached out to the paper’s co-author, Professor of McGill University in Montreal, Canada for more details on the team’s work.

The following conversation has been lightly edited for clarity and flow.

Interesting Engineering: What are the advantages of using this technique for space travel?

Professor Higgins: The principal advantage of dynamic soaring is that it is able to exploit a source of energy that is available in space for free: The wind of ionized particles that stream from the Sun. Previous concepts have been proposed to ride on the solar wind, but the fastest speed that the wind can drag a spacecraft up to is the speed of the wind itself. Using dynamic soaring permits a spacecraft to exceed the speed of the wind by circling between different regions at different speeds. This type of propulsion does not require using any propellant at all. The extreme energy requirements of interstellar travel have always been the bottleneck for flight to the stars. By beginning to think about how we might exploit sources of energy in space, we hope this work will be a small step toward realizing true starflight.

IE: In what region of space would dynamic soaring be most useful, and what would be the propulsion tech to get the craft to this region? 

Wherever there exists solar wind blowing at two different speeds, the dynamic soaring method can be invoked. For example, the wind blowing from the equatorial region of the Sun is at about 400 km/s, but in the polar regions, the wind is at 700 km/s. This wind shear in the solar wind is one of the regions we explored for this technique. Particularly promising is the termination shock and the heliopause. These are structures at the edge of our solar system where the solar wind drops from 400 km/s to almost rest, which occurs where it encounters the interstellar medium. This large change in velocity gives the greatest potential for velocity increase of the spacecraft. We proposed using this region to reach 2% of the speed of light.

To get to these regions of the solar system, a spacecraft could just be dragged up to the solar wind speed using a type of magnetic parachute called the plasma magnet. Once it encounters regions of shear, the spacecraft can begin the dynamic soaring technique.

IE: How were you inspired by the flying techniques of birds for this proposal? Did you study particular birds?

Birds and gliders were our inspiration for this project. Dynamic soaring as practiced by birds was noted as far back as the 16th Century in the notebooks of Leonardo da Vinci. There are credible accounts of seabirds, such as albatrosses flying halfway around the world without ever flapping their wings by using dynamic soaring. 

More recent inspiration has come from remote-control glider pilots, who have used dynamic soaring in the windshear as found on hilltops to reach speeds approaching 600 mph (almost 1000 kph), as seen here:

 If gliders—with no onboard propellant at all—are able to perform this feat, we were motivated to see if a similar maneuver could be done in space using the solar wind.

Check out the full study ,” published in Frontiers in Space Technology.

Study abstract: 

A technique by which a spacecraft can interact with flows of ionized gas in space (the solar wind or interstellar medium) in order to be accelerated to velocities greater than the flow velocity is explored. Inspired by the dynamic soaring maneuvers performed by sea birds and gliders in which differences in wind speed are exploited to gain velocity, in the proposed technique a lift-generating spacecraft circles between regions of the heliosphere that have different wind speeds, gaining energy in the process without the use of propellant and only modest onboard power requirements. In the simplest analysis, the spacecraft motion can be modeled as a series of elastic collisions between regions of the medium moving at different speeds. More detailed models of the spacecraft trajectory are developed to predict the potential velocity gains and the maximum velocity that may be achieved in terms of the lift-to-drag ratio of the vehicle. A lift-generating mechanism is proposed in which power is extracted from the flow over the vehicle in the flight direction and then used to accelerate the surrounding medium in the transverse direction, generating lift (i.e., a force perpendicular to the flow). Large values of lift-to-drag ratio are shown to be possible in the case where a small transverse velocity is imparted over a large area of interaction. The requirement for a large interaction area in the extremely low density of the heliosphere precludes the use of a physical wing, but the use of plasma waves generated by a compact, directional antenna to impart momentum on the surrounding medium is feasible, with the excitation of R-waves, X-waves, Alfven waves, and magnetosonic waves appearing as promising candidates. A conceptual mission is defined in which dynamic soaring is performed on the termination shock of the heliosphere, enabling a spacecraft to reach speeds approaching 2% of c within two and a half years of launch without the expenditure of propellant. The technique may comprise the first stage for a multistage mission to achieve true interstellar flight to other solar systems.