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A Fast Inflatable Sail Using Desorption

The first laboratory work on pushing a space sail with microwaves was performed by Jim and Greg Benford at the Jet Propulsion Laboratory back in 1999, with the results presented the following year at a European conference. Leik Myrabo (then at Rensselaer Polytechnic Institute) was, at about the same time, performing experiments with lasers at Wright-Patterson Air Force Base in Ohio. When you think about the problems of laboratory work on these matters, consider the fact of gravity, meaning that you are working in a 1 g gravity well with diaphanous materials whose acceleration depends on how hot you can allow them to become.

Advances in materials and in particular in lightweight carbon structures allowed the Benfords’ experiments to succeed, with the help of a 10-kilowatt microwave beam that produced significant acceleration on the test object. But I’m reminded by looking at a new paper on sail technologies using no beam at all that the Benfords also demonstrated something else. Molecules of CO2, hydrocarbons and hydrogen that had been incorporated in the lattice of their material at manufacture emerged under the high temperatures involved. Thus another form of acceleration came into play through the phenomenon called desorption.

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.

Just how useful an effect may desorption turn out to be? It seems to offer an acceleration dividend. Roman Kezerashvili (City University of New York), working with colleagues at Samara National Research University in Russia and the State University of New York at Buffalo, has now analyzed the effect of desorption on an inflated, torus-shaped sail. I had the pleasure of talking with Dr. Kezerashvili in Italy at the Aosta conference in 2009, where he discussed relativistic effects that have to be taken into account in the navigation of a space sail close to the Sun. The current work follows up an earlier paper on desorption he presented in 2016.

We have the prospect of incorporating compounds into a sail that become a kind of propulsive shell that is triggered either by microwave beam or, in the case of a close Solar pass, by the Sun itself. Think of desorption as involving a kind of propulsive ‘paint.’ In terms of beaming, it is necessary to understand that desorption can take place under either microwave or laser beam, but microwaves do not damage sail materials and so heat them far less destructively. The Benfords’ work, as Kezerashvili points out, shows that microwave beaming produces more efficient absorption in the sail’s coating materials, and high specific impulse can result from desorption.

In fact, in the Benford experiments on ultra-light carbon sails, photon pressure can account for no more than 30 percent of the observed acceleration. The rest comes from desorption.

Various materials can undergo desorption at different temperatures — the sail analyzed by Kezerashvili in 2016 uses carbon, whose properties of acceleration are there analyzed. If low mass atoms or molecules can be blown out of the sail lattice at known temperatures, the mission concept Kezerashvili suggested in that 2016 paper can emerge, one in which the sail reaches desorption temperature at a particular point in a close pass around the Sun. In this scenario, no microwave or laser beam is needed:

It is of particular interest to consider an inflatable torus-shaped solar sail as both propellant-less and propellant-based system. It is a propellant-based and a propellant-less system which create thrust by the sun-driven ejection of a flux of particles of non-zero rest mass due to the desorption of coating and solar radiation pressure, while it performs as a propellant-less conventional solar sail after the thermal desorption ends.

As far as I know, this new paper is the first analysis of desorption on an inflatable sail, but inflatable structures have a long history due to their flexibility and low volume at launch. Among the earliest inflatables to be used in space were the Echo balloons launched in the 1960s, but inflatable structures have been developed extensively in the decades since. I’ve written in the past on a hydrogen-inflated sail using a molybdenum reflector that became the basis for an early orbiting radio telescope design, a concept Greg Matloff extended and investigated along with Kezerashvili and Italian physicist Giancarlo Genta for interstellar flight purposes.

This is also the first paper that considers thermal desorption in the context of solar heating alone, which leads to an interesting class of ‘sundiver’ missions with close solar passes. Thus a conventional solar sail coated with desorptive paint is heated by the space environment.

Image: From the Aosta conference of 2009, a fine memory: A snapshot taken in the Italian Alps. Left to right: Giovanni Vulpetti, Roman Kezerashvili, and Justin Vazquez-Poritz.

On deployment matters: Most solar and beamed sails discussed in the literature assume systems of electromagnetic actuation devices, often involving guide rollers and booms. All of this, of course, adds to the mass of the resulting structure as well as its complexity. Deployment through a series of inflatable booms, as assumed in the inflatable sail concept, simplifies an otherwise intricate procedure. The paper also mentions the advances in flexible polymers and high-strength fibers that allow more mass packed into the launch vehicle at lower cost.

So inflatable sails make sense. The new Kezerashvili paper looks at the dynamics of an inflatable sail, conceived as “a thin reflective membrane attached to an inflatable torus-shaped rim.” The round, flat membrane is coated with heat sensitive materials that make the transition from solid state to gas. Pressure introduced into the rim allows sail deployment from the initial stowed configuration, with deployment occurring at the chosen heliocentric distance.

At this point the sail membrane is extended into its final flat shape, with thermal desorption occurring at a specific temperature. For the purposes of these calculations, the authors cite an acceleration time for the torus-shaped solar sail due to thermal desorption of about 1500 seconds, with the mass of the coating material pegged at 1.5 kg and a desorption rate of 1 g/s.

Image: This is Figure 1 from the paper. Credit: Kezerashvili et al.

And here is a figure from one of Dr. Kezerashvili’s presentations on the inflated sail.

Thus we wind up with two types of acceleration at perihelion, the first being the expected solar radiation pressure, the second being that caused by thermal desorption from the sail itself. The authors assume a beryllium-coated solar sail and analyze the membrane mass of the sail as well as the toroidal rim and coating material, along with the desorption rate, under varying molecular hydrogen gas fills and a temperature range varying with the perihelion approach considered. The different gas fills change the tensile stress on the sail membrane. The paper analyzes the structural strength required in the inflatable torus to support the circular membrane of the sail and considers the deflection of the membrane due to acceleration.

A ‘sundiver’ maneuver is a mission scenario we’ve discussed often in these pages — only this one occurs with an inflatable sail, and it is a mission in which the sail is deployed just as the appropriate temperature is reached. The additional kick provided by desorption produces a substantial boost in performance over a sail driven by solar radiation alone:

The present study reveals that the inflation deployed torus-shaped solar sail accelerated via thermal desorption of coating results in high post-perihelion heliocentric solar sail velocities. With the speed 20–40 AU/year, post-perihelion travel times to the vicinity of Kuiper Belt Objects (KBO) will be less than 1-3 years, while the Sun’s gravity focus at 547 AU can be reached in 13-25 years.

What interested me in particular was the idea of extending the concept into small sails and opting for a ‘fleet’ concept:

The suggested configuration of the torus-shaped solar sail fits the cube-scale size configurations. Recent research reveals that much smaller sails could be incorporated with highly miniaturized chip-scale spacecraft. It is quite possible that a single dedicated interplanetary ‘‘bus” could deploy many cube-scale sails at perihelion. Sequential deployment of a fleet of solar sails could be timed to allow exploration of many small KBOs from a single launch. The natural continuation of this work can be extended in the following directions: i. Detailed research on materials for thermal desorption at temperature suitable for solar sailing; ii. consideration of the Sun as an extended source of radiation; iii. study the influence of solar sail surface oscillations on the motion of a spacecraft performing an interplanetary flight.

Interesting issues arise with a sail like this. The coating on the surface of the sail must be uniform, and in fact one would expect that slight variations will occur that can lead to trajectory deviations during the desorption process. Remember that the time of desorption acceleration is small, around 1500 seconds, in the configuration discussed here. The authors assume there would be a need for periodic corrections in the sail’s tilting angle as these deviations occur. You can also see how neat a fit this work makes with Kezerashvili’s earlier studies on navigation issues deep in a gravity well. An error in relativistic calculations close to the Sun could lead to substantial variations in the final trajectory.

The paper is Kezerashvili et al., “A torus-shaped solar sail accelerated via thermal desorption of coating,” in press at Advances in Space Research (abstract). The original 2016 paper is Ancona and Kezerashvili, “Orbital dynamics of a solar sail accelerated by thermal desorption of coatings,” Proceedings of 67th International Astronautical Congress (IAC 2016), Guadalajara, Mexico, 26-30 September 2016. Paper IAC-16-C1.6.7.32480 (preprint). The comprehensive paper on extrasolar exploration by a solar sail accelerated via thermal desorption of a coating that grew out of this is published in Advances in Space Research 63, pp. 2021–2034 (2019).

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{ 20 comments… add one }
  • Andrei August 14, 2020, 16:17

    The idea to inflate a sail of this kind and get extra boost from ‘desorption’ is a cunning idea. And the time to reach and do a flyby mission of a KBO is amazing, a mission to the Sun’s focal point is misnamed as the instrument would be able to focus further out also.
    But there’s still a good reason for it to stop, not to mention that commands from Earth already will take a very long time. And become even worse as the years go by.
    It also depend on what the mission intend to accomplish, if it is only to show distant planets as dots of light – then the fast mission could do so. But if we would try to get a detailed image or even start searching for Earth similes – the spacecraft would need time and perhaps even do a search pattern spiral course in the optimal focus area to get the planet in view. Perhaps even wait until the planet get into the right position on the further part of the orbit so that a significant part of the hemisphere can be imagined – the more optimistic speculation is that it would be possible to get images both of continents and cloud systems from this vantage point.
    If Alpha centauri is the target, the spacecraft will only need to move a bit to cover the area near each star, while the study of something else – lets say a black hole in the same part of the sky still will require a significant course correction. And this problem get only worse the further the spacecraft get from the Sun.

  • Alex Tolley August 14, 2020, 17:50

    I love inflatable and origami structures. The use of inflatable structural elements mimics the solutions found in nature, such as the post-metamorphosis expansion of insect wings.

    This particular approach of an inflatable torus deploying a membrane has been investigated before, e.g. L’Garde’s inflatable deployable antenna

    Architects have long experimented with inflatable structures on Earth fora variety of uses. A good, but dated, survey from my library is Pneumatic Structures: A Handbook for the architect and engineer. The diversity of ideas has yet to be explored for space systems.

    Regarding the doped sail proposal, it seems to me that the torus could be partially covered in controllable, variable-reflectance cells that can be used for orientations and navigation, like those used on JAXA’s IKAROS. A mechanical inflatable mechanism for variable reflectance is shown on page 162 of the aforementioned book.

    The Kezerashvili paper includes an interesting calculation for teh distortion of the sail membrane under solar and desorption forces. The deformation is smaller with teh higher pressure in the torus. This suggests that as with airships, the internal pressure of the torus need not be fixed, allowing the sail curvature to ve focussed. This indicates that it could be used as a focussing reflector for communication, as long as there is some solar pressure on the sail. Hanging the transmitter behind the sail at the focus would be one way to communicate back from deep space. While desorption is the chosen method to generate thrust at perihelion, the sail could also be used as a solar mirror to heat propellant. While the Isp is lower than the desorption and more complex, it is much more controllable and can be used intermittently, rather than in a single all-or-nothing burst.

    If the inflatable torus is a particularly efficient way to deploy the sail, I would hope the authors of a previous post on a sail using aerographite might rework their calculations with such a structure, or alternatively, is aerographite a better “sponge” to hold the dopant for thrust.

    Lastly, whatever the application, I hope we see far more inflatable (and folding – e.g. JWST) structures in space. They can help free us from the “tyranny of the diameter of the launcher payload bay” in determining the size of structures that can be launched.

    • Mike Serfas August 15, 2020, 10:33

      I have the impression that the central mystery of rockets is in the nozzle, and it’s remarkable that this system doesn’t have one. I suppose that because we are speaking of thermal energy, the aiming of the ions in any dimension but the one away from the sail is actually irrelevant, a different degree of freedom, except in terms of how much heat total needs to be absorbed to complete the desorption. Still, the desorption must not go in the wrong direction in the one dimension you care about, so a loose structure like aerographite would need at least to have a solid backing.

      Carbon materials seem like they could be interesting. For example, nanotubes can store at least 5.5% of their total weight in hydrogen at room temperature ( https://www.chemistryworld.com/news/carbon-nanotubes-for-hydrogen-storage/3000742.article ). But though H2 is a quarter the weight of Be, with presumably double the velocity/Isp for the same thermal energy, I presume that 5.5% figure is for 1 atm, and I imagine you could put much more Be on a sail than that anyway.

      • Alex Tolley August 16, 2020, 14:56

        so a loose structure like aerographite would need at least to have a solid backing.

        You may be correct, although a 0.5-1mm thickness aerographite sail may be dense enough to prevent thermally propelled atoms/molecules to escape from the front of the sail rather than just the rear. It would depend on how much the aerographite mimicked the carbon fabric sail in the Benfords’ experiment. It is a similar problem to doping a sail with fissile material to generate extra thrust from expelled particles e.g. neutrons.

        I am not a fan of this doping approach where emission is due to a close solar perihelion as it is uncontrolled, like an SRB. At least with beaming the emissions can be turned on and off. I see this approach most useful for a scatter shot of many sails, where randomness of exact flight direction can be tolerated as the swarm of sails will cover the target area, with hopefully a few being on target, like dandelion seeds floating in local wind gusts. For larger sails, this doping approach may be a lot cheaper than providing beamed energy, assuming the cost of a sun diver delivery is cheaper and easier than just accelerating them using phased microwave beams for sails in Earth orbit. Alternatively, particle beams that are trapped or bounced of the sails might be a viable approach. None of the possible methods need be exclusive either, with different approaches all being used at different stages of the flight.

        • Michael Fidler August 16, 2020, 22:48

          This could have large implications for laser beaming.

          New class of laser beam doesn’t follow normal laws of refraction.

          “The beams, known as spacetime wave packets, follow different rules when they refract, that is when they pass through different materials. Normally, light slows down when it travels into a denser material.’
          “In contrast, spacetime wave packets can be arranged to behave in the usual manner, to not change speed at all, or even to anomalously speed up in denser materials,” Abouraddy says. “As such, these pulses of light can arrive at different points in space at the same time.”

          But there may be further possibilities!

          Yessenov’s roles included data analysis, derivations and simulations. He says he became interested in the work by wanting to explore more about entanglement, which in quantum systems is when two well-separated objects still have a relation to each other.

          https://phys.org/news/2020-08-class-laser-doesnt-laws-refraction.html

      • Paul Gilster August 16, 2020, 20:24

        On the topic of nanotubes, Dr. Kezerashvili discussed these in an earlier paper, as I just learned:

        “Today’s discoveries of strong and light carbon-based materials as graphene and carbon nanotubes can make the additional avenue to consider thermal desorption as a propulsion mechanism, since their structure seems to be ideal for surface area in absorption/desorption in addition to being lightweight and having an ultra-high threshold for tensile stress. Carbon nanotubes are ideal for absorption and desorption but have very low reflective capabilities. Desorption from carbon nanotubes surfaces at low surface coverages is characterized by high desorption temperatures [33]. Recently it was reported that carbon nanotubes can be transformed from absorber to reflective mirror when the carbon nanotubes in the forests are mechanically bent and flattened with proper control [34]. This shows carbon nanotubes’ potential application for producing monolithically integrated reflector–desorpter–absorber arrays in the material. …”

  • Neil Stahl August 15, 2020, 23:21

    Variations (errors) in orbit, even very small ones and in when/just where desorbtion starts and how fast it progresses will be magnified enormously by the speed it’s traveling at perihelion by the sun and the radial distance it will then travel. That means this will be good for projects where distance from the sun matters but it will be of no use at all for travel to a specific object or place. But I’m sure there are several of the first kind of projects. This should be doable in a few decades. Go for it!

  • Gregory Benford August 16, 2020, 13:17

    There’s been little done on desorption experiment since, though some calculations. There was an inept German experiment that dropped the sail so oould do a few-second experiment–which got chaotic results, of course.
    But the main issue this hoop sail faces is, it’s unstable in a beam. In near-sun Oberth-style, it might well tumble–in part because, the pressure of solar plasma streams may well be the bigger, vagrant force.

  • James Benford August 16, 2020, 15:33

    I’m pleased to the Roman Kezerashvili Group developing desorption propulsion of sails concepts. Greg and I published the basic ideas back in 2002, but and no one seemed to notice for quite a while until Roman took it up starting in 2015. I’d like to see it explored further, of course.

    One comment on the toroidal sail concept, his latest work on this: A flat plane sail is unstable against rotation. The rotation might be due to any nonuniformity, perhaps nonuniformity of desorption. Think of it this way: if you look at the plane sail in its plane and imagine it starting to rotate, say counterclockwise, as it rotates the solar pressure will drop going to zero when it rotates 90°. That isn’t true of desorption, because the sail material stays hot and therefore desorption continues. Then the forces are no longer aligned at all. So at 90° there is only desorption and the sail keeps rotating. It will continue to rotate. One can think of spinning it to stabilize against these perturbations. You can also shape the sail, perhaps into a cone. We’re addressing the stability questions in the Starshot program now, not in the context of desorption, but simply photon reflection.

    • Harold Shaw August 16, 2020, 18:24

      Could suspending the payload behind the sail prevent or limit tumbling? Perhaps use at least three lines to connect the payload and take in and let out lines to control sail angle.

      • Gregory Benford August 17, 2020, 18:16

        YES, parachutes & loads are stable for that reason–mass center always below pressure center.

  • Andrew Palfreyman August 16, 2020, 16:53

    All sorts of possibilities open up to us, once it becomes economical to build powered structures on the Moon. And, thanks to Musk, the time is fast approaching when we can realistically aspire to GW-scale phased laser arrays on the Moon for the purposes of directed beam remote propulsion all over the solar system. The fly in the ointment then becomes energy storage. Musk again with his Tesla battery packs?

  • James Benford August 17, 2020, 12:33

    Yes, Harold Shaw, tethering the payload below the sail will stabilize it. That’s sometimes referred to as a “parachute sail”, because that’s how parachutes work. The basic requirement of stability is that the center of force be above the center of mass. In this case the force on the sail is solar pressure and desorption. The center of mass will be between the sail and the payload. Therefore it’s stable. Note that rockets are fundamentally unstable and have to have stabilization. That’s because the center of force is at the base of the rocket but the center of mass is above it. We’ve all seen the pinwheeling rockets of the early space program!

    • Alex Tolley August 17, 2020, 19:41

      Which is likely why Godard mounted the rocket engine well above the fuel and oxidizer in his famous early test.

      With regard sails, would a hollow spherical sail with the payload mounted against, but inside, the hollow shell, also be stable, and fairly resistant to spin forces? Might this also apply to a convex sail with the outer edge forward of the center, similar to a cone with the apex to the rear? In both cases, the CoG is behind the center of force. While the Breakthrough Starshot sail is very lightweight with a very lightweight payload and huge force applied to sail, this approach is very difficult. But for large sails, with payloads and far lower forces acting on teh sails, would this approach be a viable solution to maintain stability, yet also protect the payload from the heat at perihelion, or from more modest beaming?

  • James Essig August 17, 2020, 17:21

    Solutions of tantalum-hafnium-carbide might be useful for sails provided the sails could be highly polished and made extremely reflective. Some sources quote a melting point of one solution at 7,610 F. Other conjectures suggest a related compound may have melting point of about 8,000 F.

    In the search for more refractory sail materials, there will be practical additional applications for space travel and unrelated industries.

    For example, a highly elongated polished conical shield for a starship might enable spacecraft Lorentz factors as great as 1,500 where made of tantalum-hafnium-carbide.

    Of course mechanisms to prevent erosion by massive species would be required. Some potential remedies would forwardly emit rf and microwave beams that would be blue-shifted with respect to the background to ultraviolet frequencies thus enabling a source of radiation to ionized neutral matter so that it may be electrodynamically diverted around the shield. The plasma generated might also serve as a reaction mass for electrodynamic-hydrodynamic-plasma-drives. The energy supply could be obtained by the beam to energized the above drives.

  • James Benford August 18, 2020, 12:37

    Alex, both of the sail shapes do suggest would be stable. However, they have deficiencies. A hollow spherical sail illuminated from the bottom, where the payload is, would have some of the desorption shooting out transversly instead of accelerating in the desired direction, simply because of the shape. So some thrust would be wasted. The convex sail with V-shaped cross section would have forces that would tend to collapse the sail onto itself, resulting in reduced thrust. So that would mean that the sail would have to be rigid against the desorption forces collapsing at. Indeed, the Starshot sail is a very difficult prospect. We’re dealing with it with very advanced materials with complex surfaces that enhance stability by producing a restoring force if the sail moves off the beam or vice versa.

  • Michael August 20, 2020, 4:38

    The beauty of the nano fabric is that wells can be grown into them across the surface which can be filled with material, perhaps reflective, that could be heated and ejected to generate thrust. Prehaps two lasers could be used, one to heat the material to form thrust and a second tuned to the ejected material to throw it back at it.

  • ljk August 21, 2020, 14:11

    August 20, 2020

    Upcoming space mission to test Purdue-developed drag sail pulling rocket back to Earth

    WEST LAFAYETTE, Ind. — A rocket is going up into space with a drag sail. The goal? For the drag sail to bring the rocket back to Earth, preventing it from becoming like the thousands of pieces of space junk in Earth’s lower orbit.

    The drag sail, developed by Purdue University engineers, will be on board a Firefly Aerospace rocket expected to launch in November from Vandenberg Air Force Base in California.

    This sail and six other “Dedicated Research and Education Accelerator Mission” (DREAM) payloads are flying on Firefly Aerospace’s Alpha launch, the first flight for the launch vehicle company.

    https://www.purdue.edu/newsroom/releases/2020/Q3/upcoming-space-mission-to-test-purdue-developed-drag-sail-pulling-rocket-back-to-earth.html

  • ljk August 25, 2020, 9:15

    Could carbon-foam probes bring interstellar flight within reach?

    By Charles Q. Choi a day ago

    Solar-sailing ‘bubblecraft’ could get to Proxima Centauri in 185 years, a study suggests.

    https://www.space.com/interstellar-spacecraft-carbon-foam-alpha-centauri.html

    To quote:

    The $100 million Breakthrough Starshot initiative, which was announced in 2016, aims to launch swarms of microchip-size spacecraft to Alpha Centauri, each of them sporting extraordinarily thin, incredibly reflective sails. The plan has these “starchips” flying at up to 20% the speed of light, reaching Alpha Centauri in about 20 years.

    A drawback of the Starshot project is that it requires the most powerful laser array ever built to propel the starchips outward. Not only does the technology to build this array currently not exist, the project’s estimated total costs may reach $5 billion to $10 billion.

    In the new study, astrophysicists suggested that a cheaper option could involve bubbles made of carbon foam. Probes made of this stuff could make interstellar journeys faster than any rocket while powered solely by sunlight, without the need for a giant laser array, the researchers found.

  • Roman Kezerashcili August 25, 2020, 14:19

    I would like to thank all participants of these discussions. I greatly appreciate your feedback, valuable suggestions and opinions. All these will be considered in our future research. I strongly believe that experimental work that has been pioneered by Gregory Benford and James Benford should be continued. In my opinion the experimental study should be extended by using a laser beam and light carbon-based materials such as graphene and carbon nanotubes (nanotubes forest) to create an additional avenue for the consideration of thermal desorption (TD) as a propulsion mechanism. Even for the Breakthrough Initiatives Project Starshot, the thermal desorption induced by the laser beam can add a small but additional acceleration to increase the final speed and therefore decrease the flight time. The carbon nanotubes filled by a material that undergoes TD at a particular temperature can serve as nozzles making the sail surface a jet plate [Acta Astronautica 117, 231, 2015]. Moreover, one may consider two stages for acceleration due to thermal desorption: at the first stage thermal desorption of the material which fills the nanotubes occurs at a lower temperature of desorption than carbon nanotubes. During the second, the thermal desorption of carbon nanotubes from the reflective surface of the sail occurs at a temperature that corresponds to TD of carbon nanotubes.
    Another important discussion point is the stability of the torus-shaped solar sail. In our article we addressed only the vibration of the membrane and its influence on the stability. However, during the introduction of inflation pressure into the toroidal rim, the torus will also undergo a vibration that affects the stability of the sail. We are planning, based on the extremum energy principle, to study the vibration modes of the toroidal rim when it is deployed by the gas inflated into the toroidal structure. This research will be extended to study the vibration of the membrane and toroidal rim together as one system. This is a complex mathematical problem.
    Other issues are the application of optimal control programs to the solar sail during and after desorption, refined modeling of the spacecraft movement under the influence of this control, and the dependence of the sail design parameters on the main mission ballistic parameters.
    It is worth mentioning one other issue related to a degradation of the solar sail: we plan to develop a physically based model of solar-sail degradation that takes into account the substrate and reflecting surface material, the radiation dose received, and temperature changes.

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