Launch | Keeping beam pointed on meter-scale lightsail

There are a number of effects that make this task difficult. These include beam instabilities, laser mode issues, differential forces on the sail, differential heating of the sail, and instabilities in the atmosphere induced by the energy of the beam.

The above challenges can be mitigated by spinning the sail, and by shaping both the sail and the beam. Feedback from the sail to the array helps, but the short time of flight requires a self-stabilizing system. The firing time of the beamer is on the order of 10 minutes. During this time the Earth rotates, requiring some beam agility on the order of 2 degrees. Coarse sub-element pointing and fine pointing via phase corrections at the array system can provide beam agility on the order of 30 degrees in order to accommodate various targets.

One promising approach is to shape the sail so that its position on the beam is stable— i.e. the spinning sail itself experiences torques and forces that restore its position and orientation as low-frequency pointing errors move the beam away from the sail’s centroid. High-frequency jitter degrades the overall power imparted to the sail, but the sail’s dynamics limit its susceptibility to such disturbances above a certain bandwidth.

Since a phased array would be used to form the spot, the beam profile could be shaped to maximize the sail’s ability to maintain its own position on the beam without active feedback control.

Comments (24)

  1. Nathan Bemis:

    RE: Apr 04, 2017 22:42 Breakthrough Initiatives

    Thanks again for being responsive to not only myself but all of us who post ideas here.

    The dimple idea comes from not only through fluid but through the air, Mythbusters tested the idea on a vehicle (video provided in main post) and it was successful. I'm not one to know the results of how it would work in a space environment. Which is why I pose the question here. I also try to come up with a solution to minimize the drag a sphere design would have from it's trailing side, perhaps a way to have the drag space occupied or some solution for it to negate drag. The ideas could be far off, and I'm far from knowing the science. Just posing some ideas from an average interested persons perspective.
    Personally I think a sphere design is worth the sacrifice in weight and time. I question how easy it can be to send off a flat target in comparison to a sphere. With a sphere, it doesn't matter how it moves/rotates it should stay on track and easily controlled. Plus then you can use the Coanda Effect with the beam onto the target/s. I know there are pros and cons to it. That goes with everything.

  2. Breakthrough Initiatives:

    Apr 22, 2017 17:54Nathan Bemis Posted on: Breakthrough Initiatives

    While there are certainly some analogies that can be made between fluid mechanics and the optics of the beam-sail system, at the end of the day they are quite different. The Coanda effect is fundamentally fluid-mechanical and doesn’t show up in our case.

    - Zac Manchester, Breakthrough Starshot

  3. Nathan Bemis:

    With a sphere (ish) design, what if you can manipulate the beam with a (*lens type effect) to wrap around the craft (maybe temporary film or shell around the craft?); to Also Pull the craft on the opposite side like a vacuum. I believe this can provide two main appreciative results, 1. Relieving much of the G-Force stress for structural integrity. 2. Provide more use of the beam during launch, ideally resulting with a noticeable improvement of the crafts cruising speed.
    A possible bonus effect of this idea is that the immediate path may be cleared of debris by the laser streaming ahead.

    The idea of a sphere design is for the launch phase stability and control challenge, not necessarily for the whole trip. Once the launch is complete it could be flattened or folded into a narrow shape and used as barrier from dust impacts for the craft, Then possibly redeployed to assist with stopping and/or making an eventual 180* U-turn to return when the project comes to developing that.

  4. Nathan Bemis:

    There is a concern of too much spin of the craft (at least with a sphere design), If the craft has a gyroscope on it, could it be enough to keep the spin limited?
    Perhaps you can modify the surface to allow better control to prevent overspin. There could be many ways to approach this, by the surface layer microscopically or by larger changes with dimples or otherwise.

    Microscopic surface modification example: A Diffractive Meta-Sailcraft -

    Larger scale modification Example:

    Related Article of interest:
    "Dr Yoshihiki Arita, Dr Michael Mazilu and Professor Kishan Dholakia of the School of Physics and Astronomy at the University of St Andrews were able to levitate and spin a microscopic sphere, purely using laser light in a vacuum, briefly up to 600 million RPM before it broke apart." - "The rotation rate is so fast that the angular acceleration at the sphere surface is 1 billion times that of gravity on the Earth surface– it's amazing that the centrifugal forces do not cause the sphere to disintegrate!"

    Encouraging preliminary results from the TVIW talks by Giancarlo Genta, Professor, Department of Mechanical and Aerospace Engineering, Polytechnic University of Turin, who spoke on the Breakthrough Starshot craft stability.
    His testing was basically to test stability, and was not including all factors expected. With positive results so far, he plans to continue testing and find the limits of what the beamed craft can do. Perhaps it may exceed expectations. I'm eagerly awaiting to hear his findings and/or solutions.

    On the dynamics of a massless beam with end mass rotating in the three-dimensional space
    Giancarlo Genta - October 18, 2012

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