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 (14)

  1. Breakthrough Initiatives:

    RE:
    "Sep 22, 2016 06:05 Nathan Bemis Posted on: Breakthrough Initiatives
    Edit: Some may skim through and miss some key points that are proposed.
    1. Proven techniques that may help in control of the craft.
    ..........."

    Answer:
    These are all good ideas. The biggest complications that you have not addressed is the material the sail is made from. It turns out that today the only material we can find that meets our requirements only meets them if the laser light is shown at the sail in a nearly perpendicular way. In other words, the beam needs to be normal to the sail. If we use a curved sail, then the performance of the material will decrease as you get away from the normal. It is not clear now what this means for the sail craft. It may destroy the sail. Significant additional work is required.

    - Avi Loeb, Breakthrough Starshot

  2. Nathan Bemis:

    Thank you for the reply,
    I figured the ideal material would come from the project team since they can easily figure out what is best. An idea I have is for the skeletal structure to be made of some kind of Aerogel and coated (inside and out) with the material that is necessary to work for the laser. Inlets/Outlets to capture and recycle the beam could be useful. I think this will relieve the gforce pressures that would normally crush the sail, and provide a way for excess energy to escape keeping the heat generated manageable. There has to be a way to have the craft keep its shape through its launch phase. My idea is with the beam encompassing around and through the craft, any opposing pressures will be voided and forced to keep its shape. It may be hard to find the balance to make the idea work though.

    If the aerogel is still too heavy to consider, Maybe it can be created in a vacuum type environment and have it sealed with helium or other suitable gas. (??)

    Instead of a disco ball as mentioned in the articles linked in the post, Perhaps another idea would be to use dimples around the craft. If dimples allow golf balls to achieve 2x speed and provide far more self-stability than smooth surface versions, perhaps the idea can be explored for this project. A reference for a company I believe can provide simulation testing cost efficiently is linked in the original post.

    I don't know what more I can add that is not in the original post. If anything, maybe the core idea can be improved upon by more capable and resourceful people.

  3. Breakthrough Initiatives:

    RE:
    "Dec 06, 2016 19:01 Nathan Bemis Posted on: Breakthrough Initiatives"

    Answer:
    Thanks for spending time thinking about this it is very much appreciated. Our team is small and there are many smart people not on the team that can help us and we appreciate your fine work.

    - Pete Klupar, Breakthrough Starshot

  4. Nathan Bemis:

    Sure, I don't know if many of the ideas as written are applicable but I do think there is something to the concept in some fashion. I don't have more to add to the main post. Maybe something can come out of it. Thanks.


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