Comment by jessriedel | Hacker News Reader

Comment by jessriedel | Hacker News Reader

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Agreed on the extreme skepticism warranted.

I am imagining these "solar sails" to be essentially spherical, both for structural reasons and so that their response to applied laser power does not depend on orientation. The electronics in precision guided mortars in the US army survive 50,000g.

http://proceedings.ndia.org/C347/Davis.pdf

(This is only for the fraction of a second when they are fired, and I don't know how it generalizes for longer durations.)

I conjecture that the laser profile will be "shaped", if at all possible, so the trajectory of the projectile is somewhat self-correcting: if the projectile starts to get ahead of the planned trajectory, the acceleration will decrease, while if it lags the acceleration will increase. This wouldn't require any realtime feedback by the laser emitter.

With many projectiles and a massive investment in a laser array, there will be some ability to repeatedly attempt this and iterate.

Still, this is extremely aggressive even many decades in the future.

EDIT: Their proposal confirms that the beam will need to be shaped for feedback. ("By modulating the beam modes, get greater uniformity could be attained, and feedback between the nanocrafts and the beamer array would allow real-time adjustments.") No details though.

http://breakthroughinitiatives.org/index.php?controller=Foru...

Rather than the spherical shape a supposed above, they propose a thin sail with some undefined graphene skeleton. No idea how that is supposed to survive 2-minute long explosive accelerations.

http://breakthroughinitiatives.org/index.php?controller=Foru...

F=ma

In this case m ~ 1 gram and a = 0.2c / 120 seconds =499654.097 m/s^2

F = 499654.097 m/s^2 * 1 gram = 500 newtons

However this force is distributed over the whole sail which has an area is 16 m^2, so pressure applied to the sail is 500 N/16 m^2 = 31.25 pascals. This is a fairly low pressure. The pressure change going up or down a couple meters is about equal to this. Your typical cheap garbage bags can withstand this pressure.

Yea, thanks. I think my mistake was in imagining that the sail would need to be pulling a localized payload, so that force would need to be transmitted from sail to payload using a cross-sectional area small compared to the sail. But it sounds more like the payload is integrated into the sail.

> 499654.097

Dude. 9 significant digits? You don't know m to better than 1 or 2 significant digits. Just call it 5e5 and be done with it.

It doesn't change the conclusion, but note that in a relativistic regime you can't use F=ma because the mass is no longer constant. You need F=dp/dt, where p is the relativistic momentum. The Lorentz factor here works out to 1.02, i.e. 510 N, not 500, so not a big effect if I got my maths right.

Spheres would cause a major issue - the laser would need to strike the centre of the circular visible surface.

Otherwise you'd end up with the light bouncing of at angles other than pi, which would direct the probe to the side.

A flat mirror coupled with crazy fast gyros (also powered by the laser) might be stable enough that the mirror holds directed toward the earth.

Would a slightly conical mirror with its apex pointed towards the laser be self-stabilizing in tilt? Kind of like an aircraft with dihedral wings [1] is self-stabilizing in roll - imagine the laser illumination as an analog of the aero lift vector on the wings. If it tilts wrt the laser, the asymmetric illuminated area should counter the tilt.

[1] https://en.wikipedia.org/wiki/Dihedral_(aeronautics)

An excellent idea. But then you have to worry about oscillation and damping.

The stabilization problem sounds really hard to me.

> Spheres would cause a major issue - the laser would need to strike the centre of the circular visible surface.

> Otherwise you'd end up with the light bouncing of at angles other than pi, which would direct the probe to the side.

No, because these lasers won't be focused to a point smaller than the size of the satellite. I'm not sure what the waist will be, but presumably it's much larger than the mirror.

The real reason these are sails is because they maximize the momentum transfer per unit mass. But unlike sails on boats, this force can't be transmitted to a displaced payload using tethers/struts, so the payload must be dispersed and integrated within the sail itself.

> The electronics in precision guided mortars in the US army survive 50,000g

And then are immediately 'disposed of'. That is, quite literally, a fire-and-forget problem; those electronics don't have to keep working for months or years.

Yes but they do have to keep working after those 50,000G. Which is the thing: provided they survive acceleration, the remaining problem is cosmic rays. But we're talking about such small craft here that we'll be able to launch literally hundreds, probably thousands - the real challenge is getting the 100GW laser array built. Once you have it...no reason not to use it.

Similar to XorNot, I don't really see the mechanism by which high acceleration could substantially reduce long-term reliability of the solid state electronics without breaking them completely.

I conjecture that the laser profile will be "shaped", if at all possible, so the trajectory of the projectile is somewhat self-correcting: if the projectile starts to get ahead of the planned trajectory, the acceleration will decrease, while if it lags the acceleration will increase.

This is analogous to the way train bogies go around corners.

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