Lightsail | Integrity under thrust

To inform the study, a beamer in the 100 GW class was considered. If, for example, 10-5 of the energy is absorbed by a 4mx4m sail, it will be heated by about 60kW per m2, which is roughly 60 times more than sunlight illumination on Earth. This will heat the material but not melt it. Using fully dielectric sails, we can reduce the absorption to less than 10-9 for optimized materials.

Two possible approaches to mitigate the heating challenge have been identified:

1. High reflectivity

Use a material with better than 99.999% reflectivity. Usually, highly reflective surfaces are dielectric mirrors, which are composed of ‘sandwiches’ of material, with each layer reflecting back a modest fraction of the total. Each layer needs to be at least a quarter of a wavelength thick. The weight can be reduced by using a monolayer with high reflectivity at the correct wavelength. Based on recent research, this could be achieved by a ‘hole-pocked’ layer, highly reflective for very specific angles where reflectivity caustics arise. (These caustics occur for wavelengths of light that are actually longer than the sheet thickness.) Adding the holes serves a dual purpose; it reduces the weight of the sail and it could greatly increase reflectivity. This is but one possibility being explored. Modern materials research will explore new materials such as graphene; Breakthrough Starshot aims to take advantage of this rapidly advancing field. The basic Starshot system allows a wide range of options for nanocraft masses and capabilities, all using the same array. This gives it great flexibility in optimizing the science and technology roadmaps.

2. Low absorption

Use a material (such as glass) that has a very low absorption coefficient even when not highly reflective. Such materials are used in fiber optics systems with high power applications. Without the protection of a highly reflective sail, the StarChip electronics would need to be protected from the incoming flux. But this could be accomplished by a combination of geometry (orienting the electronics ‘sideways’ with a low cross-section) and placing a very highly reflective coating only on the sensitive components. These can use the multi-layer dielectric approaches mentioned above, which have already been demonstrated in the lab. Using low absorption sail material, together with a limited use of high-reflectivity shielding for critical electronics, would protect the StarChip without increasing its mass beyond the gram scale. There are a number of high-reflectivity, low absorption materials in existence. For possible fabrication and verification, a demonstrable design of silicon microcubes on a silicon dioxide substrate is under consideration.

As demonstrated by the Japanese IKAROS mission, spinning the sail can reduce wrinkles on its surface. Special attention is needed to avoid impurities and non- uniformities in the sail composition - for example, near mechanical attachments - or accumulation of dust particles on its surface, which could otherwise lead to a localized deposition of energy. There are a wide variety of options allowing optimization of the sail design.

Apr 13, 2016 23:52 Daniele Proietti Posted on: Breakthrough Initiatives

Solution is quite simple:
The energy will be gradually increased up to the maximum.

Apr 13, 2016 23:58 Tim Peery Posted on: Breakthrough Initiatives

Did I miss the reasoning for such a short, intense time of acceleration? Why not take more time and decrease power and all related requirements?

Apr 14, 2016 00:35 Tim Peery Posted on: Breakthrough Initiatives

It seems nearly impossible to center the beam on the sail sufficiently to avoid a moment, which would rotate the sail away from the beam, and cause lateral forces that will send it off course. So wash the entire sail with uniform intensity?

Apr 14, 2016 00:38 Tim Peery Posted on: Breakthrough Initiatives

Spinning the sail could maintain attitude as well as stretch out the sail, but i doubt it would be sufficient against 80N even slightly off center.

Apr 14, 2016 02:27 Karen Pease Posted on: Breakthrough Initiatives

@tim:

You have to accelerate quickly because your beam quickly loses intensity with distance, and your craft gets up to *very* fast speeds.

Concerning the issue here: graphene is nice but it's one of those "some day...." technologies when it comes to high strength materials, just as carbon nanotube ropes before them. In the real world, your best bet would probably be M5 fiber (polyhydroquinone-diimidazopyridine). Heat tolerant and superb tensile strength (if I recall correctly, 7,2GPa, and they think they can get it even higher). Another good option would be Zylon (PBO).

If you propose to use a materials technology that isn't even close to commercialization, you're mostly engaging in fantasy.

Apr 15, 2016 12:22 Luka Marinovic Posted on: Breakthrough Initiatives

As Pete Worden suggestion, I'm posting my suggestion for a Light beam here:

1. You don't need a power for laser, 'cause we already have it! It's the Sun!
Just check the movie "Diamonds are forever": https://www.youtube.com/watch?v=xt_Kn4DggPg
In essence, a big enough Sun collector that can focus a beam can be used as a laser. Just don't turn it towards the Earth this time!
No - just joking, you can use it for:
a) moving asteroids from their trajectory, preferably farther from Earth! (plan has been made with NASA, so you can also collaborate on that research to make a powerful enough laser). Maybe NASA would like to fund such lasers for a protection of the Earth?!
b) power up the probe to Alpha centauri
You certainly wont get out of power soon, by using electrical power to power up lasers...nor will you have problem with power delivery! Just design the Sun-collector to be Solar storm proof...

2. If you manage to design such a laser, than a solar sail will have to absorb the wavelength of the Sun from the laser. It would also have the additional power from the Sun (at least up to Jupiter orbit), depending on the scale of the Solar sail. But, as the Alpha centauri 1 is similar in wavelength of our Sun, the Solar sail can be used as a:
a) Solar parachute into the Alpha centauri system - maybe not to slow down the probe much to enter the system (but why not, maybe even that trajectory can be obtained?!), but at least to slow it down to get more time for observation & more time to get pictures from another star system.
b) Can also be used to steer the probe around some objects in inner Alpha centauri system. Because it would be bad if a probe comes to Alpha centauri & crashes on planet similar to Uranus or Neptune...

Hope that the ideas interest you...

Apr 17, 2016 06:39 john.hayden1@gmail.com Posted on: Breakthrough Initiatives

Braking at the far end? Not so easy.
Under thrust, intensity is 100Gw/16m2 = 6GW/m2. Compared to sunlight (irradience 1.36kW/m2) this is a factor of about 4M, is that correct?
Then as irradience flux varies as 1/r2, this means our thrust is equivalent to sunlight at 1/sqrt(4M) = 1/2000 AU, or about 50K miles. At the far end, that's deep within the target star. So you couldn't fly the spacecraft close enough to the star to slow it down for orbital capture without actually diving into the star.

Apr 17, 2016 15:43 Karen Pease Posted on: Breakthrough Initiatives

@Luka Marinovic

As pointed out to you elsewhere on this site, the sun is not a laser, and cannot just be turned into a laser. Sunlight diverges far too fast for this purpose, and it's a fundamental limit of optics that can't be overcome; you can't take light from a broad disc and somehow, with a combination of mirrors and lenses, make it non-divergent.

Apr 18, 2016 21:13 george.dishman@talktalk.net Posted on: Breakthrough Initiatives

"If, for example, 10^-5 of the energy is absorbed by a 4mx4m sail, it will be heated by about 60kW per m^2, ... This will heat the material but not melt it."

Assuming the back of the sail is a perfect black body and hence radiating at the maximum possible rate, the Stefan-Boltzmann Law requires the sail temperature to be 1015K. The usual choice of reflector is Aluminium but that melts at 933K so the statement appears more wishful thinking than engeering.

The "solutions" mentioned make little sense. The reflectivity is already unrealistically high so looking for better than 10^-5 absorption is not a "solution". "Low absorption" would let the unreflected component of the incident light pass through but it also precludes any emissive rear coating so how is whatever energy is absorbed going to be removed?

The proper solution here is to use a much lower acceleration for a longer time, the sail needs to maintain itself centralised in the beam which can be automatic if the beam profile has a lower intensity region in the centre.

The challenge for the laser then becomes achieving an adequately high Rayleigh Length, and that is just a question of having a large enough aperture.

Using a lower thrust for a longer duration also makes the question of sail integrity much more tractable.

Apr 20, 2016 02:05 Steve Harris Posted on: Breakthrough Initiatives

Okay, simple physics:

To get to 20% c in 600 seconds you need acceleration of 100,000 m/sec^2 or a = ~ 10,000 g. Fine.

The force F available from 60 kw is only 2P/c = 4e-4 N.

If you want a=100,000 m/sec^2 from F = 4e-4 N, you need a mass of F/a = 4e-9 kg = 4e-6 grams.

So this square meter of sail has a mass of 4 MICROGRAMS. This is not a "gram-class" probe. Even for 100 m^2 it's a fraction of milligram for the whole thing, payload and all.

Tell me what I'm missing.

Steve Harris
sbharris1@earthlink.com

NB:
Somebody did. The 60 MW/m^2 is absorption by the sail and reflected power is assumed to be 10^5 times this, with 99.999% reflectivity. So it's 6 GW/m^2.

So the sail loading is not 4 mcg/m^2 but can be 0.4 g per m^2.

It's 6.4 grams mass for the whole 16 m^2 thing.

Okay, we're in the multi gram class, but a flexible dielectric sail that thin? This 0.4 grams per square meter is on the edge of what's possible with metals. This is only 0.2 micron or 200 nm thick for beryllium. Suggesting light won't get through a mirror this thin made of any other materials, gives a new meaning to "smoke and mirrors."

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