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 00:06 Alan DiCenzo Posted on: Breakthrough Initiatives

G-forces during thrusting: Did I understand correctly that the craft will reach 0.2c in 10 minutes? Under Newtonian mechanics, it takes about a year to reach c at 1 g, so the craft and sail will experience roughly around 8,000 g continuously for 10 minutes (average, NOT peak g!). Can the craft and sail withstand these g-forces?

Apr 13, 2016 00:24 Alan DiCenzo Posted on: Breakthrough Initiatives

(For comparison: I believe I heard once that when you die in a plane crash, you experience something like 300 g of force at the moment of impact, a split second, so 8,000 g for a solid 10 minutes could present a serious problem...)

Apr 13, 2016 01:11 Andrew Higgins Posted on: Breakthrough Initiatives

If you drop your iPhone from shoulder height on a hard surface, and it still works, that is about 1000 g of acceleration (well, deceleration) on impact.

Modern artillery shells, with on-board GPS and laser-guidance optics, can survived 10,000's of g, comparable to what is required of Starshot. We have the ability to g-harden electronics to withstand the 10,000's of g required. This is not a showstopper.

Apr 13, 2016 01:13 Andrew Higgins Posted on: Breakthrough Initiatives

The fact that the acceleration is continuous for Starshot is not a problem. Most damage to materials comes from the rate of application and removal of the force, not the total force applied.

Apr 13, 2016 02:41 Tom Bonofiglio Posted on: Breakthrough Initiatives

If the whole ship, i.e. sail/structure combination were experiencing 8000g simultaneously, as in a gravity field, integrity may not be a problem, but the sail must transfer most of the 8000g to the structure. Assuming the sail is at least a few orders of magnitude thinner than the rest of the structure, and therefore of much lower mass per surface area, the acceleration of the structure by itself will be tend to be much lower than that of the sail. How do you keep the sail or its connection to the structure from failing?

Apr 13, 2016 03:52 Andrew Higgins Posted on: Breakthrough Initiatives

@ Tom Bonofiglio: Possible solution would be high tensile strength fibers (carbon fibers, CNTs, etc.) will be integrated into sail to transfer load from the chip spacecraft to the rest of the sail.

Apr 13, 2016 04:54 Andrew Walker Posted on: Breakthrough Initiatives

A spacecraft weighing ~1 gram would have to withstand a quasi-static "thrust" load of ~18 lbf to sustain 8k g acceleration.

Dynamic and centrifugal loading would also have to be considered. Here's a good summary of NASA structural stress analysis standards:
http://llis.nasa.gov/lesson/819

Apr 13, 2016 15:07 John McLean Posted on: Breakthrough Initiatives

Please don't mix units. Let's go with SI units (or metric at least). 1g = 10^-3 kg. 8,000 G is approx 80,000 ms^-2. F=ma so F=80 Newtons. Not a huge force for even fine wires.

Apr 13, 2016 16:39 Andrew Walker Posted on: Breakthrough Initiatives

It will be very challenging to design a 1 gram sail structure stiff enough (or spinning fast enough) to resist 80 N with minimal deflection. That discussion belongs under the Lightsail-Structure topic however.

Apr 13, 2016 23:32 alex dale Posted on: Breakthrough Initiatives

I dont see the acceleration of the whole light sail as a problem however, the connection between the sail and the payload needs to be thought about.
during acceleration, if the payload is not subject to the same acceleration force the infrastructure holding the spacecraft together will be under strain. which has two possible problems. firstly if the holding the payload to the sail while accelerating but as the propelling laser is turned off you don't want the connecting structure to elastically snap back and shoot the payload forward through the sail!!

because 1 gram under 8,000G of acceleration is about 78N. or the equivalent of hanging 770 grams from a string. any amount of elasticity could be problematic.

I hope I made sense! its a little hard to re read my post with such a small text window!!

LLAP
Alex

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