July 08, 2015

Near term Solar sail technology for missions beyond Pluto

Here is a NASA study of the near term potential of solar sails.

The Sunjammer solar sail spacecraft will use 5 micron thick Kapton film as a sail material. An investigation into the materials available and determined that 5 micron Kapton and .9 micron Mylar are the two leading materials for this application. Mylar however does not survive radiation environments well and further study is required to determine whether this fully precludes the use of Mylar as a solar sail material.

Several manufacturers of Kapton (DuPont, 3M) said that 2 micron thick Kapton film is within manufacturing capability. We will see in a later section how this change affects Heliopause cruise times. The .5 micron thick Kapton film is theoretically possible but has not been significantly investigated because even 2 micron Kapton has yet to find a commercial application significant enough to justify the necessary modification to manufacturing facilities.

Another study will include CP-1, a potential successor to CP-1 known as CORIN™ XLS, and Thermalbright®x. CP-1 is a space-durable material developed at NASA Langley (LaRC) and exhibits a high resistance to UV radiation. It has currently been fabricated in large sheets and rolls to as thin as 1.5 micron. CP-1 has flown on Hughes HS-702 geosynchronous communications satellite. CORIN™ XLS is a potential next-generation CP-1. Thermalbright® polyimide is a high temperature highly reflective white polyimide film which is expected to be particularly beneficial for thermal control while maintaining good UV and VUV durability.

With these sail materials in mind, and an assumed spacecraft mass of 110kg, they modeled the transit time for sails with the different materials to 100AU. We also moved to a more realistic characteristic acceleration of .5 mm/s2. We chose two sail sizes to compare, the 250 meter x 250 meter size that is the limit of current techniques, and a hypothetical 500 meter x 500meter sail. Material choices are represented by the line color. We also estimated a scaling factor for the boom masses. They assumed two methods of boom mass scaling – linear (here called aggressive) and geometric (here called conservative). This factor is represented by the line style. Solid lines represent a conservative boom mass scaling factor, and dashed lines represent an aggressive boom mass scaling factor. The curve plots in figures 19 and 20 represent these results. Finally, vertical black lines represent the distance to the sun below which the listed material begins to degrade.

The below charts were derived from the initial spacecraft mass assumption of 110kg. When they completed their detailed spacecraft configuration, they concluded that a more realistic spacecraft mass is 175kg. The transit times were relatively insensitive to this change in spacecraft mass.

The estimated final mission cost for 10 long range (100AU) solar sail Spacecraft is $3.44 Billion.







Communications

Laser beams at optical frequencies can be transmitted with angular beam-widths of a few microradians. Coupled with the ability to accurately point narrow laser beams to a fraction of the beam-width, signal power densities required for communication can be delivered over huge distances. To illustrate this further a point design is presented below.

A 20 watt average power 1550 nm laser beam transmitted through a 50 cm diameter telescope will result in an angular beam-width of approximately 3.6 microrad. Pointing jitter control of ~ 0.3 microrad will result in losses relative to the on-axis peak of ~ 1.5 dB. If such a beam with the indicated pointing control were transmitted over a distance of 100 AU with a 12 m diameter collector at the receiving end a 500+/-100 bits/second communication link could be established.

On the ground the beam footprint from 100 AU would be approximately 4 Earth diameters, however, the irradiance would be a few femtowatts per square meter. A 12 meter or larger collector or an array of large collectors would be required to gather sufficient photons to overcome background and shot noise. In our preliminary analysis a 256 pixel superconducting nanowire array behind a 12 meter collector supported 500 +/- 100 bits/sec. Ground aperture would very likely utilize a conical scan to acquire and lock on the downlink signal. The use of adaptive optics and aggressive filtering to discriminate the faint signal against prevalent background light will be studied. The performance achieved with the latter would determine how close to the Sun the ground terminal can point and this would also determine the duration of possible outages. For comparison the capability achievable with an orbiting large aperture collector/receiver will be determined where shot-noise limited performance can be achieved.

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