Sails
by Solar Max Borghard
INTRODUCTION
Solar sails are a low thrust spacecraft that are driven by photons emitted from the sun. Unlike rockets that rely on mass being diffused to create thrust, solar sails are accelerated by the bombardment of photons on its huge reflective sails.
Solar sail are still in the experimental stage. Most of the development in the US is taking place in the private sector. Solar sails were first proposed by Johannes Kepler in the seventeenth century and then again in the 1920s by Friedrich Zander.
BASICS OF SOLAR SAILING A solar sail is a low thrust spacecraft
with a huge reflective driven by photons.
Photons obey physical laws of
physics and do carry momentum. This is known as solar pressure.
Solar pressure is different from solar
wind. Solar wind is when highly charged electrons and protons emitted by the sun. The forces of solar pressure are about 5000 times great than solar wind.
SOLAR PRESSURE Solar pressure was discovered by
James Clerk Maxwell (1831 1879).
Solar pressure was confirmed by
NASA when they tested for the presence of solar pressure. NASA released small metal needles into the ionosphere of the earths atmosphere.
When the needles fell back to earth, it would showed the presence of solar pressure.
Solar pressure was also utilized by the
spacecraft Mariner 10 which did a flyby of Venus and Mercury in 1973. The spacecraft used solar pressure to help control the orientation of Mariner 10 by the use of vanes. By tilting the vanes at different angles, scientists were able to
rotate the Mariner 10 on its axis without knocking it off its trajectory.
DESIGNING AND BUILDING OF THE SAIL The sail needs to be highly reflective
to help achieve an acceptable level of acceleration of 1.0 mm/sec.
Aluminum has a reflective rate of
about 85%. Aluminum by itself would
wrinkle easily. Aluminum could be sprayed onto a backing material to help prevent wrinkles and provide stiffness to the sail.
Thickness is an important factor. If
the sail is too thin, the photons can travel through the sail. The thicker the sail, the more mass is added decreasing the overall acceleration.
Durability is needed for when the sail
being folded during manufacturing and unfolded during employment. The sail also needs to withstand meteorites ripping through the sail. The sail would be reinforced with rip stops to prevent small rips becoming large rips.
New materials have been
developed by companies like DuPont
for the use of sails. Mylar is highly reflective but has low resistance to ultraviolet light, which makes not suitable for long term space travel. Kapton is however a good choice of material for a solar sail with a large service temperature range (-270 C to 400C) and long term durability to radiation.
TYPES OF THE SOLAR SAILS There are several factors to be
considered when designing the solar sail. Designing a method of controlling and steering the craft. Designing a sail that can hold its shape during its mission. Designing a method of deployment of the sail.
The simplest design is the disk sail. It
has a main mast with support wires out to the edges of the sail. To keep the sail rigid, the whole craft spins to generate a centrifugal force.
However, deployment of such a large round sail is quite challenging.
A second type is the square sail
which refers to the sail being supported by a frame of booms called spars. The sail can be designed to be any shape. The deployment would be rather simple with the framework first unfolding like an umbrella. The sail would then be
pulled open first in the X direction and then in the Y direction. Because the sail is kept rigid by the framework, solar sail doesnt need to be spun. To control the sail, the use of vanes or smaller sails on the outer edge of the sail would help with the orientation of the sail. A drawback to this design is the use of the framework that drives up the mass of the craft.
A third type of solar sails is the
heliogyro sail that has several long blades like a helicopter and spins to keep the sails rigid like the disk sail. There could be 12 blades and each blade would about 10 meters wide and 7500 meters long. This test size sail would be big enough to carry a payload of 10 kg and would have a total payload of 100 kg.
A 16 blade heliogyro solar sail provided by NASA
The design of the heliogyro sail has two key advantages, which make solar
sailing possible. First, since there is no need for a framework structure because of the spinning motion, this helps keep the overall weight of the sail down. This design provides the sail so it can be controlled by pitching the blades at different angles to steer. This design would put the solar sail at 8 grams/m and allow for an acceleration of 1 mm/sec.
FULL SCALE NUMBERS Payload at 1000 kg, sail at 7000 kg for overall mass 8000 kg. With technological advancements, a sail at 5 grams/m would yield an acceleration of 1.6 mm/sec.
Example for a Heliogyro Sail
A 6000 kg payload would require a sail with 12 blades to have a blade size of 33 km and 10 meters wide to achieve the 1 mm/sec. That would put the gross load of the craft at 36,000 kg!
STEERING THROUGH SPACE Solar sailing is much like sailing on
water. Solar sails can change its trajectory by changing the angle of its sails but change is slow and therefore the adjustments have to be quite calculated.
Solar sails have an orbital velocity,
mostly about the sun. If the orbital
velocity is decreased, the orbit will inward move and if increased, it will move outward. The rate at which these velocities are changed can be dictated by the angle at which the sail is facing the sun.
The amount of energy depends on
how far the sail is to the sun with more energy being delivered as the sail is closer to the sun. At earth, the sun
delivers 1.4 kilowatts/m, which is called the solar constant. This converts to about 9 Newtons of force.
Velocity can be added to the solar
sail by making use of the gravitational pull of the sun, planets and moons. This slingshot method can be calculated to add speed and give a new desired direction to the sail.
When traveling in space, you need
at least six measurements to know where you are and how fast you are going. Three measurements of position and three of velocity. The more measurements taking the more accurate the reading. By smoothing out the readings, known as filtering, reduces the error even further.
With the development of radar,
scientists can find the position of an object, which is called ranging. An electromagnetic signal is sent out and the amount of time is measured for the return signal. With electromagnetic waves traveling at the speed of light, the distance can easily be calculated.
QUESTION: If a radar signal is sent out to a solar sail and the return signal is heard 10 minutes later, how far was the solar sail when the signal bounced off the sail? ANSWER: Rate x Time = Distance Rate of signal is 3 x 10 m/s Time is 10 minutes = 600 seconds (3 x 10 m/s) x 600 sec = 1.8 x 10 which is divided by 2 because of the round trip of the signal which makes it 9 x 10 meters or 90 million km.
To find the velocity of an object in
space, scientists make use of the Doppler Effect that was discovered by Christian Doppler in 1842. What he discovered is when an object that is emitting a signal that is moving either toward or away from you, it will have a change in frequency in that signal. From that change in frequency, the velocity of that object can be
calculated. As an object is moving away, the frequency will become lower which is known as a redshift and as an object is moving closer, its frequency will become higher and that is known as a blueshift.
QUESTION: If the expected frequency of signal coming from a solar sail that is moving away from earth is 6.9 x 10 cycles/sec and the frequency received is 6.89931 x 10 cycles/sec, what is the velocity of the solar sail? ANSWER: (read) = (emitted) (1 V/c) c = c x 10 V = 3 x 10 m/s m/s [1 (6.88931 x 10 / 6.9 x10) ]
V = 30,000 m/s
When these two measuring
techniques are used in coordination, the accuracy of the position and velocity is remarkably high. The error of these measurements is less than one billionth and can be reduced even further with filtering. With solar sailing corrections need to be made constantly.
ADVANTAGES OF SOLARS SAILS There are major advantages of the
solar sail as compared to the conventional fuel rockets. The solar sail can be a reusable spacecraft much like the space shuttle but would not have to be rocketed back to space each time. It will have the capability to remain in the Earths orbit until it would be serviced and loaded up for its next
mission. The payload would need to be brought to the solar sail via the space shuttle but there w...