For launching the satellites we have chosen the geosynchronous orbit. Although it is relatively costlier to put satellite into geosynchronous orbit, especially when the technology is an untried one, it is still much more profitable to put one into geosynchronous orbit rather than the less costly LEO orbit.

Orbit Selection

A single kilometer‐wide band of geosynchronous earth orbit experiences enough solar flux in one year (approximately 212 terawatt‐years) to nearly equal the amount of energy contained within all known recoverable conventional oil reserves on Earth today (approximately 250 TW‐yrs). However this is highly optimistic statement because a satellite can capture only a part of this at a time. But the above estimate is useful to gauge the magnitude of energy this band can provide.

Figure 1. GEO Example

A satellite in geosynchronous orbit spends over 99% of the time in sunlight. This is the case because the earth’s axis is tilted 23 degrees from the path it follows around the sun. As a result, the satellite passes above the shadow of the earth during summer in the northern hemisphere and below the shadow in the winter. It is this tilt of the earth’s axis that causes the change of seasons. As the earth progresses on its yearly trip around the sun, summer turns to autumn. While autumn leaves are falling, the days become shorter. The earth, in its flight around the sun, is starting to lean its axis away from the sun.

Figure 2. Orbital Tilt

During the autumnal equinox period, the earth’s axis is no longer tilted towards the sun, but rather forward in its path around the sun. During this time, day and night are the same length.

For twenty days before and after the equinox, a geosynchronous satellite passes through the earth’s shadow each night. The first night the satellite will be in shadow for a minute or two. The next night it will be in shadow a couple of minutes longer, and so on until the equinox, when the maximum amount of time in shadow is 72 minutes. Then the next twenty nights follow the same schedule, but in reverse. Within twenty days of the first day of fall, the satellite will pass south of the earth’s shadow and will not reenter until spring when the same phenomenon will repeat itself.

This is the only time when the satellite is in complete darkness and the solar power will not be delivered to earth and hence conventional backup will be required.

The great advantage of space solar power in geosynchronous orbits over land-based solar power is this continuous flow of sunlight with very little interruption.

In a year it adds up to five times more energy for each solar cell in space than if that same cell was placed in the Mojave Desert — and fifteen times more than if it was placed in an average location in the United States .

(Source:  The global solution for coming energy crisis –Ralph    Nansen).

The major hindrance in employing this technology is not the relative inexperience in the use of technology but the cost involved in deploying it. This is probably the major reason why it is also explored less even though it is feasible. USA alone has invested around $21billion in pursuing nuclear fusion compared to NASA and DOE investment of $80million in exploration of this option.

The major cost involved are launching and maintaining such a heavy and high volume mass into orbit. With the price of oil and other conventional source of energy reaching astronomical levels the cost incurred in putting such a facility in space is completely justified.

 

As explained in the patent by Peter Glaser, the inventor who put forth this idea, we come to know that the foreseeable major weight contributing factors in the satellite are following.

  • 10GW power using thin film PV that delivers maximum 16.8kW/Kg=595,238kg.
  • Weight due to mirrors ( assumed equal to weight of the PV array)
  • Weight due to DC to microwave converters-(we have selected Gyrotrons owing to their efficiency) 1600Kg/gyrotron with 90%efficiency and 10Mw (approx.) output. Therefore for a 10Gw+ facility we need more than 1600 metric tons to be put into space.
  • Weight due to antennas
  • Weight due to cryogenic facilities
  • Weight due to Superconducting wires
  • Weight due to supporting trusses and columns
  • Weight due to motors for attitude control, and station keeping fuel and machinery
  • Weight due to modulating circuitry and communication link related modules .

Estimating addition of all the components in the payload ,we calculated the payload size to around 4000 metric tons(minimum) to 8000 metric tons (maximum).

In addition to this we also need to launch the rocket and hence need the fuel tanks for same. The rocket weighs a lot too. Based just on the payload size that present generation rockets an carry, we get the following cost estimates for the rockets that can put satellites into GEO. Popular choices for types of rockets that can be used for the launches to these orbits:

Table 1. Rocket Options

(Source: www.futron.com)

This is the current scene in the field of rockets. All prices are reported do not include the costs of apogee kick motors or other payload injection means. These rockets will place the satellite in geosynchronous transfer orbit (elaborated in class ECE 6390 notes) as it is more economical and then used the motors to slowly elevate the satellite to GEO orbit.

However as the project statement says we have generation 1,2 and 3 rockets that should make the launches more economical. In the case of these rockets we can assume the following payload delivery capacity numbers per launch:

  • Generation 1 – 7000kg/launch
  • Generation 2 – 9000kg/launch
  • Generation 3 – 11500kg/launch

 

The assumed rate of increase in payload capacity per launch is supported by previous reported trends in the report by AIAA (Source:http://www.aiaa.org/aerospace/Article.cfm?issuetocid=31&ArchiveIssueID=7)

Also larger number of launches per year will bring down the cost of the satellite as shown in the following graph.

Figure 3. Flights per Year, Cost

Additionally Spacex claim to develop Falcon 3 heavy reusable will reduce the cost even further.

Four satellites (each servicing two earth stations) will be launched into orbit as demanded by the project. We need to set up 8 downlinks by 2026.

Assuming the above cost trends we adopt the following strategy for putting the 4 satellite orbits where we put the satellites into space in three stages. (note data above is $/lb and calculation are in ($/kg).

Table 3. Satellite Weight

However assuming 4000 tons/satellite seems highly optimistic. Assuming a higher payload weight of 8000 tons for each satellite we come with the following numbers

 

Table 4. Satellite Weight

We are aiming to put 4 satellites into space by 2026 that will provide an output of more than 10GW to offset the losses that will occur due to device efficiencies and due to transmission through atmosphere.  However the bigger goal should be providing satellite infrastructure for providing 50% of energy that earth needs in next 60-70 years.

However for putting total 16 downlinks by 2028 which is one of our other aim we will have to double the number of satellites that we put in next 16 years. The budget will roughly double the one given in above tables for both minimum and maximum payload sizes.

Each satellite has two antennas servicing two SSP on earth providing 5GW output.(output can reduce due to various losses in transmission and conversion)

The above cost estimates seems highly encouraging since the maximum cost in the most pessimistic case is only $3.4 billion/Yr(8 downlinks) to $6 billion/year(16 downlinks) and can be achieved as we are already soending roughly $1 billion for present missions

[source: UCS_satellite_database and internet].

However, we must realize that this is just the estimate for launching payload. The other massive additions to these costs are:

  • The cost per year for the maintenance and assembly of satellites. We already have come across data that estimates these costs to go up to $18billion/year in the initial stages. The satellites may need replacement or major maintenance every 30 years
  • The cost for designing payload on earth like manufacturing of Gyrotrons, PV array antennas using space certified metals as well as supporting structures for satellite is going to add a significant amount to the cost.
  • The cost for designing circuitry and electronics that can work in space will also be incurred.

Also the launching capabilities right now show that NASA alone has capability of launching 50-60 launches in a year. Combining launching capabilities of around 21 major satellites all over the world we still fall short of the required high launching capabilities that this project demands i.e. in thousands of launches by the time generation 3 rockets are deployed.

For achieving these capabilities more launching stations need to be constructed in coming years as well as the industry needs to develop support them. This also calls for newer launching strategies to be developed to support launching of infrastructure for providing 50% of energy on earth with SSP.

The strategy that is being aggressively pursued and that can help our cause is the “slingshot strategy”. In this an enormous ring of superconducting magnets similar to a particle accelerator could fling satellites into space.

 

Figure 4. Particle Accelerator

(source: http://www.launchpnt.com/portfolio/aerospace/satellite-launch-ring/  )

The satellite, encased in an aerodynamic, cone-shaped shell that would protect it from the intense heat of launch, would be attached to a sled designed to respond to the forces from the superconducting magnets.

When the sled had been accelerated to its top speed of 10 kilometres per second, laser and pyrotechnic devices would be used to separate the cone from the sled. Then, the cone would skid into a side tunnel, losing some speed due to friction with the tunnel’s walls.

The tunnel would direct the cone to a ramp angled at 30° to the horizon, where the cone would launch towards space at about 8 kilometres per second, or more than 23 times the speed of sound. A rocket at the back end of the cone would be used to adjust its trajectory and place it in a proper orbit.

Anything launched in this way would have to be able to survive enormous accelerations – more than 2000 times the acceleration due to gravity (2000g). This would seem to be an obstacle for launching things like communications satellites, but Fiske of LaunchPoint Technologies(appointed by USAF to explore this alternative)  points out that the US military uses electronics in laser-guided artillery, which survive being fired out of guns at up to 20,000g.( refer this research paper for more information- http://www.launchpnt.com/Portals/53140/docs/2006-isdc-launch-ring-low-cost-launch-for-space-exploitation.pdf )

We also need to provide modular strategies to fold the massive PV arrays and mirrors (MYLAR mirrors-http://www.renewablepowernews.com/archives/566) Also the antenna needed for the design needs to be developed in such a way so as to fit into the payload bay of the satellite.

The often neglected costs that also needs to be considered is one involved in supporting Low earth orbit space stations that need to be developed for maintenance and assembly of these solar satellites. In future, costs will also be incurred for defense of these satellites.