September 11, 2013

Propulsion lasers for large scale deployment of solar power space satellites and reducing the startup costs and the costs to launch

Keith Henson has a new paper on Space based solar power satellites. It is called Rays of Hope: Propulsion lasers to get parts up, Microwaves to get energy down and the effect of large-scale deployment of power satellites on CO2.

Previously Keith Henson examined power satellites mostly as proposals to solve the big economic problem, the cost of transporting parts to orbit. There were unsolved problems in the last article such as how to return the launch vehicle to its runway.

Nextbigfuture covered Keith Henson's space based solar power proposal in 2011

There was also an article at theOildrum

Economic Analysis of GEO Laser Propulsion

It is not hard to get the cost for 5kg/kW power satellites down in this range if the cost of lifting parts to GEO can be brought down to $100/kg or less. Solar power on earth ties up material in the range of 500 kg/kW(average). Power plants built in space, where they get full-time sunlight and are not subject to wind and gravity; allow a hundred-ton materials reduction to 5 kg/kW and an energy-payback time of less than two months.

Given a 20-year lifetime for the power satellites, the EROEI would be around 120, good as the best days of oil. However, the cost of transporting even a greatly reduced mass to space is a big problem. One hundred dollars per kg is a hundred-to-one cost reduction compared to the current cost of around $10,000 per kg paid to put communication satellites in GEO.

Modified Skylon near the end of acceleration to LEO on hydrogen heated by 3 GW of lasers located in GEO. The exhaust velocity of the hot hydrogen is close to 7.5 km/s. Base artwork courtesy of Reaction Engines, Ltd., modified by Barbara Graham, laser additions by Anna Nesterova.

A two-orders-of-magnitude reduction in transport cost seems to be possible, but not using chemical energy (other than the first step where a Skylon-type vehicle burns hydrogen with air for about 1/4 of the velocity to orbit).

Skylon's air-breathing phase (above 25 km and Mach 5.5), it takes a 3 GW laser located in GEO to accelerate the vehicle for the last 6 km/s to orbit. This could use a simple sheet of tubes on the wings to heat hydrogen to 2700 K. The laser tracks the vehicle along the equator for ~4000 km. The long acceleration path allows a ten-fold increase in payload compared to a straight-up launch from a ground laser.

The 2700 deg K temperature gives an exhaust velocity of 7500 m/s. If you understand the rocket equation, the effect on payload of higher exhaust velocity will not surprise you. The payload fraction to orbit goes up about 5 times.

Diagram by author Keith Henson. Acceleration of the Skylon-type vehicle from ground to 25 km and 2000 m/s burning hydrogen and air plus the acceleration from 2000 m/s to 8000 m/s using laser-heated hydrogen.

The released 30-ton second stage (carried in Skylon's payload bay) separates from the Skylon before the second stage comes into range of the propulsion laser in GEOagain after one orbit. On reduced power of ~ 270 MW, the second stage accelerates, first to geosynchronous transfer orbit (GTO), an elliptical orbit with a ten-hour period. After a half orbit of 5 hours, when the second stage is approaching GEO altitude, laser energy circularizes the second stage's orbit at GEO by firing across the chord of geosynchronous orbit. Acceleration to GTO and circularizing at GEO requires a delta V of 4.1 km/s and a reaction mass fraction of 1/3. Twenty-ton second stages arrive at GEO three times an hour, carrying 1440 tons per day, ~500,000 tons per year. We scrap the second stages (72 per day) at GEO for construction material. This makes the entire dry weight of the second stage into payload. (The concept of scrapping the second stages for construction materials is my main contribution. Jordin Kare originated most of the laser propulsion concepts including tracking from GEO.)

Since the 2011 articles, Keith Henson wrote for the peer reviewed Journal of the British Interplanetary Society. The (accepted but not yet published) article analyzed a variant, putting the lasers in GEO rather than shooting the laser beams from the ground to GEO and using bounce mirrors. This solved several problems. First, we get the Skylon back to its runway by going into orbit and coming down one or more orbits later. We no longer have the PR problem of incinerating birds and bats flying through the beam.

Finally, we avoid the hard optical engineering problem of sending a high-power laser beam up through the murky atmosphere and working to keep it focused to a few meter spot after a trip out to GEO and back down to LEO. This method also avoids the problem of a cloud moving into the beam path and causing a launch failure.

The big problem with this scheme is powering the first laser in GEO. The JBIS article described an elaborate multi-step bootstrap model that built a 15,000-ton power satellite in GEO to power the first propulsion laser. The cost came in just short of $140 B and the financial breakeven took 12 years.

Early in April 2013, the model became obsolete because Steve Nixon (otherwise known in energy circles for advocating Mega-Chimney) made a suggestion that cut the cost model by $80 B and cut the financial breakeven to 8 years. At $60 B (and 500% ROI in ten years), it may be within the capacity of western finance. (Of course, there would also need to be the financial capacity for utility companies to purchase the power satellites since the financial model sells the relatively cheap power satellites to utility companies. However, they have to buy replacement power plants anyway.)

Cumulative profit and loss in millions of dollars based on 5 cents per kWh declining to 2 cents per kWh over ten years. In this model, the production company sells power satellites to utility companies for $3.5 B/GW for 5 cents/kWh falling to $1.4 B for 2 cents/kWh. Peak investment is ~$60 B.

If the western world cannot or will not finance this project, the Chinese can. It is only twice the cost of the Three Gorges Dam (22 GW).

Steve's idea was inspired; power the propulsion laser for a few months from a huge (10 km) 12-GW phased-array transmitter on the ground. Reciprocity means that the path loss will be the same if you swap the transmitter and receiver antennas. The rectenna in space from 1987, NASACR179558 indicates a 6-GW rectenna in space would mass less than1,000 tons (160 gm/kW) (50% loss in microwave transmission, 50% conversion to laser light and 76% of the laser energy goes into hydrogen kinetic energy.)

Even better, the rectenna, laser, tracking optics and heat sink are all constructed and tested in LEO, much cheaper to access than GEO. If we don't use robotics, we have man-centuries of experience working in LEO. Then we ship the propulsion laser to GEO using electric thrusters powered from the same rectenna that powers the laser.

A first-pass estimate of the mass (rectenna, laser, optics, heat sink) came in at 8,000 tons, most of it in the heat sink, plus 2000 tons of reaction mass. Ten thousand tons is about 20 times the mass of the International Space Station. Moving it to GEO takes 21 hours of thrust (exhaust velocity of 20 km/s) for the delta V to get it there. The high exhaust velocity reduces the reaction mass fraction to 20%. Powered when in view of the ground station for ten percent of the time, it could make the trip in ~10 days.

We would use this method of powering the 3-GW propulsion laser from the ground using microwaves for months to a year depending on how many laser type Skylons were available. After they bring up the factory and parts for a power satellite to replace the power from the ground, we remove the $12 B transmitter parts and rebuild as a rectenna. (A shorter time of use than the cofferdams used to dewater the foundations for dams.)

Using a propulsion laser beam from GEO to equatorial LEO does require choreographing. The operators (actually computers) need to shut the beam off for a few milliseconds while a satellite in a lower orbit passes through the beam path. This should not appreciably affect the vehicle acceleration because of the thermal averaging of the hydrogen heater.

The 3-GW laser will bring up 500,000 tons of parts per year, enough for 100 GW of power satellites, probably 20 at 5 GW each.

Is this proposal a realistic way to solve the energy problem and cap the rise in CO2? At the 100 GW-per-year production level analyzed above, certainly not, it's far too small.However, a project making large profits and with demand for 20 times that much new power per year (two TW) can grow and grow fast.

Tons of material sent into GEO each quarter. Amount remains constant after 17 years, because the market for power satellites saturates at 2 TW/year. It is uncertain how long power satellites would last. Given experience with communication satellites and some maintenance, 20 years would be a minimum. (Figure by Author Keith Hesnon)

If the power sat construction organization diverts 10% of the traffic to GEO into more propulsion lasers, the production of power satellites can double every year and that will get the human race off fossil fuels and stop the rise in CO2. In the simple model used to generate Figure below, the CO2 levels off short of 450 ppm by 22 years from the start.

At 2 TW per year, the cargo to space would be around 10 million tons per year. At some point, the environmental impact of that much traffic into space may force the use of asteroids or lunar material (even though the exhaust only makes water).

Continuing production of power satellites beyond immediate human needs would provide the energy needed to capture and sequester CO2 to any level desired, especially if synthetic fuel plants are already making large amounts of synthetic fuel from CO2 from the atmosphere and cheap energy.

This unsophisticated model plots the increase of CO2 as 400 ppm plus 0.5 ppm/quarter times (15 TW-installed power satellite TW) / 15 TW. Peak installation rate is 2 TW/year. The CO2 levels off in year 22 after construction of 15TW of SBSP. The decline over the next 20 years assumes active removal of CO2 using power in excess of 15 TW.

Technology Readiness

Is such a project realistic given the current state of engineering? I think it is. For example, the James Webb telescope mirror is big enough and accurate enough for the propulsion laser. The Strategic Defense Initiative (SDI) completed a 4-meter laser mirror (LAMP) in 1989 and pushed the technology into the 10-to-11-meter class.

Lasers in space are expected to track and hit targets because the engineering was worked out from 1984 to 1993 as part of SDI known in the mainstream media as "Star Wars." The other critical piece of technology is the Skylon rocket plane. After demonstrating the precooler (with kilometers of tubing), Reaction Engines received a two-year grant from the UK government totaling 60 million pounds in late June 2013 to build a SABRE engine prototype.

Military and Policy Issues

Unfortunately, there are unsolved problems with this proposal as well. The first is not technical, but a political/geopolitical/geometry problem. Propulsion lasers are also weapons, game-changing weapons. From GEO one of them can "see" and attack somewhat less than half the world. Would the US accept China having one? (Particularly would the US object if it were far enough to the west not to threaten the US mainland?) Would China accept the US having one? Is joint control possible, perhaps including India, China, Australia, Japan, Russia, other countries and the US? Can there be technical limits such as a laser wavelength that doesn't penetrate to the ground? Could the mechanical design prevent them from pointing off the equatorial acceleration path?

The second problem is the rocket exhaust affecting the atmosphere, especially the ozone layer. Even if it is almost all water, the amounts are large, 12 million tons for the 100 GW/year minimum, and up to 240 million tons for constructing 2 TW/year. .Even if the damage is substantial, it may be less than the damage from accumulating CO2. This needs the attention of experts.

A considerable amount of detail has accumulated over the last year and a half. If readers want to volunteer to comb through spreadsheets and the assumptions that went into them, the project could use you. If you find things to correct put them in the comments, or my email is hkeithhenson@gmail.


Here we have a project of modest cost that looks like it will solve the energy/carbon/climate problems, if not for all time at least up to a ten-to-one increase in human energy use. It will also restore economic growth on low-cost energy and end the build up of CO2. Laser beams and microwaves are indeed "rays of hope."

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