SPACE BASED SOLAR POWER IS FOR MARS Dr. Matthew L. Dalton IAC, October, 2015
Space Based Solar Power (SBSP) First proposed by Isaac Asimov in 1941 and has been under active research since the 1970 s. No demonstration mission has ever been flown and most proposals remain a pie in the sky. Notable examples are very large, risky adventures with no real proven technology, e.g. NASA Arbitrarily Large Phased Array. Many hurdles are left unanswered, how to point such an array in geosynchronous orbit always pointing nadir yet also pointing at the sun more generally, how to not interfere with Earthly communication or electronics.
SBSP: dead? Elon Musk offered perhaps one of the most damning assessments for the outlook of the field. In an interview in 2012, he declared his desire to stab that bloody thing in the heart. Full transcript: http://shitelonsays.com/transcript/elon-musk-panel-bta-2012-2013-01-28
SBSP: dead? The crux of his argument against SBSP seems to be that you have multiple power conversion steps: Photon (sun) -> electron (solar panel) -> photon (beamed power) -> electron (rectenna) The same is true, however for Earth based solar power since production often does not match consumption. Photon (sun) -> electron (solar panel) -> chemical reaction (batteries) -> electron (grid) Or perhaps solar power is produced in solar abundant regions and carried across long land lines to less abundant regions (e.g. DESERTEC) where transmission lines introduce new losses
SBSP: the real difficulty? The cost of launching into LEO (not even counting GEO!) is typically estimated to be $1,000/kg. If 1kWh costs $0.10 on Earth, you would need to produce 10,000kWh on Earth per kilogram launched just to win back the initial investment. Assuming an overly optimistic total efficiency (mirrors, power production in space, beamed power, rectified on Earth) of 10%, that means 100,000kWh of solar radiation collected per kilogram launched. Assume the reflector module in the figure below weights 100kg and has a reflective area of 2m 2, this would require 3.5 million hours or 400 years! Just to win back the initial investment, and that is not even counting solar panels or power transmission mass or refueling every 5 years.
SBSP: what about Mars? It is the cost of launching that kills SBSP from the beginning, not conversion/transmission efficiencies. The cost of launching would need to go down by a factor of at least 100 to make this financially viable (which Musk might contribute to with his re-usable launchers). Moving SBSP to Mars turns the whole problem on its head. Instead of disadvantages, we get the following advantages: 1. Instead of high launch costs we can reduce landed mass with SBSP 2. Only hundreds of watts or kilowatts are required for Mars rovers instead of Megawatts for SBSP power plants (good for technology demonstrators) 3. No international restrictions on bandwidth apply on Mars 4. Less atmosphere means less power transmission losses, but also that power station can be located much closer to the planet surface, further reducing transmission losses 5. Microwave transceivers: less mass than RTGs and less sensitive to dust than solar panels, can receive power at night or in the winter If SBSP can be done anywhere it can be done on Mars!
SBSP Mars Mission Simulation: KSP One other eye catching comment Elon Musk said in that interview was KSP is awesome!. Further inspection revealed that KSP is a spaceflight simulation software under continual development by Squad which allows the user to build and test spacecraft. The basic vanilla version simulates a fictional solar system, however, a plug-in scheme allows third party developers to introduce their own code and modifications ( mods ) which may add new technologies or improve physics simulations.
SBSP Mars Mission Simulation: KSP The new version KSP1.0 has significantly improved aerodynamics, launch/reentry and thermal physics. The most critical mods for realistic simulations and the ones used in the following simulations are the following: 1. Realism Overhaul (makes solar panels produce realistic amounts of power depending on solar facing factor, distance from sun, realistic rocket thrusts and sizes, masses, power consumption, etc.) 2. Real Solar System (RSS, replaces simplified solar system with real planets, orbits, etc.) 3. Interstellar Extended (provides near future technologies, such as microwave power transmission needed for the Mars SBSP project)
SBSP Mars Mission Simulation: KSP Planning an SBSP based mission to Mars The Mars Power Hub (MPH, right) collects solar power with PV panels and beams it down to the rover. Just like the Mars Reconnaissance Orbiter, it also serves as a data relay hub. The Mars Rectenna Rover (MRR)
SBSP Mars Mission Simulation: KSP The Realism Overhaul solar panels have an area of 13 m 2 each and sun exposure (cosine law) is updated continuously based on pointing. Solar flux near Mars is about 590 W/m 2, so the received power of 1.34 kw corresponds to a very realistic efficiency of 15.9%, however, 85% efficient power transmission to MRR is overly optimistic at a distance of 600 km! In fact, Interstellar does not include distance dependence of microwave transmission efficiency!
SBSP Mars: Implement Distance Fall-off The code for the Interstellar plug-in can be downloaded from: https://github.com/sswelm/kspinterstellar/tree/master (originally developed by Fraktal_UK, currently maintained by S.Svelm with many contributors) Under FNPlugin/Microwave/MicrowavePowerReceiver.cs We find protected double ComputeDistanceFacingEfficiency(double distance, double facingfactor) { double powerdissip = 1; if (distance > penaltyfreedistance)//if distance is <= penaltyfreedistance then powerdissip will always be 1 powerdissip = (microwaveangletan * distance * microwaveangletan * distance) / collectorarea;//dissip is always > 1 here } return facingfactor / powerdissip; The distance has no effect for power transmission when distance is less than the penalty free distance, i.e. when around only a single planetary body. So I downloaded the software, implemented distance dependent beam spreading, recompiled and re-ran the simulations.
SBSP Mars: Implement Distance Fall-off The microwave transmission module is similar to the one described in the NASA SPS ALPHA study from 2011. A communication dish is attached so that the Mars Power Hub functions also as a communication hum, just like the Mars Reconnaissance Orbiter. Transmitted microwaves are now assumed to spread at an angle of 1-10 arc seconds.
MSBSP: Landing on Mars is hard! One of the first things you learn simulating missions to Mars is that landing on Mars really is hard. At least 9 failed missions to Mars: -Mars 2 (USSR, 1971) burned up on entry -Mars 6 (USSR, 1973) landed too hard -Mars 7 (USSR, 1973) missed the planet -Mars Polar Lander/Deep Space 2 (U.S., 1999) lost on arrival -Beagle 2 (ESA, 2003) lost on arrival Reducing landed mass is important!
MSBSP: Simulating beamed power Beamed power at a distance of 300km is feasible if 1 beam divergence can be achieved. With 10 it is somewhat feasible, with 1% transmission efficiency and ~53W power received, but power hub would need to be beefed up.
SBSP Mars: Beam divergence For diffraction limited Gaussian beam, the minimum possible divergence is give by: 2θ = λ πw Where w is the width of the beam at its smallest point, λ is the wavelength. Simple testing with the MPH and MRR show that a beam divergence angle of at most 10 (or ~50 μrad) in order to have at least 1% transmission due to small receiver. 1 (~5 μrad) can avhieve nearly zero beam loss due to beam spreading at a distance of ~300km. Given transmitter radius w = 2m 10 -> λ 300 μm -> 1,000 GHz not really feasible Assume 10 GHz (30mm) 10 -> need 98m radius transmitter also not really feasible
SBSP Mars: Beam divergence On the other hand, imagine that beamed power is from a laser and the receiver is a solar panel instead of microwave phased array. 1. Solar panel functions during the day as normal, providing power 2. When laser is overhead, solar panels receive up to two or three times the solar flux on Mars in the form of laser light. 3. Laser light will be transmitted in the day, at night, in the winter adding to the normal independent functioning of the solar panels. 4. But dust problem comes back
SBSP Conclusions This presentation has shown that Mars should be considered an ideal playing ground for testing SBSP: 1. Low atmosphere (low loss, lower orbits possible) 2. No problem of interfering with communications (free range of frequency selection) 3. No danger to people (maybe environment, e.g. planetary protection?) 4. Financial incentive: less landed mass instead of more launched mass But we have seen that with microwave frequencies this is not really feasible, requires either very large transmitter or very large receiver. Laser is the last possibility, most promissing and Mars is still the best place to test/prove it. Testing giant space laser on Mars is less likely to scare the masses, good place to demonstrate safety and effectiveness. And finally, we can see that KSP, while simple, can be used for zeroth order mission feasibility testing And Elon Musk was definitely right about one thing, KSP is awesome! I am happy to take suggestions or collaborations to continue the KSP mission design/testing! Dr. Matthew L. Dalton R&D Manager, Active Space Technology, GmbH Matthew.Dalton@activespacetech.eu Tel: +49 (0) 30 6392 6072 Fax: +49 (0) 30 201 632 829 Carl Scheele Strasse 14 12489 Berlin Germany