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Big solar arrays and high specific power for SEP tugs, Interplanetary travel by Robotbeat on 11 Feb, 2010 19:29
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Okay, instead of segueing threads into this topic, I figured I'd start a new thread about it.
So, solar arrays have become more efficient, more lightweight and larger in the last few decades, while large deployable structures have been... deployed in a few missions, from Cassini to the Space Shuttle Radar mission to the ISS. There has been some recent promising research into both high specific power arrays and large deployable structures, including the canceled Space Technology-8 mission and the current FAST (Fast Access Spacecraft Testbed) DARPA program. The Ultraflex deployable array is supposed to have up to a 175W/kg specific power, while also probably being about 30% efficient (typical triple-junction efficiencies). The FAST program will demonstrate at least 130 W/kg specific weights, along with high total power (i.e. greater than ISS).
High-power nuclear reactors for Mars missions have typically been proposed to have about 250W/kg specific power, but those designs are quite ambitious (liquid metal cooling, etc) and have some overlap with solar arrays with large deployable structures (and perhaps in the future even more overlap if they use thermophotovoltaic high-efficiency thermal-to-electrical conversion techniques which require no moving parts).
Mars transfer vehicles utilizing solar power have, in the past, baselined around 350W/kg specific power, which is above the current state-of-the-art, but likely within reach. Solar power intensity available at Mars high orbit is about 50% that available at Earth high orbit.
Entech has proposed high-efficiency solar panels approaching (and supposedly eventually exceeding by 100%) 500W/kg specific power, in excess of what is required for a Mars mission, although not much new information is available beyond what's on this website: http://www.stretchedlensarray.com (Entech has seemed to move on to focus on terrestrial applications, a booming market these days). By depositing thin-film photocells on solar sail-like material (and structures), much higher specific powers are available (2000W/kg deployed specific power, >4000W/kg undeployed specific power demonstrated in the lab, with 17000W/kg possible if deposited on thin solar sail-like material).
Modern triple-junction and high-efficiency solar cells for space often have cell-level specific powers of 200-300W/kg, even 500W/kg. This doesn't count supporting structures. More progress is needed in supporting structures, since integrated solar arrays typically don't exceed 100W/kg even for modern arrays.
A deployable fiberglass boom (under bending loads) 60 meters in length was flown on the Shuttle Radar Topography Mission ( http://en.wikipedia.org/wiki/Shuttle_Radar_Topography_Mission ), the longest such boom yet demonstrated. The ISS contains many deployed booms (under combined axial and bending loads), 8 total for the large solar array.
The Space Technology-8 mission was to demonstrate SAILMAST, which would've had a greater length/diameter ratio than the SRTM's boom (176 vs. 60), but was canceled before it flew.
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#1 by Robotbeat on 11 Feb, 2010 21:57
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BTW, a 30% efficient solar array the size of the ISS's dimensions would produce over 3 MW at 1AU from the Sun:
http://www.google.com/search?q=.30*108.5m*72.8m*1366W*m^-2
And a 43% (current record holder) one would produce almost 5 MW. So, if we decide to produce a 10MW array in twenty years, it'd need about three football fields (and the dimensions of two ISSes) worth of solar array. That's not so bad. It'd likely be smaller than the radiators used for a nuclear reactor capable of 200MW. -
#2 by LegendCJS on 12 Feb, 2010 20:48
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I like this thread. Thanks for the overview. I noticed you didn't mention any space solar power concepts, I think there is a lot of overlap, and large SEP tugs/ mars transfer vehicles might be a stepping stone to conventional SSP visions.
Now I do wonder about using any kind of ion thruster or centralized drive with a large deployed structure. I know the accelerations are very low, but I just think there might be a more graceful propulsion tech to match/ compliment a large deployed space structure like what we are talking about here: M2P2 propulsion.
With the large structure already existing for the support of the solar power panels/ films, it might be possible to exert much better control over and couple much better momentum transfer from the plasma cloud/ sail of the M2P2 drive- especially if the structure was built with that in mind.
There may be a problem of premature wear on the solar elements from the plasma, but that is an unknown, and it might not be a problem with the right plasma cloud management.
-my two cents
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#3 by Nathan on 13 Feb, 2010 00:41
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I like this thread. Thanks for the overview. I noticed you didn't mention any space solar power concepts, I think there is a lot of overlap, and large SEP tugs/ mars transfer vehicles might be a stepping stone to conventional SSP visions.
M2P2 won't work until outside of Earth magnetic field - when all of the main work is already done.
Now I do wonder about using any kind of ion thruster or centralized drive with a large deployed structure. I know the accelerations are very low, but I just think there might be a more graceful propulsion tech to match/ compliment a large deployed space structure like what we are talking about here: M2P2 propulsion.
With the large structure already existing for the support of the solar power panels/ films, it might be possible to exert much better control over and couple much better momentum transfer from the plasma cloud/ sail of the M2P2 drive- especially if the structure was built with that in mind.
There may be a problem of premature wear on the solar elements from the plasma, but that is an unknown, and it might not be a problem with the right plasma cloud management.
-my two cents -
#4 by LegendCJS on 14 Feb, 2010 18:10
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M2P2 won't work until outside of Earth magnetic field - when all of the main work is already done.
Good point. So how about this: when the craft is inside a planetary magnetosphere it won't waste gasses trying to sustain a plasma cloud. Inside a planetary magnetic field the craft could use the large reach of its structure and the existing magnetic field control hardware that it normally uses to shepherd its plasma cloud to instead thrust against the planetary magnetic field directly- like a glorified version of the "tether propulsion" concept..
Maybe a reconfigurable structure would be valuable in this case, but either way I think this kind of craft would look really alien- a spiders web of conducting coils and loops and gas vents amid fields of solar panels. -
#5 by clb22 on 15 Feb, 2010 07:37
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Mhm, the last actual SEP reference mission I have heard of was Energia's updated MEK proposal (see below). They baselined a cluster of many ion thrusters, 120,000 mē of solar arrays (72 squares about 40m x 40m), the use of xenon gas and a nominal mission time of 2 1/2 years (so no VASIMR Ad Astra type claims of high-energy trajectories). The old MEK mission baselined a total mission weight of 400mt, nor sure if that is still the case for the 2005 updated MEK mission (the article below speaks of a total of 280mt xenon gas as fuel).
Anyway, I think their concept shows quite well how large the solar arrays would be compared to the crew compartment. At a 100W/mē efficiency at 1AU, this assembly would provide 12MW or quite enough for the Ad Astra 85day to Mars reference mission using VASIMR based mission IF the solar panels + supporting structure aren't prohibitively heavy.
http://www.ng.ru/science/2005-01-12/15_heart.html
EDIT: Ok, found the specs on the solar array assembly for the 1989 MEK reference mission and the 2005 MEK update http://www.friends-partners.org/partners/mwade/articles/sovctric.htm :QuoteEnergia retained the electric engines of the 1969 MEK design but dropped the nuclear ractor for its 1989 Mars expedition design. This spacecraft used the same thruster arrays requiring the same power output (15 MW) as the 1986 nuclear design. But in this case two enormous panels, each 200 m x 200 m would generate a total of 15 MW of power at earth. The use of ultra-thin (less than 50 micrometer) / low mass (0.2 kg per square meter) photovoltaic cells with a high specific power value (up to 200 W per square meter) minimised the weight of these vast arrays. The total mass of the electric engines, structure, and solar panels was 40 tonnes. The power generated would be used primarily by two ion engine clusters mounted perpendicular to the living block. In high-power mode these would have a specific impulse of 3500 seconds. They would consume 165 tonnes of xenon propellant during the voyage (of 355 tonnes total spacecraft mass).
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#6 by Nathan on 15 Feb, 2010 08:11
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M2P2 won't work until outside of Earth magnetic field - when all of the main work is already done.
Good point. So how about this: when the craft is inside a planetary magnetosphere it won't waste gasses trying to sustain a plasma cloud. Inside a planetary magnetic field the craft could use the large reach of its structure and the existing magnetic field control hardware that it normally uses to shepherd its plasma cloud to instead thrust against the planetary magnetic field directly- like a glorified version of the "tether propulsion" concept..
Maybe a reconfigurable structure would be valuable in this case, but either way I think this kind of craft would look really alien- a spiders web of conducting coils and loops and gas vents amid fields of solar panels.
That's another point though - how will that cloud of accelerated gas affect the solar panels? Regular panels degrade due to the thin oxygen in the upper atmosphere. -
#7 by Nathan on 15 Feb, 2010 08:16
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Mhm, the last actual SEP reference mission I have heard of was Energia's updated MEK proposal (see below). They baselined a cluster of many ion thrusters, 120,000 mē of solar arrays (72 squares about 40m x 40m), the use of xenon gas and a nominal mission time of 2 1/2 years (so no VASIMR Ad Astra type claims of high-energy trajectories). The old MEK mission baselined a total mission weight of 400mt, nor sure if that is still the case for the 2005 updated MEK mission (the article below speaks of a total of 280mt xenon gas as fuel).
Anyway, I think their concept shows quite well how large the solar arrays would be compared to the crew compartment. At a 100W/mē efficiency at 1AU, this assembly would provide 12MW or quite enough for the Ad Astra 85day to Mars reference mission using VASIMR based mission IF the solar panels + supporting structure aren't prohibitively heavy.
http://www.ng.ru/science/2005-01-12/15_heart.html
Any data on how they dealt with the Van Allen belts? This is key to low thrust trajectories. -
#8 by clb22 on 15 Feb, 2010 09:02
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Any data on how they dealt with the Van Allen belts? This is key to low thrust trajectories.
From the MEK 1989 reference mission profile http://www.friends-partners.org/partners/mwade/craft/mars1989.htm :QuoteIn order to clear the earth's radiation belts as quickly as possible, the mission profile began with a relatively 'high thrust' acceleration by the engines in a spiral from low earth orbit to 40,000 km. This altitude would be achieved in 29 days. Thereafter the engines would shift into their normal regime, with a lower thrust but higher specific impulse. The spacecraft would reach escape velocity after a total of 100 days of firing, followed by a 270 day coast to Mars. A 38 day braking manoeuvre would bring the spacecraft into Mars orbit. 30 days would be spent in Mars orbit, during one week of which the crew would descend to the surface. It would take 28 days to accelerate away from Mars, followed by a 250 day coast to earth. The crew would enter their return vehicle and re-enter the earth's atmosphere at 13.5 km/sec.
I think for advanced mission architectures the crew would of course get to the vehicle only after it has spiralled out to 40,000km or to a Lagrangian parking orbit. -
#9 by A_M_Swallow on 07 Feb, 2011 20:48
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If the interplanetary vehicles are leaving from a Lagrange point then the propellant and other cargo needs to get there. One way is a chemical launch vehicle to LEO followed by SEP tug to Earth-Moon Lagrange point 1 (EML-1).
A Falcon 9 can lift about 9 tonne to 600 km, so a SEP tug able to get that payload to EML-1 in about 6 months could be useful.
Ref http://www.spacex.com/Falcon9UsersGuide_2009.pdf figure 4.1
edit : spelling of unit -
#10 by Xplor on 07 Feb, 2011 22:47
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There has been a strong push for decades to develop large solar power capability in support of essentially all satellites. This has seen the maximum commercial communication satellite power level grow from a few KW to 30 kw. I certainly welcome a concerted NASA push to further this. But I remain skeptical that affordable multi 100 kw or mw level power will be launched in the next 20 years.
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#11 by Proponent on 08 Feb, 2011 02:03
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How does solar-dynamic power generation compare with photovoltaic? I'm thinking in terms of the Brayton-cycle turbine proposed for the TAAT power-beaming demo and possibly for NAUTILUS-X too. Does solar-dynamic offer superior specific power under any circumstances? Is its advantage that it is less susceptible to radiation?
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#12 by Robotbeat on 08 Feb, 2011 04:55
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How does solar-dynamic power generation compare with photovoltaic? I'm thinking in terms of the Brayton-cycle turbine proposed for the TAAT power-beaming demo and possibly for NAUTILUS-X too. Does solar-dynamic offer superior specific power under any circumstances? Is its advantage that it is less susceptible to radiation?
Solar-dynamic does not offer superior specific power to modern photovoltaics nor would it likely (in my opinion) provide better specific power than modern rad-hard solar arrays. These are the reasons solar-dynamic has never been demonstrated on orbit. -
#13 by Proponent on 08 Feb, 2011 05:51
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What about long-term durability? If NAUTILUS-X is supposed to operate for years, is it possible that a solar-dynamic system would suffer less degradation over, say, a decade?
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#14 by Robotbeat on 08 Feb, 2011 15:21
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What about long-term durability? If NAUTILUS-X is supposed to operate for years, is it possible that a solar-dynamic system would suffer less degradation over, say, a decade?
I doubt it. If the solar-dynamic system cannot be competitive on a specific power basis and is off by a factor of, say, 5 (don't know real figures because no one has flown a solar-dynamic system), then even a decade of degradation will not come even close to making the solar dynamic system better.
Besides, solar dynamic has lots of mechanical parts which could degrade, as well (both most photovoltaics and solar-dynamic have rotating joints, though not all photovoltaics do). -
#15 by Proponent on 09 Feb, 2011 03:42
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So what reason could there be for TAAT to propose solar dynamic?
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#16 by A_M_Swallow on 09 Feb, 2011 04:11
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So what reason could there be for TAAT to propose solar dynamic?
A guess. Cost? -
#17 by Patchouli on 12 Feb, 2011 03:50
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So what reason could there be for TAAT to propose solar dynamic?
A guess. Cost?
Also mass a solar reflector for a solar dynamic generator can be a very light weight inflatable structure.
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#18 by Robotbeat on 14 Feb, 2011 13:41
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The solar dynamic generator itself is very heavy. Also, no reason a thin-film photovoltaic array can't be just as light as just the reflector you posted, but without the need for the heavy generator and without the precise pointing requirements.So what reason could there be for TAAT to propose solar dynamic?
A guess. Cost?
Also mass a solar reflector for a solar dynamic generator can be a very light weight inflatable structure. -
#19 by muomega0 on 14 Feb, 2011 15:29
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The solar dynamic generator itself is very heavy. Also, no reason a thin-film photovoltaic array can't be just as light as just the reflector you posted, but without the need for the heavy generator and without the precise pointing requirements.So what reason could there be for TAAT to propose solar dynamic?
A guess. Cost?
Also mass a solar reflector for a solar dynamic generator can be a very light weight inflatable structure.
for space station, the breakeven point between photovoltaics and dynamic systems was around 10-25 kW. Thin film does not have the efficiency of PV and there are many environmental effects issues for life. The key consideration, however, was power production during locked gimbal events, where PV can produce energy approximating the area under a cosine, wheres SD cannot. The lower cost option was about 37 kW of PV and 37 kW of SD (or more). For the tug, battery life should increase significantly (over the 5 years for ISS) due to the fewer cycles required, so the cross over point would increase.
There is a trade-off between mirror accuracy and the "precise" pointing requirements. typically about 2 degrees for an array and about 0.5 for SD, far from arc sec. Since the heavy Turbine and energy storage is near the fine pointing mechanism, this creates very favorable mass properties "precision" control of the mirror.
Roadmaps have been created by dynamic power systems. It depends on the power source/level (radioisotope- sterling, sun - brayton/rankine, nuclear - brayton).
Costs: Most costs are associated with the nuclear power source. The mirrors are developed. Few know that a rotating reverse brayton cryocooler was retrofitted into Hubble Space Telescope during the mirror fix and still functions today (NICMOS). The temperature range of the SD system would be less than modern commercial aircraft, so material issues affecting life seem manageable.
http://gltrs.grc.nasa.gov/reports/1999/TM-1999-209380.pdf