Solar power to Jupiter

[BlameTheEx]

Jupiter Icy Moon Orbiter. The case for solar power.

 

The JIMO has a planned power requirement of 100kw, and NASA has decreed that this will best be provided by a nuclear reactor. Suggested weight for the reactor is 100kg to 150kg.

 

What would be the mass of a solar powered solution?

 

Spectolab is the main supplier of Solar cells for space.

 

http://www.spectrolab.com

 

For naked Solar cells the weight 840g per Square meter, with a power of 378 Watts. This gives a weight of 222kg for a 100kw bank. Given the planned payload of 1500Kg, this is not unreasonable.

 

However This is for naked Solar cells in earth orbit. The weight for solar panels is at least 624kg. It gets worse. The light intensity is only 4% of Earth's around jupiter. If efficiency was similar we would need about 7500 square meters of solar panels, weighing about 250 Tons.

 

Clearly simple solar panels will be too heavy. However all is not lost. We don't need 7500 Square meters of solar cells, we can use solar concentrators.

 

http://www.sunpowercorp.com/html/Technical%20Papers/ProgPV.pdf

 

we just need mirrors or fresnel lenses, with a combined surface area of that figure. There IS a suitable material for a mirror, with a weight of 5 grams per square meter.

 

http://www.space.com/businesstechnology/technology/carbonsail_000302.html

 

This would give a weight of 37.5kg! Given that it has the stiffness of cardboard, and would be operating in zero gravity, and very near zero acceleration, it would need little or no further support structure to be acceptably ridged. This weight doesn't seem to include the weight of the mirror coating, or mounting, but it looks promising.

 

So what about the weight of the solar cells? Solar cells work happily at light concentration of 200X earth normal sunlight, and plans are afoot for 1000X concentrations. Assuming the lower figure, we have about 1.5 square meters of solar cells, of negligible weight, and a serious cooling problem. Not nearly as serious a problem as in the nuclear power option however, for the following reasons:

 

1) Space grade solar cells currently operate at 28% efficiency, and likely to rise. This is far higher than most planned nuclear solutions.

 

2) It's a lot easier to build lots of small heat sinks that one large one, for the simple reason that the heat doesn't have to travel so far before it's radiated.

 

We have one final problem. such a large solar array must be assembled in space. It's about time we came to terms with this. Nuclear powered solutions will also need space assembly. Take a look at the heat sinks on the initial nuclear designs, and imagine launching them pre-assembled. We need to develop a robot based assembly facility at the ISS.

 

It's not only JIMO that would benefit from assembly in space. Think of the advantages of shipping satellites to the ISS, fitting them with high power, low weight solar concentrators, and bolting on the antenna. The satellites are then boosted into their proper orbit ether with onboard solar powered ion drives, or an unmanned, solar powered, reusable "space tug". ISS could host a satellite assembly, repair and maintenance facility that could revolutionise the communications industry.

[BlameTheEx]

There is an interesting paper Published on the Entech site. These are the people that created the fresnel lenses of the solar array on the Deep Space 1.

 

http://www.entechsolar.com/

"1,000W/Kg Solar Concentrator Arrays for Far-Term space missions"

 

 

The paper argues that 500W/Kg will be available by 2011 and 1000W/Kg by 2023.

 

I find the conclusions conservative, backed by both extrapolation of current trends, and technology that is under development. It assumes, as I do not, that solar arrays will be automatically deployed. My feeling is that robotic, or even manual assembly at the ISS is inevitable by then for deep space missions, and probably even commercial satellites.

 

What changes when solar collectors reach 500W/Kg? Well, obviously satellites become a lot more powerful, which is going to be great for HD TV and general communications, but there is more. If you take it in conjunction with progress in Ion Drives, lots more. Look here:

 

http://www.spacetransportation.com/ast/2003_prop_workshop/pres-pdf/9a_katz.pdf

 

Lets do some maths.

 

Assume a 1000kw array with a mass of 2 tons. Assume a 1000Kw array of ion motors with exhaust velocity of 80km/S, efficiency of 80%, and mass of 1 Ton (figures broadly compatible with the results of the Nexis program). Assume 1 ton for xenon tanks, structure ect. These are conservative assumptions.

 

The complete structure has a mass of 4 Tons, with a thrust of 10 Newtons. 10N is the equivalent of 1 mm/S2 for a total mass of 10 tons (the additional mass payload and fuel).

 

1 mm/S2 is turbocharged. It's 1km/s in only 278 hours (half that in near earth orbit due to time coasting in the earth's shadow). A "space tug" would move an assembled satellite from the ISS into synchronous orbit in about 2 months, and return in half that time for the next delivery. Xenon fuel for the round trip would be only about 10% of the satellites mass. To collect for servicing and return will take about 4 months, and 20% of the satellites mass in fuel.

 

I predict that by 2020 almost all commercial satellites will be assembled, serviced and repaired at the ISS. The advantages are overpowering:

 

1) Currently Solar Arrays and Antenna must deploy automatically If they were "flat packed" to the ISS and assembled manually, or roboticaly, they would be more reliable, lighter, and easier to develop.

 

2) Much of the cost of a satellite is design. A DIY kit of modules for assembly at the space station avoids this. The modules are simply clipped onto a standard lightweight frame that never has to handle gravity.

 

3) There is a good chance that a satellite component will fail during the high-G boost into space. But now this becomes a minor problem. If the assembled kit fails to work, the offending module is replaced from stock, repaired, or a replacement ordered.

 

4) It's a lot cheaper to boost payloads into low, loosely controlled orbits, than the high synchronous orbits needed by most satellites With the high escape velocity of ion engines, using a space tug to collect the deliveries from low orbit to the ISS, and then deliver the assembled satellite to synchronous orbit is far more fuel efficient.

 

With such a large commercial interest in earth orbit solar cells for powering satellites, NASA should consider using them for powering missions to mars, and even further afield. It's just so much cheaper to use technology that is going to be developed anyway, than develop nuclear generators that can only be used, at most, in a handful of missions. The inner planets are firmly solar powered territory. The outer planets should only be visited once a generation, at most. It takes at least that long to get funding, design, build, launch, wait for the ship to get to its destination, and finally do the observations. There is no hope of using the same power plant for a follow up mission. The technology would be obsolescent, and the parts unobtainable.

 

For the planned JIMO miss

[GAHD]

I would agree with this, but I beleive the nuclear power is being tested on this mission so that they might develop something more useful for humans to use in the future. Also, the place they should be targeting for study is the asteroid belt. I would like to see a probe with the same tech used to find resources in the earth sent to the asteroid belt . Possibly with the ability to retrieve one or more of those asteroids to isolte it for study.

 

We live in an era where a private company can design an automated deforester, shurely an automated mining and smelting machine could be built to take those retreived rocks and prep them for our use.

[Freethinker]

Solar power to Jupiter? Since it would obviously take more than 12 hours, some of the trip would have to be at night. Then what?

[Freethinker]

OK, I had a reason for that...

 

As GAHD points out, at least for space travel, solar obviously has it's major disadvatage. Put all the R&D into solar, what happens to going to outer planets? Or beyond?

 

Why duplicate the R&D. Why not just work on a technology that won't have fairly near term limitations.

 

Not to say that Solar isn;t an energy source that we shoould be paying serious attention to.

[BlameTheEx]

Ghad. The asteroid belt is well within the region where solar power is plausible.

 

Freethinker.

 

You have a point. Beyond jupiter nuclear power becomes the obvious choice for powering the instruments. However it still does not become essential for powering the acceleration for the following reasons:

 

1) There will be budgets for very few missions beyond jupiter. These can be arranged to use gravity assisted acceleration, in the same way that previous missions did. They needed no more that chemical rockets could provide.

 

2) We can still use solar powered drives. The boost required can be delivered at the beginning of the mission, before the craft gets too far from the sun.

 

Nuclear generators powerful enough to power communications and instruments have already been developed. I have shown that developing a high powered nuclear generator for thrust is not essential. It's also unlikely to be the cheap solution. Safety concerns are bound to result in expensive precautions during development and launch. Don't minimise these costs. This is why nuclear generated electricity is so expensive on earth.

 

Nasa has latched on to nuclear powered drives with the same woolly accounting that prompted the development of the shuttle. The vast majority of electricity demand in space is, and will continue to be, served by solar power. This is for good commercial reasons. Solar power will continue to develop with or without Nasa's help. Even if it isn't the aesthetic solution to boosting to the outer planets it IS the budget solution.

 

I personally think there is a deception going on here. Nasa can't possibly expect sufficient missions to the outer planets to justify the cost. I think they are hoping to use nuclear powered boosts for manned missions to mars. If so, they are making a bad error. For the inner planets, and for both nuclear and solar collector power, the big problem will be dissipating the waste heat. Solar collectors will prove easier to cool.

[Pyrotex]

...Sun Power Port is a portable generator that when used

to its full potential will pay for itself in less than two years....

Dexter,

this sounds an awful lot like SPAM, which is forbidden by our site rules.

Do this again, and you will probably get banned.

Pyro