Thursday, August 18, 2011

A Mission to Mars

Surface of Mars
Speculation about Mars missions produces an irresistable temptation to design paper spaceships, and I won't even try to resist. So here we go:

My bias, as expressed last post, is for reaching Mars and returning on fast transfer orbits, making the one way trip in approximately three months. Allowing a few weeks on (or at least orbiting) Mars, the mission can be done in six months and change.

The alternative is using Hohmann transfer orbits, more or less, and accepting an 18-month round trip. This can be done with chemfuel rockets, and broadly speaking we already know how to do that part. What we don't know is how to send humans into space for 18 months and get them back in good health.

Six months corresponds to the currently accepted mission duration for the ISS. Going much longer will be much harder on the crew. On the other hand, the 'fast' six-month mission calls for an electric drive; even nuclear thermal comes up short.

An electric drive with sufficient performance is semi-speculative. The Dawn probe has three (redundant) ion thrusters with a combined mass of 129 kg, thus 46 kg each, while its solar wings come in at 204 kg. Each thruster delivers 92 mN of thrust from 2600 watts of electric power.

The combined mass of one thruster plus solar wings thus comes to 247 kg, just about 0.01 kW/kg. We need power performance about 100 times better, approaching the figure of merit I have often mentioned here, 1 kW/kg is the standard figure of merit.

The reason for using this figure of merit shows up in sims done on the Rocketpunk Manifesto TravelPlanner. Specifically I looked at a baseline vehicle capable of putting on 29 km/s of delta v in 60 days of acceleration, allowing a 30-day coasting period. This is more efficient than a classical brachistochrone orbit, which expends energy and propellant on putting on speed, then (literally!) turning around and taking it back off again.

Exhaust velocity is a little over 30 km/s, giving a mass ratio of 2.5: given a departure mass from Earth orbit of 250 tons, the 'dry' mass that reaches Mars orbit is 100 tons.

Rated drive power is 15 megawatts. The propulsion system (thrusters and power supply combined) is allowed 50 tons, half the 'dry' mass of the ship. This corresponds to a power density of 0.29 kW/kg - or, putting it reciprocally, 3.45 kg of drive mass - thrusters and power supply - per kilowatt of drive power output. As a gearhead reference point that corresponds to 5.65 lbs per horsepower.

I allow 30 tons for fuel tankage, keel structure, and general equipment. Which leaves just 20 tons for the gross payload - life support hab, stores, and crew. This ship is half engine, not a very balanced design. But this is what you need if drive power density is limited and you want to get to Mars in a few months.

My math fu is not equal to the task of actually determining travel time to Mars for a given mission delta v. But this handy delta v calculator, can do it. According to the calculator, burns totaling 27.33 km/s of delta v are needed to get from a high (100,000 km) Earth orbit to low (500 km) Mars orbit in 90 days.

The calculator assumes a brisk 10 milligees of acceleration. The sims have a much more modest acceleration, averaging half a milligee. (And if the drive is solar electric its acceleration performance will be halved at Mars distance from the Sun. Therefore the calculator estimate quite optimistic, but clever mission design could probably squeeze out some improvements.

At least, to a first approximation, this provides some idea of what it takes to reach Mars in a few months.

But the mission profile is for a one-way trip, implying that the vehicle must refuel at Mars orbit. One day this may be a routine operation, but it will certainly not be routine the first time. Really we should be capable of a round trip with onboard propellant - requiring twice the mission delta v, about 55 km/s.

A second sim shows a lighter and more powerful drive engine, close to the 1 kW/kg figure of merit, putting out 30 megawatts and providing twice the specific impulse. This more powerful engine has a mass of 30 tons, allowing 40 tons gross payload. Realistically speaking this is close to the minimum performance requirement for a practical Mars craft - which is why 1 kW/kg is regarded as the figure of merit for fast space propulsion.


This spacecraft is strictly a crew transfer vehicle, intended to get the crew from high Earth orbit to low Mars orbit and back. Electric drive is completely unsuited to planetary landings, so any mission profile like Mars Direct is ruled out. Everything needed to land on Mars, live and work there for a time, and return to Mars orbit, can be sent on a slow orbit.

Since the mission departs from high Earth orbit, there must be a prior phase in which the ship, assembled in low orbit, spirals out, then is met by a crew ferry. In principle the ship could return to high Earth orbit and be met there. More likely at least on early missions, the payload will include an Earth return capsule for the crew (and samples of Mars material), with the interplanetary bus being expended.


This post is conceptually quite incomplete - really it only talks about the interplanetary bus. But I want to get it posted, so here it is.

Discuss.





The image of Mars' surface is from Astronomy Picture of the Day.

74 comments:

Anonymous said...

(SA Phil)

You mentions the Dawn Probe is Solar Electric - perhaps though you could do Solar Electric and get close to this 1kg/kw number.

I know the panels Nasa uses are basically silicon wafer "lightweight" glass panels.

But.. there have been quite a few experiments of amorphous silicon on plastic which produce in excess of 1 Watt/gram

The problem with amorphous silicon is it won't survive hard radiation. (I work with a Phd who studied the problem on a research grant) Which means more exotic thin film materials instead (which they also worked on - I cant remember the material at the moment only that it was very expensive).

They don't spend much money research exotic/expensive thin film materials because there's not much money in solar even with "cheap" ones. And the solar industry is busy trying develop as a power commodity, not a space power source.

However there is some newer work on Dye Sensitized Cells which is interesting. They aren't all that "popular" because they only last a few years -- but that may not be a problem with a Mars Mission.

http://en.wikipedia.org/wiki/Dye-sensitized_solar_cell

Essentially a titanium-oxide photosensitive diode.

The Solar panels would resemble an insect's wing. A thin (~100 micron) semiconductor film deposited on a one thousandth of an inch thick plastic film. You would then use a laser to turn the semi-conductor layers into usable solar cells. "Monolithic interconnect"

I would think you would have your best power/weight near earth and it would fade to an extent on the way to Mars. But thin-films also have a much better low light power than silicon wafers due to the reduced band gap.

Anonymous said...

(SA Phil)

Looking at the weight of plastic film it looks like the thinnest Kapton would require about 8% conversion efficiency to reach 1 g/watt

That's reasonably possible I suppose, Thin film in the ~12% range is available commercially. But after you add in the any supports, connection wires, etc. Your 1kg/kw will go away fast. So literally the more power you are making, likely the best power/watt.

Is this 1kg/kw number the entire mass of the space craft, or just the electrical power source?

longbeast said...

I'm not sure it's right to rule out the idea of a craft that only carries enough fuel for a one way trip.

What I'm thinking is that aerobraking unmanned supply vehicles into Mars orbit is a concept that would be worth testing, and if it was found to be reliably possible for large loads, it would vastly improve the possibility of refuelling once at Mars.

It doesn't change anything on the design of your fast transfer craft since you don't want to dive a vehicle with solar panels or heatsinks into an atmosphere, but I don't see why you couldn't try it with something that had been flown with chemical rockets and taken the slow route.

In the case of trying to deliver a fuel depot to Mars orbit, you have one extra bonus in that most of your cargo is reaction mass for an ion engine - inert gas. There's nothing to stop you storing part of it in a balloon-like inflatable structure, which gets you a huge surface area to distribute your energy as you slow down and a much much lower mass to surface area ratio. You could turn bits of your fuel tank into a gigantic parachute if you wanted to, and probably even control the volume of it to some extent, perhaps useful if you wanted to make more than one pass through the atmosphere.

Paul said...

Dye-sensitized cells have problems with UV degradation even on Earth. In space, the dye molecules would get chopped up rather quickly.

Tony said...

The efficiency of photovoltaic materials is beyond my paygrade, but I thnik it's at least likely that for a cost sufficient power density can be found. (Hardware cost not really being a disincentive in manned spaceflight.)

WRT prepositining return propellant in Mars orbit, that's a risk I'm not sure it's worth taking. Putting the lander and stay supplies in orbit makes sense. If you get there and can't make the rendezvous with them, you can always just turn around and come home. (Of course you have to build enough margin into the propellant supply to do that, but with high Isp propulsion, that's doable, if not always optimum.)

Anonymous said...

Paul said...
Dye-sensitized cells have problems with UV degradation even on Earth. In space, the dye molecules would get chopped up rather quickly.

----------

There are polymers which will protect from UV -- but they would add weight ruining the reason to use them in the first place.


So dye is out. To bad because theres a lot of titanium dioxide on the moon. Maybe as a Lunar base power source though. (PMF)

When I see the Dr. I mentioned I will find out the semi-conducter type they used that handled the radiation well.

(SA Phil)


PS - Also there was a bank error in my favor on that weight. 5% solar material is 50 Watts / Square meter not 5 watts. (At 1000 w/m^2 Irad)

Ill work out the numbers in a bit.

Anonymous said...

(SA Phil)

Okay

Gallium Indium Arsenide Phosphide or something similar. Way too expensive for your house. Fairly good deal for a space power supply.

This is currently being worked on in conjuction with silicon wafers - but there is also work into thin film only variants for space application.

Silicn wafers are nice because even though the radiation zaps them they are so thick they survive.

So back to the Saran wrap style Giant Solar power collection for a interplanetary craft.

10% efficiency would be 100 watts /Sq meter.

The plastic film would be 8-20 grams per sq meter. The thin film would maybe double that with TCO, etc.

Add bussing (low weight conductive ink or something) and you might be able to get 2-3 Watts/ Gram.

That would be for the "surface" of the panel only. You would still need frames, etc.

So assuming you could frame say 10 square meters of material with less than 10kg of carbon fiber (maybe think like the sails of a Junk) you would be golden.

So a 100 KW array like this would be 1000 square meters. And mass <100 kg.

It would only work in space of course, any wind would destroy the plastic/film in no time.

Micrometeors wouldn't be a problem. One thin film trick they have been carting around for years involves solar modules someone shot bullets in.

You would only lose some of the cells in each puncture. A tiny fraction of the whole.

Perhaps possible. It would take a real study by experts though to really determine if you could get the 1kg/watt performance with "today's" (next 20 years) technologies.

But I bet you could come close.

Anonymous said...

(1kg/kw)** -- typo

Anonymous said...

(SA Phil)

To cut down on mass - perhaps you could use a carbon fiber frame and then some really thin guy wires held at some tension to "frame" the panels.

Could even use conductive wires for the guy wires to redce connetion wires/bussing.

You wouldn't want to touch any of these panels uninsulated since there is no insulation at all on the active side in this scheme.

Thousands of volts live wire all the time. Fun.

Tony said...

SA Phil:

"Thousands of volts live wire all the time. Fun."

I wouldn't be so sure of that. I'd think the whole thing would be organized as a hierarchical map of parallel circuits. Voltage (and amperage) would only really start to add up as you got to the terminals connecting to the main busses.

Anonymous said...

(even nuclear thermal comes up short. )

Is this really true? Zubrin always says the same benchmarks when he promotes NTR for Mars travel; that it's reasonable to assume 3 months travel time but "That he'd rather use the fuel for carrying more cargo". I find myself agreeing less with Zubrin on most topics as time goes on, but he's usually very realistic when talking about near-term propulsion.

To at least double (Or triple) the efficiency compared to chemical propulsion doesn't seem unrealistic at all with NTR, and unless I'm mistaken, to just slap on an expendable stage for an NTR Mars transfer vehicle seems like an easier near-term method for relatively fast Mars travel than the slow, arduous path to develop a good electric power source.

Anonymous said...

(Sa Phil)

It would look like a giant grid of graph paper.

http://www.solarmade.com/Ascent_Images/EIPV_Module.jpg

A small scale (but encapsulated example) is pictured at that address.

On that module the series voltage adds from left to right (about 0.7 volts a square ending up 28 volts IIRC) and then the parralel current adds top to bottom. (~3 amps I think)

Since this giant saran wrap version would have 100's of thousands of cells I think the voltage would add up to be pretty high.

Also we'd probably want it to combine in series when we had a choice- just to keep wire diameter as small as we could.

Commercial systems in the US run 600 Volts and 1000 Volts in Europe (combined modules)


-----------
That pictured module is CIGS on a polymer so it isn't that much different than the type I am discussing. The polymer would be a thinner and the film more exotic - but its very similar.

Tony said...

SA Phil:

"Since this giant saran wrap version would have 100's of thousands of cells I think the voltage would add up to be pretty high.

Also we'd probably want it to combine in series when we had a choice- just to keep wire diameter as small as we could.

Commercial systems in the US run 600 Volts and 1000 Volts in Europe (combined modules)"


Presumably each module has non-negligible resistance in order to generate voltage. Hooking them up in series would cause a huge additive resistance by the time you got to the main bus. The high voltage would be a side effect of that -- for a while. But the real practical problem would be such a high resistance that the material would start generating excess heat and melt itself.

I'm looking at a sheet of these modules as essentially a giant printed circuit organized as a hierarchical map. Out towards the edges it would be mostly light collection, with narrow (but adequate) low resistance electrical transmission paths extending from each collector. Each of these paths would extend to an intermediate collection node, then the path from that node would extend to another collection terminal, etc., until you eventually reached the bus terminal. Of course, since you're doing all this in two dimensions, as you get closer to the terminal, the paths get wider, and you'd have less area devoted to light collection. You can't get something for nothing.

Tony said...

Anon:

"(even nuclear thermal comes up short. )

Is this really true? Zubrin always says the same benchmarks when he promotes NTR for Mars travel; that it's reasonable to assume 3 months travel time but "That he'd rather use the fuel for carrying more cargo". I find myself agreeing less with Zubrin on most topics as time goes on, but he's usually very realistic when talking about near-term propulsion."


If he said that, he's further off the deep end than I though. With 925 sec Isp (which is about the best you can expect from classical NTR consuming LH2) you get a mass ratio around 8 -- one way.

"To at least double (Or triple) the efficiency compared to chemical propulsion doesn't seem unrealistic at all with NTR, and unless I'm mistaken, to just slap on an expendable stage for an NTR Mars transfer vehicle seems like an easier near-term method for relatively fast Mars travel than the slow, arduous path to develop a good electric power source."

See above. NTR is useful, but it ain't magic.

Anonymous said...

Tony,

Presumably each module has non-negligible resistance in order to generate voltage. Hooking them up in series would cause a huge additive resistance by the time you got to the main bus. The high voltage would be a side effect of that -- for a while. But the real practical problem would be such a high resistance that the material would start generating excess heat and melt itself.

-----------

The resistance you are refering to is called series resistance on a Solar cell/panel.

I dont think its generally one of important sources of heat in the system in normal operation since everything is drowned out by the Sun load.

Since the Sunload is 1000 Watts/m^2 (nominal on earth) and the series resistance is in the milliohms.

Heat is going to reduce effieicncy. How hot do the solar panels run on the ISS? On Earth you are looking at 45-85C. This is where being on a clear polymer with a thin film is going to help.

Also the panels are not individually grounded. So they never see the system voltage. If a panel has a Vmpp of 100 volts then the most it sees at mpp is 100 volts. Even if it is floating 1000 volts "from ground".

Its even more complicated in Europe since they dont even ground the entire system. The whole thing floats at 1000 VDC.

If you look at some Solar Panel data sheets you will see system voltage tends to be very high compared to the system current.

A 300 Watt panel for example might be 100 Volts and 3 amps.

(SA Phil)

Tony said...

SA Phil:

"Also the panels are not individually grounded. So they never see the system voltage. If a panel has a Vmpp of 100 volts then the most it sees at mpp is 100 volts. Even if it is floating 1000 volts "from ground"."

I wasn't talking about the collector modules specifically. I was talking about the whole sheet, which presumable has power transmission paths and maybe some control circuitry imprinted on it as well. And while the system may not be grounded per se they still add resistance to the circuit. Whether or not individual modules "see" the effects of their resistance, it should still effect the circuit as a whole.

"A 300 Watt panel for example might be 100 Volts and 3 amps."

Which gives it a resisatnce of 33.3 ohms. That's not milliohms.

Skírnir said...

Hey Rick,

unfortunately I can't quite reproduce your figures with my own sims; I guess that's because mine assume constant power and not solar-electric.

FWIW, a 100t craft with constant 30MW and variable V_e up to 64km/s, R=2.5, and distance 0,50AU, the whole trip would take 59 days, half of which in drift, and expend 72TJ. This works out to 1,2 milligee acceleration and 29km/s delta-v.

A straight Tsiolkovsky rocket with fixed power and ISp could be tuned to do the trip in the same time, but expend 92TJ in the process, which comes down to 35km/s V_e.

I do not have the formulas required to duplicate your figures for variable power.

---

Anyway, I'm wondering if it's really clever to start out at such a high earth orbit. We want to escape, and as far as I know, escape burns are best done at lowest possible perigee. According to Duncan Sharpe's Deep Space Flight manual, you should always adjust energy when moving quickly, and thus have as much energy as possible stored in speed and as little as possible in altitude.

I suspect that you didn't take this effect into account because your electric drive can't deliver the necessary delta-V on short notice anyway. I reckon that would be a major selling point for NTRs -- with a good acceleration at perigee you can get several km/s for free.

Anonymous said...

Tony,

Which gives it a resisatnce of 33.3 ohms. That's not milliohms

----------
I thought exactly the same thing back in '08. And it took me a while to come around to it.

But that isn't how it works.

You are generating power not using the solar cell as a resistor.

The power you dissipate isn't based on the power you produce..

But rather the difference between what you produce and what you "should" have produced. (parasitic losses)

The circuit is shown here
http://pveducation.org/pvcdrom/solar-cell-operation/effect-of-parasitic-resistances

And even though it seems to make intuitive sense that V/I should equal Rs or perhaps Rs +RsH - that isn't what happens.

So the actual dissipation is I^2*Rs+V^2/Rsh.

Which really gets interesting because on a good solar cell Rs gets very low and Rsh gets really high .. meaning it dissipates very little waste heat.

Making Solar a very good power source for Space. Especially if you can reduce the sunload heating by making the film extremely thin and using a clear superstate... which people are working on. Although they are doing it to improve terrestial performance ofc.

It does work more like what you are thinking if you hook a power supply up to the module and Drive it backwards. (Reverse Current)


(SA Phil)

Anonymous said...

(SA Phil)

Also you don't need any control circuitry for a solar panel.

You would probably need it at the power control portion. But that would be back on the ship not on the panel.

The only electronics you need (other than the PV material itself) would be a diode to bypass current past shaded cells. Which is only needed for some technologies, basically the silicon ones.

Just don't put the diodes on the bus -- we have a patent pending for that. (My idea actually)

Anthony said...

Anyway, I'm wondering if it's really clever to start out at such a high earth orbit. We want to escape, and as far as I know, escape burns are best done at lowest possible perigee.
This is true, but it requires considerable thrust to get much benefit, and these drives just don't have enough thrust. From low orbit to a 10 km/sec transfer orbit takes:
10 m/s acceleration: 7.3 km/sec
1 m/s acceleration: 9.9 km/sec
0.1 m/s acceleration: 13.4 km/sec
0.01 m/s acceleration: 15.3 km/sec.

Anonymous said...

=Milo=



Anonymous:

"Zubrin always says the same benchmarks when he promotes NTR for Mars travel; that it's reasonable to assume 3 months travel time but "That he'd rather use the fuel for carrying more cargo"."

Unless said more cargo consists of additional astronauts, you're better off sending it beforehand on a separate ship, then sending the astronauts on the faster ship.

Anonymous said...

My vision of a Mars mission would be two phase; a chemfuel powered supply lander to leave months prior to the manned second phase. The second phase would be a propulsion module, a hab module, and a two stage lander. Part of the equipment carried by the landers would be a light-weight 'tent' to house non-enviornmental sensitive materials. The emptied supply lander would serve as addtional housing and working space for the crew. As more missions make use of the same site, it will grow in size and capabilities.

As for the ships themselves; for the first phase, I'd use an expendable chemfuel booster from Earth orbit, another for Mars orbit insertion, and then a one-way lander to the surface. The second phase ship would be 'T' shaped with the hab on one end of the cross bar, the lander on the other, with the ion drive and power plant module at the end of the base. The whole ship spins up upon leaving Earth orbit and spins down upon reaching Mars orbit. When the crew returns to the ship, it again spins up as it leaves Mars orbit; it spins down when it again reaches Earth orbit so the crew can either use the lander to transfer to the ISS (or successor), or be taken off by a transfer vehicle. I'd have the manned vehicle be powered by both Solar and something else; probably either RTG's or fuel cells to augment the Solar. I see no good reason to rely on a single power source when people are involved, especially on a multi-month voyage.

Ferrell

Tony said...

SA Phil:

"It does work more like what you are thinking if you hook a power supply up to the module and Drive it backwards. (Reverse Current)"

I'm not talking about what goes on inside a solar collector module. I'm talking about the effects of the collector's unit resistance on the entire circuit.

"Also you don't need any control circuitry for a solar panel."

Not inside the solar collector module, no. But a sheet full of such modules might require some.

Rick said...

Solar flux at 1 AU in space is about 1400 watts/m2 - considerably better than at Earth's surface. Even at Mars orbit the flux is around 700 W/m2, which ain't chopped liver.

But I haven't got a clue when it comes to the details of wiring. I just trust some electrical union guys to know their stuff.

I would send the lander ahead on a slow orbit. As I said in a comment on the last post, if you can't do a Mars orbit rendezvous you have no business going to Mars. I would not depend on refueling there, because there should always be an 'abort back to Earth' option.

Damien Sullivan said...

Totally off topic:
Rick, blog is great but long. Have you thought of compiling a "best of", or even a digest of distinctive ideas from here? Like the laserstars and the role of orbital combat.

One idea I've had and don't recall seeing here, on deep space combat: the naive SF view is Star Trek, which doesn't work. The more informed view (Newton!) contemplates passes, like jousting... which for delta-vee reasons *also* doesn't work.

But I was thinking, say a Martian fleet is coming to Earth, and for some reason you want to intercept them with ships rather than a missile barrage. You'd sent a fleet with the standard 4x delta-vee, and they'd go meet the attackers... and decelerate and match velocities, whether ahead of them or after one pass.

And at this point you have a Galaga situation, two fleets at mostly fixed distance from each other, hurtling toward Earth, and jinking around a bit as they shoot.

It's still not great, compared to missiles (1x), expendable fleet (3x, can shoot past Earth on the return), or the invaders (2x if they're optimistic about victory and refuel, 1x if they're missiles) but it's a bit different.

[Notation: x is one delta-vee chunk, you use one to accelerate, two to decelerate again, three and four to return. Missiles are 1x, resupply is 2x, self-contained return is 4x. Missiles can either carry less or go faster.]

Tony said...

Re: Damien Sullivan

If you're talking about space warfare in the next couple of centuries, there's a considerable variance of opinion among the commenters on this blog.

People that believe in the space industrialization future do foresee combat in interplanetary space, including high speed passes. They forsee independent Mars and asteroid governments, etc. It's all very much 80s libertarian space opera.

People who think that the industrial base will stay on the Earth for a long time to come foresee space forces as being owned by Earth governments and fighting for Earth government objectives. In that future, what combat takes place in space will be fought by whatever forces are in place in the various planetary and satellite orbitals. Also, combat in Earth orbit would probably be universally decisive, because once you control the Earth's orbitals, you control everybody's access to their relief crews/supplies.

WRT your specific question, my personal opinion is very much of the second type, so I would tell you that invaders from Mars -- even human ones -- was not an issue. If you go with the 80s space opera future, however, it doesn't seem tactically prudent to try to fight an attacking force in interplanetary space man-o-war style. It would make more sense, IMO, to attack them as far out as possible with a maximum interceptor strike, then fight the "leakers" from the orbitals.

Anonymous said...

Tony,

I'm not talking about what goes on inside a solar collector module. I'm talking about the effects of the collector's unit resistance on the entire circuit.

---------------

The "300 Watts / 33 ohms" would be the whole circuit of the 300 watt module

However the 300 watt module would only be about 3 ohms of that (milliohms X number of cells). The 30 would be the "load" (not the solar module)

If you short circuit a solar module it makes almost no power.

I dont think hooking up 100K of modules will be much of a challenge really, we make 100k arrays all the time on earth -- Or even larger, Larger than the spacecraft will be able to handle.

(SA Phil)

Thucydides said...

A speculative technology for high performance solar energy collection is "Optical Retenna". In theory they should be very efficient and compact (microwave rectenna have been demonstrated with efficiencies of 80%, and used to power helicopters by intercepting microwave beams). Sadly, no one has yet demonstrated this sort of collection efficiency in the optical version, but if the technology can be perfected, the mass of the collector can be drastically educed, or the available power can be increased (for the same mass), either which will improve performance. The effect of a working optical rectenna on Earth would also be considerable.

If we can arrange to meet a landing craft in Martian orbit, the idea of sending a tanker ahead to meet the transfer bus in orbit does not seem very strange. The transfer ship should keep a reserve to allow for a slower return if the tanker hookup does not work, but the benefit of not having to accelerate all that mass on the transfer ship seems to be a "no brainer".

I would also argue that planning to use aerobraking for insertion to Martian orbit should also be given serious consideration, if the extra mass to harden the spacecraft and retract radiators/solar panels etc. is offset by the reduction in mass for the power plant, fuel/remass etc.

Tony said...

SA Phil:

"The "300 Watts / 33 ohms" would be the whole circuit of the 300 watt module

However the 300 watt module would only be about 3 ohms of that (milliohms X number of cells). The 30 would be the "load" (not the solar module)

If you short circuit a solar module it makes almost no power.

I dont think hooking up 100K of modules will be much of a challenge really, we make 100k arrays all the time on earth -- Or even larger, Larger than the spacecraft will be able to handle."


Once again, Phil, I'm not talking about what goes on in the collectors. I'm talking about the resistance they add to the total circuit. So, yes, there is no doubt that hundreds or thousands of collectors could in fact be hooked up in an array, but only if you do it right. All of them in series might not work all that well.

Also, thousands of volts of direct current at the output is not exactly what you want for a ship's service electrical system. You'd rather have hundreds of volts, which is another reason you want to hook up modules in parallel.

Anonymous said...

(SA Phil)

RE: Solar System Voltage

Right now if you were to do a 100K array on Earth, on the ground.

In the US you would use 600 volts

In Europe you would use 1000 volts

Because that is the highest you can go by law, not for any technical reason, but rather a safety one.

So you hook modules up in series until you get to 600 or 1000. Then you have parralel strings of 600 or 1000 volts.

The higher voltage the better since it lets you have smaller diameter connecting wires and less line losses.

Its not really debatable - it is how it is done. If Solar makers could they would go even higher voltage.

The Inverter changes the voltage/current to a more usable level for the application.

So I don't see why you wouldn't follow the trend of the current systems in space, since the chances someone will wander out unprotected on the solar array are near zero. And the benefits of the higher voltage are mass related.

Tony said...

SA Phil:

"So I don't see why you wouldn't follow the trend of the current systems in space, since the chances someone will wander out unprotected on the solar array are near zero. And the benefits of the higher voltage are mass related."

I can think of several reasons:

1. Transformers are heavy. A step-down from say 20kV supply voltage to 220V service voltage is a pretty heavy hunk of machinery that also needs active cooling in a space application (which adds even more weight).

2. Even 1kV might be too high a voltage when run through a micrometers thick film.

3. Loads are going to vary greatly throughout the mission. One needs a simple and effective way to sector the power supply so that parts of it can be shunted offline from time to time.

Anonymous said...

Tony,

I can think of several reasons:

1. Transformers are heavy. A step-down from say 20kV supply voltage to 220V service voltage is a pretty heavy hunk of machinery that also needs active cooling in a space application (which adds even more weight).

2. Even 1kV might be too high a voltage when run through a micrometers thick film.

3. Loads are going to vary greatly throughout the mission. One needs a simple and effective way to sector the power supply so that parts of it can be shunted offline from time to time.

====================

1. I can see that - you would need to come up with a mass benefit of high voltage parts vs lower voltage parts.

2. Not an issue. This would only be a concern for degrading the "30 year" lifespan type stuff. And a Solar Electric vehicle isn't going to be traveling for many years.

3. Definitely not an issue. Even at short circuit conditions you won't hurt the module.

--------------
What sort of voltage do electric drives require?

It might be best to determine that first.

If its low voltage you would weigh the DC/DC conversion mass vs the wire/connector mass.

If its high voltage ..

(SA Phil)

Tony said...

SA Phil:

"1. I can see that - you would need to come up with a mass benefit of high voltage parts vs lower voltage parts."

I think the application favors lower voltage, for reasons that will become apparent in a bit.

"2. Not an issue. This would only be a concern for degrading the "30 year" lifespan type stuff. And a Solar Electric vehicle isn't going to be traveling for many years."

I think you're handwaving away a significant circuit robustness concern. Stay tuned for that one as well.

"3. Definitely not an issue. Even at short circuit conditions you won't hurt the module."

Once again, I'm not thinking about the collector modules. I'm thinking about the operational conditions of the entire circuit. The modules are always generating current. But the spacecraft may not need to use all of the available current -- especially when closer in to the Sun. Electric thrusters have physical power utilization limits. You can't just get more thrust by adding more power and propellant, though that would seem like a neat thing to do, and something you might want to do closer to the Sun.

"What sort of voltage do electric drives require?

It might be best to determine that first."


I took the first two columns of figures off of Busek's cut sheets for a variety of Hall effect thrusters that they offer; the other two columns are calculated:

watts volts amps volts/amps
200 250 0.80 312.50
600 300 2.00 150.00
1000 350 2.86 122.50
1700 340 5.00 68.00
8000 300 26.67 11.25
20000 500 40.00 12.50

The above shows that electric thrusters are a relatively low voltage application, and that as they get larger, the tendency is to increase current flow to get more power, not electromotive force. It may be different for something like a VASIMR. But if you're interested in just using a lot of near-term thrusters in parallel, you don't really want or need a high voltage power supply.

Also note the last line item on the list. That 20kw thruster only puts out a newton of thrust. (Which isn't as bad as you think -- the rated propulsive effcieny is 70%.) But it takes more than twice light industry voltage and a pretty hefty amount of current to do it. That right there is a load that I have serious doubts about thin film conductors handling very well. Standard copper electrical transmission wire would have to be 5 gauge (4.6mm diameter conductor) to carry a 47 amp load. Spread out on a 10 micrometer thick sheet, you would need a conductor 1.66 kilometers in width. Now that's based on a very conservative standard intended for human-occupied buildings, but even if you could shave it by an order of magnitude, that's still a conductor 166m wide.

Methinks if I was designing the power supply, I'd have all of the collector modules run in parallel into a bus bar of sufficient mass, and then take propulsion and service power off of the bus. Some of the power left over I might bank in batteries, but not too much, because batteries are heavy. What I'd probably do is use the remainder power to run electron guns to shoot it off into space.

Anthony said...

What I'd probably do is use the remainder power to run electron guns to shoot it off into space.
What I'd probably do is just shut off unneeded panels, either by covering them, or by tilting them so they don't face the sun.

Jim Baerg said...

Rick:
"My bias, as expressed last post, is for reaching Mars and returning on fast transfer orbits, making the one way trip in approximately three months. Allowing a few weeks on (or at least orbiting) Mars, the mission can be done in six months and change."

I seem to have found a point on which to disagree with you.

My bias is to accept that it is going to take several months to get anywhere beyond luna & the occasional Near Earth Asteroid, & we need to learn how to keep humans healthy in space for a year or more, if we are going to do human exploration of the solar system. This knowledge also gives us the ability to have permanent off earth habitats.

BTW, even if you can cut travel time to Mars to 3 months, what is the point of going unless you stay longer at Mars than the travel times so you can do some serious exploration while you are there?

IINM your 1kW/kg figure is what is needed to make electric drive significantly faster than Hohmann transfers, but even with current power suppplies electric drive gets stuff around the solar system with much less reaction mass than chemfuel & so has a major advantage.

One thing that will be required for keeping humans healthy is gravity, or a reasonable rotational facsimile. We need to check how much gravity is enough to keep humans healthy, so having a moonbase would combine doing lunar exploration, learning how to do In-Situ Resource Utilization, & learning whether lunar gravity is significantly better than zero gee for human health.

Lots of activities, from using a toilet to using a hammer or screwdriver for equipment repair are easier with some gravity. So if any sort of space station in cislunar space is desired, we might as well rotate part of it for martian level gravity to check how much martian level gravity helps both those activities & human health. If we find that martian gravity is inadequate then rotating space habitats should all go for 1 earth gravity, & long term Mars exploration would involve a 1 gee rotating space station in Mars orbit & explorers would come back to that station fairly frequently to maintain health.

This is all related to another bias of mine. The long term (centuries) goal of manned space flight should be to have an off earth population large enough & self sufficient enough to survive & even thrive without supplies from earth if that became necessary & to be a source of help to people on earth in the event of a planetary scale disaster (eg: supervolcano eruption, or some massive misstep with technology).

Anonymous said...

Tony,

I think you're handwaving away a significant circuit robustness concern. Stay tuned for that one as well.

========

I am not handwaving anything. It is such a small problem we have trouble coming up with good ways to test for it.

I am not speaking as an entusiast here - I had to do something after the auto downturn.

I have designed Solar modules for the last couple years.

(SA Phil)

Anonymous said...

Anthony,

What I'd probably do is just shut off unneeded panels, either by covering them, or by tilting them so they don't face the sun.
=========
Pretty much this,

The power needs will be completely predictable. No clouds or anything. So I would guess you would design your power circuits (not the solar panels) to handle these changes.

You can just use a relay to turn off the panels easy enough.

Or short them. Doesnt matter. The panels won't care.

Nothing bad will happen.

We short them (in the sun) all the time as a way to keep the cords out of the way.

It is important to remember that Solar Panels are not batteries.

(SA Phil)

Anonymous said...

(SA Phil)

RE: Thrusters

I imagine you would design the actual thrusters to meet your mission requirements.

And then design your Solar Panel configuration to match those thrusters.

You would use more thrusters close to earth since as Rick mentioned you have more power there.

Luckily voltage varies a lot less with illumination than current - and thin film voltage wont vary much at all between 700 and 1400.

You could have a smaller panel to provide power for non propulsive tasks, since 95%++ of your electricity would be going to the propulsion.

Tony said...

SA Phil:

"I am not handwaving anything. It is such a small problem we have trouble coming up with good ways to test for it.

I am not speaking as an entusiast here - I had to do something after the auto downturn.

I have designed Solar modules for the last couple years."


So, say you have a collector that generates 40 amps at 500 volts -- you don't use 5 or 6 gauge wiring to transport that power?

Tony said...

"You can just use a relay to turn off the panels easy enough.

Or short them. Doesnt matter. The panels won't care.

Nothing bad will happen.

We short them (in the sun) all the time as a way to keep the cords out of the way.

It is important to remember that Solar Panels are not batteries."


The problem with that is if a solar panel sitting out in the sunlight on Earth gets hot, it just dumps that heat into the atmosphere. (Well, up to a certain point.) If it sits in sunlight not generating power, it will just absorb the solar flux. If it can't reradiate it efficiently, oops.

That's why my system design assumes the panel will keep generating and the excess power will either be banked or sent off into space somehow. It's simpler and easier that way. Also, the other solution, having numerous trainable solar wings, leads one into diminishing returns. The more configurable your solar power system is, the more complex and prone to failure. Likewise it will be heavier, thanks to the structural overhead that has to be included in each solar wing.

Anthony said...

The problem with that is if a solar panel sitting out in the sunlight on Earth gets hot, it just dumps that heat into the atmosphere. (Well, up to a certain point.) If it sits in sunlight not generating power, it will just absorb the solar flux. If it can't reradiate it efficiently, oops.
Matters less than you might think; if you have a solar panel just sitting there, radiating on two sides, it will heat up to 325K at a distance of 1 AU from the sun. This will not particularly bother any solar panel (it will cause it to produce a bit less power when you first turn it on, but in a quite predictable way).

Tony said...

Anthony:

"Matters less than you might think; if you have a solar panel just sitting there, radiating on two sides, it will heat up to 325K at a distance of 1 AU from the sun. This will not particularly bother any solar panel (it will cause it to produce a bit less power when you first turn it on, but in a quite predictable way)."

That all depends on the thermal properties of the material. We can't presume that a material, particularly a thin film, will reradiate at a high enough rate to achieve thermal equilbibrium before it starts to degrade or even fail. In fact, the photoelectric properties of a thin film collector may in fact be the only outlet of absorbed energy efficient enough to keep the material from degrading or failing.

An illustration from the world of chemical energy, but it suffices to make the point:

Back in the 1980s, the US Marine Corps shifted from the original M60 machine gun to the M60E3. The M60 had a very heavy, thick barrel, that was rated for 200 rounds at the rapid fire rate (200 rpm) or 400 rounds at the sustianed rate (100 rpm) before it needed to be changed. The M60E3 barrel was pounds lighter, with noticeably thinner walls. The troops found out that it was probably not safe to push it past 100 rounds of rapid fire and 200 rounds of sustained fire. In fact, the M60E3 barrel would get visibly red hot at night or even in low light, something rarely seen with M60 barrels. A thin film -- just like the thinner barrel -- simply may not be able to absorb and reradiate heat with the necessary efficiency for the direct sunlight space environment.

Tony said...

Please replace "photoelectric" with "photovoltaic" in your reading of the above post.

Anthony said...

That all depends on the thermal properties of the material.
While true, anything you use on a spacecraft either needs fairly broad temperature tolerance or fairly good temperature management systems. The temperature difference between Earth and Mars is greater than the temperature difference between active and inactive solar panels.

Tony said...

Anthony:

"While true, anything you use on a spacecraft either needs fairly broad temperature tolerance or fairly good temperature management systems. The temperature difference between Earth and Mars is greater than the temperature difference between active and inactive solar panels."

The big complication is that there is no guarantee that a thin film has the necessary thermal properties. One can't just invoke such a material because it would be convenient.

Anthony said...

The big complication is that there is no guarantee that a thin film has the necessary thermal properties
There's also no guarantee that it doesn't, and no particularly strong reason why it wouldn't, unless you're using different materials (rather than merely different geometries) temperature tolerance won't change all that much.

Tony said...

Anthony:

"There's also no guarantee that it doesn't, and no particularly strong reason why it wouldn't, unless you're using different materials (rather than merely different geometries) temperature tolerance won't change all that much."

Ummm...I would call the fact that a film solar collector has to be relatively unreflective (in order to absorb light to begin with) and very thin (because we're supposedly using it to save weight) to be a "particlularly strong reason" to suspect it might not have good thermal properties. It absorbs energy and has virtually no heat sink to store energy until it can be reradiated. That was the whole point of my machinegun barrel example. In practical engineering, everything has its limits.

Anthony said...

Ummm...I would call the fact that a film solar collector has to be relatively unreflective (in order to absorb light to begin with) and very thin (because we're supposedly using it to save weight) to be a "particlularly strong reason" to suspect it might not have good thermal properties. It absorbs energy and has virtually no heat sink to store energy until it can be reradiated.
Yes, but this is totally irrelevant. It will just hit its equilibrium temperature really fast, and as long as equilibrium temperature isn't high enough to damage the sheet, it will be fine. The equilibrium temperature for a flat two-sided sheet at 1 AU from the sun is 51C, and that won't damage the materials used in a solar cell.

Tony said...

Anthony:

"Yes, but this is totally irrelevant. It will just hit its equilibrium temperature really fast, and as long as equilibrium temperature isn't high enough to damage the sheet, it will be fine. The equilibrium temperature for a flat two-sided sheet at 1 AU from the sun is 51C, and that won't damage the materials used in a solar cell."

A flat, two-sided sheet of what, how thick? 15 micrometer thick, metalized PET films work as insulation precisely because they don't absorb radiation. They reflect it. That's not the case with solar collectors.

Anthony said...

A flat, two-sided sheet of what, how thick?
A flat two-sided sheet of any generic black/grey body, where thickness is low enough (or thermal conductivity high enough) that heat can be treated as instantly conducted through the sheet. Depending on the specific visual and IR absorption spectrum of the object, its actual temperature might be higher or lower, but rarely by a lot.

Tony said...

Anthony:

"A flat two-sided sheet of any generic black/grey body, where thickness is low enough (or thermal conductivity high enough) that heat can be treated as instantly conducted through the sheet. Depending on the specific visual and IR absorption spectrum of the object, its actual temperature might be higher or lower, but rarely by a lot."

Ahhh...I see where you're going. A composite material specifically designed to capture photons and convert them to electricity is not a classical black/gray body. The atoms of the material don't absorb and reradiate photons. They absorb photons and kick off free electrons. If your material cannot relieve itself of the flow of electric charge that creates, it's going to be juuust a bit tricky to escape internal heating.

Anthony said...

The atoms of the material don't absorb and reradiate photons.
Yes they do; solar cells don't violate the second law of thermodynamics, and they don't even have particularly interesting properties in the thermal IR; they radiate heat away like any other moderately dark body. If allowed to generate power they will convert a fraction of incoming light into electricity rather than heat, meaning a solar panel will be cooler than a comparable gray body, but even the best solar panels turn less than half of the photons they absorb into electricity.

Tony said...

Anthony:

"Yes they do; solar cells don't violate the second law of thermodynamics, and they don't even have particularly interesting properties in the thermal IR; they radiate heat away like any other moderately dark body. If allowed to generate power they will convert a fraction of incoming light into electricity rather than heat, meaning a solar panel will be cooler than a comparable gray body, but even the best solar panels turn less than half of the photons they absorb into electricity."

Exactly. You have to tap the generated electricity to cool the panel. If you don't, it will generate heat. You think no big deal. I think free electrons bouncing around inside the material is less efficient in heat rejection than black/gray body radiation in a monolithic material.

Anthony said...

Exactly. You have to tap the generated electricity to cool the panel. If you don't, it will generate heat.
Right, it will generate the same amount of heat as if it were a passive dark surface. Which is what I gave the temperature for.

Tony said...

Anthony:

"Right, it will generate the same amount of heat as if it were a passive dark surface. Which is what I gave the temperature for."

Except that the heating mechanism is different. In a classic, monolithic black/gray body, heating is accomplished through absorbtion and reemission of photons, until the heat energy is radiated away. In a photovoltaic compound material, some heating is photonic, but a significant amount is also caused by electrical resistive heating. That's a slower process and allows more time for heat to build up before thermal equilibrium is achieved -- if equilibrium is achieved at all.

Also, something we've been ignoring, thin film photovoltaic collectors include in their design antireflective coatings. These not only inhibit reflection of incident radiation, they inhibit the emission of radiation.

Anthony said...

Except that the heating mechanism is different.
In an almost completely irrelevant manner. You'll get a very small delay based on how long it takes the electrons to drop to their ground states, and you may generate heat deeper inside the material than would normally occur, but neither is going to matter for a thin film, because a thin film heats up in a nearly monolithic manner anyway.
Also, something we've been ignoring, thin film photovoltaic collectors include in their design antireflective coatings. These not only inhibit reflection of incident radiation, they inhibit the emission of radiation.
There's little reason to assume optical properties in the visible and near-IR are relevant to the mid-IR, and in any case you have that backwards, anything that lowers the albedo increases emissivity. You could certainly have a heat-trapping layer, but that would simply mean your solar cell is warmer all the time, and it's hard to trap much heat with a thin film because it's thin and thus conducts heat rapidly.

I'm puzzled by why you're stuck on this point. Anyone designing high power density solar panels for use in space is aware of the space environment, and will design the panels to cope. Usually, the biggest problem is low temperatures resulting in frozen motors.

Thucydides said...

The real problem is we are using technologies that are not up to the task. There is no 1Kg/KW power system in existence, either nuclear or solar, and the current crop of high ISP drives are not particularly powerful either. Chemical propulsion would take an enormous amount of tankage without generating the required deltaV and even classical NTR isn't quite up to the task.

By analogy we are thinking about trans Atlantic air transport in 1919, and the really smart and imaginative people might dream of something like the Donier DO-X (or the Norman Bel Geddes "Airliner Number 4") when what we are really waiting for is a Boeing 707.

This takes us into the realm of speculative technology and magitech drives. Some form of very compact aneutronic fusion power plant would fit the bill, if such a thing is possible. Even using externally powered plasma drives (VASMIR or something similar)might fit the bill, so long as the receiver is light enough and the power beam is energetic enough.

Tony said...

Anthony:

"I'm puzzled by why you're stuck on this point. Anyone designing high power density solar panels for use in space is aware of the space environment, and will design the panels to cope. Usually, the biggest problem is low temperatures resulting in frozen motors."

It's because thin film solar collectors are being invoked as if they will just automatically be space flight capable, simply because somebody wants them to be. As with anything else to be used in space, it's never that simple, and some technologies that work perfectly fine on Earth don't work all that well in space.

Yes, my feet are dug in, as they always will be with the assumption that "it's gotta work, because I want it to work". Want isn't a technical qualification.

We'll have to agree to disagree on this one.

Anonymous said...

=Milo=



"It's not gonna work, because nothing I want ever works." is no more valid a reasoning.

Anthony said...

It's because thin film solar collectors are being invoked as if they will just automatically be space flight capable, simply because somebody wants them to be.
The optimistic part is achieving 1,000 W/kg at all, not the ability to use it in space. Current state of the art is on the order of 100W/kg, and pretty much all the work on improving that is either through concentrating solar power (which is way more of a heat problem than whether or not you're generating electricity) or through putting it on a thinner substrate.

Tony said...

Milo:

"'It's not gonna work, because nothing I want ever works.' is no more valid a reasoning."

No, it's not. But that's not the point I was trying to make. Thin film solar collectors may very well work in space. But one can't simply assume they will work or, if they do, that they will work in a certain way.

Anthony said...

No, it's not. But that's not the point I was trying to make. Thin film solar collectors may very well work in space. But one can't simply assume they will work or, if they do, that they will work in a certain way.
That applies to your arguments as well. Thin film solar might well have issues in space, but there's no reason to think those issues will be heat related.

Tony said...

Anthony:

"That applies to your arguments as well. Thin film solar might well have issues in space, but there's no reason to think those issues will be heat related."

Like I said, we disagree. I'm simply skeptical, from personal experience, that low mass per surface area materials have capable thermal properties under constant light/heat flux.

Thucydides said...

Most of the various numbers seem to indicate that any mission to Mars based on rapid transit by the crew really does have to be split between a crew "bus" and one or more slowboats with cargo, fuel and consumables.

The energy and mass requirments to push the crew hab and all the "stuff" just becomes overwhelming. The C-17 is hugely expensive because the plane must combine high performance (in the case of a C-17, global range and STOL performance) with the ability to carry large and heavy cargo. A spacecraft with similar performance needs will also be very expensive, when what is conceptually needed is a Learjet for the passengers and a barge for the cargo.

Plan "B" (slow flights) may be easier since all we need is a houseboat to carry everything along. this works so long as we are fairly content to spend most of the time drifting with the current...

Tony said...

Despite past disagreements on how one might implement them, solar electric ships actually make a very good platform for executing divided cargo/passenger mission architectures. The same basic solar electric bus could be used to deliver cargo to Mars over a period of two to three years, and passengers in a few months. The difference would be how much mass you try to push at one time -- a lot with cargo and as small as possible with personnel.

Anthony said...

Despite past disagreements on how one might implement them, solar electric ships actually make a very good platform for executing divided cargo/passenger mission architectures.
Eh, that's iffy. The basic problem is that Mars is actually relatively 'close'; with a 1g drive it's about 3.7 km/sec from low earth orbit, which really isn't that difficult for a chemical fuel rocket. The reusable solar-electric slow boat can manage a lower fuel fraction, but the drive hardware is much heavier and more expensive, and if you want to re-use it you need to figure out how to refuel in space and in Mars orbit and even then it's only going to make something like one trip every two years.

Tony said...

Anthony:

"Eh, that's iffy. The basic problem is that Mars is actually relatively 'close'; with a 1g drive it's about 3.7 km/sec from low earth orbit, which really isn't that difficult for a chemical fuel rocket. The reusable solar-electric slow boat can manage a lower fuel fraction, but the drive hardware is much heavier and more expensive, and if you want to re-use it you need to figure out how to refuel in space and in Mars orbit and even then it's only going to make something like one trip every two years."

All discussions of electric propulsion on this blog -- solar or nuclear -- are based on the assumption that one can somehow get at least close to the 1 kw/kg power plant. Otherwise it just doesn't make sense. But with the caveat that such a power/weight ratio could be achieved, the weight of the drive bus doesn't get out of hand, even if you include return propellant in the design.

Rick said...

Yes - when an electric drive bus is used in 'slow' mode, the propellant fraction becomes truly just a fraction, perhaps on order of 20 percent of departure mass, instead of being two thirds or more of departure mass.

Anonymous said...

(SA Phil)

Stepped away from this for a while... 6 months.

Arguing with Tony always seemed to be explicitly counter productive since he always seems to play to "win" while I more want to think about things.

He brings up some good points on heat. Since most Solar panels are designed to trap light it tends to heat up rather quickly.

On Earth the heat is basically convected away into the air or ground. You run tests on this.

So the truly lightweight panel would need to do two things.

1-Not trap many times more light than it was going to use.
2-Radiate heat away at an acceptable level.

So you could make your thin film semi-transparent to not absorb many times more light than you used. That works. It is used today in some newer technologies. There is an efficiency hit but .. read on..

As to the second point that is an interesting question since it would need to work as a radiator without adding much mass. Some expert on radiators would need to say what is feasible there. Still the more area the more it will radiate.

================
One nice aspect to this and the reason I decided to look this back up -- Kaku mentioned an interesting tidbit (he seems to be the tidbit guy actually) that Solar intensity is 8X in orbit as it is here on the ground.

That is a substantial game changer there. Since now your Solar panel has a lot more available energy to convert. And that could open the door.

As to the voltage and current stuff in order to save mass you would use as much voltage as you can get away with and design your electric thruster with that in mind.

Rick said...

I don't recall who Kaku is, but that 8x figure seems way off. As I recall, solar intensity in space at 1 AU is about a third more than at Earth's surface.

Anonymous said...

Michio Kaku

the String Theory guy.

Seems like that would be a big error to make in a book you published-- but It is definitely possible it was an error I suppose.

The nominal illumination power we use to rate panels on earth for sunlight is 1000 Watts per Meter^2.

So a 15% panel would make 150 Watts per meter^2.

A 30% would make 300 ... and so on.


(SA Phil)

Rick said...

Maybe the 8:1 was the proportion of solar flux to electric power from the cells? Or 'average' surface insolation, for all latitudes, night and day, winter and summer, etc., for a flat solar array not pointed at the sun.

Or it could just be an error that slipped through editing.

Anonymous said...

(SA Phil)

That Definitely could be- since a surface Solar Panel only provides power for a few hours a day .. 8X could be the total increase in kwH by the sun not setting.

Not something that matters for our spacecraft.


===========
So here is a translucent thin film solar panel example.

http://www.youtube.com/watch?v=7gIHMJaujMs

===========
A thought - since the film would be so light weight perhaps you could combine your gossamer solar wings/Solar Electric with a solar sail for the trip to Mars. Using the light that passed through the panel to push the sail. Starting with a healthy Chemical booster Stage of course.

And then the trip back could be slower. With a mehtane rocket push ala Zubrin. Since Mars has such low gravity it might be possible.

Damien Sullivan said...

Ground solar loses half to day-night. On average it might lose half due to latitude and angles, though we could power civilization entirely with above-average locations. And the atmosphere reflects a fair chunk as well, though half might be what's reflected by both atmosphere and ordinary ground. Throw those together sloppily plus clouds and you could get 8x.

OTOH I'm used to a 300 W/m2 average figure, which is 1/4 of the orbital 1300. Do-the-math talks a lot about equivalents of 5 hours of full sunlight, which is a bit over 1/5.