Comments on Rocketpunk Manifesto: A Mission to Mars

Yes - when an electric drive bus is used in 's...

2011-09-03T17:54:43.694-07:00

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.

Anthony: "Eh, that's iffy. The basic ...

2011-08-31T08:19:59.820-07:00
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.

Despite past disagreements on how one might imp...

2011-08-30T15:55:03.761-07:00
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.

Despite past disagreements on how one might implem...

2011-08-30T15:28:24.012-07:00
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.

Most of the various numbers seem to indicate that ...

2011-08-30T14:41:51.186-07:00
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...

Anthony: "That applies to your arguments ...

2011-08-26T10:01:07.806-07:00
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.

No, it's not. But that's not the point ...

2011-08-26T09:32:45.952-07:00
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.

Milo: "'It's not gonna work, beca...

2011-08-26T09:23:47.240-07:00
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.

It's because thin film solar collectors are...

2011-08-25T11:49:43.017-07:00
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.

=Milo= "It's not gonna work, because n...

2011-08-25T09:01:24.731-07:00
=Milo=

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

Anthony: "I'm puzzled by why you'...

2011-08-25T08:29:22.525-07:00
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.

The real problem is we are using technologies that...

2011-08-24T19:05:47.418-07:00
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.

Except that the heating mechanism is different....

2011-08-24T16:32:01.343-07:00
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.

Anthony: "Right, it will generate the sam...

2011-08-24T15:17:25.673-07:00
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.

Exactly. You have to tap the generated electric...

2011-08-24T15:04:17.948-07:00
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.

Anthony: "Yes they do; solar cells don...

2011-08-24T13:52:15.777-07:00
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.

The atoms of the material don't absorb and ...

2011-08-24T13:41:05.791-07:00
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.

Anthony: "A flat two-sided sheet of any g...

2011-08-24T12:05:33.870-07:00
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.

A flat, two-sided sheet of what, how thick?...

2011-08-24T11:35:35.062-07:00
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.

Anthony: "Yes, but this is totally irrele...

2011-08-24T10:33:07.155-07:00
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.

Ummm...I would call the fact that a film solar ...

2011-08-24T10:14:17.671-07:00
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.

Anthony: "There's also no guarantee t...

2011-08-24T10:05:51.757-07:00
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.

The big complication is that there is no guaran...

2011-08-24T09:28:29.194-07:00
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.

Anthony: "While true, anything you use on...

2011-08-24T09:02:06.431-07:00
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.

That all depends on the thermal properties of t...

2011-08-23T15:34:31.261-07:00
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.

Comments on Rocketpunk Manifesto: A Mission to Mars

Yes - when an electric drive bus is used in 's...

Anthony: "Eh, that's iffy. The basic ...

Despite past disagreements on how one might imp...

Despite past disagreements on how one might implem...

Most of the various numbers seem to indicate that ...

Anthony: "That applies to your arguments ...

No, it's not. But that's not the point ...

Milo: "'It's not gonna work, beca...

It's because thin film solar collectors are...

=Milo= "It's not gonna work, because n...

Anthony: "I'm puzzled by why you'...

The real problem is we are using technologies that...

Except that the heating mechanism is different....

Anthony: "Right, it will generate the sam...

Exactly. You have to tap the generated electric...

Anthony: "Yes they do; solar cells don...

The atoms of the material don't absorb and ...

Anthony: "A flat two-sided sheet of any g...

A flat, two-sided sheet of what, how thick?...

Anthony: "Yes, but this is totally irrele...

Ummm...I would call the fact that a film solar ...

Anthony: "There's also no guarantee t...

The big complication is that there is no guaran...

Anthony: "While true, anything you use on...

That all depends on the thermal properties of t...