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Comment: Re:Flip Argument (Score 1) 1128

by necro81 (#48457463) Attached to: Officer Not Charged In Michael Brown Shooting

I don't really know - I wasn't there, and the other party is dead

This is one of the real problems I have with "Stand Your Ground" laws, like the one in Florida that allowed George Zimmerman to escape charges in the death of Trayvon Martin. It doesn't even boil down to a "he said, he said" kind of argument - conflicting accounts of what happened, like some bad replay of Rashomon . Instead, it's "he said, and the other guy's dead," which doesn't sound like a good way to get at the truth, let alone justice.

("Stand Your Ground" is a somewhat different situation than cops shooting subjects, or Castle Doctrine laws involving one's own home. The situation is the same - one guy's dead - but the context of who did the shooting and where provide more latitude.)

Comment: Build their economy? (Score 1) 143

by necro81 (#48432955) Attached to: Coal Plants Get New Lease On Life With Natural Gas
"We couldn't let it happen. We would lose our tax base, we would lose our jobs, we would lose our future," said State Sen. Catharine M. Young. "This agreement saves us. It gives us a foundation on which to build our economy."

It seems to me that if they were going to use the local power plant as a foundation for building the local economy, they might have gotten around to that by now. The plant has been there for decades, right? Looks to me like they squandered their chance as diversifying their tax and employment base - why should they be given a second chance to squander?

Comment: Re:Gas not less CO2 on refiring coal plants (Score 4, Informative) 143

by necro81 (#48432895) Attached to: Coal Plants Get New Lease On Life With Natural Gas

If you just replace coal with natural gas in the same plant to heat the water it is not significantly less CO2

Burning coal is pretty much just turning bulk carbon into carbon dioxide. Burning natural gas (methane, CH4) creates carbon dioxide, too, of course, but also releases energy from burning the hydrogen to make water. As a result, the combustion of natural gas produces less CO2 for the same energy output.

From the Energy Information Agency - Pounds of CO2 emitted per million BTU of energy:
Coal (anthracite): 228.6
Gasoline: 157.2
Natural Gas: 117.0

[I'll apologize for the units - I'm just quoting the result. If you must know, 1 lb / 1e6 BTU is equivalent to 0.43e-3 kg/MJ. Or, just look at the number as a figure of merit: lower is better.]

more data here

Comment: Re:Who opposes cleaner sources of energy? (Score 1) 143

by necro81 (#48432827) Attached to: Coal Plants Get New Lease On Life With Natural Gas

If one were to be cynical one would suggest that environmentalists only want sources of energy that are expensive and unreliable

Or they could, ya know, maybe use less energy. There's plenty of low-hanging fruit in using what energy we can produce more efficiently, which obviates the need for any new generation, or allows old plants to be mothballed. It doesn't mean shivering in dark caves: humans, particularly Americans, are fantastically thoughtless and wasteful when it comes to energy.

Rather than forcing utility rate payers to fork over $150 million for a natural gas conversion of an older plant, why not get them to fork over half as much money to pay for efficiency measures that would 1) negate the need for that natural gas plant altogether and 2) save them lots of money in the long run.

Comment: Re:I'm quite surprised it wasn't (Score 1) 523

by necro81 (#48427093) Attached to: What Would Have Happened If Philae Were Nuclear Powered?

Check out the SNAP-19.

I know of it - it powered the Pioneer spacecraft in the early 1970s. Can I get one today? No. Can anyone take the drawings for it (assuming they can be found) and manufacture new ones for signficantly less than the cost of creating a modern design? Probably not. Could the ESA do it? Probably not.

I reiterate the points in my earlier post. Please read further.

Comment: Re:Ignorant Article (Score 1) 523

by necro81 (#48425665) Attached to: What Would Have Happened If Philae Were Nuclear Powered?

How long it lasts is dictated by nuclear physics (the half life of Pu-238.)

The decay in power output is only partly due to the half-life of the nuclear fuel. An effect of roughly equal magnitude is the degradation of the thermocouple junctions themselves. For instance, each Voyager spacecraft's RTG had about 470 W electrical output at launch in the 1970s. As of 2008, about one half of a half-life later, the electrical output had declined to about 285 W. I'm sure that the thermal output of the RTGs is following the half life of Pu-238 just as one would expect, but the useful life is limited by other considerations.

Comment: Re:but orbital reentry? (Score 1) 523

by necro81 (#48425565) Attached to: What Would Have Happened If Philae Were Nuclear Powered?

But what would happen if it would be heated up and worn down in a low angle orbital reentry? It could be subjected to melting/burning temperatures for many minutes

The Lunar Module from Apollo 13 carried an RTG, which was intended to power surface instruments on the Moon. It re-entered, along with the rest of Aquarius at around 25,000 mph, at roughly the same entry angle as the Odysee command module, which is intentionally shallow to bleed off maximum velocity with survivable g-forces.

That Pu-238 cask is now sitting at the bottom of the Pacific ocean. All seems well.

Comment: Re:Right .... (Score 2) 523

by necro81 (#48425537) Attached to: What Would Have Happened If Philae Were Nuclear Powered?

Though, this is its thermal output. If you consider that Seebeck generators have a 10% efficiency, you could get 32W electrical out of 640g of PU-238. Let's account for the 10 years trip, so let's make it 1kg

The weight of the actual plutonium isotope is but a small fraction of the weight of the finished RTG. There's the thermocouple wires, the iridium cladding, the graphite casing, the metallic casing, etc. No one has made an RTG with just 10s of watts of output since the 1970s, but those designs weighed in at 10-20 kg, which is about the same mass as Philae's (solar+battery) power system.

Having an RTG would not obviate the need for actual batteries. The RTG is good for providing average power, but the peak power draw is much higher than that, and batteries are used for that purpose.

Comment: Re:I'm quite surprised it wasn't (Score 1) 523

by necro81 (#48425421) Attached to: What Would Have Happened If Philae Were Nuclear Powered?

Plutonium 238 is really not that dangerous. It is an alpha emitter, and carried in a water insoluble form.

Radiation aside, it is also an extremely toxic heavy metal.

That said, it isn't safety concerns that kept RTGs of Rosetta/Philae. If the project had determined that the best way to achieve the science and program goals was through an RTG, they would have launched with an RTG. The safety concerns about exploding rockets or re-entering spacecraft didn't keep Curiosity, Cassini, or New Horizons from launching. Rather, if ever the engineering design and program management discussed using an RTG, they decided against it for much more pragmatic reasons (engineering, cost/benefit analysis, program risk, etc.)

Comment: Re:I'm quite surprised it wasn't (Score 0) 523

by necro81 (#48425375) Attached to: What Would Have Happened If Philae Were Nuclear Powered?

A 32W RTG would generate about 600W of waste heat, something that is easy to radiate over 3m2 into space, assuming reasonable operating temperatures for the probe (and actually, a smaller RTG is sufficient)

I think this is something that isn't particularly well appreciated - a 32-W RTG does not exist. All the designs for recent spacecraft have been on the order of 100-300 W (electric).

I practically burst out laughing when the article gets around to introducing the notion of powering Philae using an RTG. The image that the author dramatically inserts at this point is the RTG for the Curiosity rover - an assembly that is itself about the size of the entire Philae spacecraft! Cassini, Ulysses, New Horizons - all of the recent RTG-powered probes used the same design, one that is entirely the wrong size for Philae.

For Philae to use an RTG, it would need to have been a new design (something the ESA has no experience in) - a development that could have cost more than the Philae lander itself. Even with a new design, they would have needed to secure the Pu-238 to power it (assuming they didn't use some other isotope, which would have been yet another costly design effort). When the craft was being designed and built, the supply of Pu-238 was already more or less spoken for, and it would have been an enormous program risk for them to commit to an RTG without a guarantee that they could fill it.

Comment: Re:So, ion drive or something??? (Score 4, Insightful) 67

by necro81 (#48387101) Attached to: Boeing Readies For First Ever Conjoined Satellite Launch

how is this ion engine more efficient than just squirting a small amount of pressurized gas out of the tank instead

It has to do with how quickly you can throw the propellant - how much momentum you can impart to it, which in return imparts a certain change in momentum to the rest of the satellite (delta-v). With conventional satellite propulsion, like fuel+oxidizer rockets or monopropellant thrusters, the energy available to impart that momentum is chemically based. That is, the propellants undergo a chemical reaction, get hot and/or change phase into a gas, and nozzles force that gas to exit at some velocity. Details vary with engine and nozzle design, but there are limits on how much thrust you can get each fuel type. Mass in, reaction energy, mass*velocity (momentum) out. Rocket designers measure this "efficiency" with a quantity called specific impulse (measured in units of seconds) For a given mass of fuel, you can pretty quickly calculate what the total delta-v the satellite has available to it.

Ion engines can impart much higher velocities to the "fuel" than chemical rockets, in part because they are using electrical energy (of which there is an arbitrarily large supply) rather than whatever you can get from chemical reactions. Again, the details vary based on the design, but ion engines tend to have specific impulses much higher than chemical rockets. The actual thrust (i.e., total force) from an ion engine tends to be miniscule, but is provided very efficiently, and can be produced for days or weeks at a time.

The biggest difference between time and space is that you can't reuse time. -- Merrick Furst