Catch up on stories from the past week (and beyond) at the Slashdot story archive

 



Forgot your password?
typodupeerror
Get HideMyAss! VPN, PC Mag's Top 10 VPNs of 2016 for 55% off for a Limited Time ×

Comment Re:Duh? (Score 1) 304

It is, however, racist to ignore the fact that almost 100% of the results for "three black teenagers" are mugshots, while only a small fraction of black teenagers are actually criminals. It's also racist to believe that we as a society have nothing to do with creating a criminal underclass along racial lines.

Comment Re:That came in at a pretty steep angle (Score 1) 206

I think I can answer that based on my extensive training with Kerbal Space Program. And okay some stuff I read about the moon landings too. If you come straight down and find you're going to land off-target, you have to swivel the engine sideways, burn to build up some horizontal velocity, and then swivel in the other direction to cancel it out again. But if you keep some sideways velocity as you descend, it's easy to adjust your final landing point in that direction by starting your final deceleration burn a little early or late. It's still tricky in the other horizontal direction, of course.

Comment Re: More physics than economics, and it checks out (Score 1) 206

You make a very good point for, say, satellite delivery, where being in space at all is much more valuable than how many pounds the satellite weighs. But for ISS resupply, what NASA cares about for its COTS program is total tonnage reliably delivered. And SpaceX's stated goal has always been reducing the cost of bulk cargo delivery, with "dollars per pound" as the metric of success.

Comment More physics than economics, and it checks out (Score 4, Interesting) 206

SpaceX, and the people in this thread, are comparing the vehicle cost to fuel cost, which is kinda cheating. It's not the cost of the fuel that matters, it's the cost of building the vehicle larger to hold that fuel -- and the fuel needed to launch that fuel -- that matters. So let's do the math!

Most data taken from http://spaceflight101.com/spac...
Basic info:
Stage 1: 23 tonnes structure, 400 tonnes fuel
Stage 2: 4 tonnes structure, 93 tonnes fuel
Payload: 13 tonnes

When launching, the first stage burns all 9 engines at full thrust for two and a half minutes. The re-entry burn and landing happen on a single engine, and from eyeballing the videos (including this one that shows the re-entry burn) appear to take about 30 seconds total. Assuming all burns are near full thrust (which is the best way to do it), that means the landing burn takes about (1/9) * (0:30 / 2:30) = 2% of the first stage fuel. Let's double that to 4% to provide a generous safety margin: that works out to about 400 * 0.04 = 16 tons more fuel.

This fuel is carried up to the moment that the second stage separates, so it subtracts from the mass of the second stage. Second stage plus payload weighs 110 tonnes: without the landing fuel, you could have scaled that up to 126 tonnes, a 15% increase.

So, landing the first stage reduces the payload SpaceX can launch, and thus the money they earn, by about 15%. In exchange, they recover about 75% of the cost of the launch hardware. So it's worth doing, even after you subtract off the cost of recovery and refurbishing. Maybe not the game-changer Elon Musk wants it to be, but it's a win.

Comment Re:This is bullshit, right? (Score 1) 66

If I had utterly failed to consider the possibility, I wouldn't have spent a couple sentences speculating on how it might be done. Could there be some revolution in optics I've never heard of? Sure, that's why I'm asking. Could a company have patented an idea they have no idea how to implement, so they can patent troll in the future? Well gosh, that never happens.

Comment This is bullshit, right? (Score 1) 66

This has gotta be bullshit, or at least a conceptual patent (which is another word for bullshit), right?

From everything I know about optics -- and I teach college physics, so I'm not clueless -- if you put a video screen on the surface of the eyeball all you'll see is a colored blur over your whole field of vision. What matters is not the location of the light source on your cornea, but the *direction* it's coming from: any workable video screen would need to work kinda like a phased-array radar, but a million times smaller. Or something like the Lytro light field camera in reverse.

But while I can think of ways in which such a thing *could* work, with current technology this is utterly impossible. Anybody better-informed than me care to weigh in?

Comment Re:record-shattering recording instruments (Score 1) 507

Because a controlled experiment involving the actual Earth would mean telling a few billion people they have to keep their population, technology, and society constant for a century, and a) nobody's gonna agree to that, and b) if they did there wouldn't be an issue.

Comment Re:Deniers? (Score 5, Informative) 507

Just to pick the top story on that realclimatescience.com site: it's looking at NOAA's statement that 2015 had record *average* temperatures in the US, and is rebutting with data on the *frequency of hot days* in the US, which is an entirely different idea. Since greenhouse gases control the rate at which energy *leaves* the earth to cooling it down, you would predict it should warm the coolest days more than the warmest. Which is exactly what's happened. IPCC report finds, globally, a significant increase in night and winter temperatures, a statistically insignificant change in temperature of the hottest days.

The match between theoretical prediction, and basic physics is the best way to assess the truth. You'll notice that the denialists will try to poke holes in the standard global warming story, but very rarely will they show show that their revised data agrees with a physical theory. (In particular, if CO2 and water vapor concentrations are rising, why *doesn't* that cause global warming in their view? By everything we know about these gases, it should.)

Comment Re:record-shattering recording instruments (Score 2) 507

[E] is not possible when the experiment is being carried out over centuries, with a civilization growing inside the test chamber.
[D] leads to biases (chiefly the urban heat island effect) which *increase* the apparent trend (see Layzej's reply). If you don't correct for them, global warming looks *worse* than it actually is.

Comment Re:Environmentally unconscious (Score 1) 197

Having some green spots in the city is hardly an "environmental problem"

Agree, except that it's a non-renewable resource: once a cemetery, always a cemetery, and there's a social taboo against using them as public parks or letting them revert to nature. I first started thinking about this when my commute took me past some of the big cemeteries on the north side of Chicago. Square miles filled with the corpses of just one century's worth of Chicagoans. Give it another few centuries, and the dead will own more land than the living.

Comment Re:Environmentally unconscious (Score 0) 197

Let me break it down for you: I eat my weight in plants and animals about every six months or so. So the amount of biomass in my actual body is less than 1% of the biomass in the food I ate, and the inedible plant and animal parts I threw away. You can try to recycle your body if you want to, it's a nice gesture, but it's an insignificant part of the environmental problem.

(You're also wrong on the biochemistry, "useful proteins and specialized molecules" are not assembled over centuries and preserved through the food web. Plants build all their complex biomolecules from scratch; animals break the biomolecules in their food down into simple organics and reassemble them.)

Slashdot Top Deals

If you think the system is working, ask someone who's waiting for a prompt.

Working...