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Comment: Re:Lift? (Score 1) 79

by dougmc (#48902447) Attached to: NASA Considers Autonomous Martian Helicopter To Augment Future Rovers

So make them larger -- as large as you can get with the tips not quite going supersonic. (The speed of sound on Mars (probably around 540 mph at ground level) is a bit lower than it it is on Earth thanks to the low pressure, low temperatures and mostly CO2 atmosphere, so that's an even bigger problem.)

More blades as well -- not just 2, but 3, 4, 5, 6, whatever. There's diminishing returns past two (well, one!) but it can help when you don't mind using a lot more power for a little more thrust.

Go for fatter blades and higher pitches as well -- more diminishing returns, but it could still help.

If you're thinking of a multicopter as I imagine they are, go with more than four propellers. Putting propellers on top of other propellers could help as well, but again ... diminishing returns.

It's not trivial, but it should be doable.

Or maybe they could even let the tips go supersonic ... it might be less of a problem with such a thin atmosphere than it would be here. I'm not so sure about this.

so simply applying a scaling law like that isn't very accurate.

It's a good "back of the napkin" first order approximation. I'd expect NASA to take everything into consideration, model it exactly, and then actually build it and fly it in a chamber that approximates the atmosphere of Mars.

It doesn't need high performance or duration -- just enough to go almost straight up and pan around and take pictures and then land back and charge up again.

Comment: Re:Lift? (Score 3, Insightful) 79

by dougmc (#48899323) Attached to: NASA Considers Autonomous Martian Helicopter To Augment Future Rovers

Atmospheric pressure on Mars is 1% that of Earth. How're you going to get any lift?

if you rotate the blades 10x as fast as you do on Earth, you'll get the same lift.

That said, gravity on Mars is 1/3rd as much as Earth, so you only need 1/3rd the lift. So rotating the blades at 6x the rate you'd rotate them on Earth would be sufficient.

Or you could go with much larger blades.

Either way ... it's doable. It would require more power than it would on Earth, but it's certainly doable.

This is a pretty interesting discussion of how we'd fly on Mars, done in the context of the X-Plane simulator. It's written with fixed wing planes in mind rather than helicopters, but most of it still applies.

Comment: There is no right to a jury "of one's peers" (Score 1) 303

by dougmc (#48820781) Attached to: There's a Problem In the Silk Road Trial: the Jury Doesn't Get the Internet

In the US we have no right to a jury "of our peers" as we generally think of it.

It's one of those things that people think is in the Constitution but in reality is not.

We have the right to a jury trial. The jury has to be impartial.. It has to be in the state that the crime was committed. And that's it.

The only way we get a jury "of our peers" is if you consider that the American ideal says that we are all peers, regardless of gender, race, religion, education, experience, etc.

In the case of this specific trial, given that detailed knowledge of the Internet is rare, I imagine that the attorneys involved were asking questions designed to find out if any potential jurors had a deep understanding of these things, and while I'm not sure which side would be doing it, but one side or the other would decide that deep knowledge of these things was bad for their case, and since such people are rare, they'd use their peremptory challenge to keep such people off the jury.

Without this system, you might have a person or two on the jury who understands such things pretty well. But with the system ... such people would have been excluded by one side or the other.

Comment: Re:Airship one headed in the right direction (Score 1) 43

by dougmc (#48702177) Attached to: BU Students Working On a Cheaper, Gentler Suborbital Rocket

Cheaper way would be a large high altitude jet to carry the rocket to the edge of space.

The problem is - it's not really cheaper. Fuel is cheap, large high altitude jets aren't

More to the point, the high altitude jet doesn't help much.

Let's suppose we need to send something to the ISS. The ISS averages around 260 miles above sea level and orbits at about 17,000 mph.

So, our plane takes off at the equator and flies at 700 mph up to 11 miles (60,000 feet) above the ground. We launch rockets near the equator and to the East if possible to take advantage of the 1000 mph rotational velocity and our plane should do so as well -- so that means we need 16,000 mph more speed.

So, our high altitude high speed jet has provided 1/23rd of the speed and 1/23rd of the altitude needed to reach the ISS, and our rocket needs to provide the rest. (The fact that both worked out to 1/23 is just a coincidence.)

However, kinetic energy is porportional to speed *squared*, so really, the plane has only provided 1/500th of the kinetic energy needed to reach the ISS and 1/23rd of the potential energy. At the ISS. a kilogram of matter has about 30 MJ/kg worth of kinetic energy and about 3.4 MJ/kg of energy from the increased altitude (vs. sea level.) Note that the energy from the 17,000 mph is almost 10x as high as the energy from being 260 miles higher than sea level.

I haven't worked all of this out exactly, but it looks like putting your rocket on a plane and taking it up to 60,000 feet at 700 mph before launching saves less than 1% of the total energy needed to get to the ISS -- so it sounds good, but in practice it makes a lot more sense to just make your rocket a little bigger and launch from the ground.

Comment: Re:Cosmic Rays (Score 3, Insightful) 56

by dougmc (#48596685) Attached to: Raspberry Pi In Space

The reason they use older laptops is not because of the density of the chips but simply because they're known commodities -- any quirks they have have already been figured out and they get the job done. Getting anything certified (for mission critical purposes) is a very time consuming process, and once it's done ... the item is no longer state of the art, that's just the nature of the beast.

The Raspberry Pis don't have to go through the same certification process, though of course if they were expected to only work "for eight seconds" I think NASA would have told the people sending them up that to pick something older. I'm guessing that NASA knows a bit about the radiation environment up there and advises people who send up experiments appropriately.

And as others have said ... humans are living in the same environment for months at a time -- it can't be *that* bad.

Comment: Re:Cosmic Rays (Score 2) 56

by dougmc (#48595801) Attached to: Raspberry Pi In Space

The ISS is well below the Van Allen radiation belts and well within the Earth's magnetic field (which deflects many of the charged particles headed towards the Earth) so the level of cosmic radiation it gets is not *that* high, and the metal of the ISS blocks most of of that.

And if a Raspberry Pi does get its registers corrupted by cosmic rays ... it's not a tragedy. Nobody dies -- it's not mission critical.

In any event, they use pretty standard (but old -- last I heard, they still ran Windows 95) laptops on the ISS and they work fine. It would be interesting to know how much more often they experience failures and errors on the ISS due to radiation compared to how much they experience here, but I don't know if anybody has measured that. (My guess is that NASA has, though I wouldn't know where to look for the data.)

Comment: Re:Stop developing 64bit (Score 1) 242

by dougmc (#48251971) Attached to: OEM Windows 7 License Sales End This Friday

Are you trolling or what?

If you have a single process that needs to use more than 1.6 - 2.0 GB of memory ... you need the 64 bit version. And on top of that, if you've got 4 GB of memory the OS can use about 3 GB (total) due to the way Windows handles things.

The vaunted promise that 'things will run better and faster'

Who made that promise? I don't recall ever seeing that.

Comment: Re:A lot of complexity, a little gain? (Score 1) 98

by dougmc (#47509493) Attached to: Ask Slashdot: Linux Login and Resource Management In a Computer Lab?

I've used lots of multi-user linux boxes over the years and never noticed that a few bad users ruined the experience for everybody else.

I did ... but this was 25 years ago at college when hardware was scarce (we had 1 MB disk quotas!) and the computer system was used to do all sorts of things that people just couldn't do from their own personal computers (i.e. access mail, news or the Internet.)

Users policed each other back then to a degree, but there wasn't much you could do to make a bad user behave unless the sysadmins backed you on it, and they'd only back you if they explicitly broke the rules set down. And often you didn't even know who a user was -- if they sat at a console you'd know who they were, but if they dialed in you might just know their user name and often that gave no clue who they really were. (The sysadmins knew, but they wouldn't share.)

But now ... most of the things that caused problems can be done from anybody's own computer, or from a PC down in a lab somewhere. True multiuser systems are kind of rare nowadays, and most users probably don't deal with them where back then we had little choice.

Comment: A lot of this seems superflurious ... (Score 4, Interesting) 98

by dougmc (#47509353) Attached to: Ask Slashdot: Linux Login and Resource Management In a Computer Lab?

If you're giving your users access to the machines, they should be able to use them. And if you can't trust them to use them responsibly, don't give them access.

If it were me, I'd secure the boxes normally, set up some resource usage rules (guidelines?) and see what happens. If problems happen often, then maybe look into something automated to enforce the rules, but if not, then you're done.

As for renicing stuff done by remote users, I'm not sure this is a good idea, but if you want to do it you can renice sshd itself, and to be thorough you can also renice crond (if you give them access to cron/at.) But do keep in mind that nice (and ionice) can't do magic with an overloaded system -- they help, but they don't do magic.

As for commercial systems, I haven't really seen this as being a big problem outside academia. Multiuser *nix systems where different people are competing for resources is kind of rare in the commercial sector, as it seems like the trends lately are to have enough hardware, often dedicated, and to enforce limits through voluntary compliance (and have their boss talk to them if it's still a problem.)

That "have their boss talk to them" bit may not work so well for students, but still, I would wait for a problem to appear before I put too much effort into solving it.

Instead, put your efforts into proper sysadmin stuff -- stay up to date on patches, look for problems (especially security ones), make sure backups work, help users with problems, etc. If there's any troublemakers, talk to them, and if they don't shape up after a few warnings, kick them out. (And make sure the policies permit that!)

You can enforce limits on specific users through pam and sshd_config and some other mechanisms, but I'd suggest leaving that for later. Anything you do that will limit what people can do will eventually keep them from doing what they legitimately need to be doing.

Comment: Re:Cool video (Score 2) 200

by dougmc (#47390937) Attached to: The View From Inside A Fireworks Show

Crappy camera work? I take it you'd do better?

It's not like it's an easy place to put an expensive camera into. Anything bigger than a small R/C plane and they'd have stopped the fireworks entirely -- and personally, I'm sort of surprised that they didn't when they saw this craft up there. The odds of having the craft hit by a shell and crashing into the water were significant as well.

And it's quite dark, so we're stuck with high iso mode.

Personally, I thought it was quite excellent for what it was.

Comment: Re:Illegal and Dangerous? (Score 4, Informative) 200

by dougmc (#47390931) Attached to: The View From Inside A Fireworks Show

FAA limits model aircraft to a height of 500 feet

No it doesn't.

The 400 (not 500) foot figure comes from FAA advisory circular 91-57 made back in 1981, and the key thing about this is that it's *advisory*, not mandatory.

The AMA safety code says "Not fly higher than approximately 400 feet above ground level within three (3) miles of an airport without notifying the airport operator." -- but those are just safety rules for AMA members (and a good idea for everybody) -- but they do not have the force of law behind them.

Now, the FAA may change the laws in the future, but so far ... this 400 foot ceiling people talk about does not exist. (Some places have restricted airspace ... that's not what I'm talking about. I'm talking about this blanket 400 foot height limit people keep bringing up that doesn't exist.

Comment: Re:And this surprises... who? (Score 1) 191

by dougmc (#47365325) Attached to: 30% of Americans Aren't Ready For the Next Generation of Technology

I'm not sure "grandma" is synomous with senior citizens, but I guess it's as close as we're going to get real data on.

But either way, even if they "aged into being a senior citizen" ... there's still more of them using the Internet than not. Yes, they are often terrified of viruses and the like, and if they aren't they should be ... and I recall fixing up my mother in law's computer on a regular basis because it was riddled with crap ... but she still used it. She loved it.

And the "riddled with crap" problem isn't restricted to senior citizens. My children's computers have similar problems, and that's why I refuse to even let them use mine and set them up with their own ...

That said, if I had it all to do again today ... I might have set my mother in law up with a Chromebook or tablet or something instead, something that's pretty resistant to all the crap. I think it would do most of the stuff she wanted to do. My kids are digital natives and they want more than a tablet or Chromebook will provide -- but even so, that covers much of what they want too.

Mathemeticians stand on each other's shoulders while computer scientists stand on each other's toes. -- Richard Hamming