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## Comment Trust? (Score 2)103

How does a non-expert know whether this really is secure or has a NSA / FBI / Chinese etc back door. The government can easily afford to pay people to post on public forums like this claiming that any particular software is or is not secure.

Open source doesn't really help since very few people are expert enough (or have time) to review the code, and its impossible to tell if other "experts" are paid to spread misinformation.

## Comment Re:Dirk Gently (Score 1)299

A wire pendulum suspension (like used on LIGIO) is probably the most sensitive. do it in hard vacuum to get rid of convection currents. Power the device with batteries, and put it all in an insulated box whos outside temperature doesn't change significantly during a short run. Then pulse the engine at the pendulum frequency . Its a doable experiment but quite a lot of work to get right.

## Comment Re:London to paris (Score 1)55

For subsonic planes like airlines, the "jet" engines are high bypass and the majority of the thrust comes from the ducted fan . Ducted fans behave like propellers, the ducts improve efficiency at near sonic speeds, but otherwise there is not much difference. There is a little thrust from the jet exhaust, but that is low efficiency because its velocity is so much higher than the aircraft efficiency.

If you looked at the ISP of a prop plane it would be very high since the reaction mass is moving slowly. This sounds counter-intuitive becuase for a rocket a high exhaust velocity -> high ISP. The difference is that for a rocket the energy source and the reaction mass are the same thing. The momentum goes as MV, while the energy goes as 0.5MV^2 . So the energy / momentum goes as V. If you have an external medium to move (air), you want a low exhaust velocity (about the same as the aircraft speed). If you have to carry the reaction mass you want a high exhaust velociyt.

## Comment Re:London to paris (Score 1)55

Its interesting (surprising) that turbine engines are actually a bit less efficient than most piston engines. See: https://en.wikipedia.org/wiki/... Note that the 1996 turboprop engine is less efficient than the simlar size and application piston engine. The turbine engine is much lighter so the overall aircraft efficiency is better. Similarly the diesel ship engines are more efficient that turbine ship engines of similar sizes.

In turbines the maximum combustion temperature has to be within the operating range of the turbine blades. For a piston engine, the high temperatures in the middle of the cylinder don't have time to melt the metal before it is cooled on the next cycle. Piston engines can run with hotter combustion temperatures so they have better base Carnot efficient. There is a fantastic set of books by Taylor: "the internal combustion engine in theory and practice" that is great reading for anyone really interested in how engines work. I found a lot of surprises.

You mention that the high exhaust velocity of jets reduces their efficiency in low speed applications. That is in addition to the above thermodynamic efficiency.

## Comment Re:Illutrates the flaw in electric vehicles (Score 1)55

Lots of tradeoffs. Airflow over the wings helps takeoff performance, but the disrupted airflow from the props in cruise is likely top reduce efficiency. Generally for low speed aircraft you want as few total propeller blades as you can use in order to reduce the losses from blade tip vortices. (practical effects like prop diameter will often force you to more props and more blades).

## Comment Re:London to paris (Score 1)55

Ground effect planes are interesting. The Russians did a lot with those. https://en.wikipedia.org/wiki/...
I think the biggest problem is that the low altitude environment is pretty hostile - waves, birds, floating obstructions etc. In principal though they are more efficient than airplanes.

My comment really was that using buoyancy lift seems like it should provide a major fuel savings, but in practice (I've looked at other examples as well) it doesn't seem to make a lot of difference to the overall efficiency. It is very helpful if you want vertical takeoff - helicopters are very inefficient .

The examples I gave used the same technology engines so I figured that canceled out. It also turns out that aircraft piston engines are pretty efficient - its simple design, almost single operating point, so its much easier to get good efficiency than it is for a car engine) . there has been little change in aircraft piston engine efficiency since 1960. Jets are actually less efficient, but the lighter weight more than makes up for that. Going to high bypss jets has made the biggest difference. Aircraft aerodynamics has improved some. I don't know if its possible to improve the aerodynamics of buoyant craft.

## Comment Re:London to paris (Score 1)55

Lighter than air craft have a lot of problems. The energy use isn't as as low as you might imagine - there is no drag due to drift, but the large frontal area resulst in a lot of parasitic drag except at very low speeds. Winds, ice etc can be a serious problem, and they typically can't climb above weather.

one example at http://www.zeppelinflug.de/en/
carries 16 people, 80mph, 600hp total engines, range 600 miles (they don't give detailed specs).

Compare with a 1960s beechcraft baron:
6 people, 230mph, 600HP total engines, range ~800miles

person miles / gallon seems to be in the same ballpark. The airship may be a lot more pleasant to fly in, but its isn't substantially more efficient .

## Comment Re:Brief answer (Score 2)245

Most other hardware doesn't have the external and CPU interfaces. A disk driver probably cannot do anything very useful with the data it sees if you use disk encryption. It can't read the data and probably doesn't have external network access.

This management system by design has low level access to the CPU and external interfaces. It is potentially a much more capable hacking tool.

Its not easy to think of a reason that there is not a local hardware disable since the management capabilities are not needed by a substantial number of users.

If Intel knows how to access the chip, presumably they can be ordered to do so by the government, making this a universal back door.

## Comment Re: If this is correct it should be easy to check (Score 1)299

Black body radiation was measured a long time ago, back when we were just stating to be able to do complex quantitative measurements. It was measured in the late 1800s, and explained by quantum mechanics.

The problem with the EM drive and new physics is that it is operating under very normal conditions - modest frequencies, field strengths and length scales. There is a huge amount of experimentation, intentional, and as a side effect of normal engineering in this parameter range. There really is no reason to expect this particular experiment to find new physics because they are not doing anything particularly new.

## Comment Re: If this is correct it should be easy to check (Score 1)299

LIGO does get a lot out of looking for common signals between detectors but it also has exceptionally good isolation from ground motion. Of course it is looking for very much smaller signals. It also only looks at non DC effects. Its really a completely different animal. (BTW I worked on the predecessor to LIGO in 1983....awesome to see those guys finally succeed!)

The problem with the EM drive is that if it generate more thrust than a photon drive would for the same input power, without using any propellant, it violates basic physics and cannot work. If it uses propellant, then it is just a rocket. If it generates the same thrust as a photon drive, then it has the same problem of requiring an impractical amount of power to produce useful thrust.

## Comment Re:Dirk Gently (Score 1)299

I don't believe that they have corrected for all the external effects. Its a really tricky problem to get right.

I'm an experimental physicist and I can imagine how to do this experiment, but it would be a lot of effort and expense to do it correctly. There are a huge number of effects to take into account. The basic problem is that you are changing the power to the device by a lot, which can cause heating, and then trying to measure an extremely tiny force as a result.

## Comment Re: If this is correct it should be easy to check (Score 1)299

the gravity wave detectors take GREAT pains to make sure that noting touches the test masses except the suspension cables (which are single crystal fibers). Testing the EM drive requires power cables, or microwave waveguides - either of which can produce substantial forces when it warms up from power running through it.

You could do the experiment, but it would be a lot of work, and its certain to fail.

## Comment So let intel insure it (Score 1)245

If they are confident of their security, they ought to be able to get Lloyds to insure users against any break-ins or damages at the few X 100B\$ level. Oh, maybe they can't convince Lloyds that it is *that* secure?

One thing that Snowdon taught us is that even the NSA cannot protect secrets. And yes, you can fault the entire program because of a single slip up.

## Comment Re: Must be a first for slashdot RTFA skimmed summ (Score 1)299

Are you a physicist?

Looking at your link: Its true that in a waveguide electromagnetic radiation propagates at different speeds than in a vacuum. The group velocity is lower than C, and the phase velocity is higher. There is some force on each end plate, and some net longitudinal force on the tapered side walls. You could do a complicated simulation / calculation to sum up the forces (difficult to get correct due to the complex boundary conditions), or you can fall back on basic principals. Imagine that the walls are prefect conductors. Break up the volume into tiny cells. Maxwell's equations are obeyed in each volume. Those equations conserve momentum. Since momentum is conserved in each tiny volume element, it is conserved in the entire system.

The think is, they ARE claiming violation of momentum conservation. Depending on what paper you read they claim that either:

A closed system generates force without any exhaust. That is by definition violation of conservation of momentum.

OR

A system generates thrust without consuming any fuel, AND that thrust is larger for the amount of power applied than can be explained by emission of photons. To produce momentum in the rocket, you need to produce the same (opposite direction) momentum in the exhaust. If you are not consuming fuel, the mass of the exhaust must be created from the energy consumed. This is a photon drive - you create mass (photons) that are emitted at the speed of light. If you emit the mass at a lower speed it will have less momentum for the same total energy (including the energy required to create the mass).

If there was some background to push against, then the drive could work. But many many experiments (starting with Michelson Morley) have shown that there is no background ether you can push against. If there was such a background that had somehow escaped all experiments, there is no reason to think it would show up in this particular experiment.

Unruh radiation is a complete red hearing. Its only significant at extreme accelerations and it will conserve momentum

Now, to be clear: if they are only claiming the same thrust as a photon drive, then it can work through some mechanism, but its also boring - its easy to build a photon drive, just no practical way to power one.

If they are claiming that some of the mass oft he rocket is used as exhaust, then that is just a different type of rocket and that is OK.

What is not possible is for it to both not consume fuel AND produce more thrust power than does a photon drive.

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