Want to read Slashdot from your mobile device? Point it at m.slashdot.org and keep reading!

 



Forgot your password?
typodupeerror
Trust the World's Fastest VPN with Your Internet Security & Freedom - A Lifetime Subscription of PureVPN at 88% off. Also, Slashdot's Facebook page has a chat bot now. Message it for stories and more. ×

Comment Re:so is there a good theory? (Score 1) 470

With the only known reactionless drive we have, the photon drive,
First of all the term "reactionless" makes no sense. You mean: "without reaction mass".
Secondly: the photon drive has a reaction mass, they are called "photons".

Of course a "normal" rocket is no PM, and neither is a EM drive, that was my point.

Your point seemed to be that my maths works equally well for a rocket and EM drive, so if it were correct, both would have been allowing PM. As it is already known that normal rockets do not allow PM, my maths must be incorrect.

My point is that my calculations only take into account what happens on the engine side. In the EM drive, the engine is all there is to it. It converts energy and produces work all by itself. In a normal rocket, what used to be part of the system is now expelled as reaction mass. So the calculations are complete for an assumed reactionless EM drive, but incomplete for a rocket engine. It may be the case that my maths are not correct for Em drive either, but your point for its incorrectness is not valid.

I am not saying EM drive is PM, nor am I saying that it cannot work. I am saying that it cannot be an engine that converts energy into propulsive power without interacting with any kind of matter, as assuming that allows an impossible construct, a PM machine. We just don't know where its reaction mass comes from.

Comment Re:so is there a good theory? (Score 1) 470

You have to pick a mass that is worked on and a frame of reference to define work & energy. If you ignore part of the system, it is natural that you can do over unity energy gains.
A rocket has a reaction mass, initially completely in the rocket, which is accelerated by a power source and expelled. Obviously, for space travel, we are interested in the rocket only, but to do a proper energy balance, you have consider the energy balance of the expelled mass too.
For simplicity, assume that we turn on the engine in empty space in a straight line and point of "launch" is our reference point. Since the rocket's thrust is constant, the rocket seemingly gains more kinetic energy per power spent per unit time as it speeds up. However:
1) The inverse is true for reaction mass coming out of the nozzle; it has the highest amount of kinetic energy a the start and it has less and less speed relative to launch point, therefore less and less kinetic energy relative to launch point, as the rocket speeds up.
2) The rocket is heaviest at liftoff, and it has less and less mass as it speeds up.
These two balances the equation exactly, giving a constant gain in total kinetic energy per power spent per unit time.
If you pick another frame of reference, say, the rocket itself, now the expelled mass has constant kinetic energy over time, as the exhaust speed is constant. The power is also constant. The rocket does not move relative to itself, so its KE is always zero (and we do not care about it getting lighter). Again the equation is balanced exactly.
Remove the reaction mass, and it is impossible to balance regardless of frame of reference. Whether you do this as an error in calculation, as in ignoring the reaction mass in an actual rocket, or by design, as in the case of so called reactionless drive, is irrelevant. It is not possible that Energy in=Energy out without the reaction mass (in case of photon drive, you have to take into account E=mc2 to do the balance)

Comment Re:so is there a good theory? (Score 1) 470

No you cannot. With the only known reactionless drive we have, the photon drive, you cannot reach the breakeven point. With any normal rocket (ion drive whatever), your system is not closed. Both energy and mass is consumed. The thrust may be constant (which is only approximately true) but the mass of craft is not. The kinetic energy of the remaining rocket+fuel changes exactly in proportion to mass loss and the energy expended to expel it. So, no PM.

Comment Re:so is there a good theory? (Score 1) 470

Actually the argument is pretty generic without any referance to any particulars of the thrust generation device or getting useful work our of it. Any device that convert energy in to thrust without interacting anything else cannot possibly be more efficient than a photon thruster, If emdrive is conclusively shown to produce trust more efficiently than that, there must be a medium that it interacts with. Hopefully, it is not something mundane and useless like outer casing of the engine, Earth's magnetic field or air around the engine, but an exotic possiblity like axions. In the latter case there is both new physics and new space exploration possiblities.

Comment Re:so is there a good theory? (Score 1) 470

I have not read his papers. I have tried to read his basic theory but I am not a physicist and most stuff in the Shawyer's explanation is beyond me. However thrust being variable wrt. speed is also incomprehensible. What is the frame of reference for this speed and why is that frame preferred? Any external frame of reference would violate even Galileo's relativity. Since this system is at rest wrt. its parts, there is no internal speed to refer to either.

Comment Re:so is there a good theory? (Score 1) 470

Converting work and energy is not essential for the argument. If you work out the maths, you can see the same problem exists in terms of kinetic energy gain when this device is accelerating. The problem is after a certain speed you gain more energy than you put into the system. The faster it goes, the more surplus energy it generates.

Comment Re:so is there a good theory? (Score 3, Insightful) 470

The grandparent used mixed power and energy units but I think the problem is easier to grasp with work and energy:

If the reactionless drive is assumed to produce trust based on the power input only, the force produced F is some constant efficiency constant, C, times the power. Let's say C=0.000005 and F=0.000005 P.

Assume we run this engine for a time period of t. Then the total energy E spent is P.t.

The work done by the engine is force it produces F times displacement s. Note that there are no time units in the calculation. W=F.s , the same amount of work is done with a given F and s, regardless of how much time displacement s takes.

You can convert work into energy and vice versa, so if you get more work than the energy you put in, you have a perpetual motion machine. That is if Net Work=F.s-P.t is greater than 0.

The problem is the energy we put in to system directly depends on time, while the work we extract from it does not. As the engine goes faster P.t drops proportionally, while F.s stays constant. The required speed for Net Work being positive is v=P/F, that is v=(P.C)/P and that is v=1/C. With the efficiency figure you have given the breakeven speed is 200000m/s. That is a a high but physically possible speed. The efficiency could have been a thousand times less, and it would still be possible to make a perpetual motion machine.

Comment Re:Just curious... (Score 1) 232

I see your point. I think there is no getting around the fact that we would receive too little light from the planet. Discarding almost all of the photons coming from it is the best start for a solution, if the best result we could hope is to tell the planet together with all local junk from interstellar objects.

Comment Re:Just curious... (Score 1) 232

IANAA but I have an impractical idea that might work. Everything outside solar system is either redshifted or blueshifted by a large margin. If we use a very narrow band of spectrum for observation we may observe only stuff move relatively slowly wrt Earth. For example, if we are reasonable sure that any planet in our neighborhood would have some helium in it, just look at around 447.148 nm. Stuff near us move relatively slowly wrt Earth, so helium's absorption bands would be shifted only slightly. If we have a whole sky map at 447.148 nm and one at 447.147, subtracting the former from the latter would give you map of things that are something close by and has helium in it.

Comment Mr. Don Dailey (Score 3, Interesting) 107

I am disappointed to see only one mention of late Don Dailey in TFA. He is actually the guy who wrote the whole thing. I had followed his posts for years in computer go mailing list. I have learned a lot from him as an R&D engineer in an unrelated field (chemical industry). While many people adopted "improvements" only because it made sense to them, Mr. Dailey had a very systematic and methodological approach to changing the program. He had ideas and insights for improvement like many others, but he never fell in love with his own ideas. If something did not work, it did not. No matter how plausible it seemed. He also had most patience I have seen of an online person. He would carry on discussions long after it was obvious the other party was not paying enough attention or was simply stupid. He did this almost to the day he died.
Congrats Mr. Dailey. You have done it.

Comment Re: This is ridiculous (Score 1) 117

IIRC that would be two. But that is not the correct way to look at it. How many Mars landers were lost after they left the Earth orbit, but before they were restarted to enter Mars orbit? I believe that number is zero. Mr. Newton is driving these craft now. It is very improbable for a mission to fail while cruising in sleep mode.

Comment Re:Is it really "impossible"? (Score 1) 315

Conservation of momentum is just as fundamental a principle as conservation of energy. That doesn't mean that a drive that requires no fuel is impossible - because you can convert energy to matter - it just means that it has to dump the counterforce somewhere to keep momentum accounts balanced.

This device cannot work by ejecting mass it produces. The (lowest) quoted performance is 5.3 uN per W. If input energy is completely converted to ejection mass, the mass must be ejected at ~1500c to achieve the quoted force.

Comment Re:The issue is not about compliance with the law (Score 5, Informative) 94

The situation in Turkey is not just another free speech banning law, making law maker and enforcers looking ridiculous. Not at all. There is a mindbogglingly huge corruption scandal going on. The prosecutors were removed from the case, police were ordered not to obey court orders, tens of thousands of civil servants have been relocated etc to stop the investigation. The extend and the number are both unbelievable, so I will leave it to look them up yourself (you would never believe an anonymous source on internet talking about 12 digits, would you?)

Now, when it became apparent that the prime minister had no intention to actually let courts do their job, the prosecutors (quite unlawfully) started leaking dozens of voice recordings of their evidence. So far we have learned that Mr. Prime Minister ordering newspapers what not to press, ordering his son to move hundreds of millions of dollars from his house, selling valuable land to his friendly businessmen, using tax law to crush unfriendly businessmen, ordering the police to increase tension during Gezi movement etc. Tomorrow is the big day. It is said that the PM will not be able to keep his post no matter what after the recording posted on 25th of March. The leaked tapes so far has been uncovered PM's behavior so unconstitutional and immoral that I cannot image what could possibly be so much bigger. The expectation is that either PM's ordering assassination of a opposition leader or he having sex with a minor. Whatever it is, it got PM panicked. This is what got actually twitter banned. There is a cover story, but it is so hastily constructed that *the cover story itself is unlawful.* The story is that a court banned twitter on for not complying, a court which is not authorized to do so, and a court which denies doing/trying to do so.

So whatever your ideas on different lands having different customs and laws, this is not the event to discuss them. Twitter ban in Turkey is 100% wrong.

Comment Re:Don't go to college, it's clearly not for you (Score 1) 384

In the book Outliers, Malcolm Gladwell summarizes a series of research which seems to prove that while having enough intelligence for a particular task is a must to succeed in that task, having additional IQ makes next to no difference. This is true for at all levels of complexity of tasks (from winning a Nobel to keeping a daily job) inspected. Being a bright and otherwise nondescript person I am, I would love to see some research contradicting the claim. I suspect there is none.

Slashdot Top Deals

Executive ability is deciding quickly and getting somebody else to do the work. -- John G. Pollard

Working...