NewSpace 2010: Polywell and Vasimr

Point out news stories, on the net or in mainstream media, related to polywell fusion.

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neutron starr
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Postby neutron starr » Mon Aug 09, 2010 4:06 pm

Sorry, I was using my avaition definition of 1 to 1 thrust to weight, which is thrust equal to the entire weight of the aircraft (or spacecraft), as in able to accelerate in the vertical, not a very common capability in aircraft. So, yes, the entire weight of the spacecraft needs to be lifted out of our gravity well. Does that mean you are saying that this can only be done with a spacecraft that is capable of producing more thrust than the entire spacecraft weighs?


Unfortunately any orbital space craft must produce better then 1/1 thrust to weight. a space craft that hurls itself on parabolic trajectories but never reaches orbit could make it to "space" without that but would have to use wings to do so (most would call it a plane). Basically if you want to get your craft out of the atmosphere it must get better then 1 to 1. This of course means the thruster probably needs significantly better then 1 to 1, but I thought I’d be generous.

Second, while thrust efficiency is a measure of thrust to electrical power in, this could easily mislead you to believe that the thruster is a transducer that converts electrical to kinetic energy. This is not true. M-E thrusters are actually transistors, that control the flow of gravinertial energy. This is important to keep in mind because when we look at higher efficiencies, we'll be tempted to think we have a violation of conservation, should we mistake a transistor for a transducer.


For us mechanical engineers it may be a gravity amplifier of sorts (this is what you meant by your insistence that it is a transistor?) but that doesn't matter for us. We think more like yay magic black box and see electricity flowing in one side and the box surging forward under its own mysterious forces. So the net effect we see is the craft electrical system losses X number of joules per second and the craft picks up Y amount of momentum per second, whatever the technical realities may be inside the magic box. Or I don't understand what the M-E thruster is supposed to do for us. Also if it does work this way how is it not a violation of conservation of momentum? And apologies to any other mechanical engineers out there if they take a more educated view about electronics and component parts then I do, sorry.
The measure of (mental) health is flexibility (not comparison to some ‘norm’), the freedom to learn from experience…The essence of illness is the freezing of behavior into unalterable and insatiable patterns.
Lawrence Kubie

Betruger
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Postby Betruger » Mon Aug 09, 2010 6:48 pm

There are a good number of papers detailing the exact conjecture for you to evaluate.
but that doesn't matter for us. We think more like yay magic black box and see electricity flowing in one side and the box surging forward under its own mysterious forces. So the net effect we see is the craft electrical system losses X number of joules per second and the craft picks up Y amount of momentum per second, whatever the technical realities may be inside the magic box. Or I don't understand what the M-E thruster is supposed to do for us. Also if it does work this way how is it not a violation of conservation of momentum?

D Tibbets
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Postby D Tibbets » Mon Aug 09, 2010 8:21 pm

Concerning thrust to weight ratios, I'm pretty sure that it has to exceed one for ballistic launches, but not quite for atmospheric flight. A space plane could use aerodynamic lift to keep it up as it accelerates to several thousands of MPH before climbing out of the atmosphere. . At that speed, especially in an equatorial launch profile, the craft will have acheived ~ 10-20% of orbital velocity, so its weight would be ~ 10-20% less (~ 10-20% less vertical thrust necessary to keep if from falling to the ground). So at thrust to weight ratio of ~ 80-90% might serve in a pinch. It depends on the efficiency of the engines. If you are only managing to accelerate slowly, the amount of fuel needed will increase. And, of course, wings also adds weight...wings add drag...this drag adds heat (though not much concern about this if the craft is designed to withstand reentry heating).

Dan Tibbets
To error is human... and I'm very human.

Warthog
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Postby Warthog » Mon Aug 09, 2010 8:44 pm

"A space plane could use aerodynamic lift to keep it up as it accelerates to several thousands of MPH before climbing out of the atmosphere. . At that speed, especially in an equatorial launch profile, the craft will have acheived ~ 10-20% of orbital velocity, so its weight would be ~ 10-20% less (~ 10-20% less vertical thrust necessary to keep if from falling to the ground). So at thrust to weight ratio of ~ 80-90% might serve in a pinch. It depends on the efficiency of the engines. "

Once upon a time, I read a sci-fi story based on exactly that. A small company had developed a reactionless thruster, and the hero/heroine equipped an old DC-10 with it. Used the regular engines to get to max altitude for the DC-10, then kicked in the thruster and "flew" to a record altitude for a DC-10 as a publicity stunt. I forget all the details about Mach number and such, but the story covered a lot of it. Lift/drag had a lot to do with it, as I recall.

GIThruster
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Postby GIThruster » Mon Aug 09, 2010 8:58 pm

Hmmm. . .well I'll leave it to others to determine just what is possible with less than 1:1 thrust. I will say though, that IMHO, it's worth having at least 110% thrust. Then your spacecraft can afford to go slow in atmosphere and fast in vacuum. You get your 1-Gee solution in that you can fly with constant 1 Earth Gee acceleration. You can hover. No need for hypersonic exits or reentries so you have vastly reduced requirements and stresses on the craft. You can descend into any gravity well smaller than our own. This all goes to what it takes to be safe, quick, convenient and economical. save hypersonics for military craft. Civilian use doesn't require it.
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis

Betruger
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Postby Betruger » Mon Aug 09, 2010 9:31 pm

Betruger wrote:There are a good number of papers detailing the exact conjecture for you to evaluate.
if it does work this way how is it not a violation of conservation of momentum?
In fact that question's been asked and answered numerous times. IIRC at least once here on T-P and a couple of times at least on the NASAspaceflight Advanced Concepts subforum.. IIRC the Propellantless Propulsion thread that's probably linked somewhere around here.
http://forum.nasaspaceflight.com/index. ... ic=13020.0

If Paul March doesn't answer here (understandable given how many times he's done so already, esp if he has nothing new to add) you can find his posts under the "Star-Drive" handle in that NSF topic.

also http://www.cphonx.net/weffect/alt.php

GIThruster
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Postby GIThruster » Mon Aug 09, 2010 11:15 pm

The reason that M-E thrusters do not violate conservation of momentum is that they are gravinertial transistors, controlling the flow of gravinertial flux in and out of the thruster as it shakes the portion of the mass that is changing back and forth. Therefore if you want to do conservation analysis, the proper system to consider is the universe. All of it.

Mach's principle says that everything in the universe is connected by its gravinertial field, and so if you want to do a momentum exchange balance, you need to look at the entire universe.

Once you do this, any concerns about conservation vanish. Concerns only arise from looking at too small a system. The thruster pushes against the GI field much as your feet push against the ground when you walk. When you walk East, the planet responds slightly by accelerating West. We don't notice this, but the same is true when you steal momentum from the universe.
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis

Skipjack
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Postby Skipjack » Mon Aug 09, 2010 11:25 pm

Once you do this, any concerns about conservation vanish. Concerns only arise from looking at too small a system. The thruster pushes against the GI field much as your feet push against the ground when you walk. When you walk East, the planet responds slightly by accelerating West. We don't notice this, but the same is true when you steal momentum from the universe


Yeah that has always been my understanding of this too. I thought that this part of the theory has already been stated by others on this board many times (IIRC including myself).
If people would only read up on that.

icarus
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Postby icarus » Mon Aug 09, 2010 11:47 pm

When you walk East, the planet responds slightly by accelerating West. We don't notice this, but the same is true when you steal momentum from the universe.


.... and yet again .... the universe moves relative to what exactly???

GIThruster
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Postby GIThruster » Tue Aug 10, 2010 1:41 am

Well now the situation is suddenly more complicated. The whole universe does not strictly speaking move, and because of the way the momentum exchange happens, it doesn't all happen at the same time. If you try to do a careful momentum exchange calc, what ends up happening is you find you're stealing momentum from the future of the universe.

The example of the Earth and walking is just an illustration and metaphor. They always break down somewhere. For a more careful examination of this you have to see Woodward's papers.
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis

EricF
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Postby EricF » Tue Aug 10, 2010 2:54 am

icarus wrote:
When you walk East, the planet responds slightly by accelerating West. We don't notice this, but the same is true when you steal momentum from the universe.


.... and yet again .... the universe moves relative to what exactly???


Well now the situation is suddenly more complicated. The whole universe does not strictly speaking move, and because of the way the momentum exchange happens, it doesn't all happen at the same time. If you try to do a careful momentum exchange calc, what ends up happening is you find you're stealing momentum from the future of the universe.



So the Universe 'moves' relative to time?

GIThruster
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Postby GIThruster » Tue Aug 10, 2010 4:03 am

Hmm. . .not exactly sure what you're saying. All motion is relative to time. From what I remember reading the papers several years ago, the momentum that is found in the thruster, goes missing from the rest of the universe in such a way that it affects the far future--there's less momentum there. I can't venture much past that as I'm not a field theorist and this is really GR stuff. Best is read Jim's papers.
Last edited by GIThruster on Tue Aug 10, 2010 6:46 am, edited 1 time in total.
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis

icarus
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Postby icarus » Tue Aug 10, 2010 6:10 am

Best is read Jim's papers.


And you can confidently state that momentum conservation is demonstrated in these writings?

Your metaphor was practically worthless since it broke at the first and most important step.

Giorgio
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Postby Giorgio » Tue Aug 10, 2010 6:36 am

GIThruster wrote:Hmm. . .not exactly sure what you're saying. All motion is relative to time. From what I remember reading the papers several years ago, the momentum that is found in the thruster, goes missing from the rest of the universe in such a way that it effects the far future--there's less momentum there. I can't venture much past that as I'm not a field theorist and this is really GR stuff. Best is read Jim's papers.


That can be very misleading.

Motion can be considered "over a time axe" or "within a time frame", but not "relative to time" if you want to invoke an exchange of momentum. That is becouse exchange of momentum with time is mathematically impossible to describe (at least at the best of my knowledge) unless you accept the idea that our universe splits every infinitesimal time to create an alternate universe with whom exchange momentum.

GIThruster
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Postby GIThruster » Tue Aug 10, 2010 7:00 am

icarus wrote:
Best is read Jim's papers.


And you can confidently state that momentum conservation is demonstrated in these writings?

Your metaphor was practically worthless since it broke at the first and most important step.


I can confidently say momentum is conserved from the explanation I already gave. In order to conserve momentum, you have to consider the universe as the system. If you don't, you'll get what appears a conservation violation because momentum is being transferred between the thruster and the rest of the universe. If you do choose the universe as the system, it should be obvious no violation occurs.

The question of how the missing momentum is actually missing from the rest of the universe is a separate question. To understand the answer to that question, you'll want to have a graduate understanding of field theory and read Jim's papers.

The analogy is a good one used in it's proper range. The question asked of it is not obvious to most people so the analogy doesn't break down. It is still good for explaining how momentum is transferred between the two parts of a system, a person walking and the planet on the one hand, and the M-E thruster and the universe on the other. The fact that a closed universe cannot be displaced (nor can it rotate) is not salient to the issue.

But you knew that, didn't you. :-)

The question is a good one and comes up in real studies on Mach. I don't want to disparage it, but it is a separate question from the conservation issue.

Let me try again to make the conservation issue more plain. It's not just momentum that needs to be conserved, it's also matter that need to be conserved.

Think for a moment what is happening inside the active mass of the thruster. At one moment, the mass is normal. In the next quarter phase, it fluctuates positively. Lets say you have 1 gram of active mass and you fluctuate it 50%--so now you have 1.5 grams of mass. If you don't recognize the mass is coming from the gravinertial flow into the thruster, then you'll think you see a conservation violation. You HAVE to use the entire universe as your system, and Mach's Principle HAS to be true. If it's not, these thrusters absolutely cannot work.
Last edited by GIThruster on Tue Aug 10, 2010 7:25 am, edited 1 time in total.
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis


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