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Mars2010: The Vasimr Drive For Space Exploration Panel Discu
Mars2010: The Vasimr Drive For Space Exploration Panel Discu
Videos: Mars2010: The Vasimr Drive For Space Exploration Panel DiscussionPart1 of 6
Part2 of 6
Part3 of 6
Part4 of 6
Part5 of 6
Part6 of 6
They bring up a very good point: power generation alpha, and cool side temperature management.
What sort of system alpha would Polywell have?
Power generation in the p-B11 system is not a heat cycle; how much mass _would_ be needed for radiators?
Here's where the fact that a space-only system would not need a vacuum chamber really shines. Your landing craft Alpha has to include a vacuum chamber, because you're in atmosphere.
What sort of system alpha would Polywell have?
Power generation in the p-B11 system is not a heat cycle; how much mass _would_ be needed for radiators?
Here's where the fact that a space-only system would not need a vacuum chamber really shines. Your landing craft Alpha has to include a vacuum chamber, because you're in atmosphere.
Wandering Kernel of Happiness
A panel discussion about VASMIR without any VASMIR advocate present is biased. Still the main criticism seem to be the massive power generation systems, including radiators that are necessary to take advantage of VASMIR (as opposed to small ion engines that get by with solar panels). The only thing they mention as a power source for the VASMIR is thermal fission reactors which need a cold sink (huge radiators) to produce electricity through a thermal cycle. The added weight of these systems heavily penalizes the Isp efficiency of VASMIR.
This is where a PB11 Polywell would excel. Direct conversion greatly reduces the amount of radiators needed. The RFC and DPF could also excel in this application, but not as well as the Polywell. The advantage in Q (and perhaps energy density) that the Polywell may have (Q of perhaps 20, instead of ~2-3) means much less waste heat per MW of power generated. The small size and weight (depends on how much mass is needed for the X-ray conversion system) of DPF may make up some, or all (?) of the difference. Of course, you also have to consider the output of each machine. It might take 20 DPF machines to equal the fusion output of a single 100 MW Polywell. IE: a DPF may be favored for a slow accelerating robotic mission, while a Pollywell might suit more rapid transit human spaceflight.
http://www.askmar.com/Fusion_files/Iner ... ulsion.pdf
The drawing on page 9 of this article demonstrates the difference is size of the comparable systems. I think a Fission system would be comparable to the compact Tokamak example in terms of the radiators. A fission reactor would probably be smaller and lighter (?) than even a compact Tokamak. A Polywell reactor may have a similar energy density/ mass compared to a fission reactor (but without the inefficient thermal electric conversion overhead).
Of course with these systems there are other options than just VASMIR for high efficiency thrust, especially with the Polywell driven engines described by Bussard.
Dan Tibbets
This is where a PB11 Polywell would excel. Direct conversion greatly reduces the amount of radiators needed. The RFC and DPF could also excel in this application, but not as well as the Polywell. The advantage in Q (and perhaps energy density) that the Polywell may have (Q of perhaps 20, instead of ~2-3) means much less waste heat per MW of power generated. The small size and weight (depends on how much mass is needed for the X-ray conversion system) of DPF may make up some, or all (?) of the difference. Of course, you also have to consider the output of each machine. It might take 20 DPF machines to equal the fusion output of a single 100 MW Polywell. IE: a DPF may be favored for a slow accelerating robotic mission, while a Pollywell might suit more rapid transit human spaceflight.
http://www.askmar.com/Fusion_files/Iner ... ulsion.pdf
The drawing on page 9 of this article demonstrates the difference is size of the comparable systems. I think a Fission system would be comparable to the compact Tokamak example in terms of the radiators. A fission reactor would probably be smaller and lighter (?) than even a compact Tokamak. A Polywell reactor may have a similar energy density/ mass compared to a fission reactor (but without the inefficient thermal electric conversion overhead).
Of course with these systems there are other options than just VASMIR for high efficiency thrust, especially with the Polywell driven engines described by Bussard.
Dan Tibbets
To error is human... and I'm very human.
Assume a direct conversion process for pB&J.
Would the collection process be adversly impacted if the slats (grids?) were at a very high temperature?
After all, the size of a radiator is controlled by the amount of heat to reject (linear) and the temperature (fourth power IIRC). So, what temperature should the slats(grids) be run at?
Would the collection process be adversly impacted if the slats (grids?) were at a very high temperature?
After all, the size of a radiator is controlled by the amount of heat to reject (linear) and the temperature (fourth power IIRC). So, what temperature should the slats(grids) be run at?
I like the concept of a LANTR triple-mode system, with a VASIMR for a long high-ISP burn. The LANTR in power mode can be configured to use the propellant tank for heat sink, and periodicly you can "vent" through the NTR, generate some thrust, and cool down the tank (via expansion).
A similar system could be used for a first-generation D-D polywell, too.
A similar system could be used for a first-generation D-D polywell, too.
Wandering Kernel of Happiness
Both ideas sound interesting. I'm guessing that the actual collection grids in an open PB11 Polywell could be warm (how warm?), and serve some radiative cooling funtion. Though I suspect using the Polywell in a dilluted fusion product rocket mode might manage thermal loads better, give reasonable thrust efficiency and good power resulting in faster acceleration to offset the lower ISP compared to the VASMIR at it's most efficient settings.
Using the fuel as a heat sink combined with a tank that is maximized for surface area (and combined with a Sun shade) could provide adequate cooling at smaller total system weight, at least in an intermittent mode. I'm not sure what the trade offs of also producing thrust by passing fuel through the NTR would be. If you can keep the reactor cool with fuel flows such that VASMIR provides a significant percentage of the thrust the net ISP may increase from the NTR efficiency of ~ 900 Isp to perhaps 2000, 4000, or 8,000 (?) Isp. Would it be worth the added weight and complexity?
Dan Tibbets
Using the fuel as a heat sink combined with a tank that is maximized for surface area (and combined with a Sun shade) could provide adequate cooling at smaller total system weight, at least in an intermittent mode. I'm not sure what the trade offs of also producing thrust by passing fuel through the NTR would be. If you can keep the reactor cool with fuel flows such that VASMIR provides a significant percentage of the thrust the net ISP may increase from the NTR efficiency of ~ 900 Isp to perhaps 2000, 4000, or 8,000 (?) Isp. Would it be worth the added weight and complexity?
Dan Tibbets
To error is human... and I'm very human.
As far as producing ~ 200 MW with a reactor plus cooling system power to weight ration of 1 KW / Kg as at target. At 200 MW that would be 200,000 kg or ~ 440,000 pounds. If an advanced Polywell can be built to power a locamotive, because of the rapid power scaling, I would guess that 1 KW/ kg would be an easy target, provided cooling can be managed.
Dan Tibbets
Dan Tibbets
To error is human... and I'm very human.