Author Topic: Vasimr: A true game changer? Role of Vasimir in future programs  (Read 31631 times)

Offline pathfinder_01

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Why does VASIMIR have a low T/W?

Vasmir and all forms of electric propulsion have low thrust. Basically ISP and thrust are rather opposites.  It is difficult to get both high isp and high thrust in the same system.  VASMIR as plasma propulsion has more thrust than ion propulsion and has a great ability to trade thrust for ISP but it will never have as much thrust as a chemical system.

Basically in terms of thrust

Chemical>Nuculear thermal>Electric propulsion

In terms of ISP

Electric propulsion>Nuclear thermal>Chemical propulsion

Offline tnphysics

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Okay let me put it this way: why is the power/weight of VASIMIR bad vs. an arcjet?

Online docmordrid

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Robert Zubrin launched into a rant about VASIMR on July 11, 2011 calling it a "hoax" -

http://www.marssociety.org/home/press/tms-in-the-news/thevasimrhoax

PhysOrg story from July 13, 2011 -

http://www.physorg.com/news/2011-07-zubrin-vasimr-hoax.html

and Ad Astra posted this on July 15, 2011 -

http://www.adastrarocket.com/VASIMR_development_AdAstra_15July2011.pdf

I found several problems with Zubrin's article including his contention that high temperature superconducting magnets it would find useful do not exist.  Ad Astra countered that assertion in their reply -

Quote
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The technology of advanced high-temperature superconductors has also matured significantly in the last decade with the development of commercially available high current density BSCCO and YBCO conductors by companies such as Superpower Inc. and American Superconductor. These developments have made lightweight, high-field magnets for VASIMR® applications feasible. Moreover, miniaturized cryocooler technology, such as the Sunpower M series model, has already flown in space and are being considered by Ad Astra as part of the cryogenic system for the flight magnet. Ad Astra’s VX-200 test engine features the largest cryogen-free, high-field, low-temperature superconducting magnet in existence today. This magnet was developed to meet Ad Astra’s specifications and has operated quite successfully in the VX-200 engine since its integration in May of 2009. The company is currently designing a high-temperature flight version of this system.
>
« Last Edit: 07/21/2011 08:09 AM by docmordrid »
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Online alexterrell

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Okay let me put it this way: why is the power/weight of VASIMIR bad vs. an arcjet?
Mainly because VASIMR's exhaust velocity is much faster than an Arc Jet.

Assume it's 5 times faster (25km/s v 5km/s).

That means every gram thrown out produces 5 times the thrust. But it needs 25 times the energy. So with the same power supply, you can only throw the stuff out at 1/25th the rate.

So your thrust is 5 x 1/25. i.e you have one fifth the thrust. But the same fuel lasts 25 times longer.

Hence VASIMR is suited to long duration missions.

Online alexterrell

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Robert Zubrin launched into a rant about VASIMR on July 11, 2011 calling it a "hoax" -

http://www.marssociety.org/home/press/tms-in-the-news/thevasimrhoax

PhysOrg story from July 13, 2011 -

http://www.physorg.com/news/2011-07-zubrin-vasimr-hoax.html

and Ad Astra posted this on July 15, 2011 -

http://www.adastrarocket.com/VASIMR_development_AdAstra_15July2011.pdf

I found several problems with Zubrin's article including his contention that high temperature superconducting magnets it would find useful do not exist.  Ad Astra countered that assertion in their reply -

Zubrin himself did some work on magsails and in his chapter on it (written 10 years ago) stated that existing superconductors could operate with no cooling, just insulation.

Zubrin always goes over the top to try and make a point and then discredits himself. That said, both his arguments and the counter points are valid.

VASIMR is a useful electric thruster and could be useful for shipping cargos to Mars orbit in 1 year. However, chemical rockets have the Oberth effect advantage, so I don't see a significant benefit. More useful is a Low Earth Orbit to L1 tug, especially with a fuel depot at L1.

Offline A_M_Swallow

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This is the dangerous sentence.
Quote
... In contrast, after 30 years of research, the VASIMR has only obtained about 50 percent efficiency in test stand burns of a few seconds’ duration, and that is only at high specific impulse.

For a 6 month burn the VASIMR needs to thrust for 6 * 32 * 24 * 60 * 60 = 16588800 seconds.
It sounds like the current VASIMRs cannot even thrust for a full minute.  That definitely needs fixing.

Offline baldusi

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This is the dangerous sentence.
Quote
... In contrast, after 30 years of research, the VASIMR has only obtained about 50 percent efficiency in test stand burns of a few seconds’ duration, and that is only at high specific impulse.

For a 6 month burn the VASIMR needs to thrust for 6 * 32 * 24 * 60 * 60 = 16588800 seconds.
It sounds like the current VASIMRs cannot even thrust for a full minute.  That definitely needs fixing.

The current vacuum chamber can't pump to keep the necessary vacuum for more than a minute. That's why they want to test it on the ISS. The ISS can't give enough power for more than 15 min, though. But that's an order of magnitude more testing. Besides, the ISS is a microgravity research lab. Unless the VASIMR was set to exactly offset the atmospheric drag, it might ruin some experiments.

Offline 8900

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Youtube video of the Mars Society's presentation
« Reply #142 on: 12/08/2011 12:47 PM »
Youtube video of the Mars Society's presentation
http://www.youtube.com/watch?v=myYs4DCCZts

by the way, on the power per unit weight matter, I am not sure whether ISS solar panel is a good indicator. Do anyone know what is the efficiency of ISS solar panel? Given the fact that it is a Si based technology and it is produced in the 90s, I think we do have better, more efficient solar cells on the market (and surely will the efficiency continue to improve over time). So, maybe solar is the best partner of VASIMR not nuclear.
http://upload.wikimedia.org/wikipedia/commons/c/c9/PVeff%28rev111205%29.jpg
« Last Edit: 12/08/2011 01:12 PM by 8900 »

Offline spectre9

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Solar VASIMIR will not move much without panels 10x bigger than on the ISS even with new technology.

Good bump though  :P

Offline Robotbeat

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Solar VASIMIR will not move much without panels 10x bigger than on the ISS even with new technology.
...
Really? You must not have researched this, then! :)
ISS's total size is about the size of an American football field. That's 57600 ft^2. At 40% efficiency (new technology, remember?) and the insolation at 1AU of about 1500W/m^2, that works out to about 3 Megawatts. Most proposals for SEP tugs talk about power levels in the 500kW range, and even 150kW is useful.

The ISS's solar arrays are, compared to the state of the art nowadays, heavy and inefficient. A poor comparison point. Modern solar cells can operate at an efficiency level of four times ISS's cells (i.e. 40% versus 10%), and even the common commercial-level cells are 30% efficient.
« Last Edit: 12/08/2011 10:26 PM by Robotbeat »
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline 93143

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Does Zubrin really think Topaz is the best we can do?

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110009914_2011009270.pdf

Figure 5.  A 10 MW plant is predicted to have a system specific power of about 1 kWe/kg, including radiators.  Larger plants could have even higher specific powers.
« Last Edit: 12/08/2011 10:13 PM by 93143 »

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