Carbon nanotubes for "ionic wind" craft or "ionocraft".

Copyright 2016 Robert Clark

(Patents Pending) 

 

 Nanotechnology makes possible an "ion drive" for air vehicles analogous to the famous ion drive of NASA's deep space probes:

Carbon nanotubes for "Ionic Wind" Craft or "Ionocraft".
Clark R*
Department of Mathematics, Widener University, USA
Review Article
Volume 1 Issue 2 / Received Date: September 26, 2016 / Published Date: October 20, 2016
Abstract

  Peter Thiel of the Founders Fund once famously said, "We wanted flying cars, and we got 140 characters."But nanotechnology now does make possible the long desired flying cars. It's a different propulsion method though than propellers or jets however. It's propulsion by electric fields known as electrohydrodynamic propulsion (EHD). It works by ionizing air then using electric fields to propel the charged air molecules rearward, thus producing thrust. It's quite analogous to the famous space ion drive of NASA. EHD has been known at least since the sixties. Its problem is, as with ion drive, the thrust is so low. So far the EHD craft have not been able to lift both themselves and their power supplies. The ones made so far leave the power supply on the ground and connect to the craft through power cables. But the equations of EHD suggest the thrust for the power required gets larger for thinner ionizing wires. In fact if the wires are at the nanoscale then this important thrust-to-power ratio can be a hundred times higher than for the craft constructed so far. This would be enough to lift the craft and the power supply. This research is to prove what the mathematics suggests. Note that if it works then all propeller and rotor driven craft become obsolete. Also, intermediate range automobile travel would be taken over by the EHD craft, so a large proportion of carbon-emissions would be eliminated, replaced by this zero-emission travel method. In regards to space propulsion, since EHD is so similar to ion drive, using components at the nanoscale may also work to improve the thrust of ion drive. This would be important to shortening the flight times of spacecraft using such drives. This is important not just for robotic spacecraft but also satellites that use such ion drives to reach their final GEO destinations. As it is now, the ion drives used have such low thrust it takes months for such satellites to reach GEO, resulting in millions of dollars of lost revenue to the satellite companies. Being able to increase the thrust of these drives would reduce the flight time, and therefore reduce this lost revenue.

Keywords: Electrohydrodynamic propulsion; Carbon nanotubes; Nanowires; Ionic wind; Ionocraft; Plasma drive

https://medwinpublishers.com/NNOA/NNOA16000108.pdf

 

     Bob Clark

Nuclear powered VASIMR and plasma propulsion doable now.

Copyright 2015 Robert Clark

 By now all Mars advocates have heard the argument that VASIMR's 39 days to Mars promise is illusory because the needed space nuclear power sources do not exist at the needed lightweight, ca. 1,000 watts per kilo.

 This led me to propose using concentrated solar power for VASIMR or Hall effect thrusters instead:

Short travel times to Mars now possible through plasma propulsion.

http://exoscientist.blogspot.com/2014/03/short-travel-times-to-mars-now-possible.html

 I was therefore startled to read when looking at the specs of space nuclear engines that the engines themselves actually put out orders (plural) of magnitude higher power than this. See for example the specs on the "bimodal" nuclear rocket here:

Bimodal NTR.

Engine (Thrust Mode)
Thrust per engine  67,000 N
Total Thrust  200,000 N
T/W_engine  3.06
Exhaust Velocity  9,370 m/s
Specific Impulse  955 s
Propellant
Mass Flow  7.24 kg/s
Full Power
Engine Lifetime  4.5 hours
Reactor Power  335 MW_thermal
http://www.projectrho.com/public_html/rocket/realdesigns.php#id--Bimodal_NTR

 At a thrust of 67,000 N and T/W of 3.06, this means the engine weighs, 21,900 N, or 2,230 kg. So at a 335 MWthermal power this is a 150,000 watts per kg power to weight ratio. And the conversion of this thermal to kinetic energy is over 90% efficient as measured by the engine exhaust velocity.

 This means the problem with getting electrical power out of the space nuclear reactors has nothing to do with the nuclear reactors themselves. The problem is with the conversion to electrical power, specifically, the conversion/generation equipment is too heavy.

 In that vein note then there are electric motors, i.e, electric-to-mechanical conversion, with the necessary lightweight:

Power-to-weight ratio.
2.1.2 Electric motors/Electromotive generators.

https://en.wikipedia.org/wiki/Power-to-weight_ratio#Electric_motors.2FElectromotive_generators

 It turns out that electrical-to-mechanical energy conversion and vice versa is very efficient, typically in the 90% range and above. So you would run these electric motors in reverse to generate the electric power. Note then the best in that list is at 10,000 watts per kilo, sufficient for the VASIMR, and other plasma propulsion methods.

 It is important to recognize that the low electrical specific power, i.e., electrical power per weight, for space nuclear reactors is not due to the reactors themselves but due to the electrical conversion equipment. Then the focus is put on improving the electrical power generation weight efficiency. But this has importance beyond just space power systems. For instance the defense department wants lightweight electrical systems to power their UAV's. And aircraft manufacturers are investigating electrically powered aircraft for low-noise and zero-pollution aircraft. For instance LaunchPoint has produced high power density motors for UAV's in the 8200 w/kg range, which they say can be scaled up to large aircraft.

 Another area of research for high specific power motors and generators is operating them at cryogenic temperatures. According to this report 10 times as much power can be put through the windings of a motor at liquid nitrogen temperatures than at room temperature:

HIGH SPECIFIC POWER MOTORS IN LN₂ AND LH₂.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070028414.pdf

 Then using the electric motors already getting ca. 10,000 watts/kg, we could conceivably get ca. 100,000 watts/kg by running them at cryogenic temperatures(!) This has great relevance to the space propulsion application since we could use liquid hydrogen or other cryogenics as the fuel that would also serve to keep the electric generator at cryogenic temperatures.


 However, in an upcoming blog post I'll show you don't need to do the conversion to electricity and run a plasma engine. You can get the high speeds from the nuclear engines themselves, with some minor modifications.


  Bob Clark

Short travel times to Mars now possible through plasma propulsion.

Copyright 2014 Robert Clark

 Robert Zubrin wrote a critique of the plasma propulsion system VASIMR here:

The VASIMR Hoax
By Robert Zubrin | Jul. 13, 2011
http://www.spacenews.com/article/vasimr-hoax

 The primary criticism is that it would require unrealistically lightweight nuclear propulsion. However, Zubrin doesn't even like the idea of fast propulsion to allow short travel times to Mars. He argues in favor of using 6 month or more one-way travel times to allow free return trajectories at Mars. But the health disadvantages of long travel times such as radiation exposure, bone and muscle loss, and the recently found eye damage and vision loss suggest we should investigate such short travel times.

 Now we find there is also another reason: mechanical breakdowns on such missions of 2 or more years round trip length, such as found with the coolant system on the ISS.

 Note that the argument about free return trajectories does not hold with respect to planets with atmospheres. The Apollo missions did do a free return around the Moon, but there was no non-propulsive method to slow down at the Moon. On return to the Earth though, even Apollo had a trajectory that would send it off into space if the angle was too shallow or plunging too steeply into the Earth's atmosphere to burn up if the angle was too steep. The same could be used in addition to the propulsive method whose high efficiency would also allow it to be used for slow down at Mars.

 So it is important to note we may have a short term power source instead of nuclear power, for plasma propulsion such as Vasimr at the needed lightweight.

 The key point is that the power source does not need to be nuclear. According to Zubrin's article on the Vasimr it requires a power source of 1,000 watts per kg power density. This is 100 times better than what has been done with nuclear space power at 10 watts per kg. However, it is only 10 times better than standard solar space cells at 100 watts per kg. Actually more recent space solar cells get 200 watts per kg, so it is only needs to be 5 times better than those.

Now the key fact is that solar cells can put out more power if they have more concentrated light shone on them. Estimates of how much power solar cellls put out are based on the solar insolation at the Earth's distance from the Sun. But if that light is concentrated they can put out more power. In fact some Earth solar power systems get more power by using inexpensive mirrors or lenses to concentrate light over a larger area rather than using expensive solar cells over that larger area.

A disadvantage is this increases the loss due to heat and also if the light is too intense it can overload the solar cells so they don't work at all. However a recent report claims they can use concentrated light at thousands of times higher than solar insolation:

SEPTEMBER 07, 2013
Stacked Solar Cells Can Handle Energy of 70,000 Suns.

This work is important because photovoltaic energy companies are interested in using lenses to concentrate solar energy, from one sun (no lens) to 4,000 suns or more. But if the solar energy is significantly intensified – to 700 suns or more – the connecting junctions used in existing stacked cells begin losing voltage. And the more intense the solar energy, the more voltage those junctions lose – thereby reducing the conversion efficiency.

http://nextbigfuture.com/2013/09/stacked-solar-cells-can-handle-energy.html

Several reports in fact claim solar concentration at hundreds to thousands of Suns:

FEBRUARY 20, 2009
Breakthrough Solar Concentrator:low cost with high efficiency.
http://nextbigfuture.com/2009/02/breakthrough-solar-concentratorlow-cost.html

FEBRUARY 17, 2011
Concentrated solar power at half the cost of thin film solar.
http://nextbigfuture.com/2011/02/concentrated-solar-power-at-half-cost.html

DECEMBER 16, 2011
Tiny Solar Cell Could Make a Big Difference
http://nextbigfuture.com/2011/12/tiny-solar-cell-could-make-big.html

This will be dependent on having lightweight mirrors or lenses. However another key fact is that the parabolic mirrors do not have to be telescope grade accuracy. Indeed you can find on the net videos of amateurs making their own homemade solar furnaces that also require light to be concentrated to high intensity. These homemade mirrors can be as simple as aluminum foil spread onto a cardboard frame and still concentrate light to generate thousands of degrees. Not requiring high accuracy for the mirrors suggest they can be made lightweight.

DARPA is also funding lightweight space lenses:

DECEMBER 08, 2013
DARPA shoots for 20 meter folding space telescope.
http://nextbigfuture.com/2013/12/darpa-shoots-for-20-meter-folding-space.html

 Another example of how lightweight we could make the mirrors is actually to be tested in space:

Gossamer sail set to deorbit satellites.
By Jenny Winder | 30 December 2013

https://web.archive.org/web/20150618232251/http://www.sen.com/news/gossamer-sail-set-to-deorbit-satellites

 This solar sail has 25 square meters at only 2 kg weight. Let's suppose we only need 10 times solar concentration. This should already be within the capacity of currently used solar cells to accommodate since recent research is in the 100's to 1,000's of Suns range.
At 10 times solar concentration this means the solar cells have 2.5 square meters area in order for the mirror reflecting area to be 10 times greater. If they were 100% efficient this would be 2500 watts of power under standard solar illumination, i.e., without concentration. Solar cells though typically are only in the range of 30% efficient. So they would give 750 watts under standard solar illumination. At a 200 watts per kg power density now reached for space solar cells they would weigh 3.75 kg.
Now we are assuming the sail concentrates 10 times greater surface area onto the cells, so under this concentrated illumination they will put out 7,500 watts. The total weight of the cells and sail would be 5.75 kg. And the power to weight efficiency would be 1,300 watts per kg, sufficient for the Vasimr.

 The question though is would extra mass be needed to dispense the extra heat. Low power concentrators don't need these cooling systems:

Concentrated photovoltaics.
Low concentration PV (LCPV)

Low concentration PV are systems with a solar concentration of 2-100 suns.[5] For economic reasons, conventional or modified silicon solar cells are typically used, and, at these concentrations, the heat flux is low enough that the cells do not need to be actively cooled. The laws of optics dictate that a solar collector with a low concentration ratio can have a high acceptance angle and thus in some instances does not require active solar tracking.

http://en.wikipedia.org/wiki/Concentrated_photovoltaics#Low_concentration_PV_.28LCPV.29

 We only need about about 5 times concentration with currently available space solar cells without significant loss of efficiency from the solar cells to get the needed specific power.

 For simplicity and to maintain the light weight we might want to use these low power concentrators. However, there might be lightweight, passive cooling systems that could be used for the high power concentrators, that can reach hundreds to thousands of Suns, that with the higher degree of concentration would still have a light weight at high power.

These methods would concentrate sunlight onto solar cells. However, solar cells are typically low efficiency, in the range of 30%. Another advantage of concentrators is that increase the efficiency. The latest ones can get 44.7% efficiency and researchers believe they can reach above 50%.

 Another method would eliminate the need for solar cells. That is to use a solar furnace. These can get temperatures as hot as the surface of the Sun by concentrating sunlight. By thermodynamics very high temperatures correspond to high efficiency conversion of heat to other forms of energy, 90% and above.

 An additional problem with plasma thrusters though is the high weight compared to the thrust they put out. For VASIMR the thrust to weight ratio can be calculated to be in the range of only 1 to 4,000. See for example this report for the mass of the thruster per given power on p. 2  and the thrust per power on p. 3:

Low Thrust Trajectory Analysis (A Survey of Missions using VASIMR® for Flexible Space Exploration - Part 2).
http://www.adastrarocket.com/VASIMR_for_flexible_space_exploration-2012.pdf

 Other plasma thrusters however, such as the Hall effect thruster have better thrust weight ratios, ca. 1 to 200. A recent advance may even improve on that. This report discusses "nested channel" Hall effect thrusters, which have been shown to achieve the same thrust at a lower weight:

Developmental Status of a 100-kW Class
Laboratory Nested channel Hall Thruster.
IEPC-2011-246
Table 1, Example of concentrically NHT specific mass and footprint savings, p. 5.

http://pepl.engin.umich.edu/pdf/IEPC-2011-246.pdf

 The three-channel thruster in this table only weighs 320 kg. There is an inverse relationship between Isp and thrust as shown in the graph in Fig. 3 of this report on p. 3. So for the high of 5,000 s Isp in this table, the thrust would be 36 N. Still this is a 1 to 90 thrust to weight ratio, quite good for plasma propulsion. In comparison, at a 1 to 4,000 thrust to weight ratio, the VASIMR thruster would weigh nearly 15,000 kg.

 In addition to the thruster though plasma propulsion systems need a power procession unit (PPU). This transforms the low voltage put out by solar cells, usually just a few volts, to the hundreds of volts needed for plasma propulsion. The PPU mass is often comparable to that of the thruster itself.

 However, there may be methods to reduce or eliminate this extra mass. One method might be to put the solar cells in series like with batteries to build up the voltage. There is the question though if the solar cells can handle this higher voltage. Another possibility might be to use the recent advances in nanotechnology to produce a lightweight PPU. For instance quantum dots can transform low frequency light to high frequency light. It might possible to adapt this method to transform low voltage to high voltage.

 An exciting upcoming development is the Sunjammer solar sail scheduled for launch in January, 2015:

Solar Sail Demonstrator.

http://www.nasa.gov/mission_pages/tdm/solarsail/solarsail_overview.html#.UyWr7IUcbOw

 
 This sail will have an area of 1,200 sq. m. at only a 50 kg weight. At perihelion, the solar irradiance is about 1,400 watts per square meter. This would give a maximum possible power of close to 1.7 megawatts at 100% efficiency. If using the new 44.7% efficiency solar concentrator cells, this would be 750 kwatts.

 Then as early as next year we can test high power plasma propulsion systems that can make manned missions to Mars at travels times of weeks rather than months.

  Bob Clark

UPDATE, September 1, 2015:

 The use of solar concentrators to reduce the mass of solar power systems would have applications also to space solar power. Then it is notable that a recent development in carbon nanotubes may make the required mirrors 1,000 times lighter than even the Sunjammer solar sail:

Economical Space Solar Power Now Possible.
http://exoscientist.blogspot.com/2014/04/economical-space-solar-power-now_19.html