Sunday, April 20, 2014

Allan Savory: How to green the world's deserts and reverse climate change

   I found this on a climate change skeptic site but it is an important lecture whatever your opinion on that issue.
 The presenter was haunted by the decision to kill 40,000 elephants in his home country of Zimbabwe to preserve grasslands from being overgrazed. Since then he has dedicated himself to finding other ways of preventing desertification.
 He believes he has found one as he describes.

  Bob Clark

Saturday, April 19, 2014

Economical Space Solar Power Now Possible.

Copyright 2014 Robert Clark

 In the blog post, "Short travel times to Mars now possible through plasma propulsion", I suggested current solar concentrator methods and lightweight space solar sails make possible fast flights to Mars with solar powered plasma propulsion. 

 Interestingly this technology also now makes possible economical space solar power (SSP). For SSP a key detriment has been the huge weight thought needed to be sent to space. For instance solar cells typically have a 100 watt per kg weight, though more recently they are in the 200 watts per kg range. So if you wanted to get a 1 gigawatt system, about that required for a large city, you would need to send 10,000,000 kg to orbit just in solar cells alone, hugely expensive

 However solar concentrators using mirrors or lenses can now concentrate light thousands of times, requiring orders of magnitude lower weight in solar cells. Say, you had a 1,000-times solar concentrator. Then you would only need 10,000 kg in solar cells, which could be launched by a single mid-size launcher.

 BUT you would also need to send the mirrors to orbit. And that is a second key advance we have also now reached, lightweight space mirrors. The Sunjammer space mirror to test solar sail technology is scheduled to be launched January, 2015. It has a 1,200 sq. m area at only a 50 kg weight.This can collect about 1 megawatts of power. So at 1,000 times larger, it could collect 1 gigawatts of power at only 50,000 kg mass, which could be launched by a single Falcon Heavy. Another consideration though is solar cells are not 100% efficient. They are actually about 30% efficient. So you might need 3 times larger collecting area. Still only 3 launches of the Falcon Heavy. 

 Actually though some recently work on solar concentrators have also been able to use the heat created, thereby increasing the energy efficiency to 80%. So you may get close to the area size for a 100% efficient system.

 Interestingly some recent work on carbon nanotubes may be able to make the mirrors even lighter:

Researchers produce strong, transparent carbon nanotube sheets. 
Aug 18, 2005
"Strength normalized to weight is important for many applications, 
especially in space and aerospace, and this property of the nanotube 
sheets already exceeds that of the strongest steel sheets and the Mylar 
and Kapton sheets used for ultralight air vehicles and proposed for 
solar sails for space applications, according to the researchers. The 
nanotube sheets can be made so thin that a square kilometer of solar 
sail would weigh only 30 kilograms. While sheets normally have much 
lower strength than fibers or yarns, the strength of the nanotube 
sheets in the nanotube alignment direction already approaches the 
highest reported values for polymer-free nanotube yarns." 

 This is more than 1,000 times better than the Sunjammer sail. The transparent nanotubes sheets would have to be given a thin reflective layer. But this is commonly done with telescope mirrors and add little weight to the mirror. Actually it's been found that nanotube properties are highly tunable so it may be possible to create these thin, strong nanotube sheets that are themselves reflective rather than transparent. 

 Notably, this would provide a market for getting large amounts of mass to orbit for the space solar power to be applied globally for electricity generation. Then this may finally be the "killer app" for generating a large enough market for space access to bring the costs down and thereby make space access routine.

       Bob Clark

Sunday, April 13, 2014

Sample Return Missions from Enceladus, Europa, Titan, Ceres, page 1.

Copyright 2014 Robert Clark 

 Gravity measurements from Cassini have provided further evidence that Enceladus has a subice liquid ocean. It is being regarded now as a prime target in the search for extraterrestrial life. The question is how to reach that ocean through what may be 40 km of ice. There have been various proposals for drills. However, NASA has modeled the plumes seen to arise from the "tiger stripes" on Enceladus as coming from vents that attach to the ocean below. Then a simpler method may be to reach the ocean by traveling through these vents. 

This graphic shows how the ice particles and water vapor observed spewing from geysers on Saturn's moon Enceladus may be related to liquid water beneath the surface. The large number of ice particles and the rate at which they are produced require high temperatures, close to the melting point of water. These warm temperatures indicate that there may be an internal lake of liquid water at or near the moon's south pole, where the geysers are present.

 In this model the temperatures don't have to be particularly high, just near the melting point.
 This method may work to reach subsurface liquid water for other outer solar system bodies expected to have them such as Europa, Titan and Ceres.

 As a feasibility test we might try it to explore subsurface though deep-sea hydrothermal vents here on Earth. 

 According to this model the temperatures might reach a maximum of 400 °C. If we can develop a robot to travel through these conditions quite likely it would also work in the conditions for the vent systems of these outer solar system moons.

In an upcoming blog post I'll discuss how the Falcon Heavy at a 53 metric ton(mT) payload capacity and the first version of the SLS at 70 mT could each be used to conduct sample return missions from these outer solar system moons.

     Bob Clark  

Tuesday, March 25, 2014

"Golden Spike" Circumlunar Fights, Page 2.

Copyright 2014 Robert Clark

 In the blog post "Golden Spike" Circumlunar Flights, I suggested that the Falcon 9 1.1/Dragon could do a circumlunar mission with a half-sized Centaur, of ca. 10 mT propellant load, to do the translunar injection.
 Interestingly it might be possible to do without even needing the extra in-space stage. Elon Musk has said through his Twitter account that the 13 mT payload capability was actually a reduction of the F9 V1.1's true capability due to reusability considerations. Gwynne Shotwell confirmed this on a TheSpaceShow interview on Friday, Mar. 21 at about the 9 minute mark. She said the quoted payload on their web site for the F9 v1.1 is about 30% less than that of a one-use version.
 This would put the expendable version in the range of the 16.6 metric tons to LEO given on NASA's site:

NASA Launch Services Program's
Launch Vehicle Performance Web Site.

 The point is this would be just about at the payload capability to do translunar with the Dragon using just its onboard Draco, or upgraded SuperDraco, thrusters. On the "NASA Launch Services Program" site, click the link for the Performance Query Tool and select the Falcon 9 and "elliptical" orbit option. Enter in 36000 km for the altitude corresponding to geosynchronous transfer orbit (GTO) and 28.5 degrees for the orbital inclination corresponding to launch from Cape Canaveral. Then the calculator gives the payload to GTO as 5745 kg.

 As shown here the delta-v to GTO is 2,500 m/s:

 Then translunar injection (TLI) at 3,100 m/s would only require an additional 600 m/s delta-v. The Dragon has a dry mass of 4,200 kg and a propellant mass of 1,290 kg. SpaceX has not released the Isp of the hypergolic thrusters on the Dragon, but they typically are in the 320 s range in vacuum. Then it could carry 1,800 kg payload to the 600 m/s needed to reach TLI:

320*9.81ln(1 + 1290/(4200 + 1800)) = 610 m/s.

  Actually that 1,800 kg payload would put the total mass beyond the 5745 kg capacity to GTO. Smaller payload say in the 250 kg range would be doable.
 Such missions would be important to do since at a perhaps $120 million total launch price for the Falcon and Dragon it would show lunar missions are possible without requiring huge launchers such as the Saturn V, Ares V or SLS.

   Bob Clark

UPDATE, April 1, 2014:

 On forum, DocM informed me via PM that in an environmental impact statement SpaceX gave the propellant load for the Dragon as 1,388 kg. This would raise the max payload to reach 600 m/s delta-v to 2300 kg. Again though this would put the total mass outside the range that could be lofted to GTO. Likely you would still have to limit the payload to ca. 250 kg or so.



Sunday, March 16, 2014

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:

By Robert Zubrin | Jul. 13, 2011

 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.

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

FEBRUARY 20, 2009
Breakthrough Solar Concentrator:low cost with high efficiency.

FEBRUARY 17, 2011
Concentrated solar power at half the cost of thin film solar.
DECEMBER 16, 2011
Tiny Solar Cell Could Make a Big Difference

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.

 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

 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.

 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).

 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.
Table 1, Example of concentrically NHT specific mass and footprint savings, p. 5.

 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.

 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

Wednesday, January 29, 2014

Transitioning SpaceShipTwo to liquid fueled engines: a technology driver to reusable orbital launchers.

 Copyright 2014 Robert Clark

 A new book by Tom Bower on Richard Branson, “Branson: Behind the Mask”, claims the hybrid engine on SpaceShipTwo still does not have enough power to get the vehicle to the altitude for suborbital flight. Doug Messier on his blog has been reporting on the technical problems developing the hybrid engine for some time.

 There has been much speculation actually that Virgin Galactic will have to transition to a liquid fueled engine to achieve suborbital flight. In point of fact, independent studies have shown that SS2 by switching to liquid fueled propulsion, can be suborbital on its own without even needing the carrier craft WhiteKnightTwo:

SpaceShipTwo could be single stage to suborbit says ESA firm

Reusable Space Plane Idea Intrigues Europeans.
Rob Coppinger, Contributor
Date: 01 May 2012 Time: 04:30 PM ET
The Vinci suborbital space plane's structure and cryogenic fuel and oxidizer tanks are depicted in this illustration.
Credit: ESA

  This would be by using the hydrogen-fueled Vinci engine. The Vinci is soon to be introduced on the Ariane 6. However, the existing HM-7 engine used on the cryogenic version of the upper stage of the Ariane 5 could also be used. The advantage of this is that it has been in use for decades and is well-characterized. You would probably need to place an extra one on the Ariane 5 upper stage to be able to lift the SS2. Still the engine and the stage are already developed and the cost of the addition of an extra engine should be comparatively small. The development cost of the SS2+WK2 combo has reportedly reached into the few hundred million dollars range. In contrast, the addition of an already existing engine to an already existing stage should be simpler, quicker and far cheaper than creating a new engine, hybrid, from scratch.

 The reason for the choice of the hybrid for the SS2 rather than a higher performance liquid-fueled engine was the idea that a hybrid engine could not explode. However, the accident in 2007 at Scaled Composites due to a nitrous oxide explosion has destroyed that misperception. Indeed because of the instability of nitrous oxide one team involved in developing a rocket propelled car suggests nitrous should not be used for passenger flight:

Observations and comments on Cal/OSHA report (Inspection No: 31081103) on fatal accident at Mojave test site of Scaled Composites at the Mojave Air and Space Port, 26th July 2007.
While it is most advisable to apply the established safety protocols relating to liquid oxygen, such protocols, in themselves are not sufficient to ensure the safe handling of Nitrous Oxide. The unique physical properties of N2O require further protocols above and beyond those used for liquid oxygen.
Safety protocols for N2O, in a rocket motor system, should include (in addition to the protocols used for Liquid Oxygen)

   * The detailed study of materials compatibility of all components in the system
   *  Avoidance of high temperatures at all points in the system
   * Stirring of large tanks
   * Avoidance of the gaseous phase both during apparatus filling and in use
   *  Purging of lines and valves immediately prior to ignition
   * Not using any component that may have previously absorbed N2O –   especially fuel grains

We are not confident that, even with these additional precautions, that we yet know enough about N2O to consider it a safe oxidiser for use in passenger flight. In the light of what we do know, safety must remain a major concern.

  Then the SS2 hybrid engine should no longer be considered to have an advantage over a liquid fueled engine. Then the fast and low cost development possible, especially with using an already existing engine, should push the decision to using liquid fuel. In fact by doing so SS2 probably could already have been flying by the originally announced date of the first suborbital test flights of 2007.

 The importance of their making that decision then and of their making that decision now goes far beyond that of just suborbital rockets however. If you look at the specifications of the cryogenic Ariane 5 upper stage, you see it could be propelled, with the SS2 aeroshell around it, well above the speed needed for suborbital flight. In fact it could be in the high Mach range envisioned for example for the X-33. A stage like that though could be used for a reusable first stage booster for a two-stage to orbit system.

 Now, since the first stage is generally much larger and costlier than the upper stage, a reusable first stage could significantly cut the cost to orbit of a two stage system. This in fact is what DARPA wants with its reusable spaceplane program.

 So Virgin Galactic giving SS2 a liquid fueled propulsion system could have a system to satisfy the requirements for DARPA's reusable spaceplane. In fact, it could already have had such in 2007.

   Bob Clark

Sunday, December 1, 2013

Will the SpaceX push to reusability make Arianespace obsolete?

Copyright 2013 Robert Clark

 By deciding on the solid-fueled Ariane 6, ESA is, unwittingly, betting on SpaceX to fail on reusability. For if SpaceX succeeds then the solid-fueled Ariane 6 becomes obsolete, with billions of dollars and years wasted. ESA would then have to start all over again to develop a liquid-fueled version which can be made reusable:

Musk lays out plans for reusability of the Falcon 9 rocket.
October 3, 2013 by Yves-A. Grondin
“The most important thing is that we now believe we have all the pieces of the puzzle (for recovery). If you take the Grasshopper tests, where we were able to do a precision takeoff and landing of a Falcon 9 first stage and you combine it with the results from this flight where we were able to successfully transition from vacuum to hypersonic, through supersonic, through transonic and light the engines all the way through and control the stage all the way through.
“We have all the pieces necessary to achieve a full recovery of the boost stage.”

Falcon 9 first stage in a controlled descent toward the Pacific Ocean. At this point, the stage was about 3 meters (9.8 feet) above the water. (Credit: SpaceX)

  I think it's a bad bet on ESA's part.

 Arianespace has already taken seriously the competition SpaceX offers for their expendable rockets:

SpaceX Challenge Has Arianespace Rethinking Pricing Policies
By Peter B. de Selding | Nov. 25, 2013

“I have sent a signal to our customers telling them that I could review our pricing policy, within certain limits,” Israel said in an interview with Les Echos, a French financial newspaper. “I think they have appreciated this.”
Israel’s comments came on the day when Space Exploration Technologies Corp. (SpaceX), after a decade of rattling Arianespace’s cage, is preparing its first-ever launch into the geostationary transfer orbit used by most commercial telecommunications satellites, and the place where most commercial revenue is made.
SpaceX Chairman Elon Musk taunted Arianespace again on Nov. 24, the day before his company’s scheduled launch of the SES-8 satellite owned by SES of Luxembourg.
“Unless the other rocket makers improve their technology rapidly, they will lose significant market share to the Falcon 9,” Musk said in a news briefing.
SpaceX President Gwynne Shotwell added: “Competition is always a good thing. It keeps people sharp. They [Arianespace and other competitors] may not look at it that way, but hopefully they’ll come to appreciate it in the future.”
“I am looking at our pricing policy and if we must adapt it to the competition, we will,” Israel said. “We’ll look at the overall efficiency of the Ariane business with a view to optimizing it.”
Israel said there are more small telecommunications satellites being designed now than ever, a fact he attributed in part to the arrival of SpaceX, which has stimulated the market.
 IF SpaceX succeeds in cutting prices by reusability, then no readjustment of the pricing will be effective. SpaceX is already undercutting them on pricing and if reusability really does cut the SpaceX prices again by a factor of 4 to 10 then ArianeSpace simply will not be able to compete.

 This will be all due to ESA's decision to go backwards in technology and not forwards in selecting a solid-fueled version of the Ariane 6. Every other space agency in the world will be able to adapt their liquid fueled rockets to make them reusable to match SpaceX's pricing. Only ESA will be left behind - both technically and economically.

 This becomes really bad because they will no longer have the smaller satellites to partially pay for the Ariane 5 launches. This could mean they also lose their entire Ariane 5 market as well! Their entire market for any of their launches will be gone all due to the choice to move backwards in technology.

 Ironically, this would mean their real reason for selecting the solid-fuel Ariane 6 would have no meaning as well. The actual reason why France and Italy want the solid-fueled Ariane 6 is to help defray the costs of the solid-fueled ballistic missiles of the French military and the solid-fueled Vega rocket largely built in Italy. But if SpaceX succeeds in cutting costs by reusability then neither the solid-fueled Ariane 6 nor the Vega, will be used because they will be priced far outside the market. So neither of them will wind up defraying the costs of other solid-fueled rockets in Europe anyway.

 Interestingly, IF SpaceX succeeds in their next test of reusability in Feb. 2014, this might provide an incentive for ESA to at least "hedge their bets" and engage in some development research of adding a second Vulcain to the Ariane 5 core. Then they would not be years behind the other space agencies in the world IF SpaceX succeeds in cutting costs by reusability. 

  Bob Clark