The Space subcommittee of the Fusion Industry Association, of which we are a member, has prepared a new white paper recommending government funding for a dedicated fusion propulsion development program, styled similarly to ARPA-E and DARPA.
The next space race is not simply into orbit; it is to the Moon, Mars, and beyond. The global competition is fierce, and the stakes are high—from landing the first humans on Mars to harvesting the limitless wealth of asteroids, and much more. Fusion propulsion is the best path to winning this “Deep Space Race.”
Fusion Energy for Space Propulsion, FIA, June 2021
The goal is to provide funding not just for “paper studies,” but enough funding to build real hardware and start to test fusion propulsion concepts. We want the US to remain competitive in the upcoming Deep Space Race – building a human presence on the Moon, and then Mars, and beyond. Direct Fusion Drive is directly applicable to near-term, modestly sized fusion propulsion!
If you want to express your support for government funding of fusion propulsion, contact your Representatives and Senators!
PSS just finished up a research contract for NASA in which we discovered some surprising and useful ways in which Low Temperature Superconductors (LTS) may be more suitable than High Temperature Superconductors (HTS) for making light, efficient electric motors.
In short, they’re cheaper. They’re much, much easier to design, manufacture, and use. Unlike HTS, it’s easy to make LTS electrical joints that are just as superconducting as the coils. LTS experience less heating when their internal current is changed. Crucially, you can make a “persistent switch” in which an LTS magnet is charged once and the current is trapped in the coil, persisting without the need to constantly supply current. Our LTS of choice is NbTi, the “workhorse” of the LTS family.
Interested in knowing more? Then read on!
There are several big pushes toward electric aircraft. Air travel accounts for 2.5% of our carbon emissions. So what’s preventing us from electrifying aircraft like we did with cars? The problem is weight. An extra pound of motor or batteries costs much more in an aircraft than it does in a car.
That being said, there are dozens of research groups, companies, and agencies working on hybrid electric and fully electric aircraft. There are even serious advantages to having the freedom to place propulsion units (motors rather than jet turbines) wherever you want within the aircraft, concepts called Boundary Layer Ingestion and Distributed Electric Propulsion. The aerodynamics is complicated, but the gist is that you can get huge emissions savings even if you’re still using jet fuel and turbines, if those turbines are powering lots of little motors rather than one big jet engine.
As we said earlier, all parts of the propulsion powertrain need to be lightweight in order to make a practical electric aircraft. For decades now, superconductivity has been known as a phenomenon with the potential to decrease the weight and increase the efficiency of motors. The idea goes back to the 1960s, with several experimental LTS rotors being tested in the 1970s and 1980s before the programs ended.
But what happened in the 1980s that shifted focus away from LTS motors? The answer is the discovery of HTS. On paper, HTS looks wonderful. It is superconducting at more achievable temperatures, ~100 K versus ~8 K for LTS. It can create magnetic fields much higher than LTS. Plus, it can carry much more current than LTS, meaning the same motor can weigh significantly less if made of HTS.
Yet despite research programs going back to the 1980s and continuing today, there are still no HTS motors on the market. Why is that?
The LTS difference
It turns out HTS is expensive and extremely hard to use. A magnet made of HTS would cost 20 times more than one made of LTS. HTS is weak, and when it’s under strain it can’t carry as much current. It can’t be flexed in one direction. To join two cables of HTS together into one superconducting piece, you have to grow more superconductor between them; you can’t just snap them together like extension cords.
On the other hand, LTS magnets have matured since the 1980s. Most hospitals now have an LTS magnet in the form of their MRI machine. Thousands of tons of LTS are produced yearly. LTS is cheaper, stronger, more flexible, and easier to work with. Its so-called AC losses (heating that occurs when the current is changed) are lower. Two LTS cables can be joined together to make one long LTS cable.
This latter property allows the so-called persistent mode of LTS magnets. In this mode, no external current is required to power the magnet. You charge the magnet up once, then you can disconnect it and walk away. Our LTS magnet vendor, Superconducting Systems, Inc. (SSI) of Billerica MA, has magnets that have sat persistently charged for decades.
How this affects a motor design
As part of our Phase I NASA SBIR, we designed a motor using LTS. The motor design targets small aircraft like Cessna Denali or regional airliners like Beechcraft 1900. The motor’s output power is 1 MW. The total target system weight is 100 kg. The target efficiency is 99.5%.
One of the challenges of using superconducting materials is keeping them cold. Because of the low AC losses and persistent mode of LTS, we were able to cut the heat leak down from dozens of Watts to less than 1 Watt. We were able to completely eliminate the charging subsystem and cryocooler of HTS designs. We have identified four innovative technologies that are enabled by and instrumental to the use of LTS in motors. We will be developing this technology in the coming years.
One of our innovations came from the significantly reduced heat leak into the cold rotor. Rather than use heavy, expensive cryocoolers to cool the rotor, the design suddenly came into the realm of Liquid Helium (LHe) reservoirs. Our SSI partners liken it to the difference between a refrigerator and a cooler. Use the refrigerator (cryocooler) when keeping food cold for weeks or months, but use a cooler (LHe) when making a day trip to the beach.
The journey of the LTS motor has just begun. Work continues at PSS. Contact us for more information or partnering opportunities.
Watch this space! Some day soon, perhaps sooner than you think, you could be flying across the country in an aircraft as renewably powered as your electric car.
Princeton Satellite Systems has been in a leader in renewable energy with its SunStation home solar power system with battery backup. We introduced this product back in 2013. SunStation has lithium-ion phosphate batteries, the most stable and reliable batteries for home use. The core of the system is the Outback Inverter that seamlessly switches from grid power to internal power.
The solar system in the installation produces 7.3 kW of power, much more than the house needed for electric power including charging a Nissan Leaf and Toyota Prius Prime. The heating and air conditioning system was nearing its end-of-life so we decided to replace it with a geothermal heat pump. A heat pump is essentially an air conditioner that can both reject heat to a source and absorb heat from a source. The problem with both is that when the outside temperature is high, for rejecting heat, and low, for absorbing heat, the system loses efficiency. Modern air-source heat pumps are very efficient but do need backup resistance heating in some climates.
A ground source heat pump, or geothermal heat pump, uses the ground as the medium for absorbing or rejecting heat. The option we chose, due to land constraints, is to have two wells several hundred feet deep as the source. Alternatives are trenching, or a pond if you have one in your yard. The ground is always at around 50 deg F. The system was sized so that it rarely, if ever, needs resistance heating.
The geothermal system, which is made by WaterFurnace, was installed by Princeton Air. No changes to the SunStation were needed. The core geothermal system is shown below. The valves to the ground loops are in the foreground and the geothermal system is on the left.
The lines that run to the outside ground loops are shown below.
The system has a preheater for the (still gas) hot water heater. The gas water heater was less than a year old, so it didn’t make sense to replace it. The preheater is an electric hot water heater that does not have the heating coils connected.
The SunStation is shown below. The Outback inverter is on the bottom left. The boxes on top provide arc protection, which is now included in the inverter. The batteries on on the right and the battery management electronics between the inverter and the battery cabinet.
The well digging was quite a project. This picture shows the drilling rig.
This second picture shows the yard after the drilling was complete. Drilling took three days total.
The following system shows the SunStation with geothermal in operation. The Prius Prime is charging which is most of the load. The system is still sending considerable power to the grid. On average the house powers itself and two other houses.
Geothermal, with solar and battery backup is the ideal solution for new homes and for renovations to existing homes. There is no reason to even have a gas hookup anymore. Contact us at SunStation for more information
The Living Universe is both a feature film for IMAX theaters and now a four-part documentary series. We blogged about our interviews in January and the series is now available on Curiosity Stream, a service dedicated to documentaries! Episode 2,”The Explorers” features a segment on DFD narrated by PSS engineer Stephanie Thomas, in addition to discussing plasma and antimatter propulsion. Here is an article about the series from Broadway World. You need to sign up for an account on Curiosity Stream to watch, which is free for 7 days and then $3 per month.
“The Encedalus Mission” by internationally best-selling hard science fiction author Brandon Q. Morris was originally written in German, and features the DFD as the propulsion technology on a mission to study newly detected life in the Saturn system; an array of six DFDs power the spaceship. Early reviews are favorable! The book is available in paperback or for Kindle.
Send us a comment and tell us what you think if you watch the show or read the book!
We are pleased to announce that our Phase II STTR proposal, “Superconducting Coils for Small Nuclear Fusion Rocket Engines,” was one of 20 selected for award by NASA in this year’s round! The full list of winners is posted on NASA’s website.
Our briefing chart prepared as part of the proposal is shown below:
We will be building a testbed with a split-pair superconducting coil (two windings with a gap between them) and performing experiments to assess the impact of operating the magnets in the vicinity of the FRC plasma. Applications of the technology go beyond fusion reactors, for example science payloads and high-performance motors for hybrid electric aircraft.
Our colleague Eugene Evans of PPPL has had his paper, “Particle-in-cell studies of fast-ion slowing-down rates in cool tenuous magnetized plasma,” accepted for publication in Physics of Plasmas. The article is tentatively scheduled for the April 2018 issue. A quote from the reviewer:
The paper … is an interesting, well-written paper that uses PIC to build upon earlier direct numerical simulation methods based on molecular dynamics. The authors present a clearly written discussion of the scaling properties of slowing down theory to support their numerical studies. The authors do a very good job describing the simulation approach they take… Of particular note in the paper is the good agreement between their numerical data and the sub-thermal model even when the effective computational log(lambda) was on the order of 1… the authors did not stop with their results but instead applied their conclusions to the FRC reactor, predicting that the neutron production rate is 100 times lower than a conventional DT Tokamak.
This paper is key to the low radiation levels claimed for our PFRC design, and hence the Direct Fusion Drive. The fast ion slowing-down is what causes the tritium and other fusion ash to exit the machine. You can view a preprint on arXiv.
We will post again once the paper is published and available from Physics of Plasmas.
I attended the 2017 Fusion Power Associates meeting in Washington, D.C. on December 6 and 7. Fusion Power Associates is a non-profit, tax-exempt research and educational foundation, providing timely information on the status of fusion development and other applications of plasma science and fusion research.
The annual meeting brought together experts in all areas of nuclear fusion research including scientists and engineers from ITER, the Princeton Plasma Physics Laboratory, TAE Technologies, General Atomic and many others! The meeting gave a great overview of the state of nuclear fusion power generation. We learned that ITER is 50% complete and on its way to first plasma in 2025. Planning has begun on Demo, the follow-on to ITER.
There were several speakers from the University of Rochester along with colleagues from the national laboratories talking about advances in laser compression of fuel pellets. This work is for nuclear weapons research but could be applied to inertial confinement fusion.
I gave the last talk of the meeting on Princeton Satellite Systems and PPPL’s work on DFD, nuclear fusion propulsion for spacecraft.
We are pleased to report that an additional patent has been awarded for DFD! US Patent 9,822,769, “Method and Apparatus to Produce High Specific Impulse and Moderate Thrust from a Fusion-Powered Rocket Engine”, was published on Nov. 21, 2017. It’s now available from the US patent office website!
Here is a link to the patent from the Department of Energy’s Energy Innovation Portal! The inventor on the patent are Dr. Cohen, of PPPL, and three PSS engineers: Gary Pajer, Michael Paluszek, and Yosef Razin.
Fusion Rocket Engine
The first patent, “Method to Reduce Neutron Production in Small Clean Fusion Reactors,” was issued on September 9, 2017. The patent is available on Patentscope.