Electric Propulsion Close To Reality

(for those who missed out on the 2017 Aviation Week & Space Technology article 1)


by T. L. Keller




boundary layer            a very thin layer of fluid (in this case air) immediately next to a solid body, that

                                      flows more slowly than the rest of the fluid.


hybrid propulsion        a combination of means of propulsion.  In a hybrid car, this would be a                                                           combination of gasoline engine and electric motors powered by batteries.  In this                                         instance: a combination of aviation-fueled, turbofan engines and electric                                                       propulsion systems for aircraft.


power converter          an electronic device that converts electrical current.


power inverter             an electronic device that converts direct current (DC) to alternating current (AC)


power rectifier             an electronic device that converts alternating current (AC) to direct current (DC)


superconductivity        the phenomenon exhibited by certain metals and alloys (mercury, aluminum and                                         cadmium) of conducting  electrical current  without resistance, when cooled to low                                       temperatures.



Hybrid Turbojet-Electric Propulsion Systems


Sounds pretty exotic, right?  When you fly in an Airbus or a Boeing aircraft, it is propelled by what is called a turbofan bypass jet engine.  These run on aviation gasoline.  This is to say that the “fan” part is gasoline-powered.  But even at relatively low jet fuel prices that we are currently experiencing, jet engines have a very big energy “footprint.”  That’s to say that they consume a great deal of energy just to fly us from Los Angeles to New York and back.  What if the kind of technology that we now find in the Toyota Prius Hybrid could be extended to air travel?  We would arrive at the same time, but we would use less fossil fuel, the cabin would be quieter and the ticket price might be (a little) less.  A win-win situation for everyone.  That’s what this is all about.














                                                  The business end of electric propulsion 2



The Technology


A flying Prius . . . really?  No, not quite.  Here, we’re talking electrical motors, not electrical batteries.  The aircraft’s electrical motors are driven by the turbofan jet engines generating electrical power.  The electrical power is then used to drive the electrical fan mounted in the tail of the aircraft.  These electrical systems for aircraft are much more powerful than those in cars and much lighter than in ships.  This is the key to the future of such systems in aircraft: weight and operational efficiency.  What we are trying to do is to minimize the weight of the thermal management system required to deal with the heat generated by the electrical systems.  Here, we are concerned with a megawatt-scale of energy (millions of watts) and any electrical losses of just a few percent mean kilowatts of wasted energy.  We have to keep in mind that we are concerned here with aircraft, not cars or ocean-going ships.


The concept is called “boundary-layer-ingesting electric fan, partially turboelectric for a commercial aircraft.”  By “commercial,” here they’re talking one-aisle jet aircraft such as the Boeing 737.  Of course, if the design can be optimize to, say 99% efficiency, this technology could be extended to larger aircraft such as the Airbus A380 and the Boeing 747-types.



Integrating Electrical Motors Into Turbofan Engines


You’ve probably guessed that NASA is the primary mover and shaker to the research and development of electric propulsion technology.  Yes, it’s NASA . . . this time and what are they doing?  According to Aviation Week & Space Technology, “NASA’s focus is on electric machines that can be used as generators (sources) and motors (loads) and power electronics that convert AC to DC (rectifiers) and DC to AC (inverters).  Some research is also underway into wiring systems to distribute high levels of electrical power.” 3


NASA has contracted with the University of Illinois to develop a permanent magnet motor and a gallium nitride (GaN) inverter. For Ohio State University, they are to develop ring induction motors with the same power goal of 13 kW/kg (kilowatts per kilogram) and an efficiency target of greater than 93%.  NASA Glenn Research Center is also developing a third electrical machine with the goals of 16 kW/kg and greater than 98% efficiency (!).  For comparison, the US Department of Energy has set motor vehicle electric motors with a target of 1.6 kW/kg for 2020.



















              University of Illinois’ motor integrated into a Rolls-Royce turbofan engine. 4



The illustration above shows the integration of the University of Illinois’ 1-megawatt, electric motor integrated into a Rolls-Royce, electrically-variable, turbofan engine: a true hybrid propulsion system.


Of course, NASA centers, other commercial companies and universities are also involved in this program.  NASA Glenn Research Center is developing self-cooled, superconducting, synchronous motors.  General Electric has a contract to develop silicon carbide inverters and Boeing is working on cryogenically-cooled (extreme low temperature) silicon inverters.


So How Does This Work?


Two turbo jet engines (in a Boeing 737-size aircraft) work pretty much as normal except that they have an integrated, electrical motor.  The high-efficiency, high-power motor delivers electrical energy to the fan installed at the tail of the aircraft (see illustration above).  At very high power levels and efficiency, the fan provides additional thrust to the aircraft in addition to the turbofan jet engines.


This hardware is already being tested and, if funding is continued at the present rate, according to NASA, these systems will be implemented by 2035.  Just don’t hold your breath.





  1. Warwick, Graham, “NASA Moves Electric-Propulsion Components Closer To Reality,” Aviation Week & Space Technology, August 25, 2017.

  2. Image courtesy of NASA.

  3. Warwick, Graham, “NASA Moves Electric-Propulsion Components Closer To Reality,” Aviation Week & Space Technology, August 25, 2017.

  4. Image courtesy of NASA.


© T L Keller 2017




Space Junk “Incinerator”


by T. L. Keller

The Challenge of Space Junk


Worried about all of that space junk that has been accumulating in Earth orbit since 1957?  If you have signed up with Virgin Galactic for the first suborbital space tour, you may be.  Well, other folks are concerned about space junk too including The Aerospace Corporation.  This is a nonprofit corporation that operates a federally-funded research and development center headquartered in El Segundo, California.


Imagine over 500,000 pieces of space junk traveling at something like 17,500 miles per hour impacting into the spacecraft in which you are traveling.  Some of these pieces are spent spacecraft propellant cannisters, nuts and bolts and even paint chips.  For example, in 2009 the inactive Kosmos-2251 satellite collided with the operational Iridium 33 communication satellite sending debris into orbit in numerous sizes. These pose real hazards to the International Space Station (ISS) and to active satellites.  When a paint chip impacts at that speed, window glass can be cracked and a lot more damage will ensue if one collides with a missing and loose ISS wrench or power tool.  So what can be done to start cleaning up space? 1


The Solution to Space Junk Cleanup


The Aerospace Corporation has come up with one solution: the Brane spacecraft.  The Brane is like a very thin, remotely/autonomously-controlled “blanket” (see illustration below) that would be used to envelope both large and small debris and de-orbit both the Brane and the debris until it burns up upon re-entry into the atmosphere.  One Brane would be deployed to each piece of space junk.













                                        Brane spacecraft enveloping space junk 2 

NASA’s Innovative Advanced Concepts (NIAC) program is supporting a two-year grant that allows Aerospace Corporation to develop improvements to the Brane’s microprocessor and digital electronics.  One of the greatest dangers to the Brane spacecraft is apparently solar radiation exposure even during its short-term deployments.


Spacecraft Design


The Brane spacecraft is composed and powered by a very large sheet of ultra-thin solar cells.  It carries a small amount of propellant wedged in between two sheets of cells.  If any cell is damaged by, say, a micrometeorite, the other solar cells continue to function.  They can be folded for easy storage in launch vehicles and then deployed en masse against a multitude of space junk in orbit with each “bot” assigned to a specific piece of space junk.  Presumably, the Branes would be deployed in small to large sizes depending on the size of target assigned.


The Near Future


Once the initial prototype of Brane is developed, this concept would be able to “incinerate” space junk up to 0.9 kilograms (2 pounds) or less within a few years.  Keep in mind that the prototypes would be on the order of only  three (3) feet in both dimensions and the thickness of a human hair.  Don’t expect any Branes that will take down dead satellites anytime soon!  But it would be a small beginning to the solution of a big problem.




   1. Howell, Elizabeth, “This Ultrathin Craft Could Soon Envelop and Destroy Space Junk,”, September 10, 2017.

​   2. Courtesy of NASA.​

(c)  T L Keller 2017