Millisecond Pulse Load Switch Design

This summer I worked on the design of a millisecond pulse generator as part of the ARPA-E GAMOW grant. The goal of this project was to supply pulses of very high current to a fusion reactor’s plasma control antenna using solid-state power electronics. Some key design considerations were the ability to parallelize the pulse generator to scale to many power levels, to operate at high voltages, and to minimize the current rise time through the load antenna. I spent most of my time working in LTspice XVII to simulate the circuit and its response to rapid pulses of current. I wanted to make sure the circuit performed as desired, while also remaining safe for both the devices in the circuit and the operators controlling the circuit.

We based the design of the circuit on a load switch developed previously at Princeton Satellite Systems by Cindy Li and Eric Ham. Our biggest progress was in the selection of switching devices and the improvement of the gate drive circuitry. Because our goal is to switch current as quickly as possible, we did not want to rely on outdated klystrons as our switches. We decided to use many parallel MOSFETs to switch the current. The device we chose was a 650 V silicon carbide cascode JFET from UnitedSiC. This FET has low on-state resistance, meaning that it does not heat up as much when pulling large amounts of current. By using many devices in parallel, we can pull more total current while keeping each individual FET below its current capacity. Then by using multiple boards in parallel, we can reach different power levels for different fusion reactors. 

Implementing safe gate driver circuitry was another important step. The power MOSFETs used to switch the current cannot be turned on directly from a computer control signal, due to both power and safety concerns. A logic-level signal is not powerful enough to activate the FET, and directly connecting the computer to the high-voltage circuitry is unsafe. To solve both problems, I designed a galvanically-isolated gate drive circuit based on an Infineon gate driving IC. The intermediate circuit takes the computer control signal and steps it up to an intermediate voltage level high enough to activate the MOSFET, while keeping the both sections electrically isolated from each other. Each MOSFET has its own gate drive circuit, enabling independent control.

Working on the millisecond pulse generator was a great experience as an intern. I gained lots of practice working with team members across organizations and disciplines. I became much more proficient in LTspice, and I learned how to rigorously approach challenging engineering problems.

The Space Show appearance

It was my pleasure to appear on David Livingston’s “The Space Show” radio program last night, now available as a podcast:

https://www.thespaceshow.com/show/24-aug-2021/broadcast-3744-stephanie-thomas

People from all over the country called and emailed in their questions about fusion and fusion-propelled spaceflight, and we had a great discussion! David has been running this educational program for 20 years and there are almost 4000 archived episodes covering a wide range of space topics. Author David Brin, whom I met during my NASA NIAC fellowship, is going to be on next week!

So have listen and add to the conversation on The Space Show website!

Funding Options for Fusion Propulsion


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.

https://www.fusionindustryassociation.org/post/fia-proposes-funding-for-fusion-for-space-propulsion

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!

https://www.house.gov/representatives/find-your-representative

https://www.senate.gov/senators/senators-contact.htm

1U CubeSat Structural Design and 3D Print

As a Summer 2021 intern, my first project was to complete the structural design of a 1U CubeSat that will fly in orbit with and observe the NASA Solar Cruiser. The 1U CubeSat needed to follow the CubeSat design specifications set by the California Polytechnic State University; it needed to have specific dimensions, needed to weigh a certain amount, and needed to be able withstand structural loads and natural frequency/vibrational loads. In order to design and test the CubeSat, I used Fusion360’s design and simulation softwares. I based my design of the CubeSat off of the engineering drawing provided by the California Polytechnic State University’s “CubeSat Design Specification” manual.

I designed the initial model in Fusion360 as one part made up of different components, as shown below:

The top of the CubeSat faces the positive z-direction, while the front faces the negative y-direction and the right side faces the positive x-direction. The CubeSat also needed four deployable solar panels attached by hinge mechanisms to the four edges of the top face. The panels needed to start parallel to the walls and then, when deployed by some mechanism, needed to swing upward in the positive z-direction. 

After designing the idealized CubeSat, I ran multiple modal frequency analyses and structural analyses in order to make sure the CubeSat could withstand the proper loads. First, I ran modal frequency analyses, with fixed boundary conditions for a cantilever beam. The natural frequencies for the first four modes of the CubeSat are shown in the following table:

The above natural frequencies calculated in Fusion360 are very similar to the theoretical natural frequencies of a cantilever beam, given by the formula

Where “E” is the modulus of elasticity (also known as Young’s Modulus), and “I” is the area moment of inertia. This formula can be used to find the natural frequencies of a cantilever beam for any mode of vibration, “n”.

My results were also similar to other experimental results. For example, a study titled “Design, Analysis, Optimization, Manufacturing, and Testing of a 2U CubeSat” published in the International Journal of Aerospace Engineering performed a modal frequency analysis of a 2U CubeSat and found the following natural frequencies for the first four modes: 

These results are similar to the ones I found in my modal frequency analyses. 

After running the modal frequency analyses, I ran a few structural load analyses. The CubeSat frame had a honeycomb structure, which I modeled in Fusion360, and was made up of Aluminum 7075 material. The CubeSat needed to be able to withstand a maximum pressure differential of 15.2 psi (0.104 MPa) created by the Space Launch System (SLS) ascent into space, according to NASA’s Space Launch System Program’s White paper.

The maximum displacement of the CubeSat’s structure due to the applied force was 0.009 m, which is very low. Fusion360 calculates Von Mises stresses, and the maximum stress was 16.08 MPa, which is well under Young’s Modulus of Aluminum 7075 (71.7 GPa). The safety factor of the structure was 8+ everywhere on the structure, meaning the structure is much stronger than the  15.2 psi (0.104 MPa) load applied. 

After running the modal frequency and static stress analyses in Fusion360 and getting the desired results, the CubeSat was ready to be modeled as 3D printable parts and 3D printed with PLA on the FlashForge Creator Pro printer: 

The initial CubeSat design in Fusion360 had to be modified and broken up into different parts that were each 3D printable; after printing each part, I assembled them to form the whole CubeSat. I decided to break the initial design up into the following 3D printable parts: the top, the bottom, four separate side walls, four separate side rails, four deployable solar panels; finally, I needed to add four hinges plus four rods to attach each of the solar panels to the main structure (similar to a door hinge mechanism). This allowed the solar panels to rotate on a hinge from their initial position up to 180 degrees upward and back. Photos of the different 3D printed parts are shown below: 

After 3D printing all the necessary parts, I needed to assemble them. I modeled screw holes in Fusion360 on each 3D printed part in specific locations, so that I would not need to bore holes manually after the parts were printed. I ordered screws for plastic from McMaster Carr, so I knew the correct diameter and length for the screw holes I modeled in Fusion360. This way, the parts were ready to be assembled immediately after 3D printing. Images of the final assembled 1U CubeSat are shown below:

3D printing the final product was an iterative process, so I ended up assembling two different CubeSats entirely and printing a multitude of different versions of each part until I assembled the final product correctly. During the printing process I ran into many problems with the design of the parts, as well as issues with the printer itself. Some design problems included incorrect part sizes, incorrect screw hole placement, incorrect screw hole tolerancing/sizing, and incorrect dimensions of the overall assembled cube. Some printer issues included warping and two nozzle clogs. Some of my parts warped due to a lack of adhesion between the printer bed and the filament coming out of the nozzle, meaning the corners of these parts bent upward and were no longer usable. I solved this problem by reducing the heat of the 3D print bed to make sure the filament could cool down correctly on the bed. On a couple of occasions, parts would not print at all or the filament would come out tangled and would not stick to the bed; I solved this problem by taking apart the nozzle and manually unclogging it so that the filament could come out correctly. I also re-leveled the bed, to make sure the nozzle was close enough to the printer bed so that when the filament initially came out of the nozzle it would stick to the bed immediately. Photos of intermediate designs are shown below: 

Overall, this project was educational, challenging, and fun! I learned a new CAD software, Fusion360, which will be useful in the future, and I practiced my engineering design and 3D printing skills!

Overall, this project was educational, challenging, and fun! I learned a new CAD software, Fusion360, which will be useful in the future, and I practiced my engineering design and 3D printing skills!

Nuclear Fusion Power and Propulsion in the News

We just started our latest project for ARPA-E under the ARPA-E GAMOW program in which we will be build power amplifiers for fusion reactors. The goal is to lower the cost and increase the reliability of fusion reactor power electronics. We currently have grants under the DOE INFUSE program and another ARPA-E project that is part of the ARPA-E OPEN 2018 program. We just finished a NASA STTR grant to study the effects of plasma pulses on low temperature superconducting coils.

For those who have been following our work, you know that there are many articles and videos about our work. For your convenience, we’ve collected many of the URLs for them in this blog post.

2020 NJ Edison Patent Award (You’ll need to look for our award on this page. The others are interesting too!)

Popular Mechanics: The Direct Fusion Drive that Could Get Us to Saturn in Just 2 Years, Oct 21, 2020

Universe Today – Titan mission paper from Polito, Oct 19, 2020

ITER article: Space Propulsion: Have Fusion, Will Travel, July 15, 2019

Space Q, August 29, 2019 – Podcast replay of May 29, 2019 FISO

Space.com: Fusion-Powered Spacecraft Could Be Just a Decade Away, Mike Wall, June 11, 2019

PPPL press release: PPPL physicist receives funding to research improvements to unique fusion device, March, 2019

Could Tiny Fusion Rockets Revolutionize Spaceflight? June 12, 2017

Will Mini Fusion Rockets Provide Spaceflight’s Next Big Leap? Charles Choi, June 9, 2017

Video of DFD talk from DPSS 2017 (Facebook)

Essay by John G. Cramer, NIAC external council, June 30, 2016

NASA 360 video on Facebook, Pluto mission, June 13, 2017

Futurism: NASA-Funded Company Wants to Redefine Space Travel With Fusion Rockets, June 13, 2017

Princeton Field Reversed Configuration Fusion Reactor for Space Rocket Propulsion, Federal Lab Consortium award, 2018

Time.com: Going to Mars via Fusion Power, 2013

The Optical Navigation Module for the Spacecraft Control Toolbox is Now Available

Space optical navigation employs a camera for attitude determination and a second high dynamic range camera on a pan/track mount for terrain and celestial body tracking. Navigation and attitude determination are performed in a Bayesian framework using anUnscented Kalman Filter with an IMU as the navigation and attitude base. The Optical Navigation Module provides MATLAB code for implementing optical navigation. Additional measurements can be added including a sun sensor for sun distance measurements in interplanetary space, Global Positioning System (GPS) measurements near the Earth, and range and range rate from ground stations or other spacecraft in deep space. The system is suitable for both lunar and Mars landing missions and icy moon and asteroid orbital missions such as Artemis, Lunar Orbital Platform Gateway, Orion Multi-Purpose Crew Vehicle, Europa Clipper, Lucy, Psyche. It is also applicable to any situation where GPS is not available.

The Optical Navigation Module allows you to implement an optical navigation system for any of these applications. It includes dynamical models for cis-lunar and deep space missions along with measurement models for all of these sensors. Several scripts provide examples to get you going quickly.

This picture shows the camera aimed at the horizon and the stars that it can see during Earth reentry. The step counter gives the integration step. The star numbers are sequential from the file of stars but the stars come from the Hipparcos catalog.

This pictures shows the laboratory hardware for an optical navigation camera on a pan/tilt mount. Flexible cables eliminate the need for slip rings simplifying the design. The platform is driven by orthogonal stepping motors with harmonic drives.

Note the size. As with all of our toolboxes, full source code is provided.

PSS Advances in Superconducting Motors for Aircraft

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!

Electric Aircraft

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.

Superconducting motors

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

Our LTS magnet. This magnet was purchased from SSI for a separate NASA contract. This magnet is operated in persistent mode, in which the magnet does not need an external power supply once it has been charged.

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.

What’s next?

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.

Neural Space Navigator Camera in Action

Professor Michael Littman of Princeton University, who is a consultant on our Neural Space Navigator NASA Phase I SBIR, has the gimbaled camera in action! Check out the video.

The high dynamic range camera is mounted on a pan/tilt mechanism that uses stepping motors with harmonic drives. Harmonic drives have zero backlash. The camera assembly is 17 cm tall.

The Neural Space Navigator uses a neural network for terrain relative navigation during landings or takeoffs. Otherwise it uses the angles between planetary horizons or centers and stars combined with planetary chord widths for navigation measurements. The system uses an Unscented Kalman Filter and an Inertial Measurement Unit for both navigation and attitude determination. Contact us for more information!

What Makes a Reaction Wheel a Reaction Wheel?

A DC motor is the core of all momentum and reaction wheels. If you apply a voltage a, current will be produced which will cause the wheel to change speed. At the same time, the back electromotive force (EMF) will build up, eventually driving the motor torque to zero.

This is evident from the dynamical equation for a DC motor.

J\dot{\omega} = \frac{K_T}{R}\left(V - K_T\omega\right) + T_F

J is the inertia, K_T is the torque constant, V the voltage, T_F the friction torque, R the motor impedance and \omega is the angular rate of the shaft.

You can turn this into a reaction wheel by adding current feedback as shown in the following block diagram.

G is the forward gain. The input is the desired torque. This is divided by the torque constant to get the desired current. The difference between the motor current and the desired current is integrated. How do you pick the gain? If you work through the equations you will get this equation for the voltage, V

 \dot{V} + \frac{G}{R}V = G\frac{T_C}{K_T} + \frac{G}{R}K_T\omega

R/G is the time constant. The response is shown in the following plot. Even as the speed increases, the difference between the desired torque and motor torque is nearly zero.

Direct Fusion Drive Mission to Titan

Titan, a moon of Saturn, is of great interest to space scientists. Titan is the only moon with a dense atmosphere and clouds and with liquids on its surface. Universe Today reports on a masters thesis that proposes a mission using Direct Fusion Drive to put an orbiter around the moon. The thesis, “Trajectory design for a Titan mission using the Direct Fusion Drive,” is by Marco Gajeri under the direction of Professor Sabrina Corpino of the Politecnio di Torino and Professor Roman Kezerashvili of the City University of New York.

The thesis gives an excellent overview of nuclear fusion technology and space propulsion. The author then goes on to do trajectory analysis for the Titan mission using STK. He presents three different mission strategies using Direct Fusion Drive. He includes all of the orbital maneuvering needed to get into a Titan orbit. His mission designs would get a spacecraft to Titan in two years.