# 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

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?

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.

# Princeton Satellite Systems and Princeton Plasma Physics Laboratory Researchers Named 2020 Thomas Edison Patent Award Winners

Dr. Gary Pajer, Yosef Razin and Michael Paluszek of Princeton Satellite Systems and Dr. Samuel Cohen of the Princeton Plasma Physics Laboratory were awarded a 2020 Thomas Edison Patent Award for U.S. Patent 9,822,769, “Method and Apparatus to Produce High Specific Impulse and Moderate Thrust from a Fusion- Powered Rocket Engine.” This patent is for a new type of nuclear fusion reactor that is compact, making it suitable for mobile power, emergency power, space propulsion and power. Images of a mobile version of the reactor, and a version used for a rocket engine are shown below. The work is currently funded by an ARPA-E OPEN grant. NASA has also funded this work through the NASA NIAC program.

The 41st Edison Patent Awards Ceremony, themed “Transforming Hope into Action” will take place virtually on November 12th. Contact Vanessa Johnson for more information about the event.

# Visiting Planet 9

In 2015, astronomers from Caltech determined that a giant ninth planet may be orbiting the Sun. It was called Planet X and then Planet 9. The discovery was based on perturbations in the orbits of TNOs, trans Neptunian Objects. The planet has about the mass of Neptune and is in a 10,000 to 20,000 year solar orbit. Jakub Scholtz of Durham University and James Unwin of University of Illinois at Chicago hypothesize that Planet 9 might be a black hole. The orbit of Planet 9 looks something like this.

We used a semi-major axis of 700 AU, an inclination of 30 degrees and an eccentricity of 0.6. The plot shows the full orbit of Planet 9, but the simulation only shows 150 years of the other planets.

It would be very interesting to visit Planet 9. One way is to use a solar sail. The sail would start on a trajectory aiming at perigee very close to the sun and then accelerate at high speed. Another approach is to use a spacecraft propelled by Direct Fusion Drive, a fusion propulsion system we’ve been working on for several years. A 26000 kg spacecraft with a 12 MW engine and 2000 kg of payload could rendezvous with Planet 9 (based on the above orbit) in just 11 years. This is the spacecraft trajectory

Direct Fusion Drive is based on the Princeton Field Reversed Configuration reactor invented by Dr. Samuel Cohen of the Princeton Plasma Physics Laboratory (PPPL). We have a experiment running at PPPL, funded by an ARPA-E OPEN grant, to perform critical ion-heating tests. Earlier work was funded by the NASA NIAC program. Hopefully we will be in a position to send a mission to Planet 9 in the not too distant future!

# PSS Receives U.S. Patent 10,752,385, “Magnetic Dipole Cancellation”

Spacecraft with thrusters or instruments with large magnetic dipole will experience torques in a planetary magnetic field. U.S. Patent 10,752,385, just granted to Princeton Satellite Systems, uses a current loop to cancel the magnetic field of the onboard dipole. The patent text is:

“A dipole cancellation system and method may include a plurality of magnetometers for measuring a device magnetic field associated with a plurality of device coils generating a device magnetic field having a primary magnetic dipole moment. A compensating coil carrying a compensating current running a first direction that generates a compensating magnetic field having a compensating magnetic dipole moment. The compensating coil may be positioned and the first current may be selected so that the compensating magnetic dipole moment completely cancels the primary magnetic dipole moment. A method may use the system to stabilize a spacecraft by calculating an estimated torque of the spacecraft, receiving a value for an external magnetic field, receiving a value for a device magnetic field, and calculating and applying a compensating current may be then applied to the compensating coil to cancel the primary magnetic dipole moment, wherein the spacecraft is stabilized.”

Helium-3 is available in the regolith of the moon and is a possible fuel for advanced nuclear fusion reactors on Earth. It would be extracted from the lunar regolith, packaged and returned to Earth. One question is how to return the helium-3 to the Earth. One approach is to use aerodynamic braking to return the helium-3 to a low Earth orbit where it would be picked up by the Space Rapid Transit (SRT) reusable launch vehicle and delivered to an airport where it would be shipped to power plants. SRT It is a two stage to orbit vehicle with a hypersonic air-breathing engine in the first stage.

The overall architecture is shown below.

One of the major advantages of SRT is that it can land and takeoff at any major airport. The first stage can be used as a transport vehicle. Since it is fully reusable and operates like an aircraft it is potentially much less expensive than vertical launch.

The return from the Earth involves launching the helium-3 tanker into orbit and then doing a departure burn that puts the spacecraft in an elliptical Earth orbit with a low perigee. As the return vehicle passes through perigee, aerodynamic drag lowers apogee until apogee and perigee are the same. This is shown in the following plots.

The first plot show the altitude from the Earth, the velocity magnitude and the drag force magnitude. The second plot shows the orbit. The last plot shows how apogee is reduced with each pass through perigee. It takes 10 weeks to enter the final orbit if the orbit perigee is 100 km. Note that perigee doesn’t change. The simulation uses a free-molecular aerodynamic flow model. For simplicity, it does not include lunar gravity perturbations.

Ideally, the lunar return vehicle would be brought back to Earth and reused.

The maneuver uses only drag. A lifting vehicle would have an additional degree of freedom since the force vector could be controlled.

This analysis was done with the Spacecraft Control Toolbox. The function will be available in Version 2020.2 available in early fall. Contact us for more information!

# Hurricane Isaias 2020 and SunStation Solar Power with Battery Backup

Power went down when Hurricane Isaias moved in. Fortunately our customer had a SunStation solar power system with Lithium battery backup. Unlike other solar systems, this system has a transfer switch to disconnect the solar system from the grid so that the solar power system can power the house when the grid is down. The batteries provide enough power to keep critical systems going when it is really cloudy or at night.

You can see the system in operation here. The first shows the system when the solar power is insufficient to power the house.

The following shows the system with enough solar power to charge the battery and power the house.

Even on a cloudy day, you usually get enough solar power to keep the house running. The 0.2 kW load includes lighting, refrigerator, WiFi and other loads. This system has 14.4 kWh of storage, so it could run the house, without solar, for 72 hours.