Lunar Landing Control System

The first soft moon landings were accomplished in the 1960’s by the Soviet Luna 9 and the U.S. Surveyor Spacecraft. These were followed by the U.S. Lunar Module landings during the Apollo program. The Soviets had their own LK Lander lunar lander for landing humans on the moon but it never flew. China’s Chang’e-3 landed on the moon on December 13, 2013. India plans to land the Chandrayaan-2 on the moon in 2018.  South Korea intends to land a spacecraft on the moon in 2020.

The U.S. Lunar Module was flown by a crew but had a digital computer that performed guidance, navigation and control. A great new book by Don Eyles, Sunburst and Luminary an Apollo Memoir explains how that was accomplished with a computer less powerful than those in toaster ovens today. Don played a key role in saving the Apollo 14 mission when an abort light appeared on the crew’s console prior to descent. Read the book for for the whole story.

NASA intended follow-ons to the Lunar Module that would have been fully automated for delivering materials to the moon in preparation for a permanent human presence. Unfortunately, those plans never materialized.

As we are always looking for new missions for testing our Precision Attitude Control System, we added guidance, navigation and control for lunar landings. We use a really simple guidance algorithm called 2nd order guidance. It is nothing more than a Proportional Derivative (PD) controller with the landing spot as a target. You can adjust the damping ratio and undamped natural frequency of the controller to mimic more sophisticated, “optimal” guidance algorithms. The 2nd order guidance works until the lander gets near the surface and then it switches to landing algorithm that hovers, nulling any remaining translational velocities and then descends to the surface. Lidar would be used as guidance. Once it is hovering it would need to search for a flat spot for landing. NASA has developed Hazard Detection Software for Lunar Landing that uses lidar. It is available for licensing from Caltech.

Here is one simulation in our Simulation Framework. Once the descent is initiated, the spacecraft reorients so that the main engine thrust vector is in the desired direction. The display on the left shows the attitude errors (the two boxes) and the throttle setting (which is zero during the attitude maneuver.)

A close up of the attitude display. Pitch and yaw are offsets of the green rectangle. Roll is rotation of the rectangle. This is quite primitive but it is easy to add your own displays if you know a little OpenGL!

Descent starts and the throttle is about 50% at this point. The two plots are of altitude and velocity. The maneuver starts at 15 km and the target is 600 km along track. The lander has solar panels on a two-axis gimbal and a high gain antenna, also on a two-axis gimbal.

The propulsion page shows two attitude thrusters firing and the main engine.

The spacecraft has landed! You can see the terminal descent phase on the altitude and velocity plots. The lunar surface is featureless because we have not added close up maps of the landing zone to the planet display.

The descent page shows the throttle settings. You can monitor the guidance force demand and simulated force.

This is the propulsion page. The attitude thrusters get very busy during the terminal descent phase. Note that we have a lot of fuel left! We could have hovered for quite some time.

The graphics are from our VisualCommander product that runs on Mac OS X.

This GN&C system is capable of autonomous flight from LEO all the way to the moon. It uses our Optical Navigation System, developed under a NASA Phase II SBIR for trajectory determination on the flight to the moon and lunar orbit entry.

For more information contact us directly!

Living Universe Documentary

Back in early September, PSS and PPPL were visited by a film crew from Australia. The project? Living Universe: An Interstellar Voyage, which will include a feature documentary, a 4 episode TV miniseries, and a podcast. The documentary touches all aspects of an interstellar mission, from exoplanets to astrobiology, including transportation – which is where our fusion engine work comes in. The film is in production now and the producers expect to launch in late 2018.

The PFRC experiment at PPPL is the only hardware the documentary team could find with a path to fusion propulsion! Dr. Cohen was able to run the machine for the film crew, and both Mike and Stephanie were interviewed extensively. We discussed the rocket equation and the fundamental speed of fusion products, and how DFD moderates that speed with additional propellant to produce higher thrust. For an interstellar voyage, DFD would have to be much, much lighter than we know how to make it today – but who knows what innovations in magnets are possible in the future!

How will you be able to watch the film and TV series? The film should do the rounds of museums and IMAX theaters. The TV series will be available for streaming from Curiosity Stream, a service which specializes in science, history, tech & nature documentaries. We will post an update when we have a firm release date!

 

Fusion Power Associates Meeting

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 LaboratoryTAE TechnologiesGeneral 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.

The Joint European Torus plans a D-T campaign in 2019 and hopes to set new fusion benchmarks. We learned about Korea Superconducting Tokamak Advanced Research  (KStar). It has achieved longer than 70 second pulses in H-mode and has suppressed ELM for more than 34 seconds. KStar has in-vessel control coils.

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.

Second DFD Patent Awarded

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

Rendezvous with 1I/’Oumuamua

An interstellar asteroid, 1I/’Oumuamua, was discovered on a highly hyperbolic orbit by Robert Weryk on October 19, 2017 moving with a speed of  26.32 km/s. It appears to come from the direction of the star Vega in the constellation Lyra. It would be really great to send a mission to rendezvous and fly in formation with 1I/’Oumuamua to study the asteroid. The high velocity makes it hard to do with current technology.

Direct Fusion Drive (DFD) might provide a answer. We designed a spacecraft with a 1 MW DFD power plant and assumed a launch on March 16, 2030. The following plots show the trajectory and the force, mass and power of the spacecraft during the 23 year mission. As you can see we don’t have to use the full 1 MW for propulsion so we have plenty of power for data transmission and the science payload.

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The code for this analysis will be available in Release 2018.1 of the Princeton Satellite Systems  Spacecraft Control Toolbox for MATLAB.

PRISMS Secondary School Student Builds a Sun Sensor

The Princeton International School of Mathematics and Science is a private secondary school in Princeton New Jersey.

http://www.prismsus.org

One of their students, Savva Morozov, undertook a project to build a miniature 2-axis sun sensor for a CubeSat. Here is his blog post!

My name is Savva, I’m a senior at Princeton Int’l School of Math & Science. This summer I designed, built and tested a 2-axis solar tracking sensor for Princeton Satellite Systems.

The sensor can determine the relative position of the sun using a set of photodiodes. Bearing in mind that the solar sensor would be used in vacuum environment, I decided to make the sensor out of printed circuit boards (PCBs) and solder them to each other. Originally, I wanted to 3D print the sensor, but shifted to the PCB solution to eliminate the risk of outgassing.

Picture 1: solar tracking sensor, 2nd prototype.

My solar sensor design resembles the shape of a square-based pyramid that is approximately the size of a quarter. It consists of 5 PCBs: 4 sides and a base to which the sides are attached. Each side contains a photodiode, and by measuring the voltage outputs from at least 2 of the diodes, the device can determine the sunlight’s direction.

One of the initial problems I encountered was the handling and attachment of the photocells to the side PCBs. Each cell came with an anode and cathode wires already soldered to its front and back. I desoldered the cathode wire from every cell and affixed them to the PCB using a space grade silver conductive epoxy. This way I attached the cell to the device and grounded its cathode at the same time. I thought I killed two birds with one stone, but instead I killed two photodiodes: they were damaged in the soldering process. I resolved the issue in the second prototype: I threaded the cathode wire into a hole in a side PCB and then glued the diode to that same board. This way I didn’t have to use soldering iron at all, preventing possible risks. I then connected the cathode of every photodiode to a common ground and outputted the voltage readings from each cell in a single data bus.

Picture 2: Solar sensor, 2nd prototype, quarter for scale.

The diode, being attached to the outer side of the satellite, might be exposed to light that is reflected off the Earth or satellite surfaces. To partially prevent this, I soldered a shield to the edge of the sensor. Each surface of the shield would be covered with non-reflective material to further decrease the amount of ambient light.

To protect the diodes from the impacts of micrometeoroids and other space debris, I plan to cover the diodes with a thin shield of hard glass crystalline window (tempered glass or sapphire).

Testing the first prototype indicated a number of drawbacks that were solved in the second. Such problems are attaching the shield and the cells to the device, decreasing sensor’s size while increasing its aperture, and making the assembly process simpler.

I also calibrated photocell’s voltage outputs and their exact positions to account for manufacturing imperfections and for those created during the manual assembly of the device. I wrote a program to calculate the vector of sunlight direction and using Processing IDE created a visual representation of my sensor as well as its output result:

Picture 3: Visual illustration of a working solar sensor.

In order for my sensor to survive the vacuum environment, I attempted to use only space qualified materials in the device’s assembly: PCBs, solder, epoxy, and sheets of copper. I have finished working on the development stage of designing the solar sensor. Testing procedures on my last prototype showed that such device would be ready for further usage and launch.

AIAA Space Forum paper on Pluto mission is live

Sadly, the AIAA Space Forum in Orlando, FL was canceled due to hurricane Irma. So, we didn’t get to present our paper on our DFD mission to Pluto. AIAA has, however, published all the forum papers and is providing free access for a few months in lieu of the actual conference. This means anyone can download it!

Fusion-Enabled Pluto Orbiter and Lander paper:
https://arc.aiaa.org/doi/abs/10.2514/6.2017-5276

Open access to the AIAA Space Forum technical program:
https://arc.aiaa.org/doi/book/10.2514/MSPACE17

Enjoy!