It’s the end of a very productive summer with our interns, Jack and Phil. They have now returned to school, Jack to UC San Diego, and Phil to Cal Poly San Luis Obispo, to start their senior year toward their bachelor’s degree in engineering.
As a company, we made a tremendous amount of progress in a short amount of time. On the mechanical engineering side, Phil helped us determine that the spacecraft structure would hold up to the stress of launch. The largest rocket launchers typically have the lowest g-forces, around 5 G’s (five times the force of Earth gravity), while the smaller launchers have g-forces of up to 9 G’s. Our satellite structure is designed to handle 9 G’s plus some additional margin, so we can use any of the launch vendors, large or small.
We also decided that certain parts of the structure could be made of a high strength engineering plastic. These plastic pieces are strong enough to hold our telescope optics, while keeping the optics thermally isolated from the aluminum walls of the spacecraft. It’s important to keep the optics at the same overall temperature, even if one side of the spacecraft is hot from sunlight while another side is cold by radiating heat out into space.
The picture above is a computer generated rendering of the entire spacecraft with the solar panels, secondary mirror, and large light baffle deployed. The light gray or white colored panels are made of aluminum, and the darker gray panels are the high strength plastic panels. The design has five solar panels, four of them folded into the bottom of the cubesat, and the fifth one acting as the aperture cover for the telescope.
All of the major mechanical pieces have been designed, and we have been printing the pieces on our big 3D printer for an initial prototype. We have already found a few minor issues and corrected them, and we will be sending the updated designs out to our manufacturing partner to have metal parts machined out of aluminum. This will let us have two spacecraft models, one of plastic so we can make quick checks and modifications, and one of metal that we can use for strength and thermal testing.
We have also been working with a number of different vendors for the most complex pieces of our space telescope. We now have a full design of the optics, which includes the sizes, positions, and prescriptions for the three mirrors and four lenses in the main optical path. And we have been working with other vendors on the secondary mirror booms, the fine optics adjusters, and the folding solar panels. These are all things that must move, unfold, or extend at the right time and with the right amount of movement and clearance. We’ve had to make a few adjustments to our CAD drawings to make sure everything fits, but there have been no major issues.
On the electronics side, intern Jack made a great deal of progress on the laser communication circuits. The transmitter test board is working extremely well. It can switch several amps of laser current in a nanosecond, so we can transmit data down to the ground at 200 MHz or even a little faster. This corresponds to a data rate of 100 megabits per second.
Jack built two versions of the laser receiver test board. The first version had quite a bit of noise, so we tried several different receiver circuits. The second version of the board can actually receive the light pulses at 200 MHz and translate them into electrical signals. However, although the noise level was reduced, the laser receiver board still isn’t sensitive enough. We do have some ideas on how to reduce the noise and boost the sensitivity to the right level, and we’ll build and test a third version.
The picture above shows the laser transmitter board on the left, and the laser receiver board on the right, behind a light diffuser screen. The receiver board is mounted to an X-Y stage, so it can be aligned with the laser beam by turning the metal knobs.
The oscilloscope picture below shows the laser drive signal on the transmitter board in black (oscilloscope channel 1). The signal is inverted, so laser light is generated when the voltage is low -- the signal shows two pulses 5 nanoseconds wide and 10 nanoseconds apart. The laser light is beamed to the receiver board, where it is detected (converted from photons to electrons), amplified several times, then converted from an analog voltage to a digital signal. The non-inverted digital received signal is shown in green (oscilloscope channel 4), with the proper pulse timings.
As far as the laser transmitter is concerned, it is working so well that we are thinking of turning it into a product and selling it. It would be the best overall laser communication module available for cubesats - fast, low power, compact, and low priced. It could be a quick way to get some sales revenue -- but it would be a distraction from our main goal of getting our satellite into space.
Our plan is to talk to venture capital companies at the beginning of 2019. If we don’t get interest in our space telescope business plan, we’ll do a slight pivot and turn the laser communicator into a product, while still working on our space telescope. So we will be busy until the end of the year working on our satellite design, refining our business plan, and talking to prospective customers.
Good-bye to our 2018 summer interns Jack and Phil - you guys are the best!!