Introduction

A free flyer is a space drone that can move in any direction. Our goal was to design an efficient and effective autonomous free flyer for inspecting a spacecraft for damage, inspired by aspects of nature. The EVO in our name stands for evolution, emphasizing the use of solutions developed by natural evolution over many years, and found in nature today. Having said that, nature itself is not present in space, and many would say that we cannot get use nature’s designs for space exploration. We dare to disagree with that. As in any engineering challenge, the key is identifying and simplifying the problem. With this opinion we started working and came up with more solutions inspired by nature than we could possibly imagine.

These solutions include using an owl-like head for the movement of the cameras, a pied kingfisher style of surveying and landing on the spacecraft, gecko inspired grippers for gentle landing and guanine photonic crystals found in fish for radiation protection. However, not all problems found when faced with space exploration have equivalents in the nature on the surface of the earth.

Vision is key

The owl is the animal we have chosen to begin with. The owl’s eyesight is considered one of the best in nature. Its large eyes are fixed firmly in its head – so it must rotate its entire head to change view. The owl can rotate its head and neck as much as 270 degrees. The field of view of the owl is about 110 degrees, of which about 70 degrees is binocular vision – area which both eyes cover i.e. area where it has depth perception. For comparison, the human’s field of view is 180 degrees, with 140 being binocular. The owl’s vision is extraordinary, especially at night. However, it does not suffer in environments with strong light since its pupils can adjust to a wide range of sizes, allowing just the right amount of light to reach its retina. Additionally, it has 3 eyelids to protect its eyes.

Following this example, we decided that the main inspection devices which we are going to use are going to be 2 high resolution cameras, with wide sensors – to absorb more light in dark environments, and darkening filters that can be used if necessary. In addition to the darkening filters, a flashlight and additional protective covers should be added to protect the cameras if needed. These cameras are mounted on a “head” which can turn 360 degrees around the main body of the free flyer and it can pivot up and down. The head with the cameras is located at roughly the equator of the body on the flyer, and at the start it is angled diagonally ahead and below of the flyer. The reason for this is so that damage can be detected ahead of time – before the flyer arrives directly above the damaged site, giving it time to land near it efficiently.

Furthermore, we equipped the flyer head with a thermal camera, mounted near the main cameras to have the same flexibility. This will allow a faster detection of gas leaks of the main spacecraft, which not only preserves energy and propulsion fluid for the flyer, but also helps solve these types of problems faster, which could be the difference between life and death for astronauts.

Evolution fast forwarded

What is the most effective flight path the flyer should take? This depends on the model and structure of the spacecraft, as well as the configuration of the flyer. One approach is to mathematically calculate a set of predefined points that the flyer must visit for each spacecraft. However, this is difficult and requires reprogramming every time the spacecraft is modified. We believe there is a better way – to have the flyer learn to calculate a route on its own.

How could a machine learn that? We are going to teach it by running simulations with a genetic algorithm that models a neural network on a workstation down on earth. The algorithm works by trying out different combinations until it finds one that does the task most effectively within given constraints. However, it doesn’t simply brute force every possible combination. It selectively chooses the most promising combinations in each generation and does random mutations and the equivalent of DNA crossing in machine learning. To begin with we are going to split the tasks it is going to perform:

• Image analysis, by which we mean we are going to have it learn to distinguish what Micro-Meteoroid and Orbital Debris (MMOD) damage is, and basically how it looks.
• Flying around the spacecraft. To begin with the flyer is going to know how much energy and fuel is used for maneuvering, what crashing is and how to avoid it and finally the shape of the spacecraft it is inspecting. Using this information, it is going to start learning, and it’s going to fail thousands of times. However, using an evaluation function, it’s going to know how good each attempt was, pick the best attempts, and randomly change them a little each time. Evaluation is going to be done based on whether and if it crashed, how much resources it used up, how much of the spacecraft was covered and the time taken. There should be a timeout limit in place in case one of the simulations/attempts takes too long without covering the entire spacecraft.

Being careful and choosing the final move

The pied kingfisher is a bird that hunts fish from the air. First the kingfisher flies over the water to detect the fish, similar to the basic flight pattern of our flyer. Once it spots the fish, the bird hovers over the water while keeping its head still. This maneuver requires a lot of energy but allow the bird to determine where the fish exactly is and when should it sweep in and grab it from the water. Similar to this our flyer is going to stop to determine if it is necessary to land on the spacecraft and inspect the damage more thoroughly.

The landing is made safer by using gecko inspired gripers which are positioned on the feet of the lander. We decided to use gecko gripers instead of magnets, so that there is no possibility of interference with any electronics or communication on the spacecraft.

Once the flyer lands it uses two additional inspection techniques. First it scans the damaged area with a 3D laser scanner, patented by NASA, which can be positioned near the camera heads. This laser is used so we can inspect the damaged zone without any interference from shadows, and additionally this scanning allows us to have higher precision while inspecting the area, up to 0.025mm, which is going to be useful for inspecting possible cracks around the direct damaged area. By using this laser we can get an accurate 3D representation of the damaged area.

In addition to this, the flyer is also equipped with an ultrasound inspection device.This device is mounted on a robotic hand, which is mounted near the camera “head” and has the same 360-degree rotation around the body of the flyer. The ultrasound head is used to inspect for any damage hiding under the surface. This kind of damage is a known consequence of meteoroid and debris impacts on spacecraft. The robotic hand is necessary to cover with the damage hole and create a closed system so that ultrasound can work.

Radiation protection

Heat transfer on here on earth can be accomplished by conduction, convection and radiation. In space this is not the case. While an object is in space it can transfer heat to the surrounding area only with radiation. The current technology used for protecting spacecraft from the thermal radiation in space is called MLI (Multi-Layer Insulation) which in essence is the foil with a gold or silver color that we see on various satellites. This technology can be used on our free-flyer as well, but in addition to this we propose a research in the development of a material based on guanine photonic crystals. These crystals are found in fish like sardines and are basically responsible for the shining effect that some fish have, for example the sardines, and these crystals even have a part to play in the color changes of chameleons. When found in fish these crystals reflect light without polarizing it. It is our opinion that there is a great potential in developing such a material, which could in effect replace or at least just upgrade the MLI that is currently used for protection against thermal radiation.

Propulsion and cooling

For the spacecraft propulsion we have incorporated a system that can propel it and achieve cooling at the same time.As a propulsion gas, we are using liquid co2 because it can be stored in a confined space and small displacement. From thermodynamics we know that every liquid needs energy in a form of heat to evaporate. Additionally there is problem with the spacecraft cooling, because in space, heat transfer through convection cannot be achieved. With our system we managed to convert our two problems in two benefits. We are using the heat from the powerful computers and other equipment to improve the phase changing process, thus achieving and improving the propulsion efficiency.