SUMMARY

I. Introduction

a. Our Scenario

II. STAS (Space Transportation and Assembly System)

a.     Propulsion systems

b.     Maneuver systems

c.      Catch and release systems

d.     Architecture

III. The Cube

a.     Concept

b.     Docking systems

c.      Architecture

IV. Operability

a.     Control

b.     Orbital mechanics

V. Conclusion

I. Introduction

Satellite and other constructions in space are limited by the fact that we can’t build huge structure in space yet. The fabrication of the ISS cost billions and took years and even decades. The biggest problem we have to face here is the fact that to put a payload in orbit you have to spend huge amount of money and energy. For now, rockets are our only way to go in space. The bigger and heavier the payload is, the bigger and more expensive the rocket has to be. One solution to this challenge would be to send little piece in space and assemble them in orbit. Assembling satellite and even bigger structure in space could drastically reduce costs and could allow us to build huge station. 

a.    Our Scenario

In order to make our work more concrete, we decided to focus on a certain activity in space and a specific orbit.

The telecommunication satellite’s market is one of the most important in the space industry. They are usually put on a geostationary orbit (about 36 000 kilometers from Earth). They are the perfect candidate to confront our concept to the reality of the space industry and the problematic of assembling pieces in space.

We decided to create a two parts system to assemble structures in orbit. In one hand, we conceptualized STAS (Space Transportation and Assembly System). This satellite has to transport the pieces from their orbits to the construction site. This part of its mission is similar to a space tug’s mission. Moreover, STAS oversees the assembly of the structure by connecting the pieces together.

In the other, we imagined The Cube: a standardized cube containing the piece to add to the structure. The cube as its own docking systems to stick with the other elements of the construction.

II.    STAS (Space Transportation and Assembly System)

a.    Propulsions systems

The different types of engines that we could use for the propulsion

We know that the propulsion is one of the most important part of a satellite to put it in an orbit and to quit this orbit during different steps.

For our project we have to choose the type of propulsion and we have different options

Full Electric engine:

It’s working with xenon with using solar panels by collecting solar energy and expel the gaz.

All-electric motors increase the satellite's load capacity by 50%, greatly reducing launch costs.

Thales Alena space is using these kinds of engines for their missions.

There are currently two main technological options in electric propulsion. Grid ionic motors on the one hand and Hall effect motors on the other.

In the first case, the electric field is created by two polarized grids. In the second, the thrust is greater, and the electric field comes from both a magnetic field induced by two coils placed in the center and outside of a cylindrical cavity and a potential difference between an anode and a cathode. The ion grid option is very fuel efficient, but the thrust is lower and takes longer to initiate. It is therefore rather preferred for long interplanetary missions. Other variants are under study, the most powerful of which is the magnetoplasmadynamic thruster (MPD). It aims to suppress the collisions between electrons, atoms and ions that are generated in a Hall effect motor due to the circular shape of the current by creating a current aligned with the electric field.

The operation is based on the combination of two fields (electric and magnetic) to provide an axial Lorentz force. This thruster has several advantages: the thrust can be adjusted by the variation of the electric current or the quantity of gas injected, and the thrusts are 100 times greater than a basic ionic engine. The inconvenient? It requires electric currents of several hundred kilowatts and therefore has to call on a nuclear source and it erodes engine components such as electrodes much too quickly.

In fact, engines with Hall effect are the most used and could be perfect for this project because of their power and easy concept of use. 

In terms of performance, Hall thrusters have a specific impulse typically in the range 1,200 to 1,800 sec – much higher than the 300 to 400 sec of chemical rockets. However, they provide a much lower thrust. A modern Hall thruster can deliver up to 3 newtons (0.7 lb) of thrust, which is equivalent to the force you would feel by holding 54 US quarters in your hand. The high specific impulse enables a spacecraft powered by a Hall thruster to reach a top speed of about 50,000 m/s (112,000 mph). The low thrust, on the other hand, means that weeks or months are needed to attain this speed.

This Ion engine has a high efficiency, but the power of its propulsion is limited which makes the orbit transfer longer. Its pickups the sun energy with solar paddles with keep it close enough to the sun that it could acquire enough energy.

EITA AND RITA ENGINES :

EITA and RITA are two different gridded systems used in Artemis the Esas’s telecommunication satellite

EITA has been developed by Astrium UK and RITA by Astrium Germany. the difference between them is that they operate in different ways, the two thrusters function in unison with each other. They are both using solar arrays to have electrical power and make the propulsion possible.

b.    Maneuver systems

Control Momentum gyroscope (CMG)

The CMG has several objectives in the satellite:

-         lead the satellite and especially his propelling system to make an orbit change or space maneuver.

-         Orbit change to avoid partial destruction of the embedded instrument with a strong light sensibility (particularly with the Sun or the light reflection who come from the Earth and Moon)

-         Allow to the satellite to always have a good orientation with the Sun to have the best illumination for the solar panel and therefore to have the best electricity production.

-         Give the best position to the satellite instruments. For example, if the satellite is a telecommunication satellite it needs to have a very precise angle with the Earth to transmit the information around the world.

The CMG is a system working with the same functioning as the reaction wheels but with some improve features. The engineer improves the electricity consumption by deeply reduce it. Moreover, the increase the instrument measurement accuracy and the appliance robustness.

Basically, its purpose is only to lead the satellite using the CMG into the right directions to do his task or to allow it to make an orbit change.

To realize that the satellite must use the AOCS (Altitude Orbital Control System) follow several steps:

Firstly, this system is divided in three parts:

-         Some sensors that are used to permit to the satellite to identify it space position thanks to external benchmarks such as the Earth, the Sun, the Moon, … or thanks to a departure position

-         Some mechanical-electrical actuator to modify the satellite orientation

-         An onboard software making the connection between the information coming from the sensor and the actuator who will make the trajectory modification in response

The sensors are composed by three magnetometers (corresponding to the three rotation axes of the satellite) that measure the Earth’s magnetic field.

The actuator are magneto-couplers. Under an electric field they create a strength enabling to turn the satellite according to one or more axes.

The three gyroscopic actuators are the mechanical part of the actuator and thereby will make the rotation movement of the satellite. (There is a huge difference between the reaction wheels and CMG. On a reaction wheel, torque is produced by changing the rotation speed of the wheel. On a gyroscopic actuator, the reaction torque is achieved by tilting the axis.)

Finally, the last step is the OCM (Orbit correction Maneuver) using the gyroscopic actuators. It will be used for the final orbit of the satellite, for the regular maneuver of orbit correction of the satellite and for the final satellite de-orbiting

Technical benefits:

-         Possibility to create or put together structure in space that cannot be launched from Earth because of the shape and the size of the launch vehicle. Therefore, the structures are packaged efficiency into the launch vehicle compare to a normal fully integrated spacecraft

-         Possibility to create structures in space that cannot be created on Earth because of the terrestrial gravity

c.     Catch & Release systems

In the interstellar space, a satellite facing the Sun can potentially rise to +120°C, while its temperature in the shadow will fall to -150°C.

We already know there are several ideas of articulated & automated arms to hold different types of components. We went through innovative scenarios, in rupture with what already exist for the mechanism of Catch & Release system. Although we are using physical properties of materials, which can hypothetically be repurposed as propulsion capabilities.

Here the physical architecture of STAS:

·      EPS : Electronic Power System

·      GNC : Guidance Navigation & Control

·      Power Conditioning Distribution Unit (US NASA)

·      OBC : On-board computer

·      TMTC : Telemetry & Telecommand

The Telemetry is very important as it will allow us to use STAS not only as a Pushing system but also as a

Definition from Wikipedia: the Telemetry is in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots tele, "remote", and metron, "measure". Systems that need external instructions and data to operate require the counterpart of telemetry, telecommand.

Hold & release components:

We

d.    Architecture

             As the size of telecommunication satellites can vary a lot, we decided to create a probe capable of handling different size of piece. As a matter of facts, the size of the probe, its thrust and maneuver systems depend on the mass and the volume of the building parts.

             In a first part, to simplify the process of sizing, we decided to integrate of fictive thruster. An electric thruster called the X-Drive. Like a basic Ion-Thruster, it consumes 6 kW of electricity but need no gas or propellant to work. Its thrust would be relatively similar to an Ion-thruster too: around 236 mN.

           In a second part, we will simulate the sizing of the probe including pressurized xenon tank and ion-thrusters. Like the first part, the consummation of the probe would be around 6 kW. The two hypotheses include the same embedded and docking systems.

In order to assemble element of structure, we decided to simulate components by a cube of two-meter square. Thanks to this design you can deploy a large variety of geometry in space.

This concept will be described later.

A.  X-Drive probe

a.    Energy supply

Considering that, with the X-Drive scenario, the only resource needed is electricity we can work on different energy supply option.

1)    Solar panel

In order to obtain enough electrical power (6kW), we need to deploy approximately 30 meters square of solar panel. This may be a problem because it would be a huge source of obstruction on a construction site.

                           Considering a classic architecture (the satellite would be a rectangular parallelepiped), 4 solar panel could be placed. It may be: 4 solar panel of 1 meter large and 7,5 meters long.

Pros: Reliable energy source /Long life

Cons: Obstruction /   Heavy

2)    Nuclear reactor (RTG)

RTG power source are used for a long time. Unfortunately, RTG don’t produce a lot of electrical energy. As a reminder, the Voyager 1’s RTGs produce only ~470 W. In order to obtain enough electrical energy thanks to RTGs, we would need a BES-5. A soviet RTG used in reconnaissance orbiter during the Cold War. Those RTGs could produce up to 5kW using fissile material: 235U.

Pros:Compact energy source

Cons: Need a huge amount of fissile material /    Problematic if you build space hotel…

The best solution, yet, looks to be solar panel. The problem of obstruction could be solved by inclining the solar panel backward.

3)    Satellite size

Considering there is no fuel tank onboard, the size of the satellite depends uniquely on the size of the motor. It’s not relevant at all to calculate the actual optimized size of this concept.

B.  Ion-thrusted satellite

a.    Energy supply

As describe in the previous chapter, solar panels seem to be the best solution to solve the energy supply problem. Considering we are using a 6kW ion-thruster, we will need the same architecture than the one described previously.

b.    Fuel Tank

Here, we suppose we are using xenon as the propulsion gas. Indeed, xenon is the most common gas in ion thrusting.

In our scenario, our building satellite must assemble telecommunication satellite in GEO (geosynchronous equatorial orbit). Before working up there, it must go to the designated orbit. The building satellite would be released on LEO (low earth orbit). The Delta V necessary to go from that orbit to GEO is about 3,8 km/s. If the Ion Thruster is as efficient as the NSTAR Ion Thruster, it requires ~65 kg of xenon. For a long-term usage in GEO, we decided that 200 kg of Xenon would be enough to ensure every step of our probe’s journey: go to GEO, collect the pieces, assemble them. We include the fact that our probe could either go back to LEO for refueling or deorbit.

The volumic mass of xenon is 2,95 g.cm-3 in its liquid state (-109°C). Which mean we would need a 67,797 dm3 tank.

c.     Satellite size

The size of the Satellite could depend on size of the payload to assemble. Actually, this concept is made for standardized payload. The size of the payload could be blocks from 1 meter squared to 2.

Our probe has to carry one main Ion thruster on his back. Considering it is a 30 cm wide thruster, the probe has to be a minimum of 40 cm wide. Indeed, we still have to shield the probe. In our case the satellite will be a square of 50 cm per side. The core will be 1 meter tall. With the tank onboard, the probe will be 84,4 cm wide.

d.    Architecture

STAS is a small probe. Its main core composed of all the communication, telemetry, avionics systems and the main Ion-Thruster is a parallelepiped rectangle of 1 x 0,5 x 0,5 m for a volume of 0,25 m3. On its front, there is the ceramic piece that ensure this capture and the release of the payloads. On top of it, there is the radiator that is used to cool the ceramic plate to make it super conductible. This radiator is composed of two part, the thermic shield that protect the actual radiator and the radiator itself that is hide just behind it. 

On two of its laterals there is the xenon tank. On the two other you’ll find two little Ion-thruster that are aimed for extra power and maneuverability.

On every back-corner of the probe, there is one solar panel of 7,5-meter-long.

   On the back of the probe there is the main Ion-thruster that is used to change STAS’s orbit.

    The whole design has been thought to fold back during launch and be as compact as possible. During launch, STAS would be a parallelepiped of 1 x 1 x 2 meters. 

II.            The Cube

a.    Concept

One of the biggest challenges in building structure in space is the enormous variety of pieces and shapes in the assembled part. We could uniformized the pieces itself, but it causes a lot of problems. Space isn’t a friendly place for human and even machine. The constrains on the piece are huge: radiation, void, impacts, etc. Space module are conceptualized to counter those constrains. Changing the shape of those module is just unthinkable because it would decrease the integrity of the module and its capacity to survive in space. We had to keep in mind that module’s shape can’t be changed.

How to uniformized non-uniform modules? Our idea is to seal the module in an empty uniformized cube. Each module would be in its own cube. In order to assemble the module, you only would need to assemble the cubes. Because, every cube is the same, a probe could transport and assemble every type of module.

It would be like a child play to build huge infrastructure. It would actually be like playing with LEGOs. The facilitation of the assemble process could lead to a reduction of the construction time and in the same a reduction of the construction cost.

After the construction is done, cubes could be disassembled in order to alleviate the infrastructure. 

b.    Docking Systems

Not long after the “first step” of mankind in space, the problematic of docking appear. In the middle of the space race, both USA and URSS wanted to have their space station. To do so they had to develop docking port. It exists different ways of connecting modules to each other:

 Mooring system:

Concept: The device consists of an extendable probe which fits perfectly into a hollow cone. The interlocking device then maneuvers to cushion the shock between the two components and then integrates properly so that the locks engage and seal the vessel. This maneuver therefore needs a lot of precision and almost perfect damping.

There are different known models:

-         Apollo’s spaceship docking system: Designed in the 60s, it features an extending rod, which has 3 bolts on its tip. The rod heads for a cone on the other vessel. This maneuver was done manually.

-         Apas: The Apas system differentiate itself from the other Mooring systems. Indeed, it doesn’t have a male and female port but two androgynous port that oversees the same tasks during docking.

-         Low impact Docking system: LIDS is a new technology in development by NASA that improve the APAS by making it more convenient. As we’re talking here about creating the future of space assembly, we will only consider the LIDS in the study of possible docking solution.

Pros & cons:

The Low impact Docking System does not exist yet but could be used. The Apas is too big to be operated by our structures so cannot be used The Apollo model can be used on the one hand by its simplicity of structure and size. The dimensions of the different systems vary between 80cm to 200cm. The mooring system needs precision and therefore last-minute rebalancing.

Magnets:

In our case the magnets can be used for anchor or just as a means of security.

There are two types of magnets:

-         Mineral magnet made from rock such as the neodymium magnet. Those magnets are self-sustaining, it would be a great solution for an inert system. But this type of magnet can cause real damage to electronics.

-         Electromagnets, on the other hand, can be controlled. They need a power source to operate.

Mechanical docking system:

Another way to secure to assemble piece would be to lock them mechanically together. That’s why we’ve design a system that could fulfill this function.

This system is a combinaison of pistons and notchs. During the docking, pistons are in low pressure, they conduct themsleves as springs and push the notchs in the hole of the docked part. When the docking maneuvre is achieved, pistons are put under pressure to ensure that the docked part wont leave.

In order to insure security, we decided to put 2 notch and an electromagnet. Like so even if one of the systems fail, two other can handle their function and the connection between the two parts.

c.   Architecture

The Cube is a metallic structure. Its size can change depending on the need. We pack a piece inside the cube and place it in orbit. Then STAS will dock and bring it to the construction site to add it to the assemblage.

By putting multiple block together, you can create huge infrastructure. As a construction play, STAS will stack the cubes in the right order to build the planned construction.

Thanks to that architecture, every problem linked to the shape of a piece disappear. We could even send circular part by dividing them into multiple pieces. The creation of Von Braun Ring wouldn’t be science fiction anymore.

IV.            Operability

a.    Control

We currently live in a stunning time where machine can control themselves, car drive themselves, and so many other breakthroughs. Space industry is used to automatize their probes, rovers. Naturally, one of the many ways to control our builder probe would be to let it do the work. Another one would be to control it from a ground base. This application could be really interesting but is really challenging.

i.     Automatized assembly

Our progress in informatics could led us today to an automatized probe with a powerful AI. This solution has a lot of potential. Indeed, in theory, ground base would only need to enter a recipe of the building to assemble and the probe would do the rest. Dock autonomously isn’t a problem anymore. SpaceX already did it with its Dragon Crew capsule during the Demo-2 mission.

The main problem is caused by the unexpected. During the assembly of the ISS, astronauts were on site and could handle any scenario. For a probe, an unexpected event could cause the fail of the mission and the loss of the building.

Pros: Reduce cost of operating/No delay du to the send of order from a tier

Cons:   No or few adaptability

ii.    Controlled assembly

Nevertheless, astronauts don’t have to be on site to check on the construction site. They could do it from a ground base. Indeed, the building probe could be used as a drone piloted by a pilot from the ground. This solution asks for a huge infrastructure in order to be able to send and receive enough data from the probe to control it.

As the infrastructure and the ability to communicate with probes growth, this wouldn’t be a problem soon.

Pros: High adaptability / Flexibility / efficiency / secure

Cons: Need infrastructure /    Delay due to the distance with the probe

iii.    Science fiction

We even could imagine a bigger probe that could be piloted by an astronaut. After undocking from the international GEO space station, he would build the next generation of spaceship to go to Mars and even further

b.     Localization

We are using the triangulation way of geolocalisation by considering the data of 3 different satellites. We suppose our STAS is at a position X in 3 dimensions.

The principle is simple :

-         The two first satellites are considering the distance in between themselves and STAS, crossing their perimeters in 2 positions

-         The last satellite will confirm the crossing point where STAS is located

The best solution seems to be a mix between autonomous capabilities and a human pilotage if something goes wrong. This would ensure a higher rate of success, reduce human and machine error. The probe could easily take the control during the transition phase (from docking with the piece and the assembly phase) and pilot could supervise the critical phase and take part of the process manually.

V.              Orbital mechanics

The biggest challenge in the usage of smalls space tugs like STAS is the resources onboard. In order to operate the probe as long as possible, journey between pieces and the assembly site has to be the shortest possible. In our scenario our probe is in GEO. Which mean it can’t come and go from LEO to GEO, it would be way too greedy in resources. In the other hand, sending the pieces to a GEO transfer orbit could make the cost of the assembled satellite even higher. Our goal is to reduce the cost of building and sending huge satellite.

a.    LEO to GEO

Let’s imagine pieces are put on LEO. In addition to his role of assembler, our probe would have the role of a space tug. Maneuvering from GEO to LEO cost a lot of resources. Our probe has very limited fuel capacities. Doing so would definitely reduce the lifetime of the probe unless we refuel it at each pass in LEO.

Sending payload to LEO is way cheaper and it doesn’t require big rocket. We could actually use micro-launcher to send pieces into LEO.

Moreover, we are using an Ion-thruster. Even if we are in possession of the best Ion-thruster ever produced, this maneuver could take days! The pieces are inert, they have no propulsion system. Letting them in LEO for a long period of time could be risky (depending on how low the orbit is actually) because it could leave the orbit and enter the atmosphere.

b.    GEO to parking orbit

Another option would be to put the pieces on a parking orbit. By doing so, we reduce the distance between the operating orbit of our builder and the piece delivery apogee. At first it would ask a more powerful rocket at launch or launch fewer pieces at the same time. But it would save a huge amount of resources for our builder.

This solution is a kind of 50-50 deal. You save time and resources in one hand, but you have to pay more to launch in the other.

c.     Putting the pieces on GEO

The simplest way to get the pieces from ground to the construction site, from the probe point of view, would be to directly send the pieces in GEO. The builder probe would just have to dock with them and bring them back to the construction site to assemble them.

This method could save even more time and resource than the previous one. The probe could even work on different construction site during its lifetime. But the launch cost would be even bigger.

VI.            Conclusion

STAS and The Cube are two systems that can revolutionized the way of building in space. It requires technology that are available or in development, which mean that this kind of systems could exist in relatively short time. We already see prototypes and even actual mission to dock with other satellite. Our project was imagined to assemble structures in space but STAS could easily take the role of space tugs.

We are at the beginning of a new era where space isn’t a frontier anymore but the new Eldorado of thinker, inventor and dreamer. STAS is, for now, only the result of our imagination and our work during this weekend. Seeing one of these actually flying and working up there could be considered as science fiction ten years ago. But now, we are talking about the future! The future is now!

PS : You will find a version of this file with illustrations, modelization and scheme in the Team Board section. Even if, this section isn't evaluate, it could be interesting to you to see our whole vision of the STAS Project.