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At the Alpen Adria University of Klagenfurt, a team of BSc students has developed a small space project as an exercise for the “Introduction to spaceflight” lecture. While not every astronautic aspect has been defined in a detailled manner, the report still reflects the breadth of the subject and demonstrates how deep one can dive into the matter within one semester.
We, three students from the Alpen Adria University of Klagenfurt, are using our BE (Broken English) to give you a little detail about our space mission, which should theoretically start in 2037. We have to point out that we are studying computer science, and we tried to get in detail with all components of our mission. Still, you have to comprehend that not every aspect is fully realizable. Nevertheless, we reached a level of detail that is worth sharing.
Every mission starts with an idea
In the beginning, we want to give you a little overview of our mission we have conceived but not implemented. “Bumblebee” is an orbiter with a total weight of 110 kg. Its destination is the sixth planet of our solar system: Saturn. On the north pole of Saturn, a cloud pattern is spinning. The intriguing fact is that the cloud pattern forms a nearly perfect hexagon. Our goal is to analyze this hexagon to get to know what the purpose of this hexagon is, what power drives it, what effect it has on the planet and the atmosphere of Saturn, and lastly, what it’s (thermo-)dynamics are.
To make this mission a success, we had to discuss some elements of a space mission. First, we had to deal with the topic of the flight to Saturn. Once our spacecraft has reached Saturn, the next consideration was how to collect data about the hexagonal storm and, more importantly, how to transmit these data back to Earth. For these tasks, we had to equip our own spacecraft from power supply, propulsion, and controllers up to data transmission. And as if this task wasn’t challenging enough, we also had to make sure that we don’t exceed our mass budget of 110 kg. But to help us out, we had the advice of Dr. Gernot Grömer and two other experts of our choice.
What do cooking and Saturn have in common?
Figuratively speaking, the hexagon reminds a bit of a pancake. It has a width of around 29,000 km (more than twice the diameter of Earth!) and an assumed height of 300 km (1,685,393.258 Bananas in a row, or around 80 Großglockners piled). Bumblebee’s mission is to circle around Saturn in a polar orbit and release passive probes (we also call them worker bees) made of aluminum above the hexagon. Bumblebee further carries a radar that detects the reflective probes, and the resulting data is transmitted to Earth. We asked an expert in meteorology, Andreas Jäger, who helped us a lot in how and where we should release the probes. He recommended focusing on the hexagon’s special points like its eye, the corners, and the edges, where Andreas Jäger suspects jetstreams.
Bumblebee’s way to its flower
We assumed that Bumblebee could also use this planet constellation. In fact, this is possible every 20 years. Cassini-Huygens started in 1997; therefore, the next starting year would be 2037. We also assume that Bumblebee uses a launch rocket like the Ariane 6 (Or newer; looking at you, Falcon 9!) to get into the geostationary transfer orbit (GTO). For the voyage to Saturn, we calculated the needed fuel based on the data of Cassini-Huygens. In detail, we found out that Cassini weighed about 5712 kg at the start and 4641 kg shortly before reaching Saturn. The Tsiolkovsky rocket equation yields the used ∆v, which is, as mentioned, 630 m/s. (The I_sp of Cassini’s driving system is about 300 s). We used the same equation for Bumblebee to determine how much fuel it needs, depending on the I_sp of the propulsion system.
We assume that it is not necessary to reach Saturn as fast as possible, so we decided to use a Pulsed Plasma Thruster (PPT) to get the drive for Bumblebee. A PPT uses vaporized Teflon to get its specific impulse. With the help of electricity, it vaporizes and ionizes Teflon into a plasma that is accelerated out the back. The advantage is that the PPT does not handle any explosive fuel and reaches a high I_sp. Using this propulsion method, we need less than 3 kg of Teflon for our journey. The disadvantage of this drive system is the low thrust, which is about 1mN when using 60 W of electric power. Bumblebee is a comparably small spacecraft and therefore does not deliver much more power, so we have to work with the low thrust.
Around six years after launch, similar to Cassini, Bumblebee will reach Saturn. The next important thing is to change the satellite’s inclination. We want the spacecraft to orbit Saturn around the poles to be able to analyze the hexagon. Therefore, we need an inclination angle of 90° +/- 5°. This inclination can be achieved by using a swing-by around the poles of Titan (one of Saturn’s moons). Figures 2 and 3 show this thought experiment. We also use this maneuver to accelerate Bumblebee by ∆v = 4150 m/s.
The trajectory would be a highly eccentric ellipse that slowly transforms into a circle around 2950 km above the 1 bar border. We also considered altitude regulations (around 2 km per year), once Bumblebee circles around Saturn. With the PPT, less than 0.1kg of Teflon per year is necessary. When Bumblebee is circling Saturn, the next act of our mission begins.
About Samara, balloons in space and why Bumblebee has no Twitter
Now that our orbiter is in orbit around Saturn, our next consideration was how to investigate the storm. Since Bumblebee must not exceed a mass of 110 kg, we had to pay special attention to the weight of our measuring instruments. So, we first calculated how much of this 110 kg is needed for power supply, computers, propulsion, structure, and communication with Earth, and what is available for our actual scientific equipment. We consciously decided against a complex and heavy optical camera. Instead, we moved to measure the storm’s thermodynamics using exposed miniature probes, similar to the movie “Twister”. Together with our meteorology expert Andreas Jäger and Alfred Müller, a researcher on electromagnetic localization, we could agree on a radar system with small, reflecting measuring points. These measuring points are made of very light aluminum foam and have the shape of Samara seeds to be distributed as far as possible by the storm’s currents. After deployment, these elements are measured with the help of a SHARAD-based radar system on board of Bumblebee, and the recorded data can be used to measure the dynamics of the storm. In addition to the radar system, there is also a Doppler Wind Lidar sensor onboard Bumblebee to measure the storm’s upper dynamics.
We have come up with a system that uses ballutes to drop these particles from Bumblebee safely into the storm. Ballutes are a combination of parachutes and balloons. They can be made of kevlar, polyimide, and ceramic fiber, therefore eliminating the need for an extra heat shield. The ballute gets inflated in the atmosphere and thus slows down the aluminum probes, which it then ejects in small groups of 100 pieces. Since we still had 16 kg payload available, we decided to drop a total of 5 ballutes with a mass of 3.2 kg each. These 3.2 kg include the ballute system, solid fuel boosters to accelerate the ballute into the atmosphere, and 1.2 kg of probes, which are about 3400 pieces. To send the data to Earth, we use a miniature version of the high gain antenna onboard the Cassini spacecraft. This should easily reach the estimated data rates of 39 kbit/s. Since publicity is becoming more and more important in our digital era, we wanted to take this aspect into account and give Bumblebee the opportunity to post his journey to Saturn on social media. However, since antenna time of the NASA Deep Space Network is very expensive and should rather be used for “real” science, we sadly dropped this idea.
Due to the distance to the sun, the solar power in Saturn’s orbit is only 1.1% of the solar power in an Earth orbit (about 15 watts per square meter). So we decided to use an RTG (radioisotope thermoelectric generator) for power supply. This involves using 8kg plutonium-238, a radioactive isotope of plutonium that has a half-life of 87.7 years. After 10 years of radioactive decay (assuming the flight to Saturn takes 6 years, the plutonium is produced 2 years before launch and an estimated mission duration of 2 years in Saturn orbit), about 92% of this 8 kg is still available for the power supply. With an output of 20 watts per kg, we get a total output of 147.2 W at the end of the mission. Most of this power is needed for Bumblebee’s propulsion, communication, and the operation of the onboard computers. A division into time slots ensures that never more power is required than the RTG can provide, as we could not include batteries in our mass budget.
Better safe than sorry
Last but not least: Better safe than sorry! We also made a rudimentary risk assessment. There are many aspects and scenarios that may cause harm or even the end of our mission. If we ever want to build a prototype or the actual Bumblebee, we have to mathematically assess all the risks and avoid them in a way that our safety for a successful mission is (theoretically) over 95%. For simplicity, we only analyzed what risk factors can occur, what damage they would cause, and how they can be avoided. Tabular 1 shows an extract of our risk assessment. We were able to detect a total of over 40 risks, including external and internal risks.
|5||impact of meteorite (Kat2)||damage on outer shell; loss of function||Reinforcement of protective layer; radar for evasion – more fuel consumption|
|7||permanent demotion||By long-term irradiation, ionization, displacement of atoms in the crystal lattice – Destruction of electronic and non-electronic parts||Protective layer, analysis of time windows of long-term radiation|
|8||Temporary damage due to radiation||Gamma radiation, photocurrents – damage during irradiation, depending on dose rate; loss of data, loss of contact||Caching of data to allow retransmission, protection from gamma rays (problem: small wavelength)|
|13||Saturn’s magnetic field at the pole stronger than expected||Interference with radar, telecommunications and on-board systems (possible damage to conductors by induction of high voltages)||More shielding, higher radar performance, transmission only possible outside the polar orbit segment|
|14||Density of the atmosphere diverges from expectation||More Air Drag, trajectory of Ballutes incorrect||More fuel consumption, adjustment of the ballute trajectories|
|35||fuel shortage||lack of drive capability – loss of Bumblebee||Carrying a fuel buffer|
|36||Evasive maneuvers with meteorites||can lead to fuel shortages||Carrying a fuel buffer|
|40||Higher power loss of the RTG than calculated||less watts available than calculated – problems with the use of electrical equipment||Use larger RTG|
|42||Collision of orbiter with Titan or Saturn||destruction of Bumblebee, biological contamination||Why don’t we just take Saturn and move it somewhere else?|
Andreas Jäger also suggested conducting prior experiments on Earth. They could be used to test if the passive probes would help us properly analyze the hexagon and improve our probes’ design and release technique. For example, we could fly above an (Earth) Hurricane and release some worker bees. Oxford University also replicated such a hexagonal whirl in a laboratory. Working together with international Universities and laboratories would help us to be well prepared for our big voyage.
At the end of this article, we would like to thank Andreas Jäger and Alfred Müller for their professional input, and Dr. Gernot Grömer for his support in this project as well as his excellent lecture at the Alpen-Adria University Klagenfurt.
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