Sample return capsules' problematic
Sample return missions are spacecraft missions with the aim of collecting and bringing back to Earth samples from other planets, moons and asteroids. The first successful mission to ever bring back samples for another celestial body was the Apollo 11 mission in 1969. In total, the Apollo Program brought back over 380 kg of moon rocks in six successful missions. However, after that first advance in space exploration, space agencies around the world started replacing astronauts by robots for outer space exploration. Robots are safer, much cheaper and can resist more easily the harsh conditions of space travel.
Apollo 11 was the first successful mission to bring back samples from another celestial body. © NASA
OSIRIS-REx is currently collecting samples on the surface of asteroid Bennu. The Sample Return Capsule is visible at the front of the spacecraft. © NASA
Since the end of the 20th century, spacecrafts brought back samples from the Moon, comets, asteroids and even solar winds. Right now, several missions are currently collecting samples millions of kilometers away from Earth like Hayabusa2 on asteroid Ryugu, or OSIRIS-REx on asteroid Bennu. Furthermore, other missions from several space agencies are preparing to be launched to collect samples on Mars, Phobos, Ceres and even further.
All of those missions use the same type of capsule to return the collected samples safely back to Earth. Due to volume and weight constraints, those capsules tend to have quite a low profile and a bluntly shaped underside compared to the well known manned capsules. Those geometric constrains, which are helping the capsule slowing down efficiently without heating too much when re-entering the atmosphere are also making it unstable at lower speed (under the speed of sound). This instability can cause the capsule to oscillate around its center of mass and, if not constrained, these oscillations can lead to the loss of the capsule (like NASA’s Genesis capsule in 2004).
Exploded view of a typical Sample Return Capsule. Its shape has quite a low profile compared to manned capsules. © JAXA
How we came to this project
The capsule’s unstable behavior at subsonic speeds has been studied extensively at our University’s Geneva site, HEPIA. In the past couple of years, several students did their Bachelor and Master theses on the most well-known capsule body, the Hayabusa capsule. They studied its unstable behavior, making several CFD numerical simulations and wind tunnel tests.
After having studied the problem, the next step was “how can we mitigate the oscillations without changing the capsule’s shape ?” Prof. Roberto Putzu, head of Mechanical Engineering Department at HEPIA then discussed the matter with one of his friend and colleague from Fribourg’s HEIA-FR, Prof. Marco Mazza.
Prof. Mazza was studying regulation systems on inversed pendulums, making them stay perfectly upright by moving their centre of mass to compensate external forces. The idea then came to combine both projects and try to actively stabilise a capsule by moving small masses in opposition to the aerodynamic oscillations.
The HADES Team presenting their project in from of the REXUS / BEXUS Selection Panel at ESA-ESTEC (NL).
The project was then carried out by a group of students from Fribourg and Geneva, who wrote a proposal to the REXUS / BEXUS Programme. After being selected by the panel in November 2018, they started designing, testing and building their experiment, as well as doing all the project management, outreach programme and sponsoring research.
Our experiment is divided in two parts :
- The Capsule, named Hades, the core of our experiment (left).
- The Release Mechanism, named Persephone, holding the capsule during ascent and releasing it at apogee (right).
The whole experiment is located at the top of the rocket, inside the nosecone.
After exiting the atmosphere the nosecone is ejected, followed by our capsule, which will fall back to Earth at Mach 2.5.
The capsule will transmit its position during the final part of its descent and after landing, helping us guiding the recover helicopter to the landing zone. A short range detector, like the ones used in avalanches rescue will be used to find the capsule in the snow.
Inside the capsule, two linear actuators, placed on the horizontal plan 90° from one another, are moving at a maximum frequency of 3 Hz. Their own weight is sufficient to move to centre of mass by more than 3% of the capsule’s diameter. This should be enough to damp the oscillations by more than 50%, ensuring a safe return to Earth.
Hover over the capsule to see what is inside !
The whole flight will be instrumented with various sensors, and the data collected will be compared to flight predictions and simulations. This will enable us to assess our system’s performances and decide if this is a viable option for future sample return missions.