Function of the capsule

The Stratos II team is split up into different teams, each team covering an important part of the Stratos II rocket. One of these technical teams is the Capsule team; they are in charge of designing, building and testing the capsule for the Stratos II rocket. The capsule is the top section of the rocket where the electronics, payloads and deployment system are housed. The capsule needs to protect the cargo against mechanical, thermal and aerodynamic loads. On top of that the capsule and its cargo needs to land safely back on Earth in order for the payloads and collected data to be retrieved. All these functions need to fit in a shape that reduces the drag of the rocket to a minimum for the planned flight regime.

Design History

In the early design days of the capsule team, a design was made for a capsule that would go on a rocket with a diameter of 200mm (see figure 1). All the components of the capsule were housed in the nosecone shape. In the top the antenna will be housed, and below that the electronics. Below the electronics, the capsule is divided into four equal parts. In one of these parts the parachute deployment system was situated and the remaining three quarters were reserved for payloads. At the bottom of the concept launcher a separation ring was attached which connects to the engine of the rocket. This would be disconnected at apogee to do measurements and then safely return the capsule to the ground. Two concept launchers with this capsule successfully flew and through evaluating these flights, a lot of insight was brought on the design so that changes could be made to increase the ease of use and reliability of the deployment and separation system.

Figure 1: Old capsule design

The design of the rocket continued and decisions regarding the propulsion system needed to be made. For the Stratos II project two propulsion teams are designing, testing and building a propulsion system to bring the Stratos II rocket to its desired altitude. These teams are the solid team and the hybrid team. Around the end of 2011 a decision on which system would be chosen was not yet made, but the solid team decided that they could make a more efficient propulsion system if the diameter of the rocket would decrease from 200mm to 160mm. This uncertainty is not desired when designing a capsule that would be on top of an engine, however, the design of the capsule needed to continue without knowing the true diameter of the engine. Therefore a new diameter for the capsule was set to 160mm to ensure that the capsule can be mounted on either engine. It is far easier to have a larger engine diameter and a smaller capsule diameter than vice versa.
This design chose forced the capsule team to redesign the entire capsule. This came of the cost of spending more time in the design, but from the first two concept launches a lot of improvements could be implemented in the new design. One of the most important changes was to design a modular capsule where the different systems in the capsule can be housed in separate modules. These modules need to be built and tested separately, and need to be stacked like building bricks to form the capsule.  This will allow for parallel development of the systems and it will reduce the preparation time because all systems can be prepared separately and parallel.

The modular design

As explained above the modular design consist of different modules all having their own purpose. All modules are stacked on top of each other because the smaller diameter does not have enough space to have modules next to each other. The capsule consists of the following modules listed from top of the capsule to the bottom.

• Rocket electronics module, including the communication antenna
• Payload module
• Parachute/ recovery module
• Separation module

The following sections will elaborate on each module and the current design and future design plans for that particular module.

Electronic module

In the top of the capsule the rocket electronics are housed. The electronics, sometimes referred to as “the bran” of the rocket, will control the flight of the rocket. It will ignite a possible second stage of the engine and activate the separation of the capsule from the engine. The electronics will initiate the recovery procedure to safely return the capsule to Earth. Besides that, it will not only control the payloads and transmit their data but also save it on board for a redundancy. Momentarily there is not a final design for the electronic module but the development of both the electronics and the antenna is on-going.

Payload module

Underneath the electronics module, the payload module is situated. One of the goals of the Stratos II mission is to bring payloads to the desired altitude of 50km. These payloads will be housed in the payload module. The payloads will be stacked like they are in cubesats, on top of each other. A first iteration design of the modular design payload module can be seen in Figure 2. As of this moment the capsule team is working on the second design iteration. In this design the payload have more space and can be made on a standard 10 x 10 cm cubesat PCB. The structure will also be optimised so that it will be as light as possible but still be strong enough to withstand the forces expected on the capsule.

Figure 2: First iteration of the modular payload module

Parachute deployment module

The parachute deployment or recovery module will house the parachutes and the mechanisms to deploy them. In the first design iteration only a single parachute was chosen. However in the second iteration, it was chosen to have a double parachute system. With a dual parachute system a small, drogue, parachute will deployed first to decelerate the capsule and then the main parachute will be deployed to ensure a safe landing. This dual system will reduce the shocks on the capsule and simultaneously reduce the footprint of the capsule. Figure 3 shows a schematic overview of the second iteration of the recovery module. In this design both parachutes are stored in a single tube, the drogue parachute on top and the main parachute is located below the drogue. Both parachutes will be ejected like a slingshot.

Figure 3: Schematic overview of the second iteration of the recovery module

A third concept launcher was launched in September 2012 with this new recovery module. During the preparation before and during the launch some problems were discovered. The main problem was that it is hard to prepare the module and due to the complex mechanism, the risks of failure were higher than expected. With this new information the capsule team is working on redesigning the recovery module to simplify the mechanism, to reduce the risks of failure and increase the ease of use during preparing the module.

Separation module

The last module on the capsule is the separation module. The purpose of this module is to separate the capsule from the rest of the rocket. This is needed to deploy the parachutes and safely land the capsule on Earth. Figure 4 shows how the separation module looks like.

Figure 4: Left: One separation ring and the clamp band, Right: Both rings and spanning device

The separation module consists of 4 parts, two identical separation rings, a clamp band and a spanning device. The two rings lay on top of each other as can be seen in the right image of Figure 4. The clamp band will connect the two rings (see figure 4). The clamp band is one big steel spring with aluminium profiles attached to it. In its normal state it is one straight strip of steel. When it connects the two rings it will be bent in a circular shape. To hold the clamp band in this shape a dyneema wire connects both the ends of the clamp band together. This dyneema wire is guided in one of the rings and connected to the spanning device. Figure 5 shows the spanning device. By turning the bolt in the spanning device, the wire can be made shorter which will span the clamp band so that the rings will be hold together firmly.

Figure 5: Schematic view of the spanning device

The dyneema wire is run under a small electronic board with resistors mounted on it. When a current is run through the resistors the wire will melt due to the heat from the resistors. When the wire melts it will snap and release the clamp band. The clamp band will then go into its normal straight position due to the steel spring and it will release both the rings. When both rings are loose the capsule is loose from the rest of the rocket.

The separation system already flew on all three of the concept launches and performed in all flights as predicted. The system has undergone a few dozen ground tests and worked perfectly. This module is almost at its final design and the only optimization needed is to make it as light as possible and easier to use.

Future plans

The next step for the capsule team is to redesign most of the modules in order to make them lighter, easier to operate and more reliable. One of the main research topics for the team is to design a skin loading structure where most of the loads will be carried through the skin. This will benefit the inner structure by making it lighter and allowing more space for the systems inside.
When new design is made it will be tested in a wind tunnel to see how the parachute deployment system performs and based on the data acquired, minor changes will be made to prepare the capsule for a fourth concept launch planned in April 2013. The capsule for this concept launch will be, apart from minor changes, the capsule that will be launched in the final Stratos II rocket.

The Team

Joost van der Gaag, Saskia Wagenaar, Mats Kolsteeg, Jork Stapel, Siddharth Pandey, William Mulkens, Aurélien Savonet.