Like most rockets, the Stratos II+ does not only consist of an engine, it also houses some payloads. These are located in the upper segment of the rocket, which is called the capsule. The capsule protects both the electronics and the payloads and is responsible for recovery of the rocket after launch.
Protection
The main task of the capsule is to provide housing for the electronics and payloads so that they can be protected from different environmental hazards it will encounter during flight. During ascent, the air around the nose will heat up because of the rocket’s high velocity. The maximum temperature at the nose tip is expected to be over 800 ºC! The tip of the rocket is therefore made out of steel, with an aluminium ring beneath. The rest of the capsule was made out of wound glass fiber composite. This material is strong but also relatively transparent to radio waves, making sure that communication from the ground station with the rocket is possible during the flight.
Filament Winding the nosecone for Stratos II.
After the engine shuts down, the rocket will coast to very high altitudes, where the air is so thin it is almost a vacuum. If there were no air inside the capsule, the electronics would overheat because they could not lose the generated heat to the outside air. The capsule must therefore be made air-tight, such that the air inside does not escape. All radial bolts are air-sealed during assembly and several pressure leak tests were performed earlier this year to ascertain this.
At the end of the flight, the rocket will splash down in the ocean. To be able to recover the rocket, it should float. Luckily, this requirement is very similar to the one before: the air pressure should be contained inside the vessel.
Recovery
The second task of the capsule is to ensure the recovery in one piece (in fact, two pieces) of the Stratos II+ rocket. Parachutes will be deployed to slow the rocket down to a lazy descent instead of a crushing impact.
Two parachutes are aboard the capsule: the drogue parachute and the main parachute. The drogue parachute is a rather small one – only one square meter – that will be ejected at maximum altitude. Because of the high velocities it will encounter on descent, it has the same shape as parachutes used by NASA for landing on Mars – the so-called Disk-Gap-Band parachute. This type of parachute can be used at supersonic velocity. The purpose of the drogue parachute is to slow down the rocket enough for safe deployment of the main parachute.
The main parachute being folded by DARE team members.
The main parachute is much larger than the drogue, at around 6 square meters. It is cross-shaped, because DARE has had good experiences with such parachutes in previous rockets. The main parachute is deployed at approximately 3 km altitude. This relatively low altitude was chosen to limit the distance the rocket can drift while hanging from the parachute.
Sketch of parachute recovery sequence.
Upon releasing the drogue, the rocket will split into two parts: the engine section and the capsule. The lines attached to the engine are much longer, such that the engine will hit the water first. In this way, the impact velocity of the capsule is only about 30 km/h, while the engine will impact the water at twice that speed. This is because the capsule contains more critical equipment and data storage devices that the team wants to recover.
Summary
In summary, we can say that the capsule protects the electronics and payloads from the hot air during ascent, from the vacuum at high altitude and from the water after splash-down. The drogue and main parachutes are deployed in sequence to slow the rocket down such that it can be recovered from the ocean after flight.