The subsystems aboard the SPEAR vehicle are many, take a look!
The SPEAR electronic system will be the most advanced ever built and flown in DARE. The system is custom designed and tailored to the needs of the SPEAR mission. It will use technology that has not been used before making it a truly next generation electronic system. It can be split up in roughly five parts: avionics, power, sensors, actuators and communications.
The avionics are the brain of the electronic system. Its main processor is a powerful Raspberry Pi Computer 3. This small 7 x 3 cm microprocessor is more powerful than anything ever flown on a DARE mission. With the Raspberry Pi advanced real time algorithms can be used to process the sensor data and perform a state estimation of the capsule. In addition, it will process the video signal from the high definition on board camera.
The power system contains the batteries that power the SPEAR experiment. The peak power operation if all systems are turned on simultaneously can reach an astounding 50W. Roughly the same power consumption as a small laptop.
Several sensors are present inside the SPEAR capsule. An inertial measurement unit and pressure sensor are used to determine the position and altitude of the experiment. Multiple load cells measure the load of the drogue parachute and a heat flux sensor measures the thermal conditions of the shell of the capsule. Also included in the sensor system are two cameras, one on board high definition camera that will capture the deployment of the parachutes and one camera that remains on REXUS and will capture the moment of separation.
The actuators of the electronic system activate the deployment mechanisms of both parachutes. In total there are four deployment systems, one for the pilot parachute, one for the drogue parachute and one each for the two main parachutes.
The communication system of the SPEAR capsule will transmit the high definition video signal and the data from the sensors to the ground stations during flight. To do so the LimeSDR mini software defined radio is used that will connect to the Raspberry Pi. The communications system will also transmit the coordinates of SPEAR after touchdown for retrieval of the experiment.
SPEAR makes use of three different parachute deployment devices in order to deploy the drogue, stabiliser and both main parachutes in a controlled and reliable manner. The drogue will be expelled out of the vehicle with a high velocity, achieved by hot gas generated using nitrocellulose. The main parachutes deployment will be initiated by a spring system after which two pilot chutes pull out the parachutes out of their bags. Similarly to the main parachutes, the stabiliser will use a spring as a deployment mechanism.
Three pairs of wire cutters will ensure the actuation of the spring assisted deployment devices. Each one of these has a small amount of explosive charge that rapidly pushes a sharp metal through the wire. This system allows us to trigger the parachute deployment with high certainty.
The SPEAR capsule has to separate from the rocket in order to carry on with its intended mission. In order to withstand the intense vibrations during launch, the separation system should be extremely stiff while still being able to separate SPEAR from the REXUS rocket. This is why a clamp band system and conical adapter will be used.
The SPEAR vehicle will carry a total of six parachutes on board.
A small ballute type parachute will be deployed first to stabilise the capsule at supersonic velocities. It is merely 15 centimetres in diameter, which makes it the smallest parachute ever produced within DARE. This is to prevent the vehicle from decelerating too much.
The main test article of the mission is the Hemisflo ribbon drogue parachute. Although it’s relatively small size, it has to withstand an enormous pressure when deployed at supersonic velocities. Therefore it is made out of high strength TechnoraⓇ. This drogue parachute design is also used for the Stratos III and Stratos IV recovery systems.
Finally, two disk-gap-band (DGB) parachutes will slow the vehicle down to a safe landing velocity. These will be deployed by using two smaller pilot-chutes. The DGB parachutes are slightly smaller than the Stratos parachutes but made in an identical way. SPEAR has two main parachutes for redundancy and stability of the vehicle. Even when the drogue parachute fails, SPEAR can still land safely.
The sensors of SPEAR consist of five main components:
Since one of the main goals of SPEAR is to deploy the drogue parachute above Mach 1.5, it is important to know at which point during the flight this is. This is where the simulations come into place. Using tools created by ourselves (ParSim & TumSim), we can determine the trajectory of the SPEAR within 1% of the actual conditions. ParSim can also determine the parachute force, which is important to make sure the vehicle does not break apart during flight.
The envelope of SPEAR can be seen below. It is crucial for SPEAR to get the deployment of the drogue parachute in supersonic conditions. However, when first entering the atmosphere (between 60-40km) the stability of the test vehicle is unsure because the forces are quite low; therefore, the deployment of the drogue has to be done after this moment.
Because there are some uncertainties while designing the sensitivity for the minimum deployment altitude required to reach Mach 1.5 during drogue deployment. In the picture above, it can be seen that within a large range for the apogee altitude achieved during flight and the mass of the SPEAR test vehicle, there is a moment where we reach a Mach number of 1.5 or higher. With this, we hope to safely bring SPEAR back to earth whilst still completing our mission.
The SPEAR structure is built out of three main parts: the bulkheads, trusses, and the shell. The Bulkheads allow for the attachment of all subsystems and are designed to take up the loads of the parachute inflation. They are attached with the trusses. The trusses are commercial off the shelf parts to reduce the cost of the vehicle. The internal structure is covered in a composite shell. The shell protects the internals of the vehicle during re-entry, but also provides torsional stiffness. A significant challenge of the development of the shell was to allow for radio transparency. For these reasons, the shell is made out of glass fibre.
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