Stratos IV Technical Overview
Stratos IV Technical Overview
The Stratos IV Team is subdivided into several different teams that work on specific aspects of the rocket.
Recovery
Stratos IV Recovery Team aims to design, build, and test a fully functional recovery system that ensures to successfully recover the capsule of the rocket. The recovery system consists of a dual parachute system, with a drogue and a main parachute that slows down the capsule before it lands. To protect the internals during re-entry, the heat shield is used, made primarily out of cork and is jettisoned just before the drogue parachute is deployed. The drogue parachute is deployed at high speeds of around 25 m/s by a Hot Gas Deployment Device (HGDD) by pyrotechnic actuation. Before the main parachute is deployed, the drogue parachute is released via a 3-ring system with an integrated wire cutter, the main parachute is then deployed using a simple spring mechanism integrated with two wire cutters.
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The drogue parachute is a Hemisflo-ribbon parachute made from aramids, previously used during Stratos III, and the main parachute is a Disk-Gap-Band parachute made of nylon made by the Parachute Research Group (PRG). Finally, as the capsule slows down and lands, the aim of the recovery system is to successfully retrieve the stored data and footage captured aboard the rocket.
Electronics
The electronics department is responsible for the actuation of all valves, data acquisition and telemetry downstream. Every valve must open at the perfect time as a delay in the opening sequence could mean that the rocket doesn’t lift off. Furthermore, data acquisition and telemetry streams are mission critical.
The electronics department is critical in providing help at big scale tests by providing ground support.
There is a lot of data collection going on: the whole rocket is full of sensors and all this data should be logged. Therefore, we have an antenna system in place that is capable of sending the data back to the ground as well as black boxes on board to store flight data.
The main goal for electronics is to deliver a flight computer that can fulfill the above-mentioned tasks and do so reliably.
Last year, the team mainly focused on designing the flight computer itself this year a heavy focus has been set on integration and testing.
Currently, all the flight PCBs have been designed and the team is finishing the work on the software of the rocket as well as integration between subsystems.
Structures
The structures department focuses on the structural integration of everything in the Stratos IV rocket. The acceleration loads on the Stratos IV rocket will be 9 g, meaning that the rocket is subjected to extreme conditions.
There are 2 main goals for the Stratos IV structures team. Firstly weight savings. A rule of thumb used around our trajectory is that 1 kg weight saved results in 1 kilometre of altitude gained, meaning that every gram counts towards our goal of reaching space. The second goal was to make the structure more rigid then it’s predecessor, the Stratos III rocket. More stiffness results in less deflections and thrust offsets, meaning that we can more accurately simulate the flight path of the rocket.
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Last year, the team mainly worked on the design of the rocket. Compared to Stratos III, a lot of improvements have been made. The nosecone shell was changed to Twaron, a Kevlar-like material which caused this nosecone to be 3 times lighter than the Stratos III nosecone. Moreover, the parachute canisters in the recovery bay are now made of carbon fibre composites making them a lot lighter.
The engine bay had a complete redesign compared to Stratos III. Where there used to be a truss structure connecting the tank and engine, the loads now go through the shell. To make sure the connections are well aligned and really stiff, conical interfaces are used. Furthermore, a 3d-printed electronics case is designed to hold the engine control unit and FTS.
For the combustion chamber we moved to a carbon fibre design. We wind these combustion chambers ourselves, and the design has already been proven in two engine tests. This carbon combustion chamber saves a lot of weight compared to its aluminium predecessor.
After the design the first version of the Stratos IV was produced. This was done for several reasons. First of all the practicing of the production methods, since production is not a trivial task. Secondly, these parts can be used for integration tests, which will allow for slight redesigns if parts do not fit properly. Lastly, these parts could also be used for testing.
Testing was the main focus for the start of the second year. The tests that were done include, but are not limited to: a compression test of the nosecone shell, a test of the recovery bar, and a fin flutter test. Furthermore, a boat tail was designed to properly guide the airflow around the nozzle, reducing the drag on the rocket.
Currently the structures team is entering the production phase of the project. The final flight hardware will be made and integrated into the rocket. Once this is done the rocket is ready for flight, and we are confident that it will not fall apart under the extreme conditions it will be subjected to.
Roll Control
As Stratos IV is a slender rocket, it will tend to spin up to a roll rate which will match the natural frequency of its other axes, yaw and pitch. This causes an irregular motion with the rocket spinning and wobbling its nose up and down. At supersonic speeds this will cause the rocket to breakup violently in flight, similarly to Stratos III. In order to avoid roll-pitch coupling, active roll control is implemented. This system operates by decomposing Nitrous Oxide to generate heat, pressure and eventually thrust. In order to accelerate this process, rhodium coated onto a a metal foam is used as a catalyst, which is surrounded by heating cables specially designed by Thermocoax, raising the temperature up to 600-700 C, at which point Nitrous Oxide will readily decompose over our catalyst. Further testing will show whether this system responds well to pulse function. As a backup plan, cold gas control or spin up is considered, which will make use of the pressurized gas, and are also currently being studied.
Propulsion
The engine we are building nowadays was designed by the crew from Stratos III, when the opportunity presented itself that this rocket could fly to space the goal of the team became too optimise the structure and safe as much weight as possible. This resulted in a redesign of the chamber, injector manifold and nozzle. The challenges of the engine are the high temperatures, up to approximately 2500K, and high pressures of 40 bar.
Making an engine includes a lot of work from making the fuel grains inhouse, to assembling the entire engine, producing all parts and leak testing the feedsystem. Making a hybrid engine requires a lot of hands-on work and man hours, which makes it different from the theoretical engineering background that the part-timers had so far. So it is a great opportunity to combine the knowledge with practical experience. This causes an irregular motion with the rocket spinning and wobbling its nose up and down.
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Simulations
The simulations department of Stratos IV is responsible for the quantification of rocket performance and range safety. The department is needed in areas where physical testing is infeasible. As such, modeling and simulations provide a way to measure and improve the performance of the rocket at a faster and less expensively rate. Several areas where the department is active are summarised below:
– Computational Fluid Dynamics
– Simulator Design
– Stability Quantification
– Uncertainty Quantification
-Parallel Computing and Distributed Simulations
– Graphical simulations and visualization
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Its expertise is applied to determine the aerodynamics, stability and (apogee) performance of the rocket. Moreover, Monte Carlo simulations predict the rocket launch footprint and allow the team to assess the safety zone required to ensure safe flight and nominal splash zone. Several on-going projects are listed below:
– Live Trajectory Simulations
– DARE Parallel Computing
– Ordinary Differential Equation System Solving Accelerator
– Stability Analysis Simulation Toolkit
– Range Safety
Before being able to launch at the launch site, the team has to ensure the launch site provider that the launch campaign and the rocket launch can be performed in a safe manner. As such, range safety is necessary to quantify the uncertainty during launch, i.e. the performance of the rocket but also the (atmospheric) launch conditions, resulting in a footprint of possible rocket trajectories and impact points.
To cope with the uncertainty in the design and launch conditions, both a statistical and sensitivity analysis must be conducted, taking into account wind operability, nominal launch angles and rocket performance. From this analysis and the resulting footprint, a mutually agreed safety zone is established.
While relying on analytical and empirical models to determine the aerodynamic behaviour of the rocket suffices for the preliminary design the same cannot be said during the detailed design. Different tools must be used to quantify the aerodynamic properties not only much more accurately but also to verify the results obtained from (preliminary) numerical tools. As such, CFD is used which although requires more computational expense, the values can be much more accurate.
Tower
At the start of Stratos III, due to an increase in the capabilities of the rocket, an improved design for a launch tower was desired. This was the goal when the team started to design and built the launch tower used for the Stratos III launch campaign, and what will also be used during the launch of the Stratos IV.
Our biggest achievement would be the building of a tower that is capable of launching rockets of the scale of the Stratos rockets. The tower needs to remain stable as possible, as any deviation from the launch angles would impact the final trajectory of the rocket. On top of this, the tower naturally has the capability to also launch smaller rockets – this means that the number of rockets that DARE launches during its launch days are greater, as more can be launched at the same time.
Currently, the team decided to focus on a new challenge – the ability to transport the rocket from the assembly site to the launch pad. During Stratos III, the rocket was hand pushed on a cart to the launch pad, and then lifted by a group of 12 people into the tower. For Stratos IV, this was no longer an option. Therefore, a transport cart has also been added to the entire ground system. This cart has the ability to transport the rocket safely from vehicle assembly to launch pad, potentially many kilometres away. This 8m long cart also has the ability to adapt its height. This means that the rocket can also be loaded in the tower with the cart – this saves time and significantly reduces the man power for loading the rocket in the tower.
