The µHoubolt rocket was designed based on the future Houbolt rocket. Houbolt is the long term goal of the TU Wien Space Team to reach space with a self-developed liquid-fuelled rocket. The design was created in the course of the “Base11 Space Challenge”. The development of µHoubolt represents an intermediate step and will provide important insights.
Mission objective and requirements:
The mission goal is to build and launch a liquid fuelled rocket as simple as possible, due to following the Houbolt concept, the following requirements must be met:
- Propellant combination: nitrous oxide and ethanol
- Propellants fed by Nitrogen pressurant
- Flight altitude > 1 km
- Thrust: max. 500 N
- Successful recovery with two-stage parachute system
Key data of the µHoubolt preliminary mission design:
The airframe represents the supporting outer structure of the rocket, which must withstand the thrust and aerodynamic forces during flight. The body tube consists of carbon fiber reinforced plastic and has a diameter of 120 mm. The rail buttons, the connection to the launch pad, and functional openings for refueling and communication are also located on the body tube.
The rocket nose cone is made of aramid fiber reinforced plastic, which is laminated over a 3D-printed, water-soluble core made of PVA. Various concepts are currently being evaluated for the production of passively aerodynamically stabilizing fins. Fiber reinforced plastics are also to be used, which are laminated using 3D-printed or CNC-milled plastic cores.
The µHoubolt recovery system is called “Slingshot”. A line is stretched between the coupler in the body tube and a ring at the top of the nose cone. This ring is mounted on a plate, which can be tightened by a mechanism, and therefore tightens the rope and firmly fixes the nose cone to the body tube. At the peak of the flight, this rope is cut with a line cutter and springs in the coupler push the nose cone away from the body tube, which also pulls out the parachute. Since µHoubolt is an intermediate step to Houbolt, the recovery system was designed to be two-staged. With the drogue parachute, the vehicle glides down at 20 m/s until the main parachute is released at an altitude of 300 meters, leading to a relatively gentle landing at 5 m/s.
The vehicle has two separate propellant tanks for the oxidizer (nitrous oxide) and the fuel (ethanol). The nitrous oxide tank has a volume of 2040 ml, a working pressure of 45 bar and an empty weight of 1000g. The ethanol tank has a volume of 730 ml, a working pressure of 30 bar and has an empty weight of 280 g.
With nitrous oxide tanks in particular, care must be taken to ensure that all materials are compatible with the medium to be stored. Therefore it is necessary to use PTFE seals. The nitrous oxide tank shall also be openable to ensure the required cleanliness.
Pressurant Delivery System
The propellants are fed from the propellant tanks into the combustion chamber by means of pressurization with nitrogen. The nitrogen is stored in high pressure tanks and regulated to the necessary working pressure in the fuel tanks. This method represents a less complex option in comparison to other feed methods. In rockets as small as this one, it has the additional advantage that the increased mass of the more robust propellant tanks is more than compensated by the absence of other components (pump, gas generator, electric motor / battery,… ).
For µHoubolt, paintball components will be used as 300 bar pressure tanks and the associated equipment (pressure regulators, fill equipment, safety devices,…) are available in great variety commercially. The first tests about the suitability of the pressure regulators have already been carried out to determine whether they can supply not only the necessary pressure but also the required mass flow rate.
µHoubolt is powered by an engine with a thrust of approximately 500N. The oxidizer is fed into the center of the engine from above and passes through small holes that are used to regulate the mass flow. The fuel is fed into the engine from above through a cavitating venturi, which also serves to regulate the mass flow. Several injector concepts are in the works, which differ in the type and location of the mixture formation. Extensive test series on the test stand TS02-500N are planned to compare these concepts and to select the most suitable one. Due to the relatively short burn time of 8s, it is possible to choose the wall thickness of the nozzle large enough to absorb the generated heat with acceptable material heating. The disadvantage of this method is the high mass of the combustion chamber and nozzle, which is why other cooling methods are also evaluated. A promising method is film cooling, in which a thin film of liquid (mostly additional fuel) flows along the combustion chamber and nozzle wall to evaporate and thereby absorb the heat, but this leads to increased fuel consumption. Ablative cooling is also being investigated, in which the wall is slowly burned away in order to absorb the heat. In this variant, the combustion chamber and nozzle are not reusable and must be replaced after each or after several test runs.
The avionics of µHoubolt are used to control the vehicle. Data is collected at multiple places in the rocket using a wide variety of sensors and compared with expected values. “Avionics” is the umbrella term, which in our case is divided into five sub modules:
- FCU (Flight Control Unit) – Is the main control unit of the rocket in nominal operation and controls all systems according to the pre-programmed flight profile. The FCU has a variety of sensors that are used to validate the flight profile. This module also contains the connections to the engine to control the necessary valves and to monitor temperatures and pressures.
- PMU (Power Management Unit) – Serves to power most subsystems in the rocket and converts the voltage from the batteries to the necessary voltage levels. It also contains a safety pin which, as a safety precaution, only puts the rocket into a mode in which an engine ignition, and thus a start is possible when it is removed.
- RRU (Redundant Recovery Unit) – In the event of an anomaly during the flight (detected by one of the many sensors), the RRU can initiate the rocket recovery, independent of the rest of the system (and thus also if the rest of the system is damaged). The RRU has a dedicated backup battery and its own sensors in order to be able to carry out a successful recovery completely independently.
- RCU (Radio Control Unit) – The radio control unit is used to enable communication between the rocket and the ground systems during the flight. Current sensor data as well as individual images of the flight are transmitted dynamically when a favorable signal path is available.
- DLU (Data Logging Unit) – The Data Logging Unit contains its own barometer, a few temperature sensors, its own IMU (Inertial Measurement Unit), and its own backup battery so that it can continue to collect data in the event of a system failure. This module is more robust than the rest of the vehicle, so that in the event of a catastrophic crash there is still a good chance of evaluating the sensor data from the flight and learning from possible anomalies during the flight. In the case of a nominal flight, this module contains another copy of the measurement data, since the data are both sent live to the ground station and recorded in every other module – in this case, the DLU is only used for redundancy and increased robustness of the system.
The launch pad, in addition to its own control system, contains the connections for the pyrotechnic igniter of the engine and the umbilicals to the rocket (power supply, data bus and fueling hoses, which can be automatically disconnected and retracted). Thrust measuring load cells are also integrated in the launch pad, which only release the rocket via a servo motor if the engine performance is sufficiently high. Communication with the launch pad is established via a commercial long range WiFi radio link.