DanSTAR builds rocket with aluminium structure
When walking through the door of DTU Skylab, you are met by a sparkling atmosphere of creativity and nerdy focus. Here, there is room for differences of opinion and materials and installations invite exploration and challenge.
An open workshop in one side of the ground-floor welcome area is especially eye-catching. Framed by worktables, you see a long, cylindrical structure which is raised to working height in the middle of the workshop. A group of students is already hard at work around the structure. As part of the student organisation DanSTAR, which is an abbreviation of Danish Student Association for Rocketry, they are building a 4.5-meter-long rocket which will have a flying altitude of 9 km.
On closer inspection, it turns out that large parts of the rocket are constructed of aluminium.
Sandwich construction of aluminium and carbon fibre
Presently, in late April, most of the completed inner structure of the rocket is found at the DTU Skylab worktable. The meter-long structure consists of machined aluminium profiles and large aluminium rings, amongst others, which have all been finished with a view to functioning as skeleton and mounting points for other elements of the rocket. Some of the holes from the machining have also been made solely with a view to reducing the total amount of rocket material and, thus, lightening the weight of the rocket. All elements have been finished based on 3D-printed plastic prototypes.
Together, the various aluminium elements make up the inner structure of the rocket which, finally, will be reinforced by a specially designed carbon-fibre shell of a 2-milimeter thickness. Thus, the aluminium and the carbon fibre will act as a sandwich construction which will protect the shape, strength and lightness of the rocket. All three parameters are extremely important when as complicated a structure as a fully functioning rocket is to be designed and built from scratch:
- The aluminium keeps the insides of the rocket in place and it also provides good bracing but when the carbon-fibre shell is fitted on the outside of the rocket, it forms a synthesis, says Rasmus Arnt, chairman of DanSTAR.
Rasmus Arnt also explains that they have chosen to use aluminium as one of the main components of the rocket because aluminium gives the team a great advantage if it is necessary to make design-related changes along the way:
- It would have been really complicated if we were to design a custom-made carbon-fibre shell with mounting points. If we suddenly want to create a new mounting point in a carbon-fibre shell, we would need to find a solution for how to do so. Aluminium, on the other hand, we can easily process ourselves and, therefore, the combination of aluminium and carbon fibre is optimum.
From 7075 and copper to 6082 and 3D-printed aluminium motor
Building a rocket is not something you just do. All parameters are reconsidered and tested several times. For this reason, DanSTAR designed a test motor as a starting point which behaved like the rocket but did not take off. In that way, they were able to test the motor itself, the fuelling and the safety system before the rocket was finally completed. The test motor is made up of copper and aluminium in alloy EN AW-7075, respectively.
When a rocket is launched, this is done through a controlled burning of fuel. To enable the burning, the fire triangle is used as the starting point. According to the fire triangle oxygen, fuel and heat are essential factors for the burning process. Therefore, DanSTAR has designed the rocket motor based on this:
- The rocket works according to the principle that there are two types of fuel: An alcohol that is flammable and nitrous oxide which oxidises the flame. Large rockets use liquid oxygen instead of nitrous oxide. In relation to the fire triangle, we get the oxygen from the nitrous oxide, we get the burn from the alcohol and we get the heat from a filament that is turned on, explains Rasmus Arnt and continues:
- The top of the rocket motor acts as an injector where all the fuel is sprayed through small holes which atomise the fuel further into the motor. The injector has also been tested with water prior to being tested with fuel.
In the test motor, the injector is made of aluminium. A nozzle and a combustion chamber, both made of copper, are placed next to the injector. When two different metals come into direct contact with each other and a liquid, galvanic corrosion may occur where the basest of the metals acts as active anode and corrodes:
- The aluminium part is pressed up against the copper in the motor and the fuel is injected through both the aluminium and the copper part. The combination of aluminium and copper being pressed up against each other while a liquid is led past them is just really unfortunate because the materials are located at either end of the reactivity series. This means that the aluminium is the active anode in this connection which can be clearly seen on the pieces, concludes Rasmus Arnt and adds:
- It was a really annoying surprise to see how corroded the piece was but all figures indicate that 7075 has a very low corrosion resistance which we learned the hard way. One of the greatest lessons learned from this is to take corrosion and the reactivity series into consideration when putting together different materials. Aluminium has a protective layer of alumina on the surface. I believed that, to a large extent, the alumina would keep the aluminium alive; however, as soon as the aluminium was pressed together with copper and moisture was added, the corrosion started.
It has always been DanSTAR’s plan to build what they call a ’flight-grade’ successor to the test motor. The new ’flight-grade’ motor is completely made of aluminium and has been 3D printed at the Danish Technological Institute.
After finding the corrosion in the test motor, DanSTAR became extra aware of the need to take into consideration corrosion conditions within the rocket generally. They, therefore, chose to make all the rest of the aluminium parts of the rocket from EN AW-6082 instead of EN AW-7075. EN AW-7075 is a very high-strength alloy and, due to its strength, it is also referred to in some circles as ’space-grade’. For this reason, EN AW-7075 was DanSTAR’s immediate choice. However, they found that the extremely high strength was not necessary. Instead, they chose to prioritise corrosion resistance rather than strength by using EN AW-6082 which is far more corrosion resistant than EN AW-7075:
- We do not expect that corrosion will be a problem; nevertheless, we have chosen to use 6082 in the remaining parts of the rocket because we are going to screw stainless steel parts into the aluminium and all pipes, components and fittings are also made from stainless steel which means that, in any case, there will be steel near the aluminium. In order for us to hopefully get away with putting together two different types of material, we have chosen to use a more corrosion resistant aluminium alloy, concludes Rasmus Arnt.
DanSTAR started the project with the test motor in late 2017 and they have worked on the rocket itself since autumn 2018. The team has ambitions to send up the rocket in Denmark during summer 2019.
Alumeco has sponsored the aluminium for the project. We look forward to watching the launch of the rocket and wish DanSTAR all the best with their project.