REXUS-BESPIN

Hello, everyone, it is time for a new and slightly delayed blog post again. I am Jaka and I am a member of the mechanical division, responsible for the pneumatic system and designing of the largest part of our experiment, which we call the descent probe. It is a central platform from which we inflate our balloon and release the flotation probe, the heart of our experiment. But the descent probe has to be ejected from the rocket, deploy the parachute and once this has been done, release the protective shell that guards the flotation probe. Today, I will describe the pneumatic system slightly more in detail and provide a glance of the new descent probe design.

To get the overall idea of the experiment it is useful to revisit the updated general system diagram which makes the distinction between different parts of the system.

The descent probe has come a far way since the last blog post where it was shown, mainly due to the looming deadlines for the submission of the CAD files before the Critical Design Review later in June.


Its most prominent parts are three 2.0 L high pressure composite cylinders which will be kindly sponsored to us by MEYER CD. The main use for such tanks is paintball or (slightly adapted) airsoft. But because we are engineering students we can come up with arguments for fitting such equipment on a rocket as well! We know about the interfaces and the industrial standards which means that the cylinders can be nicely integrated into our pneumatic system with the use of correct fittings and piping.

 

There is also a standard way of communicating about the pneumatic or hydraulic systems, so we included the diagram above into the documentation after we specified the components in detail. You can see that the system is divided into two main parts with a very small intermediate prat in between.

The use of high pressure is justified by wanting to pack as much helium as possible into the cylinders. But why shouldn't we just open the bottles and inflate the balloon directly, from 240 bar to the atmospheric pressure? While there certainly are solenoid valves, which can be electrically opened and can handle the big pressure differences, we wanted to limit the high pressure part of our system as much as possible. This enables us to use lighter elements in the flotation probe and flexible tubing.

We decided that to use the paintball pressure regulators to lower the pressure because they are lightweight enough and they also include neat safety elements in the form of burst disks. These act as the weakest part of the system and they break if the pressure exceeds a certain level, thus preventing more serious damage or injury.

(Ninja paintball regulator. The pressure gauge will be replaced by electronic Honeywell high pressure sensor.)

Some other elements can be found in the diagram as well. It's important for us to monitor the pressure inside the tanks, the balloon and the atmospheric pressure. The pressure inside the tanks is important to see if there are any leaks and how far are we with the inflation already. But the atmospheric pressure and the pressure inside the balloon are more crucial.

After we decided to use a balloon with a thinner membrane, which can only withstand 0.3 bar pressure differences, we noticed a potential issue. We plan to start the inflation on 5 km altitude after the parachute deployment and complete it at around 1 km altitude. At 5 km, the atmospheric pressure is around 0.5 bar, while the pressure at 1 km is expected to be around 0.85 bar. The later value is also what we target for the balloon pressure if we let all the helium from the bottles go inside the balloon. The problem would arise if the balloon is fully inflated quicker than the atmospheric pressure can increase while the experiment is descending through the atmosphere. By using the pressure sensors inside the balloon and outside, the software which controls the valve can close the valve and temporary stop the inflation if the pressure differential reaches dangerous levels which would lead to balloon rupture. After the probe descends more and the atmospheric pressure is higher, the inflation can resume and the valve is opened again.

We also added bleed valves. Those are hand operated valves that can be used by us or by the recovery team to manually depressurize the whole system and ensure safe handling. Other elements include a fill line with non-return valve to fill the cylinders and a custom made manifold which connects high pressure tanks with the pressure regulators and a bleed valve. We designed the manifold ourselves and ran a final element analysis (FEA) on it to see if it can withstand a static pressure of 500 bar. So far it seems to hold up nicely, since the yield tensile strength of a typical stainless steel, from which we plan to manufacture the manifold, is around 200 MPa. (The red part is only a thin inside wall between the two compartments which will not be present in the real model. It has a shape like this because of the way the holes are defined.)



There are a lot of things about which we can write but with this we will conclude this blog post. If you have any questions feel free to send us an email or contact us via social media!