What to do if you have a flight ready capsule system and suddenly find you have too much time and not much to do. It is a luxury that seldom occurs in the aerospace world and is actually not without its own pitfalls.
Stratos II +
As you all know the capsule and recovery systems of Stratos II have not yet been flight tested and thus it remains to be seen if the system will work. We therefore decided early on that we would divide our work over three research programs and improve the current system where we could.
The first program is essentially an upgrade of the current Stratos II capsule system. During the launch campaign a lot of minor improvement points were identified. These might be as small as having a stable assembly platform for the payload to changing a connector for easier access. All in all not much work, but very valuable as it can significantly improve reliability and can reduce the time it takes to repair issues on the pad.
The second program is the program that has seen the most work over the past weeks. A shortage of time limited the amount of tests that we were able to do before the Stratos II campaign. It is standard practice in the aerospace world to test the system rigorously before flight and we plan several of these tests for the next months. They include vibration and leak tests for the capsule, and the team is still looking at the option of doing a parachute drop test in May or June. The difficulty for now rests first and foremost on estimating the load cases and finding the facilities. Therefore, If you have a helicopter then please do not hesitate to contact us!
Determining the Dynamic Load Case
One of the best examples of the verification and validation program are the vibration tests we want to do with the capsule and the electronics inside Stratos. As a student project we don’t have the luxury of building several test vehicles specifically for destructively testing the system. We therefore decided to test the system under a situation as close to the flight conditions as possible, multiplied with a safety factor.
Overview of the Stratos II capsule
The biggest challenge was then determining the actual load cases. As you probably can imagine the system is highly dynamic, and aero elastic problems are notoriously difficult to design for. So what do you do when you have no idea how to tackle the problem? – You simplify.
For the vibration load cases, called Power Spectral Densities (PSD’s), there are two major cases under consideration. The longitudinal case involves the vibrations that are mainly driven by combustion instabilities in the rocket engine. This data we already have from the pressure sensors and load cells used during the Trauen test campaign. Additionally, If we look at the system as a very simple mass-spring system we can already estimate the most basic Eigen frequencies. We are lucky; combining these spectra tells us two important things; there is not much overlap, which in turn means that at the forcing frequencies of the engine the rocket can be considered pretty much rigid.
Power Spectral Density of the measurement data obtained during Trauen test campaign
mass-spring-damper approximation of the Stratos II rocket
For the two transverse modes the problem is more difficult as it involves considerable aerodynamic effects. For a first estimate of the system we are assuming a rigid body. The driving force is in this case a sudden wind gust from the side. Luckily the people working on active control and aerodynamics could help us solve our problem by providing us linearized spring damper coefficients for a multitude of points during flight. Combined these give us a power spectral density for rotational vibrations around the center of mass.
Comparing them to payload acceptance tests done for commercial sounding rockets (NASA Sounding Rocket Program Handbook, page 104) we find that we are quite close to the levels determined for the other sounding rockets. It just shows how far you can get with a back-of-the-envelope solution.
And Further
The third program is the Future technology program. With the design of the Stratos recovery system and capsule, considerable limitations were found for the technology currently used within DARE. In other words, we found a lot of stuff that we had not encountered before. We are working on some nice new technologies such as mortar launched parachutes, reefing and inflatable Decelerators; but that is a story for some other time.
Recovery systems and Aerospace Structures. It is quite exciting.
One of the biggest challenges in DARE is not technical in nature nor is it related to a lack of technical or financial means. It is the constant flux of people that enter the society without much prior knowledge. After the Stratos II campaign from earlier this year we welcomed 5 new members in the capsule and recovery team while three of the more experienced members stepped back. Luckily, the above programs provides excellent opportunities for us to learn.
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