The Story of a Balloon that Only Wished to Float
Hello everybody! My name is François Piette, I come from Belgium and I am part of the Mechanical Division in BESPIN.
Every time I approach a deadline I feel like running a marathon. The beginning is always slow and steady, there is no pressure so we do not over-work, we want to preserve some energy because we know it is going to be a long ride. When we see the final line, everything changes and we use every ounce of energy left to finish it.
On May 28th we have to send SED version 2 for the Critical Design Review. It is called “Critical” because the final design decisions are made at that stage (normally). It is a very important milestones and I feel like it is the biggest one in this project. In the Mechanical Division we mostly discuss on What’s App or through our weekly meetings and the past weeks were similar to a marathon.
I made a small graph to illustrate the number of messages received per day during the previous months. It is highly non-linear and it took randomly an exponential shape (Oh my!). Let’s call it the exponential work increase deadline effect. What a tremendous name!
For SED v2 I have been mostly focused on the balloon design. I had to read a bunch of scientific/engineering papers on the design of natural shape balloon or super-pressure balloon.
I worked on a MATLAB code to solve a non-linear system of differential equations that satisfy the equilibrium equations of a natural shape balloon. Sound sexy right ? But it isn’t always lovely dovey with coding. There is three fundamentals questions that I always have to answer at some point :
1) How am I going to convert that theoretical problem into a numerical problem ?
2) How am I going to write it in MATLAB ?
3) How am I going to fix my bad code ?
The last question often let you think like the following image:
My teacher in Finite Element Method always told us that “A lot of software users complain that errors comes from the computer, but most of the time the error is not coming from the computer but from the user”. I feel like this picture illustrates correctly this situation.
To those of you that read up until now I have a gift for you. I made a big summary drawing of BESPIN different missions steps. I believe that we made 4 or 5 images like that already ! I feel like I needed to do my own as well. Compare to the previous versions, I wanted to focus on the mechanical environment that BESPIN will be subject to.
BESPIN will be subject to loads very similar to a satellite launched on a rocket. For example Ariane 5 produce longitudinal acceleration of 5g but REXUS rocket produce 20g longitudinal acceleration during Lift-Off. Also one of the most important loads applied on a satellite during launch is acoustic vibrations. The acoustic vibrations are produced by the rocket engines. There is also different source of vibrations, such as engine-structure interaction, aerodynamic turbulences or transonic flow, separation stages, and maybe other things I do not know yet. Anyhow these vibrations can results in loads up to 80 g and so it is very important to do modal analysis of satellites, that also applies for BESPIN
Sounding rockets are stabilized by rotating the rocket along the roll axis. It increases the inertia of the rocket and that increase the attitude stability. This centripetal acceleration results in an additional force for BESPIN.
After reaching Apogee, BESPIN will be subject to mechanical loads very different to common satellites. During the descent its velocity will increase up to Mach 3 (supersonic flow) and that will results in aerodynamic heating, high aerodynamic pressure and instabilities that must be considered for the success of the mission.
At 5km BESPIN is expected to deploy its parachute. The shock is expected to be very important because BESPIN will travel at a velocity of 150m/s before activation of the parachute and it will have a velocity of 10 m/s afterwards. This change of velocity is done over 2-3sec! It correspond to a deceleration of 40-60 g.
After that there is the balloon inflating, separating, descending etc. That’s the boring part that doesn’t interest you, right ? But if you are really interested to learn more on the balloon inflation process or anything else just leave a comment on the blog and I will happily answer to your question.
Thank you for reading.
Amazing Fact : Did you know that BESPIN mechanical division is spread in 5 different countries ? We have people living in Sweden (Kiruna and Luleå), Germany, Belgium, Taiwan and China.