Deadlines are the center of pressure
What is that over there on the 16th of July?
Comon, get closer.
...
Oh, I can see it!
...
No god, no please, no. No. No. NOOO!
It's a deadline. A deadline for an updated version of our SED. After the CDR, we got a conditional pass for our experiment. Conditional? Well, still better than no pass at all! It means that we have to update our design and information about our experiment and present this updated version to the REXUS board again.
But like with every deadline, you just wait for it to come and suddenly you realise that you only have a few days left to actually finish all the work. So we usually spend the last week before a deadline working on the project 24/7 and "sparetime" is not existing in our vocabulary anymore. But since we are well organized (mostly) and highly intelligent, we use earlier, internal deadlines so that it's possible for our supporting professors to have a look at the SED.
Problem is, that usually and especially, the mechanical division (my division) is the one finishing the latest, annoying our teamleader Anna until 5 minutes before the final submission with requests to change some text here and some numbers there. But mostly, she's okay with that.
The reason that mechanical is always late, is that we change our design every few hours and that these changes affect then the whole experiment and its properties like mass, center of gravity or aerodynamic behaviour.
I am currently working - together with François - on the aerodynamic stability of BESPIN.
Since our probe is descending from the rocket's apogee, so after ejection from the rocket, through the atmosphere (at an expected speed of Mach 3 at around 30km altitude), we need to make sure that it is oriented in the correct way so that we won't get any problems during the parachute deployment. The parachute will be ejected from the bottom of the descent probe which means that we need to have the bottom plate pointing towards the sky. If this is not the case, the parachute will be ejected in fall-direction, and would basically serve as a net to catch the experiment but not as a parachute.
In order to analyse if the probe is descending aerodynamically stable, we have to find the center of pressure (CoP) which needs to be situated behind the center of gravity (CoG). To push the CoG as far to the tip as possible, is one of our big challenges in this project since the He tanks are the heaviest part of the whole construction and are situated at the "back" of the probe and not in the tip.
There are several ways to find the CoP:
- Wind tunnel test (where we need a small scale model, quite expensive and our financial division doesn't like expensive things)
- CFD simulation (computational fluid dynamics, cheap but complicated and time consuming)
- hand calculations (simple but only yielding realtively rough estimations)
We have run some CFD simulations in ANSYS to find it (the big red dot in the 2nd picture, in reference to the yellow cross) and here is how it looks like:
OpenRocket Simulator was also used but didn't really give us good results since it's made for modelling the ascent phase of rockets and not the descent period of a bullet shaped metal block. The red dot is the CoP and the blue white one the CoG.
Since the CoP is dependent on the shape of the experiment, we will still need some time to figure it our by simulations because the final shape changes as well from day to day (pointy tip for the protective shell? or a dull one? who knows? does it even matter?). So it will take a little more time to figure that one out. Also, I never dealt with problems like that so far, which doesn't really makes it easier to solve it, but rather shows me all the things that I don't know anything about. But isn't that what's engineering is all about anyway?
Fabian