Series Overview

You can split any project into two parts. The part that you really are excited to do, and everything else. The part you’re excited about often gets most of your attention, and everything else usually gets “just enough” attention to get it done. For example, the question of “What screw should I use here?” often gets answered by “What screws do I have”, not “What screw would be best for the project”. These small decisions make sense as we only have so much free time to work on a project, but we shouldn’t forget about them entirely.

This series of posts will be about those, and other, small often overlooked parts of projects. Things that I find interesting, but don’t think warrant a long post or a series by itself. It will be of indefinite length and irregular schedule. It won’t just be about my projects but will often be motivated by seeing someone else’s project and thinking, “Hmm, I would’ve done that differently” or “I wonder why they chose to do it like that”.

Below is the first entry in the series.

Screws

In an attempt to avoid doing real work I was staring at UB’s competition rocket from last year. Something bugged me about it but I couldn’t put my finger on it.

I got a bit closer and then it hit me, can you spot it in the image below?

It’s that the screws arent flush,

Whats wrong with unflush screws?

In a word, drag.

If the flow on the rocket is laminar, the unflush screws can trip the flow and turn it turbulent. Turbulent flows cause larger amounts of drag which in turn limits the height that the rocket can reach. Friction between the surface and the flow will cause the flow to transition to turbulent flow after some distance like shown in the diagram below.

By introducing a bump to he surface, like an unflush screw, the flow will be tripped at the screw. This will look like the following image.

How can we fix it?

Simple, just make the screws flush.

Ok, maybe it’s not that simple. If the rocket body was a board we could counter sink the screws, but its round so that wont work. If we screwed them in and then filed them down we would probably mess up the finish on the fiber glass possibly damaging it. My solution would be to screw a Flathead screw in and then mark off the unflush portions with a sharpie. Once all the unflush portions have been marked off, remove the screw and then use a file to remove the excess material. Repeat until flush. This may be a sizable amount of work but it’s what I would do. If you have a better way to make these screws flush, email me at ari@gereshes.com and I’ll update this post if it’s a good idea!

UPDATE:

As was suggested in an email by J.L., just use set screws. This solution is far better than the filing solution because it involves less work without giving up any of the benefits!

Why it wasn’t fixed on this rocket

Now that we’ve identified a problem and come up with a solution, why wasn’t it implemented on this rocket?

Because perfect is the enemy of good.

Getting this rocket completed so that it could fly in SA CUP / IREC 2017 was the most important task at hand. There was only finite time to finish this project and time spent making the screws would be taken away from something else. In the end, the time was better spent elsewhere and the rocket flew.

If you want more Gereshes

There are new Gereshes post every week! If you want to receive the weekly Gereshes blog post directly to your email every Monday morning you can sign up for the newsletter here!

If you can’t wait for next weeks post and want some more Gereshes I suggest

Guerrilla astronomy

Rollout of a rocket motor test stand

How to pump a swing using math

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