Thursday, March 3, 2016

Building a Rocket (Part 1)

This semester, I am working with a team of students to design a model rocket as part of the Rocket Design Project for our local student chapter of the AIAA. And I figure I may as well write down my thoughts on the experience, as it progresses forward.

So What Is It?
This project requires teams of three to four students to design and build a model rocket with the goal of predicting and reaching as close as possible to specific target apogee – in our case, 1,750 feet. Points are awarded based on accuracy of our predictions, how close to the goal we get, and the overall aesthetic appeal of our rocket.

What Will We Use?
So far, we don’t have specific specs aside from knowing we are getting either G40-7W or G38-7FB motors. This means that the motors are the largest available without a license (G-motor), they both have a 7 second delay, and one has a specific impulse of 40 s and the other of 38 s. Basically, these are pretty big motors that should easily achieve an apogee of about 2,500 feet. The trick for us will be slowing them down, I suspect.

The rest of the parts are also given to us, excluding the fins – these we will design ourselves in SolidWorks and OpenRocket and cut from acrylic with our school’s laser cutter. This is really the only part we truly get to design; the rest are given to us.

So How Do Design It?
We will use a combination of SolidWorks and its fluid flow simulator and OpenRocket, an open source rocketry program that can also simulate flights. These can both be augmented by our own hand calculations, mostly likely solved with one of MATLAB, Mathcad, or Excel. Once we have a fin design that achieves an appropriate apogee, we will cut our fins and assemble our rocket.

So How Do You Launch It?
We will go to a sod farm where a local rocket club regularly launches. They will assist us in launching our rockets, which will hopefully perform according to our predictions. This will be done over the course of a weekend, at the end of the semester.

So What’s Next?

Next post will derive a set of simplistic two-dimensional equations for predicting apogee. It is worth noting that I have not achieved the level of knowledge needed to work out a 3D version of this or take into account things like changing propellant weight, wind speeds (let alone varying wind speeds with altitude), or varying thrust. Technically these should all be taken into account, but I also question to what degree these will affect our rockets, considering the slop in our own measurement and construction. Basically, I’m not sure if such an accurate model wouldn’t be invalidated by our manufacturing process or variations in the weather.

2 comments :

  1. Fun! How do you measure apogee - off-the-shelf radar or something more interesting?

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    1. We will be installing an altimiter in the rocket to directly measure our apogee. Considering last semester was plagued by "instability issues" resulting in at least one explosion, I'm kind of surprised they are putting that sort of gear in the rocket itself, but that appears to be the plan. Once I get more information, I'll post it. Since spring break is starting now, I suspect we will get all of that when we return (a shame because I'd like to get started on the actual design process).

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