To start off though, a brief back story about the roller coaster tradition and how I ended up being one of the people in charge of this year's 'coaster.
As far as I can tell, the first EC roller coaster, dubbed "8.01: The Ride" was built in 2004 (making this the 10th anniversary coaster). The last roller coaster, "The Reverse Cowgirl" was in 2010, and had to be taken down prematurely since they weren't permitted. So since then, EC has avoided roller coasters.
I've been thinking about the roller coaster since rush last year. Pretty early on I was joined by Jaguar and then Wesley, hall-mates and fellow Mech E. 16's. During the fall and winter, we started thinking about general track shape and construction method, and as soon as EC's Rush Chairs were elected near the beginning of spring semester 2014, we started talking with them about getting a roller coaster approved.
This post will cover track design. A construction post will come later.
Let's get started.
Track designing started out like this:
Slightly fleshed out:
So we wanted a loop.
Loops are hard for lots of reason. First, designing a loop that doesn't exert excessive g-forces at any point is non-trivial. From a structure design construction standpoint, it's challenging as well, especially given the materials and labor limitations of Rush. And convincing MIT administration, the city of Cambridge, and some professional structural engineers that our loop would be safe is an even harder problem.
The first of those problems was the easiest to deal with.
Fun Fact: going into this project, I had never ridden a roller coaster, and didn't have the faintest idea how roller coaster loops were designed, other than that they weren't just circular. I did a lot of reading about roller coaster loops, and some of the math behind them.
Before looking at any specific loop shapes though, I needed a way to look at the acceleration experienced by a person riding the roller coaster. More specifically, I cared about the normal force between the roller coaster track and cart at any given point. This number is important for both the riding sensation as well as design and analysis of the roller coaster structure.
Here's a simple mechanics view of the roller coaster with the cart at three different locations on the track. I only cared about the forces normal to the track, and didn't particularly care about forces and accelerations in the direction of the track at this point.
The negative sign if inverted above comes from the fact that my code calculated theta using the arctangent of the slope. Arctangent has a range from -π/2 to π/2, and therefore can't output an angle with a negative cosine. Yes, atan2 could have fixed this.
I cranked out some MATLAB code which, given a set of points parametricly defining the shape of the track, could calculate the normal force (and G-forces) experienced at all points along the track.
Without going into the code line-by-line, here's how that works.
First, I define the track as a set of points that looks something like [(x1,y1), (x2,y2), (x3,y3), ... (xn, yn)], where each x and y is a point along the track in meters with the origin on the ground at the start of the roller coaster.
To calculate the normal force, at any point you need the velocity, the radius of curvature of the track, and the angle of the track relative to horizontal.
Velocity at a given point is calculated using energy conservation with change in height from the starting point. A friction factor is included in the velocity calculation to account for drag and track friction.
Angle at point is approximated by first calculating the slope from point n to point n+1 by (yn+1 - yn)/(xn+1 - xn). Angle is just tan-1(slope).
Finally, the radius of curvature at any given point n is calculated by finding the circle circumscribing the triangle formed by points n-1, n, and n+1.
This is roughly illustrated in the image below, where the black dots represent the points along the track, the blue dotted line is the approximate slope at a point, and the red circle is the curvature at a point.
So, now that I had a way to analyze different track shapes, I needed to come up with an exact shape for the roller coaster track. I chose to use a clothoid loop (generated from part of an euler spiral). The curvature of a clothoid loop gradually increases until the top of the loop, at which point it decreases again and transitions to flat. Gradually increasing curvature means no huge spikes in G's experienced by the rider.
Here's a simulation of a 45 degree slope followed by a loop:
|Loop Shaping in MATLAB. How 'bout that phase margin? Oh, wait....|
And a 60 degree slope, with friction included this time:
At the bottom of the slope, G's actually go to infinity because of the instantaneous change in slope.
As shown, I was able to create reasonably sized loops with reasonable G-forces.
That's pretty cool. Unfortunately, we had to scrap the loop. One of the steps to getting the 'coaster approved by MIT and Cambridge was going through MIT's Environment, Health and Safety department (EHS). Basically, EHS said No Upside-Down People. Period.
We had very little time to come up with a new track design, so we threw around some ideas including things liked banked curves, but decided on a (relatively) simple straight, multi-humped track design. More specifically, it's an exponentially decaying cosine function plus an additional exponential decay, with a turn up at the end. Even more explicitly, the track is the function 2.124e-.055xcos(.7x - .3) + 4.676e-.055x
Here's what that looks like in the G-force simulation:
Not too bad. ~4.5 G's is pretty high though. I traced the track curve into a Solidworks sketch as a spline, and then rounded out the curve at the bottom of the first hill to reduce the acceleration there. I then exported the sketch as a DXF. I found a MATLAB script that could convert DXF files into a matrix of coordinates. I used this to pass the modified track shape into my same simulation. Unfortunately, the process of converting things into splines and DXFs made the curve much less smooth, so the resulting acceleration plot is a bit jagged. But the peaks were indeed reduced:
If you look carefully, you'll see that, with the friction estimate I used, G-forces actually go just barely negative at the top of the second hill. Negative G's mean the cart would actually lift off the track at this point. This turned out to not be the case on the actual ride, unless we gave the cart a really big push at the top.
I planned on taking some measurements to see how accurate the simulation was, but I didn't get around to securing good test equipment in time. I tried out using a smartphone accelerometer strapped to the cart, but the resulting measurements were an incomprehensible pile of noise.
There's the abridged version of how we came up with the shape of the roller coaster. Next in the series: Roller coaster mechanical design, and maybe a glimpse into the whole approval process for this project.