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By Margaret Harris
A couple of years ago, I came across what I thought was a funny (and physics-related) video about a water slide. The slide is called “Verrückt”, which my German-speaking colleagues translate as “mad” or “crazy”, and it caught my attention because it was being built at an amusement park in my home town of Kansas City. As the video shows, the slide experienced a few problems during its testing phase.
“When the rafts are loaded with more than 1000 pounds, the slide becomes unsafe,” says the video’s announcer as the test raft goes airborne. “We’re going to have to redesign the entire slide,” an unnamed official adds.
The redesigned slide opened in the summer of 2014 and operated under strict weight limits. The combined weight of riders on the three-person rafts had to be between 400 and 550 pounds (180–250 kg), and the amusement park, Schlitterbahn, established a procedure for weighing riders beforehand to ensure the limits were adhered to.
But on Sunday, something went wrong, and a 10-year-old boy called Caleb Schwab – the son of a Kansas state lawmaker – died.
The cause of Schwab’s death is still being investigated, so it’s not known whether the Verrückt’s tight weight restrictions or its testing-phase troubles had any bearing on the accident. According to reports in the local newspaper, the Kansas City Star, officials were unable to say “whether the boy fell from the slide or was even aboard a raft when fatally injured”.
We’ll probably learn more in the next few days. Until then, though, the test video is a sobering reminder of why it’s important to get the physics right.
RIP, condolences to the family.
Hope to know the cause as soon as possible.
The people who designed the ride appear not to have grasped physics. The 2nd hill would be crested at about 59mph if the heights of the hills are as published in park brochures. That would require the 2nd hill to have a radius of curvature greater than 72 meters in order to prevent weightlessness given a frictionless track. The radius of curvature of the 2nd hill seems to be much below 72 meters, but its not knowable without blueprints. But the other major point of concern is that they don’t seem to have grasped coefficients of friction. That the rafts are required to have a minimum loading weight is extremely disturbing. This suggests that the friction has several regimes due to the water-rubber-flume interface of sliding. If the raft is too lightly loaded, it appears that the coefficient of friction might considerably drop, rendering the velocity at the top of the 2nd hill too high and allowing rafts to go airborne. If this is the case, then the ride design would be incompetent – the ride should not EVER allow an airborne raft in even the frictionless-track case. By heavily loading the raft undoubtedly the coefficient of kinetic friction goes up because the contact surfaces are no longer rubber-water-flume but instead rubber-flume, thus greatly increasing frictional resistance and slowing velocity at the top of the peak. Third, if the raft has insufficient downforce at the top of the 2nd hill, air will enter under the raft, causing it to become an airfoil. We see that in the video on this page – because downforce at the top was too low, air enters the space between the raft and the flume, and then all bets are off because you have an undesigned airfoil moving at 60mph. Because the transition between whetted and attached tot he slide, and air-under-the-raft can only be controlled by downforce and air velocity, the design should never allow the regime where the raft generates air-lift to be approached. But what appears to have happened is that when the raft is insufficiently loaded, the coefficient of friction is lessened, the velocity of the raft goes up at the top of the hill, insufficient downforce exists to keep air from under the raft, air enters and aerodynamic lift ensues. That is a design failure because physics wise you must keep the air velocity below, and downforce above, the transition where the raft gets air under it. This could have been accomplished by making the radius of curvature of the crest of the 2nd hill broad enough that the centripetal force needed to stay on the curve was low. That rafts had a minimum weight loading is, in itself, an admission that the design of the ride was not understood. Because, if it had been designed properly, no airborne raft would ever have been allowed or acceptable. Reports indicate that park owners knew that rafts could become airborne on the design under certain weight loadings. This is incompetence. Further, it looks like from the tragic accident, that the weight loading on the very front of the raft was at issue, and this is in keeping with the idea that insufficient downforce existed to keep the raft in contact with the flume, thus allowing air under the raft from the front of the raft. The non-linear nature of the fluid flow, the multiple friction regimes of the contact force, the somewhat uncontrollable forward velocity vector of the raft, the side to side oscillation of the raft – all of these are incalculable and uncontrollable dynamics of a raft which on a rollercoaster are known and controlled. This ride is a dynamics experiment – it isn’t a safe engineered and controlled-repeatable engineering rig. A rollercoaster is strictly repeatable because its dynamics are linear. This water ride has many non-linear elements which can’t be calculated nor predicted. It should be permanently closed. It should have been permanently altered when the very-first raft went airborne as this indicates a fundamental failure of the design to understand, calculate, and control the physics dynamics of friction, air foil, and downforce.
There are several other tell-tale hints in the published reports. One is that you can see water going airborn over the top of the 2nd hill. That is, the water velocity shooting up the 2nd hill crests the 2nd hill and shoots up into the air instead of following the flume. That means that the downforce at the top of the 2nd hill is insufficient to prevent a near-frictionless projectile rolled on the track from going airborne. In a proper design, nothing should go airborne at the crest of the 2nd hill even if the track were frictionless. The radius of curvature of the 2nd hill should prevent that. Published reports indicates deisgners changed the height of the hill, discussed adding jets to slow down the raft, and changed the steepness of the first drop. All of these ideas display a fundamental failure to grasp the underlying physics. The steepness of the first drop does not appreciably effect velocity at the crest of the 2nd hill. Changing the height of the 2nd hill could lower the velocity at its top, but if the radius of curvature is wrong, lowering the velocity won’t do you any good. Applying water jets as a good means of slowing the craft is again incompetent because the water jets will present a non-linear resistive force which will be wholly unpredictable in its effect. The entire ride seems to have been created by people who are guessing and attempting to solve non-linear fluid-flow equations by trial and error. The bare minimum criteria for a ride like this, before testin ever began, would have been to make sure the 2nd hill was curved such as to provide a lot of down force on the raft. Laboratory testing should then have been done to establish at what downforce and wind velocity the front of the raft would lift up, and then design the ride to stay within a safety factor of 3 away from that regime at a minimum. From all the written material online, videos, etc, it appears none of that was done. The first day that 1 raft went airborne, the design should have been considered totally unuseable. This little boy died because the people who built the ride did not understand physics. Its a failure of analytical and scientific understanding in our society.
Great material for an analytical mechanics course. It isn’t just a technical question, but also a good example of why understanding the physics can be a matter of life and death in everyday life.
I agree that this verruckt ride should be permanently shut down. Caleb Schwab paid with his life due to the incompetence of the rides designers.