I think most people will agree that the Medusa heads in Castlevania are amongst the top most-hated and most-frustrating enemies in gaming. They aren’t particularly difficult to defeat, they’re just really obnoxious. While they follow a predictable path, that giant, wavy pattern is infuriating due to its wide range and knack for becoming an interference at the most inopportune times. So naturally with decades of people becoming enraged over these enemies the creators of Shovel Knight said to themselves “why don’t we do the same thing?” Thus the rocket knob was born (I know it’s not called that, but it’s the only thing I could think of without resorting to childish analogies; and I wasn’t there during development, so that conversation may have never taken place).
The rocket knob shares very distinct similarities to the Medusa heads, as they both:
travel along a distinct wave pattern;
seem to travel in the foreground to avoid objects, but swing inwards to smack the player when need be;
have some complicated means of propulsion to keep them along this path;
are annoying as hell.
Since this is the Physics of Video Games we’re only going to focus on one of the four items, which fans of the work should be able to deduce as to which we’ll choose. A lot of the work has been done for us in terms of explaining how things are done. There is an exhaust that vents from the back to move the knob horizontally forward. There is another exhaust right below that which is pointed at a 45 degree angle away from the previously mentioned vent, which pushes the rocket knob upwards when needed. It may seem like we have everything covered for our explanation, rocket vents pushing an object in the opposite direction seems pretty simple, but we have a little bit more at play here.
Our concern is the way that forces need to balance out. If we were only talking about the vent that propels the rocket knob forward all would be well so long as the weight is evenly distributed along its, uh, shaft. It would continue to move forward perfectly parallel to the horizontal and move downwards due to gravity. The angled vent is where things get complicated.
This second vent would indeed push the rocket knob upwards as depicted in-game, but there is a little bit more we need to consider as that vent would give us a little more than we bargained for. This force coming from the angled vent would force the rocket to start spiraling out of control as it’s pushing in two directions, the horizontal and vertical, but this doesn’t happen in the game. The reason for this is complex in design, but simple in explanation.
What we have is an air-intake at the tip of the knob that will continually compress air as it travels. With the collected air the rocket then vents air in whatever directions are necessary at the time. If the angled rocket is pushing upwards, there is an angled vent on top pushing in another direction to counteract the potential spinning motion. When the rocket knob is falling downwards, these vents are shooting from the bottom in order to allow for a more-steady decline as opposed to completely being overcome by gravity. This explains why there is consistency with the motion in all directions. The motion of this rocket knob is uniform, and these air-collecting-exhausting vents are the reason for it.
So there we are, propelling in many direction, collection at the tip, rocket knobs. What should we cover next in our Shovel Knight, Physics of Video Games series?
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