A modern-day catapult with an astronomical application

Posted on Wednesday, January 4th, 2023

Written by Noah Franklin

What sets humans apart from the numerous other animals on Earth is our ability to analyze, adapt to, and overcome almost any obstacle in our way. From the Stone Age to modern times, the tools we use have advanced dramatically. In recent times especially, there has been a surge of technological advancement with a wide variety of uses. A prime example of this advancement can be seen in railguns, the successor to catapults. 

A railgun can accelerate its projectile (the armature) very efficiently, leading to applications in transportation, communication, naval defense, and most recently space-launch technologies [1]. Despite current limitations, it is no exaggeration to say that railguns have advanced leaps and bounds ahead of what we had thought possible, back when catapults were the best option for when you needed something launched.

How does a railgun work? A railgun functions through properties of physics, primarily the Lorentz force. When electricity flows through the rails (see Fig. 1), an invisible field that acts on charges like electrons and protons in materials is formed. Between the rails, the projectile will act as a bridge for the current and from there, interactions between the current and field cause the armature to be propelled forward. 


Figure 1: Lorentz force produced by a railgun through the right-hand-rule [4]

The force caused by the interaction of the current and magnetic field that propels the armature is called Lorentz force. This force is produced when a current flows perpendicular to a magnetic field (as shown by Biot-Savart Laws and the right-hand rules in Fig. 1) [3]. Therefore, the rails have currents running in opposite directions so that the magnetic field is in the same direction across the gap between the rails. 

The railgun manages to reach its incredible speeds in such a short amount of time because for the entire length of the rails, as long as the current is flowing, the armature will accelerate [5]. For this current to be maintained throughout the launch, a massive energy source is required. The energy source is usually a capacitor bank (Fig. 2).

Figure 2: design of a railgun powered by a capacitor bank [5]

 The massive power consumption and the inability of the rails to withstand the high temperatures are the two biggest drawbacks to using railguns and hold the most room for future improvements [6].

When it comes to the railgun's armature, the smaller, the better. Not only does a smaller projectile lower the air resistance, but as supported by Newton’s laws, the force (what’s pushing the armature) equals mass times acceleration (how quickly it speeds up). This also means that a smaller mass will have a larger acceleration for the same force (current) [6]. 

Figure 3: railgun animation showing relation between current flow and force in a magnetic field

When trying to launch a satellite into orbit there is more to account for than just the weight. A balancing act is required for optimal efficiency. If the aperture did not have any propellant or guidance, it would be lighter and faster, but there would also be a loss of control during and after take-off [7].

Ultimately, if you omit the ridiculous energy requirements, and the wear of the rails after each use, you would be left with a perfect piece of equipment capable of controlled accelerations on a global scale. This may seem like a major stipulation, and nothing more than a pipedream, but if you told people a hundred years ago what would be around today, they would have said the same thing. 


Noah Franklin, an undergraduate student at the University of Guelph, produced this article in the context of the 3rd-year course IPS*3000 on Science Communication in the Fall 2022 semester (course instructor: Alex Gezerlis, TA: Carley Miki).

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