Mass Driver

A Mass Driver (MD) is a platform that is installed on a solid surface of a planet, moon, or asteroid. It is used to shoot payloads into space at high velocity. The payload may be as simple as a rock or as complex as an interstellar probe - it really doesn't matter. The purpose is to fling an object as fast as you can without using any engines or fuel - this is a very attractive option since we can use a land-based electric power-station that can be heavy and powerful while the spacecraft that we launch stays lightweight and nimble.

There are two main types of the Mass Driver: a centrifuge (rotary) and railgun (linear). The centrifuge MD uses a central electric motor to spin a payload on a tether to very high RPM (SpinLaunch recently unveiled such a design). When the needed velocity is obtained, the payload is released from the tether and continues on a linear trajectory. This is similar to the Hammer Throw that athletes perform during Olympic Sports. The centrifuge MD is compact, but the payload will experience very high G-forces during spin-up and the timing of the release must be perfect to ensure that the payload ejects exactly on the right path. On the other hand, a rail-gun MD is a linear device that employs a superconducting rail to accelerate payloads (similarly to maglev trains in Japan). Although the railgun is much larger than the centrifuge, the G-forces acting on the payload are smaller and there is no need for precise ejection mechanisms - when the rail ends, the payload simply continues in a straight path.

But whether we use a centrifugal MD or a railgun MD to fling an object into space, there are several important drawbacks that should be taken into account. The Mass Driver suffers greatly when operating within an atmosphere because of one reason - drag. Even if we put the entire platform into a vacuum chamber, ultimately, the ejected payload will hit the atmosphere at an incredibly high velocity which would likely result in over-heating or supersonic shock failure. For this reason, it is smart to build MD systems off-word in vacuum environments like Moon or Mars. Let's not forget that an MD must also contain a power-plant be it a Solar Array or a Nuclear Reactor, which adds to the overall cost and complexity of the system. Lastly, we can slow down rocks and simple cargo containers with nets or other catching mechanisms, but large spacecraft will require engines to slow-down, begging the question of why not use these engines in the first place.

In my opinion, there is one killer application that is not often considered - Planetary Defense. According to Newton's second law: every action produces an equal but opposite reaction. In other words, a Mass Driver installed on an asteroid would shoot rocks in one direction and accelerate the asteroid in the opposite. Imagine a scenario where a large cobalt-filled asteroid is on a collision course with Earth. If we can deploy a Mass Driver to its surface well in advance, we could mine it for precious metals and at the same time, deflect it sufficiently so it does not impact Earth - a truly elegant solution, as I like to say - "kill two birds with one stone".

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