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Sublight Engines 2

Contrary to the first sublight engines list, this article covers the engine types that do not carry their own power source on board and as such are not subject to the tyranny of the rocket equation. They offer better complexity-to-performance ratios than regular engines but depend on external parameters that may not be under the ship's control.

Engine types are ordered from least to most complex.

1 - Lightsails

A lightsail is a propulsion device leveraging radiation pressure produced by light on large mirrors to create forward momentum. It can be understood as the equivalent of a sailing ship in space. The light source being used can be a close star (in which case it is a solar sail) or a fixed laser beam (in which case it is a lightsail proper). Lightsails produce ridiculously small amounts of acceleration but are simple and streamlined designs that can be built at a minimal cost. They are used aboard satellites for station-keeping and orbital manoeuvers. The constant acceleration guaranteed by a solar sail may enable a light craft to reach high velocities providing time is not an important factor. Lightsails are ubiquitous on small satellites and probes. When passing by an unexplored system exploration ships often deploy swarms of lightsail probes that they then accelerate with their lasers. As they do not emit an exhaust plume solar sails are also remarkably stealthy and are known to be used on low-tech military drones. The only obstacle to using a solar sail is that they are somewhat tricky to deploy.

Aside from their low acceleration lightsails have the main drawback of being heavily geography-dependent: their efficiency is directly correlated to their proximity to the sun or a laser emitter.

2 - Electromagnetic sails

Magsails use the same broader principle as lightsails though instead of light they use charged particles in the solar wind to create forward momentum thanks to superconducting strands deployed around the spaceship. Their main characteristic is that they are capable of sailing against solar winds and planetary magnetospheres, using the resulting drag to slow down an incoming ship. This is useful for interplanetary vessels that need to decelerate for orbital insertion after a faster than light translation. Instead of using their engines they may just deploy a magsail and bleed speed using electromagnetic drag thus sparing their precious delta-v.

Magsails are more complex than lightsails and are always found mounted on ships with additional means of propulsion. Mundian ship manufacturers in particular are known to manufacture exquisitely complex magsail arrays that border on works of art. 

3 - Solar moth

The solar moth is a simple emergency engine design. It consists of a pair of mirrors that are used to ignite propellant stored in a tank, thus creating thrust. Much like for a solar sail the efficiency of a solar moth depends on how far it is from a star. It also has a limited range due to its reliance on propellant reserves. Not needing fuel nor an onboard energy source means that the solar moth has a very good weight-to-power ratio making it ideal as a backup engine for bigger starships.

4 - Q-drive

The Q-drive is a hybrid design that is based around the plasma magsail: a type of solar sail that does not require a large physical structure but instead relies on a magnetic sail created by an electromagnetic engine. Instead of using the sail to propel the ship directly a Q-drive leverages the momentum and energy of the incoming plasma wind to transfer momentum to onboard propellant, generally water ice. Though more complex to build and operate than a magnetic sail, the Q-drive combines the advantages of both a fission/fusion drive and a magsail, with very high specific impulse and good thrust combined with the use of an external energy source which greatly reduces the ship's mass requirements. As the solar wind (or ionized interstellar medium) is only used to transfer momentum, the Q-drive can be used to propel a ship against the solar wind, which is unique among other magsail designs. The fact that the Q-drive doesn't require an exclusion zone the way torch drives do and that it can be "folded" inside a relatively small ship make it ideal for courier and passenger ships. Fusion drives are however preferred when raw thrust is the most important factor, such as in military vessels. 

Solar sail illustration by Andrezj Mirecki. Q-drive illustration by Adrian Mann. Solar Moth illustration source unknown. All other pictures are NASA public domain. 

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