Photon Rocket

😐	PLAUSIBLE:
	This is a realistic concept that likely could be built, but has not been yet.

This is it - the be-all and end-all of reaction drives. No other drive competes with a photon rocket's metrics of output, efficiency, and complexity. Effectively a flashlight of sufficient power to generate practical thrust, the photon rocket is of the same species as chemical and nuclear rockets, but evolved beyond either. Instead of chemical bonds or nuclear fusion and fission, a photon rocket uses one-to-one antimatter-matter annihilation to convert the fuel's mass into radiation. Directed in a single direction, this light is sufficient to create thrust.

The rule for any torch drive is - "Don't point this at planets and other friendly objects." For photon rockets, the rule the same, but in all capitals.

Introduction

Reaction drives are all about energy and efficiency. The more energy you can extract and apply to a reaction mass, the greater your efficiency, the mightier your rocket. A chemical rocket might combine two compounds, burning them, and using the released energy to eject the fuel's mass as fast as possible. A nuclear rocket extracts that same energy from nuclear fusion or fission, and due to the nature of nuclear reactions, manages a much greater energy density potential. For example, in the case of hydrogen-hydrogen fusion, 0.645% of the reaction's mass is carried away in the form of radiation - less than a percent of the mass in play becomes light.

A photon rocket converts 100% of its fuel into energy. Reacting 0.5 grams of antimatter with 0.5 grams of matter results in 0 grams of either, and ~9×10^13 Joules (21.5 kilotons-equivalent) of energy. A nuclear bomb's worth of energy is generated with the mass as an average pen cap. Where batteries, chemical fuels and nuclear bombs all store energy in their mass, antimatter is the ultimate battery, because the entirety of its mass is the energy potential in play.

Some designs choose to apply that energy to accelerating matter, ejecting it to create thrust. Instead, a photon rocket uses the generated radiation directly, reflecting it off the rocket's drive and casting into one direction, thrusting the craft the other way. No mass is ejected, just a cone of radiation. It's an elegant, efficient, and incredibly complicated method - after all, we need to reflect that radiation without absorbing almost any of it, lest we become hot, expanding plasma. The radiation in play is of a vast variety, up to and including gamma and neutron radiation that penetrates meters of lead and water, although workable designs strive to generate as little of those as possible. We also need to store antimatter without allowing it to touch matter, for many months or years, in a acceleration-resistant and secure way that doesn't have the crew constantly fearing an instant and unexpected death.

Theoretical Performance

The exact mathematics of a photon drive depend on its design.

Design and Function

Storage

By its nature, antimatter containment is unlike any other form of storage imaginable. The fuel tank itself cannot be allowed to touch the fuel it stores. Air, loose particles, evaporation and unfortunate dust coming into contact with the fuel would cause the same reaction as is meant to be within the engine's focus - annihilation. If your vessel is meant to have significant acceleration or withstand sizable impacts, your antimatter containment needs to reflect that with a huge margin of error.

Perfect vacuum, near-zero-kelvin temperature and acceleration resistance are a must.

The primary theorized means of antimatter storage is based around electromagnetic levitation. For this, the antimatter in question has to be both paramagnetic, and cold enough not to evaporate too much.

That's where hydrogen ice comes in. Hydrogen has a melting point of 14.01 Kelvin. You've probably met more people named Kelvin than the degrees required to keep hydrogen solid. However the act of electromagnetic levitation injects energies into the frozen anti-hydrogen, while the only way of cooling it remains black-body radiation. Levitating your anti-hydrogen will cause it to heat up and melt or evaporate, becoming non-paramagnetic and escaping containment.

A possible solution is keeping pellets of anti-hydrogen ice wrapped in a thin later of anti-lithium, preventing evaporation and sublimation.

Performance / Capabilities / Applications

An antimatter rocket as we know it would never be used in an atmosphere. Not because of our unwillingness to irradiate continents, but because excess of gasses in such an environment would tend to flood reaction chambers and risk annihilation with the antimatter at inopportune times.

In Fiction

'The Land of Crimson Clouds' by Boris and Arkady Strugatsky is set around a prototype photon rocket's mission to Venus. Where nuclear rockets have failed to consistently survive the Venusian environment, due largely to their limited freedom of spare acceleration, the spacecraft Khius is meant to overpower the planet with a hyper-efficient and powerful photon torch.

Khius uses an 'absolute mirror' of mesa-matter to reflect all forms of emitted radiation away from the craft. Khius possesses five fusion rockets around the primary antimatter photon drive, primarily designed for earth-side landings that don't allow for the photon drive's collateral. During the cross-system flight from Earth to Venus, the craft abuses its excessive freedom of motion to set a speed record, at which point the crew experience anomalous behavior in their equipment, attributed to relativistic distortion.

Previous prototypes had a history of failing mirrors and instant vaporization. Later stories - "Way to Amalthea" and "Probationers" - note the insanity of the prototype's single-layer mirror, which, upon failing at even a single crack or fault, would have vaporized the entire craft. In "Way to Amalthea", a photon rocket freighter falls into Jupiter. After re-calibration of a partially broken mirror and drive, the captain risks igniting a chain fusion reaction, with a possibility of turning Jupiter into star, in order to escape.