What would it take to make a fuctioning photon rocket?

For reference closet counterpart to this would be the "wings of light" and Minovsky Drive System of the Gundam metaseries, engines that release what looks like pure light as a propellant.

• Just to clarify, are you looking for a mechanism/maths which would allow for light to be used as a propellant? or a proper design for a rocket? As a note, you would not be able to see the light from a photon rocket unless it was interacting with particles in the eject path, or if you are somehow in the blast zone of the light. – Shadowzee Aug 22 '19 at 1:44
• I would post a picture of a astronaut holding a flashlight, but apparently I can't put pictures into comments. – TheDyingOfLight Aug 22 '19 at 1:57
• Literally anything that can emit photons can act as a rocket in space for items of small enough mass, so "what it would take" to be useful is just a lot of photons. – stix Aug 22 '19 at 3:00

1 Answer

You can point any light source out the back of your ship and get some acceleration, but for all modern-day light-producing technologies the acceleration of the ship would be incredibly low. If you want to get decent acceleration your best bet would probably be matter-antimatter reactions powering a gamma ray laser, see the pdf of F. Winterberg's paper on the idea here and a shorter summary of his scheme here. The reason you want photons in the gamma ray frequency range is that the momentum $$p$$ of a photon is given by the equation $$p = \frac{hf}{c}$$ where $$h$$ is Planck's constant, $$f$$ is the frequency, and $$c$$ is the speed of light, so you want the frequency to be as high as possible so that the momentum added to the ship when it emits the photon (which is equal and opposite to the momentum of the photon, due to conservation of total momentum) will be as large as possible.

The nice feature of a photon rocket is that since the effective exhaust velocity is close to light speed (somewhat lower because the fuel won't be converted to photons with perfect efficiency, and the photons won't always exit exactly parallel to the axis of travel), according to the rocket equation this means the mass of fuel needed to achieve a given delta-v (increase in velocity over the acceleration period) is much smaller than for other types of rockets with lower exhaust velocities. Even so, you'd need a huge amount of fuel to get to a distant location like the center of the galaxy--if you scroll about 2/3 of the way down on the relativistic rocket page until you get to the section "How much fuel is needed?", there are some charts in that section about the mass of the fuel you'd need for every kg of payload mass to get to various locations, assuming an ideal photon rocket with exhaust velocity equal to the speed of light. To get to the center of the galaxy, if you want to accelerate at 1G for half the trip and decelerate for the second half so you arrive at low speed, the chart says you'd need 955,000 metric tonnes of fuel mass for every kilogram of payload mass (you could get by with less fuel if you used a smaller acceleration so your maximum velocity at the midpoint of the trip would be smaller, but this would extend the time of the trip--according to another chart on the page, 1G acceleration and deceleration allows you to reach the center of the galaxy in only 20 years according to your own clock, thanks to relativistic time dilation as you approach the speed of light).

A nearby star would be a lot more manageable though--to get to Vega 27 light years away, a photon rocket would need 886 kg of fuel mass for every kg of payload mass, and the whole trip would only take 6.6 years of time according to onboard clocks. For comparison, it says here that the Saturn V rocket that was used to get astronauts to the moon had a mass of 2.8 million kilograms, and that the payload was 43,500 kilograms, so that's about 64 kg of fuel mass for every kg of payload mass.

BTW, although the photon drive is an idea that's often discussed in the context of far-future technologies for interstellar travel, I looked for some info on the Minkovsky drive in Gundam and it doesn't actually seem to be one--the "laser propulsion system" section of this wiki page on Gundam technology says they just use a laser to heat the propellant, they don't fire the laser out the back of the ship to provide acceleration on its own.

• Using a laser to heat a propellant gives you a lot more thrust, but much lower exhaust velocity. It's a useful tradeoff -- for instance, if you don't want an engine that can burn holes in planets... – Zeiss Ikon Aug 22 '19 at 13:15