Can increasing the speed of light lead to more efficient photon rockets?

[Bab5space]

Radiation pressure that light provides is weak.

The photon rocket only gives thrust on par with rockets if you are willing to exhaust way more energy than normally used for rocketry (antimatter/matter annihilation). By that time your exhaust beam will be loaded with enough photons to nuke anything in it's wake.

An altrrnative for scifi would be to increase the radiation presssure by raising it's speed artificially, rather than simply throwing more photons out the back like a bomb.
Result? Flashlights without covers that exert radiation pressure on par with rpcket exhaust pressure.

 

[anorlunda]

An altrrnative for scifi would be to increase the radiation presssure by raising it's speed

Ah raising the speed of photons? You have heard about the speed of light, correct?

 

[Bab5space]

Ah raising the speed of photons? You have heard about the speed of light, correct?

Why would'nt I?

What I am saying is that theoretically, if the speed if light could be increased by a factor of oh I dunno... a zillion, then you would not need to generate as many to provide rocket-level thrust.

https://media.istockphoto.com/photo...=UleG3UwGPIAECkspNn8-OvmDabi7kTR2HrKFGulAXgU=

 

[russ_watters]

Why would'nt I?

What I am saying is that theoretically, if the speed if light could be increased by a factor of oh I dunno... a zillion, then you would not need to generate as many to provide rocket-level thrust.

Theoretically is reality: the speed of light is the speed of light is the speed of light. It is what it is.

 

[Janus]

Why would'nt I?

What I am saying is that theoretically, if the speed if light could be increased by a factor of oh I dunno... a zillion, then you would not need to generate as many to provide rocket-level thrust.

View attachment 251826

https://media.istockphoto.com/photo...=UleG3UwGPIAECkspNn8-OvmDabi7kTR2HrKFGulAXgU=

The problem is that even if you ignore Relativity's limit of c and use strictly Newtonian physics, increasing the speed of photon by a factor of a zillion involves increasing its energy by a zillion squared. If you were worried about nuking things in the exhaust path before, you just made it a lot worse.
The momentum of the exhaust is proportional to the its mass and velocity. So increasing an exhaust particle's speed by a factor of ten gives it 10 times the momentum. So you could get the same total momentum with 1/10 as many particles. However, the KE of those particles go up by the square of their velocity; increasing any given particle's velocity by a factor of ten increases its energy by a factor of 100. So 1/10 as many particles moving 10 times faster take 10 times the energy to get up to speed, and have 10 times the kinetic energy while not actually having any more total momentum.

While increasing the exhaust velocity increases the efficiency in terms of the amount of reaction mass you need to carry in order to reach a given speed, it has the opposite effect on the efficiency in producing thrust in terms of joules of energy expended.

Assume you have a 1 kg "payload" you want to get up to speed and a set energy budget of 10 joules to work with. That 10 joules can be used to either expel 10 kg of reaction mass at a speed of 1.414 m/s or a 1 kg reaction mass at 4.47 m/s.

in the first case, the delta v of the payload = 4.264 m/s
in the second case, it equals 3.16 m/s

For the same energy budget* you get a better result tossing a larger mass out slower than smaller mass faster.

When you expel reaction mass for a rocket, the energy doing so is split between the rocket and the reaction mass. The higher the velocity the exhaust has, the larger the fraction of this energy it carries away. *With a chemical rocket, the fuel and reaction mass are one and the same. Adding more fuel adds both more reaction mass and at the same time increases your energy budget.