Sci-fi engine thrust calculations

[jbriggs444]

Backing up a second - 1,020 zettawatts, what's that equivalent to in joules?

Does not compute. A watt is a unit of power. A joule is a unit of energy.

Power is a rate at which energy is expended. One Watt is one Joule per second. A 100 watt light bulb (back when they were incandescent) uses up 100 joules every second.

So one zettawatt is one zettajoule every second.

 

[Deleted member 690984]

Does not compute. A watt is a unit of power. A joule is a unit of energy.

Power is a rate at which energy is expended. One Watt is one Joule per second. A 100 watt light bulb (back when they were incandescent) uses up 100 joules every second.

So one zettawatt is one zettajoule every second.

If I have it right, then the engines would need 1,020 zettajoules? Assuming that the antimatter annihilations aren't just fired out as thrust but power some kind of sci-fi engine, so isn't a "photon rocket" so to speak.

Am I correct in that a half-gram of matter and a half-gram of antimatter would produce 21.5 kilotons of energy? I read that 1 kiloton is about 4.184 terajoules, so a 1g annihilation would produce 89.956 terajoules, correct? So for that kind of energy, you'd need 11,300,000,000 grams, or 11,300 metric tons, correct?

 

[jbriggs444]

If I have it right, then the engines would need 1,020 zettajoules?

Right. That's the 1.5 zettawats per engine times two engines times 300 seconds for a total of 1020 zettajoules.

Assuming that the antimatter annihilations aren't just fired out as thrust but power some kind of sci-fi engine, so isn't a "photon rocket" so to speak.

If it is not a photon rocket, what is it? Does it work by hurling out reaction mass? If so, it is a photon rocket.

If it works by grabbing hold of some sort of cosmic fabric and thrusting against it then that takes much less energy but it flies in the face of the principle of relativity. Something that physicists are unwilling to part with.

Am I correct in that a half-gram of matter and a half-gram of antimatter would produce 21.5 kilotons of energy? I read that 1 kiloton is about 4.184 terajoules, so a 1g annihilation would produce 89.956 terajoules, correct? So for that kind of energy, you'd need 11,300,000,000 grams, or 11,300 metric tons, correct?

E=mc^2. One gram is .001 kg. Times 3 x 10^8 squared is 90 terajoules. Yes.

But now you are mixing scenarios. If you have an anti-matter engine, why not throw photons out the back? Why carry inert reaction mass that will be thrown out the back when you could just haul some more antimatter and have yourself a photon rocket.

And if you are not using helium three fusion, why are you throwing reaction mass out the back with an exhaust velocity characteristic of helium three fusion?

 

[Melbourne Guy]

How about antimatter as a fuel? Say, deuterium, and antideuterium.

Are you intending that your ship is physically possible with current engineering and physics, @paulthomas? I'm assuming not, in which case you'll have to invent the parameters of your sub-light engines. In that regard, pretty much anything that requires reaction mass will be an engineering struggle, so it may be worthwhile considering reactionless engines and assigning them an acceleration value sufficient to get the ship moving to your top speed in a reasonable amount of time (which is likely hours rather than minutes or days). Or if you don't have artificial gravity, you can just set the acceleration to one gee and work out how long it takes to come up to speed.

 

[Deleted member 690984]

Are you intending that your ship is physically possible with current engineering and physics, @paulthomas? I'm assuming not, in which case you'll have to invent the parameters of your sub-light engines. In that regard, pretty much anything that requires reaction mass will be an engineering struggle, so it may be worthwhile considering reactionless engines and assigning them an acceleration value sufficient to get the ship moving to your top speed in a reasonable amount of time (which is likely hours rather than minutes or days). Or if you don't have artificial gravity, you can just set the acceleration to one gee and work out how long it takes to come up to speed.

No, the ship isn't physically possible with current engineering and physics. I'm trying to make it as grounded as possible, but there are still some super-technologies in there, such as the ship's warp drive.

I think I may have to go with what you suggested, go with a reactionless engine of some kind, as using any kind of reaction mass is just going to slam head-first into the wall that is physical engineering difficulties.

 

[Deleted member 690984]

If it works by grabbing hold of some sort of cosmic fabric and thrusting against it then that takes much less energy but it flies in the face of the principle of relativity. Something that physicists are unwilling to part with.

On that topic for a moment, I read that Dr. Harold White managed to reduce the amount of antimatter needed to 500kg to generate a warp engine that would take a ship to Alpha Centauri in just shy of 5 months. However, are there any figures for this? What I mean is, does this imply only 500kg are needed if the engine is in constant use for the full five months, so, a use of 100kg per month?

The ships in my world do have a warp drive, however, they are still limited by the light speed barrier - i.e., a ship's maximum speed is the speed of light, but the warp engine negates time dilation effects. How would this factor into energy usage?

 

[Melbourne Guy]

Dr. Harold White managed to reduce the amount of antimatter needed...

As far as I know, he's merely postulated a way to reduce the energy demands for an Alcubierre Warp Metric. It's all theoretical and maybe not even physically possible, and is as handwavium as you're likely to get for a sci-fi story, but in terms of how much antimatter etc., nobody knows.

...but the warp engine negates time dilation effects. How would this factor into energy usage?

However you need it to, really. This is not physics as we understand it operates in the universe, so you get to set the scene, @paulthomas. My only advice is to think your plot(s) as far ahead as possible in broad terms so you don't lock some ship attribute now that trips you up later.

 

[Deleted member 690984]

Thanks @Melbourne Guy and @jbriggs444 for your help in this area! It's definitely been most helpful!