Ion Engines in Sci-fi

[paladin]

Two websites, one on B5 and one on Galactica, state that both use a form of ion engine that accelerates the exhaust to near light speed to increase the amount of thrust the engine produces.

The reading I've done on ion engines says they're have low fuel consumption but their thrust is also low. Keeping the fuel comsumption the same, would accelerating the exhaust to near light speed have much of an effect on the thrust produced?

[Enlightenment]

The performance of sci-fi ion drives is unphysical. Materials and physics limitations do not allow ion drives to accelerate their exhausts to unreasonably high velocities nor will they allow for the acceleration capabilities shown in scifi.

[GrandMasterTerwynn]

Two websites, one on B5 and one on Galactica, state that both use a form of ion engine that accelerates the exhaust to near light speed to increase the amount of thrust the engine produces.

The reading I've done on ion engines says they're have low fuel consumption but their thrust is also low. Keeping the fuel comsumption the same, would accelerating the exhaust to near light speed have much of an effect on the thrust produced?

Here is how an ion drive works:

1 - Charge some particles (strip 'em of their electrons and make 'em positively charged.)

2 - Run these particles through an accelerator.

3 - Throw particles out back of ship, while neutralizing their charge so they don't lose momentum being pulled back to our ship which would be carrying a hefty negative charge if we didn't neutralize the charge.

Yes the individual particles come out the back of the engine going at really high velocities. Unfortunately you need the equipment to accelerate the particles to said high velocities, and you can't throw too many of them out the back at once due to that charge thing. As a result, an ion drive has piss poor acceleration charactaristics.

[SyntaxVorlon]

Acceleration is low, but ION engines are meant for LONG hauls, such as intersystem travel. Thus acceleration isn't a problem, just the time. That fact is why there will not be a peopled ship headed off into the reaches of deep space any time soon, the rocket scientists have to make a starship big enough to power a proportional Ion drive, and to support a human crew, which would require some form of large reactor that would work for centuries.

[GrandMasterTerwynn]

Acceleration is low, but ION engines are meant for LONG hauls, such as intersystem travel. Thus acceleration isn't a problem, just the time. That fact is why there will not be a peopled ship headed off into the reaches of deep space any time soon, the rocket scientists have to make a starship big enough to power a proportional Ion drive, and to support a human crew, which would require some form of large reactor that would work for centuries.

Very true. An ion drive has poor acceleration but high specific impulse. If you want a drive with high acceleration, you'd want something that can throw a lot of propellant out the back quickly and at high delta-v. Ideally you want something that gets up to a high velocity quickly and back down again. Antimatter rockets are cool . . . if dangerous . . .

[Crown]

Actually... there is an Ion engine that does expel it's propellant at the speed of light.... It's called a torch! And yes I am being serious, but do you see the problem? The expelled 'fuel' is so small, that thrust is negligable.

[Gunner]

How long would current ion drives have to be firing in order to accelerate a ship to the kinds of velocities (high fractions of C) where voyages to nearby stars could be accomplished in a reasonable portion of a human lifetime?

Let's use Alpha Centauri as an example. This system is 4 lightyears distant, so it would take 8 years if the ship was going at half lightspeed, 16 if it made one quarter, etc. How long would the ion engine have to be thrusting in order to get the speed up to such levels?

[kojikun]

long enough that 500 years down the road the ship would be about to ACentauri and still only at 0.1c

[Enlightenment]

Actually... there is an Ion engine that does expel it's propellant at the speed of light.... It's called a torch!

Torch = flashlight, for puzzled readers in the US and Canada.

Extremely bright lights are photon drives, not ion drives. The construction of the drive system is completely different in a photon drive and shares no commonality with ion propulsion systems.

[Sea Skimmer]

Actually... there is an Ion engine that does expel it's propellant at the speed of light.... It's called a torch!

Torch = flashlight, for puzzled readers in the US and Canada.

Crap, Europe has the battery! Quickly my North American comrades, we must begin hoarding technological advantages at once and make ready for war!

[SWPIGWANG]

Don't modern ion engines have such crap trust because it uses electric fields instead of uber magentic fields that is possible with some more advanced tech?

[SyntaxVorlon]

The reason real Ion drives use electric fields is because with a nuclear battery you can have reliable if slow superplanetery travel, that's why they use Ion drives on special satelites, the one that went to the asteriod Eros I believe.(If Eros became a planetery satelite, would they have to reshape it into a dogbone?)