Flare Sci-Fi Forums: Impulse speeds

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Posted by Bernd (Member # 6) on :

Does anyone have a good definition of "full impulse" or "half impulse"? Is it referred to the speed of light or another physical quantity, or just a convenient yet arbitrary unit?

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Posted by David Sands (Member # 132) on :

Full impulse (I believe) simply refers to the highest non-warp speed of a starship, which, of course, varies by class and individual ship. Speeds above .75c are possible, but usually avoided for energy and relativistic considerations.

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Posted by Bernd (Member # 6) on :

I think I've read something like that in the TNGTM, yet, it was not satisfactory. Maybe there is some kind of cut-off speed above which the efficiency of the impulse drive drops significantly, keeping in mind that a subspace field is used to facilitate propulsion, so it is already a non-Einsteinian propulsion. Another, more "engineer-like" explanation is that "full impulse" refers to the nominal power output or consumption of the impulse drive and not to the speed.

Posted by Sol System (Member # 30) on :

I think that it has more to do with acceleration than velocity. Full impulse means opening the engines up all the way, and accelerating very quickly. Half impulse is, well, one half that acceleration, and so on. The actual speeds reach probably vary from ship to ship and situation to situation. When Kirk orders one quarter impulse in spacedock, he can't mean one quarter of their total power. Otherwise I would imagine very bad things would happen to the place.

Instead, the actual speeds are determined by the helmsman offscreen, like many other minor tasks.

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Posted by Baloo (Member # 5) on :

<unsure>I believe I have read somewhere that "Full impulse" is arbitrarily set at 1/4 lightspeed to avoid excessive relativistic effects.

Full impulse should not be confused with maximum sublight velocity. I presume that all military vessels retain some excess capability for emergency purposes.</unsure>

--Baloo

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[This message was edited by Baloo on June 01, 1999.]

Posted by Elim Garak (Member # 14) on :

Yes, many sources do seem to indicate that it's one quarter light-speed. Not that I always trust those sources!

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Posted by Bernd (Member # 6) on :

I think no matter how much full impulse actually is, a quarter impulse is definitely not possible inside the spacedock, at least not for more than a few microseconds

Posted by David Sands (Member # 132) on :

Well, again, if impulse was referred to as increments of the ship's maximum speed, 1/4 impulse in STII may be just fine, but by TNG it may have been incresed with more efficient energy-saving engines that can afford to go faster for longer periods of time.

I do agree with you, Bernd, that there is almost surely a cutoff point where it just isn't very efficient to try and go faster. That's what I think would be considered full impulse.

It may be that helmsmen know enough in non-tactical situations to realize that when the captain orders full impulse, they are being told to take the ship to a speed that avoids relativistic considerations if it were to be used for any extended period of time. That may be the origin of the "typical" "full impulse" speed of .25c.

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"Warfare is the greatest affair of state, the basis of life and death, the Tao to survivial or extinction. It must be thoroughly pondered and analyzed."

"...attaining one hundred victories in one hundred battles is not the pinnacle of excellence. Subjugating the enemy's army without fighting is the true pinnacle of excellence."

-Sun Tzu, The Art of War, 6th century B.C.E.

Posted by Bernd (Member # 6) on :

So "full impulse" is the maximum *useful* impulse speed. This might take into account ship specific limits as well as relativistic considerations.

Posted by TSN (Member # 31) on :

I think we have to assume that anything that refers to "relatavistic considerations" is just plain wrong. For one thing, it wouldn't make sense for the ship to not have a warp field at impulse. Not only would it remove time dilation, it also makes the ship's relative mass go down, making it easier for the engines to move the ship (à la moving DS9 in "Emissary"). Besides, if time really did dilate every time they used the impulse engines, everything would be all screwy.

Captain: "We have to be there in half-an-hour, or everyone will die. How long will it take to get there at full impulse?"

Ensign: "About five minutes, sir."

Captain: "Good. Make it so."

*five minutes later*

Captain: "All stop. Scan for lifesigns."

Ensign: "They're all dead, sir."

Captain: "Dead? But they shouldn't be dead for 25 more minutes!"

Ensign: "No, sir, it was five minutes for us. It's been an hour on the planet."

Captain: *thwaps the ensign*

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Posted by Cargile (Member # 45) on :

Everything should be screwy. The Trek writers are not very good at writing HARD sci-fi. One quarter of the speed of light is the fastest you can go while incuring the least impacting tau factor. Point 25 should be the standard Full Impulse velocity. Anything higher (without the majicaly subspace field to reduce the tau factor) should be considered emergency speeds where life is more important that time dilation effects.

At .25 the tau factor is 5.164 minutes.
At .5, it is 5.7 minutes.
At .75 it becomes 7.56 minutes.
At .98 it is 25.13 minutes.
At .99 it is 35.44 minutes.

Why go .98c when you can go Warp 1 and shave off a few minutes?

75,000 kilometers per second is still pretty fast.

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Posted by TSN (Member # 31) on :

What do those tau numbers mean? Is it the objective equivalent of a subjective minute? And what's the equation?

BTW, if they didn't use a subspace field to counteract relativistic effects, would the Fitgerald-Lorentz contraction at full impulse be enough that we should see it on screen? If so, it's another arguement for why they must be using the field...

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Posted by Bernd (Member # 6) on :

I assume the tau factors are the dilatation if the normal time is one hour(?).

The TNGTM mentions the possibility of using a warp field to alleviate sublight propulsion, but it is argued that the warp coils do not work efficiently at <1c. The impulse drive has subspace field coils too, but at sublight speed the time dilatation is not supposed to be decreased. The ship's apparent mass, however, is lowered. I have to look it up again, right now it sounds contradictory.

Posted by Montgomery (Member # 23) on :

*head explodes*

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Posted by TSN (Member # 31) on :

Well, onscreen evidence, while not coming right out and saying it, pretty much shows that the writers don't consider time dilation a problem that starships face, so I think we can safely assume that it doesn't happen...

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"Flying Thompson's Gazelle of the Yard!"
-Inspector Fox, the Arguement Clinic sketch, Monty Python's Flying Circus

Posted by Bernd (Member # 6) on :

I wonder why the TNGTM says warp drive is not efficient at sublight speed so the impulse engine needs its own field generators. Maybe Sternbach feared someone could question the impulse drive on the whole.

Posted by JC2astro on :

Bernd is correct.

Here's your "Einstein's Theory of Relativity fact of the Day:"

The "Tau" factor is equal to the square root of the quanity of one minus the ratio of the relative velocity squared and the speed of light squared, or:

Tau * Tau = 1 - (v * v)/(c * c)

(simplify and solve for Tau gives the equation described above. I can't write a square root or squared sign).

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Please excuse the bad spelling.

Posted by TSN (Member # 31) on :

Ah! That equation! *thinks he just saw that one recently when reading about the Alcubierre space warp* However, I thought it was something else... Hm...

Oh, and no-one ever answered my question about the contraction the ships would experience at those speeds. Would it be enough for us to see on screen?

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"Flying Thompson's Gazelle of the Yard!"
-Inspector Fox, the Arguement Clinic sketch, Monty Python's Flying Circus

Posted by TSN (Member # 31) on :

Okay, I figured out what I was thinking of. The expression I saw was 1/[l-(v/c)²]^1/2 which does look a lot like that tau equation...

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"Flying Thompson's Gazelle of the Yard!"
-Inspector Fox, the Arguement Clinic sketch, Monty Python's Flying Circus

[This message was edited by TSN on June 07, 1999.]

Posted by Bernd (Member # 6) on :

Yes, it's the same factor. I think tau is used for convenience because it can be written: t'=t/tau, where t' is the dilated time (observed outside the ship) and t the time inside the ship.

The reciprocal factor applies to the length contraction. It should be visible at full impulse speed, but only for a ship passing by which would be too fast to be perceived anyway. It's been some time ago I've read a book where relativistic effects already become apparent at a few km/h ("Mr. Tomkin's..."?). Quite funny when a passing bicycle is flattened, or if you get on a train, travel a few hours and reach your destination years later.