Impulse drive, how fast is it?

Throughout star trek we see various ships travelling at fractions of 'Impulse drive'. An impulse is a unit of force and thus converting to speed is difficult.

Here are some approximations I found on the memory alpha article on Impulse Drive:

In The Motion Picture, The Enterprise traveled at warp 0.5 ... or roughly 1/3 light speed

No mention of what fraction of impulse this would be though.

According to Jo'Bril in the episode "Suspicions", the shuttles aboard the Enterprise-D had a maximum impulse velocity of approximately 2.5% of light speed – he specified that at ¾ impulse the shuttle would travel a distance of one million kilometers in approximately three minutes (approximately 12,400,000 miles per hour)

12,400,000 miles per hour is 0.01849c making 1 Impulse drive 2.46% c. But is this inconsistent with the above value? 1 Impulse = ~2.5%c = 0.5*(1/3*2.5%) = warp 0.004. Is that the same for all of the ships?

A reference made in "Fair Haven" indicated that USS Voyager's impulse power would not be enough to outrun an approaching neutronic storm that was traveling at a velocity of 200,000 kilometers per second (447,387,258 miles per hour), or roughly 2/3 the speed of light. However, it is also commented in "Timeless" that at full impulse, Voyager could travel at roughly 80% light speed.

Now we have an inconsistent source (1 impulse <2/3 speed of light AND 1 impulse ~8/10) that disagrees with the above (1 impulse is 1/40 speed of light).

Is this inconsistancy explained by the differing inertias required to change the momentum of the space ships (I know, it's in space, but how else would an 'impulse' drive work?)?

Is impulse drive an absolute measure, and If not what is it relative to?

  • 7
    My understanding was that "impulse speed" was whatever maximum speed could be obtained by a ship's available impulse drives (fusion reactor pushing out plasma). It is always sub-light speed, and it is likely different for every ship (size, number of drives, etc.). Sep 5, 2012 at 20:42
  • 4
    In space any propulsion would translate to an acceleration, not a constant speed. Anything that could brake the ship is most likely negligible, so the whole concept of "impulse speed" doesn't make literal sense. It makes more sense to call it "impulse acceleration". The resulting speed should therefore increase linearly with time (classically). Typical suspension of disbelieve is in place.
    – bitmask
    Sep 5, 2012 at 22:35
  • 1
    It would have to be limited at some point to avoid relativistic effects.
    – NominSim
    Sep 5, 2012 at 23:10
  • 2
    The original series gives the impression that they use impulse to accelerate up to (nearly) the speed of light, and then warp to go faster (warp 1 being the speed of light). Which doesn't make much physical sense, since without some wibbly-wobbly spacey-wacey stuff it would take infinite energy to approach light speed. In Elaan of Troyius, they traveled from one planet to another at "sublight factor .037". I believe the first explicit use of "warp" for sublight speed was in The Motion Picture, when Kirk ordered "Ahead warp point five". Sep 6, 2012 at 0:28
  • 1
    @Pureferret - I understood "half impulse" meant "one half of maximum impulse", but my point was that although I (and bitmask) thought it would seem more natural to treat these as fractions of some maximum acceleration, it seems from the lines of dialogue above that the are fractions of some maximum impulse velocity. This just seems a little weird to me since in space, there's no friction or air resistance to keep you from going faster and faster as long as you have fuel to burn, but I guess "maximum impulse" could be more like a self-imposed rule than a limit of the impulse engines.
    – Hypnosifl
    Feb 3, 2015 at 13:25

5 Answers 5


In the official Star Trek Universe, outside of TOS, it is implied strongly that Impulse Drive only works at sub-light speeds. The top impulse speeds seem to vary by ship, and "Full Impulse" seems to be whatever the peak designed safe impulse speed is.

Several TOS references, however, imply that an impulse drive can be used for FTL travel. It is most clear in Balance of Terror that the ship has to have something more than slower-than-light travel.

Extended Universes

The extended universe, including the tabletop role-playing games, gives full impulse a specific speed, usually varying by timeframe. In the Decipher Star Trek RPG, maximum sustained impulse speeds are around 0.75C (75% of the speed of light).

In the Star Fleet Universe line of Games (Star Fleet Battles, Federation and Empire, Prime Directive), which are based upon TOS and TAS but developed differently from the core Trek Universe, such FTL impulse speeds as are implied by Balance of Terror are given a name: Non-Tactical Warp. They rely only upon impulse drives, and are capable of speeds up to about warp 5.5. (GURPS Prime Directive, page 160.)

Canonicity of FTL Impulse

It is not clear whether the reference in Where No Man Has Gone Before refers to colonies out to the edge of the galaxy, at which point the barrier might be only a few light years out, or to bases further in. It is clear that the ship is travelling on impulse drives at speeds approaching C, in that Gary isn't shown jumping in power massively as they cross 5 light-days distance on impulse drives.

It also is implied in that same episode that SS Valiant used impulse drives for deep space travel. It seems to be sloppy dialogue as much as anything else.

Combined with Balance of Terror, it's pretty clear that "Impulse Drive" is at worse high sublight. The logical requirements of a "pure impulse" vessel race making war upon a warp-capable race across a multi-light-year neutral zone pretty much requires some form of FTL integrated into the Impulse Drives.

Interpretations of Canon

Steven Cole of Amarillo Design Bureau interpreted the canon to include "FTL." FASA didn't in writing their first Star Trek RPG. Both predate TNG, where it's clear that FTL travel is Warp Drive. Both also provided for FTL combats, unlike the later series, based upon TOS and TAS evidence. In this case, it's best not too look too deeply at canon, as what you find is "Speed of Plot."


According to page 78 of the TNG Technical Manual: normal impulse is limited to 0.25c for time dilation concerns. 0.75c for high impulse is not recommended (p75) as it involves lengthy recalibration of the onboard clocks.

A maximum of 0.92c is in the design spec on page 2.


In your answers and calculations you assume a linear scale; "Impulse 0.5" = 0.5 * "Impulse 1". As we know, the Warp scale is non-linear; it's cubic or even - to be more exact - 10/3. Warp 2 = 10 * Warp 1. (only true for Warp factors between 1 and 9) Let's assume the Impulse scale is non-linear as well. Let me propose the following formula: "Impulse factor" ^ 13 = factor of c. Impulse 0.75 = 0.75^13 c = 2.3% c. With this non-linear scale "Normal" Impulse is roughly 0.9, "High" Impulse is 0.98 and "Max" Impulse is 0.99.


I've always thought this was holdover from (sea) ships with Engine Order Telegraphs, which would signal "Half Ahead" or "Dead slow astern" etc. The actual speed relating to the specific ship.



In "The Cage" and "Menagerie" Pike specifically tells the crew "Time Warp Factor" instead of "warp factor". Possibly it was shortened to plain warp factor by the series.

In "The Cage" and "Menagerie" Lt. Tyler also says that the "time barrier" his been broken and new ships are faster. Thus it would seem that the "space warp" discovered by Cochrane ("Metamorphosis") was used in the original warp drive but newer warp drive also has a "time Warp".

In "Where No Man Has Gone Before" the "space Warp" was disabled but it seems certain that the Enterprise was still able to use the "time warp" generators to either ONE) travel several light days at faster than light speeds or to TWO) travel several light days at slower than light speeds but slow down the passage of time aboard so that less than a single day passed aboard, or THREE) possibly both.

This "time warp" ability explains how doing something new and different with the engines sent the Enterprise back in time in "The Naked Time".

If the "space warp" in warp drive is similar to the theoretical Alcubierre Warp Drive it would warp space around the ship and created a warped bubble of space time to propel the ship faster than light as seen from outside the warp bubble.

The "time warp" which was introduced about 13 to 31 years before the first season of TOS may have been used in conjunction with the "space warp" to either one) make interstellar travel seem faster to outside observers, or two) make interstellar travel seem faster to the crew aboard the ship or three) do both.

When the Enterprise's "space warp" capability was disabled in "Where no Man has gone before" the speed of the Enterprise was dropped to just a few percents - or even less - of its previous speed.

Bu the Enterprise was still apparently able to use its "time warp" capability along with the impulse engines to travel a distance of several light days while only a few hours pass on the ship (and possibly also in the outside universe). The alternative is to imagine that the Enterprise's impulse engines could achieve faster than light speeds.

Nobody seems to believe that the TOS Enterprise had a faster-than-light impulse drive or that it could warp time with its engines, but the dialog in "Where No Man Has Gone before" seems to prove that one or the other, or possibly both, is correct.

If, as some people think, different generations of warp drive were described as "space warp", "time warp", "transwarp" etc., it is possible that an early form of warp drive was called "impulse warp drive" for some technical reason, and by Kirk's era "impulse warp drive" was sometimes called "impulse drive" (confusing it with the current impulse drive) and sometimes called "warp drive".

Or it is possible that impulse drive was always totally different from warp drive and that at one time impulse drive was a rival and alternate form of faster-than-light drive but by Kirk's era warp drive was clearly superior for faster-than-light travel and impulse drive was only used for slower-than-light travel and the impulse engines used were no longer designed to go faster than light.

David Winfrrey in "Falling Out of Standard Orbit" in The Best of Trek # 16, 1991, is one of several writers who have pointed out contradictions between common theories of treknology and the evidence of the episodes. Winfrey cites events in several episodes and movies to claim that impulse drive must still be capable of hyperlight speeds even during the eras of Kirk and Picard.

He ends with a quote from The Next Generation episode "Conspiracy":

Riker: "Increase to warp six." LaForge: "Aye, sir. Full Impulse."

If LaForge made a mistake, maybe he meant to say "full warp". But full warp for the Enterprise D is over warp factor nine in the new scale so he couldn't go to full warp to reach warp six.

Perhaps the various warp factors of the Enterprise D were combinations of space warp and time warp and LaForge increased either the space warp or the time warp to full strength in order to reach warp six and so meant to say either "full space warp" or "full time warp" when he said "Full impulse".

Or perhaps the Enterprise D uses combinations of warp drive and impulse drive to reach various warp factors and LaForge increased the impulse drive to full power to achieve warp factor six. Then LaForge was correct to say "Full impulse" and the next generation Technical Manuel is incorrect about how the Enterprise D functions.

  • 3
    This doesn't address the question asked, except tangentially. If anything, you seem to be answering a completely different question ("why is it called Warp Drive", perhaps?)
    – Valorum
    Mar 25, 2015 at 21:55
  • Ricard - I cited a source which claims that there are examples of faster-than-light travel with impulse power in TOS and TNG. I also showed that in "Where No Man Has Gone Before" the Enterprise either travels faster-than-light with impulse drive or else slows down the passage of time aboard with a time warp, or perhaps even does both. For anyone who denies the use of time warps, "Where No Man Has Gone before" PROVES that the Enterprise can travel fast-than-light with impulse drive. Mar 28, 2015 at 5:15

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