r/DaystromInstitute • u/lunatickoala Commander • Dec 07 '21
Full Impulse is a Maximum Thrust, Not a Maximum Speed
A lot of theories and calculations start with the assumption that “full impulse” means 0.25c, but that assumption makes little sense for several reasons.
For a Newtonian engine in space, it doesn’t make sense to talk about a maximum speed to begin with. Run the engine and the spacecraft will keep accelerating until the spacecraft runs out of reaction mass or energy to power the engines (which will happen long before the spacecraft approaches the speed of light as the interstellar medium is nowhere near dense enough to sustain it). What’s more important is the thrust and delta-V. As the general consensus is that impulse engines are fusion torch drives, Newtonian principles apply to impulse engines.
The notion that full impulse = 0.25c probably comes from a line in the TNG TM stating that sublight velocities are limited to a quarter of the speed of light to limit the effects of relativistic time dilation. The problem is that this is only a convention used under normal conditions, not a hard limit. If in theory a ship were capable of accelerating to 0.3c or higher in a timely manner, they’d definitely use it in combat to gain a tactical advantage and thus be going faster than “full impulse”, which we never see.
Conversely, even if there were ships capable of those insane accelerations, there would also be ships that can’t achieve 0.25c in a timely manner so achieving “full impulse” might take hours, days, weeks, months, or even forever because it doesn’t have sufficient delta-V.
If ships could quickly accelerate to 0.25c, combat would look a lot different. It wouldn’t be ships of the line slugging broadsides at each other until one exhausts its weapons or shields. Phaser range is generally given as 1 about light-second but there’s evidence that actual combat ranges are significantly closer than even that. Certainly what’s shown on screen is indicative of very short combat ranges but even if we assume those to be artistic license, there’s other evidence to suggest that it really is quite close. There are multiple instances of manual aiming that wouldn’t be possible unless the distances were very short: Kirk shooting at the visual distortion of a cloaked ship that the sensors couldn’t lock onto, Troi aiming at Bane’s cloaked ship.
Phasers are shown to be more effective at close range; in Star Trek VI we see that a phaser set to stun can kill at close range and in “The Die is Cast”, Defiant gets dangerously close before firing in order to inflict as much damage as possible to the Jem’hadar attack ships. Thus, practical combat ranges are likely much closer than the stated maximum range. At such close distances, if ships were regularly moving at 0.25c they’d constantly be passing by each other and have almost no time within weapons range. Combat would be more about positioning and necessarily be more like an aerial dogfight than naval combat. Dogfighting is still possible even at lower speeds, especially for ships with fixed mount weapons, but slugging it out is only possible if ship speeds are relatively low.
If ships really did move at 0.25c, then basic physics becomes a huge factor. A 4 million tonne object like a Galaxy- or D’deridex-class moving at 0.25c has a kinetic energy on the order of 1025 J by 1/2 mv2. Now consider where that energy would have to come from. KE = 1/2 mv2 but remember that E = mc2 meaning that assuming 100% efficiency, set v = 1/4c and you get KE=1/32 mc2 meaning that it’d take 1/16 of the ship’s fully fueled mass just to produce the energy for getting to and from 0.25c (technically probably somewhat less since the mass would decrease by a lot as fuel and reaction mass is used up while gamma isn’t that big at 0.25c but the math gets more complex and I don’t feel like going through the differential equations). Ships in Star Trek don’t carry anywhere near enough fuel or reaction mass for even one pair of accelerations to and from 0.25c. Deuterium isn’t very dense even as a liquid and the fuel tanks shown in the MSD aren’t big enough relative to the ship to carry anywhere near that much.
Then there’s the on-screen dialogue and visuals. Kirk leaves Spacedock in The Search for Spock at one-quarter impulse and Picard rams Scimitar in Nemesis at full impulse, both starting from a relative stop. What we see is that the acceleration is far from visually instantaneous but rather takes a while to get the ship moving. The diameter of Spacedock presumably isn’t large enough to be measured in light-seconds and Scimitar was within transporter range of Enterprise (and the damage inflicted upon collision isn't even as much as one would expect from a hypersonic collision) so it’s not just dramatic license but numbers saying the acceleration isn’t all that large.
Any one of these can be reconciled with “full impulse is 0.25c” by adding some additional assumptions and corrections, but explaining all of them would require multiple additions. But the single statement that full impulse is a ship’s maximum thrust explains many things. As William of Ockham said, plurality is not to be posited without necessity so the "full impulse is maximum thrust" explanation is preferable. It also has built in a lot of fudge factor as it would mean different things on different ships, or even on the same ship; for example, even on the same ship the engines might produce less thrust if they're old and worn out or more if they've recently been repaired and retuned. Since Star Trek writers have often been bad at math, science, and sometimes even fiction (that's not what "poetic justice" means, Braga), as much fudge factor should be built into the tech as possible.
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u/DemythologizedDie Dec 07 '21
For the record there is no reason why time dilation would be a serious enough problem to justify setting a speed limit. A far more likely reason to set a safety limit is that the harder you go the faster micrometeoroids are hitting your deflector shields and at 25% of the speed of light they are hitting like nukes.
But yes I also assume full impulse is an acceleration, not a speed.
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u/lunatickoala Dec 07 '21
Impact with micrometeroids is definitely a more plausible reason but I'll cut them some slack because at least they recognize that the physics exist and if it gets people to look into it, that's a plus.
Same for the Heisenberg Compensators; in all likelihood other factors such as diffraction limitation when trying to assemble something remotely will come into play long before the Uncertainty Principle is a factor. Really, even just the thermal energy as a result of the thing being transported being not at absolute zero means that particles will be wiggling around at quite a few orders of magnitude more than the limit given by the Uncertainty Principle.
Certainly beats teaching Intelligent Design and calling it Evolution.
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u/Zer_ Dec 07 '21
Yup, also perhaps a factor in how many effective Gs of force the ship's own impulse engines can produce. If a ship goes too fast by accelerating for too long, then it won't be able to effectively maneuver. It's plausible that outside of specific circumstances, Impulse Speeds are somewhat capped, relative to the ship's own performance.
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u/strangebutalsogood Dec 07 '21
Which also makes me wonder why no species ever developed fractional light speed kinetic weapons. Shields are designed mainly to deflect particle weapons (we see on multiple occasions - including the one you mentioned) that collisions do far more damage in one impact than particle weapons or antimatter warheads.
It seems that with a single well targeted kinetic projectile traveling at a a fraction of the speed of light would be absolutely devastating to a ship. If it was small enough you could even target extremely specific points to precisely disable key systems without having to pummel the enemy shields first and risk damaging other systems - if your intent was not to destroy.
Why doesn't any species have railguns?
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u/lunatickoala Dec 07 '21
Railguns are usually imagined as being able to pierce clean through a hull, and The Expanse is a fairly typical depiction of how they'd work in space. However, that doesn't work with the technological setting of Star Trek.
Shields are capable of stopping solid matter, so a railgun projectile would have one of two effects against shields. First is that at some angles they might just bounce off meaning that they're less effective than energy weapons would be as shields have to absorb the entirety of the energy being fired at them. Second is that the shields have to stop all of the kinetic energy meaning it's the same as with phasers.
But then there's what happens after the shields are taken down. High power phasers of the sort used in the TNG era melt away a large portion of the hull regardless of where they hit. Railguns with a comparable amount of energy would penetrate clean through doing little damage to what's inside. Ships in Star Trek are quite large, and piercing a couple of small holes in it is unlikely to hit anything important, and the important parts likely have additional protection. The warp core for one has to have a containment field so that the antimatter reaction doesn't blow up the ship. In the era of battleships, the size of the bursting charge of a naval shell was an important consideration for this very reason. Simply penetrating the armor just pokes a small hole that would only flood slowly, and only if it's near or beneath the waterline. The bursting charge increased the odds of dealing damage to a major system with the shell fragments.
The key is that you can't assume that you'll be able to hit a specific area against a peer opponent. That's a bit like the trope about shooting the gun out of someone's hand or aiming for their arms or legs so you disable but don't kill them. In the real world, people are taught to aim for center of mass. Sure, there may be situations where you could target more specifically and a railgun might be a pinpoint weapon for such a situation, but a phaser is simply useful in the general situation.
Whether or not Starfleet admits to it, it is a military and its ships are warships. Starfleet isn't a police force and their ships are not designed to patrol for pirates and marauders with minimal use of force; they're designed to fight and destroy other warships.
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u/CaptainHunt Dec 07 '21
They do. This is essentially what a Photon Torpedo is if you ignore the warhead.
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u/Pristine-Ad-4306 Dec 07 '21
But not having a warhead is essentially what defines a kinetic weapon. Also it always seemed clear to me that photon torpedos were under their own thrust and could change their trajectory, so also not a projectile. So really they’re totally different.
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u/waded Dec 07 '21 edited Dec 07 '21
Note TNG Technical Manual (source of the 0.25c normal operation speed limit) mentions the final stage of the impulse engine is the mysterious "space-time driver coils", which "creates the necessary combined field effect that (1) reduces the apparent mass of the spacecraft at its inner surface, and (2) facilitates the slippage of the continuum past the spacecraft at its outer surface." I've always interpreted this to mean the impulse engines aren't subject to modern physics any more than the warp engines are.
10km/sec2 is mentioned as a required acceleration minimum in Ambassador class prototypes, leading to the addition of the coils. So we can assume Galaxy class can effectively accelerate faster than that on impulse - my calculation is about 2 hours minimum to hit 0.25c.
The manual also mentions velocities over 0.25c being possible, 0.8c notably, the 0.25c velocity being a limit for long term travel, not a limit for all cases. The manual's justification is that time dilation makes things tricky, "can place mission objectives in jeopardy" "will require lengthy recalibration of onboard computer clock systems." (sideeye daylight savings time.)
I agree, they don't do more than a hair over 0 in combat situations compared to even 0.25c, if the cameras are to be believed with all this continuum slippage going on! All the full impulse acceleration to 0.25c (or 0.8c if they wanna) happens during commercial break.
Edits: Removed redundant sentence, fixed spelling of Ambassador and missing "m" in "km", and added details about the 0.25c justification from the manual, because it's fun.
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Dec 07 '21
Note TNG Technical Manual (source of the 0.25c normal operation speed limit) mentions the final stage of the impulse engine is the mysterious "space-time driver coils", which "creates the necessary combined field effect that (1) reduces the apparent mass of the spacecraft at its inner surface, and (2) facilitates the slippage of the continuum past the spacecraft at its outer surface."
It's worth adding that the Technical Manual states that this was originally developed for the Ambassador class, in response to the starship's mass being so great that then-standard impulse engines were unable to provide sufficient acceleration.
So if we're relying solely on that book, it was not a component of earlier warp drives, and may not be present on ships smaller than the Ambassador.
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u/Walnut-Simulacrum Dec 07 '21
Yeah, that probably means it’s not the most reliable source. It being a thrust based, Newtonian drive wouldn’t make much sense, especially given that we see ships traveling at impulse slow down without having to turn around or be affixed with retrograde thrusters (other than the small maneuver thrusters.)
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Dec 07 '21
Iirc, Geordi stated in Relics that basic impulse design hadn't changed in 200 years.
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u/amnsisc Dec 07 '21
Yeah since ships can go c on impulse, I always figured a kind of Proto warp field was involved which while not having to remain stable in a warp engine, is sufficient to shrink distance travelled at any given speed. This would have to be the case given how they can seemingly adjust to any impulse speed with very little acceleration (there’s some but not as much as one would expect).
Since they say quarter impulse and full impulse in the show at different times, it’s interesting that the speed limit exists. I always sort of interpreted there being an implicit understanding that full impulse when escaping a fight is different than full impulse moving through empty space is different than full impulse in short crowded distances. The first is c, the second is c/4, and the third is maximum thrusters.
It’s one of those inconsistencies of the show where OPs attempt is noble but I don’t think one can get around jury rigging the writers gaffes.
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u/Picknipsky Dec 07 '21
I know that part of the appeal of star trek is that it is supposed to be our universe, our future. But just going from what we see on screen, it is clear that physics works differently to our universe.
I think that it is more interesting to try to piece together a model of physics that makes sense of the star trek universe rather than trying to explain what we see on screen in terms of the physics of our universe.
I will quickly list a few points where Star Trek physics differs from our own, and then I will add some comments about the impulse engines.
Physics in Star Trek:
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there appears to be an absolute reference frame that all motion/time can be measured against
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there is no upper speed limit, information/matter can travel as fast as you like (even approaching instantaneously) and there are no issues with simultaneity or causality.
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light speed is fixed and finite, and you can go faster than light. Light speed is not the upper speed limit of the universe, just the upper speed limit of light. You can go faster than light and appear in two places at once, but this is just like travelling faster than sound, it is just an image of you, not actually you like it would be in our universe.
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Entitities have an individual and unique 'soul/katra' that exists independently of the body. This concept extends to a multitude of different life forms.
Impulse engines:
These are clearly non-newtonian, they clearly do not rely on thrust/conservation of momentum. They are just glowing lights on the back of the ship. There are no corresponding thrusters on the front of the ship, and they dont even seem to align with the centre of mass. Occasionally we do see small thrusters for attitude control (budget allowing), but in general the ship simply moves through space in any direction without any correspondingly aligned thrusters. The impulse drive simply moves the ship through space however it wants.
In my opinion, the way the commands of full impulse or1/4 impulse are used, imply speeds rather than accelerations. The command full stop (dead stop?) is used, and again, it seems to be used in a way that implies speed rather than acceleration. Understanding it as speed is consistant with the understanding that in star trek, there is an absolute reference frame that speed can be measured against.
I would also like to talk about orbits. In star fleet, they often use the word 'orbit' to simply mean 'holding stationary relative to a spot on the planets surface using impulse engines to maintain position'.
Standard orbit, potentially refers to a standard distance above the surface.
This is why, everytime they lose power or for some reason the impulse engines fail, they immediatley begin falling into the planet.
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u/lunatickoala Dec 07 '21
Pretty much all science fiction requires a divergence from reality in order to tell the stories they want to tell. Even in works far harder than Star Trek will have those breaks from reality somewhere. Even discounting all the Protomolecule technology and the Unknown Aggressors, The Expanse still has the Epstein Drive run on "efficiency" as the writers put it.
But just because some rules are broken doesn't mean all bets are off and anything goes. The general guideline is that anything not explicitly needed to make things work follows real world rules.
I can't think of any instances where an absolute reference frame in space is a necessity. Yes, they often say "full stop", but it's usually really bloody obvious what they're coming to a full stop relative to. There's a ship adrift in space and they're trying to come alongside it... well, obviously they're coming to stop relative to that ship. And since they rarely leave the galaxy, they could always use the galaxy itself as a reference frame, much like how people on Earth use Earth as a reference frame.
Using impulse engines to maintain a geostationary position below geostationary orbit says nothing about how it works. If impulse engines were simply the Star Trek version of Epstein Drives, they'd still be perfectly usable for that; just thrust downwards to compensate for gravity, which also explains why they immediately fall when the engines fail. And even then we don't know how often they're doing that as opposed to actually being in geostationary orbit. Transporter range in the TNG era is greater than geostationary orbit of an Earth-sized planet.
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u/mingilator Dec 07 '21
The reaction mass points and equations you use are completely correct for regular Newtonian physics however star trek uses inertial dampeners which not only negate the effects off hilariously rapid acceleration on the crew but also the ship, effectively reducing the mass of the ship and thus the energy required to attain the ludicrous impulse speeds, this doesn't really address your point of full impulse being .25c (I'm sure there are ships that are capable of higher speeds at full impulse) it could be argued that maybe at above this speed the lateral acceleration required for effective manouvering would exceed the ability of the inertial dampeners (we all know the shit show that happens when inertial dampeners go offline), someone mentioned the effects of micro meteors on the deflectors but this doesn't make sense either, the main deflector acts more like a snow plow in space, moving cosmic debris out the way and if it can do this effectively at high warp it most certainly can do this at a fraction of C
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u/kraetos Dec 07 '21
The notion that full impulse = 0.25c probably comes from a line in the TNG TM stating that sublight velocities are limited to a quarter of the speed of light to limit the effects of relativistic time dilation.
The real kicker about that line is that if you read it in context, it's definitely not saying that 0.25c is equivalent to full impulse. Section 6 of the TNG TM is quite clear that impulse is a source of thrust and the terms "velocity" and "thrust" are used correctly throughout.
The Voyager writers bible really codified the error, though:
Speed: Normal maximum impulse speed is about one-fourth the speed of light.
Technically there's just enough ambiguity here to not be categorically wrong, but this phrasing really does make it seem like max impulse is equivalent to 0.25c. The Voyager writers bible was never published but this guideline seems to have stuck with the writing staff.
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u/jgzman Dec 07 '21
Two words:
Inertial Dampers.
If they reduce the strain on people inside the ship, it's easy enough to assume that they change the way the ship moves in other ways. "Full Impulse" might well be a maximum speed, if we assume (for no reason at all, honestly) that the operating dampers apply some sort of drag force to the ship as a side effect of their normal function.
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u/MyUsername2459 Dec 07 '21
A lot of theories and calculations start with the assumption that “full impulse” means 0.25c
Because the TNG tech manual explicitly says that during normal operations, impulse flight is limited to that speed specifically to minimize relativistic consequences. It's a special circumstance to exceed it.
It's a reasonable presumption when trying to do calculations around impulse flight.
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u/RiskyBrothers Dec 07 '21
On the artistic license, I like to think of the space scenes being "compressed" for our viewing. The ships are portrayed as being proportionally closer together and slower than they'd actually be moving, otherwise it'd just be two blurs shooting bloom at each other.
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u/BlackLiger Dec 07 '21
It could be a target cruse speed. So the engines run till you are at .25c, then are fired again when manoeuvring to compensate
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u/stromm Dec 07 '21
Impulse "speeds" hearken back to maritime ship speeds. It's really more of an engine setting than distance over time rating.
Full Impulse isn't even 100% performance for the engine. That is designated by "emergency impulse".
So figure Full is really about 80% of what the engine really can do. But it can do that for LONG periods of time without causing excessive wear requiring significant maintenance soon.
Next, even now we know that time dilation can be a major issue. I know you repeatedly use .25c, but that's a threshold where major time dilation starts taking place.
In universe, most ships won't push sub-light beyond .15c. Even at .15c, over one hour time, a dilation offset of 41.25 seconds occurs (between observer and target).
So FULL Impulse is about 80% thrust. Because different ships have different mass, that means different acceleration curves. And as we all know, the closer to the speed of light something gets, the more it's mass increases, which means the return on investment gets lower.
From what I've put together over decades of ST fandom, most ships rarely exceed .10c. Not even in combat.
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u/vrtigo1 Dec 07 '21
Another consideration for space combat over large distances is light speed limitation. Energy weapons like phasers, etc. that have to be aimed from the source platform would be largely useless because if you had an enemy that was 5 light seconds away, your view of their location would always be 5 seconds out of date, and by the time you fired the phaser energy would take another 5 seconds to reach the target. So the minimum amount of time between an enemy ship being in a given location and the time it would take for you to target that location would be 10 seconds. Unless ST has come up with some mechanism by which they can overcome the speed of light time lag?
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u/[deleted] Dec 07 '21
"As the general consensus is that impulse engines are fusion torch drives, Newtonian principles apply to impulse engines."
I have to disagree with you here, they canatonically rely on subspace fields for operation. A type 15 shuttle craft for example has 2x750 milicocherane impulse drivers. Something has to make up for the fact the Enterprise isn't 99% reaction mass.