Reverse Impulse

[DanGovier]

I saw a post on Facebook asking how Federation ships can fly backwards using impulse engines, and it got me thinking about what the impulse engines are actually capable of. Memory Alpha states;

"In Federation starships, the impulse drive was essentially an augmented fusion rocket, usually consisting of one or more fusion reactors, a driver coil assembly, and a vectored thrust nozzle to direct the plasma exhaust."

That's pretty much how I pictured impulse engines. They are basically vectored rocket nozzles, but in orders of magnitude more powerful than basic chemical rockets. So, if that's the case;

1. How can you reverse a ship using impulse engines? Does that ever happen on screen?
2. How do you slow down from full impulse? Do you do a 180 and thrust retrograde?

The RCS thrusters are obviously fairly powerful because they can land a ship on a planet, but are they powerful enough to do the above?

 

[XCV330]

the whole concept is repulsive

I had to say that.

Enterprise makes a point of showing the SS Emmet as actually having glowing rocket nozzles on her stern, whereas the later Warp Delta, basically the same 3d model, does not. I always took impulse engines to just be some other not-specifically explained technobabbl idea, but since they specifically mentioned thrust vectoring, they're getting all Newtonian on us for once.

It would be something equivalent to thrust reversers on a modern airliner's fanjets.. They use them to slow down in a hurry. You can feel it whenever you land. That's not brake pads you intially feel pusing you forward, its the thrust reversers kicking in.

 

[CorporalCaptain]

One theory I picked up somewhere, don't remember where at the moment, is that thrust could be redirected using deflectors. In conservation of momentum, what matters is the final direction that the rocket exhaust is ejected from the ship. In other words, the deflector system would absorb the change of momentum in changing the direction of the thrust and transfer it in turn to the ship. It would be just as if the rockets were facing forward.

 

[DanGovier]

I was just watching an aviation video on thrust reversal, and on that particular model of commercial jet engine the cowling retracts to allow the thrust to be vectored forwards by about 45 degrees. I actually assumed all this time that they were simply running the turbine backwards.... you learn something new every day!

Looking at a quick selection of federation ships, most do seem to have at least some form of grille in or around the impulse engine exhaust. All the components are there for vectoring, and I think I may actually look at my Refit model to see if I can vector the thrust mechanically rather than having to rely on energy based deflectors etc. Generating an arc-shaped force field behind the impulse engines to vector thrust certainly works, but feels like a bit of a bodge :P

Edit: Good examples:

To view this content we will need your consent to set third party cookies.
For more detailed information, see our cookies page.

Accept third party cookies

 

[Santaman]

The impulse engine exhausts are there to get rid of the stuff the fusion reactors spit out and to get rid of heat, the engines themselves are coil drives like the warp engines are so they can move the ship around any which way you want, they're not rockets, I mean some ships have them in the most idiotic places like Ent B and Ent E, if those exhausts would actually produce thrust they would cleanly blow the warp engines off the pylons or burn through the pylons themselves others would never be able to fly in a straight line and do loops continuously and some ships simply don't have the glowy exhausts at all.

 

[DanGovier]

The impulse engine exhausts are there to get rid of the stuff the fusion reactors spit out and to get rid of heat, the engines themselves are coil drives like the warp engines are so they can move the ship around any which way you want, they're not rockets, I mean some ships have them in the most idiotic places like Ent B and Ent E, if those exhausts would actually produce thrust they would cleanly blow the warp engines off the pylons or burn through the pylons themselves others would never be able to fly in a straight line and do loops continuously and some ships simply don't have the glowy exhausts at all.

Is there a canonical reference which confirms or at least hints towards that being the case? The USS Franklin's impulse engines make a real mess of these trees :P
Edit: the embed didn't save the timestamp, it's at 2:28.

To view this content we will need your consent to set third party cookies.
For more detailed information, see our cookies page.

Accept third party cookies

Regarding reverse impulse capability though, there seems to be a fair number of references to it on screen;

The Apple [TOS]
SCOTT: We're ready here, sir. All available power has been channelled into the impulse engines. We have twelve minutes before entering atmosphere.
KIRK: All right, Scotty, put her in full reverse. Get her out of there.
SCOTT: Full reverse, Mister Kyle, all engines.

Evolution [TNG]
PICARD: Reverse impulse engines.
LAFORGE: Initiating reverse sequence, now.

Galaxy's Child [TNG]
PICARD: Reverse power, full impulse.
RAGER: Impulse engines at full power.

Relics [TNG]
SCOTT: Look at the momentum distribution of the ions. It would take an impulse engine at full reverse to put out a signature like that.

Parallax [VOY]
KIM: Power to the tractor beam is down eighty percent. The gravimetric force of the singularity is pulling us in!
JANEWAY: Impulse engines, full reverse!

So I think that's pretty conclusive. Impulse engines do go backwards.

 

[CorporalCaptain]

Is there a canonical reference which confirms or at least hints towards that being the case?

No.

This is from the TOS Writer's Guide (The Star Trek Guide, Third revision, April 17, 1967) p. 8 [any transcription errors are mine]:

The Enterprise has a secondary propulsion system. These are the impulse power engines (same principle as rocket power), located at the rear of the "saucer section". Vessel speed, when using the impulse engine is, of course, less than the speed of light.​

That really couldn't be any clearer as to what the original intent was.

As for the placement of the engines on the models being used as hard evidence of their capability, that's ridiculous. This is a TV show/television & movie franchise. The starship designs aren't the result of actual real world engineering. There's a general eye on capabilities based on story needs, but looking cool trumps actual physics every time.

 

[Kamen Rider Blade]

Don't forget that that the vessels have some form of Anti-Grav or Gravimetric (Babylon 5 term) Propulsion on top of Impulse.

You can see it with Discovery when it jumps into the Cave and still keeping it self afloat.

The same with Voyager landing on a planet.

Anti-Grav / Gravimetric (Babylon 5 term) Propulsion allows a large vessel to go in any direction, but it's speed / acceleration is directly tied with the mass of the vessel.

Where as the Impulse Exhaust nozzles on the back is a form of directional thrust using the plasma from the Fusion Reactors to give significant speed in a generally forward direction.

When they back up using only "Gravimetrics", they aren't capable going at True full Impulse speeds.

When going forwards, they're capable of going at True full Impulse speeds.

True Full Impulse = Gravimetric + Fusion Thrust forced out of the Impulse Exhausts facing the rear of the Star Ship.

 

[DanGovier]

Don't forget that that the vessels have some form of Anti-Grav or Gravimetric (Babylon 5 term) Propulsion on top of Impulse.
You can see it with Discovery when it jumps into the Cave and still keeping it self afloat.
The same with Voyager landing on a planet.

I don't think that's ever mentioned on screen though is it?

I've always taken Voyager's landing sequences as incomplete CGI, because the bridge dialogue clearly refers to atmospheric thrusters;

The 37's [VOY]
PARIS: Captain, I think I should tell you I've never actually landed a starship before.
JANEWAY: That's all right, Lieutenant, neither have I. Bridge to Engineering. We're going to land the ship, Miss Torres. Take the warp core offline, vent all plasma from the nacelles and standby to engage atmospheric thrusters.

Demon [VOY]
JANEWAY: Janeway to engineering. Vent all plasma from the nacelles. Transfer any available power to atmospheric thrusters and stand by to commence landing sequence.

Course: Oblivion [VOY]
JANEWAY: Harry, vent all plasma from the nacelles, transfer available power to atmospheric thrusters and stand by to commence landing sequence.

 

[SpyOne]

We don't know a whole lot about how the impulse engines work.
Since we know they can be used in reverse, it seems a safe bet that either some of them are mounted facing backwards or there is a way to deflect their thrust. Or that thing about coils; that was good. I'll have to dig out my Tech Manual and see if that fits with their description.

I am reminded of taking physics in high school. We were doing basic mechanics and got to the formula that acceleration is equal to force times time. The teacher told us that the side of the equation with force times time is called "impulse", and then said "and so we know that the warp drive on Star Trek doesn't work like this because they call the other drive 'impulse' to differentiate it."
Because the man knew what sort of people sign up for Honors Physics when Freshman Biology covered their science requirements for graduation.

 

[Mytran]

No.

This is from the TOS Writer's Guide (The Star Trek Guide, Third revision, April 17, 1967) p. 8 [any transcription errors are mine]:

The Enterprise has a secondary propulsion system. These are the impulse power engines (same principle as rocket power), located at the rear of the "saucer section". Vessel speed, when using the impulse engine is, of course, less than the speed of light.​

That really couldn't be any clearer as to what the original intent was.

That might have been the original intent, but it has little bearing on how the technology is depicted on screen. Early scripts and behind the scene memos can be helpful guides, but are not authoritative.

As for the placement of the engines on the models being used as hard evidence of their capability, that's ridiculous. This is a TV show/television & movie franchise. The starship designs aren't the result of actual real world engineering. There's a general eye on capabilities based on story needs, but looking cool trumps actual physics every time.

That's as maybe, but actual rockets situated right in front of nacelle pylons are a recipe for disaster, even in a "general eye on capabilities situation"

My money is on Impulse Engines being more akin to exhaust pipes than propulsive rockets (and the gas venting is actually a plot point in ST6 as well)

Don't forget that that the vessels have some form of Anti-Grav or Gravimetric (Babylon 5 term) Propulsion on top of Impulse.

You can see it with Discovery when it jumps into the Cave and still keeping it self afloat.

The same with Voyager landing on a planet.

Anti-Grav / Gravimetric (Babylon 5 term) Propulsion allows a large vessel to go in any direction, but it's speed / acceleration is directly tied with the mass of the vessel.

Where as the Impulse Exhaust nozzles on the back is a form of directional thrust using the plasma from the Fusion Reactors to give significant speed in a generally forward direction.

When they back up using only "Gravimetrics", they aren't capable going at True full Impulse speeds.

When going forwards, they're capable of going at True full Impulse speeds.

True Full Impulse = Gravimetric + Fusion Thrust forced out of the Impulse Exhausts facing the rear of the Star Ship.

True: we see time and time again that Trek vessels are masters of gravity manipulation, from deck plates to tractor beams to inertial dampeners.
It's not too much of a stretch to think that the tech was expanded into propulsion as well

 

[CorporalCaptain]

Well, there's zero chance of my agreeing with that.

Exhaust is the name of what comes out of the nozzle of a rocket, by the way.

 

[Santaman]

Same as the exhaust coming out of a car which does nothing to propel the vehicle.
With the fuel consumption and generally how they accelerate ships to high speeds within seconds it is imposible that it is somesort of rocket propulsion fusion powered or otherwise IMO.

 

[CorporalCaptain]

Yeah, it's science fiction leaning towards fantasy, and not actually engineered.

 

[Santaman]

^^ I guess that's part of the fun.

 

[blssdwlf]

FWIW, In TOS they refer to "rockets" to "blast out" in "The Cage". But it was never called "impulse".

SPOCK: Mister Spock here. Switch to rockets. We're blasting out.​

Later, in "The Naked Time" and "Where No Man Has Gone Before" the phrase "blast out" of wherever they are was used so we can potentially link "rockets" to the impulse engine system.

KIRK: And if we can't? We'll be trapped in orbit there. We haven't enough power to blast back out. [They are on impulse because the star drive is out.]
...
KIRK: Impulse power then. Blast us out of this orbit​

TOS also has many references to applying "thrust" to slow the ship down or increasing/maintaining speed at warp ("The Ultimate Computer", "Court Martial", "The Voyage Home") and at impulse ("The Apple", "Who Mourns for Adonais"). In that same vein, this thrust could be coming from "rockets" so they could be the same as thrusters.

I'd argue that based on the episodes and the first few movies (but not strictly the writer's guide) that the TOS/TMP Enterprise uses it's "thrusters aka rockets" in a newtonian fashion for attitude and simple maneuvering. When impulse or warp engines are active, the ship's movement is magnified so that any thrust applied will accelerate the ship faster and to a higher, fantastical speed. All IMHO.

And back to @DanGovier's question about reversing impulse - apply power to the impulse engine for the magic to work and use any of the thrusters/rockets facing forward to apply reverse thrust. Voila, reverse impulse.

Remember in "The Corbomite Maneuver" they also applied impulse power for a 90 degree lateral move. So it's not just forward and back that impulse power can be applied.

 

[Mytran]

Interesting...so, an Impulse Engine is just the mechanism by which the overall mass of the ship is lowered, thus permitting the regular thrusters to propel the ship at more and more fantastic speeds? That's certainly food for thought!

It also works with the parlance used: one-quarter impulse, one half impulse etc refer to how much of the maximum output of the Impulse Engines are being used. This means that the terms have no bearing on the actual speed being achieved (since that is dependant on the thrusters), thus closing the lid on various Impulse speed inconsistencies!

 

[DanGovier]

Hmm very interesting. What if... the impulse drive assembly could augment the output power of the RCS thrusters? So without impulse they are simple chemical rockets, but when the impulse systems are online additional power can be fed through to the RCS ports?

I'm torn... I'm in love with the scene from Beyond where the USS Franklin's impulse engines are generating hurricane force winds, and I do prefer the notion that the impulse engines accelerate plasma to incredible speeds for propulsion.... but... they do seem capable of far more interesting feats than simply generating thrust.

 

[Mytran]

The scene from Beyond needn't be dismissed at all! Those old engines hadn't been used in a long time - what we saw was simply the impact of Scotty clearing the exhaust pipe of solidified waste gas and other debris (pretty potent stuff!)

 

[blssdwlf]

@DanGovier - Since rockets / thrusters aren't defined as anything specific then they could be anything and shouldn't be limited to simple chemical rockets. They could be some plasma or ion thruster or something else that hasn't been invented yet.

But an interesting thing to ponder is this, when a ship takes off, say like in "The Voyage Home", she's on thrusters only and it creates hurricane force winds around the take off area. But when the impulse engines are engaged, you barely know she's there. They were almost complete stealth when they dived and hovered over the fishing boat at the end.

So following my thinking above, when you turn on impulse engines, for an equivalent action like hovering in TVH it actually requires *less* thrust from the thrusters. And if you switch to warp drive, super minute changes like trying to hover might be beyond the precision possible with their thrusters (which is why impulse is favored.)

Edit: I do think that the impulse engine exhausts at the back of the ship are really just exhausts for radiating heat or exhausting radioactive waste/plasma and probably don't contribute thrust.

This doesn't necessarily translate to the Kelvin or TNG universes though as I haven't really looked at all the possible examples. You could think of the thrusters on the Franklin still firing aft to move forward and as the impulse engines are ramping up in power the exhaust pipes are blowing out debris/heat and igniting things behind the ship (ala @Mytran)