RCS Systems TOS

[Crazy Eddie]

We pretty much know what they are, a type of highly specialized thruster system that uses subspace driver coils or "unified space energy fields" or some other fuckery to achieve an absurdly-high specific impulse

In other words, "not a rocket."

Inasmuch as a photon torpedo is "not a missile."

You can't have impulse without thrust. You can't have thrust without a reaction. A drive system that expels a reaction mass to produce thrust is, essentially, a rocket.

Impulse engines are not that simple, and they do a lot more than simply "expel thrust." That, probably, is the DIFFERENCE between an impulse engine and a conventional rocket (much like the difference between, say, a rocket and a turbojet).

 

[uniderth]

No, we really can't. This is a line from "The Cage" at which point Rodenberry hadn't worked out the more advanced terminology that went into later episodes of the series. Much like your own analogy: "Phasers" were called "Lasers" and "deflectors" were called "meteorite beams" and "warp drive" was called "time warp" and moved the ship by "time warp factors."

So we can either conclude that Pike's ship was carrying a set of bizzarely anachronistic technologies for some reason -- including conventional rockets in the impulse deck and handheld laser pistols -- OR we can just chalk it up to the language barrier.

I say run with it. There's nothing wrong with lasers and time warp factors. I'll take these unique terms over TNG cliches any day.

 

[Mytran]

Lots of items and technology can be referred to by more than one name - why should the Star Trek universe be any different?

As regards the operation of Impulse Engines; the creator's intent is all very well but it is trumped by the whims of the script writers in the episodes that actually appear on screen. We have decades of episodes depicting Impulse Engines acting in a certain way and not always compatible with the technical manuals
The dialogue from The Cage was taken from a series bible that still featured nuclear powered engines from a routinely detachable saucer that was 20 storeys high. Any episode (especially early ones) needs to viewed as part of the wider universe that came later. Spock said "rockets", which in the context of the rest of the franchise is a piece of tech most akin to the RCS thrusters - i.e. something with basic Newtonian propulsion.

Having said all that, I agree that Impulse Engines most likely combine traditional thrust and some sort of subspace and/or gravity manipulation to produce high speeds with only minimal fuel use. Does make them rockets? Only by a very broad stretch of the definition, IMO.

Finally, it wasn't Impulse Engines that O'Brien used to fly DS9 to the wormhole, but ordinary thrusters:

KIRA: Mister O'Brien, what would it take to move this station to the mouth of the wormhole?
O'BRIEN: This isn't a starship, Major. We've got six working thrusters to power us and that's it. A hundred sixty million kilometre trip would take two months.
KIRA: It has to be there tomorrow.
O'BRIEN: That's not possible, sir.
KIRA: That wormhole might just reshape the future of this entire quadrant. The Bajorans have to stake a claim to it. And I have to admit that claim will be a lot stronger if there's a Federation presence to back it up.
DAX: Couldn't you modify the subspace field output of the deflector generators just enough to create a low-level field around the station?
O'BRIEN: So we could lower the inertial mass?
DAX: If you can make the station lighter, those six thrusters will be all the power we'd need.

 

[Crazy Eddie]

I agree that Impulse Engines most likely combine traditional thrust and some sort of subspace and/or gravity manipulation to produce high speeds with only minimal fuel use. Does make them rockets? Only by a very broad stretch of the definition, IMO.

The definition is actually pretty broad to begin with and includes everything from bottle rockets to jet engines. They all work on basically similar principles, even though they all do very different things.

Finally, it wasn't Impulse Engines that O'Brien used to fly DS9 to the wormhole, but ordinary thrusters

Obviously. The difference between an "ordinary thruster" and a Starfleet impulse engine is that the latter produces the needed mass-reducing subspace field and thrust vector control as part of its normal operation. DS9 had to supply the field via its deflector shields and directional control by pure force of will.

This is a bootstrappy way of providing impulse-like performance without an actual impulse engine, and it gives us a VERY solid idea of how impulse engines work and why they're built the way they are.

 

[Egger]

I wonder how the Impulse propulsion system works when the ship is backing up- no forward facing ports/vents. They specifically say 'reverse impulse' several times in TNG...

It's not so mysterious. The Enterprise uses thrust vectoring for steering under impulse power, which makes it more agile at high speeds than it otherwise would be with thrusters alone.

Reverse thrust with an impulse engine probably works the same way it does with a turbofan:

Stick a properly-shaped forcefield over those engine exhausts and problem solved.

I don't think that this would work in space.

Let's imagine a perfect 180° thrust reverse. Then - in space - the thrust that is generated in the engine and the reversed thrust would just cancel each other out.
Everything lower than a 180° thrust reverse would work for sideways movement, but you could never reverse it 180°, even if you would try it by changing the direction of the thrust bit by bit with several reversers.

The reason this all works on planes is that there is additional thrust generated by the air coming out of the engine pushing against the surrounding atmosphere (imagine the reversed airflow in your picture pushing against the air coming from ahead). Since there is no atmosphere to push against in space, a thrust-reverser like this one shouldn't work (for "stopping" the ship).

 

[Crazy Eddie]

I don't think that this would work in space.

Actually it would work a lot BETTER in space since there's no gas flow coming into the engine from the front end. All of the reaction mass is directed forward and none aft (unless the forcefield allows some leakage through for some reason).

You could actually calculate exactly how much thrust the impulse engine would produce just by knowing something about the shape of the forcefield (effectively temporary nozzle) and the angle of the exhaust relative to the hull. Unsurprisingly, it would be significantly less than it would be for normal "forward" impulse operation, but then impulse engines point aft for a REASON.

Let's imagine a perfect 180° thrust reverse. Then - in space - the thrust that is generated in the engine and the reversed thrust would just cancel each other out.

Uh... no. The final vector of the exhaust gasses as they separates from the ship is the inverse vector of the force acting on the ship. If the thrust is applied 180 degrees (forward) then the ship's motion is aftwards. If the gases are being expelled at, say, 30 degrees forward in a conical shape, then you have a cosine loss for the angle of the gases and that can be calculated too.

The reason this all works on planes is that there is additional thrust generated by the air coming out of the engine pushing against the surrounding atmosphere

Nope. The thrust from a jet engine pushes on the engine housing itself, which in turn transfers those forces into the airframe of the craft. The "push" against the surrounding atmosphere is called drag; that's produced by an airplane's control surfaces, namely flaps and airbrakes. The engines provide thrust that has nothing at all to do with atmospheric drag; they do not "push" on the surrounding atmosphere to produce thrust.

This exact same jet engine setup would work just as well if we were looking at a bipropellant rocket with no airflow. If the direction of the gas is (mostly) forward, then the direction of movement is (mostly) aft.