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Sub-light Speed in Star Trek

Is Impulse Drive Possible?


In the television and film series Star Trek man, part of an intergalactic federation of races, travels the galaxy exploring strange new worlds. And while the Warp Drive gets most of the attention, and is the subject of much discussion, it is the impulse drive on the Starship Enterprise that allows it travel at sub-light speeds.

What is Impulse Drive?

The most common means of propulsion in space travel is currently the chemical rocket. They efficiently generate great amounts of thrust, a requirement for launching vehicles into space and accelerating them to orbital speeds.

However, chemical rockets have several drawbacks. They require massive amounts of propellant (fuel) and are generally very large and heavy.

Impulse engines, like those depicted to exist on the starship Enterprise utilize a slightly different approach to accelerating a craft.

Instead of using chemical reactions to propel the space craft, a nuclear reactor (or something similar) is used to supply electricity to the engines.

The engines than use the electricity to either power large electromagnets that use the energy stored in the fields to propel the ship or, more likely, superheat plasma that is then collimated by strong magnetic fields and then spit out the back of the craft, accelerating it forward.

Effectively, impulse engines represent a step forward from current chemical powered rockets. But at the same time utilize well understood physics to achieve propulsion.

Technical Considerations of Impulse Drives

There are several problems problems with impulse drives, at least as how they pertain to those depicted in science fiction.

  • Time Dilation: Any time a craft travels at relativistic speeds, concerns of time dilation arise. Namely, how does the timeline stay consistent when the craft are traveling at near-light speeds? Unfortunately there is no way around this, and it is for this reason that impulse engines are often limited in science fiction to about 25% of the speed of light where relativistic effects would be minimal. Note: this is not a concern for warp drive technology as technically the craft is stationary relative to the space-time continuum and therefore would not experience relativistic effects.

  • Acceleration Effects: On science fiction shows the crew of the starships never seem effected experience the effects of the craft accelerating. If you've ever ridden a bus you ender stand the effects of trying to stand up while the bus is accelerating. But this never seems to be a problem in shows where the acceleration rates are extremely high. One explanation is that the starships are fitted with an acceleration compensator to correct for this effect. However this device never seems to be working when the ship is impacted by an enemy weapon, sending everyone flying. Strange that.

Could We Someday Have Impulse Engines?

The basic premise of impulse-type engines is scientifically sound. However, there are some considerations.

In the films, the starships are able to use their impulse engines to accelerate to great speeds (a significant fraction of the speed of light) using these engines. In order to achieve such speeds with impulse engines the power generated would have to be significant.

Currently, even with nuclear power, it seems unlikely that we could produce sufficient enough current to power such drives, especially for such large ships.

Also, impulse engines are seen being used in planetary atmospheres and in regions of nebulous material. Every design of impulse like drives relies on their operation in a vacuum. As soon as the starship entered a region of high particle density (like an atmosphere), the engines would be rendered useless.

So on the surface things may not look promising. But the reality is that impulse drives may very soon be a reality.

Ion Drives

Ion drives, which use very similar concepts to impulse drive technology have been in use aboard spacecraft for years.

However, due to their high energy use, they are not efficient at accelerating craft very efficiently. In fact, these engines are only used as primary propulsion systems on interplanetary craft. Meaning only probes traveling to other planets would carry ion engines.

But they do cary one benefit. Since they need only a small amount of propellant to operate, they can operate continuously during their operational lifetime. So while a chemical rocket may be quicker at getting a craft up to speed, it will also quickly run out of fuel.

Where as an ion drive will continue to accelerate a craft for days, months and even years. Ultimately allowing the craft to reach a greater top speed which is important for trekking across the solar system.

But while ion drive technology is certainly an application of impulse drive technology, it fails to match readily available acceleration ability of the engines depicted in film.

Plasma Engines

A more promising technology then is plasma drive technology. These engines use electricity to superheat plasma and then eject it out the back of the engine using powerful magnetic fields.

Such plasma engines, like those under development by the Ad Astra Rocket Company, could very will be the next evolution in rocketry.

They bare some similarity to ion drives in that they use such little propellant that they are able to operate for long periods of time, especially relative to traditional chemical rockets.

However, they are much more powerful; able to propel craft at such a high rate that a plasma powered rocket (using technology available today) could get a craft to Mars in little over a month. Compare this feat to the nearly six months it would take a traditionally powered craft, it is quite an improvement.

Is it Star Trek levels of engineering? Not quite. But it is definitely a step in the right direction.

And with hundreds of years more development, who knows, maybe impulse drives like those depicted in movies will one day be a reality.

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