Why the Death Star has Exhaust Ports
Next week fans of the movie STAR WARS will be celebrating the original film’s 40th anniversary. On May 25th, 1977, Lucasfilm released a movie that became not only one of the most iconic space opera films, but refreshed the science fiction film genre and paved the way for others to follow (according to some, the STAR TREK revival never would have happened without the resurgence of science fiction popularity granted by STAR WARS). The franchise is so popular that we’ve seen a near-constant stream of content with new films entering the canon every year since 2015. In fact, last year’s Rogue One: A Star Wars Story revisited the setting for the original. But in 1977, the original shot that destroyed the Death Star was a grand event that left a few people asking, “why leave such a vulnerability in the first place?”
The Power to Destroy a Planet is not Insignificant
It takes a lot of energy to destroy a planet. In STAR WARS, the Death Star destroys a world in a single shot that takes only a moment. From the imagery in the film, the laser travels to the core, and superheats the center, forcing the internal pressure to force the world apart. But just how much energy would that take? Randall Munroe of XKCD has an amazing article about whether or not you can light up the moon with laser pointers. If you’ll allow me a moment to summarize his article:
If you gave everyone on earth a focused version of the most powerful laser on the planet (a 500-terawatt laser used in fusion torch experiments) and pointed them all at the moon, after solving how to fire the laser for longer than a picosecond. You’d begin carving up the moon at roughly four meters per second.
But, the surface of the moon takes an interesting turn here. The surface doesn’t just vaporize, it turns into plasma which rockets off into space. Sustained fire would not only block the laser bore (plasma becomes difficult to cut through with a laser) but it would also turn the entire planet into a plasma-rocket spaceship, sending it off into space before getting a chance to blow it up.
To reach the core of the planet in the span of a second, you’d need a very focused beam with an energy output of roughly 2 petawatts, that manages to shunt the plasma away in much the same way our drills move rock back to the surface as they bore into the Earth. Even then, it wouldn’t rupture the planet (although you’d be exerting enough pressure on the inside of the planet to speed it up if you aim the laser right and that, according to another article by Munroe, would be enough to destroy the planet eventually).
So what kind of power plant do you need to deliver 2 petawatts of sustained power? The Death Star uses a fictional hypermatter reactor which, if it only delivers enough power to fire the laser, would require just over 88,000 Three Gorges hydroelectric plants (the power plant with the greatest output on earth). Of course, you could use charged batteries to power a single short burst, like the Sandia lightning facility does. But don’t forget that you’re not just powering the laser, you’re also powering lights, heating, cooling, and electrical systems throughout the 120-km diameter station.
Heating and Cooling in Space
Electronics give off an amazing amount of heat during their operation. Wikipedia sums Joule’s first law about this nicely:
“Joule’s first law, also known as the Joule–Lenz law, states that the power of heating generated by an electrical conductor is proportional to the product of its resistance and the square of the current”
But it isn’t just the electrical heat output (and it’s possible we can ignore most of the output if we assume that the station is constructed with super conductors that limit the output of heat from electronics to near zero), when an HVAC technician is sizing a home for heating they include what’s known as the people load. For every occupant of the home you need to add roughly 400 BTUs of heating and cooling. According to one site, the Death Star carries a minimum crew complement of nearly 343,000 people (more than Spokane and Coeur d’Alene combined). That’s a heat output of 137 million BTUs in a relatively small space.
And getting rid of heat in space isn’t as simple as it is on Earth. Air conditioners remove heat by transferring it outside via conductance and radiating. In space, heat can only be removed via energy radiation, which is a very slow output. Aboard the ISS, special panels exist to radiate heat away from the station. Internally, heat pumps regulate some heat to keep the sun-facing side cool while it warms the side of the station that is not in direct sunlight (direct sunlight can heat metal up to 250 degrees). So the temperature variance in space ranges between -250 to 250 degrees depending on proximity to the sun and which side is facing it.
Between the power output of the primary reactor (which will generate a lot of heat from electrons, photons, and energy reactions) and the heat provided by the mass of people inside the station, all of that heat needs to radiate somewhere else. The ISS is designed with many of its spaces having ready access to heat exchangers. A spherical station would need a way to release heat, either in the form of heated gasses or in special radiators. Because of that, building direct pipelines with a reflective surface to channel radiated heat out from the core of the station makes sense.
Images from promotional items of STAR WARS – Lucas, G. (Director). (1993). STAR WARS [Motion picture on laser disc]. USA: FOX.