Current distance from sun: 0.029 lightyears

Despite hitting what I thought was a record speed at solar periapsis, it seems that this ship is actually significantly slower than the Polymakria I. Which is really sad, because I thought making it look cool would also make it faster.

But much like with racing stripes on a car, or blue LEDs on a computer, I found the spacecraft’s performance unchanged. It exited the system at around 46 km/s, a full 10 km/s slower than the first version. I’ve run the numbers and if everything stays stable, we should expect to hit the lightyear mark right around St. Patrick’s Day.

I should have lit the cargo bay green instead of red, apparently.

Here’s the ship now:

I even expended all my monopropellant to give it just a tiny bit of extra speed. For some reason, the resources panel is showing that I have 0.02 units of liquid fuel left… not sure how that happened. It’s useless without oxidizer anyway, even though my engine is a nuclear thermal rocket… which, when you think about it, doesn’t really make sense. An NTR should be able to run on just about anything that you put in it, as long as you can get it hot enough – hydrogen, oxygen, water, dirty water, spaghetti, Kerbal waste products, you name it.

Since a nuclear thermal rocket works by heating up a propellant and allowing the expanding gas/plasma to escape from the back, it’s pretty much the simplest design imaginable. In fact, Hero of Alexandria built a (non-nuclear!) version of one some 2,000 years ago called the Aeolipile, which used steam jets to make a little wheel spin around. (And if the ancient Greeks were experimenting with steam, then I can almost guarantee that the very first rocket to ever take flight was an unintentionally exploding boiler)

But while such devices are exciting, they run up against the limits of materials science if you try to squeeze too much efficiency from them. A basic rule of rocketry is that the faster your rocket exhaust is going, the better your rocket is. The exhaust velocity is correlated with the temperature – hotter means faster. No matter how good your fuels might be, if you end up melting your engine then You Will Not Go To Space Today. The practical upper limit for chemical rockets is around 4,500 m/s, which is okay, but not great. Nuclear thermal can hit upwards of 8,000, which is better. Ion engines, where the propellant is accelerated using electromagnetic fields and doesn’t come into direct contact with any physical part of the engine, can run much hotter and send those ions spewing out the rear at 200,000 m/s. (I really should have included some ion engines on the Polymakria II)

Anyway, with an exhaust velocity of 0.07% of the speed of light, give or take, ion engines are on the edge of what one might term “interesting” spaceship propulsion systems. But let me tell you a story about the other type of nuclear rocket.

Turn the clock back 60 years. It’s the mid-1950s, and atomic energy is the way of the future. Uranium mining is ramping up, nuclear power plants are being constructed, and A- and H-bombs are being tested all over the place. Imagine being a rocket scientist and witnessing one of those tests, and comparing its truly awesome power with the piddly little chemicals you’re trying to use as fuel to get something – anything – into orbit. Your spaceships keep exploding, your boss is a Nazi, people keep trying to get you to use chlorine trifluoride… basically, a bad work environment all around.

And so, maybe as a joke, you scribble a diagram onto the back of a napkin. In your drawing, a nuclear bomb explodes behind a spacecraft, pushing it forward. Nuclear detonations are tricky things to achieve, but they can make a whole bunch of plasma that is travelling really really fast. If that plasma becomes your rocket’s reaction mass, why, you could get to Mars in a week.

Or, considering the political climate at the time, you could build an honest-to-god SPACE BATTLESHIP and give those Soviets what-for!

“But that’s completely and utterly bonkers,” you might exclaim, entirely correctly. But this is Cold War-era Department of Defense we’re talking about. There was no proposal too off-the-wall that they wouldn’t give it serious consideration. And so, Project Orion was born. The surprising thing, really, is that it would have worked. It’s entirely possible to construct a ship capable of withstanding multiple nuclear explosions, if you design it more along the lines of a deep-sea submarine than a normal make-it-as-light-as-possible spacecraft.

Well, the nuclear test ban treaty put an end to the idea of carrying thousands of city-killer bombs into space, and so it was never built. On one hand, sure, we might have had colonies all over the solar system by the 1980s, but on the other hand, we didn’t destroy the world with nuclear war, so you take what you can get I suppose.

By some estimates, an interstellar ship with an Orion drive could potentially reach speeds of several percent of the speed of light. That’s way beyond the capabilities of any other current or near-future propulsion technologies, and it’s interesting to imagine how a probe launched in the 60s could be swinging by Proxima Centauri right about now.

Ahh, if only I could get up to, say, 5% of the speed of light with my beloved Polymakria ship series. The lightyear challenge would be done in a little under 2 hours (with 100,000x time acceleration). I’ve spent longer than that trying to get across the city in bad traffic.

Anyway, I think that’s about it for today. Tomorrow I’ll be taking my laptop downtown, and I’ll need to keep Kerbal Space Program running the whole time. Will I make it there before the battery runs out? Will I accidentally put the computer to sleep and lose everything? You’ll have to wait for my next post to find out!

A quick note on terminology: when talking about rockets, “fuel” and “propellant” are not interchangeable. Propellant, or “reaction mass”, is what shoots out the back in order to push the spaceship forward. Fuel is what gives the propellant energy. In chemical rockets, fuel and propellant are the same. But in an NTR, for example, the fuel would be the radioactive material in the nuclear core, while the propellant would be the hydrogen (or whatever) that is being heated up and expelled from the rocket nozzle. (It can also be useful to distinguish between “fuel” and “oxidizer”, especially when dealing with things like hybrid air-breathing rockets)