Burn Rockets Burn

We’re long overdue for a post about the rocketry of the game itself, so here it is finally.

Reaction engines are the cornerstone of any exploration of space warfare. Zero-propellant drives such as solar sails, laser sails, electromagnetic tethers, and the like, are not explored by Children of a Dead Earth due to certain limitations, particularly thrust. As you’ll see soon enough, thrust ends up being a heavily limiting factor for space travel.

Design for a Nuclear Thermal Rocket, which is more or less a nuclear reactor jammed into the thrust chamber of a thermal rocket engine.

The primary drive of Children of a Dead Earth is the Solid Core Nuclear Thermal Rocket, though a number of other technologies are supported. Many futuristic and experimental technologies were not included because a full treatment of these technology’s limitations has been published in scientific literature. Implementing the basic equations for a technology’s abilities, without fully implementing the mechanical and thermal stresses of that technology, would be disingenuous towards the end goal of the game. From my perspective, only exploring what a technology can do without keeping tabs on what it can’t is no better than inventing fictitious technologies altogether.

But anyways, why is the Nuclear Thermal Rocket (NTR) the go to drive in use? If you’ve been following the blog, you’ll find that it’s constantly brought up that delta-v is the limiting factor on just about everything. And due to the rocket equation, the easiest way to get more delta-v is to get a better drive with a better exhaust velocity. Well designed Solid Core Nuclear Thermal Rockets achieve 4 – 9 km/s, better than chemical propulsion, but mediocre in comparison, for instance, to ion thrusters, which can achieve 100 km/s or more. Or if you go for laser propulsion, or fission sails, or many more options, you can achieve orders of magnitude better exhaust velocity.

Hey look, those Magnetoplasmadynamic Thrusters have great exhaust velocities. Why don’t we use them instead?

You can also scale up the thrust of a rocket, but not the exhaust velocity. If you stick two identical engines together, your thrust doubles, but your exhaust velocity stays constant. So isn’t exhaust velocity the most important attribute of an engine?

Not exactly. You can work with a mediocre exhaust velocity with a greater mass ratio (though this maxes out too, this will be discussed in future posts) and staging. Counterintuitively, trying to get around mediocre acceleration is actually far more difficult.

After implementing the Nuclear Thermal Rocket, I looked into ion thrusters, and settled on the Magnetoplasmadynamic (MPD) Thruster, because it had some of the highest thrust out of all of them, and thrust is quite nice for dodging in combat. I built a few thrusters in the megawatt range, tried them out on a few missions, and their limitations became immediately apparent.

Thrust, and by extension, acceleration, is not simply important for dodging in combat. Low accelerations not only prevent you from using standard orbital maneuvers like Hohmann Transfers or Orbit Phasing, they vastly increase burn time and ultimately travel time. Getting between planets, for instance, might require a longer burn time than the actual period of the planets themselves, years, or even decades! Getting cargo anywhere in the solar system was prohibitively slow. NTRs and chemical propulsion turned out to be far superior.

It doesn’t matter that you have tons of delta-v, if it takes you years or decades to use it all.

With combat spacecraft, it only got worse. The accelerations are so poor that orbital evasion maneuvers are impossible to execute against high-g missiles or anything else really. In range of weapons fire, enemy projectile range is limited primarily by the target’s areal cross section, and its acceleration. With accelerations as low as the MPD Thruster was yielding (micro-g’s at best, nano-g’s at worst), my warships were basically immobile, sitting ducks for the enemy. And I should emphasize: The MPD Thruster yields one of the highest thrusts of any of the ion drive designs.

Okay, though we can just crank up the power consumption, and get higher thrusts, right? You can actually, as long as you keep an eye on the various stresses of the design. In particular, Onset Phenomenon of MPD Thrusters gets rather nasty at megawatt levels of power.

Designing a MPD Thruster. Compare it to the NTR above. This MPD Thruster has 4 times the exhaust velocity, yet the NTR has over 10,000 times the thrust.

But more power requires more nuclear reactors, and more reactors require more heat radiators. The amount of heat radiators by mass became overwhelming, and reduced the spacecraft’s overall acceleration faster than adding more MPD Thrusters could increase it.

At this point, the Rocket Power equation (it’s further down in the link) should be pointed out. For a given amount of power, the thrust is inversely proportional to the exhaust velocity. This equation became very evident with MPD Thrusters. One could increase the mass flow of the engine (and thus, the thrust) at the cost of plasma excitation (and thus, exhaust velocity). The two quantities were directly opposed.

I designed a MPD Thruster with exhaust velocities comparable to NTRs, and found the thrust still lacking. NTRs had a rocket power in the multi-gigawatt range, and my MPD Thrusters were in the tens or hundreds of megawatts range. They’re both run via nuclear reactors, so why were MPD Thrusters so much worse in this regard?

It wasn’t until I designed resistojets powered by a nuclear reactor (Nuclear Electric Propulsion) that it hit me. The additional step between the nuclear power generation and actually utilizing this power is the problem. NTRs and Combustion Rockets expel most of their waste heat through the exhaust itself, while nuclear powered resistojets and MPD Thrusters must expel most of their their waste heat through heat radiators unconnected to the drive. Making a MPD Thruster or resistojet with the same power as NTRs requires a staggering amount of radiators while NTRs do not. Counting the mass of the radiators needed for such a high powered MPD Thruster into the drive’s total thrust-to-mass ratio yields abysmal ratios.

In fact, most drives suffer from this same limitation that NTRs and Combustion Rockets avoid. Only a few drives, like Nuclear Pulse Propulsion manage to sidestep the issue of requiring enormous amounts of radiators to have comparable power. But ion drives, and nearly all other high-exhaust-velocity counterparts, fall flat. As it turns out, thrust is hugely important to spacecraft.

It was at this point when it finally clicked for me why aerospace engineer Robert Zubrin, inventor of the Nuclear Salt Water Rocket, has long since called ion thrusters (in particular, VASIMR) a hoax. I personally wouldn’t call them hoaxes, but in order for them to be the future of space travel would require a hypothetical advance in technology to fix their glaring flaws. As it stands, ion thrusters don’t appear to be viable for any sort of bulk space transportation. For scouts and tiny probe spacecrafts, ion thrusters are great. But for moving cargo, passengers, and military ordnance around the solar systems, ion thrusters and electric propulsion simply aren’t going to cut it.

Leave it to Zubrin to conceive of an engine that requires continuously detonating nukes inside your rocket’s thrust chamber.

Extremely high thrust propulsion looks to be the way to go, unless the radiator limitations can be solved somehow. Discounting far future drives like antimatter engines, this cleanly kills off a huge portion of potential drives for bulk space travel: ion thrusters, most sails and tethers, electric thrusters, photon thrusters, fission fragment thrusters. All you are left with besides chemical propulsion is nuclear, nuclear, nuclear. Maybe I should’ve listened to Robert Zubrin from the start.

That’s a quick run through on the rocketry of the game, next time we’ll explore one of the most overlooked, yet extremely important parts about space travel: the propellant tanks themselves, as well as the pros and cons of different propellants to use. As it turns out, neither of these are as simply as you might imagine.