Maneuver drives in Traveller are one of the “magic” technologies that live in the setting, but let’s try to scrap that and try something new. This setting is an attempt to make a “hard” interstellar science fiction setting for Traveller, and so we’ll look at more realistic maneuver drives.

Rockets!

First thing we’re going to decide: We’re using rockets. Reaction drives. Just like all spacecraft now, our ships will move by blasting stuff really fast out one end, and then Newton’s Second Law will push us the other way. The next question, though… What kind of rockets?

We’re not going to use chemical propulsion. That’s what we use today. Combine fuel and oxidizer in an engine, and hot gasses blast out of the end of it. The problem with chemical propulsion is it’s just not good enough. An Atlas V, which is a pretty common modern rocket, is 590,000 kg of metal and angry chemicals, and it can get at most 18,000 kg into orbit. Once. If you want to get something to Mars, then that 18,000 kg has to include another small rocket that will propel your spacecraft to Mars, so your final spacecraft mass will likely be something on the order of 1,000 kg in Mars orbit… After a journey of years. That’s not going to do it.

We’re not going to use ion engines, either. The problem, you see, is that current rockets really come in two flavors. High thrust, or high efficiency. A chemical rocket produces high thrust, which lets the rocket lift off from the Earth and produce a bunch of acceleration. For a very short time. An ion engine, on the other hand, is very efficient, but it produces minuscule thrust.

No, what we want is a torchship drive! A drive that is both high thrust and high efficiency! One that will let us travel from one planet to another in a reasonable amount of time! We want something that’s in the general ballpark of what a standard Traveller maneuver drive can do… Just with added realism. And that realism will include thermodynamics.

Thermodynamics

Oh God, here we go.

Nothing is 100% efficient. Especially rockets. They generate heat. Often a tremendous amount of heat. Chemical rockets get rid of that heat by sending it out with the exhaust. This works for them because they chew through all their fuel in a very short time – there is a lot of hot mass going out the business end of a rocket engine. Since we want a spacecraft that can produce a reasonable amount of acceleration for a long time while not filling up all the internal space of the ship with fuel tanks, we can’t get away with this. So we’ll need to deal with the heat our engine produces. However much heat that is…

So, let’s start to work out a better definition for a torchship. I’m going off of Winchell Chung’s definition: A torchship is “a spacecraft with more than 300 km/s total delta V and an acceleration greater than 0.01 g.” We know what acceleration is, but now we’ve added delta V. That’s a concept that describes how much a vehicle can change its velocity. It’s essentially a measure of fuel capacity. If our torchship can accelerate at 0.01 g, that’s about 0.1 m/s^2. 300 km/s of delta V, at that acceleration, just means that the ship can fire its thrusters for 300,000/0.1 = 3,000,000 seconds. That’s almost 35 days. The standard Traveller starship can also maneuver for about the same amount of time, so that’s a good definition.

There’s an equation for determining how much power a drive produces: Thrust * exhaust velocity / 2 = power. If our spacecraft masses 1000 tons (which feels like it’s in the Type A Free Trader range, assuming each dton masses 5 tons on the average), then our drive is producing 10,000 newtons of thrust. It’s pretty common with theoretical fusion engine designs to have an exhaust velocity somewhere in the range of 1,000 km/s, so let’s use that. Our drive, then, produces 5 gigawatts of power. Let’s be generous, and assume that we’re only losing 10% of that as waste heat. We still need to radiate 500 megawatts of heat for the entire time the drive is operating. That’s a lot, but it’s doable, especially with exotic, theoretical radiators.

So what do we choose?

I’m going to gloss over all of the time I spent pouring over the Atomic Rockets website. I highly recommend it, though. I wanted to have something that was a low end torchship that was feasible with future technology – something that wouldn’t break the known laws of physics. I wanted something that used some sort of hydrogen fuel, because so much of Traveller revolves around that. That led me to fusion engines. One design that’s fairly well developed is the gasdynamic mirror fusion engine. There have been a couple of theoretical designs done with these as proposed propulsion for manned Mars missions, and they have surprisingly decent performance. Exhaust velocities above 1,000 km/s and thousands of Newtons of thrust. So we’ll pick that. We want our setting to be farther in the future – I picked 2600 AD out of a hat – and so I used that as an excuse to beef up some of the numbers a bit, mostly in the heat dissipation area, and ended up with a drive massing 108 tons and producing 60,000 Newtons of thrust with an exhaust velocity of 1,200 km/s. This drive produces 36 GW of thrust power, and dissipates waste heat through a type of exotic radiator called a Curie Point Radiator which is based on the magnetic properties of hot metal. Most metals lose their magnetic properties when they’re heated above a certain point, called the Curie point. The Curie Point Radiator, then, just flings hot metal out. When it cools down below its Curie point, then it is drawn back by powerful magnets. There are electrostatic versions of this, as well. These are buildable with existing technology, and they’re capable of radiating several MW of heat per square meter, which is pretty good. The gasdynamic mirror drive is a long tube, so there’s plenty of room to mount these things on a framework running along the drive. The drive itself uses powerful electromagnets along the tube, so it’s entirely possible those could do double duty as the magnets in Curie Point Radiators.

The gasdynamic mirror fusion engine fuses deuterium and tritium, both of which are refined from hydrogen. Tritium has a short half life – 12 years – but we don’t care too much about that, because we’re looking at trip times shorter than a month. The only problem with this is that deuterium-tritium fusion produces a lot of neutrons, which are deadly and hard to shield against. And by “a lot,” I mean nearly 80% of the fusion energy is in the form of neutrons, emitted in all directions. So we’re going to bring in a little unobtanium…

Unobtanium

Unobtanium is technology that doesn’t seem like it’s impossible – it doesn’t violate laws of physics – but we have no idea how to make. It’s almost cheating, but not quite. I’m allowing myself a little unobtanium in the setting.

The unobtanium that we’re bringing in is spin-polarization. When a deuterium atom and a tritium atom slam together and fuse, they emit an alpha particle and a neutron in opposite directions. And it turns out that these directions are based on the magnetic spin moments of the atoms. So if you can align them… you can control the direction the neutrons and alpha particles are emitted! You can direct all the neutrons out the nozzle of the rocket! You can direct some of the alpha particles through a magnetohydrodynamic generator for electricity, and use magnets to direct the rest of them back out the nozzle! This not only cuts way down on the amount of shielding you need for this engine, it effectively doubles the thrust! We’re totally doing that!

We’re also going to allow the thrust to be augmented by injecting additional hydrogen as propellant. Basically, we’re using the energy from fusing some hydrogen to heat more hydrogen, boosting our thrust. We can double the thrust of the engine by injecting four times as much hydrogen, reducing the exhaust velocity by half. This gives us some choices, trading off fuel efficiency for thrust.

Extras

This drive produces power, so we can potentially use this as a replacement for both the maneuver drive and power plant. One of the reference designs for this drive described the power required to start it – 1,000 MW-sec – and described this as coming from a capacitor bank that was initially charged up with fuel cells. Once the drive was operating, it would produce the electricity needed to maintain the reaction, as well as hotel power for the rest of the ship, and power to “recharge” the fuel cells. Our drive is smaller and lighter than this reference drive, so I’ve scaled this down to about 250 MW-sec, which can be provided by a 7 ton bank of capacitors, using Book 5 rules. 2 tons of fuel cells can charge these capacitors in about 1000 seconds, which is 1 CT space combat turn, so that seems like a good option.

One downside to this drive is it produces a dangerous neutron stream out the business end of the rocket. This makes decelerating for rendezvous with a spaceship or station tricky. To get around this, we’re going to simply say the standard drive also has a pair of ponderomotive VASIMR thrusters firing along the axis of the drive for “last mile” maneuvering.

The final drive

Here’s what I’m calling our reference drive – something suitable for mounting in a Type S scout/courier or a Type A free trader:

Cost: Mcr 20

Mass (including radiators): 108 tons

Length: 44m

The VASIMR thrusters on this drive produce 1,280 N of thrust, which isn’t much, but it’s better than giving everyone radiation poisoning at the station you’re docking with.

This isn’t really a drive that’s useful for taking off from or landing on planets, but that’s fine for us. This setting is a station-heavy setting, so we’ll assume that our spaceships never land. If players want to go to a planet, they’ll take a shuttle from a station in orbit.

We are keeping jump drives in this setting, and we’ll want to work out the accuracy of those jump drives. It seems reasonable to assume that a typical pilot can jump into a system and land about a million km from a station. That’s a bit more than double the distance from the Earth to the Moon, and a bit less than 100 diameters from the Earth. That’s the sort of distances we’re going to be dealing with in day-to-day use.

Performance-wise, lets imagine a typical ship. We’re using a near stars starmap, and we’ll need a minimum of jump-2 to be able to get around in the vicinity of the Sun, so we’ll call that the standard. In Classic Traveller, a 200 dton far trader is barely profitable, with a 300 dton variant a much better option, so we’ll go with that. 300 dtons. That’s about 1500 tons, using our 5 ton per dton napkin rule. This drive, running at 60,000 N thrust, can travel a million km in about 3.5 days. At 120,000 N, it cuts it down to 2.5 days. 240,000 N gets us there in under 2 days – 40 hours or so. 600,000 N drops it to about 25 hours, but there’s a big drawback. 600,000 N gets us into the station with dry tanks, while 240,000 N only consumes about 30% of our fuel, taking 15 hours longer. 240,000 N feels like the sweet spot, then, for a working trader. With the wimpy “last mile” VASIMR drive, we dock in 2 days. If the station is outside of the 100 diameter limit of a planet, which seems like a reasonable choice for a trade depot, we can just undock and jump, so we get 5 days at the station.

Next Post: What does this drive do to our setting?

This is the first in a series of posts detailing a new Traveller setting I’ve been playing around with.

It’s been a long time since I’ve updated this blog, mostly because I haven’t had a lot of time to mess around with Traveller recently. A few months ago, though, I had a hard science fiction itch and I started thinking about how to scratch it with Traveller. While Traveller as a game, especially Classic Traveller, sits more on the “hard” side of things, it does rely on a handful of what I would call “magic” additions: Psionics; torch drives that break the laws of thermodynamics; and faster than light travel. Could you strip some of those things away and still have a fun and viable game? Could I do that while portraying a universe that felt gritty and realistic?

Psionics is a no-brainer. They have always felt like add-ons in the rules, and quite a few settings ignore them entirely.

Likewise, faster than light travel is a no-brainer. You pretty much have to leave it in. Without it, there’s no good way to go from system to system. There are settings that leave it out – Zozer’s Orbital 2100, for instance, does a good job of presenting a setting without FTL travel – but I’m interested in a multi-system setting. So I will be leaving jump drives in.

Which leaves the torch drives. The nature of these things isn’t really gone into in the little black books, but you can make some guesses based on the rules around them. In the little black books, they range from 1 to 6 gravities of acceleration, and they’re capable of constant use for about a month on most ships. That’s quite a bit. The bare minimum 1-G drive will take you from Earth to Mars in about half a week even when they are farthest apart. And these drives are magical, in that they produce no heat to speak of, no radiation to speak of, and consume no fuel. Later editions and custom settings sometimes go into reaction drives, but these are still magic: Tremendous performance for fairly small fuel consumption with no heat to worry about.

So I started thinking about a setting with “future realistic” maneuver drives and standard jump drives. What would that look like? Would it be fun?

Future realistic?

What I wanted to do with this is to come up with something that would appeal to the sort of people who enjoy Winchell Chung’s Atomic Rockets website. Something that would appeal to people who play games like Children of a Dead Earth. So the first thing wanted to do is define what it is I’m going for. This is not going to be a modern realistic setting. I don’t want this to be Traveller with Atlas rockets. I don’t want multi-year travel times. Longer travel times are fine, but I want to keep the Traveller feel of interplanetary and interstellar mobility. So I’m looking to the future. I’m looking towards propulsion systems that enable that.

Gritty, realistic universe

I’ve experimented in the past with revamping the Traveller world generation rules to make shirtsleeve worlds rare, and I’ve always enjoyed the result. One of my favorite authors is C.J. Cherryh, who takes this tack with her Alliance/Union setting, so I felt there was a lot of potential for an interstellar setting that doesn’t depend on a lot of planets where you can walk around without a space suit. The last setting I did was an attempt to reproduce the Alliance/Union setting in Traveller, using near star data to create a sector centered on the Sun, and that was really fun. I decided to basically take the exact same physical setting, and use it for this one.

Next post: Traveller Fusion maneuver drives!