Tuesday, March 5, 2013

Traveller Tuesday: How Drives Work

I've been running an online game of Traveller using the Mongoose ruleset since December, and I have found myself writing a fair amount of stuff for it. Since this was already written, it was a busy day today, and "Traveller Tuesday" makes for a nifty bit of alliteration, I figured I'd just post it.

Besides, I think the folks over at RPGBA are getting annoyed at me for talking about guns & ponies all the time.

OK, so before I begin, I'm sure some of you are asking "Just what the heck is Traveller?"   The long version may be found here.  The shorter version is "It's a science-fiction role-playing game that came out in the 1970s, roughly at the same time as D&D. It does not have nearly magical technology a la Star Trek, but neither is it space opera like Star Wars. It's a lot like the Firefly TV show, where a motley crew of murder hoboes in a small starship tramp about a chunk of space, looking for jobs and getting into trouble."

So anyway, here is the first in an irregular series of articles whereupon I expound with most profundity on background details most of my players likely don't care about.

How Drives Work in Traveller

Ship design in Traveller is based upon the Displacement Ton. I do not know why this is so, but it is. Everything, from fighters to dreadnaughts, has a hull measured in how many volumetric tons of liquid hydrogen it displaces.

Obviously, this means that cargo is likewise based on dtons. This has the curious effect of creating ships where a cargo hold full of feathers and a cargo hold full of uranium require exactly the same thrust to move at the same speed. If I were to try to make sense of this, it would
  1. result in far more math than I find enjoyable, and 
  2. require a complete refit of the starship rules. 
I am a lazy writer who hates math. Therefore, I Made Stuff Up. 

Since a ship with 2G acceleration moves at that speed regardless of what's in its hold, clearly this means that some kind of field is being generated by the maneuver engines to move a select volume through space. It doesn't matter how much that volume weighs; what matters is if things fit inside that volume or not. Since so much of Traveller tech is based upon manipulation of gravity, this seems like the way to go if I am to reasonably fake it. 

I posit that the maneuver drive is tied to the gravity plates and to the intertial dampeners, such that the field actually absorbs the kinetic energy that a mass would exert upon on the rest of the ship. In other words: You can walk around all you want and that's fine, but if the kinetic energy differential between you and the ship's movement would result in you becoming chunky salsa spread across a bulkhead, that energy is absorbed by the field through those grav plates. This also means that if momentum is applied to the ship by anything other than the maneuver drive, like something striking the hull, the crew is going to be tossed around a bit as the inertial dampeners have to compensate for something outside the grav envelope.

Where does this energy go? you may ask.  This is an excellent question, and it ties into another oddity about Traveller (which, I should point out, is something I Do Not Like): their engines somehow create thrust without using reaction mass. If this were Star Trek or Star Wars it would not bother me, but since Traveller at least tries to be relatively non-soft Sci-Fi, this sort of magical thrust does not suit the genre.

I decided to fix this using my skill at technobabble*, and posited that the kinetic energy absorbed from inertia/momentum is directed outward from the grav envelope around the ship. This is how an air/raft works, and it's what Star Wars called repulsorlift. It's great for atmospheric maneuvering, takeoffs, and landings, because it manipulates the gravity field of a planet in the same way that airfoils manipulate an atmosphere in order to achieve flight. However, while this is suitable for low-speed applications, it doesn't give a lot of thrust for reaching orbit in a reasonable amount of time, and it definitely is not optimal for interplanetary travel. 

Edit: This was sent to me via email by a helpful reader and I thought I'd include it. I'm clearly no engineer, but it seems to compliment what I said, only with more tech jargon:
Why the Displacement Ton? 
Theoretically, the possibly recently discovered Higgs Boson is what gives matter mass. Mass is apparently where inertia comes from. If you can change the boson's characteristics, you can change your vessel's mass and inertia. So... Posit a gadget (like a high energy particle accelerator) that creates a field that alters the Higgs Boson spin. Crank it up, and your ship's mass drops. Crank it enough, and you go to zero mass. Very handy when you want to accelerate a ship. 

Now assume that matter shows some resistance to the spin altering field. Low density matter (hydrogen) has high permeability. High density (uranium) has low permeability. A field of a given strength can cover a larger volume of hydrogen than it can a volume of uranium. So the displacement ton becomes shorthand for how much mass you can shift for a given energy input to your boson spin modulator. 

Nitpickers may wonder what happens to matter in the field when the mass goes away. Doc Smith rather blithely blew that off in the Lensman books. You can, too. Tell them that since all matter within the field takes on the same boson spin condition, then RELATIVE to everything else IN THE FIELD it acts normally. You only go "massless" relative to the external universe beyond the field.

For higher thrust applications, such as reaching escape velocity or interplanetary travel, I figure the engines can just vent hydrogen onto (or pipe it near) the plasma created from the fusion engine, and the resulting explosion is channeled along gravitically-shaped nozzles. While it would be possible to get propulsion from just venting the plasma fusion directly aft, the combination of "low thrust" and "radioactive output" makes this a non-optimal choice. Contrast this with burning hydrogen exhaust in an oxygen environment: 2H2 + O2 --> 2H2O. It's eco-friendly!

More math from engineer friend:
Stripped to basics, you simply inject reaction mass (hydrogen or what ever; my own spacecraft ran on anything from H2 to methane to water) into the reactor plasma exhaust, which is VERY hot. More thrust. A nice thing about this is that the heat distributes through all the reaction mass, so the overall temperature drops enough that you can direct this exhaust with fairly conventional venturis (rocket nozzles) instead of electromagnetic focusing.
For jump drives, large amounts of boiling hydrogen are gravitically held in the shape of a plasmatic cocoon while a wormhole or miniature singularity is generated to pull the ship out of N-space and into J-space. If you've plotted your course right, your hydrogen envelope evaporates when you've reached the proper coordinates, and you're dumped into N-space with the last of the jump energy (this has the useful side-effect of killing all momentum of the ship relative to the solar gravity well, so no coming out of jump at an effective speed of several thousand gees). 

If you've done it wrong, well, things go badly. Your field collapses before you reach your coordinates, and your ship disintegrates as the harsh reality of jump space interacts poorly with matter. Or maybe you miss the coordinates, and you all starve to death. Or you come out of jump thousands of parsecs in a random direction. Or time passed at a different rate, so maybe your week in jump was a hundred years, and everyone you know is dead. 

So far, no misjump has ever resulted in a crew travelling backwards in time. 

That you know of. 

A misjump is a Bad Thing, is what I'm saying. 

* Also known as handwavium bullshittium.


  1. If I might be permitted to ponder on your positions?

    You first noted a reactionless drive, but then suggested a reaction drive (channeled explosion, vented). Perhaps thus: gravity manipulation and apparent Mass manipulation can be thought to go hand in hand. If so, and if the field can only cover a certain area, then that first explains the 'cargo of feathers = cargo of depleted uranium' conundrum. In addition, cycle the field from supermassive to near zero. At supermassive, launch a significant but relatively small real mass in the direction you wish to go. It could even be a very small artificial mass. As you are (apparently) supermassive, your negative thrust is negligible. Cycle to near zero mass (and cycle your launched mass to supermassive) and let gravity pull you forward for net thrust. Cycle masses back to start positions and catch launched (artificial) mass. Rinse and repeat until at desired velocity, perhaps keep artificial mass in front of you as a radiation / microdebris shield.

    Thrust with no reaction mass. Viola. Also, power limits can limit the effective artificial masses, which limits the Gs produced, which dovetails nicely with your accel limits. 

  2. Please, ponder away!  But first:

    I can't really explain why I feel strongly about this, but the idea of "reactionless drive" just bugs the everloving heck out of me when it comes to Traveller.  I could accept it for Trek and SW, but since Trav makes at least a token effort towards real physics, I can't in good conscience just throw Newton out on his ear.  So I say "Grav plates work in gravity fields, so you can have your antigrav cars, but in space you need reaction mass."

    I do like your theory, though, as it is almost the reverse of the Orion dive. I'll keep it in mind if I ever need something weird and alien!

  3. LK Wiseman has talked about it at length with GURPS Traveller fans.

    The dTon comes from real ship tonnage being in water displacement and they conjectured that hydrogen would become the new standard of ship sizing since so much hydrogen was used by the jump drives.

    The maneuver drive is a reactionless drive.  The OT rules the power plant and jump drives consume fuel, not the maneuver.  Marc Miller once said outright that the drive creates a bubble that's inertialess to the drive, so mass is irrelevant.  He's reversed that a couple of times and his current iteration has thrust and mass, but still reactionless.

    The implications of the inertialess part also give a contragravity effect allowing a M1 ship the ability to vertical lift at the 1g indicated.  A 1g drive on a 1g planet is hovering, not lifting and a Free Trader with a 1g drive can lift from larger than 1g planets according to the rules as written in both OT and Mongoose.

    I read this and think, has it really been 33 years since I first played Traveller?

  4. I likes Traveller stuffs.

  5. Okay, I knew the part about ship displacement in water.  The thing is, that is utterly pointless for Traveller mechanics, because a 400 dton Subsidized Merchant is always going to displace 400 dtons regardless of how full or empty its hold is. This is not how traditional ships work, as they displace more water when they are fulled loaded.

    And according to Rules As Written, a dton of feathers takes up the same volume as a dton of uranium, and yet there is no difference in performance between a ship fully loaded between one or the other, which is also not how traditional ships work.  If Traveller were a magical high-tech universe like Trek, it wouldn't bother me.

    Likewise "Thruster plates" and "reactionless drives".  They bother me, so I changed them IMTU.

  6. I'm reminded of Alan Dean Foster's gravity drives in his Humanx universe. They project a singularity gravity point ahead of the ship and then fall towards it - given that the projected point moves as the ship does, they're always falling toward it. If we can manipulate gravity (as implied by grav plates) then that sort of interaction might be part of how Trav drives work. OTOH I rather like what you've done with alternative drive possibilities.

  7. I can think of a few solutions to this kind of problem, if I'm reading it right. Knowing nothing of Traveller, I may be way off, but the ideas I present here may be adaptable in some way.

    The first thing that comes to mind is Alcubierre drive, the real-world explanation of Trek's Warp: move the space that the ship is in. This would make the ship have two effective speeds while in use, the speed of the bubble and the speed you're going while in the bubble. The numbers posit'd for Alcubierre drives are for near- or superluminal speeds, so if it's necessary to dial it back, that should reduce the power reqs quite a bit. You could be going 40mph in unmod'd space, but move the space thousands of times faster than that. It's also worth mentioning that a recent redesign of the warp field's shape and pulsing it instead of running continuously would require far less energy (estimates run to about the annual energy output of the USA). All of this would mean that the volume of the ship is all that matters, and load'd or unload'd it would move the same.

    If I'm reading you correctly, that last line is a problem, but easily fix'd. A drive that distorts space must work over the top of the distortion already there from the cargo, and the difference in magnitude between the gravity of a tank of H and a hunk of U is negligible because distance is the more important factor for local-scale objects that aren't made of neutronium. That could be taken as-is, or because the distortions are additive rather than multiplicative because of how the field-strengths map to each other. The distortions get quadratically weaker with distance, so volume is more important than mass for affecting gravity fields.

    You could use this to displace inertia if you tried hard enough, or use a similar drive that crawls through space rather than drags it. A drive that got its thrust from an effect like Frame-dragging would also work to solve the inertia issue, though not the displacement mass problem. While either of these can be used to dampen inertial shock, a Frame-drag based drive imparts actual velocity, meaning you don't stop moving if your engines die, just stop accelerating.

    As for some more traditional thrust, if your plasma reactor is two-stage, it could create safe waste for direct thrust. Use the D and regular Protium to make He-3, then use that in your plasma thrusters so that you don't give everyone radiation poisoning. There's some talk in the present of using this instead of chemical rockets to make going to space more efficient, such that we could go to the moon or mars etc much more casually.


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