Saturday, November 7, 2015

Reactionary Drives

Rockets get no respect in a lot of SF these days. People would rather use technobabble to get where they want to go. Add to this the fact that a lot of game systems suffer from the One major world plus junk per star system trope. You've reached your destination world,  unloaded your cargo and/or sought other means to make your expenses back and show a profit, every other planet there is an ice/dust/rock ball and about as interesting as your dog's average day.

Okay, some people want reaction drives in Traveller. I wanted reaction drives in it many times for you know, reasons. Nothing freudian there at all. Nope. Move along in your air/raft ya hippies.

Okay there are some rules for reaction drives in Traveller. You just have to look for them. I'm referring to Special Supplement 3 Missiles in Traveller from JTAS #21 (published back in year 1110 of the Imperium).

Missile propulsion systems come in three types: continuous burn, limited burn, and discretionary burn. Continuous burn rockets are turned on and blast at full acceleration until they are used up and are to solid rockets. Limited burn rockets have their thrust set when they launch and seem to be a form of solid rocket. Discretionary burn rockets are similar to liquid propellant rockets.

A fifty kilogram missile can accelerate to 10 kilometers per second on one kilogram of solid propellant. That means we're talking a specific impulse of (50 * 1000) 50,000. A quick scan of engine types on the Atomic Rockets website tells me the technical term for a rocket like this a torch ship. My personal term is 'Holy F*ck!' (I'm going to forget about what in hell a solid fuel torchship would be exactly.)

Solid propellant chemical boosters aren't that good. Not by TL 8 and probably not ever as we understand things. The only engines with that kind of bang for your kilo involve fusion, anti-matter or captured demons being tortured (with anti-matter or fusion). Saying it is some kind of fusion power doesn't help either. Something with that much boost would irradiate the ship launching it and the target. It doesn't need a warhead.

I'd like to point out this causes about as many problems as a reactionless drive. For one thing you don't really need nukes with chemical fuel sources that powerful. A hand grenade could knock out a tank. Even if you wanted to explain away Traveller's stealth in space due to exhaustless reactionless drives, the aliens at Tabby's Star are going to see one of these engines 3500 years after it fires up and take a break from building their Dyson Sphere to watch (once again, let me say, "Holy F*ck!")

The reasons for these incredible rockets are solid. The designer knew what he was doing. See a chemically propelled missile would never get anywhere near a ship with an M-drive. A nuke powered missile might get close if the pilot was narcoleptic or had a latent death wish. More to the point Traveller is a system based game. Realism isn't as important as giving people decisions to make, and keeping the game fun.

So the rockets are really variations on the maneuver drives some love to hate. They are smaller and as a trade use much more limited power sources than the fusion power of starships. Fine having gotten this far I'm damned if I'll stop now. I'd also like to point out that this is one way to limit players who want to use a gig boosted to .99c to kill off the dinosaurs on a target planet. It also gives a neat Firefly feel to your game. So let's go with the Limited Drive ancestor of the M-Drive (or loser cousin).

L-Drives are introduced at TL 8. Some cultures never make the leap of engineering that lead to the M-Drive or have a deficient reserve of lanthanum to build M-Drives (lanthanum ... it's not just for Jump Drives.)

L-Drives cost 1/2 that of a comparable M-Drive. Three quarters of the L-Drive are composed of fuel cells, energy cartridges or whatever to power it.

The L2-Drive is introduced at TL 9. It has greater efficiency similar to the Discretionary Burn engine for missiles.

Traveller ships mass roughly 10 tons per displacement tonne according to Megatraveller and various grognards. A 100 tonne Scout will weigh in at 1000 tons on a standard 1 gee field. Propelling it to 10 kps (100 millimeters in standard combat scale) will take (1,000,000 kg/50,000 =) 20 kgs of fuel or propellant or whatever (f/p/w). That's per second. One gee for one standard turn (1000 seconds) takes 20 tons of fuel. Wow.

That's for a solid fuel model which has the drawback of staying on after you light the fuse.

Discretionary burn models are more efficient at TL 9. They use their fuel up at 40% of that rate so the Scout uses 8 tons of f/p/w per 1 gee turn of movement. That's a little better. Our Scout's type A L-Drive has a measly .75 tons of fuel letting it boost for short bursts for docking and such. It'll have to dip into the 40 tons that would normally be for jump fuel to power more burns. Twenty tons of fuel will let it thrust at 1 gee for 2.5 turns (it goes 50 millimeters in the third turn.)

What does this mean for a campaign?

Change your nomenclature. If the L-Drive runs on easily depleted energy sources then that isn't a fusion plant you're lugging about to power jump. Then again some people were never happy with a free trader putting out enough energy to melt the polar ice caps to jump  a parsec. (There's no technology that doesn't have a destructive application in the hands of a player character.)


Scouts become more a courier than a than an explorer with their low capacity for propellant. Almost every ship is going to need drop tanks for extensive maneuvering or travel. Ships are going to fall into two classes: ftl ships that go from one jump limit to another and in system ships with more copious tankage that zip around. Jump drives become more important for those really long trips. It will be more fuel efficient. A Scout Courier might be fitted with external tankage dropping its jump range to 1 parsec and flit about on internal tankage. That would give it 12.5 boosts or a delta vee of 125 kps which would let it travel an AU in a month. Not bad by 21st century standards.

Missiles are suddenly terrors in their own right. No ship will have enough boost to dodge more than a couple of waves of the things. ECM, Anti-missile programs and sand suddenly become very important.

Those 'solid fuel' can also find use as a a strap on booster system to let ships take off from larger worlds or increase delta vee for the really long hauls. Any ship that can carry a landing vessel will to conserve its precious burns. Our Scout Courier might do most of its exploration by jumping in, firing off a bunch of RCVs and probes from its turret receiving readings for follow up missions and then jumping back out.

Planetary systems with several worlds settled will make use of space stations for fuel/propellant depots and other infra structure. Infra structure is good for giving a setting a lived in look and adventure hooks. A number of sessions can be run in the mostly abandoned or unexplored outer regions of star systems.

As a final note most of this obsession with space craft speeds and travel times is not necessary in running an actual game. Look what Ken Burnside has to say:


Worrying about the speed of your spaceship for an RPG is like focusing your WWII platoon RPG on the voyage over the Atlantic in a troop transport...


It's usually sufficient to say the characters are there and set them lose on the poor locals. All this detailing is mostly for referees. You'll have a better idea of what an outpost is like if the major world(s) is hours, or months away.