# Handling extensive in-system travel

I'm going to run a campaign that spends a significant time in a system with no jump capability. The players will be similarly limited.

This means that for most of the time they are going to travel between celestial objects in this one system. In system travel seems to be covered rather briefly by the core books. It seems to me that the idea is that you arrive 100D from where you want to be and travel in a direct line with a flip and burn strategy.

Since the focus for several sessions will be this system, I'm looking to give it a more realistic feeling of having celestial objects actually orbiting the star, which in turn should make objects change position relative to each other. And I would like to be able to have travel not be a matter of pointing the nose of the ship in the direction of where you want to be and hitting the throttle. As I understand it, in space you would increase your orbit until you reached escape velocity and then maybe do a braking burn to enter the new orbit around a different object.

The perfect answer is a reference to a book or text that provide a RPG system that solves this, but it is also an acceptable answer to explain how you have handled this in the past and/or what is the Traveller tradition of handling this.

• A question about the degree of realism you're going for: Hohmann transfers (the type of orbit-based space travel you describe in the question) take months and years between planets, mostly regardless of engine power (because the engine maneuver time is a fraction of 1% of the travel time); is months/years to travel between planets, or days/weeks between local moons, the kind of time scale you are planning this campaign around? Jan 20, 2017 at 8:04
• That is a good question SevenSidedDie. Is the M-Drive of a regular Traveller ship powerful enough to make normal orbital transfers irrelevant? If that is the case then I'm happy to stick with it, but I would still need to objects to move and a reasonable easy way to track distance/travel time between them at any given point. For instance mars can be "only" 60 million km from earth or it can be 400 million km from earth. Realistic assuming that the M-Drive works as described in the books. If that makes hohmann transfers irrelevant then I'll be happy with that Jan 20, 2017 at 8:29
• @SevenSidedDie Hofmann transfers are used because they are the most fuel efficient (excluding using the solar superhighway which can take decades instead of years) and fuel is the no 1 limiting factor on real spacecraft. If fuel is not an issue, then accelerating as fast as possible to halfway and decelerating as fast as possible the other half is the quickest transition but horribly, horribly inefficient. Jan 20, 2017 at 11:57
• @DaleM Yes, that's why I asked. Jan 20, 2017 at 15:25
• I feel like relativity would come into play with that sort of accel/decel profile. Dec 14, 2017 at 21:04

An excellent way to learn about real orbital mechanics (and a lot of fun, if a terrible time sink) is to get a copy of Kerbal Space Program.

By the time it's eaten all your RP gaming time for a few weeks, you'll know all you want to know about what's involved in getting from planet to planet the real world way -- right down to how much more fuel it takes to get somewhere fast compared to the minimum-fuel Hohmann transfer, waiting for a transfer window, and sitting in your spaceship while you wait through half an orbit (which can be decades for, say, a trip from Earth to Saturn).

Canonically, the maneuver drive in Traveller was a constant-boost type, and even at a "mere" 1G, such a drive can take you anywhere in our solar system in a matter of weeks (Neptune is six or seven weeks at 1G -- and it doesn't make much difference which side of the Sun the Earth is on relative to Neptune). A faster drive gets you there proportionally faster.

Therefore, there's no sense, if you have Traveller maneuver drives, in trying deal with Hohmann transfers or even cometary orbits, unless you have a fuel limitation (ships built with tiny tanks, so they can't operate the maneuver drive continuously -- a possible optimization for cost/profit) or something like system laws against using fusion drives near planets, where they do the most good. Failing limitations of that sort, anyone who has access to a constant-boost drive, even one good for a small fraction of a G, ought to use it, and if it's better than about a quarter G, point-and-burn, flip-and-brake is the most effective way to use such a drive.

BTW, this was known as long ago as the 1950s, at least. E.E. "Doc" Smith used this kind of travel model in Spacehounds of IPC, originally published in 1947.

Now, original Traveller had (as I recall) an orbital movement system for intership combat with miniatures or counters, but that's most likely more detailed than you want. It should be sufficient to say "It'll take you 374 days to get from Systema to the fourth moon of Gigantor -- fill out your time use cards." The problem with that is that few if any starships have that level of life support endurance and nobody in their right mind, in possession of a working jump drive, would spend that kind of time as opposed to making an intrasystem jump and getting there in one week, or using a constant-boost drive and getting there in three or four weeks (if jumps are impossible inside the system for some reason).

Even with a constant-boost drive in the 1G range, orbits will affect travel time a bit -- it's close to a week to travel from Earth to Mars, on average, but that figure varies by several days depending on relative positions -- nearest approach is less than a quarter what it is when the Sun is between. The only practical way to track this is by knowing the orbital distances and periods, and using geometry and trigonometry to keep track of distances. Not my idea of fun, but should be possible with a spreadsheet...

• I did play KSP a lot, and that is one of the things that got me thinking that I should include a more realistic way of travelling between celestial objects. Jan 20, 2017 at 14:30

This was solved pretty comprehensively for GURPS Spaceships by an article in Pyramid #3/79, entitled "Half-way to Anywhere". It comes with an Excel spreadsheet, which is necessary given all the calculations involved. It isn't specifically for Traveller, but it's done in real-world units, so you should be able to join it up to your game.

Orbital 2100 has a nice system that is designed to integrate with "Classic 2D6 SF games" (i.e., Traveller.) It's setting is at TL-9 with TL-11 electronics and no jump drive.

In Orbital 2100, the ref keeps track of where the planets are in relation to each other by moving the planets on a track where each sweep equals 1 month and you determine travel distance by counting the number of sweeps and adding a constant. (This is only done with the inner planets, the "constant" used for the outer planets is so huge that there isn't a point in tracking orbital minutia.

They have two different maneuver drive systems. One is a constant G reaction drive (up to 1G) and one uses an "accelerate-coast-decelerate" mechanic.

• I'll second Orbital 2100. It's a neat supplement for playing a "lower tech" Traveller game.
– rje
Dec 18, 2018 at 5:08

In Astrosynthesis v3, software from NBOS, you can model a solar system (and it will generate them, but warning few will be habitable without massaging) and by date see where the planets are and use a measuring tool to figure distance, but that's not the whole story. Your target is moving too so you likely would need to use a numerical methods approach to solve for your intercept time and distance. An abstraction would probably be more useful to you for gaming.

If you have limited fuel mass, older drive types, and long distances, you'd better have early cryosleep options and a willingness to sleep away years of time.