DIY space travel.

As this is way above my pay check, I can't contribute much except with my encouragement: very useful thread! I will stay tuned and see what the rocket scientists on this forum comes up with.

May I add that this is soooo much more helpful than just arguing about whether the game developers should do this or not as we are doing here http://www.eclipsephase.com/eclipse-phase-space-ship-combat-game

Good luck.

My suggestion is that, unless "EXACT" is a preference, that boiling travel times down to a set of tables might be worth considering. Start with the vessel is and wherever it is, then cross-reference to destination, for which there would be a set of (very approximate) travel times. I'm thinking maybe three separate ranges (best case, worst case, and middle case), and the Referee determines (by whatever means is preferred) which set is applicable at that time.

Not sure if it is correct, but this calculator could be useful (although we do not know the acceleration of the EP rockets).

http://home.att.net/~srschmitt/script_starship.html

[edit] We do know the acceleration, but not the Delta V [/edit]

So, from pg 384, "Anti-matter drive fast couriers are vessels designed for this specific purpose...is able to reach as much as one half of one percent of the speed of light. To manage this, the spacecraft must also carry 6 tons of antimatter..."
So if it can accelerate at 0.05g (roughly 0.5 m/s^2) and reach a speed of 0.005c (roughly 1,500,000 m/s) it has to have a total acceleration time not less than roughly 3 million seconds, or roughly 34.72 days on continuous acceleration. Assuming that's the suicidal no-way-to-stop option and you normally want to accelerate for only half of that, but doesn't that that get us the burn endurance and delta-V that we wanted?

Using the Spacecraft Travel Time simulator with a burn endurance of 833 hours to go the mean 6.5 AU from Mars to Jupiter (acording to Wolfram Alpha), which pg 348 advertises will take a month, takes a calculated 32 days, accelerating/decelerating the entire time, so that's just about perfect.

Of course, I can't make the advertised week-long trip from Venus to Mars in less than 12 days, unfortunately.

Wait, Wofram Alpha also says that the distance from Venus to Mars is less than the distance from Earth to Mars. Houston, we have a problem.

Seems reasonable. In any case, it seems like we've arrived at a reasonable burn endurance for a fully-fueled fast courier, so that's useful, right?

For Mercury, just set the minimum time frame, +1 month, +/- 1 month.

(But yeah, a travel chart would STILL be useful. Assume every ship will use approximately the same route, then just make a multiplier based on ship acceleration and delta-v. Since this isn't NASA, if we're a few weeks off it doesn't make a huge deal.

Better still, a computer program which adjusts with the date. If you spent 3 years to get to Neptune, your chart is now out of date.)

In an attack of utter, irresponsible overdoing things I have hacked together a Matlab program for running orbits. I based it on this page and this Matlab package for solving the Lambert problem. I can now calculate porkchop plots of the necessary Delta-V for going from one planet to the next, given a certain start date.

With jsnead's numbers for total delta-V I can figure out what trajectories are possible for different kinds of ships. It is interesting to see that bulk carriers and slow scum barges are actually often constrained in where they can go (they need to consider suitable launch windows, gravitational assists or make use of the ITN), while standard transports move around pretty freely in the inner system. In the outer system things take time, even with pretty good engines: even the fast courier does need a few weeks to get around in the outer system (for example, a typical time from Jupiter to Saturn is 14 days).

A few issues I need to decide on, where I would like to hear suggestions:

How close can a ship go to the sun? Obviously not closer than 700,000 km, but likely there is a much larger safety distance.

Does it matter that I can't calculate close encounters with Jupiter efficiently?

What start date ought I use?

How to present the data so it is usable from a gaming perspective? One possibility is that I create a script that generates porkchop plots for all possible trips between a number of destinations (the major planets, key asteroids, some of the Lagrange points) and then puts them onto the web. It would be nicer to just have a web interface, but I am not sure how to port these scripts to that. PHP is not known for its numerical power.

I also ought to expand it to use http://ssd.jpl.nasa.gov/?sat_elem for calculating trips within the Jupiter and Saturn moon systems.

I wouldn't stress too much about Jupiter. We just need to know approximate numbers. Does it take a week? A month? Six months? A day here or there isn't going to make or break things.

I would tend to write it up in Java, but that's because I'm more of an app programmer than a web programmer. It's been a while since I've meddled with these things, so I might not be much help at the actual coding side. Is it possible to use these matlab things without having to go out and buy anything crazy? Without having to learn a whole new system?

2147 would be a fairly good date to use. Its far enough into the future that the 60BF events could easily occur (And some of them line up very well) and the planets are also in alignment as per the System map in the Core rulebook. If I remember right it is also one of the dates that a Tour of the System flight could occur.

Okay, pretend I'd never seen a porkchop plot chart like this before (hint: I haven't) and pretend I'm stupid (no hints there).

What's the red mean? What's the blue mean? What do all the squiggly lines mean and why do they have numbers on? Also, why is travel time on the X-axis instead of say... delta V? I'm pretty sure if I'm taking my little freighter out for a run, I'm more likely to already know the delta-V and the starting day than I am to know the trip duration.

OK, here is my attempt at a "how to use a porkchop" tutorial:

You are at planet X and want to go to planet Y. You select the right plot. This plot will tell you how much energy you are going to need if you want to go from X to Y at a certain time and arrive at another time.

The colours denote the total amount of delta V needed for that particular combination of start and end time: red means a *lot* of delta V, while dark blue is a small amount. If your spaceship can produce a certain amount of delta V, then you can make the trip in the regions that require less. If you are a real cheapskate you will try to use one of the bluest regions (where a little white dot denotes the cheapest possible trip in the year). If you have resources to spend (hurray for your Oversight expense account!) you can deviate from this. However, if the required delta V is too large for your ship you will end up stranded: either you cannot catch up with the destination, or you cannot match speed with it and will just go past.

Now you find today's date on the vertical axis (I just used "days since 1 Jan 10 AF" here, but imagine it labelled with a nice almanac). If you want to start today, go right from this point. The horizontal axis denotes how many days the trip will last. The leftmost part denotes super-fast trips taking just a few days. They require enormous energy and go nearly in a straight line from X to Y. Most likely your ship is only rated for a certain delta V, so you can't make these trips. Continue right until you reach the contour corresponding to your delta V rating (say 400 km/s for a standard transport). This point is the fastest trip you can make to your destination that your ship has fuel and power enough to do. If you continue to the right you will likely find slightly longer trips that take less fuel, up to a point. Beyond that lies less useful orbits: slow trips that take more fuel (and sometimes a really slow trip that happens to be cheaper because the destination lines up nicely with your approach).

However, sometimes it is smart to wait until later to launch. Especially for very weak ships like an old scum barge with plasma rocket there might not even be a feasible trip today. But if you wait for a while the planets will move and new trajectories become possible. If you go upwards in the diagram you can find regions of launch date/trip length that are feasible. Even if you have a pretty powerful rocket it is sometimes smart to wait a bit for a good alignment making the trip shorter.

Had this been a real interactive program you would have seen nice graphs of the suggested trajectory, navigation hazard information, slingshot possibilities etc. Generally the thin brighter green/cyan lines (sometimes looking like isolated points) represent trajectories that go pretty close to the sun. The diagrams could of course be extended arbitrarily far into the future, these just show trips starting within 10 AF that last less than 1 year (and doesn't round the sun more than once; back in the early days of space travel people often traded speed for fuel efficiency by taking very roundabout trajectories). For travel in the outer system pretty hefty drives are required for trips shorter than one year - if you can egocast it is usually the best alternative.

In practice this is the kind of stuff your navigation AI solves, with no need to think too deeply about delta V or any plots (unless you happen to be professional/old-fashioned about it, like a sea captain who likes to know how to navigate by stars besides GPS). You just tell the AI where you want to go and it gives you a series of likely options and their price-tags/risks. Some bickering later, and your course is set.

{ For a taste of how one could do this interactively, check out
http://astrojava.com/ballistic-trajectory-planner
It is a somewhat temperamental program that has many options I have little or no clue about, but at least one can doubleclick at a point in the porkchop and then switch to the selected trajectory tab to see how the orbit looks. }

Partially tradition, since this is how the charts that NASA and ESA use look (more or less). But also because it helps figure out the speed-energy trade-off. Knowing that I can get somewhere in X days is not quite as informative as knowing that if I go a bit later or travel for X+1 days I save a lot of expensive fuel.

However, I decided to check what happens if you plot delta V vs travel time. Here are two results:
http://www.flickr.com/photos/arenamontanus/4933292966/
This one is pretty clear. But Mercury to Venus gets complicated because of the wide choice of low delta V orbits:
http://www.flickr.com/photos/arenamontanus/4933292910/
Personally I don't find them very illuminating. But they do answer (roughly) how fast one can get from A to B with a particular ship.