Quote from: Bren;854885I do to. For Star Wars. But SW is space opera. You can tell by William's leitmotifs.

Or the ship is in atmosphere.

Quote from: Bobloblah;859145You know, this stuff is the sort of thing that makes a great RPG supplement, basically: Faking Orbital Mechanics, or some such similar title. Practical, in-game solutions and shortcuts for these kinds of questions. Maybe you need to branch out in your writing?

Thanks, I have some stuff put together and slowly working on it amid my other projects. It will be pretty graphic (but not math) intensive to illustrate the various concept.

There is already a similiar on-line source Winchell D. Chung's Atomic Rockets

http://www.projectrho.com/public_html/rocket/index.php

However he goes for comprehensive, and includes some math. Good for world building not so good for at the table.

Also a comment on Traveller's burn, turn, and burn default.

A brachistochrone orbit is the quickest route from point A to  point B. It is the curve that result from constantly accelerating, flip, and constant decelerating to the target. This can be done by any thruster that has a thrust rating above that of the local gravity field. Drives that put out .01 G (actually slightly lower) can use these orbits.

So wait does that mean I am all wrong?

The orbit always works when you taking off from the central body of a system for example from the Earth to the Moon or anything else orbiting.

Where it is inaccurate is if you are taking off from one body orbiting a central mass to another body orbiting the same mass.

The inaccuracy is that the central mass can act as a barrier and block the brachistochrone orbit thus increasing the travel time as another orbit needs to be used.

In game terms this amounts to the fact that for a certain percentage of time you can't get to the destination the fastest way. Which is how it should be handled. Just roll if it is low then increase the travel time. I don't remember how much the increase is off hand but from what I remember it can be a lot depending on the position of the two planets.

Also it comes into play if you are landed on the central body in this case the ground beneath the craft is the barrier that creates the longer travel time.

Also for gaming it is only something that needs a ruling when timing or positioning is important. For the earth moon system there the longest wait for a window is never more than 24 hours and in most cases there are multiple opportunities throughout the day.

Quote from: estar;859188...
In game terms this amounts to the fact that for a certain percentage of time you can't get to the destination the fastest way. Which is how it should be handled. Just roll if it is low then increase the travel time. I don't remember how much the increase is off hand but from what I remember it can be a lot depending on the position of the two planets.
...
Also for gaming it is only something that needs a ruling when timing or positioning is important. For the earth moon system there the longest wait for a window is never more than 24 hours and in most cases there are multiple opportunities throughout the day.

The relative positions and motion of planets in a system make a big difference for travel between planets, unless you have extremely fast propulsion. And they change in complex but predictable ways (orbits) that place over months/years/decades or more, so "Just roll" only works if/when your game context is just doing a hit and run in its involvement with the system (or the players don't care). If it is relevant to the game's interests (e.g. if the players care about the travel conditions in the system and their changes over time), then planetary arrangement is a long-term condition which would make sense to have a schedule for, even if it's abstract and initially generated by rolls.

I would say attempting to work out orbital mechanics for a star system to work out less than general distances only works if you are setting your game in a single, or very very limited set, of solar systems.

For a game like Traveller, where the 'setting' is potentially hundreds of star systems (thousands if you REALLY want to play the entire setting...), that's impractical.

While I recognize that a difference of a few thousand, or hundred thousand kilometer differences between potential gas giant orbits may be a big fucken' deal, there is a practical value in having generic orbital ranges that are true for every system.

It depends on what is important for your sense of immersion. I will tell you, with some experience over at the Citizen's of the Imperium forums, that there isn't a practical level of realism that makes everyone happy. I think Marc Miller did a pretty good job of guessing the approximate level for a gamist focused system. The biggest complaints amongst superfans seems to be about the economic model of the game, not the space mechanics.

Quote from: Willie the Duck;859345It depends on what is important for your sense of immersion. I will tell you, with some experience over at the Citizen's of the Imperium forums, that there isn't a practical level of realism that makes everyone happy. I think Marc Miller did a pretty good job of guessing the approximate level for a gamist focused system. The biggest complaints amongst superfans seems to be about the economic model of the game, not the space mechanics.

Definitely a bad news / good news situation. The economic model is flawed. But at least Traveller has an economic model.

I would not complain about a referee working out the orbital distances and rates of an entire star system and keeping track of where all the various bodies in that system are. It is not a level of detail I would want to track most of the time.

Yes, I know that using traveller maneuver drives it could be quite significant where insystem body A is relative to body B, and some adventures could hinge on plotting the most time efficient course.

Most of the time though I am more interested in what happens planetoid.

As for the intersteller economics of Traveller, I just play that game as a game. There are plenty of other things in RPGs that make me twitchy. I am happy that Traveller at least recognizes that economics will be important to the setting.

Quote from: Skarg;859208The relative positions and motion of planets in a system make a big difference for travel between planets, unless you have extremely fast propulsion. And they change in complex but predictable ways (orbits) that place over months/years/decades or more, so "Just roll" only works if/when your game context is just doing a hit and run in its involvement with the system (or the players don't care). If it is relevant to the game's interests (e.g. if the players care about the travel conditions in the system and their changes over time), then planetary arrangement is a long-term condition which would make sense to have a schedule for, even if it's abstract and initially generated by rolls.

In a tabletop RPG it is crucial in order to adjudicate a river crossing to know the rainfall, the slope of the terrain, the type of stream bed? All of these important factors in determine current speed in a river.  

While a referee could come up with all these factors and do the math to figure out the current speed, it is overkill and in my opinion doesn't add much to the game if anything at all.

However if you know that there was recent rainfall, that the slope is steep, that the river bed in this section is rocky that is sufficient to rule that likely what the PCs encounter is a set of raging rapids. The same if referee said the river is moving slowly if the PCs encounter a section of the river in a flat terrain, on a mud bed, with normal rainfall.

Coming up with the relative positions of multiple bodies along with the attendant delta-vee/distances number take a similar amount of work to calculating the exact current speed of a river.

It is deceptively simple in it looks simple with a sun in the center and planets orbiting around it. But when you try to do anything with it you quickly get into what I would call overkill math. To be precise it because of how the math for orbital mechanics works.

The exact distance isn't the central piece of data for the "terrain" of a solar system. Just as the exact current speed isn't for the terrain of a river.

For a river all you really need to know is if it is slow, swift, or raging.

For solar system travel all you really need to know the minimum travel time, the maximum travel time, the length of the windows for the two, and how often the two windows cycle. You then just roll to see where you are in time relative to the two windows. That will give you a percentage to
compute the actual travel time.

Many sci-fi games, including traveller, use standard orbits which mean making this in to a easy to use table straight forward.

Finally my point is that you can have more realistic abstraction that is no more difficult to us then using a continuous acceleration formula on a fixed distance.

As a side note it is perfectly understandable why Marc Miller and the other Traveller designers opted for the acceleration formula. It wasn't until the early 2000s that there existed practical software that a layman can use to get a sense of how orbital mechanics work. Prior to that you had to learn the math and even with the computer crunching the number that wasn't the same as actually seeing what is happening.

But today with programs like Universe Sandbox, Orbiter Space Simulator and the Kerbal Space Program you can see the practical effects of different orbital manuevuers and use that knowledge to make a more realistic ABSTRACTION for RPGs.

Quote from: Willie the Duck;859345It depends on what is important for your sense of immersion. I will tell you, with some experience over at the Citizen's of the Imperium forums, that there isn't a practical level of realism that makes everyone happy. I think Marc Miller did a pretty good job of guessing the approximate level for a gamist focused system. The biggest complaints amongst superfans seems to be about the economic model of the game, not the space mechanics.

In my experience most people don't get orbital mechanics. Not because they not knowledgable but rather a lot of it is counter-intuitive or just that math heavy. So even the hard core traveller fans don't debate it much. Plus the focus of Traveller is rarely on in-system travel. But interstellar trading has always been a popular type of campaign for Traveller. So a lot of attention has been focused on that.

For the former, you have to slow down in order to catch up with a target in orbit*.  For the latter, computing travel windows is very math heavy. The best a designer can do to make something practical is to present a table.

*This is because lower orbit travel faster around the central mass than higher orbits. So to catch up you need to drop to a lower orbit which means you need to slow down as lower orbits have less energy. The trick of actually intercepting a target in orbit involves waiting until the right moment when you can put yourself into a eccentric orbit (an orbit that more of an ellipse than a circle) that intersect the target. Then when you reach target burn to match it's velocity and orbit. If you are close enough (500m or so) you can treat the situation as if taking place on a flat table.

Quote from: Elfdart;859156There are screenwriters better than Tom Stoppard out there? Better editors than Roger Barton? Can you name a few? I'm genuinely curious.

I'm curious where you get Stoppard credited with the screenplay or script.

A change in the relative positions in orbits of planets changes how far you've got to go and the difference in vectors. Constant acceleration works if your starting vector is what you want to have at the end; otherwise you need some additional at one end or the other.

As metaphor, driving Highway 65 might always work, but the matter of getting to and from Highway 65 can make a big difference in the trip.

A table of average travel times does not represent the great variation at different times.

Depending on the details, you might get there not just faster but sooner by delaying the trip. (That's counter-intuitive since Highway 65 is always first in first out, "rush hour" snail crawl or not.)

Quote from: Phillip;859401As metaphor, driving Highway 65 might always work, but the matter of getting to and from Highway 65 can make a big difference in the trip.

Except in the case of in-system travel the highway is always bending towards the central mass. Which is why trying to compute a travel time once you are on the highway is not simple let alone figuring out how to get on or off.

Quote from: Phillip;859401A table of average travel times does not represent the great variation at different times.

You are right and that not the table I am talking about.

Regardless of the numbers a in-system trajectory works for a specific departure time with the two bodies in specific positions. This which work out to a specific arrival time using a specific amount of energy over time.

Most RPGs use an abstraction of computing a straightline distance and a constant acceleration profile with a turn in the middle.

A better abstraction is to use a table of windows for maximum travel and minimum travel times,  and how often they occur. You roll to see where you are in the time period and use that to determine the travel distance.

For example period of windows appearing for Hohman trajectories is little over 2 year between earth and mars. You could go to mars at any time but the window for a minimum energy transfer is only roughly 2.1 year. Any other time involves more energy.

But for Traveller we are not talking about Hohman trajectories but rather the continuous acceleration profile of brachistochrone trajectories. And like Hohman trajectories there is a window where if you launch you reach your destination in the shortest time. If you launch at any other time the voyage will take longer.

And the distance travelled is not when the two planets are closest to each other. Nor it is the distance between the two a certain degree apart. But it is periodic and that allows a table to be created and dice rolls to be use to see what the situation is at a certain time.

And if a referee wants to track relative positions you can supply the degrees of separation at which the maximum and minimum windows appear and then use a proportional system to figure out the in-between times.

For example for Classic Traveller using Book 6 where Terra is in Orbit 3, and Mars is in Orbit 4. One entry of the table will have from Orbit 3 to 4, the minimum travel window occur every X weeks and is open for X days/week and will take X hours/days. The maximum travel time takes X hours/days/weeks.

For the guys who think tracking the relative positions of planets important you would include the following.
The degree of separation for minimum travel time is X degree, and extends +/- degrees. The degree of separation for maximum travel time extends +/- degrees.

So if you determine are a quarter of the way between minimum travel times. You figure out the travel time as (Max Time - Min Time /2) + Min Time.

Quote from: Phillip;859401The same for the other orbits.
Depending on the details, you might get there not just faster but sooner by delaying the trip. (That's counter-intuitive since Highway 65 is always first in first out, "rush hour" snail crawl or not.)

For every acceleration rating or amount of fuel if you are using rockets there is always a minimum travel time or energy expenditure where you can't get any better result. Obviously if you get a drive with a better acceleration or more fuel (to a point) you will get better results.

I understand that calculating actual travel times in a solar system is quite complex.

Here's the thing though. Even if it were a Star Trek game, planets move in orbits predictably on a scale of months and years. There are certain reasonable questions that even someone who understands what that means at a pre-pre-algebra level might reasonably ask and expect an answer more consistent that rolling dice on a table each time it's asked, even if the table is brilliant and accurate. Questions like what's the orbital period of each planet (i.e. how long does it take to go around the sun)? Because it should be possible without too much math to at least have planets that move in orbits and have that determine their distance, and not have the GM roll each time it's asked even about the same planet later in time .

Now, I realize that the distance isn't all that's needed to determine the travel time for ships without FTL drives, and the less thrust they have, the more relevant all the other orbital math is, but it would make sense to be have the travel times fluctuate on an appropriate schedule, and to have the values be predictable. Because, the changing travel times between planets could be very relevant for planning what happens where when, whether it's a simple itinerary for the players, or for trade patterns, or for fleet deployments, or whatever.

On a related but different note, I wonder if a (perhaps simplified) system for doing such math couldn't be (or hasn't already been) put into a handy software tool somewhere. I know gamers like to be able to roll some dice for everything, but computers can be great for doing complex game math and record-keeping...

Quote from: Skarg;859409I understand that calculating actual travel times in a solar system is quite complex.

Here's the thing though. Even if it were a Star Trek game, planets move in orbits predictably on a scale of months and years. There are certain reasonable questions that even someone who understands what that means at a pre-pre-algebra level might reasonably ask and expect an answer more consistent that rolling dice on a table each time it's asked, even if the table is brilliant and accurate. Questions like what's the orbital period of each planet (i.e. how long does it take to go around the sun)? Because it should be possible without too much math to at least have planets that move in orbits and have that determine their distance, and not have the GM roll each time.

Actually you don't have to roll each time just the first time. Because you know how long windows appear you just keep track of in-game afterwards. However the point of a roll is to eliminate trying to figure where each planet is when the players arrive in the system. It not an important point as most in-system travel involves one or two destination outside of the main world.

Quote from: Skarg;859409On a related but different note, I wonder if a (perhaps simplified) system for doing such math couldn't be (or hasn't already been) put into a handy software tool somewhere. I know gamers like to be able to roll some dice for everything, but computers can be great for doing complex game math and record-keeping...

Yes there is however they are not a in a form easily usable during a RPG session. For example Orbiter Space Simulator has a Interplanetary Travel MDF that will give you a time in the future where you can initiate your hohmann transfer.

Kerbal has even a niftier function window that will display a delta-vee map based on your craft's capabilties. However it also focused on hohmann transfers.

My personal recommendation for a person serious about getting this right for a RPG campaign is to get Kerbal Space Program and practice orbital manuevuers and interplanetary transfer for a couple of session and you will develop a sense of how it all hangs to together. Learn the basic formulas of orbital math to get a sense of how long things take and that pretty much all you need to rule on the fly.

For myself I was deep into add-on development for the Orbiter Space Simulator for several years and created a complete simulation of the Mercury Space Capsule and worked some on the Gemini Space Capsule. I also helped the guys doing the Apollo Capsule.

http://www.ibiblio.org/mscorbit/

I have done hundreds of of maneuvers for my own enjoyment and in playtesting my add-ons. In general I can summarize the experience as figuring out the exact numbers is very hard, learning how things generally work is not, but it is not intuitive and there is a learning curve to master the details.

As for why I never wrote up anything because while I am good at creating hardware simulation, I always relied on more savvy people for the formulas. I know enough to do a writeup for hohmann transfers and windows but not enough for brachistrone trajectories involving constant Gs. I have built simulations of Traveller craft and flown them enough to learn that it is not point and shoot. You have still plot out a trajectory and you still only have limited windows in which to do them. Of course it is way better than kick and burn rockets. But again it is also not just point and burn either.

For example if you are trying to fly a Free Trader with 1G thrust or a Scout Ship with 2G thrust from the surface of the Earth to orbit you still have to following an ascent path.

If you can lift straight up to say 150 miles and starting thrusting. But you have to do it just right and maintain your altitude. I can fly by hand a ascent path by using a table of values called a pitch program. It tell me what angle my pitch needs to be at at what time to go into orbit.

The fly straight up and then thrust method not only requires automated control, it also take longer than following ascent path. Because too much of your thrust is devoting to pointing down to maintaining your altitude.

The more Gs your drive can put out as acceleration the easier it gets and the less time it takes but it never goes away. And at the higher Gs atmospheric heating becomes a factor. We are talking temperatures and plasma heating reaching level found at the surface of the sun. And there is the brute force beating of the atmosphere slamming into your craft.

Anyway my view is that playing RPGs is more about the experience in being someone else in another time and place than the numbers. So as long as you get the experience right, then the referee has done his job.