# Reasonable size of a sci-fi game planetary system

I have developed a 2D space trading game, yet in the final stage of the development of the game engine, I am struggling to find the optimal size of my planetary system(s) and the maximum velocity of my spaceship. Setting the correct dimensions is crucial for the further development of the project, as it lays the foundation for many elements, technology and story line. I encountered a few shortcomings during the worldbuilding stage and I would appreciate any help with it.

1. How can I creatively reason to a player an imposed speed limit of his or her spaceship, given that in reality with constant acceleration a spaceship is only limited by the speed of light? I chose to limit the maximum velocity of a vessel, as it otherwise becomes (technically) impractical to engage with other spaceship in a (dogfight) combat.

2. Ultimately related to speed, how can I determine the ideal size of an average planetary system (i.e. usually one star and planets in orbits), the number of planets and the time it could take on average to travel from a planet in the outer orbit to a planet closer to the sun, both in game time and real time? Ideally I would like the game to feel realistic and exciting to get across the planetary system, yet not feel too boring with longer periods of nothing happening. Currently I have set 1 (real) minute minute to be 1 (game) hour; what is a reasonably good conversion ratio?

3. I would like the trade to be challenging. However, if people can simply see the prevailing prices on each planet beforehand, there is not much challenge for finding the optimal trade. I can cut the communication but then how can I reason to a player that they can only see the prevailing prices of goods when they visit a planet, however, not when flying in space? Is there any other way to make trade more challenging and reason it to the player?

Notes: (1) Players control a ship or even a fleet of ships using mostly their mouse, something similar to that that (early demo of my game). (2) There will be asteroids to visit that can be mined etc. (3) I want to include around 200 AI players per planetary system, but can technically go up to as many as 2000.

Edit: To narrow the scope down, I though I will provide a little bit more info on the game so far--what it can do, what it can't and what I can implement.

• A key element of the game is that it contains an entirely simulated real-time economy with prices that are determined by supply and demand through AI generated traders and human players. There is a plan to include a interplanetary stock market, derivatives, commodities and the possibility to mine asteroids and build factories on planets that transform commodities in more complex products.

• In terms of propulsion technology, there will be two essential methods to get across. The first is within a planetary system by means of normal propulsion such as nuclear fusion, solar sails, anti-matter, impulse etc. This will be used for interplanetary exploration, trade and sometimes fights. There will be RPG/Adventure elements such as exploring ancient civilizations, galactic phenomena, nebulas etc.

• The other propulsion technology is enabling interstellar flight. The current working idea is that ships can enter a higher dimensional plane by creating a temporary rift in space. A ship then enters into a (nearly) higher spatial dimension where it is possible to travel light years in a relatively short time (e.g. days). As mentioned, this technology is independent from the first interplanetary propulsion and it only works when there are no major gravitational disturbances around the ship such as planets, asteroids or other ships. So you need to be flying a bit out of the planetary system to activate it and therefore it won't be possible to escape from a fight or as long there are objects around at a certain radius.

• Other ship technologies include different type of energy generators, shields, weapons, probes, radars, scanners. The ship is capable to have a fight, but not just dumb-down clicking around and about. With the correct speed it should rather feel tactical despite being in real time. It may take a minute to take another ship down, and therefore fights are not be overly frequent but if one wishes to engage in more fight than rpg/trade, that's up to them. As there will be RPG elements, gaining experience will increase maneuverability, aim accuracy etc., to make fights even more interesting.

• With regards to the story line. The current working version envisages that earth experienced an attack from an unknown force and while being transported out from the solar system with a small vessel (as slave/prisoner), there was an unknown energy burst on the vessel and you find yourself in a different star system with all but you dead. You manage to get out of your cell and learn to take control of the severely damaged ship and get to what looks like a space station. While this seems not to original, what people will realize during the game, that your ship not only had been transported hundreds of light years away, but also around 7 years back in time. So, in a time frame of seven years, you can explore the reasons behind this attack and prevent it from happening by means of either becoming very powerful individual (trade+diplomacy) or tactical combat (up to controlling a fleet to fight against the invading force).

• HDE. The Edit is misleading. I am asking for a solar system not stellar system. The difference is that a stellar system is according to Wiki: "A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction." That's not what I have and originally asked for as I am looking for a world around one star (solar system). Oct 27, 2015 at 0:03
• "Solar system" is misleading, too, because it refers specifically to our system - which these systems clearly are not. Would "planetary system" be better? Oct 27, 2015 at 0:06
• It looks like you have at least three questions here. One about system size, one about maximum speed, and the last about trade. The first two go together a bit and may require similar expertise, but the trade question seems like it would draw from a completely different area. Maybe reduce this down to one question and follow up later? Oct 27, 2015 at 0:26
• @Majte - "in reality with constant acceleration a spaceship is only limited by the speed of light?" That's only true for infinite fuel/energy. With a limited fuel supply, a ship can only use half its fuel to accelerate, reserving ~1/2 its fuel to decelerate on arrival. Fixed fuel capacity means fixed maximum speed. Oct 27, 2015 at 1:42
• None of these are really world building questions. There is a game development SE that may hadnle this better. In nearly every game (except perhaps Kerbal Space Simulator), the actual laws of space physics are ignored, because they're honestly not all that fun for people. Not even EVE really tries to get them right. Oct 27, 2015 at 23:15

There are 3 basic methods of transferring between orbits.

## Minimum energy

Involves expending enough energy to get to the "closest" (in energy terms) Lagrange point. From there, small course corrections to steer your vessel between the planetary system's Lagrange points can keep the overall energy expenditure much lower than even a Hohmann orbit transfer.

A Minimum energy orbit transfer will likely take decades or more for a Earth to Saturn voyage. It would be suitable for the transportation of bulk materials that do not spoil. You might consider this the "ground transportation" version of space travel.

## Hohmann Transfer

A Hohmann orbit transfer is the lowest energy direct orbit transfer method. Hohmann orbit transfers are sometimes called "bang-bang" transfers because they typically involve 2 main propulsive maneuvers (one to change your current orbit to an ellipse intersecting the new orbit & one to change the ellipse at the other end to the new orbit).

A Hohmann orbit transfer from the Earth to Saturn might take 6 years. It would be suitable to material needed soon but not ASAP. You might consider this as the "priority" shipping.

## Brachistochrone

Is a direct thrust orbit transfer - in which the vessel thrusts constantly during the whole voyage. Brachistochrone trajectories are limited by the amount of propellant, the amount of energy, and the maximum thrust of the engines of the ship.

There are very few engine, propellant, & fuel combinations that can accomplish brachistochrone trajectories. Realistically, only extremely low thrust propulsion systems (e.g. solar sail, ion thruster) or extremely energetic systems (e.g. fission, fusion, or antimatter) systems can accomplish them.

A Brachistochrone trajectory to Saturn might take months. You might consider this the equivalent of "over night" delivery.

For the purposes of your game, you must consider the costs associated with each of these propulsion schemes. Clearly even if your society can and will use fission, fusion, and anti-matter propulsion; the ship using those fuels must pay a premium to get them. Fuels like oxygen-hydrogen would be far FAR cheaper per unit mass and much easier to get but wouldn't be as energetic.

So you could limit your ships based upon technology (they can't do fusion or anti-matter), resources (not very many places have the refining capabilities to extract the correct isotopes), and/or cost (if you're moving asteroid rocks, you're not going to pay the premium of high energy fuels).

Hopefully, the game will allow the players to chose which things he moves using which trajectories. If he always selects methods using the high energy trajectories, maybe they eventually run out of those resources.

For more information go my favorite world building resource, Atomic Rockets. This is the link to the "mission tables" which shows the $\Delta V$ required and time it takes to complete each trajectory.

Just remember though that the minimum energy trajectory is essentially the Hohmann transfer to the nearest Lagrange point plus some additional $\Delta V$ for adjusting the trajectory at each subsequent Lagrange point.

• That's already an excellent post. I was already thinking of player being able to chose between Brachistochrone transfer (mouse click and go) and Hohmann transfer (by setting a course) which then will use the fuel differently. Is there a approximate way to calculate the energy usage of these two methods? I have already my planets and orbits in place by using Newtons law of gravitation. What would be the max. speed achieved of a realistic Brachistochrone transfer? Oct 27, 2015 at 2:06
• It's late night and maybe I am tired but I can't find the link :) Oct 27, 2015 at 2:39
• projectrho.com/public_html/rocket/appmissiontable.php Even with the link, it'll take a lot of reading and thinking to find the answer, I think. Oct 27, 2015 at 2:43
• According to the site, with 1.0 g we have a 4 days Saturn return trip! I think this sounds quite realistic and I can assume that having an energy sources that could constantly propel from 1 g to let's say 5 g would be an realistic assumption, if we also assume that there are some inertial dampers that can absorb some but not all of the forces. Oct 27, 2015 at 12:24

How can I creatively reason to a player an imposed speed limit of his or her spaceship, given that in reality with constant acceleration a spaceship is only limited by the speed of light? I chose to limit the maximum velocity of a vessel, as it otherwise becomes (technically) impractical to engage with other spaceship in a (dogfight) combat.

If you're sure you want to limit the player to always having to do dogfight(I for one am an explorer type, so I'd just accelerate away every time), then you could for example limit the fuel in following way: The spaceships generate their fuel while not using their engines. This could be rationalized with a system of them bringing the components on board, but having to do some catalyst-process in order for them to create the actual compounds used as fuel. You can therefore only speed up for a limited amount of time before you require to "refuel".

How can I determine the ideal size of an average planetary system, the number of planets

I'd go for as random as they can get. Maybe a young star only has a huge asteroid belt and no planets, while some stars can sport a dozen or so planetoids and moons to boot. There's no way to determine "realism" for the average planetary system, as we don't have the technology to really determine other star systems currently. But for the explorer, vastly differing systems would increase the interest.

the time it could take on average to get a planet in the outer orbit to a planet closer to the sun, both in game time and real time? Ideally I would like the game to feel realistic and exciting to get across the planetary system, yet not feel too boring with longer periods of nothing happening.

This is going to have to be either/ or, but not both. If you go for any sliver of realism, then it's going to be boring for the action players. As an example, Elite Dangerous. Even with the automatic docking and star drives, the game still suffers from boredom of the huge distances, if you don't like the serenity of cruising around in empty space. The universe is huge and on average, empty. Take the known planets to have life divided by the planets that don't have life and we'll approach zero quite fast. The distances are at best only mind bogling, and this translates to hours of boredom, even with your accelerated time frame. Kerbal Space Program also provides a good idea on how boring it really can get with any amount of realism: KSP has the whole solar system with its furthest away planet closer to the Kerbol sun than the Earth is to Sol, and still it takes several hours to transit even to the closest moon. Granted, the craziest missions where they just basically go with solid rocket boosters to the Mun and back take but minutes, but that's besides the point. Don't worry on the realism, create a balance of fun and "that's just ridiculous".

In a 2D game, the only realism to be had might be to pre-plan your escape from and entry to any solar system, using planetary masses to do a gravity assisted boost. But it only makes sense if the fuel for long distance traveling is scarce enough to justify not just simply turning the accelerator to 11 and aiming for your target.

However, if people can simply see the prevailing prices on each planet beforehand, there is not much challenge for finding the optimal trade. I can cut the communication but then how can I reason to a player that they can only see the prevailing prices of goods when they visit a planet, however, not when flying in space? Is there any other way to make trade more challenging and reason it to the player?

You can make the tonnage affect the acceleration, therefore slowing them down and making it risky to try and haul a huge mass of cargo across the stars for a profit.

The communication can be limited to either the nearest star systems. This can easily be justified with "signals can't go faster than light". This way you could have a system in play that shows a spreadsheet for the player on the known prices, coupled with the "age" of that information. These prices could even be updated on the fly, so as your flying closer to a star system, onboard computers update the price from signals coming from that planet(4 weeks old, 2 weeks old, week old, current).

You could only show the player prices from their last visit to any known system, and a list of planets with a shortage on some cargo. Combine this with the previous signal limitation idea, and long hauls would almost always be a gamble in terms of profit.

1. You could tell the player that using more fuel will, of course, make them fly faster, but they may not have enough fuel for a deceleration burn to go to another planet (or use any fuel to turn in a dogfight)

2. The size of the system will depend on the speed and acceleration you allow your spacecraft to have. Even if you could travel at the speed of light, it would still take several hours to fly from Earth to Pluto (at its closest!).

3. Limit the range of communication to some distance from each planet, and disallow superluminal comms. If they can't see the price until they are close (or if it takes say 5-10 real-time minutes for them to get a price update from a faraway planet), then there may be some trade challenges. Other methods could be locking in a price before taking off - for example, flying cargo from A to B, doing a flyby of C and seeing the price is better at C, selling there (and paying a penalty for cancelling the A->B route).

• The price locking is what I don't want as I cannot possibly reason that it is not possible for a player to get the information of the current price on a planet, while having a modern spaceship that can do incredible things and even interstellar flights ;) Oct 27, 2015 at 0:17
• What I meant by price locking is like how Euro Truck Sim 2 works - they have jobs in a queue you can take, set to expire at some time in the future, without having to communicate to the recipient that you're taking the job (as long as it's picked up from its start point, it's fine). The job price can't change during the flight, but you might get a better price by dropping it off earlier in the flight elsewhere Oct 27, 2015 at 0:20
• I see, that could be an interesting concept, however, it is also based on not being able to communicate with other planets during flight, right? Oct 27, 2015 at 0:26
• Sort of -- the idea is that each planet has a radius of communication (it can communicate maybe every 1-90 game minutes), and can update with every other planet less frequently (maybe 1-3 times per game day). it would allow a craft near a planet to get updated info about that planet, but about nowhere else Oct 27, 2015 at 0:29
• Would that sell to the audience without making the game feel too plain, you reckon? One strong part of my game is that the economy is entirely simulated with stock markets and forces of supply and demand (I am an economist by profession). Oct 27, 2015 at 0:31

I would explain the speed limitation as a limitation of the deflector system. At very high relative velocities even tiny specks of spacedust pose a serious perforation risk. Just say your deflector shields can't reliably deflect dangerous objects moving faster than # relative to the ship. (This also gives you an excuse to force players to enter asteroid belts with the orbital flow rather than against, which might be an interesting tactical mechanic.)

• Interesting idea. As it is now, my ships have shields that can deflect missiles and other deadly weaponry that have high velocities. It would be less convincing to argue with dust particles. Oct 27, 2015 at 11:44
• Good point, but there's strong sci-fi precedent for combat deflectors being much more effective than standard flight deflectors. ("Shields up!") Oct 27, 2015 at 11:53
• Maybe it could be argued with Inertial dampers (like in the Star Trek franchise), where speed is limited as otherwise you would end up splattered on the wall or the systems will fail... Oct 27, 2015 at 12:22