Gravity, or Electricity and Magnetism.

you could theoretically do an impact just like the trailer, if you killed all horizontal velocity relative to the planet, and then reoriented the asteroid to face towards the target planet and begin to thrust again, but your right, the planet would have to stop spinning for you to hit your target

The N-Body problem was mentioned. It is a hard problem to *predict* the outcome of an N-Body problem, but it is *not* a hard problem to simulate it, which is what a computer would do. Even doing the calculations for a few orbits and then just using that orbit (so it is a realistic orbit but can never change) would be easy.

Some simulation would probably be needed if you want realistic looking KEW trajectories. But the UI can be as simple as choose asteroid, chooset target, choose damage level. Then it generates a few possible trajectories and you choose one based on weather it passes something you don't want it to pass, etc.

And like someone mentioned, there would be float precision roundoff errors so you couldn't run this simulation for *millions* of orbits, but I'm ok with that. If you were worried about long term stability (e.g. for galactic war), always start off a solar system in some preconfigured state when you visit there.

I'm saying this as a person who has done a few N-Body orbital simulations, and I'm not even a game developer or a physicist. Just an interested CS grad that enjoyed his physics classes.

It is good for gameplay (IMO) because it makes players take hard choices about getting an attack in fast but maybe not with high damage, and doing a more long term slingshot maneuver that ends up obliterating the planet.

This was an excellent explanation thank you! Hmmm I see what you mean regarding the two systems. I have a few questions.

In your example for Hohmann transfers you had a single object being orbited, so you were igniting the burn to drop out of orbit. The trailer had the KEW doing a gravity assist maneuver around the star. Given that could the KEW be heading towards the planet in a more head on manner sci fi style (engines aside) without violating orbital mechanics?

Also what level of player input are you thinking for something like this? I would rather some method of displaying orbital mechanics be used rather than straight line vectors from one planetary body to another, but I also think that the player should be presented with possible firing solutions to implement rather than having to think about when to ignite burns to adjust orbits.

putting aside that the engines in he video were firing in the wrong direction for the orbital change observed in the video, even with a gravity assist maneuver (which dont work around stars when in system) around another orbital body its still a single impulse burn maneuver to move towards the body, you could have an intercept almost like the one in the video but you would not be able to choose a specific location on the planet as your target, you could only hit the planet directly at a location dependent on its rotation,(this is hard to explain).


in reality to hit a specific point on the planets surface it would probably have to do something like this; do a burn to drop into a lower orbit around the sun burning again to circularize the orbit, then wait until the correct phase angles (angle between current position in current orbit and target body in its orbit, centered on the sun) are achieved and then do a second burn to intercept the target body, a third burn to circularize the orbit around the target body, and a fourth burn to drop onto the target when its at the correct point in its orbit, if you just want to smash the whole planet omit steps three and 4.

not concerned too much about the inputs beyond the select destination for asteroid maybe display the patched conics for the intercept, and the time to impact and each burn, allow some tweaking and have the simulation work out the rest without too much input from the player beyond designating the target body and/or position to crash into or the desired orbit and/or orbital position to take up, problem is that if you do this based on reality it could take some time before your rock hits your target, dependent on the time scale uber decides to apply to the star system to make it work. with the ability to change the orbit of a body in the game the advanced users could create their own intercepts without the assistance of the UI target selection if they wanted.

some people may look at the maneuver results from any reality based system and start screaming at the machine that they could have done it so much better if the rock just headed straight at the target, but thats pure scifi without any reality, which dont get me wrong sure is fun too and ill be reasonably happy whichever way uber decides to do it as long as they're consistent


i dream of building my entire bas on a group of asteroids and constantly moving around the system

Ok I see the issues with the way that the engine was depicted, but why does gravity assisting around a star not work? I would imagine that the rockets firing initially were to put the asteroid into an orbit around the star that intercepts with the planets orbit.

Where the location issues come in I guess is where some liberties are taken with the orbital mechanics for the sake of gameplay.

Yeah I would be pretty happy with that – although the development of custom intercepts might be getting a little complex when compared with the rest of the game as an RTS (just an opinion).
Other than that I would be happy if when selecting the target the UI would throw up more than one option if there was one (ie slingshot around on planet on the way to intercept rather than another) in order to take the KEW through safer areas of the solar system and avoid interception by enemy forces. I would even be happy with a KEW taking a fair amount of time to reach the target, it gives the enemy the chance to attempt intercepting it (provided they knew it was on the way)

I would be happy with it either way really, I wouldn’t mind stylistic interpretations of orbital mechanics as depicted in the trailer, they have already taken an extremely stylistic approach to the appearance and scale of planets so as long as it fit in that aesthetic. I guess this is something that they test both ways to see which way plays better in game.

Considering the talk surrounding using asteroids as a carrier to pull into orbit around an enemy system I’d say that this isn’t an unreachable dream

a gravity assist maneuver borrows the orbital velocity of the body used in the assist, as stars are stationary (for our purposes anyway) they don't have any velocity to borrow

Ok I get that (damn why didn't I think of that). So with this in mind the easier thing to do in the trailer would have been to use the rockets to drop the asteroid out of orbit into the planet directly rather than throwing it around the star first.

I am guessing that in the context of throwing an asteroid from one planet to another one could still use the star to curve the KEW into the orbital path of the target planet despite not gaining any speed in the flyby?

depends, if it was orbiting the planet then yes, just drop it
if it was orbiting the star not the planet then it becomes a more involved process of changing the objects orbit and getting to the right angle relative to the target, then either orbiting the target and dropping onto the target for a targeted strike of a specific location or just ramming into the planet at a location determine by the planets rotation

sure but using only 1 burn would make it an untargeted strike against the whole planet rather than a specific point on the surface


[Edit] Its an interesting problem, if you include planetary motion and rotation, how do you fudge the orbital mechanics to allow targeted asteroid impacts in reasonable time periods? it will be really interesting to follow as uber faces and conquers this and other challenges they will face during the creation of this game!

This sort of seems like the kind of game where the asteroid's rockets never stop burning until they crash into a planet along with the rest of the asteroid. Then they set on fire.

Something to remember is that most of the maneuvers and transfer orbits and what not are designed for very short burst of high thrust engines. With electric propulsion we have been figuring out low thrust long time maneuvers.

For PA we can fudge away the tech and use high thrust long time maneuvers, which we don't currently have a basis for.

As has been pointed out I think, you can hit anything, it's just a matter of how much time does it take (based on the need to sync up with what you want to hit). Right now, most of the time would be spent ensuring syncing.

BUT!!! With strong enough engines and enough fuel (assume for a sec that asteroids have unlimited fuel) you can force the synchronization by running your engines a lot. For example, the fast, inefficient version of the hohmann orbit has been proposed for some Manned mars missions, you use extra fuel on both ends so your transfer orbit is more a straight line, and hence, shorter and at higher speeds (because of the extra fuel). With the excess fuel you can do things that were never considered because they weren't realistic. The thing to worry about is if you are trying to destroy ONLY the base, then it gets harder because not only do you have to hit the right place, you have to do it 'slowly'.

For moving around a solar system it's all about the efficiency(fuel used) vs time trade off, in terms of things you can control. Planetary slingshots while cool, but the reason they are used is because you don't have fuel, so you have to take a detour to save fuel.

You can end up with something close to the engines full on going straight down on a target, it's just a matter of is it actually going to do more damage(i.e. land at higher speed) and how much running around you need to do to set it up.

tldr:Most orbital mechanics is about efficiency, we have lots of options if we ignore efficiency.

It would be interesting to see this energy efficiency vs. transfer time trade be used as a mechanic in the game i.e. use less of your energy resource and do a slower Hohmann transfer or use more of your energy resource and do a fast transfer. This would allow you to trade energy to hit a target faster and decrease the enemy’s chances of intercepting or avoiding your strike. It would also allow you to hit planets at different points in their orbit other than the optimal position for low energy Hohmann transfers.

In the universe of PA over the top delta Vs will surely be possible. So that would open the door to transfers that would never be considered in reality because they require so much energy.