Are Kerbal Space Program orbital physics accurate to real life?

[Red Squirrel]

Been playing that game for a while, one thing I find interesting is how orbital physics work, I had a general idea that's how it worked but one thing that surprised me is that you never really are out of an orbit, you can make an elipsal orbit that goes past the moon even, but you are still in the planet's orbit even that far out, and eventually get sucked back in and go around or even crash into it, depending on your periapsis. You can also change your periapsis from all the way out there by driving against the orbit. I never figured that's how it worked. I always figured you could just fire your engines away or closer from/to a planet, turn them off, and keep accelerating away, and once you make an opposite motion to stop you're in a new orbit, but that does not seem to be how it works at all. It seems the orbital energy trumps your engines so you really have to keep accelerating to change it and not just give yourself a nudge. I also figured that once you're far enough away you're not tied to an orbit at all.

My question is, is the game actually accurate to how it works in real life?

 

[dave_the_nerd]

Pretty much accurate, yes.

Gravity exerts force on you no matter where you are. Eventually, you get captured by another larger object, which is itself captured by another object, which is all orbiting around the center of mass of the galaxy. It's quite beautiful.

 

[Red Squirrel]

That's pretty interesting then, shows how much I know about orbital physics. What causes you to eventually be taken over by another body to orbit to, I'm guessing the direction you're going (inclination etc of current orbit) plays a role? Like if you want to go to the moon you need to set your orbit around the earth on more or less same orbit as the moon, but let it catch up to you or vise versa, then you eventually fall on it's orbit?

 

[[DHT]Osiris]

That's one way to do it, yep. If you've 'overprovisioned' your thrust/lsp for the mission, you can power your way to say, Mun, catch up with it from behind or something, then reverse thrust to slow down and fall into a stable orbit that way. A more 'elegant' approach is to have just enough thrust/fuel to get you to a point where you get captured by the approaching body. Have to be careful with how the approach happens though to not a) run into it, or b) get thrown out.

If you approach in a way that you will travel around the side of the body, in the direction it's rotating (so if it's rotating clockwise, approaching in a way that you will move clockwise around it) you will capture some of it's rotational energy and accelerate your way away from the orbit. Other side, you slow down and can potentially fall in.

If you escape the local orbital system, you will still be captured in the next most local orbital system (aka the escape the mun, still in Kerbin, escape kerbin, still in the nearest star). In Real Life(tm), escape velocity of the moon is 2.4 kilometers/s, earth is 11.2 km/s, the Sun is 42.1km/s (from earth/moon orbit... from the surface it takes 617.5km/s according to wikipedia, but you're not likely to be launching from the surface of a star, so it's less useful). Escape from our solar system looks to be around 492-594km/s, and the galactic escape velocity is roughly the same from there (little pull from the galaxy as a whole I guess). There's gobs of math behind all this, and as you go further out there's still tons of gravitational effects happening (local group, megastructures, etc). There's a reason there is a whole science built for this KSP is a great 'tutorial' into it all.

 

[Jaepheth]

KSP uses simplified models to avoid the complex mathematics of 3 body problems.

Basically, each celestial body has a sphere of influence and when you cross a certain threshold you leave one body's sphere and enter another's.

This means that there are no Lagrangian points in KSP.

 

[[DHT]Osiris]

The scales of everything is also much smaller.. I think technically 'space' starts at like 20 miles up from Kerbin instead of 100 from Earth... Distances are smaller, etc.

I didn't know KSP had no Lagrangian points though, how does it handle very close orbiting bodies? I haven't played enough to get to some of the other planets where you'd technically be under the influence of a planet/its moon at the same time.

It'd be fun if they released a 'supercomputer' mode where it was basically using whatever NASA uses to calculate orbital physics. You'd probably never be able to launch a basic tub due to system requirements

 

[ElFenix]

last i played it, KSP used 2 body mechanics. i'm guessing the crossover point is hardcoded into the game.

It'd be fun if they released a 'supercomputer' mode where it was basically using whatever NASA uses to calculate orbital physics. You'd probably never be able to launch a basic tub due to system requirements

 

[[DHT]Osiris]

Touché, good sir.

 

[Red Squirrel]

Some pretty cool info. Learning new stuff just playing this game. haha. Now that I think of it it does make sense though because gravity is kinda like magnetism, the field gets weaker the further you go but it's never truly gone. So you need to fall into a stronger orbit to get out of the one you're already in. I'm guessing the direction of travel probably matters too? Like you want to ease into the next orbit and you'll get caught in but if you're not going at the right angle you just go right past the body, ex moon or other planet.

So in sci fi movies where they just point the ship in the general direction, turn engines to accelerate then stop, then let the ship go straight in a line until they have to correct is not how space travel really happens then? Or I guess if you are very far away from any planet then that probably works as all gravity is too weak.

 

[exdeath]

You are never truly going straight. It's always series of orbital transfer burns to rendezvous with objects in other orbits. It needs no mention that movies over simplify everything.

In reality you would be on a galactic orbit on a solar injection trajectory to a target star, go back into a solar orbit with a long burn to be captured by that star, then the same thing again on a smaller scale to transfer again to a planet, and so on. With a fixed amount of fuel you'd have to calculate all of this and know your burn schedule for each orbital transfer for the entire trip start to finish before you left Earth. This also implies that you know the positions of multiple bodies at the start and the end as well as the trip time, etc.

To make it even more complex, you'd realistically be using a ton of gravity assists as well both in leaving the local system and arriving at the destination system, implying you know where all these other bodies will be and when they will be there over the course of many many many years.

It gets mind boggling complex really fast, despite it being so deceptively simple in principle. A little bit of extra fuel for very minor correction burns is due to the math being approximate and not possibly being able to account for every body and variable (eg: mass of other bodies is approximate and rounded, orbits are not perfectly circular, planets are not perfect spheres, a single mass is actually a cluster of smaller masses, velocities rounded, etc). The longer the distance and trip time, the greater all these small errors add up.

Movies get around this by assuming infinite energy and fuel where they just drive like car with a constant propulsive force rather than being on a free trajectory, particularly movies with warp drives and such that by design must be powered constantly to maintain a constant velocity. When you have infinite fuel and engine power you can do whatever want, including going in straight lines, since you can constantly negate any other forces acting on you. In this instance you'd be flying straight through space using constant engine power to override orbital mechanics in much the same way a plane can use constant engine power to fly straight in crosswinds, eg you have the ability to negate any other external forces at all times.

Anything else with real world reaction engines where they shut off engines and coast is always going to be on some kind of intermediate orbital trajectory within an orbital system hierarchy, and the engines themselves are only for orbital transfer delta v maneuvers from one orbit to another up and down the hierarchy. eg: planet > star > planet > moon, or planet > star > galaxy > star > planet

"It seems the orbital energy trumps your engines" < engines are for changing orbits and that's all, pretty much sums it up. We are pretty much just along for the ride swirling around with everything else and already moving pretty fast with no way to really stop or go elsewhere. You'd never have enough propellant to achieve or negate these velocities on your own. The delta v provided by even the biggest human rocket burn is really only a very minor +/- to the many km/s you are already moving. Spaceflight isn't so much moving under applied power, but more that you already moving all along and only applying a change in relative momentum to hop tracks.

 

[destrekor]

You are never truly going straight. It's always series of orbital transfer burns to rendezvous with objects in other orbits. It needs no mention that movies over simplify everything.

In reality you would be on a galactic orbit on a solar injection trajectory to a target star, go back into a solar orbit with a long burn to be captured by that star, then the same thing again on a smaller scale to transfer again to a planet, and so on. With a fixed amount of fuel you'd have to calculate all of this and know your burn schedule for each orbital transfer for the entire trip start to finish before you left Earth. This also implies that you know the positions of multiple bodies at the start and the end as well as the trip time, etc.

To make it even more complex, you'd realistically be using a ton of gravity assists as well both in leaving the local system and arriving at the destination system, implying you know where all these other bodies will be and when they will be there over the course of many many many years.

It gets mind boggling complex really fast, despite it being so deceptively simple in principle. A little bit of extra fuel for very minor correction burns is due to the math being approximate and not possibly being able to account for every body and variable (eg: mass of other bodies is approximate and rounded, orbits are not perfectly circular, planets are not perfect spheres, a single mass is actually a cluster of smaller masses, velocities rounded, etc). The longer the distance and trip time, the greater all these small errors add up.

Movies get around this by assuming infinite energy and fuel where they just drive like car with a constant propulsive force rather than being on a free trajectory, particularly movies with warp drives and such that by design must be powered constantly to maintain a constant velocity. When you have infinite fuel and engine power you can do whatever want, including going in straight lines, since you can constantly negate any other forces acting on you. In this instance you'd be flying straight through space using constant engine power to override orbital mechanics in much the same way a plane can use constant engine power to fly straight in crosswinds, eg you have the ability to negate any other external forces at all times.

Anything else with real world reaction engines where they shut off engines and coast is always going to be on some kind of intermediate orbital trajectory within an orbital system hierarchy, and the engines themselves are only for orbital transfer delta v maneuvers from one orbit to another up and down the hierarchy. eg: planet > star > planet > moon, or planet > star > galaxy > star > planet

"It seems the orbital energy trumps your engines" < engines are for changing orbits and that's all, pretty much sums it up. We are pretty much just along for the ride swirling around with everything else and already moving pretty fast with no way to really stop or go elsewhere. You'd never have enough propellant to achieve or negate these velocities on your own. The delta v provided by even the biggest human rocket burn is really only a very minor +/- to the many km/s you are already moving. Spaceflight isn't so much moving under applied power, but more that you already moving all along and only applying a change in relative momentum to hop tracks.

:thumbsup:

I would like point out that for some of the warp engine fiction and theories, they often like to play the "subspace" or "hyperspace" card, wherein some aspect of the spacecraft taps into dimensions of space that we haven't technically found yet (and have only theorized their existence). I believe the common framework is that this other dimension is exempt from space-time. And regardless of proposed dimensions, if there were true FTL travel, that too would be, for all intents and purposes, immune to the effects of gravity and/or distortions in space-time.