If KSP 2 allegedly can handle burn during non physical warp, wouldn't other pertubations also work?

[paul23]

Well allegedly KSP2 supports burn-during-time-warp.

 

While this is amazing in making certain manoeuvres and engines useful. (Helical orbit using ion/very low thrust engines). There are more effects that are even "better" to use. I'm mainly looking at gravitational pertubations due to non spherical shape, those effects are not "hard" to calculate (like n-body dynamics). And they are fun.

 

While at first glance those might make things very hard. You can actually use those effects for the greater good. In real life for example the J1 effect (oblate sphere) is used for station keeping. Any non equatorial or polar orbit will have their longitude of ascending node shifted due to this effect; it's a "free" shift. The amount depends on altitude and inclination, but it's both used to launch satellites into correct orbit without having to wait for the transfer window (allowing multiple at once), and it is used to have a geosynchronous orbit at low altitude, for weather observation satellites.

Similarly J1.1 (more mass east of meridian than west) allows for "skipping ahead" in orbits, while I don't know any place where this is actually used, I know GPS satellites have to take it into account.

 

Basically each pertubation is a mathematical "harmonic" over the gravitational field. J0 is just the standard gravity. J1 is a half period north south, J2 a full period (so if positive north has more mass than south) etc etc. The Jn,m effects are similar but east-west.

 

The good thing is that each level is always magnitudes larger than the level below: J0 >>> J1 >>> J1.1 / J2 etc. And since they are just harmonics, in real life there are infinite many, but in game they can be modelled by (say) only using J1 (and maybe J1.1). The function (courtesy of wikipedia) is slightly "complex", but not that hard:

 

The left summation is for the north south terms (J1, J2) etc. The right for J1.1 etc. The theta and phi are the location of the current object (spherical coordinate system), and the P_n^m and C_n^m terms are just constants based on the natural gravitation (so one can just use tabular data here, which can be calculated during game compilation).

 

J_n and J_n^m terms are the interesting ones that "define" how a body looks (gravitational).

 

A better explanation than I can give is at wikipedia.

 

Wouldn't this be amazing, actually making different *type* of orbits possible. Opening new manoeuvre methods. If only using J1 pertubation it's also really simple, there is a "extra phantom acceleration" acting on the craft (positive is "away" from the planet) given by:

 

u = J_1 * sin(theta) / r^2

Edited November 15, 2019 by paul23

[Nuke]

good, this will make ion engines more fun.

[mcwaffles2003]

Call me crazy but I dont think this will make it into stock...

That said non homogeneous gravity could be cool but calculating that into the future could be difficult

[paul23]

5 minutes ago, mcwaffles2003 said:

Call me crazy but I dont think this will make it into stock...

That said non homogeneous gravity could be cool but calculating that into the future could be difficult

As I just showed in the first post: it's no more difficult than adding an acceleration to the object. That exactly the same as a constant thrust, but instead of prograde (or whatever) it's radial in/out.

[mcwaffles2003]

Just now, paul23 said:

As I just showed in the first post: it's no more difficult than adding an acceleration to the object. That exactly the same as a constant thrust, but instead of prograde (or whatever) it's radial in/out.

It's been a while since I've worked with spherical harmonics but that thrust isn't constant and varies over a map. As you've stated earlier, it shouldnt be as hard as calculating N-body (since the gravity field is almost stationary, with the exception of the constant rotation of the planet, still very predictable) but does this really offer that much variation in mechanics, I feel like unless there was a very large perturbation the effects would be too subtle to notice. Also wouldn't stable orbits stop existing using this?

[paul23]

1 hour ago, mcwaffles2003 said:

It's been a while since I've worked with spherical harmonics but that thrust isn't constant and varies over a map. As you've stated earlier, it shouldnt be as hard as calculating N-body (since the gravity field is almost stationary, with the exception of the constant rotation of the planet, still very predictable) but does this really offer that much variation in mechanics, I feel like unless there was a very large perturbation the effects would be too subtle to notice. Also wouldn't stable orbits stop existing using this?

Perfectly "equal" orbits will stop existing. However the effect is, unless going into an orbit that elongates this effect self cancelling at larger distance (acceleration vector reduces with distance squared for J1 & J1,1 - where gravitational acceleration reduces with distance linearly). Hence an object will still be "captured". Only very low orbits will be unstable (again depending on the constants to which part).

For earth:  This paper shows the clear results on the orbital parameters of J2, solved numerically - so just like kerbal space program would do. As can be seen in figure 2 of the linked paper, the effects on all except the longitude of ascending node (RAAN) cancel out again. Where the longitude of the ascending node slowly diminishes over time.

This effect is actually often generalized by just a linear function when designing a mission, and the steepness depends on altitude + inclination.

Using both you can make sun synchronous orbits - an orbit that will rotate around earth but always face the sun at the same point, practical example is a molniya like orbit that rotates just enough so that always the fast/periapsis is at the shadow side of the body you revolve around. Or you can make Geosynchronous orbits at low altitude - by making sure the change in RAAN has the same average change per time as the angular velocity of earth. Or you can "change" the plane to match that of another station, without expending delta-V and just by make a small nudge to the altitude and later changing it back.

 

There are many possibilities for advanced maneuvers with this. Really some higher level planning can be done instead of making each solution "more delta-v". And the good part is: it is not chaotic at all, unlike n-body systems the effect of J1 (and many other perturbations) "cancel" out, even the ascending node will come back to the original, after it changed 2*pi.

 

PS: notice that the paper also included effect of the sun on orbits around earth, the timescale is massive, from "orbits periods/hours" to "years", for similar effects. Also notice that the simulation there is done at geostationary altitude, the effect of J1 (J2 in paper) grows with distance cubed, so at altitude of ISS (1/6 of the distance) the effect is more than 200 times as large.

 

Edited November 15, 2019 by paul23

[Incarnation of Chaos]

Considering there's a mod for "Lumpy" gravity in KSP1; this will likely be modded in pretty quickly by someone. Since KSP2 will make it so much easier to do, and it's something realism freaks would want.

[paul23]

10 minutes ago, Incarnation of Chaos said:

Considering there's a mod for "Lumpy" gravity in KSP1; this will likely be modded in pretty quickly by someone. Since KSP2 will make it so much easier to do, and it's something realism freaks would want.

Of course that's possible, however that would mean it's not integrated in the rest of the game. It would be nice if certain missions actually depend on this. That you require putting the object in an orbit that keeps it in the sun lest the item cools too much and breaks.

 

Or if the system is extended by custom made planets: if the gravitational pertubations were part of the core KSP system each custom planet would have those set. However if it is just an add on, those extra planets will not have those set and will still be considered perfect spheres.

[Incarnation of Chaos]

1 hour ago, paul23 said:

Of course that's possible, however that would mean it's not integrated in the rest of the game. It would be nice if certain missions actually depend on this. That you require putting the object in an orbit that keeps it in the sun lest the item cools too much and breaks.

 

Or if the system is extended by custom made planets: if the gravitational pertubations were part of the core KSP system each custom planet would have those set. However if it is just an add on, those extra planets will not have those set and will still be considered perfect spheres.

Hm.....they did have to pretty much craft each planet's anomaly map by hand for the stock KSP system; you make a great point here. But i find it very unlikely Star Theory will do anything beyond the standard patched conics implmentation and whatever "Solution" they have for rask and rusk. Just because of time, along with the possiblity it would throw newer players off.

[garwel]

I'm pretty sure at this point that no such effects will make it into the stock game. It's supposed to serve a wide audience, and even basic orbital mechanics is already hard enough for most. Just look at the number of guides that explain how to do a Hoffman transfer or synchronize orbits.

[Mr. Peabody]

13 hours ago, paul23 said:

...wouldn't other pertubations also work?

I'm here because the slightly irregular title word caught my attention.

According to the Merriam-Webster dictionary, pertubation is not a word in the English language. Did you, perhaps, mean perturbation? Or am I wrong and you correct?

[paul23]

4 hours ago, garwel said:

I'm pretty sure at this point that no such effects will make it into the stock game. It's supposed to serve a wide audience, and even basic orbital mechanics is already hard enough for most. Just look at the number of guides that explain how to do a Hoffman transfer or synchronize orbits.

Well they could make the nearby bodies perfect spheres. And only have this effect when further away - that way it's easy to start. However once you get to the outer system it isn't "just more of the same". That currently haunts ksp: once you can escape kerbin and can do a transfer window planning, each other body is the same. Whether you get to jool or eve or moho, the orbital mechanics won't change anymore.

 

46 minutes ago, Mr. Peabody said:

According to the Merriam-Webster dictionary, pertubation is not a word in the English language.

Well english isn't my native language....

[mcwaffles2003]

48 minutes ago, Mr. Peabody said:

I'm here because the slightly irregular title word caught my attention.

According to the Merriam-Webster dictionary, pertubation is not a word in the English language. Did you, perhaps, mean perturbation? Or am I wrong and you correct?

https://en.wikipedia.org/wiki/Perturbation_(astronomy)

Perturbation (astronomy)

 

"In astronomy, perturbation is the complex motion of a massive body subject to forces other than the gravitational attraction of a single other massive body.^[1] The other forces can include a third (fourth, fifth, etc.) body, resistance, as from an atmosphere, and the off-center attraction of an oblate or otherwise misshapen body"

 

Given our context clues, I think it is safe to say yes, you found a typo