Shouldn't objects fall back down to the planet eventually when in orbit?

[guitarxe]

I'm pretty new to all of this space stuff, but I was under the impression that things in orbit will usually eventually get pulled back down? But in the game, if you establish orbit you're up there for pretty much forever?

Also another question - how does Kerbin compare to Earth in terms of scale? If, for example, I wanted to stage the launch of Sputnik into orbit, I know that the apoapsis of Sputnik was about 939 KM from the wikipedia. If I wanted to replicate this in KSP, how much would that be equivalent around Kerbin, since I know that it's smaller in size than Earth?

Edited July 21, 2014 by guitarxe

[FenrirWolf]

KSP uses the Patched Conics method of modeling orbits. That is, only the gravity of the nearest planet/moon affects a ship at any given time. In real life, the gravity of every object affects every other object simultaneously. The atmosphere of a planet also has no sharply defined end and so there is always some degree of aerodynamic drag even in low earth orbit. These factors are why real-world orbits do decay over time and why they do not in KSP.

As for your second question, Kerbin (and all the other bodies in the game's solar system) are somewhere around 10-11 times smaller than their real-world counterparts.

[Pecan]

What goes up must come down - UNLESS it's in orbit. The definition, and difficulty, of orbitting is that something is going around a 'world' fast enough to keep missing the ground as it free-falls back towards it.

In a perfect vacuum - such as KSP emulates - there is nothing to ever slow this orbital velocity so the satellite, or whatever, keeps going around forever.

However, if any part of the orbit is low enough to enter the atmosphere - even the highest/thinnest part of it - the velocity will be slightly reduced, lowering the orbit even more so, eventually, the orbit will degrade so much the thing will crash.

In real-life everything affects everything else and even an apparently-perfect orbit might be deflected into the atmosphere by the famous chaos butterfly - you know what I mean here ^^. For simplicity and, mainly, to keep the game going at a reasonable speed KSP 'just' calculates your vehicle vs the body it's orbitting so a 'perfect' orbit will stay perfect. It's always a good idea to provide propulsion for every craft though so it can perform "station keeping" adjustments if it needs to, just as in real life.

There is no specific relation between the KSP universe and Earth but the scale is roughly 1/10th. "Low" orbit around Kerbin is usually taken as 75km, although the atmosphere stops at 70km. The reason Sputnik fell is that it never assumed an orbit that was entirely outside Earth's atmosphere so if you want to replicate it in KSP keep the periapsis (low point of orbit) below 69km.

[Jodo42]

In real life, there's quite a few reasons an orbit can decay. Atmospheric drag is the biggest problem in LEO. Uneven mass distribution in a body will also decay an orbit over time without having the spacecraft ever touch an atmosphere , as most famously seen by the Apollo missions. Even tides can degrade orbits. (Un?)Fortunately, these things aren't modeled in stock KSP. If they were, it'd make the lives of noobs a lot more difficult, and the devs don't like doing that. Once you're above the cutoff for the atmosphere on a planet (69,078m for Kerbin), you don't experience any atmospheric drag and your orbit will never decay, aside from floating-point precision bugs.

As for Kerbin's size, it's about 10.9 times smaller than Earth. That would make your apoapsis roughly 86km.

[razark]

For a clue on how much atmospheric drag can affect a spacecraft in orbit, note that when ISS is on the dark side of Earth, the solar panels are turned edge-on to the direction of travel to minimize drag.

[guitarxe]

Oh wow ok, that makes sense now. I didn't know that real spacecraft in low earth orbit experience any kind of atmospheric drag.

Also about the apoapsis thing, is that measured from the center of the planet, or from something like the sea level?

[Pecan]

Also about the apoapsis thing, is that measured from the center of the planet, or from something like the sea level?

A VERY good question that often trips-up people trying to calculate orbits by hand! The display is relative to sea level. Some cockpits include a 'radar altimeter' that will also show you the current true altitude above the ground. Several mods will give you both figures. (If you are trying to do it by hand, you need to add on the radius of the planet/moon to get the altitude relative to the centre).

[dryer_lint]

Also about the apoapsis thing, is that measured from the center of the planet, or from something like the sea level?

It's measured from sea level, at least in KSP.

And yeah, I didn't know about the LEO atmospheric drag either. It made me sad, we can't have infinite orbits in real life

[Reactordrone]

Yes, apoapsis and periapsis are measured from sea level or the planetary equivalent.

[cantab]

Even in the real world, while satellites in low Earth orbit may have their orbits decay rapidly, those in higher orbits will take much longer to come down. For example the LAGEOS satellites, http://en.wikipedia.org/wiki/LAGEOS , will stay in orbit for several million years.

[Drew Kerman]

Slightly related, and what I thought this question was actually about - if you have a craft "orbiting" with a periapsis of something like 40km and you are not switched to it, the game will not apply physics to it. This is an issue most common with debris. You'll detach your stage just prior to achieving orbit (say you end up giving it a 75x50km orbit) so it will burn up eventually, then days later you happen to check the debris objects in the tracking station and find that it's still there! Unless it is in focus, physics won't be applied. The only time debris is auto-deleted is I think when it falls below 35km or so

[Dkmdlb]

The only time debris is auto-deleted is I think when it falls below 35km or so

23 km on Kerbin. Different altitudes on different bodies.

[jeb_Carney_tx]

So, in relation to OPs question, IF we scale sputniks orbit (or the ISS, even, which is what just brought me here), we'd need to orbit at 90000-100000m for sputnik and about 40000m for the ISS.

[Pecan]

90 - 100km will give you an orbit fully outside Kerbin's atmosphere so it'll stay in space forever, whereas the real sputnik didn't.

40km, on the other hand, is fairly deep inside the atmosphere so a station there will need constant station-keeping burns or its orbit will degrade pretty quickly.

KSP's universe is not Earth's or even a scaled version of it, trying to replicate sci-fi missions from this mythical 'Earth' is rarely useful, accept KSP for what it is. If you want something to orbit indefinitely set its periapsis to at least 70km, preferably 75km as a margin for error. If you want it to degrade then use a lower periapsis and, possibly, apoapsis.

[Soda Popinski]

For people who want to better simulate Earth, there are mods like "Real Solar System (RSS)," which I for one am too wussy to try. Deadly Re-entry is tough enough for me.

[KerikBalm]

Most things in the kerbin universe are roughly 10x smaller than their analogue (when they have an analogue).

Atmospheres are not one of those things. Kerbin's atmosphere is approximately 1/2 scale if I am correct. The traditional definition for "space" on Earth is 100km. Kerbins is roughly 69 km.

Relative to its diameter, kerbin's atmosphere is really really thick (as is that of even duna, and especially Eve)

You can orbit just fine at 68 km for quite a while in the game. Drag is very low at that altitude, but not non-existent.

For the sake of gameplay and due to computational limits, it is non existent above that.

There'd be only the physical 4x timewarp if it wasn't implemented this way (and we'd still have the problem of treating craft 2.5km from the active one as "on rails", or burining up your cpu trying to similate everything at once)

[Rhomphaia]

So, in relation to OPs question, IF we scale sputniks orbit (or the ISS, even, which is what just brought me here), we'd need to orbit at 90000-100000m for sputnik and about 40000m for the ISS.

Depends on the scale you use,

If you compare the orbits in terms of in terms of altitude as a fraction of the radius of the parent body then yes. though for Sputnik that is the apoapsis, Periapsis would be at 21.5km

If you compared the orbits in terms of orbital period as a fraction of the rotational period of the parent body then the ISS would be underground, and Sputnik would have an apoapsis of 806m with the Periapsis well underground.

If you compared them in terms of the Atmospheric pressure that they would experience if KSP modeled pressures below 0.000001atm then the ISS would orbit at 249km and Sputnik would have Periapsis 126km apoapsis 552km

[KingPhantom]

everything in the sphere of influence (SOI) will fall back to the object when it's not enough fast... only when the speed of the object can beat the gravity it "stays up" in a stable orbit.

thats in real world and ksp the same... just slow the orbit speed down and watch in orbital map whats happen... it's the same when you slow down a plane during flight

thats physics... science, b**ch!

[cpast]

Atmospheres are not one of those things. Kerbin's atmosphere is approximately 1/2 scale if I am correct. The traditional definition for "space" on Earth is 100km. Kerbins is roughly 69 km.

Relative to its diameter, kerbin's atmosphere is really really thick (as is that of even duna, and especially Eve)

Yes and no. The standard boundary of space on Earth is 100km, but that's just the altitude at which the speed you need to stay aloft through lift is orbital velocity -- it is not an altitude at which you can orbit. NASA actually considered entry to start at 122 km for shuttles, because that's where atmospheric drag became noticeable and control surfaces started working (that's when the shuttles stopped using RCS and started using aerodynamic controls). In KSP, an orbit at 70 km will last forever; on Earth, an orbit at 101 km will not last a single orbit, and any orbit below 300 km will only last days. It's not really accurate to compare 69 km in KSP with 100 km on Earth.

[LethalDose]

I think it's also worth noting that real orbits can be unstable for reasons other than atmospheric drag. For example, low lunar orbits are typically unstable in the long term because our moon has areas mass concentrations, or "Mascons", that cause anomalies and unevenness in it's gravity field, which distort the orbits of lunar satellites over time.

KSP treats all gravity as coming from a single point (which is how basically everyone is taught to do physics), which works great for most math and KSP, it's just not a perfect representation of what's really out there.

Yes and no. The standard boundary of space on Earth is 100km, but that's just the altitude at which the speed you need to stay aloft through lift is orbital velocity -- it is not an altitude at which you can orbit. NASA actually considered entry to start at 122 km for shuttles, because that's where atmospheric drag became noticeable and control surfaces started working (that's when the shuttles stopped using RCS and started using aerodynamic controls). In KSP, an orbit at 70 km will last forever; on Earth, an orbit at 101 km will not last a single orbit, and any orbit below 300 km will only last days. It's not really accurate to compare 69 km in KSP with 100 km on Earth.

Good points. It almost seems likes the equivalent of the "standard boundary of space" around Kerbin would be somewhere in that 35 - 45 km zone where it really feels like you're escaping the atmosphere (at least when running FAR, I haven't used the stock aero model in so long, I forget where that is).

[bdito]

Here's a very cool image someone created regarding Kerbin vs Earth.

http://imgur.com/ckadxCa

[Bobnova]

Or install RSS and all the realism mods and give it a whack yourself.

Very different.

[Mr Shifty]

For people who want to better simulate Earth, there are mods like "Real Solar System (RSS)," which I for one am too wussy to try. Deadly Re-entry is tough enough for me.

For an even better simulation of orbital mechanics, try Orbiter. It's a free space-flight simulator that's been in continuous development since 2000. HarvesteR played Orbiter long before he ever got started with KSP. He posted the first ever announcement about KSP on the Orbiter forums and even before that had been soliciting expert opinion there for help fleshing out ideas for how to do mechanics in KSP. Orbiter simulates n-body gravitation, exosphere drag, gravity gradient torque, non-spherical gravity, orbital precession, solar radiation pressure, etc. It's got a fully modeled, real-life, real-time solar system available for exploration. If you put a satellite in LEO, with non-spherical gravity and exosphere drag turned on, its orbit will decay over time (and its LAN, perigee, and apogee will fluctuate constantly due to gravitational variation.) Orbital rendezvous becomes a little bit more of a challenge, and conic solvers require constant corrections to get you anywhere. I'm not sure why people recommend RSS for this; RSS's purpose is not really to make piloting more realistic; it's to make building more difficult. You have to build much more dV into your crafts. But, it doesn't at all simulate any of these non-ideal orbital mechanics characteristics; I don't even think it simulates axial tilt. If you want realism, there's no substitute for Orbiter. And it's free.

[KerikBalm]

Yes and no. The standard boundary of space on Earth is 100km, but that's just the altitude at which the speed you need to stay aloft through lift is orbital velocity -- it is not an altitude at which you can orbit. NASA actually considered entry to start at 122 km for shuttles, because that's where atmospheric drag became noticeable and control surfaces started working (that's when the shuttles stopped using RCS and started using aerodynamic controls). In KSP, an orbit at 70 km will last forever; on Earth, an orbit at 101 km will not last a single orbit, and any orbit below 300 km will only last days. It's not really accurate to compare 69 km in KSP with 100 km on Earth.

True, the definition of "space" for real life, and for KSP, are not the same, but I will point out a few things:

You could orbit in theory, with a low enough drag coefficient.

The orbital period for earth is about 90 minutes, so you'd have to go 90 minutes without drag bringing you PE down too much.

So the question is, what altitude can you "orbit" kerbin at for 90 minutes.... at least with stock aero, it seems to be more than 40km (I don't think you'll even make 1x 30 minute orbit, maybe I need to fire it up to test this).

I don't think an orbit of 68km will last for days in KSP either.

If we use 40km for KSP, and 122 km for real life, its still about 3x smaller instead of approximately 2x smaller.

In RL, you have to consider how long an orbit will be stable, on the oder of weeks for most cases, in KSP, if its stable for a few hours (maybe longer? I really am not sure how long a 68x68 km orbit will last), its stable forever. I wouldn't use the engine limitations to argue the thickness of KSP's atmosphere relative to RL.

*When* KSP is modelling atmospheric drag, it seems to be modelling an atmosphere that is much thicker relative to its planet's diameter than ours is, and I think its about a factor of 3, as opposed to the ~1/10th of the actual celestial objects

For reference, the Apollo missions, prior to trans lunar injection, went to an orbit of 185km, which is less than 3x the height of kerbin's atmosphere

bdito: That summary is nice, but its also worth noting that things in KSP are unrealistically heavy. RL rockets engines have a much much higher TWR, the chemical rockets can get better ISPs (H2 LOX fuelled), and the mass fraction for entire stages varies between 13:1 and 17:1, when the best tanks in KSP (with no engines or anything) are only 9:1.

Yes, it took a lot of engineering to get parts to those specs, but in KSP, we don't engineer the parts, we plan missions with the performance of the parts available to use. Our parts performance is sub par compared to what Apollo had, of course, as your diagram shows, this is offset by the required dV being much less.

It "somewhat" balanced out in the end.

I'll note that the Saturn V was essentiall 2 stage to orbit*, which is quite hard to do with any reasonable payload fraction in KSP (at least before the nasa parts release, and buffing of the mainsail and skipper)

* The third stage was needed to get to orbit, but the vast majority of the burn time on the third stage was for trans lunar injection

Edited July 24, 2014 by KerikBalm