De-orbit mechanics question

[Omegalegion]

So I'd like this answered in two parts: one as it pertains to the game that is KSP and another as things behave in the real world:

I achieve an orbit with a now unnecessary engine/fuel tank stage that needs to be separated from my ship/station. If I "aim" the stage at the surface of the planet and separate it with the use of sepratrons to push it towards the atmosphere, will it inevitably de-orbit? Or will it simply carry on at a distorted orbit. Let's say for this example my initial orbit from which I'm launching the now useless stage is 80km or so...

My thoughts are that since there is no resistance in space and the piece is now moving away from a stable orbit station, the debris should continue traveling downwards in an ever lower orbit that will eventually cause it to hit atmosphere and reentry. Is this also possible even with very minor "pushes" as there is no resistance? Will parts eventually move closer and closer and finally de orbit inevitably?

So. Wise ones, how does this work in game/real life, throw some knowledge at me.

[regex]

You use velocity to counteract gravity when orbiting. You are essentially falling in towards what you are orbiting. If you reduce your velocity you will fall further towards the orbited object; reducing orbital velocity is a retrograde maneuver. If you increase orbital velocity you reduce the effect gravity has on you and "fall less"; increasing orbital velocity is a prograde maneuver.

Burning towards a planet isn't exactly a retrograde maneuver, it actually just alters your orbit's shape. To deorbit, reduce orbital velocity by burning retrograde.

In real life many things will interact with the craft to reduce its velocity and alter its vector. Eventually it will encounter a greater force (atmospheric drag, gravity, solar winds, micrometeoroids) that will degrade its orbit. KSP has a simplified system so orbits will not automatically degrade (at this time).

[stupid_chris]

IT works exactly the same as with your craft. If you aim to the planet and thrust forward for a few seconds, are you gonna re-enter the atmosphere? Probably not, but you will get an eccentric orbit.

If You just burn downwards, yes you will start going down, but you're not actually lowering your speed, as you are burning 90 degrees from your movement vector. So you will go down, pcik up speed from gravitational acceleration, and start going up and even higher again from that speed you picked up.

To deorbit something, you have to lower it's orbit until it enters the atmosphere. And how do you lower an orbit? The exact opposite way as how you raise or put yourself into orbit, by thrusting retrograde. See it like this: shooting prograde makes you go faster --> makes you go higher. Shooting retrograde makes you go slower --> makes you go lower. An orbit is just about speed. To orbit, you go faster, to deorbit you go slower.

So, the best way to deorbit this spent stage is to have it thrust retrograde. So if it's at the back for your rocket, just point the nose forward and when you stage, the spent booster will go retrograde. Although I must say, it'S very very unlikely it will re-enter the atmosphere. The best way to do it, is to put a probe core hidden in your booster. You stage it with separatrons to lower it's orbit, and then you finish it by controlling the booster, and using what's left of the fuel to burn retrograde and reenter.

Hope I could help!

[Vanamonde]

Remember that reaching orbit required accelerating your ship to around 2000m/s. Any delta-V the sepratrons impart to the ejected piece is only going to be a small fraction of that, meaning that its velocity will remain quite similar to your own, and therefore its orbit will remain quite similar to your own.

[Omegalegion]

Thanks so much folks, it makes a hell of air more sense now when I realize those little engines are not consuming the entire velocity of the orbit!

[Sleipnir]

Altough it is possible to deorbit light weight spend stages, i have done it before. just add enough separations and use them in increments, ie burn two at time for added effect.

[Omegalegion]

And I guess another question is in order then, how do I change this thread to be marked "answered" or is that a mod thing?

[regex]

And I guess another question is in order then, how do I change this thread to be marked "answered" or is that a mod thing?

Edit the OP and change the drop-down.

[Mystique]

Remember that reaching orbit required accelerating your ship to around 2000m/s. Any delta-V the sepratrons impart to the ejected piece is only going to be a small fraction of that, meaning that its velocity will remain quite similar to your own, and therefore its orbit will remain quite similar to your own.

You don't need to burn all those 2000 m/s, only 50-150 m/s depending on altitude in order to make periapisis below 70 km (the lower the better, because at high speed debris may bounce and go up again, but in any case atmosphere interaction must slow them down on each bounce, so eventually they'll go down on ballistic trajectory; I usually deorbited space station part pushers from 200 km orbit with such d-V, they just need to be as light as possible, actually empty tanks are already light enough). After they will enter atmosphere - they're already finished. So all that's needed is to add enough sepratrons for a small retrograde burn. Disadvantage is that number of sepratrons is individual and they also reduce total d-V of your vessel.

[Cashen]

Another good idea that I use somewhat often is to have a small probe core and a battery/solar panel on the upper stage, so I can separate it and have a short time to do maneuvers with it, like point retrograde and burn off any remaining fuel, usually enough to get the orbit to intersect Kerbin, which will eventually auto-delete the stage.

[SSSPutnik]

If you get it below 55km or so, drag will reenter it pretty quickly.

Like others, I put a probe core on it, spin it retrograde and use remaining fuel to deorbit it. Didn't think of using sepratrons. Probably insufficient in a lot of cases.

PS: to mark answered, you need to edit the original post and click "Go advanced" to see the drop down answered bit.

[Kasuha]

If you get it below 55km or so, drag will reenter it pretty quickly.

55 km is a little bit high to get the deorbit soon, it might take a few orbits to deorbit something with such periapsis.

During Career mode I found that if I don't want to land on night side of Kerbin while returning from Mun, I can keep myself at around 40 km height using small thrust downwards and the initial speed will be enough for me to pass almost the whole night side.

So for safe deorbiting I think 30 km periapsis is just about right.

[Taki117]

Putting in my two cents: In order for your seprotron separation to be most efficient (And have a greater likely hood of causing your piece to renter) you need to fire it retrograde (so if it's on the back of your rocket, point your rocket prograde) at apoapsis as this will have the greatest net effect on your periapsis such that it has a greater possibility of reentry than if you did it at any other time. This is mainly due to the energy required to raise/lower your apsis in that it takes less energy to raise/lower an apsis at the opposite apsis (i.e Raise periapsis by thrusting prograde at apoapsis) than it does to try and to the same thing by the same amount anywhere else in your orbit. (If you don't believe me plop a maneuver node at apoapsis and raise your periapsis to a set altitude and make a note of the dV required, then plop another one down elsewhere and repeat.)