Calculating the fuel required for simple deorbiting?

[Dave Kerbin]

My Kerbals have been conducting a series of proving missions in preparation for the construction of a refueling space station. This is mostly through the time honored way of trial and error. I've sort of figured out how to dock and how to efficently do some orbit changes though lift off probably still has a lot more brute force then needed.

My Kerbals want to keep the planets orbit clean of debris so anything that goes up has to attach to the station or come back down. But since the goal is to get as much fuel up there as possible I want to minimize how much needs to be left behind in the boosters when I detach them and then turn them around to do a deorbiting burn.

Given my engine (impulse in vaccum), dry weight and orbiting altitude what would the math be to figure how much fuel I need to add to make a deorbiting burn at the apoapsis (I assume this is something "Delta V" related). My Kerbals aren't really concerned where the debris crashes since the planet seems to be nearly devoid of any settlements beyond the KSC.

Edited August 18, 2013 by Dave Kerbin

[zxczxczbfg]

I'm not sure how to calculate it, but it takes VERY little thrust to deorbit something, about 2-3 seconds of retrograde burn on a Poodle engine to deorbit from a 100km orbit. You might also want to take a look at the Pirated Weapons Mod and the Lazor family of mods for some good debris-obliterating equipment, Lazor Sunbeam especially.

[Captain Sierra]

Calculating: There is no math I know to do it beyond delta V. To truly de-orbit, you need periaps on the ground since KSP won't calculate the atmosphere unless you are switched to the debris.

orbital velocity is something like 1800m/s in a 75km parking orbit. It takes >50m/s to bring something down to a sub-orbital trajectory. If your debris has 6-8 retro rocket sepatrons, it should be able to de-orbit itself. If you find that to not be enough, add more. I recommend getting Editor Extensions. You can then mount radially in up to 50x symemtry in 1x increments. I can get 50 sepatrons worth radially onto a 2.5m diameter part.

[Specialist290]

To truly de-orbit, you need periaps on the ground since KSP won't calculate the atmosphere unless you are switched to the debris.

Not strictly true; once it's below a certain altitude above Kerbin (~20km or so), the debris will be deleted automatically as long as you don't have another craft within a little over 2km of it.

From a roughly circular orbit at 100km or so above ground level, a few backward-firing sepratrons should effectively deorbit just about any discarded stage.

[Frederf]

Without intervention the magic altitude to have your debris deleted by atmospheric reentry is 23km. If you have the will to make them the active vessel and watch babysit them with physics you can do this a lot higher (35 km on a single orbit, 50 km on several).

Assuming your station is in a 100km circular you need 68.3 m/s retro to lower your pe to 23km. Since dV = Isp*g0*ln(M/m) you want your mass fraction to be about 2% fuel for the deorbit burn.