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deltaV loss in orbit



hi all. So im playing RSS and with the incresed difficulty I have a question that ive never thought about before and feel a little foolish for asking however. ...

my moon lander will need say 5000dV to land, take off and reorbit. It will then need a burn of about 3400dV to reach Earths SOI.

8400dV is heavy to launch from Earth in one launch im finding so thinking of sending the return to Earth stage up first then docking the lander stage on a separate launch.

my question is will my dV remain at 8400dV when the two dock?

I feel I should know the answer however RSS has made my brain work hard!

Sorry if im asking an obvious question. 


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If you launch one craft with X amount of dV, when you dock it to another, the dV of the pair will be significantly less -- if they have equal mass, it'll be half what it was.  This is because dV is mass dependent -- you can see this in the VAB if you use Mechjeb or Kerbal Engineer; take any craft you like, add some mass, and watch the dV drop.  You'd have to have both craft carrying X dV in order to still have the same amount when you dock.

Of course, this is more complicated than that simple case; if the two craft have engines with different Isp, the same fuel that would give one 8400 m/s might give the other only, say, 6000 -- or might give it 10,000 if it has much more efficient engines that use the same fuel (are you using Real Fuels?).  But bottom line is that dV is impulse divided by mass (impulse is thrust times time).  Your impulse is constant (unless you spend some of it by burning propellants), but the more mass it has to push, the less dV it produces.

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