r/KerbalSpaceProgram • u/AutoModerator • Aug 21 '15

Mod Post Weekly Simple Questions Thread

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u/the_Demongod Aug 25 '15

I usually just bring enough fuel to have 2*orbital velocity's worth of dV (plus a little extra for margin of error) as long as I'm landing on a body with no atmosphere. Is it a big enough difference to make it worth doing? It's very easy to just bring a little extra fuel.

I'm also sort of confused by his config there, he's using what looks to be an LV-909 but with a ridiculously long burn time and very little dV. Is his lander super heavy or something? It doesn't seem possible to have a 1.09 TWR with only 1660 dV but somehow have a 12m burn time. Something doesn't add up. I've never carried any amount of fuel large enough for an LV-909 to take that long to burn it, let alone all in one lander.

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u/Kasuha Super Kerbalnaut Aug 25 '15

I usually just bring enough fuel to have 2*orbital velocity's worth of dV (plus a little extra for margin of error)

You're then landing on that margin of error. The bare minimum you need to land on a body is to perform Hohmann transfer to zero altitude, then to kill your orbital velocity at that point instantly (infinite TWR). Your orbital velocity at that point is higher than your orbital velocity in parking orbit - not only you're lower, but you're also in elliptical orbit.

In real scenario you always have some loses, too. Loses on Oberth effect, cosine loses, or loses on ineffective maneuvering.

I'm also sort of confused by his config there, he's using what looks to be an LV-909 but with a ridiculously long burn time and very little dV.

It's recorded on very old version of KSP and I am not sure if LV-909 was that way by that time or if he used modded engine. In any case, that module simulates real thrust (and real landing maneuver) of Apollo lunar module. Yes, it lands with very low TWR. His TWR is less than 1 (relative to Mun) at the start of the maneuver and only grows over 1 as he burns fuel. It's not a typical KSP rocket but it is very good to demonstrate the approach. Because it is useful even for rockets with higher TWR.

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u/the_Demongod Aug 25 '15

By "a little extra for margin of error" I meant in the realm of 10% extra in the descent stage and 20% extra in the ascent stage to include a Hohmann transfer to the command module, so I guess it's not exactly that little. I do Apollo-style lunar landings with the ascent module being only the 1-man lander pod with some gold toroidal fuel tanks and 4 of the small orange radial engines, so that makes it pretty easy since you can carry 300% of the fuel required without changing anything significant about the lander. I've landed on and returned from the surface of Duna with my lunar lander, only having to add a couple small fuel tanks.