"Different" delta v estimates in Kerbal Engineer - what's up with that?

[MerlinsMaster]

So I'm designing a spaceship to get to Eeloo, and I'm trying to create a design that has the required delta v (I'm figuring about 7000 to get there and back), but on this screen in Kerbal Engineer...

...there are different buttons for each planetary body, each having a different delta v estimate. Can anyone tell me what this means? If the estimate is one value when you press the Kerbin button, and is a different value when you press the Eeloo button, which one is the correct one?

Edited August 7, 2014 by MerlinsMaster

[UmbralRaptor]

Isp varies with atmospheric pressure. KSP does something a bit silly and alters fuel consumption rather than thrust, but it still means you need to consider the lower values for Kerbin, Eve, etc. The vacuum ÃŽâ€V should remain unchanged (And be the same for all airless bodies for the obvious reason), though TWR will be different for every body.

Also, 7 km/s from where? It might be enough from LKO, but is definitely not from the surface of Kerbin (unless you're doing some very clever flying)

[MerlinsMaster]

Isp varies with atmospheric pressure. KSP does something a bit silly and alters fuel consumption rather than thrust, but it still means you need to consider the lower values for Kerbin, Eve, etc. The vacuum ÃŽâ€V should remain unchanged (And be the same for all airless bodies for the obvious reason), though TWR will be different for every body.

That was my thought, but there's no button for vacuum, and I'm pretty sure the buttons for different airless bodies have different values.

Also, 7 km/s from where? It might be enough from LKO, but is definitely not from the surface of Kerbin (unless you're doing some very clever flying)

Yeah, I build my interplanetary ships in orbit, so it's 7000 from LKO.

[kfunk]

umbral raptor is correct on this. if you leave with 7000 m/s you'll have just enough power to slingshot past eeloo and go the way of the voyager (koyager? .. needs work) satellite.

[kfunk]

i still don't see 7000 being enough... maybe i'm reading the map incorrectly 950 to break orbit, 1150 to intercept eeloo, 2100 to establish an orbit, 4200 total, double that to get home, 8400. 7000 should leave you stranded on some bizzare orbit in the middle of the kerbol system.

[itsmagic2009]

Switching between the body buttons should not change your dv, only the TWR. KER shows always vaccum dv unless you activate the "Atmospheric stats" button. And kfunk is right, you won't make it back with 7000 m/s. You might be able to safe 950 m/s on the return by slamming into Kerbins atmosphere, but you will still be ~500 m/s short. The cold equations win. Also the wiki cheat sheet is highly optimistic, i would add at least 10% margin to the theoretical value.

[Bedi]

I think he would make it with gravity assists.

[itsmagic2009]

Pure luck, but possible. But the correct (Kerbal) way is to add more boosters .

[Jouni]

i still don't see 7000 being enough... maybe i'm reading the map incorrectly 950 to break orbit, 1150 to intercept eeloo, 2100 to establish an orbit, 4200 total, double that to get home, 8400. 7000 should leave you stranded on some bizzare orbit in the middle of the kerbol system.

The return trip from Eeloo orbit to Kerbin aerocapture is a bit over 1000 m/s.

[itsmagic2009]

I think Jouni is right on the return. Eloo escape is only ~230 m/s and much cheaper than Eloo capture. After all those trips to the Mun i should have known better.

[MerlinsMaster]

Switching between the body buttons should not change your dv, only the TWR. KER shows always vaccum dv unless you activate the "Atmospheric stats" button. And kfunk is right, you won't make it back with 7000 m/s. You might be able to safe 950 m/s on the return by slamming into Kerbins atmosphere, but you will still be ~500 m/s short. The cold equations win. Also the wiki cheat sheet is highly optimistic, i would add at least 10% margin to the theoretical value.

Here is the delta v map I use most often.

http://forum.kerbalspaceprogram.com/threads/41652-A-more-accurate-delta-v-map

Now reading this, I originally thought I'd need about 10km/s for the round trip and have a little bit of breathing room. But Metaphor and a few others told me that it won't take as much delta to return. Here's a link to that thread if you're interested: http://forum.kerbalspaceprogram.com/threads/88381-How-to-make-a-round-trip-HEAVY-reusable-interplanetary-rocket/page2

[itsmagic2009]

Dang, the value for Eloo capture in the wiki map is wrong, should be 210 m/s instead of 2100 m/s. I justed tested it with a "spare" rocket in Eloo orbit and getting away is pretty cheap. I think i will use your map in the future :-).

[MerlinsMaster]

Switching between the body buttons should not change your dv, only the TWR. KER shows always vaccum dv unless you activate the "Atmospheric stats" button.

Okay, I just clicked off Atmospheric Stats and now the delta v appears to be what it is when I launch the ship. Thanks, that's exactly what I needed.

Edited August 7, 2014 by MerlinsMaster

[MerlinsMaster]

Dang, the value for Eloo capture in the wiki map is wrong, should be 210 m/s instead of 2100 m/s. I justed tested it with a "spare" rocket in Eloo orbit and getting away is pretty cheap. I think i will use your map in the future :-).

Yeah, it's a great map, the best one I've found so far. It's also nice because it shows you all the places where you can use aerobraking, which is pretty handy.