ISP Graphs

[Northlight]

I have long wondered what ISP the different engines have at different altitudes.

That would make it easier to choose the best engine for a stage. Especially that 2nd stage burn halfway up the atmosphere.

If you have done the same, this graphs may be useful.

I have grouped the engines into 3 groups. the 1.25m, 2.5m and 3.75m.

I have choosen the most common engines.

UPDATE: Switched atmospheric model to NASA model (80%), for more accuracy.

Edited May 9, 2015 by Northlight

[Superfluous J]

This matches my guesstimates (I thought the 909 started beating the other low-tier engines at roughly 10km). My Mun rocket uses 4 Reliants as boosters up to about 20km, and at about 10km I kick on a central 909 (at the same time I lose 2 of the Reliants) that eventually gets me all the way to Mun.

Are you going to do the same with the smaller engines? I'd be curious to see how the 48-7s stacks up.

[Northlight]

Shure I could make som graphs for the smaller engines too. Which ones would be good to group together?

[Superfluous J]

Hmm, the funny thing is I tend to always be deciding between the 909 and the 48-7S. Not sure what a 3rd engine would be. Maybe just add the 7S to the first graph?

I don't know if it's the same now (as so much has changed) but it used to be the 7S was the go-to engine for anything you didn't use Nukes for.

[Northlight]

I decided to trow in the LV-N "Nerv"

Edited May 9, 2015 by Northlight
Removed erronous stuff.

[Superfluous J]

Cool thanks. That confirms another suspicion of mine, that the nuke is still pretty good even as low as 10km up. And it looks like the 48-7S can still be a workhorse at any altitude with its competitive Isp and low mass.

[NathanKell]

Nice work!

Just a note, in 1.0 Kerbin's atmosphere doesn't have a fixed scaleheight; instead it appears to follow an 80% standard model of Earth's atmosphere up to about 65km, after which it trails off.

(80% as in 8km on Kerbin == 10km on Earth, etc).

[Northlight]

NathanKell:

Cool! When I use the standard atmospheric model found on a NASA-website, i get a slightly different curve.

Basically my model gave engines too high ISP in the lower atmosphere. And the bigger the difference in vac and asl-ISP,

the bigger my error was.

Edited May 9, 2015 by Northlight
Graphs no longer relevant.

[Skorpychan]

Why is the X axis 'kilometers below vacuum'? Why not just have 'kilometers ASL' so it's easy to read?

[Northlight]

Why is the X axis 'kilometers below vacuum'? Why not just have 'kilometers ASL' so it's easy to read?

Don't have a good reason really, it just made sense at the time...

I'll change the axis-label and update the graphs with NASA atmosphere.

All graphs in post #1 and #5 updated to NASA Atmospheric Model (80%)

Edited May 9, 2015 by Northlight