Why can't I take my rocket into orbit?

[Rryk]

I've been using the Rocket Equation to estimate produced delta-V to estimate whether I can get into orbit or not. For example, my Mun Savior NG (prototype) rocket has a RE-I5 Skipper Liquid Fuel Engine with a single Rockomax X200-32 Fuel Tank on the first stage. Fulled mass is 28.045 and dry mass is 12.045, therefore, assuming maximum thrust at MSL, produced deltaV should be 568.75 * 9.8 * ln(28.045 / 12.045) = 4710.71 m/s. According to Kerbal Space Program Δv Map, this is more than enough to escape Kerbin atmosphere and reach low orbit (70-80km). Despite this, I haven't so far managed to reach orbit without using my second stage.

Is it because my ascent profile is so inefficient or did I make mistake when calculating deltaV or reading the map? Is there a way to compute most efficient ascent profile without using MechJeb mod?

EDIT: I've just realized I've been using Max. Thrust in place of Isp, therefore my first stage actually produces 280 * 9.8 * ln(28.045 / 12.045) = 2319.12 m/s, which is significantly less than 3400 as suggested by the map.

Edited September 7, 2016 by Rryk

[Superfluous J]

You already found the problem, of course, but I'd like to note we'd not have been able to answer it because you didn't say what your ascent profile was. However, if your ship HAD had 4700m/s of dV, it'd have been pretty hard to NOT get it into orbit. The only way I can think of for you to need more than that is to thrust straight up until your Ap is 80km, then turn East and thrust sideways until your orbit is circular. Even doing that, you might make it.

Note that the 3400 suggested by the map is an estimate, and in my opinion a low one (which in my opinion is good. These should be lower bounds). I tend to need closer to 3500 and try to bring 3600. Also note that this number is VACUUM dV, and not ATMO dV. The convention around here is to list Vacuum dV for everything and assume the player knows to use engines with good Atmo Isp.

Once you're 10km up you're mostly in Vacuum anyway, so it's better to use that number than Atmo, if you're going to choose one.

[GoSlash27]

30 minutes ago, Rryk said:

I've been using the Rocket Equation to estimate produced delta-V to estimate whether I can get into orbit or not. For example, my Mun Savior NG (prototype) rocket has a RE-I5 Skipper Liquid Fuel Engine with a single Rockomax X200-32 Fuel Tank on the first stage. Fulled mass is 28.045 and dry mass is 12.045, therefore, assuming maximum thrust at MSL, produced deltaV should be 568.75 * 9.8 * ln(28.045 / 12.045) = 4710.71 m/s. According to Kerbal Space Program Δv Map, this is more than enough to escape Kerbin atmosphere and reach low orbit (70-80km). Despite this, I haven't so far managed to reach orbit without using my second stage.

Is it because my ascent profile is so inefficient or did I make mistake when calculating deltaV or reading the map? Is there a way to compute most efficient ascent profile without using MechJeb mod?

EDIT: I've just realized I've been using Max. Thrust in place of Isp, therefore my first stage actually produces 280 * 9.8 * ln(28.045 / 12.045) = 2319.12 m/s, which is significantly less than 3400 as suggested by the map.

Rryk,

 You just answered your own question

 As a freebie... you should split up your lifter into 2 stages. The upper stage producing 1,700 m/sec and 0.7g vacuum and the lower stage producing 1,800 m/sec (1/2 atm) and 1.4G (1 atm) with all aero control surfaces and streamlining. It'll make your life easier.

Best,
-Slashy