How long your first stages last?

[Xyphos]

I've found out the hard way, SSTO rockets aren't very profitable because you'd have to build them big, heavy, and use more fuel than a staged rocket would. even if you were to recover your first stage, it's not really worth the effort, and it's very limiting on the payload due to that pesky rocket equation.

that said, boosters typically stage off around 8-10Km, lower stage drops around 30Km maybe a bit higher, middle stage drops off somewhere outside the atmosphere, usually around the 120 Km mark, then I continue to climb and circularize at 2000 Km using LV-N engine(s) from there, I can target any celestial body and safely perform the transfer burn with LV-N engine(s)

Edited August 8, 2017 by Xyphos

[DerekL1963]

I don't judge first stage performance by time or altitude - but by Ap at burnout.  (We're not trying to spend time or gain altitude after all, we're trying to gain velocity.)  Generally I'm looking for at least 20-30km for conventional designs, rather lower for the first pair of asparagus stages in a cluster design.  If I can't hit that, generally I'm not going to space today.

[wumpus]

18 hours ago, Xyphos said:

I've found out the hard way, SSTO rockets aren't very profitable because you'd have to build them big, heavy, and use more fuel than a staged rocket would. even if you were to recover your first stage, it's not really worth the effort, and it's very limiting on the payload due to that pesky rocket equation.

Note that you cheat the rocket equation by building a two-stage rocket that lets the first stage get to orbit* once it drops the second stage.  This is profitiable in KSP, but probably not enough to justify the player's time.  I certainly burned out trying to do this (without mods) and even with mods I'd expect to be spending well over four times the time it takes just to get into orbit on each flight.  Not worth it either.

You can often have time to boost both first and second stages to orbit (I suspect the only advantage is that you can use high-Isp fuels for your second stage, otherwise the rocket equation shouldn't change), and the main use is to allow the first stage to orbit Kerbin to come down on KSP.  It would make a lot more sense to use a checkpoint mod or even just something that gets rid of the "delete things outside the physics window".

[Aru]

I'll quote another post of mine, "But of course the real Kerbin density changes rapidly, it doubles going from about 5300 km to 0 km. Some rough density "half lives": 5300, 9700, 13200, 16800, 20200, 24000, 27500, 32000. So that's about 1/256 of surface density at 32000 m. This also gives an idea of how quickly engines change from their ASL stats to their VAC stats, though that depends on pressure rather than density."

Density and pressure are closely related. Drag depends on density, engine efficiency on pressure. Similar numbers for pressure: 4400, 8200, 11800, 15300, 21300, 22500, 28400, 30100. About 1/256 surface pressure at 30100 m. So at 30100 m, the Isp and thrust are 255/256 * VAC and 1/256 * ASL. Halfway from ASL to VAC at 4400 m, three quarters of the way at 8200 m, etc. The ASL stats really only matter for the lowest part of the ascent. But to be fair, you do spend a somewhat disproportionate amount of time in the lower atmosphere because you start at 0 speed and accelerate.

Edited August 9, 2017 by Aru

[septemberWaves]

Working in 4x scale I tend to use 3-stage or 4-stage rockets. The first stage usually does about 25% of the work to get to orbit for a three-stage rocket, or less for a four-stage rocket. That said, if I'm using a 2-stage rocket the first stage will often consist of about half the required delta-v (boosters included).

[Tyko]

I try to time my first stages to last until I've increased my time to AP to about 55-60 seconds. At that point I want to reduce thrust to maintain that time to AP most of the way to orbit. This works out to around 1500-1600 m/s DV. So my first stage (or SRB burnout if I'm using them) Is set up with 1500-1600DV. This also happens to put me at a high enough altitude that vacuum engines are at or very near 100% efficiency. 

So, first stage with a lifter engine and 1600DV, 2nd stage with vacuum engine and just about 100 DV short of full orbit. This lets me drop the 2nd stage when it's on a sub-orbital trajectory (no space trash). The payload then uses 100DV to circularize.

Edited August 11, 2017 by Tyko

[Nicias]

3 hours ago, Tyko said:

I try to time my first stages to last until I've increased my time to AP to about 55-60 seconds. At that point I want to reduce thrust to maintain that time to AP most of the way to orbit. This works out to around 1500-1600 m/s DV. So my first stage (or SRB burnout if I'm using them) Is set up with 1500-1600DV. This also happens to put me at a high enough altitude that vacuum engines are at or very near 100% efficiency. 

So, first stage with a lifter engine and 1600DV, 2nd stage with vacuum engine and just about 100 DV short of full orbit. This lets me drop the 2nd stage when it's on a sub-orbital trajectory (no space trash). The payload then uses 100DV to circularize.

I usually do exactly this except I use one LF engine all the way to orbit with combined SRB/droptanks.

[RoadRunnerAerospace]

depends on if it explodes or nor

[Tyko]

2 hours ago, Nicias said:

I usually do exactly this except I use one LF engine all the way to orbit with combined SRB/droptanks.

Yep, I've done that too...my second stage doesn't have to be stacked and could just be a central core with drop tanks/SRBs. Whatever is most efficient