Delta v and ascent to orbit

[LN400]

For a rocket starting at the launch pad, and heading for a low Kerbin orbit (71,000m or thereabout), and with minimum 2 stages, maximum 3 excluding the payload, what are your rule of thumb numbers for each stage in terms of delta v? I would like to hear from you what figures you go for.

[dire]

Depending on how big the rocket is, and how top-heavy/draggy it is, I usually try to have at least 3200 dV to get from launchpad to orbit. If it's a big, ugly, draggy monster I wind up needing at least 3500, sometimes more. I recently plopped a 100-ton payload onto the munar surface, and that took about 3700 to orbit I think, but that's pretty much my upper limit in terms of drag and other losses that will actually get to orbit.

More experienced/better pilots than I will tell you 3k dV to get to orbit, which is doable.

In terms of staging, for lighter loads I usually just throw a big orange+twin boar on the bottom and that will get a pretty sizeable payload to orbit.

Edited August 15, 2016 by dire

[LN400]

Thank you for that. I am however particularely curious about each stage in a multi stage rocket, that is how you distribute the dv among the stages, from the lifter to the injection stage.

[eddiew]

For me, this depends on TWR rather than delta-v. I like to lift off with 1.5-1.6 and want to maintain that all the way for a comfortable gravity turn. I will use as many stages as needed for this to happen. Sometimes that means just 1 for small payloads, it could be 4+ for very big ones.

Actual fuel distribution makes no difference to the flight if TWR is kept sane

[foamyesque]

I prefer two-stage + booster layouts, with the delta V split roughly half and half between the two sustainer stages. It lets you use all the engines in their optimum environments and tends to yield manageable but good TWRs throughout.

[LN400]

1 minute ago, foamyesque said:

I prefer two-stage + booster layouts, with the delta V split roughly half and half between the two sustainer stages. It lets you use all the engines in their optimum environments and tends to yield manageable but good TWRs throughout.

50/50, that's interesting and something I haven't considered. Mine has been more a 1/3 for the first stage then 2/3 for the final stage, in a 2 stage design.

[dire]

Well, the great majority of your losses due to drag and gravity tend to be under 10km, with an especially costly patch going up to 3km. So the higher your TWR in that region, the smaller your losses due to gravity. Drag is much less of a dV loss and more of a control issue; if you can have a highly streamlined nose and a little bit of a draggy tail on your rocket you can afford a higher TWR. A good two-stage design should have at least 800-1200 dV in the bottom stage, and probably not more than 2k dV in that stage (or else it's really more efficient to make it a one-stage design) because you only want to pay for one set of big engines, and you should keep them until you're out of the zone that gives you big penalties to dV. A three-stage design should be similar; just tack on some SRB's to get you through that first 3km with fewer losses.

My rockets tend to be absurdly draggy and I've found that keeping the SRB's after they're burnt out instead of discarding them helps keep my nose pointed in the right direction, and I also add wings and fins to my bottom stages, but all of that comes at a delta-V penalty.

Edited August 15, 2016 by dire

[Red Iron Crown]

1 hour ago, foamyesque said:

I prefer two-stage + booster layouts, with the delta V split roughly half and half between the two sustainer stages. It lets you use all the engines in their optimum environments and tends to yield manageable but good TWRs throughout.

I do similar, but my rule of thumb is that the first stage should have twice the fuel mass as the second. This often gives close to a 50/50 delta-V split. Add enough SRBs to get the core sustainer TWR high enough and we're go for launch.

(As a sandbox player I should probably use LFBs instead of SRBs, but SRBs are awesome.)

[OhioBob]

Here are my rules of thumb for designing rockets:

 

[GoSlash27]

First stage: 1,800 m/sec calculated at 1/2 atm and minimum 1.2 t/w at 1 atm.
Second stage: 1,700 m/sec vac and minimum 0.7 t/w vac.

This puts the staging right about 27 km altitude and 800 m/sec, allowing staging with no fear of tumbling. As a result, all the aero stuff and shells can be jettisoned with the first stage.

Best,
-Slashy

[Reactordrone]

I usually budget about 2000/2000m/s based on vacuum delta-v for first and second stages. However, if I've made an upper stage with extra delta-v I'll sometimes just make a single stage with about 3000m/s and finish off with the upper stage/payload engines.

[cantab]

3.5 km/s to start with. First stage measured as atmo, further stages as vacuum. But I'm in science mode so I don't mind overengineering.

[Superfluous J]

I'm in the 50/50 crowd. If I need a third (non-SRB) stage my ship's usually really weird and not really worth trying to categorize - and probably doesn't work anyway. But usually I try to have 3000-3300m/s, roughly evenly split between the two stages, and SRBs for a bit of an early kick off the pad pushing the total dV up to about 3500 (vacuum) dV between those stages.

Sometimes I shoot for about 3000 total dV (including SRBs) and get to orbit with some of the payload's fuel, but that's only in cases where the payload is small and I'm bringing extra fuel because I just can't make the ship any smaller and get any real gain. It's easier to add 500 m/s of dV to a tiny probe than its launcher. But that's another of those edge cases where its just best to do it and not think about it much.

Addendum: One common >2 stager I still use to this day sometimes is asparagus staging. The nice thing about that is you keep a relatively consistent TWR all the way up, but it can be expensive (in Funds) especially on those first few stages where you get only a couple hundred m/s and then drop 2 really expensive engines. But if your payload requires it, hey it's an option.

[OhioBob]

4 hours ago, 5thHorseman said:

I'm in the 50/50 crowd.

The real life rule of thumb is that we should give more Δv to the stages with the higher specific impulse.  Liquid fueled engines in KSP generally don't differ all that much in Isp, so yeah, splitting it 50/50 is a pretty good way to go.  In theory we should probably give a little bit more to the second stage, but @GoSlash27 make a good argument for why we might want to weigh it a little bit more toward the first stage.  I generally don't look at the Δv distribution as much as the propellant distribution.  I typically try to give my first stage twice as much propellant as my second stage.  If I get that balanced about right, then the Δv should be well balanced also.  If I'm using SRBs for the first stage, then I definitely like to give more Δv to the liquid stage, which we should because is has better Isp.