Lifting Capacities

[gc1ceo]

Is there a good way to judge what you need to lift something of a specific weight without pure trial and error?

Basically I want to know specifications I need to lift a given payload of a certain weight before I construct the lifter, or at least a general figure.

[MockKnizzle]

Well, if you know how much delta-v you need to get where you're going (about 4500 to LKO) and have picked an engine (you need to know the ISP), you can calculate required propellant fraction with the Rocket Equation and use that to make a rough estimate of the fuel mass you'll need based on your payload. It might take a few design iterations to get both the propellant fraction and TWR where you want them, but it's more systematic and less prone to failure than pure trial and error.

Alternately, download either MechJeb or Kerbal Engineer and use the VAB data readouts to quickly and easily see your dV and TWR information on the fly.

[annallia]

I generally just build the core stage and then check my TWR (mechjeb or kerbal engineer) and start adding stages until it gets where I want it to be.

That said you need approximately 9 kN to lift 1 ton (real world not sure how that translates to game) so if you were trying to lift something that was say 10 tons (including fuel/engine weight) you would need at minimum a 90kN engine. However that really does little for getting it orbital... that is just so you can get it off the ground.

[blizzy78]

How to calculate what you need (or you could just use one of the Zenith rocket family lifters)

[Crater]

A good rule of thumb is that your payload should be about 10% of the total mass of your launch vehicle.

In other words, if you build the payload first, you can expect to have to have to "put a naught on the end" of the mass before you launch.

Or, if you build a lifter, you can expect it to put about a tenth of its own mass into orbit as a payload.

In the real world, the figures are more like 5% mass for a typical launch payload. In KSP, you can sometimes push it quite a bit further, especially with jet engines. But for a rocket, 10% is a reasonable ball-park figure.

[blizzy78]

A good rule of thumb is that your payload should be about 10% of the total mass of your launch vehicle. ... Or, if you build a lifter, you can expect it to put about a tenth of its own mass into orbit as a payload.

You know, percentage math doesn't work like this

Also, the payload fraction can be higher than 10%.

[Crater]

You know, percentage math doesn't work like this

Also, the payload fraction can be higher than 10%.

Yeah, which is why I sprinkled words like "about" throughout the post. I don't see that it would have helped to tell him that the payload could be up to 11.1111111% of the lifter mass, and I definitely mentioned that you could push it further. But if someone is asking what size a lifter needs to be in relation to the payload, then they are probably relatively new to the game, and are not likely to be building massively effective vehicles (no offence to the OP).

10:1 is a rule of thumb that helps to know that you are in about the right ball-park, and only depends on maths that almost anyone is capable of. It doesn't need a calculator, and doesn't give you multiple significant digits of precision for a perfect circularisation at exactly your target altitude just as the engines flame out from lack of fuel in your ascent stage.

[capi3101]

How to calculate what you need (or you could just use one of the Zenith rocket family lifters)

Argh!!! You beat me to it!!!

10% payload is a good estimate for serial-staged rocket designs (i.e. what all of us usually think of and build first; i.e. Apollo-style boosters). As Temstar points out in that link though, asparagus gets you up to 15% or so.

I personally have been having some serious issues with the Zenith family in 0.21; it's like the fix that Temstar used for 0.20 didn't carry over. Haven't checked for updates, though.

Edited August 14, 2013 by capi3101

[HeadHunter67]

How to calculate what you need (or you could just use one of the Zenith rocket family lifters)

For someone who doesn't want to struggle with mathematics while having fun, the Zenith lifters are a godsend. Not the prettiest launchers, but VERY reliable - and reasonable part count for what they do.

Using the SelectRoot plugin, I added all of these lifters in Subassembly Manager so I can slap them on underneath my chosen payload. However, even with Temstar's recommended technique for saving the subassemblies (and, in fact, using his subassembly files), it seems sometimes the struts and fuel lines get borked on the larger lifters. A small price to pay for power.