Payload Capability?

[sodopro]

How do you tell how heavy of a payload your rocket can lift?

[thatoneperson8]

Why you test it ofcource

[EndlessWaves]

Divide the total rocket weight (mass * local gravity) by the thrust of all of the engines (that are active at once). If it's higher than one then it'll lift off the ground.

If you're talking about getting it into a specific place then it's much harder to say as payload and weight are linked - the more payload you have the shorter distance your fuel will take you. Your best bet is to grab a mod like Kerbal Engineer Redux that shows you range (DeltaV) and load up the weight on top until it's the minimum figure that'll do the job in hand (around 4600/4700m/s DeltaV to get to a low orbit from KSC).

[ThatBum]

There's a fundamental problem with rockets: they need to lift their own fuel. Say you want to lift a payload of a given weight to orbit. Right, so you need some propellant to get it there. But, the propellant weighs something as well, so you need more propellant to lift that propellant. Then, that additional propellant weighs something, so you need more...

Fortunately, you're saved from an infinite loop in that the rocket gets lighter as you burn fuel and shed stages, and air resistance decreases as the atmosphere gets thinner. It still stands to show that additional weight can get out of control quickly, and bigger rockets don't always help.

[ROFLCopter64bit]

You could just take a look at my application Carbon

[Peenvogel]

Mechjeb also shows this. And Engineer Redux even gives info on TWR for the planets and moons! Works really well. For a Kerbin orbit, I always aim for 2 to 3 TWR.

[capi3101]

How do you tell how heavy of a payload your rocket can lift?

Thrust to weight ratio is the easiest way to figure out how much your rocket can lift; that's simply taking the total mass of your rocket, multiplying it by Kerbin's surface gravity and dividing the total stage thrust you're outputting by the result. A result greater than one means you can lift it; less than one means you're not going anywhere.

Now, if you're asking "can this rocket get my payload into Kerbin orbit", that's a tricky question; it's dependent on a lot of different factors, including the specific impulse values of your engines, the amount of delta-v you want your rocket to have in aggregate, the amount of thrust you want to be able to generate, and the mass of everything. The best way to test it is to take the delta-v you want, divide it by the specific impulse of your engine, divide that by 9.81 and take the inverse natural logarithm of that result; you wind up with the ratio of wet mass to dry mass. For example, if you're using Toroidal aerospikes (388 Isp) to acheive Kerbin orbit (4500 m/s delta-V), the mass ratio you want is 3.26 (4500/388 = 11.579/9.81 = e^1.82 = 3.26). As long as you can keep the ratio above that amount, you should have sufficient delta-V to reach orbit (but be sure you still have a TWR over 1!!)