Calculating maximum lifting capacity

[Frafor]

Using Subassembly loader I have made a group of premade launchers of different lifting capacities and now I want to calculate how much weight can they get into orbit. Is there any way of doing this fast and easy or I have to do trial and error?

[42undead2]

What I would do in your situation is install Kerbal Engineer Redux, and check the TWR. If it goes below one, if won't lift.

[Shrike42]

Well, there's Kerbal engineer, but it hasn't been updated for 0.20. It gives you the TWR, and as long as you can keep it above 1 for Kerbin, you can get it off the ground.

[SinPistones]

1.- Divide thrust (in Newton) by weight (also in Newton.)

2.- ???

3.- If result (TWR) is more than 1 = PROFIT.

[MDBenson]

I'll backup the TWR (Thrust-Weight Ratio) check, I use the Delta-V stats panel in MechJeb 2.0 to calculate mine. You need to make sure that at every stage the TWR is above 1 until you get into orbit then it can be less than 1 (I try and keep it above 0.7 though or things don't accelerate very quick) as you are in free-fall.

MechJeb's Delta-V panel analyses your staging setup to determine the stats for each stage giving the Min (starting) and Max (right before the fuel is finished) TWR values so you can see if it will increase. Don't worry if you are close to 1.00, as soon as you hit the gas on launch the TWR will start to increase as fuel is burned and the rocket lightens. I've had a couple of rockets sit above the pad for a fraction of a second before going and most of mine are loaded so that they lift off the pad veeeeerry slowly!

[tavert]

TWR is important but not the only thing that matters here. You also need to have enough delta-V to get a payload to orbit - MechJeb or Kerbal Engineer will tell you how much you have. The required delta-V is a function of TWR, but a good target is around 4500 m/s to get from Kerbin into a low orbit.

[SinPistones]

Yeah, well you could also calculate the available Delta-V but the math gets quite more complex (for me, I barely passed calculus) and then Kerbal engineer or mechjeb's vessel info window become super useful

[mcstalker]

Here is a formula to calculate the addition lift capacity for a single stage rocket. I have not been able to get the math to work with a multi stage or multi engine with different burn times.

Ml = Mass the rocket can lift.

Delta-V = Since Delta V and Mass have an invers relationship as you add mass to your rocket you will loss Delta V. You need to determine the delta V you wish to have with the addition mass.

Mt = is the total Mass of the rocket with out the addition of Ml

Mf = the mass of the fuel

T = Thrust of the engines

Bt = burn time

[technicalfool]

....an image from the local machine...

You might want to upload that to imgur or similar so the rest of the Internet can see it.

HTH.

[O-Doc]

I prefer using trial an error. KER will probably help you out in the design phase with dV calculations. Where's the fun in that?

[SRV Ron]

Lifting capacity can also involve being able to land on Mun or Minmus with enough fuel for a safe return.

In fulfilling a contract to test a turbojet engine on Mun, I selected to use my manned Mun lander replacing the capsule with a probe and the heavier turbojet engine. That choice appeared OK since a return from Mun was not required and there should be fuel enough for a one way flight.

While the flight did take more fuel as expected with the higher payload weight, experience gave me a more efficient flight profile to Mun with only a tiny amount of lander fuel needed to finish the low orbit. (Normal flight leaves enough fuel in the transfer stage to deorbit it during landing.) Locating a flat landing spot helped with the results that after dumping the turbojet after testing, the lander has more then enough fuel for return. But with no parachutes, I am writing it off and leaving it there should I get a contract to test the probe in a Muner escape of something as rediclous.

[Red Iron Crown]

I prefer using trial an error. KER will probably help you out in the design phase with dV calculations. Where's the fun in that?

OP is specifically asking for a method easier and faster than trial and error.

Best bet is to install Kerbal Engineer Redux or other data-providing tool. Keep adding mass at the top of the rocket (fuel tanks work well but don't forget to disable them), when your TWR falls below 1.2 or so or your total delta-V falls below ~4500m/s or so, you've reached the limit of that rocket's capabilities. (If you're using the FAR or NEAR aerodynamic mods, use ~3500m/s instead.)

[Pecan]

Necro guys - OP and last post before today were May last year.

For anyone else looking for the formula, it's on wikipedia (so it must be true ^^): http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

[Claw]

Necro guys - OP and last post before today were May last year.

Yep, there's been a rash of these lately. While a necro itself isn't bad, it sometimes confuses people. So in this particular case, I think this one is played out.

Cheers,

~Claw