How to determine how much a rocket can lift?

[ouion]

Is there a equation that can determine how much tons can a rocket lift.

Edited July 13, 2015 by ouion

[Randazzo]

See here: http://wiki.kerbalspaceprogram.com/wiki/Thrust-to-weight_ratio

You can turn the equation around to figure out maximum mass for a specified TWR.

(F_T / g) / TWR = m

You will need to know the thrust of the engines, and you will need to choose a TWR. The gravitational constant (g) for Kerbin is 9.81 according to the wiki.

So let's use the Skipper as an example. It has 568.75 kN of thrust at 1 atm, or sea level. Let's assume we want a TWR of 1.3.

(568.75/9.81) / 1.3 = 44.597

So, for a TWR of 1.3 on the Launchpad, your skipper powered rocket must be less than approximately 44.6 tons.

[MoonMan22]

There is no easy way of calculating that. You do have a couple of options.

1. You can make dummy payloads of different weights and then use Kerbal Engineer to check the dv/TWR numbers it generates.

2. You can use this Rocket Calculator to see the payload capabilities of different rocket designs.

3. You can calculate it yourslef using the Tsiolkovsky rocket equation to figure it out. (lots of crazy math)

Good Luck!

[Chaos_Klaus]

3. You can calculate it yourslef using the Tsiolkovsky rocket equation to figure it out. (lots of crazy math)

Well, lots of crazy math .... no.

The only fancy thing in there is the natural logarithm and your average calculator even has a dedicated button for that.

The thing about these calculations is that you have to do them for every stage seperately and that tends to get tedious. That's why we use something like Kerbal Engineer Redux to do the calculations for us.

You need to worry about two things:

1.) Do you have enough thrust to lift your payload off the ground and acellerate fast enough. For that you need to look at your Thrust-to-Weight ratio. And that is really just your Thrust devided by your weight, which is your vehicles mass times g. TWR = T/(mg) You propably want your TWR above 1.5 at least. So that puts a boundary on your max payload mass. You can counteract this somewhat with adding more boosters. Many real rockets come in different configurations for different payloads.

2.) Do you have enough delta v to get to orbit. If you look at the Rocket equation, you will see that delta v is dependent on your dry mass. Payload is dry mass and when you bring more of it, your delta v decreases. That means you can get into a higher orbit with a lighter payload. On the other hand, a very heavy payload might give you not even enough delta v to reach orbit.

Now to actually calculate the payload mass for a given delta v requirement, you have to rearrange that equation to solve for the part of your upper stage drymass that is your payload mass. Then iterate this through all your stages. This is not hard, but takes time to do on paper. I'd just slap on a fuel tank as a dummy payload and adjust the fuel level to simulate different payload masses. Make sure there is no fuel crossfeed though.