A silly method of payload mass estimation and launcher composition.

[kerbiloid]

Sorry, if this looks too primitive to be a thread.

It is simple, but enough long to tell, so I've created a post about how I solve this problem  to easily find it in future if ever tell all this again.

It's an answer to this thread:

http://forum.kerbalspaceprogram.com/threads/101459-Payload-to-orbit-mass?p=1567857

The method is:

Assumptions:

1. We have a desired payload mass.

2. Acceleration would be 2..4 g (less means less efficiency, greater means a structure crash).

3. So, any subrocket would have TWR=2 when full and TWR=4 when empty.

4. Any launcher would be designed only for one purpose: launch a payload to 80 km circle orbit.

5. Any other orbits: use additional stage inside you payload.

6. In KSP a launcher has 2..3 stages.

7. Delta-V for Kerbal launch ~4.8 km/s.

So, designing a launcher from top subrocket to bottom.

1. Estimated thrust ~= Mpayload * ~1.25 * 4 * 9.81.

2. Choose an engine(s), according to this value.

3. Summarize their thrust values and get a total thrust of subrocket.

4. Empty subrocket mass ~= (Mpayload + Mengine) * 1.1.

5. Full subrocket mass ~= Total thrust / (2 * 9.81).

6. Subrocket fuel mass = Full subrocket mass - Empty subrocket mass.

7. Collect fuel tanks for subrocket according to fuel mass.

8. Calculate exact mass of the subrocket.

9. Calculate a subrocket Delta-V = ISP * 9.81 * ln(Mtotal/Mempty)

10. Calculate rest of total Delta-V = Total Delta-V (initially 4.8 km/s) - subrocket Delta-V.

Calculate the previous subrocket in the same way.

Of course, its payload mass = next subrocket mass.

If possible, split it to use the next engine to (i.e. use a parallel stage scheme), in this case you would subtract the thrust of the next subrocket engine from the total thrust.

In this case always put fuel pipes from the previous subrocket to the next one.

If the rest of Delta-V is yet positive, do magic with extending fuel tanks or adding launch boosters.

When you have this finished, you would test your LV.

Collect a set of fuel tanks of total mass equal to your desired payload mass.

Place these tank on your top stage instead of a payload.

Do not use decouplers, just extend the last stage, allowing it get the fuel from these dummy payload tanks.

Launch this to 80 km.

You have an orbit launch guaranteed due to these additional tanks and no real payload.

Calculate the rest of fuel when your tanks are on orbit.

You get a precise value of your launcher REAL payload mass.

Now you can decide:

- either to change your launcher composition to tune your payload mass.

- or to let it be so, as the a real payload mass is OK for you.

That's all.

Edited December 1, 2014 by kerbiloid