How to calculate a booster's payload capacity?

[Pie Guy]

In real life, NASA designs several different booster designs. These boosters are flexible and can be fitted either with/without boosters and any payload can be mounted on the booster. Each booster is designed for a different max payload capacity,  like a 15t booster or 30t booster.

I'm trying to create a similar system in KSP, making many boosters and saving them as subassemblies. One problem: I don't know how to calculate how much payload a given booster can actually carry into LKO.

Anyone have a calculator I could use? Or just give me an equation and how to put it into a spreadsheet?

[The Aziz]

Build a lifter and keep adding mass to dummy payload (ore tanks are the easiest to work with) until you don't have enough dv to reach orbit. That's your limit.

[Guest]

It's easier to use the in-game calculator. 

Work out a minimum deltaV and TWR amount you need to get into orbit. Let's say 4k (so spare) and 2. 

Build your lifter with excess of those minimums, then in the editor add something you can change with mass of like a fuel tank to the top and adjust the fuel level until the calculated deltaV and TWR (from the games calculation is accurate enough in general) reaches your minimum needed. 

Note the mass (in the engineer’s report) then remove the tank and note the new mass. Find the difference between the 2 and that's your payload.

[Wobbly Av8r]

While I am sure that if one has the proper mathematical acumen this can be determined 'on paper', on the planet of Kerbin such calculations are left to trial and error - I'm pretty sure it's in their Konstitution!

The steps to developing a meaningful chart/table that determines lifting capability needs a consistent launch profile, launching a specific mass (I use full 2.5m ore tanks) and verifying that you can 1) Achieve orbit, 2) Determine how much fuel is left over. The left-over fuel can either be off-loaded or payload increased, as necessary.

As an example, I have a booster system that is based upon the Twin-Boar with a Jumbo-64 tank on top.  That arrangement will place a relatively steamlined payload of 18t-19t into LKO. As I strap certain combinations of SRB's to it, it increases the amount of payload...

Spoiler

			LKO			Reentry
Booster			Tons	Kerbucks	Fuel dV
----------------------  ----	--------	--------
Twin-Boar Solo							Twin Boar flown @30% profile, full thrust except where noted
								100% Recovery = 21,955
Twin-Boar + Rokomax 8
Twin-Boar + RokoMax 16
Twin-Boar + RokoMax 32

Twin-Boar + RokoMax 64	18t	27,830		73 m/s		w/95% Recovery, actual cost 6,980
			19t	27,830		14 m/s		(Lift 2.72 kg per $)

TBRM64 + 2x Hammer	19t	29,830		98 m/s		w/95% Recovery, actual cost 8,980
			20t	29,830		47 m/s		(Lift 2.22 kg per $)
	
TBRM64 + 4x Hammer	20t	32,030		76 m/s		w/95% Recovery, actual cost 11,180
			21t	32,030		27 m/s		(Lift 1.87 kg per $)

TBRM64 + 2x Thumper	21t	31,470		64 m/s		w/95% Recovery, actual cost 10,620
			22t	31,470		16 m/s		(Lift 2.07 kg per $)

TBRM64 + 4x Thumper	23t	35,310		62 m/s		w/95% Recovery, actual cost 14,460
			24t	35,310		17 m/s		(Lift 1.66 kg per $)

TBRM64 + 2x Sidekick	26t	35,170		76 m/s		w/95% Recovery, actual cost 14,320
			27t	35,170		35 m/s		(Lift 1.89 kg per $)

TBRM64 + 4x Sidekick	30t	42,710		145 m/s		w/95% Recovery, actual cost 21,860
			32t	42,710		32 m/s		(Lift 1.46 kg per $)

TBRM64 + 2x Pollux	42t	41,700		28 m/s		Thrust Q-limit 40000 pa
								w/95% Recovery, actual cost 20,850
								(Lift 2.01 kg per $)

TBRM64 + 2x Throbred	50t	61,198*		430 m/s		Thrust Q-limit 40000 pa
								w/95% Recovery, actual cost 39,243
								(Lift 1.27 kg per $)

TBRM64 + 2x Clydesdl	85t	85,200*		232 m/s		Thrust Q-limit 40000 pa (28% Launch profile)
								Thrust limit 95% until circularization
								w/95% Recovery, actual cost 64,245
								(Lift 0.76 kg per $)

 

 

Edited March 26, 2021 by Wobbly Av8r