How to calculete the mass and fuel of the SRB

[Kaleu]

Well i am making a mod and i don't know how much of fuel it has to have and the mass

I am making a real life SRB: http://www.astronautix.com/s/s-43.html

but want to make it in KSP standards, Like Tantares do or Bluedog etc

[Felbourn]

You need to know the dimensions of the part and the ratio of fuel to structure. That gives you the liters of solid fuel that it will contain by using (circle area) x (length) which you may recall is PI * radius^2 * height. A cubic meter is of course 1000 liters.

For example, when I search that link, I see the SRB is 1m diam and 9m long. That's 7068 liters (pi * 9 * 0.5^2). The gross mass is listed as 8425 kg and dry as 1245 kg. That's 7180 kg of fuel. I'm not sure what fuel it uses but common ones are PBAN and HTPB. I used to pretend all my custom rockets used HTPB so I'll pretend it's that. There are various formulations of solid fuel (even of HTPB) but we need to pick one so I usually go with 1.77 kg per liter. I think that was the same value used by RealFuels too. Anyway, given that we can tell the SRB has about 4056 liters of fuel (7180 / 1.77).  Seems like it's 57.4% fuel to volume (which seems low but I think I did the math right).

Now convert real life version into KSP by scaling it down and changing to the KSP mass of solid fuel (unless you're using a different fuel). KSP solid fuel is heavy as I recall. But anyway from here is takes needing to know the size of your part. You'll need to finish the math.

[sumghai]

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