MechJeb and dV?

[Taki117]

So I use MechJeb for vessel dV planning. In the VAB it lists the dV by stage, and I know it takes 4550m/s dVto reach LKO (According to the cheat sheet on the wiki) My question is how do I figure out how much total dV I have for an asparagus staged rocket? I have developed two possible formulae:

1.) dV_T = (dV₁+dV₂+dV₃+dV₄+dV₅+dV₆) +(dV₁+dV₂+dV₃+dV₄+dV₅)+(dV₁+dV₂+dV₃+dV₄)+(dV₁+dV₂+dV₃)+(dV₁+dV₂)+dV₁

2.) dV_T = (dV₁+dV₂+dV₃+dV₄+dV₅+dV₆)

Which is correct? Or is there some other formula?

[blizzy78]

Formula 2 is correct.

[Faark]

Jop, its 2. You could have just tested your rocket with and without fuel lines... if there is a difference, MJ does have to somehow consider it.

[Taki117]

But option 2 doesn't make much sense to me. If I have an asparagus staged rocked how is it only producing a few hundred dV in the first stage withe somewhere on the order of 49 LV-45 engines firing simultaneously?

[killamoves]

It's because the 49 engines are being fueled with the outer 2 tanks and so the fuel runs out very quickly.

[blizzy78]

It's because the 49 engines are being fueled with the outer 2 tanks and so the fuel runs out very quickly.

Exactly. In your first stage, all engines are being fueled by only 2 tanks, depleting them quickly. Once those tanks and a few engines are ditched, less engines are now being fueled by 2 tanks, depleting them less quickly. Once those tanks and some engines are ditched, even less engines will be fueled by 2 tanks, and so on. This is the reason why upper stages get empty slower than lower stages. Also, the Isp of engines usually goes up while ascending, leading to even slower fuel depletion.

[Taki117]

ah, ok. It makes sense now. thank you all!