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Question

Hi, in ksp 1.1+ is there any way of guessing how much dV can your RCS thruster give? I believe that neither vanilla nor K engineer can do it? Or do they?

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RCS thrusters have a vacuum ISP of 240 s, therefore, using the rocket equation, we have

Δv = 2354 * LN( total vehicle mass / (total vehicle mass - RCS fuel mass) )

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RCS Build Aid does it. Or you could use ye olde way and calculate it yourself using the rocket equation.

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MechJeb can also calculate RCS delta-V for you, though I don't think it shows it by default.

It's a bit complicated though, as RCS typically isn't used as a stage by itself after the other propellant is expended. I usually end up using it throughout the mission at varying vessel masses, so the practical dV falls somewhere between the dV with other tanks empty and with them full.

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4 hours ago, Red Iron Crown said:

RCS typically isn't used as a stage by itself after the other propellant is expended.

I have on a few occasions made an upper stage for a small payload out of a FL-R10 fuel tank with some rearward facing "place anywhere" RCS ports.  It was highly efficient in older versions of KSP because the RCS ports were massless.  That's not the case anymore.  My old v0.90 command seat Eve launcher had an RCS upper stage that could deliver something like 2000 m/s dV.

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19 hours ago, Gaarst said:

RCS Build Aid does it.

 

Where does it show it?  That sounds useful.

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12 minutes ago, Invader Jim said:

Where does it show it?  That sounds useful.

Don't remember off the top of my head, try messing with the RCS Build Aid UI in the VAB, it's probably under Translation.

EDIT: yes under translation: (image from the RCS Build Aid thread OP)

2yXRr1K.png

Edited by Gaarst
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19 minutes ago, Red Iron Crown said:

@OhioBob Of course it's possible, just not typical. :) 

I find it a highly useful alternate system, both in stock and RO.  It may not be the most efficient in terms of ~KSPKash~ but it does the job quite well when you want to make minute adjustments in any desirable plane without noddling with craft orientation.

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11 minutes ago, regex said:

I find it a highly useful alternate system, both in stock and RO.  It may not be the most efficient in terms of ~KSPKash~ but it does the job quite well when you want to make minute adjustments in any desirable plane without noddling with craft orientation.

The system is useful (particularly in RO), but the dV meter for it is not overly so. About half the time I'm using it for ullage, and those minute adjustments might be at any mass state of the craft.

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One of two: either  RCS Build Aid  is not compatible with 1.1, or i don't know how to use it. I don't see any window or option in the VAB...

5 hours ago, OhioBob said:

I have on a few occasions made an upper stage for a small payload out of a FL-R10 fuel tank with some rearward facing "place anywhere" RCS ports.  It was highly efficient in older versions of KSP because the RCS ports were massless.  That's not the case anymore.  My old v0.90 command seat Eve launcher had an RCS upper stage that could deliver something like 2000 m/s dV.

I'm using the RCS for leaving Minmus surface with barely the command pod. I think it saves some weight. But i wanted to know the TTW and dV before trying different combinations. 

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