Is there a way to calculate Delta-V for RCS?

[themonk]

So I find myself in a craft that has just picked up Herbal Kerman from the surface of the Mun, and I have changed the inclination of my orbit to meet up with a Science Station, and I discover that most of my fuel is gone...

But I notice I have a bunch of RCS monopropellant  left and I wonder, Can I do this rendezvous with just RCS?  Yep... It worked!  Still have half the monopropellant left as I finally docked.

Now my question is, as the title asks, Can you calculate DV for RCS?

[Tex_NL]

Yes you can. You could even calculate dV for chickens thrown out of an airlock if you wanted to. Just use the same formula as you would use calculating it for regular fuel.
[picsnip]
(https://en.wikipedia.org/wiki/Delta-v)

Edited March 11, 2017 by Tex_NL

[Streetwind]

@Tex_NL that equation calculates dV expenditure at a given thrust over a given period of time. It doesn't calculate dV available for use.

 

@themonk RCS thrusters are just tiny engines, like any other rocket engine. So you can use the Rocket Equation: dV = isp * 9.80665 * ln(wet mass / dry mass)

Insert the Isp of the thrusters (260s in vacuum IIRC). Plug in the current mass of your vessel (visible in map mode via the info button) as wet mass. Figure out the total mass of monoprop you're currently carrying (4 kg per unit), subtract this from your wet mass, and put the result in as dry mass.

Example: a 5-ton vessel carries a single FL-R25 monoprop tank with 250 units. So your Isp is 260, your wet mass is 5 tons, and your dry mass is 5 - 0.004 * 250 = 4 tons.

dV = 260 * 9.80665 * ln(5 / 4) = ca. 569 m/s

 

Edited March 11, 2017 by Streetwind

[Tex_NL]

@Streetwind You're right. I wasn't paying close enough attention to which formula I posted.
But my point stays the same. dV can be calculated for ANY form of 'rocket' propulsion.

[themonk]

Thank you for your responses....   I never thought about using the wet mass/dry mass to figure remaining DV.

[fourfa]

RCS Build Aid calculates RCS dV in its little window in the VAB/SPH editor.  I know you're already in flight - so load up the exact craft in the VAB, adjust the fuel tanks to exactly what you have in flight, and read out the dV remaining.  That is, if you don't want to bother calculating by hand.

[Chaos_Klaus]

7 hours ago, themonk said:

I never thought about using the wet mass/dry mass to figure remaining DV.

Well ... that's how calculating delta v works.

[Freshmeat]

On 11/3/2017 at 1:19 PM, Tex_NL said:

Yes you can. You could even calculate dV for chickens thrown out of an airlock if you wanted to. Just use the same formula as you would use calculating it for regular fuel.
[picsnip]
(https://en.wikipedia.org/wiki/Delta-v)

Can I quote you on that one in my sig?

[Tex_NL]

On 3/12/2017 at 7:16 AM, Freshmeat said:

Can I quote you on that one in my sig?

[snip] I can't really stop you, now can I?  

Edited March 14, 2017 by Vanamonde
Language.