Delta-V of RCS?

[Daiya]

So I'm trying to make a little suborbital Munar cruising pod powered solely by RCS for fine control. I'm so used to the Engineer plugin, though, that I have no idea how to calculate delta-v for RCS. The wiki says that a Kerbal jetpack has about 600, so I'd like to design something with at least four times the performance.

I guess I could turn off gravity and fly straight up, but that'd be rather time-consuming for each redesign.

Edited November 3, 2013 by Daiya

[UmbralRaptor]

Isp*9.82*ln(initial_mass/final_mass) The vacuum Isp of stock RCS is 260 s.

edit for details: You'll want to sum up part masses in each case. Go with full tanks for initial mass and empty ones for final. If you have a kerbal in an external seat, make sure to include its mass (0.09375 t) also.

Edited November 2, 2013 by UmbralRaptor
moar info

[Dave Kerbin]

Wait, is this for RCS on a spacecraft, or the little jetpack on a Kerbal?

[capi3101]

The 600 m/s delta-V? That's for a Kerbal's jetpack.

Yes, that does mean that there are worlds in the Kerbol system from which it is possible for a Kerbal to take off and achieve orbit on just jetpack thrust alone - Minmus, Gilly, Bop and Pol. I may be missing one or two but I think that's all of them.

As for RCS, it too provides delta-V and its calculated the same way as any other engines - with the Tsiolkovsky Equation UmbralRaptor posted above. The fuelmass of each RCS tank is as follows:

FL-R10: 0.2 tonnes

FL-R25: 0.4 tonnes

FL-R1: 3 tonnes

Stratus-V Roundified: 0.16 tonnes

Stratus-V Cylindrified: 0.6 tonnes

RCS typically doesn't give you a lot of delta-V, but it is enough to do precision docking maneuvers and in a pinch it can be enough to save a mission from drifting endlessly through space. A quick puff or two right after a long interplanetary transfer burn can improve the closest approach to an encounter; done correctly for a world like Duna or Jool, it can prevent you from having to do a long and costly correction burn.

[Daiya]

Thanks, this has all been very helpful! I think my RCS idea was waaay too optimistic in outperforming the plain Kerbal jetpack, sadly.

[SRV Ron]

The delta v for RCS will, like any ship, depend on how much mass is being moved. For very small probes, RCS can be used for maneuvering burns and landings. On big ships, it can still be used for fine tuning orbital corrections. In a pinch, the RCS can deorbit a ship that is out of fuel.

Lander using RCS fuel only.

RCS used to circularize a Mun orbit with a huge payload. Edit, fixed photo.

Build one and see what it can or can't do.

Edited February 24, 2014 by SRV Ron

[DMagic]

Don't let the low ISP for RCS thrusters discourage you from trying this. That value is a little misleading because the mass of RCS fuel is lower than liquid fuel. For low gravity planets/moons you can actually build a decent lander using only RCS.

[MK3424]

Don't let the low ISP for RCS thrusters discourage you from trying this. That value is a little misleading because the mass of RCS fuel is lower than liquid fuel. For low gravity planets/moons you can actually build a decent lander using only RCS.

Yeah, look at the Phoenix Mars lander: they used puls motors.

You can simulate it using rcs because there is no throttling in the RCS thrusters.

[UmbralRaptor]

Not sure that was worth bumping the thread.

Anyway, if you're looking at using RCS as primary propulsion, I would also recommend looking at the LV-1.

[Streetwind]

Don't let the low ISP for RCS thrusters discourage you from trying this. That value is a little misleading because the mass of RCS fuel is lower than liquid fuel.

Despite this being old as heck, let's at least correct this piece of misinformation... mass per unit does not matter, because units are arbitrary. If you have 1 ton of RCS fuel to burn, it will give you the delta-V of 1 ton of RCS fuel - whether that 1 ton is measured in 100 units or in 5000 units does not matter. The rocket equation asks for mass only.

I used to be confused by this myself, which is why I think it's worth pointing out

[Kerba Fett]

If you want to see dV for your ships RCS system MechJeb will show it, but you need to add the function to a window using the custom window editor.

That said, if you take a mk1 pod, add a parachute, 1.25m RCS tank, 4 quad thrusters, 4 probe landing legs, and a shielded docking port it will have an RCS dV of 688m/s.

I use a ship just like that as a data courier for my Minmus operation. Once it's transfer stage has it on the way to Minmus the RCS is enough to enter orbit, dock with the station there, then get back to Kerbin.

I never tried getting to the surface with it, but it should be able to manage it without any trouble.

If you want to play with RCS powered hovercraft, Minmus is probably a better place than the Mun.

I tried to do the same thing as you've got in mind and it didn't work well on the Mun. 4 RCS quad blocks will lift a small rover or ship, but the fuel doesn't last that long on the Mun and it was a bit unstable to fly.

[Dangerous_Beans]

i know i am a bit late, but i wanted to answer this:

Thanks, this has all been very helpful! I think my RCS idea was waaay too optimistic in outperforming the plain Kerbal jetpack, sadly.

so our target is ~2,400m/s, based on your first post, which using the rocket equation and the isp of RCS thrusters (and some trial and error cause my maths isn't that good) gives us a mass ratio of slightly over 2:1.

if we use a big RCS tank (though the tiny ones would also work, same mass ratio), one seat, a kerbal, and four blocks we get a full mass of 3.74, empty of 0.74, T/W of 0.1, and a dV of 4,126. you could use that to return from low Moho orbit.

if we add more RCS blocks (say, 10) we have dV of 3457.9, and a T/W of 0.25, which should be plenty to land and return from the Mun.

going for faster than a jetpack, we need over 12 RCS blocks. at 16 rcs blocks we get 3.6m/s, and still have nearly 3km dV. hell, at 96 rcs blocks (if they fit) we have >1km dV and a T/W > 1 on Kerbin.

(unless i have messed up my maths somewhere. that has been known to happen )

so depending on how you define "outperform", it's pretty easy to do.