Delta v and rcs

[Spaceman83]

Sorry for noob question how to count delta v for rcs engines? I have 3 stage rocket last stage has 2 rcs and middle stage 1 rcs do i calculate them separately? Can't use mods playing console!

[Snark]

Depends how and when you use them.  RCS thrusters have an Isp just like any other engine, so you calculate it the same way you'd do for any burn-this-fuel-first, then-burn-that-fuel situation.

Most people don't bother calculating the potential dV contribution for RCS because typically it's not used for big burns, just tiny nudges for docking purposes.  And most ships only carry a fairly small amount.  (Doesn't mean you can't, just that most folks don't.  The Isp for RCS thrusters isn't all that great, you'd be better off saving the monopropellant and spending the mass on LFO.)

If you do want to calculate the dV, it matters what order you burn the fuel in.  If I have one stage that has two different fuels, and those two fuels have different Isp values, burning fuel A before fuel B will give a different dV answer than the other way around.

If you could describe your order of operations of what you burn when, then I could tell you how to go about calculating the dV.

But honestly, unless you're schlepping around a big load of monopropellant that you're counting on for your dV budget, it's probably not worth calculating.

[dire]

Unless you absolutely need the ability to have lateral thrust (like you're doing docking maneuvers, for example) the math suggests you are better off with a battery+SAS stacks for rotational control or more LFO for delta-V.

Now, monoprop is a little bit lighter than liquid fuel by itself (you get 250 units per ton compared to 200 units of LF) but that's more than offset by the much lower ISP of RCS blocks -- 240 compared to the 345 for a terrier. The terrier engine does give you a tiny mass penalty, though.

Just for kicks I put together a command capsule, parachute, 1.15 ton monoprop cannister, and 4x "Puff" monoprop engines (Mechjeb doesn't read normal monoprop thrusters when calculating dV, but Puffs act as normal engines). In vacuum, you'll get 3.15 TWR for 31 seconds, giving you a net total of 1321 dV. Slap a heatshield on, and that goes down to 1136. That's a viable munar return vehicle (detach it from your lander when you're ready to go home). If I switch from the monoprop to a FL-T200 with 4x "Twitch" radial engines and the same heatshield, I get 2.39 TWR for 44s, yielding 1295 dV.  A Terrier (which doesn't need the heatshield for munar return, and can't equip it anyhow) yields 2.39 TWR for 56 seconds, producing 1672 dV with the same fuel.

The reason the Terrier gives you better dV than the radial engines is simply because it has an ISP of 345, rather than the Twitch's 290. 

Finally, I put chute, pod, heatshield, decoupler; FL-T400 with no oxidizer (giving me the same fuel mass as the other test options) and a NERV, and Mechjeb says it's 1419 dV at 1.12 TWR. So, Terrier beats NERV for your munar return stage, but NERV beat both radial engines (its extra mass perpetuates down the rocket though). It's more intended as a deep-space engine anyhow.

So in conclusion, this experiment yielded the surprising (to me anyway) result that monoprop and nuclear are both viable for your return stage, but LFO (especially with the high-efficiency engines) still beat it by a significant margin.

Edit: Now, bear in mind that dV scales to the log of your fuel, not in a linear manner. So you can't just say "Oh, one ton of fuel was 1672 dV, so two tons of fuel are 3344 dV and that's plenty to get me from LKO to the Mun, down and back." (Two tons on this rocket with a terrier is 2642, which  is likely to leave you stranded in high Munar orbit unless you bring a transfer stage).

Edited August 17, 2016 by dire

[Pecan]

1 hour ago, dire said:

...In vacuum, you'll get 3.15 TWR...

Nice experimentation :-)
Note though that TWR is dependent on, er, weight - so it changes according to which body you're orbiting.  Normally that doesn't matter in the slightest but for something that has to land on or launch from a planet/moon you'd better make sure those engines are powerful enough as well as fuel-efficient.

[Not saying there's anything wrong with your analysis, just making the point that TWR, unlike dV, depends on where you are]

[Spaceman83]

Thanks for info! Yes they really not matter much my heavy mun rocket get only couple of hunred dv for those and use those just random manoeuvres but nice to know!!! If i need to calculate those some other rocket!

[dire]

3 hours ago, Pecan said:

Nice experimentation :-)
Note though that TWR is dependent on, er, weight - so it changes according to which body you're orbiting.  Normally that doesn't matter in the slightest but for something that has to land on or launch from a planet/moon you'd better make sure those engines are powerful enough as well as fuel-efficient.

[Not saying there's anything wrong with your analysis, just making the point that TWR, unlike dV, depends on where you are]

True, but obviously, for hangar analysis we're using Kerbin TWR; it's helpful in comparing the relative "oomph" of the different options, and the ratios of TWR are what I was really interested in. Monoprop actually had the highest TWR of the tested engines, and will continue to have the highest TWR whether you are on Kerbin, the Mun, or elsewhere. NERV's had the lowest TWR, and since their thrust varies greatly depending on atmosphere that was an upper bound for your relative NERV strength -- you could probably hit 10km with the Puff engine but you won't even get off the ground at KSC if you test these engines in flight. Even on the Mun, where gravity is almost negligible for launching into low orbit, you will incur significantly more gravity losses with NERV than you would with any other option for a return vehicle.