Delta-V and RCS

[THobson]

Does the DV hit from transporting monopropellant outweigh the DV that can be obtained from using it?

I have a ship that I am going to land on a planet.  Fuel is tight.  Should I send it does with full RCS tanks and use the DV from the RCS to help get it back into orbit or is it better to send it down with empty RSC tanks?

I am happy to work it out if someone can tell me how.

Many thanks

[mk1980]

generally speaking, RCS is always a loss of deltaV. even if you don't need the monoprop for maneuvering and use it up for acceleration/deceleration, it's a net loss because RCS thrusters have very bad specific impulse (fuel efficiency) which is one of the main components of the deltaV equation. every single rocket engine has better Isp. think twice if you really need RCS and at all. if it's a large ship, it may be convenient for docking, but a small lander should be quite easy to dock without RCS.

[icedown]

The only small ships I make with RCS are station tugs.  I do equip all of my larger ships with it, but then I usually have a monoprop fuel cell on board in case I end up in a dark corner for to long.

[THobson]

Many thanks.  Very helpful.

[Reactordrone]

If you have Monoprop and liquid fuel engines you'll always get more delta-v by using the monoprop first to reduce weight before using the liquid fuel engine. So if you have a lander with excess monoprop on board you're better off burning the monoprop for the de-orbit burn rather than dragging its weight around until the end. if you're docking you should retain a small amount of monoprop for final translation manoeuvres but that's about it.

[ibanix]

On the other hand, even a tiny amount of monoprop and some RCS can make close orbital manuevers for docking much, MUCH easier. If you have a tiny craft it may not be worth it, but for a larger craft the control can be exceptionally useful. 

[fourfa]

8 hours ago, THobson said:

Does the DV hit from transporting monopropellant outweigh the DV that can be obtained from using it?

Strictly speaking, the answer is a qualified 'no.'  Assuming you're making a vacuum landing (or Duna, which is essentially vacuum), it's possible to make a ship powered completely by monoprop, Puffs and RCS thrusters - even on Tylo.  Therefore the dV obtained is more than the hit of transporting it.

But the better answer is - RCS engines/thrusters have the very worst efficiency in the game, so your best option is have no monoprop or RCS whatsoever.  You should only consider having RCS if docking is an explicit part of the mission plan - and there's no reason it should be for many basic missions.  Even if docking is required, only one of the two vehicles needs to have RCS.

Try to use a single set of thrusters at the center of mass, Apollo CSM style.  Deactivate yaw/pitch/roll controls in the right-click menu to save monoprop.  Then make a point to practice docking maneuvers until you can dock with very minimal amounts of mono.  With some practice, the default amount in the crewed pods is usually enough all by itself.  The reaction wheels will keep it straight even if your CoM wanders around some through the mission.  If you're not opposed to mods - RCS Build Aid is a brilliant tool for seeing what happens to your CoM, and how the RCS thrusters are placed relative to it.

Most other players' ships I see have way, way more mono than they need, and at that point is really is just dead weight with a big dV hit.  If perhaps you're reliant on MechJeb to dock - know that it is egregiously wasteful with mono and you'd do well to learn to do it yourself if you're looking for efficiency.  

If it's a larger vehicle, have a look at the Vernier thrusters instead, and get rid of all the mono and mono tanks to save some dead mass.  At a minimum you need four - three radially at the CoM (or four for a bit more efficiency), and one pointed retrograde.  Use a blip of engine for prograde.

(And yes, it is possible to dock with zero RCS but based on the question, that's probably too advanced right now)

[Warzouz]

Don't forget that Pod-RCS internal reserve is sufficient to dock regular ships. You can easily dock a 60T ship with the MK1-2 onboard RCS reserves. and a 15T ship with a MK1 lander can RCS. Just be gentle on the RCS thrusters and don't be hasty (use timewarp intead).

Never add RCS tanks, they are useless, except on space station to refuel other ships (and you rarely need the biggest one...). Keep RCS pack to a minimum (I usually use 2 packs + 2 anywhere for a 10T ship).

If you want to dock without RCS, you'll need the target ship to be able to roll. then orient its docking ring toward your own ship. Switch back and go forward.

 

 

[icedown]

Is there a way to lock RCS to only be used on translation?

[THobson]

Okay thanks everybody.  I realise there are many ways to play this game. My vessels are not of the subtle Scott Manley type.  Mine are industrial.  

 

My 3 Man lander for example consists of a Mk1-2 pod;  1x Rockomax x200-16 tank; 4x (sometimes 8x) FL-T100 tanks; 4 LV909 engines; plus assorted other stuff, science and the like.  It weighs over 47t and is connected to its transport via Senior docking ports.  Once in orbit around a planet/moon it undocks lands then re-orbits, docks with its transport and heads for home.

 

My current mission:

On the surface of Moho I have a miner/refinery, a fully fuelled 3 man lander and a fully fuelled shuttle capable of taking significant quantities of fuel into orbit.  Also in orbit i have a fully fuelled tanker used to transfer fuel between orbiting vessels.  One cycle is:

3 man lander into orbit.

refuel from the orbitting tanker

land the 3 man lander at a new biome do lots of science and then re-orbit.

launch the fuel shuttle and use it to refuel the 3 man lander.

both then land at the refinery and refuel.

the shuttle then goes into orbit and refuels the orbiting tanker, lands and then refuels.

We are now back where we started and can repeat.

 

Once the mission is over the 3 man lander will go into orbit and dock with its transport, get back to Kerbin orbit, undock and land.

 

So I do need to do a lot of docking.  I usually do it manually but it is getting a bit boring now so I will occasionally set up MechJeb and go and do emails while it sorts it out for me.

 

I realise others play the game differently but this is what suits me.

[evileye.x]

RCS is very underrated in stock game. Because of magical REACTION WHEELS. 

To make my game somewhat more realistic and challenging, I use patch do reduce RW torgue to 2,5% of original. And of cause I use Persistant Rotation too. 

And now RCS become not just dead weight, but really important. 

[monstah]

4 hours ago, icedown said:

Is there a way to lock RCS to only be used on translation?

On the VAB, check the sliders on the RCS thruster blocks' tweakable menus.

[Pecan]

8 hours ago, icedown said:

Is there a way to lock RCS to only be used on translation?

 

3 hours ago, monstah said:

On the VAB, check the sliders on the RCS thruster blocks' tweakable menus.

 

That all seems too easy to forget.  There are separate keys for translation and rotation after all.  I've always been happy to leave RCS switched off except when docking, which is also the only time I'll need the translation keys.

RCS off.  Rotation keys.  WASDQE.
RCS on.  Translation keys. IJKLHN
Repeat.

The main point is, though, that monopropellant is only really useful for RCS and RCS is really only useful for docking anyway.

Edited June 14, 2016 by Pecan

[pincushionman]

On June 13, 2016 at 2:01 PM, THobson said:

Does the DV hit from transporting monopropellant outweigh the DV that can be obtained from using it?

I have a ship that I am going to land on a planet.  Fuel is tight.  Should I send it does with full RCS tanks and use the DV from the RCS to help get it back into orbit or is it better to send it down with empty RSC tanks?

I am happy to work it out if someone can tell me how.

Many thanks

If you're firing anything larger than a Spark  or a pair of Twitches, the increased thrust you get from the RCS systems will be darn near meaningless, and the impact to total ISP will be similarly small. The only thing you'll be accomplishing is reducing the mass of the ship, and not carrying any RCS fuel to begin with would be far more effective.

[icedown]

8 hours ago, Pecan said:

That all seems too easy to forget.  There are separate keys for translation and rotation after all.  I've always been happy to leave RCS switched off except when docking, which is also the only time I'll need the translation keys.

RCS off.  Rotation keys.  WASDQE.
RCS on.  Translation keys. IJKLHN
Repeat.

The main point is, though, that monopropellant is only really useful for RCS and RCS is really only useful for docking anyway.

Well, when trying to dock, I'd like to leave rotation to the reaction wheels and just have rcs for keeping it centered.