RCS: monoprop vs. LF+O

[NFunky]

So I've recently been messing with shuttle's, which has made me hyper aware of post-orbit delta-V. Right now I use monopropellant with RCS thrusters and two O-10s for on orbit maneuvering. This works pretty well sinceit uses a common fuel for both, but monoprop is heavy and has a low Isp. I finally realized I could use LV-1Rs and verner thrusters so I would only use LF+O.

So tell me, if this is a viable option, does it make monoprop basically obsolete? This just seems like, if it works and is more efficient, why isn't everybody doing it?

[Nich]

I would think because all command pods have mono prop. Even if you drain it your still paying for the tank space so might as well use it. Why people bring extra monoprop is beyond me

[Warzouz]

Verner might not be too easy to use with small ships. They have 12x more thrust than RCS.

Further more, there isn't any "pack" so you need a lot of verner to control your ship in all direction.

With RCS packs, you only need 2 packs and 2 inline.

Again, that depends on what you want to do with all that stuff. For one or 2 docking, you can do with only the RCS included in the command pod/can. Some docking rings even include a small RCS canister. Dedicated RCS tanks are only usefull for reserve or if you intend to do a lot of docking without refuelling.

Personnally, I use RCS on lander ships to let them dock with local space stations (I've one around Mun, Minmus, Duna and projecting one for Eve and Vall). I never needed additionnl RCS tanks. 15 units of RCS is enough to dock a 20T lander once. Then I refuel from the space station and depart again for another landing.

I don't have used Verner yet. I assume they are more an alternative to reaction wheels for big and heavy interplanetary ships which are hard to rotate.

[Nich]

Especially since multiple reaction wheels tend not to play well with stock SAS (face palm as they should be used in pairs) I always disable all reaction wheels other then the biggest one. Takes EC usage down 10 fold

[Red Iron Crown]

I use Vernors for big stuff and monoprop for smaller stuff. Nice that most pods include some "free" tankage for monoprop.

Nich, I haven't had trouble with multiple wheels (as long as the amount of torque per ton is appropriate), nor do I see why they should be used in pairs. Can you clarify?

[cantab]

For a small ship an RCS setup will offer lower part count, with 3 RCS blocks enough for complete translation and rotation (though 4 is preferred). You'd need six Vernors just to do complete translation, and even more if you need them to handle rotation as well. For a large ship though the extra thrust of the Vernors makes a difference, you'd need to spam RCS blocks to get the same performance and that undoes the part count benefits.

As far as a shuttle goes I think either can work. But you might find it easier with RCS to have downward-facing thrusters that are on the upper side of the orbiter protected during re-entry. The Vernor model makes that a bit difficult.

[Tatonf]

Monopropellant is good only for docking and turning big spacecraft if you don't like reaction wheels.

But as you said LF+O have better ISP, and therefore is always a better alternative for everything else.

Actually if you could get RCS powered by LF+O instead of monopropellant, it would make the monoprop completly useless.

[Red Iron Crown]

Actually if you could get RCS powered by LF+O instead of monopropellant, it would make the monoprop completly useless.

That is what the Vernor part does, though their Isp is the same as the monoprop thrusters.

[Tatonf]

Yes you're right, I was only thinking of the blocks.

[Nich]

Reaction wheels create torque by spinning faster and faster. After a while they will reach a speed where they will blow up. At this point the ship will be extremely stable in this plane due to gyroscopic forces. Now the other momentum wheel will spin up at a 90 degree offset. The ship will not rotate because of the gyroscopic forces from the first momentum wheel hold it stable. Now the first wheel can slow down because the second wheel is stabilizing the ship with its gyroscopic forces. The further the moment arm the two wheels have from each other the more efficient the process. A single momentum wheel with 3 dof can do this with its self however because the moment arms are 100-1000 times smaller it is 100-1000 times less efficient.

I generally run VERY large ships (30-50 tons) on the smallest reaction wheels or command pods alone and probes use the .3Nm probe core until about 5 tons. The only exception are landers where control is a must for landing. Most of my problems occur when I have two equally sized reaction wheels (5 Nm) put very close together it will drain 2-3 E/S and if I turn off 1 it will only drain .3 E/S

Edited July 24, 2015 by Nich

[Snark]

Monoprop is useful for small ships and small dV corrections. I find that I practically never use the Vernor engines-- even when I'm maneuvering 100-ton monsters, I use RCS blocks. That's because I only need to nudge them by a few centimeters per second, so the mass of monopropellant isn't significant.

I like RCS because those 4-way RCS blocks are super convenient for placement.

- - - Updated - - -

Reaction wheels create torque by spinning faster and faster. After a while they will reach a speed where they will blow up. At this point the ship will be extremely stable in this plane due to gyroscopic forces. Now the other momentum wheel will spin up at a 90 degree offset. The ship will not rotate because of the gyroscopic forces from the first momentum wheel hold it stable. Now the first wheel can slow down because the second wheel is stabilizing the ship with its gyroscopic forces.

KSP reaction wheels don't work anything at all like how real-world ones do, so it's kind of a moot point. FWIW, I think Squad totally made the right decision for playability here; making KSP reaction wheels work like real ones would have added a lot of player confusion without really helping realism enough to be worthwhile.

I assume you're referring to "real-world" reaction wheels in the above statement. That's actually not how they work. There are two differences from what you've said:

- The usual purpose is not to work by gyroscopic stabilization, but to rotate a ship without expending reaction mass. (They can be used as gyroscopic stabilizers for one axis, but then they're generally called "momentum wheels".)

- Even if they did use gyroscopic stabilization, they wouldn't work the way you describe-- specifically, having two wheels at 90 degrees wouldn't hold the ship immobile. Instead, the two of them working together would act like a single wheel at 45 degrees between the two.

In their normal "rest" state, reaction wheels aren't spinning at all. Let's say you decide "I'm pointing in this direction, when I want to be pointing in that direction that's rotated by N degrees around the X axis." In that case, you turn on your X-axis reaction wheel and start it spinning. While it's spinning, the ship rotates in the opposite direction (due to action-reaction). When you reach the desired orientation, you stop the wheel spinning, which also stops the ship at its new orientation.

Note that the above assumes that the ship has no angular momentum when the wheels are at rest (i.e. the ship isn't tumbling). If the ship does have a tumble to it, then the only way you can fix that is by expending reaction mass. (This is where KSP reaction wheels diverge from reality.) If the tumble is very slow, you could "temporarily" eliminate it by spinning up your reaction wheels in the opposite direction to compensate... but then you'd have to leave them permanently running (and drawing power) in order to maintain that state.

However, this is a "real-life rocket physics" question rather than a gameplay one, so 'nuff said here.

Edited July 24, 2015 by Snark

[Empiro]

So I've recently been messing with shuttle's, which has made me hyper aware of post-orbit delta-V. Right now I use monopropellant with RCS thrusters and two O-10s for on orbit maneuvering. This works pretty well sinceit uses a common fuel for both, but monoprop is heavy and has a low Isp. I finally realized I could use LV-1Rs and verner thrusters so I would only use LF+O.

So tell me, if this is a viable option, does it make monoprop basically obsolete? This just seems like, if it works and is more efficient, why isn't everybody doing it?

You'd need to put a bunch of ports, but it would work.

However, if you limit your RCS usage for docking, the command pod probably has enough monoprop without having to carry extra.

It comes down to whether you want to follow the real life space shuttle closely, or do the most efficient thing in KSP. In KSP, you don't need to worry about boil-off or having to re-ignite liquid fuel engines, so LFO engines are just strictly more efficient than the monoprop engines.

[NFunky]

Thanks guys. This has given me a lot of food for thought. I love these forums.

The main reason I have a whole seperate OMS is all for jumbo tanks are fully drained and jettisoned JUST below orbital velocity is reached (Kerbol 3.7). This was actually just a happy coincidence, but it's pretty perfect for a shuttle.

I wanted a low mass system I could complete orbital insertion with and use for docking. I honestly don't know if my docking skills are good enough for me to only use 15 mono to dock this thing, with the docking port dorsally mounted and off the CoM.

Anyway, no real point to make, but I thought I'd explain my reasons for asking this question a little better.