On Monopropellant...

[Homusubi]

I'm a bit confused as to the distinction between monopropellant and RCS. Do Vernor engines count as RCS, even though they use normal fuel? Do non-RCS monopropellant engines count as "RCS" for the purposes of e.g. placing fuel tanks, i.e can you place fuel tanks for non-RCS monopropellant engines anywhere?

Also, is monopropellant more or less efficient than normal fuel? Would it be a good idea to build a ship exclusively using monopropellant tanks instead of normal ones?

[Vanamonde]

Within the game, "monopropellant" and "RCS" are different names for the same parts. Whether or not the efficiency ratings of the two propulsion systems differ is less of a consideration than the fact that RCS engines and tanks are very small, making them useful for probes regardless of their relative efficiency. They are far too weak for all but the tiniest ships, though.

[wibou]

Yes, Vernor engine count as RCS, although they are only going 1 direction (so you would need at least 6 to cover all translation direction).

And RCS engines really are LESS efficient than liquid fuel engine.

If you look at their specific impulse (or Isp, simply "efficiency of an engine"), you will notice it is way less than most liquid fuel engine and way WAY less than ION engines or LV-N nuclear engine. As such, powering a vehicle, even a small one, with only RCS would be less efficient than powering it using the same amount of liquid fuel or Xenon gas.

The wiki might help here:

http://wiki.kerbalspaceprogram.com/wiki/RV-105_RCS_Thruster_Block

http://wiki.kerbalspaceprogram.com/wiki/Place-Anywhere_7_Linear_RCS_Port

http://wiki.kerbalspaceprogram.com/wiki/Vernor_Engine

[SanderB]

As a further measure of efficiency, the mass ratio of the fuel to its tank is also a consideration, and in the case of monopropollant the mass ratio is usually lower than for liquid fuel. This is however more of an issue if you want to use mono prop as your main fuel...

Edited January 31, 2015 by SanderB

[GoSlash27]

RCS is the system that translates and points the craft through the timed firing of thrusters.

Monopropellant is the reaction mass that is ejected by most RCS systems.

Vernor engines would count as RCS subsystems because they are actuated and controlled by the RCS system.

Mono engines (O-10) would not be considered RCS, since they are operated through the primary flight controls.

Best,

-Slashy

[Technical Ben]

Within the game, "monopropellant" and "RCS" are different names for the same parts. Whether or not the efficiency ratings of the two propulsion systems differ is less of a consideration than the fact that RCS engines and tanks are very small, making them useful for probes regardless of their relative efficiency. They are far too weak for all but the tiniest ships, though.

One is the fuel, one is the engine/control mechanism. "RCS" is generally any thruster which controls rotation/translation etc (are those terms correct?) and "monopropellant" is a fuel, mainly used for these engines, but also at times used for fixed rockets or other use.

[Invader Myk]

Monopropellant is a fuel (irl it is just compressed air). A RCS or Reaction Control System uses small bursts of decompressing gasses (usually compressed air) to control a ship's attitude and, for fine adjustments, velocity. You may also see RCT or Reaction Control Thruster, which is a component of a RCS. The Vernor engine is a RCT that uses liquid fuel and oxidizer to power a ships RCS. The O-10 Monopropellant Engine is a main thrust engine that uses monopropellant and is not part of a ships RCS.

The efficiency of liquid fuel and monopropellant should not be compared. This is because each is used for very different functions. Monopropellant is more efficient for small manoeuvres, but liquid and solid fuels are more efficient for high power thrusts.

[FlyingPete]

Monopropellant engines (the O-10 engine) respond to normal throttle controls but consume monopropellant. They also don't care about fuel tank placement- they will draw from any onboard supply with no need for connecting fuel lines. The vernor engine is essentially the opposite. AFAIK they need to be connected to a tank with fuel in it (or a fuel line connecting to a tank with fuel)

The actual usefulness of the monoprop engine is fairly restricted. For general rocketry, it's almost always more efficient to use liquid fuel due to the higher ISP. I've found a couple of exceptions though.

-Orbital SSTO aircraft: Here you're using jet engines most of the way, then using another propulsion system to nudge the plane into orbit. Here it can be convenient to use monopropellant, since then you can use a single fuel type for both orbital maneuvers and docking. Simplifies the craft and makes it smaller.

-Small landers (for Minmus etc): Here the monoprop capacity in the command pods comes in useful, tipping the balance slightly in favour of monopropellant in some cases. Particularly as the 24-77 engines which you'd typically use on a small landing vehicle are not that efficient.

-Space station emergency escape pods: Same idea here- all you need is a command pod, docking port and a couple of engines. You can use it for a launch abort system too- assign the action group to shut down all engines on the lifter, separate the command pod and fire the monoprop engines attached to it.

-Orbital assembly tugs: The sort of 'tractor' with lots of RCS thrust which you use to move space station parts etc around. Having a couple of main engines can be useful and is slightly more efficient than the RCS quads when used for thrust.

[Ten Key]

Monopropellant is not compressed air. It is usually a chemical that reacts violently when forced through a catalyst. Most modern probes and spacecraft use hydrazine.

[FlyingPete]

Monopropellant is not compressed air.

In practice, no. Though there's nothing theoretically stopping you using compressed air. I like the idea of an electric or nuclear steam rocket. Such a space-kettle would technically be a monopropellant thruster. I guess the ISP would depend on the pressure you could generate.

[Wanderfound]

The actual usefulness of the monoprop engine is fairly restricted. For general rocketry, it's almost always more efficient to use liquid fuel due to the higher ISP. I've found a couple of exceptions though.

The O-10 monoprop engines are also very good for small probes, to the point of being rather dramatically overpowered. You can go interplanetary off less than a single FL-R10 tank.

Edited February 1, 2015 by Wanderfound

[Homusubi]

I think that's answered it, thanks everyone. I might post a challenge based on monopropellant-only rockets later.

[OhioBob]

Monopropellant is not compressed air. It is usually a chemical that reacts violently when forced through a catalyst. Most modern probes and spacecraft use hydrazine.

While what you say is entirely true, compressed gas is sometimes used for RCS. It is usually just referred to as "cold gas". The decomposition of monopropellant is exothermic, meaning that it produces heat. Cold gas is used in applications where the hot gases expelled from a monopropellant thruster could be damaging or dangerous. An obvious example is a manned maneuvering unit. In KSP the Kerbals' EVA packs are said to contain monopropellant, but it's likely that in real life they would use cold gas. Cold gas is far less efficient than monopropellant but it's much safer. The most common and most efficient monopropellant used today is hydrazine, though many early spacecraft used hydrogen peroxide (such as Mercury). In those early days of the space program, engineers hadn't yet found a catalyst that worked with hydrazine without being destroyed during operation. Based on the ISP of monopropellant thrusters in KSP (260 s) it's probably assumed hydrazine.

- - - Updated - - -

The O-10 monoprop engines are also very good for small probes, to the point of being rather dramatically overpowered. You can go interplanetary off less than a single FL-R10 tank.

In an application like you show, I often just use point-anywhere RCS ports and angle them aft-ward. They're cheap, massless, and provide plenty of thrust on a small probe. There are three main disadvantages: (1) RCS ports have lower ISP than the O-10 engine [260 vs. 290], (2) you have to hold the H-key down throughout a burn, and (3) you have to calculate the ÃŽâ€V by hand because KER doesn't compute it for RCS.

Edited February 1, 2015 by OhioBob

[Ten Key]

I stand corrected.

For what it's worth, the warning diamond on the side of the Mark 1-2 command pod is accurate for hydrazine. I thought that was a nice touch.

[MircoMars]

In an application like you show, I often just use point-anywhere RCS ports and angle them aft-ward. They're cheap, massless, and provide plenty of thrust on a small probe. There are three main disadvantages: (1) RCS ports have lower ISP than the O-10 engine [260 vs. 290], (2) you have to hold the H-key down throughout a burn, and (3) you have to calculate the ÃŽâ€V by hand because KER doesn't compute it for RCS.

this is actually my plan-C: grab science data, decouple commandpod, enable RSC and return home.

MechJeb calculates RCS deltaV and also somehow predicts time of RCS-burns. holding the h key is a pain but alt-. timewarp helps and i can imagine a simple mechanical "finger" to keep the key pushed (I remember times when I ducktaped my playstation controller for the 100 rounds on a superspeedway...

[Renegrade]

An obvious example is a manned maneuvering unit. In KSP the Kerbals' EVA packs are said to contain monopropellant, but it's likely that in real life they would use cold gas. Cold gas is far less efficient than monopropellant but it's much safer.

Yeah, the Shuttle MMU uses compressed nitrogen as a propellant. The user's guide states that the specific impulse is approximately 60 (a big hit). Definitely safer than some hot-gas hydrazine-based deathtrap... However, I don't see any reason why Kerbals would restrict themselves to safe EVA~

Note though that RCS systems aren't always monopropellant systems - the Apollo systems were generally hydrazine/NTO bipropellant. (R-4D engines, 0.44kn, 312s, MMH/NTO)

[OhioBob]

MechJeb calculates RCS deltaV and also somehow predicts time of RCS-burns.

I haven't used MechJeb so I wasn't aware that it does that. Of course computing the ÃŽâ€V of an RCS system is pretty simple to do by hand.

However, I don't see any reason why Kerbals would restrict themselves to safe EVA~

Good point. Kerbals do seem to be less concerned with safety than us humans.

Note though that RCS systems aren't always monopropellant systems - the Apollo systems were generally hydrazine/NTO bipropellant. (R-4D engines, 0.44kn, 312s, MMH/NTO)

The Space Shuttle also used bipropellant RCS. I suppose that for large spacecraft like Apollo and the Shuttle, the extra ISP of bipropellant makes it worth the added complexity. Of course these systems used pressure-fed hypergolic propellants, so they were still relatively simple in design. I recall that some of the early plans for Orion called for oxygen/methane RCS, though this was switched to hypergols. The first thing I thought of when I heard that was that it would require some type of igniter. I'm not quite sure how that would have worked for something requiring frequent short pulses like RCS. Presumably the Vernor engine in KSP also requires an igniter.

[numerobis]

In an application like you show, I often just use point-anywhere RCS ports and angle them aft-ward. They're cheap, massless, and provide plenty of thrust on a small probe. There are three main disadvantages: (1) RCS ports have lower ISP than the O-10 engine [260 vs. 290], (2) you have to hold the H-key down throughout a burn, and (3) you have to calculate the ÃŽâ€V by hand because KER doesn't compute it for RCS.

FYI, the O-10 is massless, and similarly the vernor. In the cfg of each of those two parts, the line

PhysicsSignificance = 1

means the part has no mass.

[Renegrade]

Good point. Kerbals do seem to be less concerned with safety than us humans.

As a bit of an off-topic aside, I rather admire their space-crazyiness. They give the impression of having this "space at all cost" exploration drive....

The Space Shuttle also used bipropellant RCS. I suppose that for large spacecraft like Apollo and the Shuttle, the extra ISP of bipropellant makes it worth the added complexity. Of course these systems used pressure-fed hypergolic propellants, so they were still relatively simple in design.

OH yeah, that's right, the Space Shuttle had it too. And yeah, the extra specific impulse would help with such heavy craft, whereas a little probe might have more delta-v with a lighter RCS system (similar to LV-909 vs 48-7S).

I recall that some of the early plans for Orion called for oxygen/methane RCS, though this was switched to hypergols. The first thing I thought of when I heard that was that it would require some type of igniter. I'm not quite sure how that would have worked for something requiring frequent short pulses like RCS. Presumably the Vernor engine in KSP also requires an igniter.

Yeah.. Igniter, and LOX is cryogenic... that sounds a bit crazy. I have a datasheet for some Moog-ISP monoprop engines, and the smallest one, the 1N MONARC-1, is rated for 375,000 pulses - an igniter-based system would have to have incredible endurance and speed...

[OhioBob]

FYI, the O-10 is massless, and similarly the vernor. In the cfg of each of those two parts, the line

PhysicsSignificance = 1

means the part has no mass.

True, but I consider that to be an error. The O-10 is the only stock engine that is massless. It is equivalent to the Rockomax 24-77 in size and thrust, yet the 24-77 has mass like all other engines. I always edit the cfg file to change PhysicsSignificance = 0. I do the same thing for the TR-38-D stack decoupler and the FL-A5 adapter. It makes no sense that those parts should be massless; it goes against the apparent intent. I wish Squad would change them. In the meantime there's no reason you can't exploit it.

Edited February 1, 2015 by OhioBob

[numerobis]

Personally I find all massless parts to be a bit dodgy. I can construct huge scaffoldings with massless cubes.

[OhioBob]

Personally I find all massless parts to be a bit dodgy. I can construct huge scaffoldings with massless cubes.

I agree. At some point I'll likely remove the 'PhysicsSignificance = 1' on all the parts, along with changing/rebalancing some of the part masses. I'm just waiting for Squad to complete their rebalancing before I consider tweaking it further.