Meanwhile at KSC...

[Enture]

Hey!

So this morning I decided to go back to work on my MunScout family of probes: various unmanned landers, used to search interesting landing sites on the Mun and Minmus.

Now the MunScout-Alpha was first conceived as a serie of minimalistic impactor probes (much like the Ranger Program), just fitted with cameras and no fuel at all. This morning I was working on redesign it in a very lightweight unmanned lander, and with that in mind, decided to use RCS as the probe's sole propulsion system during the descent. I was wondering if anybody ever calculated the specific impulse of the RCS block and linear thrusters, and since I didn't find anything about it, I decided to see by myself what RCS thrusters were exactly capable of.

So I fitted my 800kg lander with one single RCS tank and 4 linear thrusters, and went to the launchpad...

5809km max altitude! Is this some kind of bug exploit, or just due to the low mass of the spacecraft? Some experienced players have an opinion on this? And does anybody have values for the RCS thrusters' specific impulse?

[Grand Lander]

What in kerbal! never seen that before! wow i dont know if its a bug or what but thats epic. LOL i guess now you dont need to worrie about the amount of fuel to land with RCS LOL.

I think its because it doesnt weight much but i make small landers to test but they dont even leave the atmosphere. It could also be cause you put the RCS under the rocket as i can see? Pretty cool though.

You can also use dev tool to have unlimited fuel but i dont encourage its use.

Edited September 15, 2012 by Grand Lander

[swiftgates24]

It's a bug that makes craft's mass go down when it's just an RSC tank because it has such low mass already, and the RSC ports are way OP anyway. This is all I could find about them that you might want.

Linear RSC:

// --- standard part parameters ---

mass = 0.05

dragModelType = default

maximum_drag = 0.01

minimum_drag = 0.01

angularDrag = 0.1

crashTolerance = 50

breakingForce = 50

breakingTorque = 50

maxTemp = 3400

fuelCrossFeed = True

// --- rcs module parameters ---

fuelConsumption = 0.6

RSC Tank:

// --- standard part parameters ---

mass = 0.5

dragModelType = default

maximum_drag = 0.2

minimum_drag = 0.2

angularDrag = 2

crashTolerance = 12

maxTemp = 2900

// --- rcs fuel tank parameters ---

fuel = 200

dryMass = 0.1

fullExplosionPotential = 0.7

emptyExplosionPotential = 0.1

As you can see they both have very low drag which is also causing it to go very fast. Add 2 more RSC tanks 12 more thrusters and your in solar orbit.

Edited September 15, 2012 by swiftgates24

[Enture]

So the game is making the craft's mass go down faster than it should, while burning its fuel, because there's only a RCS tank involved, and nothing else?

I'll try adding some parts to the spacecraft, and see what happens; thanks for those first infos!

Oh, and thanks also for the raw data, I'll see what I can make with them

[Nibb31]

The linear RCS thrusters are totally imbalanced.

[swiftgates24]

Here is a lander that is a lot less cheaty. Using all it's fuel on Kerbin can only get on Apoapsis of 13km. Can't take off while the gear is down. Enjoy!

[Enture]

The linear RCS thrusters are totally imbalanced.

Yep, I investigated further, and the same lander I showed before fitted with block RCS can't takeoff. So I guess I'll forget about using linear RCS for now...

@Swiftgate: On my way to check this out!

[swiftgates24]

It proves if your a good enough pilot with the lander:

[Enture]

Swiftgate, your lander is an interesting one, but too heavy for the background I had in mind (although nice landing site! one of the far side's canyons, I guess?)... So I sticked with my original design, and opted for RCS blocks rather than linear ports, to avoid the bug-thing... Piloted carefully, a 37t-launcher is sufficient to bring that little scout in an equatorial munar orbit, and down to the chosen target.

•MuSA-2 (MunScout Alpha-2) touched down safely at the planned target: , that little crater exactly half-way between the two enormous ones from the near side (in my RP-ish background, those big craters are Tsiolkovsky and Newton, and the landing site one is Curie):

•First picture retransmitted from the surface of Curie crater:

•TMI burn:

[swiftgates24]

Nice You landed! I have a name for big craters and canyons and the one I landed in was Huldge Canyon.

[EndlessWaves]

As you can see they both have very low drag which is also causing it to go very fast. Add 2 more RSC tanks 12 more thrusters and your in solar orbit.

All fuel tanks have 0.2 drag.

It does seem to be some sort of bug

The wiki and the part files agree that the Linear RCS has 3 thrust and 0.6 fuel consumption (RCS fuel units per second). 20 of them would be 60 thrust per ton.

The rocket thrusters have the following values for thrust/ton

LV-909: 33.75, 40

Mark 55 radial: 53.33

LV-T45: 133

LV-T30: 171, 172

Aerospike: 250

Clearly something not right here, four RCS thrusters with 12 thrust on your 0.7 ton machine should produce comparable results to an large LV-909 and 600L of fuel (135 thrust, 7.6 tons). Instead they're far faster. It's not drag, the thruster machine actually has more at 0.156 drag (77 thrust per unit drag) compared to the rocket with 1.528 (88 thrust per unit drag).

A quick test shows 8 linear RCS thrusters start to hover with around 2.2-2.4 tons which would suggest a thrust of about 3 as well (gravity/thrust). I did notice around 2.1 tons they 'jump', going off the pad a little and then dropping back down which a rocket or jet engine never does.

4 thrusters at 0.6 units each should burn through 200 units of fuel in 83 seconds. I tested your machine and it took ~1:25 to burn through the fuel so that's about right.

Efficiency:

A Mark 55 (280 ISP) uses 5.825L per second at 1atm with 200L per ton

Thrusters should use 0.6 units per second with 500 units per ton

weight of fuel per unit thrust that is:

Mark 55: 0.0728L per unit thrust = 0.364Kg per second per unit thrust

Thrusters: 0.2 per unit thrust = 0.4kg per second per unit thrust

(400 isp is 0.255kg per second per unit thrust)

RCS tanks are only 80% fuel by mass rather than the 89% fuel that the rocket tanks are, so you would have more dead weight there.

It definitely seems to be a scaling issue of some sort, the thrust/acceleration going up faster than the power to weight ratio.

[swiftgates24]

Yeah, didn't think to look at the wiki. That's a lot of thrust though for that little guy...

[Enture]

Thanks EndlessWaves for having taken the time to dig deeper in the problem! I'll take the time to read your math in details soon