Docking with two Advanced SAS modules (how to turn on off?)

[Elsig]

Hey all,

Is it possible to turn off one SAS module on my craft? (I have 2)

Mission: Deploy a refueling station in orbit around Moho and land a spacecraft on the planet.

I wasn't able to design anything with enough Delta-V to get there in one go so my plan is to dock with my Kerbal refuelling station and then head off.

The problem I'm having is docking with the refueling station! As soon as I turn on SAS and RCS, the thrusters are constantly on.

I think the problem is that I have 2 Advanced SAS modules on my craft and they are fighting eachother. One for the spacecraft and one for the Moho Orbiting station.

Is there any way to turn one SAS off?

[JaySmoka]

As far as i know it's not possible to turn off only one SAS on a craft. When docking it's preferable to turn off SAS on both vessels to avoid the corrections.

[stupid_chris]

Having more than one ASAS module doesn't affect your craft at all. The second one is only a dead weight.

ASAS modules use what is available on the ship (command module torque, RCS, control surfaces) to steer the ship. So having a second one doesn't change anything because there's no new things to control with and the first module already does the job. and having two doesn't double the effect.

Your problem is twofold: first the ASAS modules are quite overreacting. This is known to the devs and they will eventually fix it. But essentially it has no dampening value and if it isn't pinpoint on the target heading, it will go full steering to correct it. But then it oversteers, so it goes full steering in the other direction, and then the other, and then... you see the pattern. Second, your craft is unbalanced on the RCS thrusters. My guess is that this cluster of LV-Ns is so heavy the CoM of the ship must be quite low. Your RCS thrusters aren't evenly spread compared to your CoM and it makes rotation and translation more expensive and inefficient with RCS. Combined to the ASAS, this is even worse.

Your best bet is to only tap the RCS button to do short corrections or to tap the SAS button to hold headings for short periods.

Also there is a differents between SAS modules and ASAS modules: ASAS as explained above holds a heading using the ships's control. SAS modules will try to negate any rotation of the ship when activated using it's own torque. Try using the right term when posting for help else it could confuse people

[Elsig]

Having more than one ASAS module doesn't affect your craft at all. The second one is only a dead weight.

ASAS modules use what is available on the ship (command module torque, RCS, control surfaces) to steer the ship. So having a second one doesn't change anything because there's no new things to control with and the first module already does the job. and having two doesn't double the effect.

Your problem is twofold: first the ASAS modules are quite overreacting. This is known to the devs and they will eventually fix it. But essentially it has no dampening value and if it isn't pinpoint on the target heading, it will go full steering to correct it. But then it oversteers, so it goes full steering in the other direction, and then the other, and then... you see the pattern. Second, your craft is unbalanced on the RCS thrusters. My guess is that this cluster of LV-Ns is so heavy the CoM of the ship must be quite low. Your RCS thrusters aren't evenly spread compared to your CoM and it makes rotation and translation more expensive and inefficient with RCS. Combined to the ASAS, this is even worse.

Your best bet is to only tap the RCS button to do short corrections or to tap the SAS button to hold headings for short periods.

Also there is a differents between SAS modules and ASAS modules: ASAS as explained above holds a heading using the ships's control. SAS modules will try to negate any rotation of the ship when activated using it's own torque. Try using the right term when posting for help else it could confuse people

That was a very helpful answer. Thank you!

I ended up using mechjeb to dock it. I've docked plenty of ships before without issues and I always have an ASAS module and I've used that to keep the ship steady whilst docking. It's worked really well, but the ships were a lot lighter (usually one or smaller engines). I didn't factor in the ridicolous weight of my LN-Vs.

It's all docked and refuelled now ready for the trip to Moho. The 7 LN-Vs have 3500 Delta-V and the lander around 2000. I'm guessing that's enough to get me there and land? But it probably doesn't have enough to get back to Kerbin. The brave explorers will be stuck there until a rescue mission can be done.

[stupid_chris]

That has WAY ore than enough to land and go back

Moho has a ridiculously small gravitational force for a planet, landing on it will be a breeze, espescially if you land on the engine bells, which shouldn't be too hard considering the amount of them you have Landing and going back in orbit is nearly 3000m/s and going back to kerbin with aerobrake or direct landing shouldn't be more than 1200m/s! It's only getting there that's hard!

[RoboRay]

The concept of multiple ASAS modules "fighting each other" is pure myth.

All having one (or more) ASAS modules does is change how your "SAS Mode" button (the T key) works.

The real problem (as noted above) is that when an ASAS is installed, the steering it does is all-or-nothing. Nobody told the ASAS that you can angle a control surface part of the way; it instantly goes all the way to full. Same for RCS nozzles... they fire full-force when a tiny nudge was all that was needed, which leads to the system over-correcting and oscillating back and forth, burning through all your monoprop.

Edited June 11, 2013 by RoboRay

[Elsig]

For my smaller crafts using ASAS and RCS has worked perfectly for docking. This craft is bigger and it just kept wobbling.

Anyway. I failed on my missions to Moho. I tried using the Olex calculator to time my transfers but i ran out of fuel every time... Once I reached Moho i needed another 4500 delta V to slow down! I only had 300 left in the main shuttle and 2000 in the lander so that wasn't going to work.

I ended up reverting back to a save and went to Eve instead. Now orbiting eve. I'll deploy the lander another day.

No fuel left in the main craft but lander has 2000 and a parachute so that should do nicely. Taking off again is not an option as the gravity is too high. Poor Kerbanauts.

[stupid_chris]

For my smaller crafts using ASAS and RCS has worked perfectly for docking. This craft is bigger and it just kept wobbling.

Anyway. I failed on my missions to Moho. I tried using the Olex calculator to time my transfers but i ran out of fuel every time... Once I reached Moho i needed another 4500 delta V to slow down! I only had 300 left in the main shuttle and 2000 in the lander so that wasn't going to work.

I ended up reverting back to a save and went to Eve instead. Now orbiting eve. I'll deploy the lander another day.

No fuel left in the main craft but lander has 2000 and a parachute so that should do nicely. Taking off again is not an option as the gravity is too high. Poor Kerbanauts.

Unlikely of what most delta V maps say, Moho is by far the WORST planet or moon in the entire game. It starts off normally, with a small 1200m/s burn for the transfer. But then you often need a huge 1000m/s of orbital adjustments because theres really no way to have a perfect Hohmann transfer with it's inclination and eccentricity. Then, the capture burn turns between 2000m/s and 4500m/s, depending on if you're lucky enough to catch it at it's apoapsis.

This is due to two reasons: one, Moho is insanely close to the Sun, it's twice as close as Mercury actually is if I remember correctly. This means that to approach it, you need to go very deep in the sun's gravity well, and you pick up an INCREDIBLE amount of speed going down. Second, Moho's gravitational force is pretty low for a planet, meaning it's escape velocity is also very low. There's no aerobrake possibility, meaning you have to burn up a huge amount of relative speed to get a catch around Moho.

There's the same problem with probes trying to reach Mercury. If you look up on Wikipedia, it's the only planet that they talk about the insane amount of delta V required to reach it. Mercury is actually the planet with the smallest orbital inclination, but the largest eccentricity: it's nearly 1.5 times further away from the sun at it's apoapsis than it's periapsis. The delta V required to reach Mercury is even higher than the one require to escape the sun :l

So yeah, basically Moho is hard.