RCS Problem (Pitch not aligned)

[Nayah-]

Hi everyone, I've got an issue with my last probe. RCS activate themselves although SAS is disabled. I think the problem comes from the pitch not aligned (bottom left of the screenshot). What am I doing wrong ?

Thanks

http://i.imgur.com/GnYNpNC.jpg

[Gargamel]

RCS does not need SAS enabled to be functional.  If RCS is activated, they can be fired at any time. 

I'm not familiar with most of the mods on your screen, one of those might be enabling the RCS and firing it.

You also may have tweaked your trim a bit with Ctrl-S (I think, I don't use trim very often).  Try hitting Ctrl-W to level off.  

But you should be having stability issues even with RCS turned off then (I think, as I said, I don't use trim often).  Do the reaction wheels slowly spin your ship? 

[Tex_NL]

3 minutes ago, Gargamel said:

You also may have tweaked your trim a bit with Ctrl-S (I think, I don't use trim very often).  Try hitting Ctrl-W to level off.

Better yet, hit [Alt]+[x] to remove all trim settings.

Edited July 29, 2017 by Tex_NL

[Gargamel]

3 minutes ago, Tex_NL said:

Better yet, hit [Alt]+[x] to remove all trim settings.

Well I learned something today!  Weeee!

The best way to get a correct answer on the internet is to post an incorrect one.    Or in this case, slightly inaccurate and inefficient.

[Tex_NL]

Just now, Gargamel said:

Well I learned something today!  Weeee!

The best way to get a correct answer on the internet is to post an incorrect one.    Or in this case, slightly inaccurate and inefficient.

Your answer wasn't incorrect. It would have led to a similar result eventually. I merely offered an easier and more definite solution.

[Spricigo]

3 hours ago, Nayah- said:

What am I doing wrong ?

Ok, that is intended as contructive criticism: From what I can see, assuming stock parts and solar system,  what you are doing wrong (as in I wouldn't  do this way) is using a heavy and expensive Nerv engine for this craft. A terrier(and the oxider that can be put in this tank)  will give considerable more deltaV and more TWR. Actually I suspect that a spark+fl400 combo will offer enough performance to this probe.  I didn't see a docking port anywhere so I don't see the need to RCS also, a small reaction  whell can handle the attitude control.

 

Now about the RCS: it should not activate unless the RCS mode is active. If RCS is active the thruster will only respond to a the accutation toggles enabled for that particular thruster (yaw, pitch, roll, port/stbd, dorsal/ventral and fore/aft by defoult, fore by trottle can also be enabled). So if RCS is enabled and SAS disabled only pilot imput. 

 

[Gargamel]

3 minutes ago, Spricigo said:

Ok, that is intended as contructive criticism: From what I can see, assuming stock parts and solar system,  what you are doing wrong (as in I wouldn't  do this way) is using a heavy and expensive Nerv engine for this craft. A terrier(and the oxider that can be put in this tank)  will give considerable more deltaV and more TWR. Actually I suspect that a spark+fl400 combo will offer enough performance to this probe.  I didn't see a docking port anywhere so I don't see the need to RCS also, a small reaction  whell can handle the attitude control.

 

I had the same thought, why use RCS at all if you aren't planning on docking.  And it looks like the NERV is a booster stage for an ion type probe.  He's got a full tank of Argon, and no LF left.  I think he just hasn't decoupled that stage yet.  That, or there was an issues getting the two parts to separate, but that just could be an optical illusion from the angle of the solar panels. 

[Spricigo]

31 minutes ago, Gargamel said:

 why use RCS at all if you aren't planning on docking.

Personally I had used RCS to fine tune the rendezvous of my rescue ship. But that was enterily for convenince, since just 2 thrusters (aft and fore) and a small amount of monopropelant remove the need of 180° turn and adjusts of thrust limits.

 

[Nayah-]

Thanks for your responses.

1 hour ago, Tex_NL said:

Better yet, hit [Alt]+[x] to remove all trim settings.

This fixed my issue. I don't know how I manage to add some trim to my probe.

1 hour ago, Spricigo said:

Ok, that is intended as contructive criticism: From what I can see, assuming stock parts and solar system,  what you are doing wrong (as in I wouldn't  do this way) is using a heavy and expensive Nerv engine for this craft. A terrier(and the oxider that can be put in this tank)  will give considerable more deltaV and more TWR. Actually I suspect that a spark+fl400 combo will offer enough performance to this probe.  I didn't see a docking port anywhere so I don't see the need to RCS also, a small reaction  whell can handle the attitude control.

I create this probe to be in orbit of neidon, a planet of the mod "outer planets mod" far away from kerbol. On my screen we can see the two last stages of my ship, the nerv (empty but didn't decouple yet) and a argon thruster (near future technologies mod).

 

I though the nerv was a good engine because of the high ISP in vacum. This is a bad idea ? And for the RCS, I use them to turn my ship for manoeuvre node. I can do these only with reaction whell even with the nerv (200h on KSP, never used reaction whell )?

 

Thanks

Edited July 29, 2017 by Nayah-
added a smiley

[Lelitu]

1 hour ago, Nayah- said:

I though the nerv was a good engine because of the high ISP in vacum. This is a bad idea ? And for the RCS, I use them to turn my ship for manoeuvre node. I can do these only with reaction whell even with the nerv (200h on KSP, never used reaction whell )?

For very light probes, the NERV is a bad idea, because it's very, very heavy. 3 tons for just the engine. For comparison, the LV-909 is a chemical engine delivering the same thrust for 0.5 tons. That 2.5 ton mass will completely outweigh the extra 455 seconds ISP when it's a large fraction of the total craft mass. 

For this design, the NERV works out even worse, since the FTL-800 carries LF and Oxidizer. NERV engines don't use Oxidizer, so the Tank is only half it's usual propellant volume. From the screenshot, you've also left the Oxidizer in the tank.That's another ~2 tons dead mass sucking away at your delta v. 

The reason the engine mass is such a problem is that with the lighter LV-909 you can carry an extra FLT-400 for the same craft total mass, but more than 2 tons less dry mass.

Assuming a 10 ton vessel fully fuelled, with your current design, or a 909 and an extra tank, you get from the Tsiolkovsky rocket equation, isp * G * ln(Mfull/Mempty) = deltav 

current - 800 * 9.81 * ln(10/8) = 1751m/s

with ox stripped - 800 * 9.81 * ln(8/6) =  2257m/s (not as bad as it looks, since you'll get more out of all earlier stages, from the 2 ton less wet mass.

with lv-909 - 345 * 9.81 * ln(10/4) =  3101m/s nearly twice the deltav, for the same mass to orbit.

with mk1 lf fuselage - 800 * 9.81 * ln(10/6)  = 4008m/s best of all options for a 10 ton vessel. 

If the estimates of the dry mass are high, it'll tip in absolute favour of the lv-909 fast. 

Edited July 30, 2017 by Lelitu

[Spricigo]

Was about to reply, and @Lelitu just made it easy. Exactly what he said.

My only nitpick is that instead of for more deltaV for the same mass we are more often aiming for the same deltaV for less mass.

Edit: about Reaction Whells: stock reaction whell use Electric charge to produce torque. It dont care about what else there is in the craft and not even for real life laws of physics.

Also, when build a ship look what KER says about deltaV and TWR

 

 

 

Edited July 30, 2017 by Spricigo

[Lelitu]

13 minutes ago, Spricigo said:

Was about to reply, and @Lelitu just made it easy. Exactly what he said.

My only nitpick is that instead of for more deltaV for the same mass we are more often aiming for the same deltaV for less mass.

Edit: about Reaction Whells: stock reaction whell use Electric charge to produce torque. It dont care about what else there is in the craft and not even for real life laws of physics.

Also, when build a ship look what KER says about deltaV and TWR

that prompted me to do the back of the envelope math for just swapping out the NERVA for an LV-909

345 * 9.81 * ln(7.5/3.5) = 2579m/s, and the lightest total stage mass..  it's saved 2.5 tons total stage weight, That's going to appear as deltav gains all through the booster stack used to get that there.. might even save enough mass to drop 2 tons of fuel and use a FLT-400 tank, or just drop that stage entirely and use a slightly bigger ion stage, for the same total performance.

KER makes doing the math for all this much easier, since it does the math for you.