Do I need to toggle RCS for Vernors?

[MaxwellsDemon]

Do I need to toggle RCS on at launch to enable my Vernors?  And if so, what's to stop the monoprop RCS farther up the stack from firing if I don't want them to?

[Cpt Kerbalkrunch]

3 minutes ago, MaxwellsDemon said:

Do I need to toggle RCS on at launch to enable my Vernors?  And if so, what's to stop the monoprop RCS farther up the stack from firing if I don't want them to?

Yes (if you want to use them at launch) and nothing. If you want to separate the use of thrusters and Vernors, just set them to action groups. This way, you can turn them on and off at will. Otherwise, you'll need to disable and re-enable them by hand. Which takes awhile.

Edited February 1, 2018 by Cpt Kerbalkrunch

[MaxwellsDemon]

hm.   Is it possible to enable the Vernors by action group without toggling RCS?  Or do I need to add a "suppressive" action group for all other RCS systems?

[Cpt Kerbalkrunch]

They turn on together, so you'd have to action group them separately. By doing so, you could actually enable RCS before launch and then just kill them both with their action group numbers, and turn on the one you need when you're ready. You could keep RCS enabled all the time.

Edited February 1, 2018 by Cpt Kerbalkrunch

[Nicias]

You could also turn off the RCS monoprop *tanks*.

[MaxwellsDemon]

2 minutes ago, Nicias said:

You could also turn off the RCS monoprop *tanks*.

I like!   I will give that a try.   

[Spricigo]

The RCS toggle is the master switch for the system, but you can still activate/deactivate individual thrusters.

If you enable advanced tweakables in the setting you can make so the thruster respond to only some of the control inputs . (e.g I set mine to not respond to yaw, pitch and roll commands)

 

Also, using both regular and vernor RCS forfeit the possible advantages of both alternatives. Regular have the advantage of lighter trhusters (0.3t linear, 0,5 block vs .8t vernor) while vernor have better TWR and better tank mass fraction.

[MaxwellsDemon]

I put the vernors on a booster to help steer it on the way to orbit-- not intending to use them at the same time as the monoprop RCS on the payload (in fact, trying to avoid that scenario).

[Guest]

Yep advanced tweakables is key.

I use a pair of vernors on the (long) tail of my heavy SSTO lifter, to counteract thrust torque. They're keyed to an action group and set to react to pitch control only. A bit brute-force but it works.

[Aegolius13]

Also, you can also set Vernors (and other RCS thrusters) to respond to throttle via a tweakable in the VAB.  You do have to have RCS enabled to do this, but at least it will avoid firing thrusters where you have not enabled this option.  

But i believe this only works to make you go forward (relative to your control point, of course).  Does not help if you want to translate in other directions, or rotate. 

As noted above, action group toggles sound like the easiest way to get what you want.  

[Foxster]

Are you sure you need RCS at all?

I never ever use it except for docking and I've put a lot of craft in space. 

[linuxgurugamer]

9 minutes ago, Foxster said:

Are you sure you need RCS at all?

I never ever use it except for docking and I've put a lot of craft in space. 

Try playing withoutt reaction wheels.   Use the mod MandatoryRCS and have some fun

[Foxster]

1 minute ago, linuxgurugamer said:

Try playing withoutt reaction wheels.   Use the mod MandatoryRCS and have some fun

No thanks. Very happy without

 

[Spricigo]

9 hours ago, MaxwellsDemon said:

I put the vernors on a booster to help steer it on the way to orbit

 

Well, whatever works for you. My preference is for aerodynamic stable rocket that go to the orbit without the need of active steering. Fire and forget.

[foamyesque]

2 hours ago, Foxster said:

Are you sure you need RCS at all?

I never ever use it except for docking and I've put a lot of craft in space. 

 

I've found them useful in a few scenarios other than docking: Orbit trimming (where even turned way down the main engines aren't precise enough and/or would require rotating the whole craft a bunch), landing on small bodies (e.g. Minmus, where they can effectively be your main engine and/or allow different landing configurations that a main-engine/RW only combo would not), soaking thrust offsets in certain asymmetrical craft, and -- and this is actually my primary use outside of docking! -- control during a cobra reentry.

Vernor RCS in particular are the lightest way to manouver really big things, because you don't need a dedicated tank for them and they have much more control authority for their mass than reaction wheels manage.

 

10 hours ago, MaxwellsDemon said:

I put the vernors on a booster to help steer it on the way to orbit-- not intending to use them at the same time as the monoprop RCS on the payload (in fact, trying to avoid that scenario).

 

If that's the scenario, just disabling the tanks on the payload should give you the result you want. Be thematically appropriate, too.

[Geschosskopf]

4 hours ago, Spricigo said:

Well, whatever works for you. My preference is for aerodynamic stable rocket that go to the orbit without the need of active steering. Fire and forget.

It depends on how you define "aerodynamically stable".  Stability comes in many flavors, depending on both the situation and the language of the technical discipline you're speaking in.  You can make a rocket that is so "stable" that it will stay pointed the way you last turned it regardless of the changes in the external forces of aerodynamics and gravity. in which case you have to apply control forces to steer it into the gravity turn (except it's not really a "gravity turn" this way because control forces, rather than gravity, are doing it).  OTOH, you can make a rocket that so lacks stability (however you define that) that it wants to swap ends during ascent.  The "fire and forget" type of rocket stability is somewhere in between these extremes, where the aerodynamic forces on the rocket don't exceed the influence of gravity, so that the rocket gradually topples over in manner carefully choreographed with changes in velocity and altitude. 

This is a beautiful thing to achieve.  But it's quite common to go to far at avoiding instability that you require active control to make the turn.  In that case, Vernors are arguably the best option, as long as you disable any mono tanks further up the rocket that you want to save for later.

 

Edited February 2, 2018 by Geschosskopf

[Spricigo]

5 minutes ago, Geschosskopf said:

It depends on how you define "aerodynamically stable". 

no, its not a personal definition. This is a technical term used by physicists.

aero: relative to air

dynamic: relative to  motion under forces

stable: not likely to change.

 

aero.dynamic stable= not likely to change relative to the motion of the air.

or in colloquial  kerbalese: It stay pointed to prograde.

21 minutes ago, Geschosskopf said:

 You can make a rocket that is so "stable" that it will stay pointed the way you last turned it regardless of the changes in the external forces of aerodynamics and gravity.

F=m.a    and   =I.α    Stability comes from balance of forces, when we sum up forces(/torques) from all the sources it results in 0 for a given point of equilibrium an the rocket don't suffer change in it's state of movement there.  Your statement is akin to "2-x=0, regardless of how much is x" it don't make physical sense.

 

1 hour ago, Geschosskopf said:

in which case you have to apply control forces to steer it into the gravity turn (except it's not really a "gravity turn" this way because control forces, rather than gravity, are doing it).

Gravity acts in the center of mass. A force acting at the rotation axis causes no torque. So, unless you can define a  rotation axis that don't pass through the CoM, you can't say the gravity is turning the rocket.

 

2 hours ago, Geschosskopf said:

The "fire and forget" type of rocket stability is somewhere in between these extremes, where the aerodynamic forces on the rocket don't exceed the influence of gravity, so that the rocket gradually topples over in manner carefully choreographed with changes in velocity and altitude. 

That is a classic misinterpretation of forces and movement. You think about gravity as a motive force that 'wants to' cause movement but is stoped by a resistive force of the air, and then conclude that only when the motive force is stronger than the resistive force movement will occurs.

It don't work that way, there is no difference between motive and resistive forces. And the movment will change in the direction of the resulting force. Assuming an object is affected by only aero eand gravity forces,  if aero are stronger than gravity the object will accelerate in the direction of aeroforces.

 

That is how we explain a aerodynamic stable rocket maintening it's orientation, as the rocket moves (dynamic) through  the air (aero), the air forces the rocket to stay in a stable (unlike to change) position. Unlike gravity, aeroforces are not constant, It changes depending on how far appart the rocket is from the stable position. A movement in one direction will cause a change in movement in the oposite direction.  BTW you don't need to take my word for that, fire up KSP.

 

 

2 hours ago, Geschosskopf said:

This is a beautiful thing to achieve.

Then you realise how easy is to do it, and that all the trouble you had before was because the silly idea that you need 'control' the make the craft follow a better trajectory.

Of course, you need well designed craft to do it. Example:

Spoiler

 

3 hours ago, Geschosskopf said:

....  But it's quite common to go to far at avoiding instability that you require active control to make the turn.  In that case, Vernors are arguably the best option, as long as you disable any mono tanks further up the rocket that you want to save for later.

Quite the contrary. Every time I see a lifter using RCS or Vernor  is because otherwise it will flip. 

 

 

 

[linuxgurugamer]

3 hours ago, Spricigo said:

Quite the contrary. Every time I see a lifter using RCS or Vernor  is because otherwise it will flip. 

I never use RCS or Vernor for lifters, except when they are in space.  I either use the thrust vectoring on the engine, or the fins which would be on it.  And, since I play with Mandatory RCS, I don't use reaction wheels either.

I've never gotten a true fire'n'forget rocket.  It really isn't worth my time, although I applaud those people who can do it

[MaxwellsDemon]

You know, talking about whether I ought to be using Vernors or not is really beside the point.  

[Rocket In My Pocket]

6 minutes ago, MaxwellsDemon said:

You know, talking about whether I ought to be using Vernors or not is really beside the point.  

What I do with Vernors typically is unbind them from the RCS Action Group and assign them to something else, so you can toggle the two independently from each other.

Just select the "RCS group" in the Action Group menu and click the Vernors to take them off that list. Then add them to a different one.

Edited February 2, 2018 by Rocket In My Pocket

[Geschosskopf]

6 hours ago, Spricigo said:

no, its not a personal definition....

Yes, it's your personal choice of which one of, I dunno, maybe 30 different technical definitions of the word "stability" to use, whether or not you hang the qualifier "aerodynamic" in front of it or not.  The way you're using the word "stable" is in the sense of "positive stability" in the reference frame of the rocket with respect to the airflow.  In this case, the rocket weathervanes, so that if perturbed to have a non-zero AoA, it will return to having a zero AoA all by itself. 

However, this way of defining "stability" does not take into account what's going on with gravity, it's just looking at the airflow going by the rocket.  An observer on the ground sees the rocket toppling over from vertical to horizontal due to gravity working on the lever arm between the rocket's CoM and aerodynamic center, even though the rocket's body is always parallel to the local airflow.  To this observer, the rocket does not have "positive stability" with respect to the ground.  It instead has "neutral" or "marginal stability", in that external forces cause changes of orientation that are not self-corrected.  If the external force was to vanish, the rocket would stay oriented where that force left it.

Stability with respect to the airflow and to the ground are both important when designing flying machines.  But they do not mean the same thing and having one does not necessarily mean having the other at the same time.  And it is not necessarily trivial to achieve "fire and forget".  If you pitch over too much and/or too soon for the thrust you have, then gravity works too much for too long and the rocket, despite being totally "stable" as you define it, will reach a horizontal orientation prior to escaping the atmosphere, and then come back down.  Or you might have the opposite problem and leave the atmosphere still with a significant nose-up attitude with the respect to the ground.  That you find it easy to pull off "fire and forget" is quite admirable.  Salute!

 

6 hours ago, Spricigo said:

  Reveal hidden contents

 

Quite the contrary. Every time I see a lifter using RCS or Vernor  is because otherwise it will flip. 

Hmmm.  Do you launch with SAS (or some other orientation-controlling thing like MJ) turned on, or not?

If you have some orientation-controlling thing active, it will use the means at its disposal (torque, control surfaces, gimbal, RCS, Vernors) to keep the rocket pointed in the direction it thinks you want.  All such systems have a PID controller algorithm that decides when and how much to use the available orientation control mechanisms.  Because players can build rockets of countless shapes and sizes, these algorithms are not always tuned to a specific rocket's characteristics.  And the authority of the various control mechanisms is not always tuned to the rest of the rocket, either.  Thus, it can easily happen that the rocket might weathervane and settle down if left to itself, but the algorithm keeps triggering and over-controlling slightly, essentially fighting itself, so that you get a constant, small oscillation with Vernors firing (or other controls like gimbal activating) alternately all the way up.  This really has nothing to do with whether the rocket itself is "stable" however you define that, but with the limitations of the PID controller used.

[Cpt Kerbalkrunch]

Being a KSP fanatic seems to make Google think I'm a space nut, which isn't really true (of course, they also think I love Emily Browning, which is absolutely true ). They send me every space related story out there. All are interesting, but most I don't bother clicking on for time reasons. However, yesterday I read the story of the Japanese probe to Venus. The pics were spectacular (as a fan of Eve, I'm very interested in its real life inspiration; though I know they're vastly different), but the story was better. In the States, we don't pay as much attention to the rest of the world, so I had never heard this one. Evidently, the engine crapped out during the capture burn at Venus in 2010 and the probe hit solar orbit. In spectacular fashion, they salvaged the mission by using RCS to finally capture back in 2015. I loved the story because it reminds me of the early days of my space program (though not nearly on the same scale, obviously ). I used to put RCS on everything (whether it needed it or not), and several times it saved me when I ran out of fuel and used the monopropellent to get back home. The irony, of course, being that if I had just brought more fuel instead of RCS I wouldn't have run out in the first place. Still brings a smile to think about it, though.

For @MaxwellsDemon, these threads will often go a bit beyond the scope of the OP. With a game like this, there's technically no "right" way to play, and not really a "best way" to do anything. Of course, "technically" and "not really" leave plenty of room for interpretation, discussion, and (sometimes) argument. It's a community after all, and each of us loves the game and have our own ideas on how to play. I don't think anyone was knocking your play-style for using Vernors. We just like to discuss these things and give our opinions. When your stuck at work or school and can't play, talking about the game is the next best thing. 

[MaxwellsDemon]

1 minute ago, Cpt Kerbalkrunch said:

We just like to discuss these things and give our opinions. When your stuck at work or school and can't play, talking about the game is the next best thing. 

Stipulated.  Especially since I'm doing that right now (don't tell the boss)...   

[Spricigo]

3 hours ago, linuxgurugamer said:

I've never gotten a true fire'n'forget rocket.  It really isn't worth my time, although I applaud those people who can do it

TBH doing a gravity turn in a true Fire'n'Forget manner is quite rare, and not really worth anyone's time except for the challenge/cool factor. But IMHO that is reason enough to try to do it al least once.

The effect may be small, but gimbals, active fins, reaction wheels,  RCS still help to "cut the corners" and make the path a bit closer to the ideal. And those are things that you often already included in the design because of "reason" , so we use it if we can. (e.g. You took the skipper engine for his thrust, and it comes with gimbals; you have a probecore that can provide a small torque and prograde hold)

Another issue is error propagation, starting the gravity turn with just a little difference (e.g. +0.5° or -100kg)  and no correction may easily prevent a capable rocket to reach orbit or even leaving the atmosphere, so you need extreme precision (we know that stock controls, in flight and at the editor, don't help much with this). Is particularly problematic (both challenging and inefficient, still possible) to make the rocket perform a correct circularization without any control input after launching.

So, for the sake of precision and clarity: My approach take all those factor in consideration and, because of that, my rocket are designed to do the gravity turn with little pilot input.  This input is limited to: 1)staging, 2)change of SAS mode, 3) Orientation for circularization(only if the craft cannot hold prograde)  4) throttle control for the circulation. My launch vehicles tend to have little to no gimbals, reaction wheels and control surfaces.

 

1 hour ago, Geschosskopf said:

Yes, it's your personal choice of which one of, I dunno, maybe 30 different technical definitions of the word "stability" to use,

again:  Stable == not likely to change.

In fact is exactly what you mean when you use that word. Our divergence resides in a different point.

 

1 hour ago, Geschosskopf said:

whether or not you hang the qualifier "aerodynamic" in front of it or not.  The way you're using the word "stable" is in the sense of "positive stability" in the reference frame of the rocket with respect to the airflow.  In this case, the rocket weathervanes, so that if perturbed to have a non-zero AoA, it will return to having a zero AoA all by itself. 

And that is where our divergence is. Interestingly enough, you show there that you comprehend exactly what I'm saying:

The qualifier aerodynamic defines that my frame of reference is relative to the airflow.

To say anything in physics is necessary to define a frame of reference. I'm defining a frame of reference where all the position are measured relative to the CoM of the Rocket. That is the context where my statement need to be considered. A person in the ground is not in this context, thus his observation are meaningless for what I'm saying.

1 hour ago, Geschosskopf said:

However, this way of defining "stability" does not take into account what's going on with gravity, it's just looking at the airflow going by the rocket. 

And there an excellent reason for me to not take gravity in account: the frame of reference.

Any position is considered relative to the CoM of the rocket, thus we immediately see that there is no translation of the rocket. We don't need to consider any force for that conclusion this is already given by how is defined the frame of reference. In fact, if you sum all the forces acting upon the rocket and the result is different of zero that is because we are not considering a force (or several forces) that actually exist in that frame of reference (we may even give a name for that force(s) e.g centrifugal, coriolis, sugar, spice, everythingNice,...)

For rotation we need to consider not only the force, but the  sum of Torques relative to the CoM. But then we notice that the force of gravity (a.k.a. weight) is acting exactly at the CoM, thus producing no torque.

So yes, I'm not taking in account what's going on with gravity. Because in the frame of reference I took, gravity does nothing.

 

3 hours ago, Geschosskopf said:

Stability with respect to the airflow and to the ground are both important when designing flying machines.  But they do not mean the same thing and having one does not necessarily mean having the other at the same time.

 

All my considerations were about aerodynamic stability (with respect to the airflow). Whatever happens relative to the surface have nothing to do whit what I was saying.

In fact, I used a frame of reference that is moving relative to the surface (with changing velocity!!). With make evident that what is stable in my frame of reference cannot be stable relative to the surface.

 

4 hours ago, Geschosskopf said:

Hmmm.  Do you launch with SAS (or some other orientation-controlling thing like MJ) turned on, or not?

As explained above, for usual gameplay I take some shortcuts. But I did it for real: no autopilot, no SAS and not even a control point.