n.b.z.

Commercial break: reap the powers of the TiltProbeCore™!

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Obviously, I'm not actually trying to peddle some real-world commercial crapola here.

Instead, I'm advertising, or rather proposing, the simple act of adding a stock probe core, or even two, to a vehicle that may already have one. If that already sounds silly to you, that's only because I haven't yet told you that I want the new probe core(s) to be rotated slightly OFF the center axis; preferably using the snap function of the rotate tool for reproducible results.

Why?

A probe core is, among other things, the attitude sensor of a vessel. The SAS function relies on this data for doing stuff like pointing prograde.

By having more than one core to choose from ("control from here"), with variations in their orientation, we can modify the data that goes into the SAS, which will in turn modify how the SAS modes orient our vessel. I have come across numerous occasions, where this either helps with controlling a vessel, or even adds some semi-automated "autopilot" function. For those who aren't going all-out MechJeb or KOS, but could occasionally use a bit of flight assistance here and there, this may be useful.


USE CASE 1: Implement "AOA HOLD" on airplanes

The angle of attack is the difference between where you're pointing, and where you're going. In other words, between your plane's longitudinal axis and the prograde marker.

In KSP, we always need some angle of attack to create lift. If a plane with "flat", non-angled wings is oriented exactly horizontally, it will be losing altitude. To stay at a given altitude, we'll need to point a bit upwards. Now, we can arrest this orientation with SAS hold, but due to the curvature of Kerbin, this will require constant re-adjustment over time.

But if we have a probe core that's pointed a bit downward, we can use the prograde mode! We are tricking SAS to believe that "prograde" is a bit above its actual place. If our TiltProbeCore™ has, say, a 10° angle downwards, our plane will pitch to 10° above the horizon when travelling exactly horizontally.

But why does this rock? Because with increasing altitude, and decreasing air pressure, we'll need an ever bigger angle of attack to create a given amount of lift.

If we keep the throttle constant, this means:

  • if we're too high to maintain this altitude, prograde will move downwards as a result of too little lift. We descend.
  • if we're "too low", our lift will be plenty, and we don't need all of the angle of attack that's "hardwired" for our probe core just to stay at level flight. Thus, prograde will move upwards. We climb.

And... yes! You are right! Really!

I know you were hoping that in the above situation, the oscillation would eventually dampen out, settling in a stable config, with a plane nicely cruising the skies of Kerbin all by itself, while you're making another coffee, even getting annoyed if after 5 minutes, your plane requires a 3° course correction. And, yes, that happens to be exactly the sort of thing I'm trying to sell you here.

While I'm on a marketing spree, please allow me to point out a secondary use of this. (This is when some sales dude tries to make you believe that the product isn't just useful, but once you "buy" it, you'll be a much cooler person all around.) EVERY tilted probe core on an aircraft (if not constrained by low thrust at altitude, says my lawyer) comes with an EXTRA that you get FOR FREE, and that extra is called AUTOMAGIC CRUISE CLIMB! That's to say, your plane will gradually climb as a result of depleting its fuel.

All of the above can be yours, for the modest price of slapping an OKTO2 onto your plane. Maybe two. Deal?

Exhibit 1:

DUdu5TV.jpgThe "White blackbird". My first plane to use the Whiplash, and probably one of the most obvious ways to put a Mk2 craft together for these engines. Once positioned at 20000m and 1200m/s, this thing travels around all of Kerbin with just a little occasional correction for roll. If thrust is reduced by one notch ONLY TWICE during an entire Kerbin circumnavigation, this plane stays between 20000 and 21000 m the entire time with no pitch input whatsoever - thanks to the secret powers of the TiltProbeCore™.


USE CASE 2: Flying precise profiles with wobbly rockets

If your launch stack happens to be of the rather wobbly sort, the old instinct of adding MOAR STRUTS has turned out to add considerable drag. Wobblyness makes it harder to fly a precise ascent profile. Often, it helps to just add a probe core to the stack bottom, so that SAS is not fed the data from the top of the rocket, which may be bending and oscillating all over the place.

But sometimes, more precision may be desired.

The stack shown below is extremely wobbly, and even more so after shedding the boosters. Which also leaves it with a very low remaining TWR. (There is a huge amount of fuel stored inside that fairing.) The margin between going up way too steep, and being too shallow to produce an apoapsis of 70 km before going downhill again, was hard to attain with manual control. Also, to prevent SAS from violently overcorrecting, I had to severely restrict gimbal authority on the engines.

Instead of adding 64 struts, and then 6 moar booster tanks to carry the struts, I added, you know, a TiltProbeCore™. I now launch with the stack controlled from the *regular* probe core, SAS hold activated, straight up 90°. I change to SAS prograde around 70m/s, and at exactly 100m/s, I switch to the TiltProbeCore™, which produces a smooth and predictable turn to the east. At a certain pitch, I change back to SAS hold, switch to the regular core again, and watch as the prograde marker - which was slightly lagging behind during the turn - aligns with the rocket's center axis again, at which point I go back to prograde for the remainder of the ascent.

Exhibit 2:

jA5nKEG.jpgA very wobbly stack. You just know that by looking at it. This post isn't about what this particular rocket was designed for. (Too bad I named the darn thing "Outer Planet Stack". I'll work on that.)


USE CASE 3: Dealing with cases where center-of-mass changes relative to the thrust vector

Sometimes, there's some special need for a special solution. For example, you may find yourself building your first Shuttle (this recently happened to me), asymmetric stack and all, and you realize that the dudes who came up with that config for real must have been every bit as bonkers as you and me. Thrust line changes at booster sep. Cargo bay also off center. Tank gets lighter during ascent, as they all do, but that asymmetric stack makes that a bit more fun. Further up the trajectory, they did a 180° roll with the SSMEs still burning enthusiastically, probably mostly because there was no way to do anything even cooler than that. (I mean... seriously... that's the only possible maneuver that preserves the thrust vector... but I digress.)

Those shuttle engines are angled for a reason: it is very helpful if your thrust vector happens to pass somewhere near your center of mass. Actually, preferably, going right through it.

So what do you do, when all has been slapped together, and it is only after having left the launchpad that you realize that the "this is forward" pointer on the navball, derived from the orientation of the Mk3 cockpit, is further away from your thrust vector than Eeloo is from Jeb's mom? Well, presumably, you revert, or regret, or find a way to improvise yourself to orbit using SAS hold only, probably requiring a lot of fiddling and adjusting.

Wouldn't it be nice to actually just hit prograde, and fly your shuttle like a proper rocket? You bet. Well, obviously, your design needs - you guessed it - a TiltProbeCore™.

Actually, in a special case like this - boosters, cargo bay, OMS engines and all - I'd go even further. While previously, I was arguing for adding one or two probe cores, I'm going to really shove the point across here. And I'll admit it outright.

It's not even three. I really, actually, added four probe cores. Seriously. And I'm not even ashamed about it, nope, I'll actually say it straight to your face. Five degrees, ten degrees, fifteen degrees. And, twenty degrees. MY shuttle is a whopping FOUR probe cores cooler than YOUR shuttle. There you have it!

Exhibit 3:

DJAvI14.jpgSome shuttle. It's really just another boring shuttle clone, produced by slapping the most obvious stock parts together in the most obvious fashion. Yet it's still 4 probe cores cooler than yours. This is why I get to roll mine 180° into a heads-up position at 30K just because I can, while you sit there and think I'm out of my mind.

 

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hey! that's actually a really good idea. You're not even paying much of a mass or power cost to have a whole bunch. And as you show, with the new pinnable menus you won't be frantically scrambling to select the right one.  The only thing that might be a bit confusing is not having a read on true pitch during ascent, but if you know which core is which that should be manageable. Pity we can't label parts! 
 

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I'd heard of multiple cores at 90 degrees before, but never at slight offsets. It's a pretty simple solution, actually. Elegant!

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... huh. I need to find an elegant way to do this at lower tech-levels for career. Don't want those fatter probe-cores to look all ugly now.

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The airplane one would be better served with *actual* aircraft AP modes (attitude hold, altitude hold, wing leveler, etc.).

The other ones I can't argue with.

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The one teeny tiny issue regarding this is that in 1.1.2, the prograde hold has a bug and overcorrects at high frequency. There's also a little bit of funny business about how to mount the cores is the most aesthetic way. Otherwise, I love the idea.

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This.  Would be a great mod.  I say mod because the idea is so crazy for something that wouldn't be done by the real NASA people (cause they have proper equipment and all that), that Squad unfortunately wouldn't put it in, or in the form you recommend.  Fully support any mods with this concept.  Great idea. 

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I was actually thinking of adding a suggestion to squad to allow control from here on engines (for shuttles) but your trade marked cores solves every problem perfectly.  Although on airplanes I prefer tilted wings.  Does your tilted core prevent oscillations while following prograde?  That has been my major challenge

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NB: Docking ports also exist as a controllable point and require no power.  And can dock.  Some of my docking craft actually have the ports in the bottom (that's a workaround for having a centered goo on the top, or an EAS "craft", etc).. I just 'control from here' the port and dock away happily, without having to worry about drivin' backwards.

I'm not sure how they compare to actual probe cores in terms of aero.. but they do have another advantage: they can be surface mounted directly.  Well, except the big one.  Also the Jr. port is fairly early, around the tier of the original octo.

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I live in china, near shenzen, sooooooo.......   Its Totally not me producing knockoffs of your product.

 

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6 hours ago, bewing said:

the prograde hold has a bug and overcorrects at high frequency.

 

6 hours ago, Nich said:

Does your tilted core prevent oscillations while following prograde?


Well, unfortunately, all the usual issues one may run into with SAS modes apply here. Often, these can be somehow worked around. For example, if your airplane suffers from roll oscillations, you may try deactivating the ailerons when all you want is a straight-line cruise. The reaction wheels built into cockpits are often enough for a little roll correction here and there.

(It's my impression that this has recently been made easier - that turning off the aileron on one side, in flight, will also turn off the one on the other side, if symmetry was used for placing it. Not really sure, though.)

 

5 hours ago, Renegrade said:

Docking ports also exist as a controllable point and require no power.  And can dock.

Of course they do, and my idea should not keep anyone from exploiting this fact whenever that's useful.

For example, I have a rover that can also fly using monopropellant. It has a front-facing probe core, and a docking port facing up. For driving, I'm using the front-facing core, and for flying, I'm controlling from the dock port, so I get to use nice things such as a working retrograde marker.

 

7 hours ago, RandomUser said:

This.  Would be a great mod.  I say mod because the idea is so crazy for something that wouldn't be done by the real NASA people (cause they have proper equipment and all that), that Squad unfortunately wouldn't put it in, or in the form you recommend.  Fully support any mods with this concept.  Great idea. 

This isn't crazy! (Says the dude with four Okto2's on his shuttle.) This is actually a stock part approximation - admittedly a very crude one - of what NASA could do better than us because they have "proper equipment and all that". I'm quite sure that the shuttle's guidance software is acutely aware of the center of mass, and thrust vector, at any given time during ascent. In our world of KSP stock parts, a probe core oriented along the thrust vector probably approximates this closer that one oriented along the vehicles length axis.

So what would a mod look like, that wanted to implement my idea? It would be a probe core like any other, except that you could control the tilt angle independently of placement, so that people like me wouldn't slap four of the things on a craft. Sort of like being able to use the rotate tool on the thing during flight. But I would guess that anyone willing to spend time on such a mod would probably quickly realize, that...

9 hours ago, pincushionman said:

The airplane one would be better served with *actual* aircraft AP modes (attitude hold, altitude hold, wing leveler, etc.).

... at which point we are entering the world of things like MechJeb, KOS, and so on. Nothing wrong with that, but my point was rather to show how just a stock part, placed wisely, can sometimes go a long way of getting us some helpful flight assistance here and there.

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I was talking about the dynamics of an aircraft following a set AOA.  If your loosing altitude you will pull up too far and start loosing speed until prograde dips low enough that you start loosing altitude and gaining speed.  In my experience my planes have had very little, negative, or no dampening to this mode of oscillation

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Super interesting idea!  I suppose the smaller cores are still able to harness the torque from all the command modules, so even a tiny OKTO2 can hold a shuttle's heading.  How are you attaching your TiltProbeCore™(s)?Via Cubic Octagonals and gizmo work?

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10 hours ago, Renegrade said:

NB: Docking ports also exist as a controllable point and require no power.  And can dock.  Some of my docking craft actually have the ports in the bottom (that's a workaround for having a centered goo on the top, or an EAS "craft", etc).. I just 'control from here' the port and dock away happily, without having to worry about drivin' backwards.

I'm not sure how they compare to actual probe cores in terms of aero.. but they do have another advantage: they can be surface mounted directly.  Well, except the big one.  Also the Jr. port is fairly early, around the tier of the original octo.

Yup, that's what I have used a few times on some skinny rockets to cure the shakes....

W6ZT3Xc.jpg

Never thought to tilt them though. Gonna try that on an SSTO I'm building as a possible alternative to angling the wings. 

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32 minutes ago, Nich said:

I was talking about the dynamics of an aircraft following a set AOA.  If your loosing altitude you will pull up too far and start loosing speed until prograde dips low enough that you start loosing altitude and gaining speed.  In my experience my planes have had very little, negative, or no dampening to this mode of oscillation

Hmmm. Admittedly, I haven't really tested this systematically. But I have been slapping tilted cores onto planes pretty much routinely, and my impression is that you can usually find a cruise config that's "stable enough". Oscillations may not completely stop, but are usually just a few 100 m in magnitude.

How well this works probably depends on a huge amounts of factors. Different engines will have a different change of thrust with increasing altitude, maybe some have a "curve" that would de-stabilize this oscillating system?

I routinely leave the room for small chores around the house or another coffee while a plane is being flown on "oktopilot".

 

32 minutes ago, klesh said:

I suppose the smaller cores are still able to harness the torque from all the command modules, so even a tiny OKTO2 can hold a shuttle's heading.

Yes, you have all the torque of your vessel available from all parts that let you "control from here", even if they have no torque, such as docking ports.
 

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Ahh! Nice write-up and description. I do something similar, but only with SSTOs where I often have the cockpit tilted off axis from the direction of thrust.

That shuttle of yours is absolutely gorgeous BTW!

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This is the solution to most of my problems, thank you! The plane attitude hold will greatly benefit my high altitude jet experiments, and  I might actually build a core into my shuttle. @n.b.z., you deserve all the rep of these forums :D

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Very clever use of limited parts, but I generally use mechjeb for attitude hold (Smart A.S.S. in surface mode is awesome!).  If someone were going to create a mod based on this idea, I would suggest the Variable Attitude Probe Orientation Rectifier (VAPOR); A single probe core that would allow for in-flight adjustments to it's internal orientation.  And that's something I suspect you'll find NASA already has without much fanfare about it.  All you modders are free to use that name, but please credit me for it if you do. :wink:

Danny

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