Any mod for extending the thrust vector in the VAB/SPH?

[Nazamroth]

Greetings,

Some background: I built myself a thing recently. Unfortunately, dou to the aforementioned "'thing" being a 370 ton spaceplane, I had to get somewhat creative with the design...also, to make it look nice... Taking off with a building sized sheet of metal on the runway may work, but landing it on a rather unregulated "landing field" on another body under less than ideal circumstances, would be kind of bothersome... especially if I want to take off again.

Due to this, the engines ended up out of line with the main but and eventually, the thrust vector is somewhat off from the center of mass. This has obvious ramifications which I temporarily solved by stuffing the cargo bay full of SAS units. However, I would prefer to have things properly set up without having to do a 5 minute test after every minor adjustment.

 

Real question: Is there some sort of mod that would allow me to extend the thrust vector in the designer? Preferably even have some indicator as to how close I actually am to the COM with it? I failed to find it so far.

Yours sincerely,
Nazamroth 

Nazamroth Industries - The havoc of tomorrow, today!

[JadeOfMaar]

@Nazamroth Kerbal Engineer Redux and RCS Build Aid together are essential for any craft design with any hint at complexity to therm. They will enable you to see in the hangar:

• Thrust torque at a given altitude at any body with atmosphere
• Thrust torque at each and every engine stage
• Locations of wet and dry CoM with whatever combination of toggled resources
• Total RCS power per vector and RCS torque
• dV in each stage, in total, and (RCS Build Aid, MonoPropellant only) in RCS itself
• Velocity when descending with all chutes deployed

Unfortunately, this alone won't do you much good. You may want to consider your craft design philosophy (I'm lightly suggesting it, I don't mean to urge you to). A super-sized spaceplane will have a hard time landing anywhere except right around the KSC without the ability to shed horizontal speed very quickly, and without some means to control vertical velocity (VTOL). And it will have a hard time holding its CoM and CoT together after a long time mid-flight as landing gear are significant weight. Once fuel mass decreases, the CoM will move down towards those many, heavy wheels.

Once you install these mods, be warned, you will quickly join the thousands of happy customers asking why these aren't stock!

[Nazamroth]

Ah, you see, I considered the issues you mentioned as well. As inefficeint as it is, the thing(built mostly from OPT K parts) has 24 tons of extra thrusters for slowing and VTOL. It has a TWR of only 0,7 when fully fuelled if you only use the VTOL engines, on Kerbin, but it should work well enough on moons, and planets tend to have atmospheres to use for a conventional landing. For these cases, I have added a giant retrothruster to the front and airbrakes to the back. So If i am coming down too fast, I fire the VTOL and it will slow my descent down even if I cant hover on the current planet, and if I am rolling towards a chasm at mach speed, fire the brakes.

The COM moving issue is solved by the Fuel Balancer mod, so all tanks drain equally under normal operation. I use stage recovery to check descent speed with chutes per stage.

 

I will install RCS control though. Thanks for the suggestion. (Kerbal Engineer is a must, so I had it already.)

 

PS.: Any idea why Kerbal Engineer keeps saying that when I add more reaction control wheels, my torque is going down? Does it matter where I place them or something?

[JadeOfMaar]

18 minutes ago, Nazamroth said:

PS.: Any idea why Kerbal Engineer keeps saying that when I add more reaction control wheels, my torque is going down? Does it matter where I place them or something?

Every part has mass. It always matters where you place something. The place where you add reaction wheels, therefore, seems to be good if not ideal to add dead mass and counter-weigh the craft.

[Nazamroth]

I suspected as much, although I thought that the seemingly decreasing toque would be a glitch in Kerbal Engineer like the delta V sometimes glitches. I can't seem to find a pattern. Putting a control on one pair of engine housings increases torque greatly, but putting it on the one right next to it, pretty much the same distance from the COM and the same alignment, decreases it. Slapping a pair on the wingtips decreases it, but putting one on the top of the fusealge increases it. Putting it on the bottom in the same cross section decreases it again. Am I really not getting how this works for some reason? (Added an ilustration where green increases, red decreases toque) 
https://s15.postimg.cc/avr3n2wtn/K_pkiv_g_s.png

[JadeOfMaar]

What you have, right thee, is a monstrosity.  It will hep you a lot to turn on the CoT marker and try to visualize its alignment with the CoM. Also attach a pair of full Ore tanks (they can be the smallest ones) in mirror symmetry to your craft and offset them vertically, very slowly and carefully. Watch how the torque value changes. See where it stops rising or falling and you may have the sweet spot. Also, thrust limit the upper pair (or lower pair) of engines to deal with that torque. You lose a little on TWR but it can save you the trouble of setting up counter-weights. Since you're using Dark Drives (which have 0 gimbal) the best thing to do is thrust limit. Spamming reaction wheels is not the answer for engines with no gimbal.

In-flight, KER (its VESSEL panel) can also show your torque in real-time so you can (with whatever ease or difficulty) adjust thrust limits.

[Nazamroth]

Actually, I already solved the engines torquing issue. Just needed to rotate them a few degrees to make the COT go through the COM. (also added probe cores to control from there for precision). Might be a pain to deal with it later, but I will probably redesign this at some point anyway.

The sweet spot thing seem to work well, but for some obscure reason, it is way above the craft(could not even reach it with just moving the part). The higher up I put the mass or control wheel, the more torque I get out of it.

Thanks for the advice. Gotta go to the dentist now or there will be an endless line later. Bye

 

Ps.: It is not nice to call someone's (brain)child a monstrosity... you mean person.

Pps.: What are those engines? Seem to be OPT E series but in my OPT mod, OPT J is the smallest I have.(J, H, K)

Edited July 4, 2018 by Nazamroth

[JadeOfMaar]

@Nazamroth Heh. Sorry. I didn't mean it as an insult when I said monstrosity.  The body class letters are not based on size but on shape or purpose. [H, J, K] are for fuselages. But OPT-E is for Engine and OPT-W is for Wing.

[FleshJeb]

9 hours ago, Nazamroth said:

The sweet spot thing seem to work well, but for some obscure reason, it is way above the craft(could not even reach it with just moving the part). The higher up I put the mass or control wheel, the more torque I get out of it.

You're having an understandable misconception. The torque values shown in KER and RCS Build Aid are for the engines being off-axis. They have nothing to do with the reaction wheels. Reaction wheel torque is a fixed number (https://wiki.kerbalspaceprogram.com/wiki/Reaction_wheel), and it doesn't matter where you place them on the craft, except that they may cause the craft to flex).

For VTOL craft, you're going to want to examine the torques for your drive and lifting engines separately. Just slide the thrust limiters to zero on the engines you're not looking at.

In RCS Build Aid, if you have engines that use different fuels, or monoprop, you're going to want to balance those seperately as well. You can look at individual fuels (like LF and Oxidizer) in isolation.

I strongly recommend trying to minimize the torque around the ACom (1/2 empty fuel).

f you choose to get deeper into spaceplane design, try CorrectCOL in addition to those other tools. The stock lift indicator doesn't account for body drag and lift, so the true aerodynamic center can be well ahead of where you think it is. This will cause really frustrating instability.

 

[Nazamroth]

On 7/4/2018 at 5:59 PM, FleshJeb said:

You're having an understandable misconception. The torque values shown in KER and RCS Build Aid are for the engines being off-axis.

 

.................As much as I am thankful for the insight, especially after testing it and optimizing engine rotations and thrusts......... I feel like bashing my head into a wall repeatedly now..... I have been working hard "increasing the torque" and now it turns out that I have been increasing the problem instead.... I feel like there is something poetic hiding in there... I need to commission Jeb to write something about this....

[FleshJeb]

On 7/5/2018 at 10:36 AM, Nazamroth said:

.................As much as I am thankful for the insight, especially after testing it and optimizing engine rotations and thrusts......... I feel like bashing my head into a wall repeatedly now..... I have been working hard "increasing the torque" and now it turns out that I have been increasing the problem instead.... I feel like there is something poetic hiding in there... I need to commission Jeb to write something about this....

Oh, you're not the first person to experience this type of frustration.

That's actually one of the things I've come to value and enjoy about KSP--The fact that I can be very wrong (ignorant) about something, and have the opportunity to learn from it. A momentary facepalm, my ego is intact, and then on to new and better mistakes

Spaceplanes offer a lot of opportunities like these, and I hope you continue to enjoy them. Yell if you need help.