How to design and control vehicles with off-center thrust vectors

[devikwolf]

Hi all,

I've been lurking for a while without posting or contributing. Veteran KSP player here. I've done a lot of bonkers stuff in this great game, but designing -- and controlling -- anything with off-balance thrust is still harder than absolutely anything else I have done in this game.

For the entire time I've played KSP, I've struggled with designing and flying vehicles with off-center thrust vectors, EG an STS-styled space shuttle. I've designed and flown a few of these, but the most manageable versions were all pre-KSP1.0. SSTU offers a beautiful set of building blocks to make a ship, but there's still a ton of limitations from the core mechanics.

1. The display for thrust vectors and CoM during vehicle design seem to not be useful for something like this; I have to eyeball it, by rotating the camera until the thrust vector is aimed right at the camera, and then I have to gauge whether the thrust vector 'ball' is lined up centrally with the Center of Mass. Maybe I'm totally missing something, but this "eyeball it" method seems to be the only option in this case? I can't even find a mod which will draw a ray along the thrust vectors so I can see if it properly intersects CoM. So it seems the only option is to eyeball it, throw it into a test flight, revert, eyeball it again, test again, etc. There's got to be a better way, even if it's through a mod, but I've never found a mod that will allow me to either do this -- or better still, just let me hit a button or a key to rotate the engine to align it to the center of mass.

2. During flight, the space shuttle's main engines would gimbal themselves towards CoM, to account for how the CoM would change as fuel burned off (despite the stack being engineered to keep the CoM change as in line as possible with the center of thrust, it still moves). KSP of course doesn't have anything like this stock, but I also can't find this behavior in any sort of mod. Being able to keep the engines gimbaled to a changing center of mass is a critical ability of the space shuttle.

3. Navigation in KSP is always aligned through the control part's forward vector. I've found some older mods that give better handling options, but I've never been impressed with them. In the past I've usually tucked a probe core between the SSME cluster and angled it along the thrust vector, and use it as a control point, but again, this is just a very eyeball-and-test process. I can't find any mod that offers a "control from thrust vector" 

 

I've watched users voice these same frustrations for 3-4 years and I've not seen any good options besides "wrestle it to orbit by manually doing things that the computer would be doing a thousand times a second" which is ultimately more frustrating than fun, to me. I've managed to get perfectly stable and flyable shuttles in the past, but only after spending dozens of hours making tiny, tiny changes to part positions.

 

[FleshJeb]

1 & 2) RCS Build Aid will show you all kinds of uneven torques for both dry and full tanks, and recalculate in real time. Kerbal Engineer Redux will show you the total amount of torque, but I've never looked at that readout.

What I do in these situations is use Editor Extensions in combination with RCS Build Aid to precisely rotate my engines to their gimbal extent to see what effect that has. RCS-BA also has a gimbal submenu that I haven't really used.

2) In a well-designed craft, trying to hold the SAS WILL result in thrust through the CoM (plus aero forces). As long as the current CoM is within the limits of your gimbals to put the thrust vector through.

2 & 3) Throttle Controlled Avionics is THE gold standard of controlling "wacky" craft. It's got a pretty steep learning curve that I haven't put the effort into mastering yet. I definitely need to.

[devikwolf]

Thanks for the input @FleshJeb!

I'll give RCS-BA and KER a try. I've been using MechJeb for ages because of the details it gives during design, but I haven't used Kerbal Engineer in a few years (I liked what MJ did, at the time, better).

However, my experience with SAS is that the craft usually starts thrusting with the engines at neutral gimbal, and then once it begins torquing off to one side, it will begin to correct. This occurs in orbit so I'm ruling out any aero influence. However, I MIGHT be mis-attributing the behavior to the fact that my probe's control vector is only aligned with my thrust vector in the crudest of senses.

I'm pretty sure I've heard of TCA in the past, but I'll check it out this weekend. If it's the one I'm thinking of, I played with it a lot, a while ago. I might be thinking of a different mod, though -- I was looking for something that could balance engine throttle and gimbals to put a skycrane into an autonomous hover mode while lowering my Curiosity clone to Duna's surface, something I'm still trying to accomplish smoothly.

Edited March 9, 2018 by devikwolf

[FleshJeb]

14 minutes ago, devikwolf said:

Thanks for the input @FleshJeb!

I'll give RCS-BA and KER a try. I've been using MechJeb for ages because of the details it gives during design, but I haven't used Kerbal Engineer in a few years (I liked what MJ did, at the time, better).

However, my experience with SAS is that the craft usually starts thrusting with the engines at neutral gimbal, and then once it begins torquing off to one side, it will begin to correct. This occurs in orbit so I'm ruling out any aero influence. However, I MIGHT be mis-attributing the behavior to the fact that my probe's control vector is only aligned with my thrust vector in the crudest of senses.

I'm pretty sure I've heard of TCA in the past, but I'll check it out this weekend. If it's the one I'm thinking of, I played with it a lot, a while ago. I might be thinking of a different mod, though -- I was looking for something that could balance engine throttle and gimbals to put a skycrane into an autonomous hover mode while lowering my Curiosity clone to Duna's surface, something I'm still trying to accomplish smoothly.

KER isn't going to give you that much more information specific to this problem. Although I've always used both it and MechJeb for various readouts.

You're probably correct, it's been about a year since I flew anything that had gimballed engines in space.It should settle out after the initial mis-torque (provided that it's small enough).

TCA is a super-complex thrust-balancer. Eventually I'll get around to learning it.

MechJeb has a hover mode, but it's not fantastic. I think it's the Translatron. I try to push MJ to it's limits to find out what they are, and they can be fairly narrow without getting into PID tuning. So I try to design my craft to operate within the limits of what MJ can do well and reliably. Which means limiting the gimbal range on my engines and keeping my TWR within reasonable bounds (>2 <3) by limiting the thrust sliders, or the max acceleration in the Utility settings.

EDIT: You can also get into automated control via KOS or KRPC.

Edited March 9, 2018 by FleshJeb

[GoSlash27]

 The trick I use when designing shuttles:

 Not only do I aim the thrust at the CoM, but I also juggle the ET's flow priority so that the CoM moves toward the engines as the tanks drain. This is why the engines need to be well- below the main tank instead of next to it.

 For control... handling is a bit wonky. Yaw and roll inputs basically swap and reverse.

Best,
-Slashy

[GoSlash27]

Behold: the Next- gen Asymmetrical Navigable Rocket (NANR)
 

Spoiler

 

 

The fact that the thrust is pointed at the CoM and the CoM falls toward the engine as the fuel drains makes this rocket controllable... Barely. Roll control is achieved with small yaw inputs.

HTHs,
-Slashy

[Human Person]

18 hours ago, FleshJeb said:

1 & 2) RCS Build Aid will show you all kinds of uneven torques for both dry and full tanks, and recalculate in real time.

Second that. RCS Build Aid is IMO the most useful mod you can have, even more useful than Mechjeb.

[devikwolf]

KER doesn't seem to be much help to my current problem, but I definitely wish I had installed this mod a few years ago. Thumbs up for this recommendation!

Seems I can't get RCS-BA running properly. The button shows up in the toolbar, but pressing it does nothing. Hm. I'll do some more digging into what's keeping that from working properly.

Tried out TCA; it hindered more than helped, ultimately. It made the shuttle extremely controllable at certain parts of the flight, but most of the time it was wobbling around. And I definitely had used this mod in the past, too, at some point. I don't recall exactly what it was for.

I spent a while last night playing with the shuttle some more. By locking gimbals on the SSMEs during initial ascent and just relying on aero surfaces and SRB gimbal to maneuver, I eliminated the ascent wobble. Gimbals go back on shortly before SRB sep. Some extreme fine-tuning with the position of my probe core control points have helped a lot, but I still suffer from the fact that SAS doesn't make engines gimbal towards CoM until after the vehicle is already torquing over. It's far more controllable now than it was before, but fine rendezvous is a bit tricky because of that wobble. Selecting the probe cores to be a control point is also somewhat frustrating because of how they're tucked up in the engine cluster, so just getting the cursor to highlight them is a trick. But that's always been the case with my shuttles.

Also, and I'm certain this is a limitation of SSTU: I had to mount my shuttle very much lower on the tank than I would like. I believe this is because, unlike the real shuttle which had the heavier oxidizer on the top of the tank, SSTU doesn't know how to do that, so the tank's COM is a lot lower than the real shuttle's would be. I think, anyways. I could probably solve the problem by building the tank in two parts, a separate piece for LH2 and one for LOX.

[FleshJeb]

12 minutes ago, devikwolf said:

I spent a while last night playing with the shuttle some more. By locking gimbals on the SSMEs during initial ascent and just relying on aero surfaces and SRB gimbal to maneuver, I eliminated the ascent wobble. Gimbals go back on shortly before SRB sep. Some extreme fine-tuning with the position of my probe core control points have helped a lot, but I still suffer from the fact that SAS doesn't make engines gimbal towards CoM until after the vehicle is already torquing over. It's far more controllable now than it was before, but fine rendezvous is a bit tricky because of that wobble. Selecting the probe cores to be a control point is also somewhat frustrating because of how they're tucked up in the engine cluster, so just getting the cursor to highlight them is a trick. But that's always been the case with my shuttles.

Also, and I'm certain this is a limitation of SSTU: I had to mount my shuttle very much lower on the tank than I would like. I believe this is because, unlike the real shuttle which had the heavier oxidizer on the top of the tank, SSTU doesn't know how to do that, so the tank's COM is a lot lower than the real shuttle's would be. I think, anyways. I could probably solve the problem by building the tank in two parts, a separate piece for LH2 and one for LOX.

Do the trim controls affect gimbals before you light the engines? (Alt-W,S,A,D,Q,E. Alt-X to recenter.)
"Control From Here" can be set via action group now--No clicking necessary.

Changing your tank setup would definitely help.

[FleshJeb]

@devikwolf 

This will be worth checking out, even though it's not currently updated.

[Cap Arcona]

I'm sure you've already tried this, but I've found RCS Build Aid to be invaluable. You can make some really precise designs if you put in the time. I lost all my good shuttles with the update, because I'm a dingus and didn't save my 1.3.1 installation, but I had some that could launch and land with little to no input or MechJeb (getting a stable glide on your shuttle is a pain, but getting a stable launch vehicle is murder). I didn't know about Thruster Trim, I should hang out in the forums more often.