fourfa

Adjustments to the M.E.M. command pod

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I have a few items to discuss about the M.E.M. lander pod.  I already posted this twice, sorry for the spam but now this seems like the correct place for this discussion?

I am a little annoyed that the center of mass of the new Making History M.E.M. Lander command pod is defined at the very bottom edge of the pod.  The donut tank texture at the bottom is greeble texture only - no collider.  The entire point of the weird bulges on the sides of the real-life Lunar Module is to mount fuel tanks near the center of mass, so the CoM wouldn't shift much through the ascent, rendezvous, and docking process.   Likewise, the RCS quads are mounted mid-ships so they're near the average CoM (as copied on the new M.E.M).  It's especially noticeable as the pod (like the Soviet style pods) doesn't include reaction wheels, so we're more likely than usual to use the RCS extensively for attitude control.

Since we face all the same issues they did, and the stock CoM is nonsensically at the edge of the actual structure, I worked out a fix:

// Adjust center of mass of the MEM Command Pod new in KSP Making History for 1.4.1
// Author: Fourfa
// x, z, y relative to VAB door?

@PART[MEMLander]:FINAL
{
	CoMOffset = 0, 0.675, 0
}

It might not be dead center depending what else you strap onto it (docking port, engine, batteries, more fuel/monoprop, etc) but it's pretty close.  Make small changes in the range of 0.6-0.75.

Original:

7ZDlqXo.jpg

Adjusted:

6nOskaP.jpg

Second thing that bugs me is the overall scaling on this guy.  It's very nice all by itself, but when stored in a Saturn V replica or docked up to an Apollo CSM replica with the new MH parts... it's just far too small.  I'm going to try playing with rescaling via Module Manager patches, but it seems rather more complex than the simple CoM adjustment above.

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22 hours ago, fourfa said:

I have a few items to discuss about the M.E.M. lander pod.  I already posted this twice, sorry for the spam but now this seems like the correct place for this discussion?

I am a little annoyed that the center of mass of the new Making History M.E.M. Lander command pod is defined at the very bottom edge of the pod.  The donut tank texture at the bottom is greeble texture only - no collider.  The entire point of the weird bulges on the sides of the real-life Lunar Module is to mount fuel tanks near the center of mass, so the CoM wouldn't shift much through the ascent, rendezvous, and docking process.   Likewise, the RCS quads are mounted mid-ships so they're near the average CoM (as copied on the new M.E.M).  It's especially noticeable as the pod (like the Soviet style pods) doesn't include reaction wheels, so we're more likely than usual to use the RCS extensively for attitude control.

Since we face all the same issues they did, and the stock CoM is nonsensically at the edge of the actual structure, I worked out a fix:


// Adjust center of mass of the MEM Command Pod new in KSP Making History for 1.4.1
// Author: Fourfa
// x, z, y relative to VAB door?

@PART[MEMLander]:FINAL
{
	CoMOffset = 0, 0.675, 0
}

It might not be dead center depending what else you strap onto it (docking port, engine, batteries, more fuel/monoprop, etc) but it's pretty close.  Make small changes in the range of 0.6-0.75.

Original:

7ZDlqXo.jpg

Adjusted:

6nOskaP.jpg

Second thing that bugs me is the overall scaling on this guy.  It's very nice all by itself, but when stored in a Saturn V replica or docked up to an Apollo CSM replica with the new MH parts... it's just far too small.  I'm going to try playing with rescaling via Module Manager patches, but it seems rather more complex than the simple CoM adjustment above.

Personal opinion - drop the CoM slightly so that the RCS Build Aid vectors are directly through the center of the lateral RCS nozzles (assuming 1/2 RCS fuel) - that's why they were located there on the real LEM. They should be positioned for ascent mode, as the rotation is critical for ascent docking - docking prior to descent is controlled by the CSM in real life, and RCS positioning isn't as critical on the MEM descent so long as it's "close enough" so that translation RCS adjustments don't induce too much rotation in the full MEM. Maybe that's how you already have it set up, it just looks slightly off in this pic.

Great adjustment, thanks for posting the text for a patch!

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This is pictured vs the average center of mass (ACoM) of fuel and monoprop of the bare command pod.  0.005kNm of torque is essentially zero - they basically are directly through the center of the lateral RCS nozzles.  There are slight differences between starboard/port and ventral/dorsal, but the 0.675 Z offset is very close.  If you find there's a better setting, I'm sure we'd all like to know what it is

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I noticed this problem as well with my 2-stage lander. I'll try the fix.

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I used slightly different values, also accounting for the longitudinal axis offset of the thruster quads to minimize uncommanded pitch while thrusting foreward/aft.

CoMOffset = 0, 0.6998, 0.029

The quads are actually assymetrically attached along two different axes (vertical and longitudinal), so pretty much any chosen COM offset values are going to be a compromise.  Compensating for the fore/aft thrust issue makes the horizontal thrust issues worse (but still far better than the stock placement).

The only real fix is to replace the part model with one having symmetrical thrusters.  Then the CoM could be fine-tuned to perfection.

And I'm sure that ain't happening.

Edited by RoboRay

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Well, yeah, but anything other than the Spark is overkill unless you're adding more fuel tanks... and if you're doing that, you're adding room for more RCS nozzles to balance with the built-in ones.

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Looking into this a bit myself I have only really ever used this MEM with a Spark on the bottom and a Jnr Docking Port on the top. I imported the MEM .mu file into Blender and found the average central x, y, z co-ordinates of each of the 4 RCS thrusters and then found the average central co-ordinates of those 4 points. This result gives you and x, y, z location of 0.000019, 0.0313, 0.676581.

Knowing the mass and CoM positions of the Spark and Jnr Docking Port and the node positions on the MEM you can do a little resolving of moments and you end up with a the CoM offset positions that suit the MEM to give you, in theory, the best average position to suit the 4 RCS thrusters. Granted, because they are not located all on the same plane aligned to the axis (Roboray has pointed out their asymmetry above) you cannot resolve all of the final torque down to basically 0kNm in each direction of travel but you can get it low enough for good use.

Secondly, as the CoM needs to be offset in the y-axis it also causes any engine connected to the bottom node to create a fair bit of torque so I have shifted the bottom node over to align through the CoM.

Here's my patch:

// Poodmund's MH M.E.M CoM Patch
@PART[MEMLander]:FINAL
{
	CoMOffset = 0, 0.70281219, 0.0313
	@node_stack_bottom = 0.0, -0, 0.0313, 0.0, -1.0, 0.0, 2
}

// Poodmund's MH M.E.M CoM Patch for use with no node movement
//@PART[MEMLander]:FINAL
//{
//	CoMOffset = 0, 0.70281219, 0.03311012
//}

Here's a .gif showing the resulting forces in RCS Build Aid:

FoolishBlushingAquaticleech.gif

  • Port/Starboard Torque: 0.066kNm
  • Dorsal/Ventral Torque: 0.031kNm
  • Fore/Aft Torque: 0.046kNm
  • Engine Torque: 0.005kNm
Edited by Poodmund

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It is also backwards

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@Poodmund ...lipstick on a pig, but nice job none-the-less...  It'd be a lot easier if the part designer had paid attention to RCS placement in the first place.

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Old thread, I know. Figured I'd post my tweak here to keep it consolidated for future googlers.

I found that using a CoMOffset of 0,0.7125,0 combined with fullThrust = true gives me 0.000 induced thrust on all the attitude ranges with a spark on the bottom and a docking port jr on top.

I use the tiny ReStock+ RCS nozzles to adjust the translation torque and am now left with a beautifully balanced MEM. Until I use fuel and shift the CoM at least.

Hope it helps! Here's the MM patch if you want to try it for yourself:

@PART[MEMLander]:NEEDS[SquadExpansion/MakingHistory]
{
	%CoMOffset = 0, 0.7125, 0
	@MODULE[ModuleRCSFX]
	{
		%fullThrust = true
	}
}

Note that I my MEM is a bit heavier than the stock one due to added life support supplies, but I tweaked it so it stays balanced even if there's a slight change in MEM mass (slightly more/less won't induce torque/thrust). So it shouldn't be an issue.

If you find that it does induce thrust (pitch is the first to deviate) you can try 0.7120 or 0.7115 instead.

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Posted (edited)

Bumping this thread rather than starting a new one; if I should do the latter, let me know.

My pet peeve with the M.E.M. is, to me, obvious at a glance:  the scaling.  (Fourfa said as much at the end of their OP, too.)  There are lots of reference images for how big it should be relative to the Mk 1-3 pod (the Apollo CSM), so why couldn't the part designer get that one basic thing right?  The actual part looks fine, IMO, so it bugs me to a possibly irrational extent that they'd spend so much time on the expansion and then get something so simple and immediately visible wrong.

At this point I've given up on SQUAD putting out an official fix, so does anyone know of a mod that does it, or even a set of Tweakscale numbers to bring it up to the proper size?

Edited by Commander Zoom

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On 7/5/2019 at 7:06 PM, Commander Zoom said:

My pet peeve with the M.E.M. is, to me, obvious at a glance:  the scaling.

Indeed, it seems to fit the Mk2 pod better.

reW7tyf.png

 

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