RCS Placement + Electric charge questions

[N_Danger]

 Hello there,

I'm building a docking trainer while playing in sandbox and I have a couple of questions:

1) I am placing RCS blocks on my space craft and the only way I can see to place them equidistant from the COM is to use another part as a reference guide like so.

Is there a better way? (without mods for now, I only use MecJeb for the moment and will mod up with the upcoming 64 bit support for windows with ALL the mods )

2) I have found on a un-kerbeled re-entry using SAS and one of the probe cores to hold retro-grade that the 150 charge of the capsule is used up very quickly. The capsule stabilizes eventualy without power but I worry if I have a re-entry from interplanetary speeds with a live crew it will not do that in time before it explodes. Is this something to worry about?

Thanks in advance! 

[Phil deCube]

That's actually a pretty clever way of measuring the distances. I'll have to start using that. On the other hand, I would put the RCS jets at the COM, if I could. Your solution looks good though.

As for reentry, I always make sure I have enough battery onboard for just that purpose. If you craft isn't well balanced for coming in rear first put on a bigger battery and some reaction wheels.

Edited March 8, 2016 by Phil deCube

[FullMetalMachinist]

My view on the question of balancing RCS thrusters is this: don't worry too much about it. There are a couple of reasons for that.  Fist is that for a ship that size, you really don't even need RCS for attitude control. The reaction wheels inside that command pod should be plenty to turn your ship.  That means that they're only useful when docking. 

Now, for docking there is a little-known trick for translation movements: use fine controls. If you're unaware, you can press Caps Lock at any time and it will put you into "fine" control mode, and the little red arrows in the bottom left corner will turn cyan/light blue.  The important thing this does for RCS and docking is to automatically adjust the thrust of each RCS block to keep it within what the command pod reaction wheels can counter, as long as they're fairly close to balanced to begin with.  This means that RCS thrusters closer to the CoM will thrust more, and ones further away will thrust less, automatically.  So as long as you're close enough to being balanced, the fine controls will take care of the rest for you.

Now, if/when you're willing to look at mods, there's a great one called RCS Build Aid that has lots of features to get your RCS balanced just so. But for most situations, it's really not necessary thanks to the handy "fine controls" trick.

As for your second question, very light ships (like small unmanned probes that you mentioned) are pretty terrible about wasting monoprop while trying to hold a certain orientation.  This comes from the PID controller for stock SAS being very twitchy and getting itself into a feedback loop where it tries to correct, then overcorrects, then tries to correct the overcorrection, etc., etc. My suggestion would again be to not rely on RCS for attitude control, even on reentry. (As a side note, the ship you have pictured will do fine on reentry, assuming that's a heat shield between the pod and decoupler. That configuration is extremely stable during reentry, so much so that you'd have a hard time getting it to point any direction but retrograde.)

[RizzoTheRat]

From LKO you' don't actually need a heatshield on the bottom of a pod.  As FMM says pods are very stable, if you do the deorbit burn and then turn SAS off it'll orient itself the right way before anything gets hot enough to cause problems.

For things that aren't quite as stable, I find that holding retrograde seems to use more power than just holding the current attitude, so if power is marginal you can save some by setting it retrograde then switching back to hold attitude and manually adjusting it occasionally.

[Snark]

The fine-control mode that Machinist mentions takes care of RCS placement issues. Note that there's often little point in trying to get your RCS thrusters *exactly* centered in the VAB, since your CoM is going to move as you burn fuel anyway.

Also note that for the "two rings of RCS thrusters" approach, such as you're using here, you only really need to have one ring of the 4-way thrusters-- the other ring can be the 1-way.  A handy way to save 80 kg, plus the one-way thrusters are less visually obtrusive (for me, anyway).

As for EC, the simple answer is to just bring more. 150 is actually quite a small amount, it's not even two of the smallest battery. Batteries are fairly cheap to add, mass-wise. You can add a 1000 EC battery for only 0.05 tons.

Edited March 8, 2016 by Snark

[Plusck]

2 hours ago, Snark said:

Also note that for the "two rings of RCS thrusters" approach, such as you're using here, you only really need to have one ring of the 4-way thrusters-- the other ring can be the 1-way.  A handy way to save 80 kg, plus the one-way thrusters are less visually obtrusive (for me, anyway).

+1 to this.

Also - a ring of four thrusters will fire 2 nozzles in each of the lateral directions (total thrust = 2) and four thrusters forwards or backwards (total thrust = 4). If you add a second ring of just the single "place-anywhere" ports, they will add 2 thrust in each lateral direction (giving a total of 4 thrust) but zero forwards or backwards, so each direction will now have exactly the same total thrust. The only downside is that the single ports can't help with rolling your craft, unless you place them off-centre, but rolling generally needs a lot less effort anyway.

Edited March 8, 2016 by Plusck

[Snark]

1 minute ago, Plusck said:

Also - a ring of four thrusters will fire 2 nozzles in each of the lateral directions (total thrust = 2) and four thrusters forwards or backwards (total thrust = 4). If you add a second ring of just the single "place-anywhere" ports, they will add 2 thrust in each direction (giving a total of 4 thrust) but zero forwards or backwards, so each direction will not have exactly the same total thrust. The only downside is that the single ports can't help with rolling your craft, unless you place them off-centre, but rolling generally needs a lot less effort anyway.

Not only does rolling need a lot less effort, but it's also usually a lot less important, given that docking ports don't care about your roll angle.  When docking, your roll orientation usually doesn't matter, unless you care about aesthetics (or you're doing some sort of fancy-shmancy multiple-port simultaneous docking).

 

[Wemb]

7 hours ago, FullMetalMachinist said:

My view on the question of balancing RCS thrusters is this: don't worry too much about it.

And this makes sense if you think about it - there are two types of movement that RCS is usually used for - one is rotation and the other is translation.  For rotation, the best place to put the RCS thrusters would be as far as possible from the CoM, but for translation, the best place would be at close as possible to the CoM.   And for docking, you're quite likely to need both.  But FMM has said, with a ship that small, your reaction wheels can probably handle the the rotation - but they'll also counter the rotation caused by having off-center RCS ports when translating.  In short - it won't be a probably on something that small. 

As for re-entry - attitude might be critical - it really depends on what speed you're doing and your angle of re-entry.  one really useful feature of heatshields that are often overlooked is that that they are bloody heavy - having one of those strapped to the bottom will not only protect you from the heat of re-entry, but will also help to ensure your ship is orientated the way you want it to be.  This has been useful to me on early orbital crated whose CoM was badly affected by the number of SciJr pods I was returning.  A heatshield wasn't remotely necessary to shield me from heat, but it certainly helped ensure I kept the correct attitude.

Wemb

[Val]

Like others have mentioned, a ring of 4 of the 4-port RCS around CoM should be enough.

Personally I prefer a ring of 4 of the Place-Anywhere Linear RCS Port plus one each for forward and backwards translation. It's lighter than a ring of 4 4-port RCS blocks.

Also the Linear RCS port has much higher thermal tolerance than the 4-port, 2600K vs 1500K, which is important especially for airbreathing spaceplane ascents.

[Val]

And sometimes if there isn't room for a full ring of 4 I'll use a set of these on either side.

[N_Danger]

 Thanks everyone for your help! Back to the VAB!

Docking and rendezvous are the two skills I haven't mastered yet and since MecJeb will probably be broken for a while with the new patch it looks like a good time to learn them. 

 

Edit: How do I mark this as answered?

Edited March 8, 2016 by N_Danger
Asking about closing question

[RizzoTheRat]

The key thing with docking is learn to use the nav ball.  Rotate the ship to point the nose at the target marker and use lateral RCS to keep the prograde vector on the target marker.  It's also a lot easier if you switch the camera to fixed to the RCS directions match the picture you're looking at.

Love Vals space scooter design, reminds me of those WW2 human torpedo craft. 

[Wemb]

1 hour ago, N_Danger said:

 Thanks everyone for your help! Back to the VAB!

Docking and rendezvous are the two skills I haven't mastered yet and since MecJeb will probably be broken for a while with the new patch it looks like a good time to learn them. 

MJ can help with the rendezvous, but unless you want it to do the docking for you, I'm not sure it's much more help there - other than the TGT SmartASS functions - the PAR +/- can be useful - but even more so is the Docking Port Alignment indicator mod - the Nav ball is a bit small for my eyes, and that mod really helps.

Wemb