Balancing the final stage for tank delivery to a space station

[redneon]

I'm building a refuelling space station. The space station is in orbit (design shamelessly stolen from someone on here)...

...and now I'm building a rocket to deliver and dock the fuel tanks to it.

The problem I'm having is that I don't want to put RCS rockets on the delivery tank itself which means I need to balance the final stage of the rocket so the centre of mass isn't on the delivery tank. That way I can position RCS rockets around the heavy end. Currently I've been looking at something like this:

This will work, I'm sure, but it's major overkill to use two orange tanks with a poodle on it just to counterweight the delivery tank at the end. I did preliminary tests launching like this...

..and it does look like it'll work but it required a metric-ton of struts to stop the "shaft" from destabilising. I can't help thinking there'd be a better way.

[SpaceGremlin]

You don't need to counterweight your payload like that, just fly with unbalanced RCS and add in some reaction wheels to muscle it around.

Or if you're dead set on keeping your RCS balanced, you could save a lot of weight by running some girders from your orbital insertion stage up alongside the payload and placing RCS on that. You can then strut the payload down from the girders to stabilize it.

[redneon]

Ah, reaction wheels. I'm not familiar with them. I'll have to do a bit of reading. I was under the impression that to keep the ship balanced when translating using RCS you had to position them evenly around the centre of mass.

[SRV Ron]

A design like this, but with a 2.5m control module and RCS placed where it can be staged and return to Kerbal after docking. Easier to have the dual pair of thrusters that can be disabled after docking.

Heavy lift fueler;

Image to be posted later since Imgur is frozen.

Edit, finally Imgur is unlocked. Here is the lifter for the big fuel can to orbit;

And the final stage in mun orbit;

I would recommend having a second controller on the last stage of the launch vehicle with parachutes so it can be reused. Second recommendation is to keep the thruster and control pack on the fuel can so that when it is emptied, you can undock it and return it to Kerbal for reuse.

Edited January 30, 2014 by SRV Ron
to add pictures

[redneon]

I'm confused. According to the Wiki, the command modules automatically have reaction wheels now. So why do I need to add them? At the minute, if I go up with one bank of four RCS thrusters around the delivery tank then when attempting to translate it ends up rotating around the centre of mass.

[KerbMav]

I'm confused. According to the Wiki, the command modules automatically have reaction wheels now. So why do I need to add them? At the minute, if I go up with one bank of four RCS thrusters around the delivery tank then when attempting to translate it ends up rotating around the centre of mass.

They have, but are less powerful then the dedicated parts.

You basically answered your question yourself.

To translate more or less without any rotation, you can use reaction wheels to some extend, but the better way is to place two banks of RCS thrusters with equal distance to the center of mass (COM) of your craft - try placing them relative to the average of the COM when tanks are full and empty, you can try this since .23 added the ability to empty your tanks in the VAB.

Sorry, misread your question, you want to deliver a tank without pockmarking it with thrusters.

Have you tried building your lifter/the last stage around your cargo?

Upper stage engine below wide but short tank.

Attach smaller, longer tanks around it up along your cargo, feeding fuel into the tank below.

Let gracious space between the tanks and your cargo - remember struts disappear when decoupling/undocking.

Edited January 30, 2014 by KerbMav

[Rickenbacker]

This happens when your thrusters aren't centered around the CoM.

The easiest way to balance things, is to build the part that will dock with the space station first, then turn on the CoM marker and place the thrusters. Two four-packs of thrusters, equidistant from the CoM, is usually best, and they don't weigh that much compared to a whole tank of fuel. You can also play around with the amount of fuel in the tanks, if you expect to use any of it before docking.

Reaction wheels can't be used for translating, only rotating. And they're so weak that they can't really compensate for much off-center thrust. Better to balance things from the beginning.

[redneon]

Yeah, I know about the CoM thing. I mentioned it in the original post. The reason I was asking about the reaction wheels is because SpaceGremlin said I don't need to counterweight, I can use unbalanced RCS with reaction wheels.

[KerbMav]

You can also play around with the amount of fuel in the tanks, if you expect to use any of it before docking.

Interesting idea, pump the fuel out of your cargo tank into the delivery stage tank and pump it back after docking. This will not reduce rotation to zero, but maybe it will be manageable (sp???).

Alternatively you could attach a docking helper probe to the top of your cargo with two rings of thrusters - build your delivery craft with two rings also for balance. When you are lined up for docking, quickly undock (you should not use decouplers here!) the probe, fly it ayay and simply push the cargo the last few meters.

I can use unbalanced RCS with reaction wheels.

Kill gravity with ALT-F12 on the VAB and test your cargo stage without the need to launch it. Let us know how many reaction wheels it took to stabilize the craft while translating.

[SRV Ron]

I'm confused. According to the Wiki, the command modules automatically have reaction wheels now. So why do I need to add them? At the minute, if I go up with one bank of four RCS thrusters around the delivery tank then when attempting to translate it ends up rotating around the centre of mass.

If just delivering an orange fuel can by a remotely controlled launch, the built in reaction wheel is way too weak for the job of launching such a payload. Best to design the payload as follows;

1. docking clamp.

2. jumbo fuel can

3. decoupler.

4. reaction wheel

5. remote command module for 2.5 meter payload.

6. RCS tank

7. 2 sets of thrusters, one in the front and one in the rear.

8. Parachutes, optional, to return to Kerbal and recover for reuse.

Have enough launch vehicle to place the payload into the desired orbit, then return it to Kerbal. (Optional, place a command module and parachutes on the booster for controlled return and recovery.) Use the thrusters on the payload for final interception and docking to the station. Once docked, stage the command module and return it to Kerbal.

Option, take it a step further. Leave the command module connected to the fuel can. When the fuel can is empty, undock it and bring it back to kerbal using thrusters for a controlled landing and recovery.

Edited January 30, 2014 by SRV Ron

[KerbMav]

Last tip: Install KAS and remove the no longer needed thrusters on your next EVA.

[Kasuha]

I definitely vote for using unbalanced RCS. To handle one orange tank, you may not need any extra reaction wheels (or maybe one). The trick is to phase all RCS bursts to two - initial burst which will get the tank moving and rotating, then turn RCS off, watch torque compensate, and then turn RCS on an do another burst when the tank is in counterrotation. That will make it stable and moving at the same time. Use short bursts, not long ones. That will give reaction wheels time to compensate.

I remember building a large orbital complex consisting of long girders with just a tiny probe and there was no problem at all. Of course orange tank is heavier than that, but your tug is also going to be heavier than that probe.

[Claw]

The problem I'm having is that I don't want to put RCS rockets on the delivery tank itself which means I need to balance the final stage of the rocket so the centre of mass isn't on the delivery tank. That way I can position RCS rockets around the heavy end.

Like you, I don't like putting RCS thrusters on my delivered orange tank. (Although I've never thought about why...)

In any case, I have found that the large Advanced SAS module helps a lot, but doesn't solve all your worries if you have nothing attached to the bottom. I also send mine up unmanned, but it's all what you like.

Under my payload I connect via a Sr docking port for smooth disconnect. Under the docking port, I place the reaction wheel with a set of four RCS thruster blocks. Under that, I have the large RC-L01 remote guidance with spherical RCS fuel attached. Then under all that I have my rocket's upper stage.

So docking this I find that the reaction wheel is enough to help stabilize it, but it might still wobble (trying to rotate) a little when translating. Sometimes it'll take a burst of forward/backward RCS to kill unwanted drift. You can also briefly turn off RCS to use reaction wheels to kill rotation when it gets out of hand, then turn it back on. Alternatively, as someone suggested, I have also transferred fuel from my payload to the upper stage to balance CG, then moved it back after docking.

Bear in mind, reaction wheels are the weak link in the rocket stack. If you are accelerating to hard the SAS module will break, much to the surprise of your poor kerbals. When I deliver anything larger than a single orange tank, I move the reaction wheel to the top of the payload, which isn't ideal either but works.

EDIT:

I definitely vote for using unbalanced RCS. To handle one orange tank, you may not need any extra reaction wheels (or maybe one). The trick is to phase all RCS bursts to two - initial burst which will get the tank moving and rotating, then turn RCS off, watch torque compensate, and then turn RCS on an do another burst when the tank is in counterrotation. That will make it stable and moving at the same time. Use short bursts, not long ones. That will give reaction wheels time to compensate.

Yes, this is more eloquent than how I put it.

[WhiteKnuckle]

I have to ask, why are you against having RCS thrusters on the tank?

Personaly I like my LKO tanks to have thrusters and a small RCS tank on them so I can de-orbit them once they're empty without needing to use a tug.

[radonek]

I'm confused. According to the Wiki, the command modules automatically have reaction wheels now. So why do I need to add them?

Builtin reaction wheel do not have much torque. Full orange tank is heavy, thus to counterbalance rotation from being pushed off-axis, you will need A LOT of torque. I'd say this is bad way to do it. If you really really need this, add rcs thrusters where you need them on assembled station, then add another set to counterbalance it. After you dock, simply turn-off counterbalancing ones. If you ever need to redock, you will find them handy anyway. Or, you can do better by desiging your station as whole, launch it with empty tanks and refill them later from tankers.

But, you are IMO on bad track all along – you do not push space stations around anyway. They are by definition a passive side during docking.

[redneon]

Thanks everyone. I'll give some of these a try later and see how I get on. I'm still learning so much about this game so it's all helpful

I don't really know why I'm against having RCS thrusters on the delivery tank. As you say, deorbiting would be much simpler with them, should I need to.

[the_bT]

I did it like this the last time I build a space station:

(Was a while ago hence the asas module, you can switch that for a battery i guess)

Once you are in a stable orbit and decouple the launch vehicle, you simply flip the thing around and control from the docking port. The I-Beams can also be strutted to the launch vehicle to increase stability. One thing I added later on was unfolding solar panels on both sides (like wings) and an antenna on the top as visual orientation references. That was after I almost crashed 40 tons of fuel into my habitat because I was flying upside down and didn't notice

[Rickenbacker]

If you really, REALLY don't want thrusters on your tank, it'll be much harder. I have done it, by using a tug at each end (basically a docking port, an RCS tank, a probe body and a few symmetrical RCS thrusters in the middle). Once the tank was lined up perfectly, I detached the "front" tug, and used the rear one to push the tank in to dock. It was really not a very neat way to do it, but it kinda worked . Like you, I did experiment with tugs with more or fewer RCS thrusters, but no amount of reaction wheels can keep an orange tank straight when you only have RCS on one end, in my experience.

The simplest way, if you can do it, is to use the last stage to deliver the fuel tank to the station, so that the fuel tank stage is exactly the same weight as it was when you designed it in the VAB. That way your balanced thrusters will still be balanced and you don't need to move fuel around. You can then use RCS for the approach. When I do this, I also like to put a probe body on the last stage, so that I can move it away from the station and deorbit it safely when I'm done.

Not sure what the deal is with the reaction wheels. I normally put one on the last engined stage, to help it turn for orbital maneuvers, but the tank bit doesn't need reaction wheels as long as the thrusters are put on symmetrically and no fuel is used.

Edited January 30, 2014 by Rickenbacker

[WafflesToo]

I was going to suggest something similar to Rickenbacker and use a robot with a quad of RCS thrusters to balance it front-to-rear. It can just live there at the station when not being used to guide fuel tanks into docking ports.

Another idea would be to pump fuel between the upper stage and the fuel tank prior to docking and then pump the fuel back into the storage tank once docked in place (assuming the upper stage motor would be empty by this point of course).

Edited January 30, 2014 by WafflesToo

[Randox]

I usually use a single tug to bring in the heavy loads. I don't send payloads directly to a station from launch, because by the time I rig the payload to be rendezvous capable I might as well just add docking capability as well. Instead, I use retrieval tugs which start at 33 tonnes (the standard for a tug that can go from Minmus to Kerbin and tow a payload back to Minmus). Since the tugs need refueling anyway, I top them up periodically to keep moving the CoM forward rather than backward.

I also normally keep a small RCS boat (just rcs thrusters attached to rcs fuel) which can move ship pieces around (constructing and arranging space stations or multipart ships) at fuel stations. Sometimes I attach it to the back of a payload for the last 100m or so to make things easier.

[Vanamonde]

You can put RCS thrusters on things even if you don't want to keep the thrusters. I often use the trick of placing then on decouplers,

which you can eject.

leaving no traces to mar the beauty of your ship.

[redneon]

Did it! I utilised the_bT's design and it was easy. Apart from the docking. I'm still having trouble with that. I always end up pressing the traslation for up when I mean down etc. Very frustrating but I'm sure I'll get used to it. Maybe I should try using the docking mode...

Here's a docking I just completed:

I do have one more question, however. I'm sure it's an incredibly stupid one that I should know the answer to but when the final stage is in space like this:

I can't rotate the craft unless I have RCS turned on or I'm using the engine. Why is this? I thought with the reaction wheels I'd be able to turn the ship in space? Or could it be that the remote controlled pod has run out of electricity, maybe? I do have a rechargeable battery on the ship, though, and that must be powering the RCS.

[KerbMav]

Go to settings (main menu only I think?) and change the keys to your liking - remember to do it again after an update!

It is very dark, so I cannot tell if you have an extra reaction wheel module on your craft. But if you do not, then the probe core alone may not have enough torque to rotate the heavy tank.

Also, if you have SAS turned on, as soon as you release the key it will try to keep its orientation - either keep the turn button pressed or (more power effecient*) turn of SAS to keep rotating until you are close to your desired orientation.

*While SAS tries to keep your crafts orientation it will draw power for the reaction wheels - and you will use power for the time you press the turn keys - so while you and the SAS fight for who gets to steer your craft you will be wasting power. (Not a big issue with enough batteries, solar panels etc. but can be problematic in career mode during the first missions.)

RCS does not need power to function, but without power your probe core will no longer be able to steer the ship. With a Kerbal on board you can fire your RCS tanks dry without any power at all, but reaction wheels wont work.

Anyway, you have no Kerbal on board and can still use RCS, so you still have power, therefore I am guessing it is your SAS fighting your control input.

[radonek]

Stay clear of docking mode – it's useless. Swapped control keys are camera issue. Your keys are fixed, but default camera is not. Switch to chasecam mode – camera stays fixed relative to your vehicle, thus translation keys remain same.

[Kerba Fett]

I don't like to have thrusters on stations either. They're just a problem waiting to happen.

I've used 2 different approaches. One is to put a pair of girders on the delivery stage that give me a mounting point for the RCS. I use a short girder to come out from the hull of the delivery vehicle then a long one running parallel to the module I'm delivering. This way you can mount the RCS in line with the COM and strut the payload to the girders.

You dock with the station, separate the delivery vehicle and back away from the station. The delivery vehicle takes the girders and RCS thrusters with it.

The other thing I've done is to put radial decouplers in line with the COM and mount the RCS thrusters on those. Once you dock you decouple them and let the space junk float away. Of course then you've got the decoupler mounting hardware left on your station. (they all leave little stubs after you decouple)

Hadn't seen the_bT's design when I posted. I like it. Looks much cleaner than the girders at 90' I was using.

Once your station is built you can just refuel those tanks rather than swapping them out. Your refueler would be pretty much the same ship minus the I beams. You probably should add some lights though. Sooner or later you'll find yourself wishing for them.

You might also want to add a marker so you can tell which is the top of your ship. I use the friendly green light of my MechJeb module. While the chase view works well to keep the top of your ship pointed to the top of your screen, I find it makes the camera a bit too twitchy for my liking.

If you're OK with mods, a couple that make this a lot easier is the Docking Alignment in Indicator by navyfish and MechJeb. I've never tried the docking autopilot in MJ, but the attitude holding features make manual docking much easier. It's also easy to make custom windows to show you info like closest approach distance and relative velocity at closest approach. Some of it's default windows can be a bit cluttered with info so its nice to make your own with just the info you want to see.

One other thing that took me a while to learn is that the distance readout to your target is to it's COM. (labels are F4) If you want an accurate distance between the docking ports the docking alignment plugin does it and also has a more accurate readout of your approach speed.

http://forum.kerbalspaceprogram.com/threads/12384-PART-0-23-Anatid-Robotics-MuMech-MechJeb-Autopilot-v2-1-1

http://forum.kerbalspaceprogram.com/threads/43901-0-23-Docking-Port-Alignment-Indicator-%28Version-3-0-Updated-12-18-13%29

Edited February 1, 2014 by Kerba Fett