Large interplanetary spacecraft rendezvous maneuvering impossible with stock parts?

[Jason_25]

I am currently working on an interplanetary ship that I am running into maneuvering trouble with. The ship weighs about 50 tons once in orbit. It currently has 87 RCS jets and 4 large RCS tanks.

The maneuverability is not great even with all the RCS jets. Pitch and roll is ok but yaw is not. One problem is the amount of jets stuck to the side to increase yaw authority causes the ship to look extremely ugly. It is supposed to be a sleek ship and this makes it look like it has a disease. Do these RCS jets have any real world basis anyway? The shuttle RCS is integrated and aerodynamic and looks great for instance.

The RCS fuel usage currently is enormous. All 4 tanks are dry in about 30-45 seconds or so. As an aside to this, the RCS and or Mechjeb are extremely inefficient with fuel usage. Are there any real world designs that have the RCS and main engines share fuel? I understand the shuttle does not since the RCS it has uses hypergolic propellant.

The ship is able to maneuver to make burns just fine. Rendezvous is the problem. Specifically, there is not enough linear RCS authority to "line up" when you are closing on a target and the RCS retros are too weak and the ship is too slow to "flip" around so I can fire in reverse. Basically, fine maneuvering in it is extremely frustrating. Are there any low-maneuverability real world designs out there that are able to launch and maneuver for burns but not able to do rendezvous? Basically, should I resign the ship to the scrap yard for this problem or is it "safe" to fly to other planets anyway? Thanks for the help.

[Whatshtisthis]

I got a 66 ton space colony which turns, without RCS, well. It might help to have it roughly cylindrical.

[Jason_25]

Thanks for your reply. I am not willing to compromise that much on the design right now. That brings up another point that I wanted to make. That the ships can turn AT ALL without RCS is a pretty serious bug. I'm sure I don't have to explain why. I have not seen anyone else bring this up on the forums either!

[sanity]

87 rcs props?

youtre doing something wrong there, i have some ships in the 100 ton range with only 8 rcs props and they maneuvre fine, granted they're not fighter jets but no ammount of rcs is going to make a 100 ton + vessel respond quickly, its just to much mass moving to fast to slow down in time when turning around, take it slowly on rcs maneuvers.

only place rcs along your main axis, (left , right, up down) and always balance it around the centre of mass

ships can turn without rcs, or roll rather, not sure about turning really

Edited October 18, 2012 by sanity

[Rickenbacker]

Thanks for your reply. I am not willing to compromise that much on the design right now. That brings up another point that I wanted to make. That the ships can turn AT ALL without RCS is a pretty serious bug. I'm sure I don't have to explain why. I have not seen anyone else bring this up on the forums either!

It's been brought up a thousand and one times, and it's NOT a bug. It's simply the reaction wheels that each capsule has, that enables even RCS-less ships to turn a bit.

http://en.wikipedia.org/wiki/Momentum_wheel

Also, I think you're expecting a bit much out of RCS. If a ship was a few hundred meters long in real life, you would HAVE to turn it slowly, otherwise it'd turn itself apart.

[Accelerando]

Thanks for your reply. I am not willing to compromise that much on the design right now. That brings up another point that I wanted to make. That the ships can turn AT ALL without RCS is a pretty serious bug. I'm sure I don't have to explain why. I have not seen anyone else bring this up on the forums either!

It's not really a bug. The standalone SAS module is pretty powerful too, for a gyroscope... Kerbal RCS is very bad at conserving fuel, as you yourself have noted; most real-world designs accomplish what they set out to do with much less fuel, much less total maneuverability, and far fewer burns and touches on the throttle, both because the RCS systems themselves tend to be better built and programmed so as not to use up as much fuel stabilizing the craft, and because the spacecrafts' trajectories are thoroughly precalculated by computer tools that are also available to help when the time comes to actually perform docking.

KSP doesn't have any of those  at least not yet  and so in order to account for the possibility that inexperienced players (or those not inclined to carry out the mathematics outside of the game) will make many correction burns to sort out their trajectories, KSP affords players some leeway in the form of magic flywheels that provide much more significant torque than what is usually possible in real life.

At least in my estimation.

[Cykyrios]

As a note, the International Space Station uses 4 gyroscopes to keep its attitude relative to Earth, and these gyroscopes are almost 1 meter in diameter.

In KSP, gyroscopes are indeed way too efficient, but we won't complain about that

As for your problem, RCS is generally supposed to be used in short bursts, to start and stop a maneuver. Their use for rendezvous will necessarily drain RCS fuel quickly, especially with large ships.

Maybe you should mount a few engines on the sides (that you can decouple afterwards if you want) that fire backward, or even sideways, before you actually use RCS to fine-tune your approach.

[Temstar]

Why do you need your big interplanetary spacecraft to do docking anyway? Wouldn't the interplanetary spacecraft be the mothership and all the other little landers and tankers dock with it instead?

My interplanetary carrier isn't as big as sanity's. And even though it does have 12 RCS block I would not consider it maneuverable enough to perform docking. Any why would it need to? It just need to be able to transfer between planets. Docking capability should be built into the landers and tankers and spacecrafts that want to dock with it and not the other way around.

[Crater]

well, the tanker that I use to refuel my space fuel depot has two large RCS tanks, four large fuel tanks (it gets there with around 10,000 litres of fuel, that's 3+ full tanks), and a mainsail for thrust. I don't use the RCS much for docking, the tanks are there to refuel the station, for onward fuelling of other craft. It has 8 rcs blocks, but I just leave them mostly switched off, apart from for stabilizing after a turn.

I can dock it with the station manually - it isn't easy, takes around 30 minutes (but I'm not that great a pilot), just using the mainsail for thrust, gyros for turning, and rcs for stabilization.

So it sounds like there is something odd about your design if you are having that much difficulty handling your craft

[bsalis]

Not sure it will help much with rendezvous. However you should be able to conserve RCS fuel by using the Physics Warp to help turn the craft. Help as in to help deal with impatience.

[Martonaut]

use ordas auto pilot, he don't care about rcs fuel

[Punk]

They use gyros to stabilise mega-tankers and cruise ships on earth in rough seas. Granted they are several tons of spinning steel but for station orientation and small craft manouvering surely they are fine. I think use less rcs thrusters further from your central axis' (out on wings or the rcs booms from novapunch) and manouver the smaller ship to the larger one. They wouldn't fly the ISS to a supply craft.

Edit: Also can we see a pic of your craft?

Edited October 18, 2012 by Punk
request

[azazel1024]

I almost never use RCS for actual orientation changes. I use SAS or the capsules built in SAS for orientation changes (that or gimballing engines under thrust when doing big orbit changes sometimes, for my heavier ships). I only use RCS systems for linear translations and/or stability on landing to help keep the lander upright.

I did my first orbital rendezvous last night. I don't have the screen caps of it, but I do of the space station in question (Freedom Station I). The sucker is orbiting at 320-322km above Kerbin. It took me roughly 45 minutes from the time I had a succesful orbital rendezvous ship put together and tested to the time I was within transfer distance of the space station.

I don't have any of the docking mods installed (though I'll probably be giving them a try soon).

It took me around a day and a half of in game time to match the orbits and orbital planes up to get to 1.8km out from the space station and then I used used RCS for linear translations to park the space ship within 15m of the space station. I transfered a pair of Kerbals over and in the 5 minutes between that, doing some screen caps and some monkeying around (I got a really bad lag spike at one point so I had to get one of my Kerbals back after he had floated maybe 80m out or so. The other transfer was flawless) the ship and the station had maybe floated all of an extra 10-15m further apart. So relative motion was well under .1m/sec.

Anyway, it was my first time for any rendezvous. Next mission, placing a full up Mun base on the Mun. I have a little tiny one composed of a cosmetic radio dish, a since empty 1m module, some landing legs and a couple of lights that I managed to place there (it took shoving with a kerbal for about 15 minutes to get it righted though...thankfully it is light). This time I am going to get a real station placed there. It is roughly the size of Freedom Station I and I have the launcher built that should be able to manage the task, though I have to tweak the design since in testing after the SRBs had seperated and then the 1st stage dropped (cluster of 6 main sails with 4 tanks per engine) the 2nd stage was uncontrollable up around 60km. Not enough torque with no SAS modules added to prevent the left Yaw. Just barely enough to keep it from accelerating, but not enough to reverse it. So I need to go back and add a SAS module or two to add some extra torque, and add it closer to the center of mass.

As mentioned, don't manuever the big ship to a smaller one, do it the other way. I'd also rely on SAS and just take it slow. Plan well in advance for any attitude changes for manuevering.[ATTACH=CONFIG]34974[/ATTACH]

[sanity]

big ship maneuvers aren't really that hard, or even any harder than small ship maneuvers, you just have to go slow, remember that your not flying some fast responsive fighter but a large mass tanker, move slowly, take your time, its not really difficult

[Jason_25]

Thanks for the info on reaction wheels.

I have reduced the RCS jets down to 48, removed 2 large tanks and have added 2 sas modules. I have also adjusted my expectations in terms of craft rotation speed and rendezvous behavior.

Linear RCS does function now, though maybe still not enough for rendezvous. The RCS is not perfectly balanced either but the ASAS seems to correct well enough for that. That is, whenever it isn't trying to spin the spacecraft out of control.

As others have suggested, I am going to use other spacecraft to rendezvous with it. Just now I had a thought that maybe rendezvous would be easier farther out in orbit, say several thousand kilometers or so because of the lower speeds. I was attempting rendezvous with my light fuel tanker "Gaspipe" at about 75 kilometers altitude before.

I was also having trouble with the ASAS flipping the boost stage high in the atmosphere but I decided to just pilot it manually rather than go through a lengthy redesign process.

As a result of these developments, the prototype vehicle "Eclipse" has cleared initial safety testing and the extended mission as a trial run to one of Kerbin's moons and beyond is a go.

[Khrissetti]

Sounds to me like you're not placing the RCS jets properly. The nearer to the centre of gravity they are, the less force they turn with. If a jet producing 1N of force is placed 1m away from the cetre of gravity it will exert a turning moment of 1Nm. However, a 1N jet placed 100m from the centre of gravity will produce 100Nm.

place 6 RCS jets symettrically around the furthest points along your ship as you can get away with, that should be more than enough maneuverability.

Could be that you have so many RCS jets around your ship that the force one puts out is being cancelled out by one or two on the other side.