Modular interplanetary vessel physics questions

[Mister Kerman]

Hey fellas. I have a question about placement of engines, stability enhancers, and any other tips you guys feel like sharing when designing and constructing interplanetary vessels in sections.

I'm not in any kind of shape in my space program to attempt something like building an interplanetary ship, but when I do I'm going want to know a few things about it. I plan to contruct it in seperate pieces in LKO with jumbo docking ports, so this is what I'd like to know:

What kind of relation to center of mass and thrust do I have to take into consideration when designing my ship? Is it alright to have a tug engine like a train or is it best to push from behind like a traditional rocket? And with each of those options where should my stability enhancers be?

Edited January 2, 2015 by Mister Kerman

[Postron]

A puller design is definitely superior to a pusher design because the pusher tends to make your ship bend (and break) while the puller pulls your ship straight (imagine trying to push/pull a spaghetti from its ends).

Regarding SAS modules, I'd try to use as few of them as possible, sure it's gonna make turning excruciating but at least you won't have to worry about your falling apart due to too much torque applied from digital on/off buttons (maybe that's a different story if you use a stick to fly).

[Axelord FTW]

Puller designs are usually much less wobbly, which, if you have a relatively high TWR, can be important.

Disabling gimbals on puller engines is a must.

I've taken a liking to Near Future Technologies and Remote Tech; my long range unmanned missions are getting pretty hectic but I stick with pusher designs.

Kerbal Joint Reinforcement is a MUST!

[Empiro]

The consideration is basically the same -- make sure your ship has rotational symmetry or you'll have a tough time keeping it straight as you burn.

Using the large docking ports is a good idea since those have pretty high strength and resistance to flexing. You're still likely to get lots of wobbling though. The key to turning such a craft is patience. Turn off SAS, use RCS, turn on fine-control mode. Only fire the RCS briefly and let your ship slowly drift to the right orientation. If your ship still wobbles too much, you might need a mod like Kerbal Attachment System that lets you add struts in space, but personally, I've never had issues with just being careful and gentle with the turns.

Puller designs work well if your ship is very long, and will reduce flexing. Make sure that the exhaust doesn't hit any part of your ship, however. One piece of advice is to turn off the gimbals on the engines of the tug (and any engine that is in front of your center-of-mass). If you don't, the engines will gimbal in the wrong direction and steer you away from where you want to turn. This bug/quirk might be fixed at some point, but is still present as far as I know. This means that you'll need to have several SAS modules or use RCS to maintain altitude control.

[Mister Kerman]

So make a tugger with SAS on the front and possibly back with gimbal deactivated.

Seem good?

[Skorpychan]

Example:

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I make them in segments, with a Sr docking port at each end, standardised to a certain size. This was designed before we had the Kerbodyne parts though, so future efforts will have a much wider tug.

[Signo]

Hi, if you assemble it in space a puller is definitely better, but if you make just one single big launch you can afford a pusher too. Moreover, the heavier the ship the less strict you are with "simmetry". I made quite a big asymmetric pusher myself where I usually have very different radial loads.

[Mister Kerman]

How's that thing on gas? =3

[Jason Patterson]

So make a tugger with SAS on the front and possibly back with gimbal deactivated.

Seem good?

Just in case you haven't experienced the fun yet, rocket exhaust can be blocked and/or destroy parts that it hits, so keep that in mind when you build your tug.

[cantab]

Engines anywhere can work really, but ensure their exhaust plumes are well clear of other parts or you'll get no thrust from them. That's not an issue for rear-mounted engines but ones mounted forwards or amidships will need putting on outriggers.

Try and make the "chain" of docking ports as short as possible. For example, consider docking your various modules around the main drive section all directly to it, instead of one to another all in a line.

On the other hand, be wary of docking heavy modules by a single port on their side. The forces will make them bend backwards and in extreme cases can destroy the ship.

If you have an engine cluster, you can use thrust limiting to compensate for an off-centre Centre of Mass. This really increases your design freedom, there's no need for perfect symmetry.

[Mister Kerman]

If you have an engine cluster, you can use thrust limiting to compensate for an off-centre Centre of Mass. This really increases your design freedom, there's no need for perfect symmetry.

This is something I'll keep in mind. I've never been a fan of symmetry

[Signo]

How's that thing on gas? =3

The nuke core is supposed to stay at the destination as a gas station. The crew come back with a small single nuke.