Interplanetary tug sizing - how do I know?

[mabarry3]

I am trying to make an interplanetary tug using LV-Ns. How do I know if I oversized it or undersized it? I've never made a tug before, so not sure how big it should be or how high I should get the TWR. Also, can I presume I should use large tanks and clear out the oxidizer rather than using LF only tanks (which don't come in massive sizes)? Does what I have look adequate for getting around Duna/Ike (and other places, primarily Jool, in the future) and cleaning out its biomes of science, as well as being able to set up a return trip for every kerbal I plan on bringing?

Pic 1 is the tug itself, pic 2 is the tug with its cargo (to be docked in LKO) of 2 unmanned miner landers on the top and a kerbal lander on the bottom. I could switch that to one miner and one lander, each on one side of the tug, but there's an 11.4t difference in mass between the two so I'd have to balance it out first.

Edit: my staging was off so KER gave results that don't match. The TWR with a full tank while in LKO burning should be around .15(.20) rather than those numbers.

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Edited November 9, 2015 by mabarry3

[Snark]

How big the tug needs to be depends entirely on how big the thing you want to move is. So start with the intended payload and work backward from there.

Generally speaking, you want a pretty low TWR so that you're not wasting too much mass on those heavy LV-Ns. There's not really any such thing as "too low" from an engineering perspective, it's more a matter of how much patience you have to sit there through really long burns.

Using LFO tanks and leaving out the oxidizer is a really bad idea, you waste many tons of mass that way. Use the large LF-only tanks (there are some big ones in Mk3 format).

One thing you definitely want plenty of on a tug is torque authority-- I'd suggest adding a fair number of reaction wheels. Also make sure you have a reasonable distribution of RCS thrusters.

Having some lights that self-illuminate the area around the docking port comes in handy for when you need to dock in darkness (which is annoyingly often).

Be prepared for the fact that docking-port connections are very wobbly and bendy; this can cause a problems for tugs that are pushing big loads. Some ways around that:

1. Use larger-size docking ports for docking to big-size vessels; they're stiffer.

2. Use Kerbal Attachment System. They have some nice deployable struts: basically, "strut sockets" that are attached to the two ships involved. After docking, send a kerbal on EVA to connect them by right-clicking, and it puts struts in place to stiffen the connection.

3. Design your tug so that it pulls the load rather than pushing it (i.e. the tug's engines are way off to the side).

4. Just run the tug at really really low TWR so that it's pushing so gently that the bendiness doesn't cause a problem. This is very easy from an engineering point of view, but requires a lot of patience on your part during long burns.

Edited November 9, 2015 by Snark

[ForScience6686]

Use the rocket equation to determine fuel needed. The way I do this is to take the payload and attach the engines I will use on the tug to give me the dry mass. Them use the following equation dv=(ISP*9.81)*ln(1.125x+ dry mass/.125x+dry mass). Dv will be the amount required for your travels, then solve for x. This accounts roughly for tank mass as well as fuel. I use that as an estimate, and design with drop tanks to improve the numbers.

[Warzouz]

Tug is not trivial to build

Here is how i do it

1- I Fully build my payload. The payload itself can have to dV

2- I Add a core engine block which will provide most of thrust except escape *

3- I Add booster because I don't like low TWR when quitting Kerbin (those boosters are usually heavy). *

4- Check dV with KER,

5- build fairing around payload

6- Add a launch to LKO stage (one of my recoverable ones)

* For most target (except Moho), I only use one Rhino and a one tank. I don't rely much on LVN

As Snark said, you should pile up some reaction wheels or Vernors or your ship will be a pain to move.

Here is a 5700m/s tug to push a station of 70T around Moho (in addition to the 900m/s dV available in the station itself).

It has a 8 LVN (in addition to the 4 station LVN which can provide thrust in that configuration) and 2 rhino (because I don't like low TWR to go to Moho).

http://tof.canardpc.com/view/017d2bde-716d-4ca8-a3b2-050147fd4dff.jpg

And Eeloo with a simplier interplanetary tug just ditched

http://tof.canardpc.com/view/bed0d9ca-1299-4584-b1c1-611849fead44.jpg

[mabarry3]

Didn't know there were large (1000+) LF only tanks. I also haven't unlocked mk3 parts yet, so that's probably why. I guess I'll wait until that becomes available to launch this tug.

I'll add some lights and a fair amount of reaction wheels and RCS engines; those are in progress right now. My payload should be about 100/155 tons depending on whether I bring 1 or 2 miners along with. I'll probably take it down to 1 and balance out the weight. I'll probably end up dropping the TWR down to about 0.1 but I don't have the largest patience for long burn times (that may change as I play more)

For using LV-Ns: How many radiators do I need to run nuclear engines for long burns? Just as a general idea of active radiators:engines.

I'll adjust the tug to a very wide base on the top and I'll probably change the docking ports location to the bottom of the rig to reflect that, as well as to make it so payload is pulled rather than pushed. (that's also a more convenient location, anyway)

As always, thanks for the help. I'll make some changes later today or tomorrow and see if they look more closely to what a stable tug "should" look like.

[ForScience6686]

I use the small lf tanks, that way I can shed dead weight faster than with mk3 parts. I've found using one small radiator per engine to be effective, but I also attach engines to tanks for more heat absorption. Other than that, make sure it's built for the mission. A craft I sent to duna I accidentally put the non retractable panels on, but also designed for aero breaking, so I'll end up losing those and may need to pop my fairing early to access the panels on the payload.