Interplanetary Tug Design

[spudman2]

With the new changes to the nuclear engine, I seem to be unable to pull off any of my old feats with it. (Duna SST-Kerbin). I've realized that the nuke should be used as an interplanetary tug, that just pushes a chemical lander back and forth. What designs has everyone else put into action for their reusable nuclear tugs?

[juanml82]

A large central fuel tank, usually 3.75m or MK3, typically with an adapter to 2.5 for control probe, RCS, batteries, reaction wheels and a docking port. A heat shield goes beneath the central fuel tank

Four radially attached MK1 fuel tanks with the nukes beneath them and nose cones for take off. Depending on the mission, I may have extra docking port on those MK1 fuel tanks or use them for solar panels - but if this is going beyond Duna, I just attach a bunch of rtgs to the central tank.

[Red Iron Crown]

Here's my standard 4-LV-N tug from 1.0.2, the Mule:

I should update it for 1.0.4, the Mk1 fuselage has a better mass ratio now.

[Snark]

If you're used to using LV-N's pre-1.0, the two big changes that will affect ship design are:

1. Much increased mass, it's 3 tons now. So watch out if your design has several LV-N's on it. You may want to get by with a lower TWR than previously (i.e. put fewer LV-N's on). Otherwise, the increased mass of the engines could offset much of the benefit from having higher Isp.

2. The need for LF-only, instead of LFO, means making new decisions about fuel tanks. Mk3 has some decent LF-only tanks, or you could go with an approach like Red Iron Crown's above.

[Xannari Ferrows]

As weird and totally boring as it might sound, I actually use ion tugs, but only for Hydrogen delivery units.

[imaspacestation]

If it only tugs between planets/moons, I don't see how anything but the most simple designs would be of any benefit. Slap on as many engines as you can bear (the more, the less burn time), enough fuel for the intended destination and payload, and RCS that's balanced for the tug itself.

Now if the tug was used to perform orbital construction or any other kind of maneuvers, then you have a bigger challenge - your RCS would need to be balanced with the extra payload mass in mind, and that's no easy feat. In those cases, the way I usually do it, is the tug's RCS is balanced for itself, and then when I design the payload in the VAB, I load up the tug as a subassembly and attach their docking ports - then I add RCS on the payload in such a way as to balance them together. The payload does not need any RCS fuel, only the RCS thrusters. That way when they are docked in space, they are balanced for maneuvers. And if you don't want the RCS left over after construction is done, you can place them on pylons or octagonal struts attached to small decouplers that you can jettison at the end of construction.

[spudman2]

Ahh, I see, these are good ideas. Red Iron Crown, what is that prograde-pointing nozzle for? How much Dv does a full tug have by itself? With a 20t payload?

[CalculusWarrior]

Ahh, I see, these are good ideas. Red Iron Crown, what is that prograde-pointing nozzle for? How much Dv does a full tug have by itself? With a 20t payload?

I'm not Red Iron Crown, but I'll answer your questions. The 'nozzle' is actually a light, presumably there for assistance when docking.

From my calculations, the delta-v a full tug would have is roughly 5700 m/s, and with a 20t payload, just shy of 3000 m/s. I assumed the tug had four tanks and engines.

[Red Iron Crown]

Ahh, I see, these are good ideas. Red Iron Crown, what is that prograde-pointing nozzle for? How much Dv does a full tug have by itself? With a 20t payload?

The little nozzle is a light to help with dark docking and for remembering which way is up. The tug itself has ~5.4km/s of dV when unloaded, with a 20t payload it should be ~2.8km/s. Here's the craft file if you'd like to mess around with it (disclaimer: not tested in 1.0.4).

Edit for the ninja above: I think the numbers are a bit lower than you expect because there's some utility parts hidden between the LF tanks, an RTG, battery and monoprop tank.

[CalculusWarrior]

The little nozzle is a light to help with dark docking and for remembering which way is up. The tug itself has ~5.4km/s of dV when unloaded, with a 20t payload it should be ~2.8km/s. Here's the craft file if you'd like to mess around with it (disclaimer: not tested in 1.0.4).

Edit for the ninja above: I think the numbers are a bit lower than you expect because there's some utility parts hidden between the LF tanks, an RTG, battery and monoprop tank.

Agh! Your part clipping is messing with my idealized equations!

[ajburges]

Agh! Your part clipping is messing with my idealized equations!

Some helpful tips:

Design your tug to be propulsion only with a docking port. You can use the port to attach any extras. My standby SSTO cargo (for when I fly crew missions) is a dockable claw with chutes. Works great for recovery contract and limits the potential harm of claw use.

800 units of LF an engine is the break even point for mass efficiency vs chemical rockets. Design to at least that tankage and run light until you need that dV if possible.

Use more than one tug design. My current work horse is 2 Mk1 tanks, 1 engine, airbrakes, RCS, and a docking port. Can accelerate my RCS scan sats by about 4 km/s and has enough power for every recovery contact. My Minmus habitation ring OTOH is being delivered by a tug that has 4 times the engines (and fuel) with some extra fuel stored in the ring's structure. My true interstellar tugs will be multi section behemoths so I can get 80+ t accelerated by 4-5 km/s with â…“ initial TWR. (Pending requirements review)

[Phil deCube]

These are from an older version of KSP (0.90 I think). The first is a simple, three engined one, pushing a Duna lander stage on top of one of its two senior docking ports:

The other is two bigger, 12 engined beasts, pushing a manned asteroid catcher top stage:

[spudman2]

Okay, thank you all, I see what directions my designs should go now. I should have realized the light was a light; I've been playing for what, two years now?