Taxi for bringing payloads to station?

[AaronLS]

TLDR, A craft that is just a very small engine, protobodobodyne, and docking port, which is used to go and retrieve a station module from low orbit, say 70km and bring it to add to the main station at 120km. Good idea? Bad idea?

1) Detach from station.

2) Burn to reduce orbit from 120km down to 70km

3) Rendezvous with new module

4) Burn to push orbit back from 70km to 120km

5) Refuel/repeat to bring future modules from 70km.

Getting a ~1t taxi from 120km down to 70km seems like it'd be almost nothing(based on experience pushing tiny satellites around), and then pushing the combined 5t craft back up to 120km seems like it'd be many times cheaper than the alternative, which is to push it with the 15t dry weight SSTO that originally put it in the low orbit(total 19t with payload).

Long story:

I've built an SSTO that's capable of putting one station module at a time into low orbit(~72km), so now I'm thinking about how to build the station in orbit. My SSTO would be barely capable of rendezvous each module with a 70km station, but it's dry mass is 14.9t(without payload or fuel).

If a module such as a lab is 3.5t, then it would seem more efficient to detach the module once stable orbit is attained, and let a small engine on the module get it the rest of the way there.

A side effect of my SSTO design is each module has a "nose cone fuel tank" which is a 2.5m to 1.25m adapter with a detachable cone. (Oh well, the station will double as a fuel depot.) So fuel tank is not a problem really.

But then every module needs its own controller, engine, RCS. (as well as a few other things like solar panels/batteries but those would be useful on a station).

Plus the engine will be centered somewhere that would likely be better as a convenient docking port location.

So it seems like it'd be easier to have a small craft dedicated to grabbing modules and bringing them up from low orbit.

What approach do others take?

If you wanna see my craft, this is it(linking to steam so hopefully will work). You can see the docking port on the nose cone, and there's another on the backend of the lab. The fuel tank with the two winglets are permanent fixtures of the module. There's a small storage bay where the SSTO's controller is mounted, and there'd be room for the small taxi there. So it could be delivered with my first module. I don't have access to larger storage bays yet, hence the nose mounted module...

[Grumman]

People do build orbital tugs to move their stuff around, but they tend to be ion or nuclear powered for their better Isp. An ion tug in particular has the advantage of being hooked up to a hunk of space station when it's doing the most work, meaning that you can augment the tug's solar panels and batteries with the station's own.

[LostOblivion]

Put the station at 70k? It's farther out than the ISS if you do the ratios of Kerbin and Earth. (Earth is roughly 10x Kerbin, so 420 km (ISS orbit) would be at around 42 km on Kerbin; and atmosphere would only be like 10 km!)

Might be a bad idea though if you need to catch up to it. :-)

[AaronLS]

you can augment the tug's solar panels and batteries with the station's own.

Good point. Was afraid ion might be painfully slow, but for something that's gonna have such a long term use it'd be good. It'd also minimize the chance of running out of fuel mid tug.

[delder92663]

I use a tug to move stuff from my LKO (80km - 90 KM) station to my HKO (350 km and out) stations. 70 km seems too low for a station. One little bump and you'll be kissing atmosphere. The fuel to get from 80 km - 120 km is so little I don't use a tug. I also use a tug to take probes to from LKO station to Mun and Minmus.

[problemecium]

I build "space tugs" for basically every KSP version I play, and in my case they tend to be RCS-only since they're only making small orbital corrections to carry stuff, so precision is more important than fuel efficiency (and with some practice rendezvousing, I've realized that the game makes it easy to provide massive amounts of extra Monopropellant).

These were for 0.90, but if you're in Sandbox Mode they should still work fine: http://forum.kerbalspaceprogram.com/threads/68977

[AaronLS]

70 km seems too low for a station.

I agree. The station is at 120km. The ~72km orbit is just the initial insertion orbit for a new module.

[Gargamel]

Put the station at 70k? It's farther out than the ISS if you do the ratios of Kerbin and Earth. (Earth is roughly 10x Kerbin, so 420 km (ISS orbit) would be at around 42 km on Kerbin; and atmosphere would only be like 10 km!)

Might be a bad idea though if you need to catch up to it. :-)

Sooooo..... you're recommending a suborbital space station?

[adinfinitum]

I would definitely recommend an orbital tug, another benefit being that you end up with less parts on your lifting rockets, because all the maneuvering equipment is already up in space. Less parts means less weight, which means cheaper launches, if you're playing career.