Tug Depot

[G'th]

So lately I've been toying around with reusable rockets in KSP (because of dat SpaceX bandwagon naturally) and something occurred to me about reusable upper stages (namely in the style of a Falcon 9 upper stage). What if instead of expending them or devising some complicated method of bringing them back down, you designed your upper stages to have a use as an orbital tug once its delivered its payload? And then, simply had it meet up with an orbiting propellant depot to sit and wait until theres a use for it?

Obviously this would be mostly pointless unless you had good reason to require so many random tugs sitting around in orbit (Such as near constant satellite/probe deployment or re-supply to beyond LEO locations, whereupon they become expendable for obvious reasons), but I would think it would make for a potentially cost saving method of devising a reusable (or at least, a limited reusable) upper stage if there was a need for tugs. Considering the upper stages would be expendable anyway, and provided the additional cost of turning these stages into tugs post payload deployment was acceptable (which again, depends on how much of a use you'd get out of them), you might not even need to design them to be that necessarily reliable as the relative large number you'd have sitting in waiting would always guarantee a backup.

[Nibb31]

- Typically, upper stages aren't designed to loiter on orbit. Preventing boiloff would require storable propellant or cooling techniques, which eats into your payload fraction.

- You would also need a power source, either solar panels or some form of generator, which eats into your payload fraction.

- In order to reuse your tug, you're going to need docking hardware, including avionics, radar, thrusters, which eats into your payload fraction.

- You're also going to need to carry quite a lot more fuel (and empty tankage) for your extended mission, which eats into your payload fraction.

So yes, it could be done, but you would have a much less capable upper stage for its primary role, which is to put a payload into orbit. In real-life there is very little margin left once the upper stage is spent.

 

Edited May 16, 2016 by Nibb31

[G'th]

^ Indeed. And the more I think about it, it occurs to me that the idea made more sense originally because all of my KSP upper stages are typically overengineered and I end up throwing some 4-700 dV away.

[sojourner]

37 minutes ago, G'th said:

So lately I've been toying around with reusable rockets in KSP (because of dat SpaceX bandwagon naturally) and something occurred to me about reusable upper stages (namely in the style of a Falcon 9 upper stage). What if instead of expending them or devising some complicated method of bringing them back down, you designed your upper stages to have a use as an orbital tug once its delivered its payload? And then, simply had it meet up with an orbiting propellant depot to sit and wait until theres a use for it?

Obviously this would be mostly pointless unless you had good reason to require so many random tugs sitting around in orbit (Such as near constant satellite/probe deployment or re-supply to beyond LEO locations, whereupon they become expendable for obvious reasons), but I would think it would make for a potentially cost saving method of devising a reusable (or at least, a limited reusable) upper stage if there was a need for tugs. Considering the upper stages would be expendable anyway, and provided the additional cost of turning these stages into tugs post payload deployment was acceptable (which again, depends on how much of a use you'd get out of them), you might not even need to design them to be that necessarily reliable as the relative large number you'd have sitting in waiting would always guarantee a backup.

You've just stumbled upon ULA's Aces upper stage concept:

https://en.wikipedia.org/wiki/Advanced_Cryogenic_Evolved_Stage

[magnemoe]

50 minutes ago, Nibb31 said:

- Typically, upper stages aren't designed to loiter on orbit. Preventing boiloff would require storable propellant or cooling techniques, which eats into your payload fraction.

- You would also need a power source, either solar panels or some form of generator, which eats into your payload fraction.

- In order to reuse your tug, you're going to need docking hardware, including avionics, radar, thrusters, which eats into your payload fraction.

- You're also going to need to carry quite a lot more fuel (and empty tankage) for your extended mission, which eats into your payload fraction.

So yes, it could be done, but you would have a much less capable upper stage for its primary role, which is to put a payload into orbit. In real-life there is very little margin left once the upper stage is spent.

Yes, add that the upper stage is mostly designed to get you up to orbital speed, its overkill for taking satellites to GEO or even interplanetary.
Might make sense for an manned Moon mission using two falcon heavy, first you launch an empty upper stage as an fuel depot then another upper stage with the lander, dock the depot and fill up the transfer stage before going to the moon, note that the only benefit here is that you can use two rockets who is too small for an manned moon mission rater than an sls or saturn 5.

An dedicated tug with nerva or an more advanced engine like vasmir makes more sense. satelite and fuel tank to LEO dock with tug who then get fuel from tank, take satellite to GEO and return to LEO and drop tank. This makes sense if the tug has far higher ISP than the upper stage. 

In KSP tugs makes sense as you can refuel them with mined fuel from minmus.  

[Basto]

I think the biggest barrier is going to be plane. Plane changes are extremely costly so in order to make this viable you would either need multiple holding stations at various planes or only launch things into a single plane. 

 

[wumpus]

The only obvious "space tug" I can think of would be something like the Dawn spacecraft.  It used an ion engine with 10km/s delta-v, enough to move a few of satellites [assuming they weigh less than Dawn] up into GSO (if it was feeling cheap it could probably do some fancy flying to get back to LEO at low cost.  Or perhaps move them to GTO and aerobrake back to LEO).  Two reasons this isn't done:

A:  These engines are even slower than they are in KSP.  The interest on the satellite costs even more than 4000m/s of fuel launched to LEO.

B:  Nobody wants to have their expensive communications satellites hang around in the Van Allan belts for months if not years.

Note that plenty of satellites already use ion thrust for station keeping and that most GSO birds appear to use station keeping engines for circularization (the launch vehicle sends them to GTO).  So presumably if the bird's owners wanted to use ion propulsion to get to GSO, they would simply buy a ticket to LEO and then do the rest themselves (note this probably requires much bigger thrusters and solar panels than normal, but technically is an option).  I'm somewhat curious that ion circularization appears to be standard practice.  Presumably the delta-v is low enough to get done quickly, and the other options are tricky (hyrdrazine or low-boiloff LH2 systems).

Edited May 18, 2016 by wumpus
hang not hand

[AngelLestat]

I guess a tug should be light, low thrust with high isp. If they are manned, the thrust should be much higher than today ion engines but no so much like many merlin engines.

So even if you adapt these second stages to fulfill this role, I don't see much benefits vs launch a special tug to fulfil that purpose. Yeah, you save the deorbit fuel that can be used in solar panels but does not seems enough. But I give you a point for the idea.

 

[Rakaydos]

A better use might be to repurpose the O2 tanks as habitable construction material.

[kerbiloid]

And 30 years later we get a New Generation Orbital Station:

 

Spoiler

 

Edited May 17, 2016 by kerbiloid

[Admac]

If the stage 1 reuse costs get low enough it might make sense the throw up a company owned launch of a specialized stage 2 spacecraft that gathers up a half dozen regular stage 2s, deploys a big heat shield and land them somewhere, maybe even propulsively.

The gathering them up might be a nightmare, assuming they are all on their own LEO or GTO orbits with different inclinations. Probably more economical than setting up a landing system for each one, but you'd need more advanced space robotics than exist now to get them put into a single vehicle. It would be incredibly awesome though, so maybe they'll run the numbers!

I could do it in Kerbal with docking ports, but I have a feeling the real world is less forgiving...

[James Kerman]

I think this could be feasible for preventing Kessler syndrome - use the tug to de-orbit a defunct satellite each launch.

[Bill Phil]

It's doable. But your regular rocket engine these days is pretty hard to start again. We can add in a few restarts, but they would all need refubishing after they run out. Unless we use pressure fed engines.

[magnemoe]

8 hours ago, Bill Phil said:

It's doable. But your regular rocket engine these days is pretty hard to start again. We can add in a few restarts, but they would all need refubishing after they run out. Unless we use pressure fed engines.

Rocket engines are tested before use so they can be started multiple times, however most first stages can not be restarted in flight as its pointless anyway. 

[Bill Phil]

3 hours ago, magnemoe said:

Rocket engines are tested before use so they can be started multiple times, however most first stages can not be restarted in flight as its pointless anyway. 

Depends on the type of testing. 

They might be testing the geometry, or the flow, or it could be a static test. They're not necessarily testing all components. Starting a rocket engine involves quite a few things, spinning up the Turbopumps, opening valves, providing heat sources to incite combustion...