Space tug, refuelling crafts - where are they?

[kiwiak]

What is stopping space agencies from developing space tugs and tankers for unmanned satellites?

Isnt it worth a try? Tugs woudl allow to use smaller launch vehicles for regular sattelites and simplify their design allowing to get rid of propulsion systems. Tankers woudl lenghten life of sattelites on LEO.

And there are also theoretical maintenance crafts though doing it remotly probably woudl be much more difficult than just docking and refueling.

[Nibb31]

I think it's a matter of trade studies not making it worth the effort.

Let's take your run-of-the-mill GEO comsat. It uses an upper-stage (Fregat, Briz-M or the Ariane EPS/ESC) as a tug to take it from LEO to GEO. These tugs go up with the payload, are relatively light, and standardized to reduce production cost. Once they have done their job, they are jettisoned and that's it. For example, the Ariane EPS weighs 1.25 t empty and carries 9 tons of propellant to put two comsats into GEO.

Now, if you wanted to reuse one of these as a reusable tug, you want need to add some sort of standard docking interface, RCS, and a more complex avionics package, similar to the ATV for example. You would also need to bring it back to LEO for the next mission, which means that it would need twice the delta-v, and therefore over 2.5 times the propellant to do the same job.

Now to refuel it between two jobs, you would need to send up a tanker with 25 tons of propellant to refuel your 1.25t tug. However, your tanker itself needs propulsion, docking hardware, avionics and attitude control, to be able to dock with the tug, so basically it will be quite similar to an expendable upper stage itself.

So your reusable tug saves you 10 tons on the comsat launch. However, it requires a separate 25 ton to launch the reusable tug, and another 25 ton launch to refuel it for each mission. In addition, your comsat still needs attitude control, propulsion, and docking hardware to be able to rendez-vous with the tug. You've also introduced a lot of mission complexity, mission control work, and lots of possible failure modes.

[Nibb31]

Oh, and ULA has a proposal to develop the ACES upper stage into a tug/fuel depot infrastructure, which is cool. Again, I'm not sure how feasible it is and there isn't any real requirement from NASA to go forward.

http://en.wikipedia.org/wiki/Advanced_Common_Evolved_Stage

[kiwiak]

Thank you for this reply.

What about just refuelling LEO satellites with their own propulsion system, to extend their station keeping capabilities?

[Themohawkninja]

Pumping fuel in microgravity via docking has engineering issues associated with it. I do know that NASA, or at least a private space company is working on in-orbit fuel transfers (I'll see if I can find the video), but the reason why it hasn't gotten to the point of KSP yet, is just due to (A) lack of demand, and ( engineering/physics issues.

EDIT: That other post was meant to be an edit of this one. I don't know why it posted by itself.

Edited January 14, 2014 by Themohawkninja

[Themohawkninja]

Pumping fuel in microgravity via docking has engineering issues associated with it. I do know that NASA is working on in-orbit fuel transfers, but the reason why it hasn't gotten to the point of KSP yet, is just due to (A) lack of demand, and ( engineering/physics issues.

[maccollo]

I would guess there's also the issue of added complexity. Just putting everything in one rocket is less complicated than doing that and then successfully rendezvousing in orbit, and then successfully docking, and then the chance of the tug malfunctioning at some point in the process. More complexity means higher risk. The risk might be very small, but it's still added risk, which is what you really don't want when your satellite costs hundreds of millions of dollars.

[rdfox]

NASA had a functional "space tug" design completed in the 1980s that would have used a small nuclear thermal rocket to retrieve failed satellites from geosynchronous orbit and bring them down to an orbit where they could be retrieved and either repaired or returned to Earth by the Shuttle, but never actually built it as A) they were a bit leery of launching an NTR in the Shuttle's cargo bay, and there was never a demand for it. IIRC, it was really a leftover from the late-60s planning for an on-orbit assembly manned mission to Mars, but when the Mars mission was cancelled, they managed to save that part of it by recasting it as an orbital transfer vehicle... though it ended up never happening because it would have cost so much more than other options.