SpaceX and "expendable mini tugs"?

[Beast@Tanagra]

This is a proposal for optimizing SpaceX's expected future reusable launch capacity.

Assumptions:

1) Resuable launch vehicles don't make economic sense without high launch rates.
2) Larger reusable vehicles will be more cost-effective than smaller ones, given a full payload.
3) Reusable vehicles may initially struggle to gain acceptance with outside customers.

Problem:

SpaceX may struggle to fill the payload of a large reusable often enough to launch at a profitable rate.

Solutions:

Others have proposed that SpaceX find low-value cargo to haul to orbit as the company "shakes down" their equipment and builds a client base.  This low-value cargo would later serve to fill out any unused capacity on a given launch. 

The company's actual shake-down cargo has so far consisted of cheese. 

Fuel is an oft-mentioned alternative, but without any existing infrastructure of fuel depots and vehicles capable of using these depots, this idea looks dead on the pad for now. Musk has also been cool on the idea of depots in general, on the theory that lower complexity is better for mission success. Besides, who would want to be the first to create a depot, without knowing which orbits, fuels, and transfer interfaces would be desired by future clients?

Cubesats are one way launch providers currently pad-out their manifests.  But will there be enough demand for cubesat launches to fill future launch capacity?  I'm not ruling it out.  Planetary Resources, for example, might soon be mass-producing cubed (or otherwise small) prospecting satellites.  They would probably be willing to fly on stand-by.

But why leave the launch manifest to chance?  I propose that SpaceX ensure full payloads on every flight by mass producing and launching its own "expendable mini tugs".

The Expendable Mini Tug (EMT):

In this proposal, assume that an EMT is larger than your typical cubesat, but smaller than a primary payload. The key features of an EMT are as follows:

1) Efficient, long shelf-life propulsion -- probably solar-electric. 
2) Manipulator arms and/or standardized hard points/clamps for latching onto other assets. 
3) Maneuvering and axis-control sufficient to allow it to orient captive vehicles with acceptable performance.
4) A communications relay package, allowing it to serve as a node in an orbital network.

In my scenario, SpaceX builds its own mini-tugs in much the same way Google builds its own servers: en masse, according to proprietary in-house designs, and using commodity parts where suitable. Remember, these are expendable mini tugs.

EMTs would be less efficient than fuel depots for bigger missions, but would be far more capable in every other respect.  EMTs can come to the customer.  They don't care if the customer uses a different fuel, or wasn't designed for refueling.  And the greater they are in number, the more valuable EMTs become.

Possible Uses:

--Deorbiting end-of-life assets and other space junk.  This would probably be the first use of EMTs, as they would not be trusted around expensive toys right away.
--Boosting satellites.  This could enable heavier sats on existing launch vehicles to reach useful orbits. It might even let SpaceX "fix" launches that don't quite hit the intended orbit.
--Extending life of operational satellites.  Let an attached tug do your station-keeping until it runs out of gas, then let it go.
--Resurrecting useless satellites. If your orbital asset becomes non-functional solely for lack of fuel or axis control, buy a tug for permanent attachment.
--Assisting missions beyond earth orbit.  EMTs could chain together for bigger jobs.
--Data relay with other tugs and other customers.  Excess EMTs might be sent beyond earth orbit simply to enhance the Deep Space Network.  SpaceX and its customers would probably be the biggest users of this network as they turn to Mars.
--Justifying the initial creation of larger fuel depots. The tugs could refuel at these depots to extend their service lives. Once the depots are built, bigger missions might be designed to use them.
--Unknown future uses.  When you have a cloud of mini-tugs up there, who knows what you might decide to do with them? Divert a deadly asteroid on relatively short notice? Assemble space-to-earth solar power arrays? Advertise by flying in formation over high-population areas just after sunset?

To be cheap, EMTs must be standardized and mass produced, but might still come in variants:

--High-thrust variant.  For urgent missions, boosting manned missions beyond Van Allen Belts, etc.
--Extra fuel/long endurance variant. This should be a very cheap modification.
--Attached commodity cargo variants.  Did someone order a bag of ice? A tank of LoX?
--Specialized one-off variants.  Fix Kepler by latching on with a special high-precision gyro kit?
--"Bus" variant.  Instead of building a complete satellite, just attach your payload to a SpaceX tug core on the ground. Compare to SHERPA.

Challenges:

--Can EMTs be small/light enough to make good secondary or tertiary payloads (for a large launcher) and still be useful?
--Can EMTs be built cheaply enough, even if mass produced?
--Could EMTs hurt launch demand more than stoke it, by reducing satellite turnover?
--Would there be enough paying customers at first to justify EMT development and mass production?
--Are EMTs too far outside of SpaceX's core competencies? (And worth making a core competency?)
--Could EMTs be hindered by geopolitical considerations? After all, a fleet of mini tugs could easily bring down an adversary's orbital infrastructure on relatively short notice.

I look forward to your feedback on this concept.

[MP99]

I think you've missed the simplest market.

Assume the LV delivers the payload to some contracted GTO, eg -1500 or -1800 m/s.

The payload will then need to expend a large part of it's dV budget to move itself to GEO.

However, there would seem to be a certain value for a subsidiary service for your "mini tug" to perform some or all of the burns to deliver the payload to GEO.

Assuming the payload has the prop that could lift itself to GSO, the value to the customer would be to increase the sat's prop residuals, which gives the chance that the sat's lifetime may be increased. There seems to be an opportunity for a secondary payment based on the amount of dV delivered, with the payload performing the whole GTO-to-GSO transition if the tug fails.

cheers, Martin

[ClaytonBirchenough]

Why not go simpler and simply send up tanks of water?

 - Tanks can be filled with as much water needed
 - No rendezvous necessary

When SpaceX gets the chance, they can then go back and rendezvous with the tanks and utilize the water.