Thursday, May 10, 2012

On Commercial Flights to the ISS and "space tugs".

Copyright 2012 Robert Clark 

The SpaceX flight to the International Space Station has been delayed multiple times:

1st Private Rocket Launch to Space Station Delayed. 
by Denise Chow, Staff Writer 
Date: 16 January 2012 Time: 01:51 PM ET

SpaceX launch to station faces delay.
CBS News 
"KENNEDY SPACE CENTER, FL--The long-awaited launch of a commercial 
cargo ship bound for the International Space Station almost certainly 
will be delayed from May 7 to at least May 10 and possibly longer, 
sources said late Tuesday, to give company engineers additional time 
to complete pre-flight tests and checkout..." 
05/01/2012 11:18 PM Filed in: Space News | Commercial Space | International Space Station

 SpaceX has said they need to do additional testing of the software controlling the link-up, understandable when a $100 billion asset in the ISS could be at risk.
 It would seem prudent for SpaceX to do several test runs rendezvousing with orbiting satellites before attempting the link up with the ISS. The current plan is not for Dragon to perform the final link-up with ISS under its own power and navigation capabilities but just to get close enough for the robot arm to grapple it and pull it to the station to dock with it. So to test this, all the Dragon has to do is demonstrate the ability to get close enough to some orbiting satellite without colliding with it, to within a similar distance as it would be to the ISS. It might be able to do this several times with different satellites to further demonstrate this capability.  
 With several commercial companies proposing private spacecraft to dock with the ISS this would appear to be a common problem. Robert Zubrin proposed an interesting solution in his book Entering Space: Creating a Spacefaring Civilization. He suggests having a "space tug" attached to the the station specifically to ferry in approaching cargo and crew spacecraft:

 "NASA managers are understandably jittery about the idea of just anybody flying up and attempting to dock their Radio Shack-wired cheapo spacecraft with one of these assets.
 "One solution that might get past this problem would be the development, by NASA, of a proximity operations vehicle (a "proxops" stage) that would hang around the station and grab resupply payloads delivered to the general vicinity by commercial suppliers."
Entering Space, p. 68.

 This would provide another use of the "space tugs" that have been proposed for orbital satellite servicing:

Satellite Refueling in Orbit, Coming Soon? 
By Steve Rousseau 
October 17, 2011 5:00 PM

Space Infrastructure Servicing. 

thus providing another income stream to the company(s) that produce the tugs.
  As for the satellite servicing business plan, according to the "Satellite Refueling in Orbit, Coming Soon?" article a satellite may last 10 to 15 years. And according to the "Space Infrastructure Servicing" wikipedia page, 200 kg of fuel may provide an additional 2 to 4 years of life. So it might take 100 kg per year for fuel, and over 10 years would require 1,000 kg.
 The cost to get anything to GEO, including this fuel, is in the range of $20,000 to $25,000 per kg. So for 1,000 kg of fuel to get to GEO for satellite refueling it would cost perhaps $25 million. But this would double the life of the satellite since it would again have a full fuel load for 10 years. So for $25 million you saved the satellite companies from paying, say, $300 - $500 million, to purchase and launch a new satellite.
 So even if you charged 4 times the usual price to get to GEO for this fuel, the satellite companies could still consider this a bargain.
 You would need a small reusable servicing spacecraft to be launch from the payload bay of the launch vehicle to transport the fuel to GEO. If you use LH2/LOX propellant for this spacecraft like the Centaur upper stages, then it takes about the same amount of propellant to get from LEO to GEO, as the mass of the (spacecraft + payload), the payload being the refueling fuel in this case. The dry mass of the spacecraft is only a small proportion of the propellant as indicated by the Centaur upper stage, about 1/10th.
 So a ca. 20,000 kg cargo capacity of the current largest launchers could be made up of half LH2/LOX propellant for the space tug and half the refueling fuel for the satellites. That's 10,000 kg of refueling fuel. If you do charge the satellites companies 4 times the usual rate to GEO to $100,000 per kg for this fuel, then that's potentially $1 billion revenue from that one launcher flight.
 The estimate of a charge of $100,000 per kg of the refueling fuel to be delivered to satellites in GEO may seem high but it's actually less than a price that has been quoted by a company planning on doing such refueling missions. On that "Space Infrastructure Servicing" wikipedia
page is given this ref. to an article on satellite servicing:

de Selding, Peter B. (2011-03-14). "Intelsat Signs Up for Satellite Refueling Service".
Space News. Retrieved 2011-03-15. "If the MDA 
spacecraft performs as planned, Intelsat will be paying a total of
some $200 million to MDA. This assumes that four or five satellites
are given around 200 kilograms each of fuel. ... The maiden flight of
the vehicle would be on an International Launch Services Proton
rocket, industry officials said. One official said the MDA spacecraft,
including its 2,000 kilograms of refueling propellant, is likely to
weigh around 6,000 kilograms at launch."

 So for this company they are actually charging $200,000 per kg of the refueling fuel.

      Bob Clark 


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  2. Smart move, but I am doubtful that LH2/LOx is the fuel of choice.

    I am also sceptical that this concept will gain much traction before other developments make it cheaper to send up an entire new satellite.

    Also, there is an upper limit on the utility of extending satellite useful life. There are upgrades to consider, and many components, such as solar panels, are designed to be cheaper and with a lifetime expectancy matching that of the satellite. Telecom are not under the same constraints as the Voyager 1!