On 10/27/2011 08:03 AM, Jeff Findley wrote:

I'd think that Delta IV Heavy or Falcon Heavy could loft large masses to
ISS when used in conjunction with a Dragon service module

Dragon doesn't have a service module, as that term is usually
understood. The "trunk", as SpaceX calls it, contains only the solar
arrays, radiators, and external cargo accommodation. *All* Dragon
propulsion capability is integrated into the capsule itself.

or the service
module that Orbital Sciences is using on their ISS resupply craft. The
Russians have done the same thing with a Progress service module to
deliver their docking modules to ISS (launched on a Soyuz launch
vehicle). That and they've used TKS derived service modules to deliver
large station modules (launched on Proton). Certainly the US can do the
same.

You can't just slap an inert module on the end of a service module and
expect the stack to be controllable. If you look carefully at Orbital's
Cygnus, the pressurized module has RCS clusters arrayed around the front
end. They're fed by propellant lines running *through the module* to the
service module in the aft end.

Ditto the Progress-derived modules and the TKS. They've got RCS clusters
at both ends for controllability.

Bottom line is that the ISS module must be fairly tightly integrated
with the service module.

Very little opportunity? I think not. HTV is flying and has provisions
for unpressurized cargo. Dragon does as well (14 m3 volume for
unpressurized cargo). I'm not sure about OSC's vehicle...

Current version of Cygnus has no unpressurized cargo accommodation, but
a variant could be developed that would do so.

If the number
is four Dragon flights to equal a typical MPLM flight, then SpaceX only
needs to cost less than $360 million dollars.

I think they'd have to screw up a lot, or be NASA regulationed to
death to hit that high. Unfortunately, both are completely within the
realm of possibility.

True, but to me, this is still better than SLS. The one silver lining
to NASA screwing the pooch on Ares I and Orion is that they *have* to
rely on commercial providers for ISS resupply.

Well, no, they *could* just continue to use Soyuz and Progress provided
Congress continues to waive INKSNA. Might even be cheaper if ISS ends in
2020.