In article ,
says...

Brian Thorn wrote:
If they have a big payload they're dreaming about and NASA has SLS
in service, they'd be able to launch it at no R&D cost to
themselves,

Little perhaps but not no R&D cost. At the very least, NASA would
look to extract at least some funding from them for SLS R&D.

I decided to dig a bit to see what was "out there" and found this on the
Delta IV Wikipedia page:

The possibility of an extra-heavy variant was indicated in a 2006 RAND
Corporation study of national security launch requirements out to 2020,
[22] which noted, "...only the Delta IV Heavy has the performance to
lift the ten NSS launch requirements that require a heavy-lift
capability... the production capacity for Delta IV, with one possible
exception, can satisfy the entire projected NSS launch demand. The
exception involves the requirement to increase the Delta IV Heavy lift
capability to accommodate a single NRO (National Reconnaissance Office)
payload. The best solution to this requirement is currently under
study."

The above reference points to:

National Security Space Launch Report
http://www.rand.org/pubs/monographs/2006/RAND_MG503.pdf

Interesting reading. The paper seemed to indicate relatively minor
upgrades to Delta IV Heavy would be needed for the single planned NSS
launch which (then) exceeded the Delta IV Heavy payload capacity. See
this quote:

Currently, the U.S. Air Force indicates that the Boeing Delta IV
Heavy falls slightly short of meeting the performance needed for
an NRO mission scheduled to launch before 2010. The Air Force is
confident that modifications to the Delta IV will provide
sufficient lift. The cost of these modifications to attain the
required performance improvement is estimated to be on the order
of $200 million.

$200 million in development money isn't enough for a *huge* increase in
lift capacity, which is what SLS would provide.

Also, the heavy launch needs of NSS weren't huge. From what I gathered,
when the paper was written in 2006, there were 10 payloads requiring
"heavy lift" (Delta IV Heavy) through the year 2020, which is less than
one per year.

Jeff
--
"the perennial claim that hypersonic airbreathing propulsion would
magically make space launch cheaper is nonsense -- LOX is much cheaper
than advanced airbreathing engines, and so are the tanks to put it in
and the extra thrust to carry it." - Henry Spencer