Congress chose the motors?
Really?
Kinda doubt that .... instead they constrained the budget.
See also:
https://www.govinfo.gov/content/pkg/PLAW-111publ267/html/PLAW-111publ267.htm
"
SEC. 302. <<NOTE: 42 USC 18322.>> SPACE LAUNCH SYSTEM AS FOLLOW-ON
LAUNCH VEHICLE TO THE SPACE SHUTTLE.
(a) United States Policy.--It is the policy of the United States
that NASA develop a Space Launch System as a follow-on to the Space
Shuttle that can access cis-lunar space and the regions of space beyond
low-Earth orbit in order to enable the United States to participate in
global efforts to access and develop this increasingly strategic region.
(b) Initiation of Development.--
[[Page 124 STAT. 2815]]
(1) In general.--The Administrator shall, as soon as
practicable after the date of the enactment of this Act,
initiate development of a Space Launch System meeting the
minimum capabilities requirements specified in subsection (c).
(2) Modification of current contracts.--In order to limit
NASA's termination liability costs and support critical
capabilities, the Administrator shall, to the extent
practicable, extend or modify existing vehicle development and
associated contracts necessary to meet the requirements in
paragraph (1), including contracts for ground testing of solid
rocket motors, if necessary, to ensure their availability for
development of the Space Launch System.
(c) Minimum Capability Requirements.--
(1) In general.--The Space Launch System developed pursuant
to subsection (b) shall be designed to have, at a minimum, the
following:
(A) The initial capability of the core elements,
without an upper stage, of lifting payloads weighing
between 70 tons and 100 tons into low-Earth orbit in
preparation for transit for missions beyond low-Earth
orbit.
(B) The capability to carry an integrated upper
Earth departure stage bringing the total lift capability
of the Space Launch System to 130 tons or more.
(C) The capability to lift the multipurpose crew
vehicle.
(D) The capability to serve as a backup system for
supplying and supporting ISS cargo requirements or crew
delivery requirements not otherwise met by available
commercial or partner-supplied vehicles.
(2) Flexibility.--The Space Launch System shall be designed
from inception as a fully-integrated vehicle capable of carrying
a total payload of 130 tons or more into low-Earth orbit in
preparation for transit for missions beyond low-Earth orbit. The
Space Launch System shall, to the extent practicable,
incorporate capabilities for evolutionary growth to carry
heavier payloads. Developmental work and testing of the core
elements and the upper stage should proceed in parallel subject
to appropriations. Priority should <<NOTE: Deadline.>> be placed
on the core elements with the goal for operational capability
for the core elements not later than December 31, 2016.
(3) Transition needs.--The Administrator shall ensure
critical skills and capabilities are retained, modified, and
developed, as appropriate, in areas related to solid and liquid
engines, large diameter fuel tanks, rocket propulsion, and other
ground test capabilities for an effective transition to the
follow-on Space Launch System.
(4) The capacity for efficient and timely evolution,
including the incorporation of new technologies, competition of
sub-elements, and commercial operations.
"
And ...
"
SEC. 309. REPORT <<NOTE: 42 USC 18327.>> REQUIREMENT.
Within 90 days after the date of enactment of this Act, or upon
completion of reference designs for the Space Launch System and Multi-
purpose Crew Vehicle authorized by this Act, whichever occurs first, the
Administrator shall provide a detailed report to the appropriate
committees of Congress that provides an overall description of the
reference vehicle design, the assumptions, description, data, and
analysis of the systems trades and resolution process, justification of
trade decisions, the design factors which implement the essential system
and vehicle capability requirements established by this Act, the
explanation and justification of any deviations from those requirements,
the plan for utilization of existing contracts, civil service and
contract workforce, supporting infrastructure utilization and
modifications, and procurement strategy to expedite development
activities through modification of existing contract vehicles, and the
schedule of design and development milestones and related schedules
leading to the accomplishment of operational goals established by this
Act. The Administrator shall provide an update of this report as part of
the President's annual Budget Request.
"