An uncrewed side-mounted concept of the Space Shuttle named Shuttle-C was investigated between 1984 and 1995. The Shuttle-C cargo only option was not funded in the 1980s and 1990s due to NASA's budgetary constraints. After the Space Shuttle Columbia disaster, a two-year industry study was prepared in 2004 and 2005 to further investigate the concept as a Shuttle replacement. The Exploration Systems Architecture Study in 2005 also investigated a Shuttle-C option for Project Constellation, again only in an uncrewed version. All these concepts intended the side-mounted carrier to be an autonomous spacecraft which would detach from the External Tank after main engine cut-off, similarly to the Space Shuttle. Some of the studies included the reuse of the Space Shuttle Main Engines on this side-mounted carrier. None of the concepts involved in-ascent fairing separation. The HLV proposal presented on 17 June 2009 was partly based on the original Shuttle-C proposal. The main differences were that the side-mounted carrier could not detach from the ET, and proposing to also carry crews on the HLV. The proposal included work from about 60 NASA engineers.
HLV specifications
The HLV was proposed to be a vehicle at liftoff with two 4-segment Space Shuttle Solid Rocket Boosters weighing about providing a total thrust of at sea level and the Space Shuttle External Tank weighing about fueled. The side-mounted carrier was to include a Shuttle-derived 'boattail' carrying the three Space Shuttle Main Engines and other propulsion elements. A diameter payload carrier with a separable fairing weighing would take up the space usually occupied by the rest of the orbiter. The basic vehicle would not have an upper stage, requiring the payload to perform orbit circularization and possibly trans-lunar injection burns. The only completely new hardware development to be required for the HLV was the side-mounted carrier. All other components used on the HLV were previously in use with the Space Shuttle, and up to the first six flights of the vehicle would have reused spare parts and salvaged functioning hardware from the orbiters, including existing avionics modules, flight software, and SSMEs. Virtually no change to the existing Space Shuttle infrastructure, from the Vehicle Assembly Building to the External Tank barge to the launch pads, was to be required.
Upper stage
To be usable for the envisioned lunar flights, the HLV would require an upper stage. The use of the J-2X engine that was under development for the Ares I launch vehicle was proposed for this upper stage. It would have provided nearly and was intended to have a specific impulse of 448 sec. Alternatively, the United Launch Alliance proposed that their Dual Thrust Axis Lander could fit in a side mount payload shroud. The ULA ACE 41 and ACE 71 upper stage/fuel depot concepts could have also fitted inside a side mount payload shroud, and the ACE 71 at was well within the side mount shuttle derived vehicle's payload capacity.
Performance
The HLV's 4-segment SRBs were to deliver a specific impulse of 267 sec and a thrust of and burn for about 155 seconds. The SSME main engines were to be flown at 104.5% and deliver a specific impulse of 452 sec and and burn for about 500 seconds. The payload mass for different missions was envisioned as follows:
Block I vehicle without an upper stage – and to a × reference orbit from Kennedy Space Center
Block II cargo vehicle with an upper stage – and to a × reference orbit from Kennedy Space Center
Block II crew vehicle with an upper stage – and to a × reference orbit from Kennedy Space Center
Block II lunar missions: to TLI with the lunar lander and to TLI from Kennedy Space Center.
Mission profile
In contrast to Shuttle-C, no part of the vehicle would have been recoverable and reusable. The HLV could have used a different flight profile than Shuttle because of a lack of wings and associated load limits. The payload fairing was to be jettisoned 185 seconds into the flight at about altitude. The SSME main engines were not to be reused and thus could be simplified, and new engines would have to be produced for each vehicle. For lunar missions, the HLV proposal envisioned suborbital staging at × of the vehicle to increase mass through TLI with two burns of the upper stage.
Lunar mission architecture
While the HLV was designed to provide crew and cargo missions to the ISS, its primary aim would have been to replace the Ares I – Ares V lunar architecture. The rudimentary mission architecture used a Lunar Orbit Rendezvous profile. Two HLVs were to be launched for the completion of one mission. The first HLV was to be launched with the lunar lander and immediately place the lunar lander on a trans-lunar injection. The lunar lander would have had a net mass of 35 metric tons after TLI, and would have inserted itself into a low lunar orbit. In LLO, the lunar lander would weigh about 28 metric tons. The second HLV was to place an Orion spacecraft and crew to trans-lunar injection. The 20 metric ton Orion spacecraft would remain attached to the upper stage, which was to insert the Orion spacecraft into LLO and dock with the lunar lander.
Growth options
The HLV would have had limited growth option. While 5-segment SRBs could have been used on the vehicle, they would have required significant re-engineering to yield 7 metric tons more to lower Earth orbit. Other growth options included an upgrade of the SSME to 106% or 109% thrust level or a switch from the J-2X upper engine to an air-startable SSME.
