|SLS core vehicle and EUS without SRBs (Modified after NASA)|
The SLS-B would only consist of the SLS core vehicle plus the Exploratory Upper Stage (EUS). Five expendable RS-25 engines would be used for the SLS-B, one more than is utilized for the SLS heavy lift vehicle. And such a basic SLS configuration should be capable of deploying at least 20 tonnes of payload to LEO.
Twenty tonnes of payload capability would, of course, make the SLS-B fully capable of launching a variety of crew modules into orbit such as: Boeing's CST-100 Starliner, Space X's Dragon, and possible future crewed spacecraft such as Sierra Nevada's Dream Chaser. Such crew modules on top of the SLS-B would also allow several tonnes of additional payload within an expansive payload fairing with a maximum payload diameter of 7.5 meters, much larger that the 4.6 meter internal fairing diameter you'd get aboard the Falcon Heavy or the Atlas V.
The SLS-B could, therefore, add an additional crew launch vehicle to America's fleet of Earth to LEO spacecraft, competing with the Falcon 9, the Atlas V, and the future Vulcan and Glenn launch vehicles.
A cluster of small solid rocket boosters currently used to enhance the payload capability of the Atlas-V could also be used to increase the payload capability of the SLS-B up to 50 tonnes to LEO. This would give the SLS-B a payload capability close to that of the Falcon Heavy while offering a substantially larger internal payload fairing diameter.
As a purely liquid hydrogen and liquid oxygen fueled vehicle, the SLS-B would have the environmental advantage of being carbon neutral-- if its fuel is exclusively derived from nuclear or renewable electrolytically produced hydrogen and oxygen. The the SLS-B would, therefore, be one of the most environmental benign launch vehicles in operation by not contributing to global warming, ocean acidification, and global sea rise.
The increase in the demand for the SLS EUS, core stage, and RS-25 engines for sub-heavy lift flights should help to significantly reduce the vehicle cost for the SLS super heavy lift vehicle.
Of course, there is still the question as to whether there will be enough launch demand for so many launch vehicles.
As far as crew launches are concerned, current demand by government agencies is relatively meager, with only four crew launches in 2017. However, the potential passenger demand for space tourism could be substantial once private spacecraft of crew launch capability. There are currently more than 50,000 people in the world wealthy enough to afford a $20 million to $50 million ticket to orbit. If just 1% (500 individuals) of such individuals traveled into orbit every year, the demand would require at least 100 to 125 crew launches annually.
Commercial launch demand could be further enhanced by:
1. Starting a space lotto system: allowing adults to purchase dollar tickets for a trip to a private space station in orbit.
2. Deploying propellant producing water depots at LEO and EML1 (Such depots would require hundreds of tonnes of water to be launched from the Earth's surface annually)
3. Creating an international space agency that requires only $50 million in annual dues from its member nations (If such an international agency only had twenty members and spent only half of its revenue on purchasing crew flights into space then ten to twenty people could be annually launched to LEO)
4. Allowing the US military to deploy astronauts from the armed services into space aboard facilities owned and operated by the DOD (Department of Defense): A mere $1 billion a year could allow ten to twenty military personal to fly into orbit annually.
5. Starting a guest astronaut program for deep space missions that charges foreign astronauts $150 million for participating in a deep space mission. So two foreign astronauts participating in a deep space mission could cut NASA's cost per mission by $300 million, allowing more missions to be funded.
Private operation and commercial utilization of the SLS and SLS-B (Boeing, Orbital ATK, and Aerojet Rocketdyne?) would allow both vehicles to compete with launch vehicles deployed by Space X, the ULA, and other countries around the world.
Links and References
The Case for an International Space Agency