It should come as no surprise that NASA has selected a "shuttle-derived" vehicle with two existing LOX/LH2 stages as its reference design for the new heavy-lift Space Launch System (SLS) ordered by Congress and to be used for exploration missions beyond low Earth orbit (LEO).

Over the past few years NASA had supported the use of solid rocket boosters (SRBs) as strap-on motors for both the now-cancelled Ares I and Ares V launch systems.

Many experts have opposed the use of SRBs for these applications, because of limited energy efficiency and expensive post-flight refurbishing. NASA has now decided to hold a competition between liquid-propellant and solid-propellant boosters for the SLS in order to satisfy a Congressional mandate.

The use of liquid-propellant boosters is not a new idea. The original proposed Space Shuttle design had included reusable fly-back liquid boosters as far back as the early 1970s.

Of course, the Shuttle system design was changed many times in the 1970s due to political and financial pressures. The end result was a lower cost development and higher per-flight costs for the Shuttle system.

Earlier this month, NASA Administrator Bolden endorsed the possible use of LOX/kerosene liquid boosters for SLS.

Such boosters could significantly increase the energy efficiency of the SLS boost phase and could allow this vehicle to offer a 130-metric-ton payload capability that is specified in the 2010 NASA reauthorization legislation.

The booster competition will likely be between ATK and Aerojet. It appears that ATK will propose the five-segment version of the four-segment SRBs used on the Space Shuttle. The five-segment variant was to be the first stage of the Ares I crew launch vehicle.

Aerojet will likely propose a U.S.-built version of the Russian-based NK-33, but renamed as the AJ-26.

Orbital Sciences will be using the AJ-26 as the main-stage engine for its new Taurus II commercial launch vehicle.

Launchspace speculates that had the liquid booster option been adopted for the original Space Shuttle, the U.S. might have saved several hundred million dollars and already have a working booster for SLS. Unfortunately, we will never know.

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