In 1997 Borowski at NASA/LRC proposed combining liquid oxygen mined from the moon with a LOX-Augmented Nuclear Thermal Rocket earth-to-moon shuttle to achieve dramatic reductions in launch requirements for a lunar base. In this concept a reusable Lunar Landing Vehicle and reusable LANTR nuclear shuttle acted as tankers for each other. The LLV transported liquid oxygen mined from lunar soil to the LANTR in lunar orbit, while the nuclear shuttle delivered liquid hydrogen from Earth.
In 1992, NASA's Nuclear Propulsion Office at the Lewis Research Center (LRC) in Cleveland started a joint American-Russian program to develop a small nuclear thermal engine (NTR) for President Bush's Space Exploration Initiative. The SEI effort quickly foundered, but LRC's experts, led by Dr. Stanley Borowski, nonetheless continued to investigate the concept while promoting nuclear propulsion as a realistic alternative for future missions to the Moon and Mars. Borowski's small NTR workhorse would provide 67KN of thrust and produce a specific impulse of 940s using liquid hydrogen fuel. It could be developed in about seven years at a cost of $1.5 billion. The engine would initially be used on a new 4.6-meter diameter upper stage for the Titan IVB, greatly boosting the rocket's payload capability. A single Titan IVB launch could quickly and easily deliver an orbiter probe to Saturn, Uranus, Neptune; a large 350-kilogram scientific payload on a Pluto "fast flyby" mission; or alternatively land an unmanned cargo package on Mars (rovers, sample return probes) A dual Titan/Shuttle launch and rendezvous in Earth orbit would be sufficient for sending a small Early Lunar Access-type manned spacecraft to the Moon and back. Thereafter a Shuttle-derived unmanned heavy-lift launch vehicle and a large 7.6-diameter NTR stage would be introduced to support lunar commercialization & settlement and piloted missions to Mars...
Perhaps the most unusual of Borowski's designs, the 36-hour lunar commuter shuttle would be reusable and permanently based in space. Two heavy-lift Shuttle-C type launch vehicles would deliver the liquid hydrogen propellant from Earth. A smaller 10 meter long cylinder served as a 15-25t Passenger Transport Module. The PTM was basically an independent spacecraft containing its own auxiliary propulsion, docking, control and life support systems for 18 passengers and 2 pilots. Normally stored at the International Space Station between missions, it would provide the brains for the lunar commuter shuttle. The empty nuclear shuttle had a dry mass of about ~25 metric tons. Its four-engine NTR cluster would produce electrical power for the spacecraft subsystems as well as sufficient propulsion for a quick 24-36 hour trip from Earth orbit to lunar orbit. The delta-V would be up to 6.9 km/s vs. 4.1km/s for a normal 3.5-day journey to lunar orbit. The total mass varied slightly depending on the mission requirements, e.g. the initial mass in low Earth orbit would be 249 t (including 59 t of liquid hydrogen; the oxygen tank has a capacity of 155 t) for delivering a 25,000-kg one-way payload to lunar orbit in 24 hours. The NTR engine was so efficient that no expendable components or aerobrake would be required to save weight and fuel.
Borowski and his team envisioned an ambitious, phased lunar resource utilization and settlement program. It would be based on two key elements: lunar oxygen mining, and an innovative trimodal NTR engine concept developed by NASA/LRC and Aerojet in 1994 -- the LOX-Augmented Nuclear Thermal Rocket (LANTR) engine. NTR space vehicles would be gradually upgraded for improved performance and economy. The expected development cost of the basic cis-lunar nuclear shuttle was $2.5 billion in 1994 dollars. The point of comparison was a Boeing-developed lunar transfer vehicle concept developed in the early 1990s. It would be powered by chemical rockets and be totally expendable in order to save weight. Despite this, it would require the development of a new large-diameter carrier rocket. This approach was dubbed Phase A.
Borowski proposed instead a Phase B nuclear thermal rocket fueled by liquid hydrogen (LH2) brought from Earth. It would be lightweight enough to be launched fully fueled on two comparatively small heavy-lift rockets derived from the existing Space Shuttle. Following rendezvous and docking in Earth orbit, the spacecraft would transport 9,000-kg moon base and oxygen mining facility payloads to the lunar surface. The entire spacecraft (NTR stage and Lunar Landing Vehicle) would be expendable in order to maximize the size of the payloads while minimizing the number of costly infrastructure elements in space (propellant depots etc.).
Once the surface base became operational, the Lunar Landing Vehicle (LLV) could be permanently based on the Moon and reused for several missions. Its chemical engines would burn oxygen propellant mined from the lunar soil, and liquid hydrogen brought from Earth by a Phase C improved reusable NTR shuttle. The NTR shuttle would transporting 8,800 kg of cargo from Earth orbit to lunar orbit, where it would rendezvous with the LLV. The mass of the entire spacecraft in low Earth orbit was reduced from 132 t to 98 t since no new LLV must be transported to lunar orbit on each flight. A single Shuttle-C type tanker launch would thus be sufficient. Unlike previous all-chemical Space Exploration Initiative designs, there would be no costly expendable aerobrakes or drop tanks for the fuel.
Liquid oxygen propellant exported from the Moon ("Lunox") would be introduced in Phase D. This increased the payload carrying capability by almost a factor of three, and the amount of hydrogen fuel that must be launched (at great cost) from Earth was also reduced. The NTR shuttle was now refueled in lunar orbit by the LLV, which delivered up to 25 t of LUNOX for the nuclear shuttle's return to Earth orbit. Meanwhile, the LLV returned to the lunar surface with 24t of cargo brought from Earth. Its engines were powered by 14t of hydrogen delivered by the nuclear shuttle, and oxygen produced on the Moon.
The LANTR engine relied on an innovative scheme to increase engine thrust while reducing the size and weight of the bulky liquid hydrogen fuel tanks. In LANTR mode, liquid oxygen was injected into the large divergent section of the nozzle where it burned spontaneously with the nuclear reactor-heated hydrogen exhaust. The specific impulse was thereby reduced, which meant more propellant must be carried. However, the oxygen component could be brought relatively easily from the Moon, the size and weight on the propellant tank was drastically reduced and the thrust of the engine could be increased -- which made it possible to obtain "big engine performance" from a comparatively small and cheap nuclear engine. Finally, the operating temperature determined the lifetime of the engine. For a reusable nuclear shuttle operating in pure hydrogen mode, a temperature of 2800 degrees Kelvin translated to about 10 lunar missions before a set of new engines was required. However, the engines would last twice as many missions if the shuttle operated with a LOX/LH2 ratio of 3.0, because the higher thrust (182.45KN vs. 66.75KN) reduced the total mission burn time from one hour to about 30 minutes. The 24-hour LANTR "commuter" scenario required 43t of Earth-supplied LH2, 105t of LO2 from Earth and 92t of lunar oxygen. The optimum engine performance would then be an oxygen/hydrogen mass ratio of 4.0 (Isp=607s; thrust=431.65KN) on the outbound leg, and inbound MR=6.0 (Isp=545s; thrust=542.9KN). The payload capability would be 15,000kg and the total burn time=47 minutes. LOX-Augmented Nuclear Thermal Rocket (LANTR) performance tradeoffs were as follows: At operating temperatures of 2900, 2800 and 2600 K (corresponding to engine lifes of 5, 10, and 35 hours:
The Lewis Research Center plan featured a strong private sector element, since the lunar oxygen mining and production units (oil rig-like assemblies) would be maintained on a for-profit basis by private investors. The accrued cost savings and industry profits from LUNOX would gradually be invested to develop further infrastructure such as propellant depots in orbit, to support full reusability of the LLV and nuclear shuttle. A typical base site near Mare Serenitatis (21 deg. N., 29 deg. E.) would have abundant iron-rich volcanic glass resources for mining. Lunar soil would be used to cover inflatable domes for added living space for the government and industry personnel working at the site.
Borowski's plan turned out to be NASA's last grand-scale lunar exploration proposal of the 1990s as the agency increasingly focused its future manned spaceflight planning efforts on Mars. LANTR had many features in common with the old NERVA nuclear shuttle proposals of the late 1960s, but the use of smaller engines and lunar oxygen would have reduced the initial and marginal cost of supporting a moon base. Reusability appeared highly desirable for such vehicles, e.g. the 1970 nuclear shuttle would have cost about $200 million per copy in 1999 dollars. But nuclear propulsion had a political "image problem", and Borowski's plan simply proved too costly and ambitious to Dan Goldin's NASA which was struggling to assemble the International Space Station on cost and on schedule. Future NASA administrators would hopefully return to the lunar outpost concept after the ISS was completed.