Researchers at the Los Alamos National Laboratory have also considered another interesting NTR propulsion scheme, known as a “gas core nuclear rocket (GCNR)”. In a GCNR, hydrogen is pumped into one end of a cylindrical reaction chamber, with an exhaust at the other end. The hydrogen expands as it passes through the chamber, and not all of it goes out the exhaust, instead flowing back up the chamber. This creates a toroidal vortex of hydrogen gas that can be used for fission reaction containment. Dust-sized particles of uranium are injected into the toroid and accumulate at its center. A number of long cylinders are mounted on the interior of the reaction chamber outside the toroid. These cylinders normally absorb radiation emitted by the uranium, but they can be rotated to reflect it, initiating a fission reaction.
The cylinders are the equivalent of control rods in an Earth-based reactor and were apparently featured in earlier NTR designs. Once fission begins in the center of the hydrogen gas toroid, the high temperatures heat the gas into a plasma, which flies out the exhaust at high velocity to provide thrust. A small magnetic nozzle could be used to ensure that the uranium remains in the reaction chamber, while allowing the hydrogen plasma to escape. GCNR was considered during the NERVA program, but the approach was clearly beyond the technology of the time. In 1991, Los Alamos sponsored a government-industry workshop on GCNR, and the participants concluded that the computer horsepower and software skills were finally available to perform useful studies and simulations on the approach.
The Los Alamos studies on GCNR were theoretical and speculative, and no program to actually develop one is planned. However, the studies have have shown that a GCNR engine is much more efficient than a traditional NTR engine. A GCNR engine could be used to power a manned Mars mission that could reach Mars from Earth in 90 days, half the time required by a more conventional “solid core” NTR-powered vehicle. This would reduce mission cost and exposure of the crew to radiation.
