An arxiv paper looks at possible engines for a manned Mars mission that are currently being tested or with flight proven capabilities. One propulsion system that was examined that is unproven is the “Pure Electro-Magnetic Thrust” (PEMT), presented in 2002 by Carlo Rubbia. In his proposal, the engine produces thrust as a Solar Sail, but uses a nuclear reactor as the power source.
The propulsion systems considered are:
• Common Extensible Cryogenic Engine (CECE) – a classical chemical propulsion;
• Nuclear Engine for Rocket Vehicle Application (NERVA) II – a nuclear thermal propulsion;
• RIT-XT, a modern electric propulsion;
• Pure Electro-Magnetic Thrust (PEMT) – a concept propulsion.
Cryogenic chemical systems have higher thrust then regular chemical rockets. Cryogenical chemical systems have Isp of about 465 seconds.
Nuclear thermal engines have been developed since the 1940s. A single propellant, usually hydrogen, is heated by the nuclear core and is expelled through a nozzle while expanding. The core, usually an uranium derivative (like dioxide or carbide) or plutonium, releases heat due to the nuclear reaction, providing energy to the gas expansion, and resulting in an Isp approximately the double of the chemical engines. The heat released is limited by the melting point of the materials.
NERVA II produced the required power with a uranium inventory of 360 kg, and had 2 m in diameter. Temperatures of the hydrogen fuel could reach 2, 755 K. One of the requirements of the program was an endurance of over 600 min. These engines can also be treated as impulsive, as in the case of the chemical ones, due to their high thrust and Isp ≈ 900 s
Modern examples of electric engines are NASA’s Evolutionary Xenon Thruster (NEXT), an evolution of the already tested NASA Solar Technology Application Readiness (NSTAR) used in the Deep Space 1 mission, the radio frequency ion thruster RIT-XT, which works by generating ions using high frequency electromagnetic fields (and is being developed by Astrium), and the PPS 1350-G, which is a plasma thruster with flight proven capability (SMART-1 mission)
Using the RIT-XT engine as baseline, we also briefly discuss the possible gains in performance if some future technologies would enhance the specific impulse or the thrust-to-weight ratio. We considered a specific impulse of Isp = 10, 000 s and a thrust-to-weight ratio of FT /w = 0.0142 N/kg, more than two times and three times the corresponding value of the RITXT engine, respectively. While the considered specific impulse increase would correspond to a direct technology enhancement in the engine, the increase in the thrust-to-weight ratio was based in foreseen developments in solar panels and energy storage efficiencies, and cell density.
Results show that the pure chemical CECE propulsion system performs better than the nuclear thermal NERVA propulsion, for total mission masses lower than ∼ 1, 250 t. Between about 1, 250 t and about 1, 600 t both options are fairly similar. Whereas, for total mission mass higher than about 1, 750 t the NERVA system yields slightly better results.
There are surprisingly small increase in performance suggests that large gains cannot be achieved even with future technological developments of the continuous propulsion. We therefore focus our analysis in the RIT-XT engine when comparing with other types of propulsion.
SOURCES- arxiv, nasa
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