As first reported on this website, NASA is considering changing the baseline LOX/methane engine for the Crew Exploration Vehicle (CEV) Service Module – and the Lunar Surface Access Moduleâ€™s (LSAM) ascent stage – to a hypergolic or hydrocarbon engine.
The specter of the removal of methane has raised some concerns in the aerospace community, mainly because the ESAS team considered it to be ideally suited to the task of acting as the fuel for the CEV and LSAM ascent stages.
Because methane could be manufactured in-situ on Mars, it is both more efficient and much safer than hypergolics – while it is much denser and more storable than liquid hydrogen, Dr. Doug Stanley, the ESAS Lead, believes that the decision to drop the methane engine could create long-term problems for the ESAS. ‘I believe dropping methane in favor of hypergols would not make sense for a number of reasons,’ he said.
â€œThe effect on the total architecture and sustainability must be properly weighed.â€
A switch to hypergolic fuels would prevent lunar ISRU, because liquid oxygen extracted from the lunar regolith or mined from suspected lunar ice could no longer be used as rocket propellant. Dr. Stanley believes that in the long term this could increase the life-cycle cost of the lunar landers used to support a base.
‘Eventually the lunar lander will be re-used in a single stage (hopefully with LOX ISRU),’ he explained, while a lack of liquid oxygen-fueled engines would make such a design much more expensive to operate because fuels would have to hauled all the way from Earth.
Because a methane/oxygen system is more efficient than hypergolic propellants, Dr. Stanley believes that using hypergolics will vastly increase the mass of the CEV.
‘Switching from methane to hypergols could add about 1500 lb to the CEV system weight,â€ he said. Additionally, â€œ[the] LSAM weight could grow over 3klbs to provide [the] same payload capability [to the lunar surface].’
However, Dr. Stanley says that a third alternative, a LOX/ethanol-fueled system, is still in the running for the CEV/LSAM engines.
‘LOX/ethanol is very much on the table and preferred by many in NASA,’ he said. Because ethanol has a higher Isp than hypergolic propellants – and is about ten times denser than liquid hydrogen – â€œswitching to another LOX/hydrocarbon would not add nearly as much [mass],’ to the CEV system as would a switch to traditional storables, he explained.
However, ethanol is still not as efficient as methane, and a switch to LOX/ethanol for the CEV would still increase combined CEV and LSAM mass by ‘a couple of thousand pounds’, Dr. Stanley noted.
Because of this, he hopes to verify that top-level NASA management is properly considering all alternatives in the coming weeks.
He added that the jury on the CEV/LSAM propulsion system is still out.
‘The final decision on green [hydrocarbon fuels] vs. hypergols has not been made.’
For updates and debate, head to our CLV/CEV section: