First, could you squeeze more fuel in the descent stage? There is certainly room (see below). Then how about mass margin?
Instead, JPL could use the pressurization sequence to move liquid fuel from tank to tank, changing the center of gravity, then move it back again.
This would reduce the needed ejectable masses
A bi-propellant descent stage would only need 300 kg of propellant as opposed to the 400 kg they used. (Plus the tanks would be lighter both due to smaller size and lower pressure -
Finally, their descent profile is designed to be as safe as possible, not to use the minimum fuel. A more hoverslam-type landing would use less fuel, leaving more for post-rover-dropoff maneuvers.
SpaceX's plans are irrelevant to any future Skycrane developments. I think your point may be retro-propulsive landings will obsolete everything else. Maybe...maybe not...that is a possibility, however current missions like sample return are paced towards later this decade.
<snip>SpaceX's plans are irrelevant to any future Skycrane developments. <snip>Even if SpaceX succeeded with landing on Mars tomorrow, nothing is in the NASA, ESA, JAXA or any other agencies pipeline for science missions that would take advantage of that. ( I could be unaware of it , but haven't heard of anything)
Once again, I don't think these changes are a good idea. But there is certainly slack in the system - the descent stage had reliability in mind, not the utmost possible performance.
Quote from: LouScheffer on 02/28/2021 11:38 pmFinally, their descent profile is designed to be as safe as possible, not to use the minimum fuel. A more hoverslam-type landing would use less fuel, leaving more for post-rover-dropoff maneuvers. Wrong. A hover slam does not save propellant. It is used when the engines can't throttle low enough and still have a T/W>1. It actually uses more propellant because a larger than necessary engines are used. [...]
At the beginning (Red Dragon ?) Musk wanted to go 100% propulsive, just like on the Moon - and a bit like Falcon 9, using retropropulsive rocketry (note: the paper mentions supersonic retro-propulsion, nearly a decade before Falcon 9 2013-2016 gradual mastering of it !) Doing as if Mars atmosphere was thin enough, it simply did not existed. But Mars atmosphere did not agree, and would readily burn any such lander. Alternatively, it would burden it with an enormous amount of propellant. In both cases: Mars versus Musk: 1-0 for Mars. Even Musk had to concede to Mars, and accept some hypersonic glide. In turn, this led to the present SN-8 / SN-9 flight profile - with the skydive manoeuver, and hypersonic entry / glide, Shuttle style. Except there is no Shuttle Landing Strip on Mars, so back to a vertical landing... Mars EDL is hard, because Mars is so damn unforgiving.