I'm curious, but are all the Mars landers you envision Mars Orbit Rendezvous types or would you ever consider a direct ascent lander? I ask because the rough estimates are that a 9-Raptor per core Falcon X Heavy would fling more than 110 mt through TMI. It's so much mass you could feasibly pull it off. Of course the problem I see is getting back off of Mars.
http://www.angelfire.com/md/dmdventures/orbitalmech/DeltaV.htmFrom To Delta-V (km/s) LEO Mars Surface 4.8LEO Lunar surface 6.2Mars LMO 4.4LMO Mars 0.05LMO Earth return 3.4Lunar surface LEO 3.2Total delta-v required To Mars surface and back to Earth: 22 km/s (12.6 km/s required for everything beyond LEO) To lunar surface and back to Earth 18.8 km/s (9.4 km/s required for everything beyond LEO) It seems pretty clear from the math that Spacex or anyone else for that matter would be hard-pressed to pull off a Mars direct ascent approach. It'd work superbly if all they cared about was getting payloads to the surface of Mars though. Matter of fact, it requires only 77.4% of the delta-v needed (beyond LEO) to land on the Moon for you to land on Mars. Now if only there wasn't that dire fact about half of all missions to Mars ending in failure adding a huge asterisk to that. My guess is if that if anyone wants to mount a round-trip mission, pretty much all the landers will have to be Mars orbit rendezvous types. Otherwise you're adding a lot of unnecessary propellant and structural mass to the mission that could otherwise be put into useful cargo and habitat mass.
Quote from: Hyperion5 on 11/09/2013 01:58 amSee the 2:40 mark of the video in the first post. The lander, which clearly masses considerably more than the Apollo Lunar Module, is deploying not one but 3 parachutes. These deploy after the back aeroshell is left behind, allowing the parachutes to unfurl. As the parachutes unfurl the descent engines kick in. I believe Steven Pietrobon mentioned that this approach chops the descent delta-v required from the engines to a mere 500 m/s. That's an impressively low figure for landing something 50 mt or more on Mars. I don't know what the figure would be doing an all-propulsive approach, but it'll be significantly more than that. PAge 19 of this document talks about this.Mars Exploration Entry, Descent and Landing Challenges (paper):http://www.ssdl.gatech.edu/papers/conferencePapers/IEEE-2006-0076.pdf Similarly, a 50 t vehicle requires asupersonic parachute diameter on the order of 90 m. Whileclustered supersonic chutes are an option, the size of suchsystems would still result in large timeline penalties foropening. As such, an all parachute approach for Marshuman exploration vehicles, similar to the concepts nowused for robotic landers, is likely impractical."
See the 2:40 mark of the video in the first post. The lander, which clearly masses considerably more than the Apollo Lunar Module, is deploying not one but 3 parachutes. These deploy after the back aeroshell is left behind, allowing the parachutes to unfurl. As the parachutes unfurl the descent engines kick in. I believe Steven Pietrobon mentioned that this approach chops the descent delta-v required from the engines to a mere 500 m/s. That's an impressively low figure for landing something 50 mt or more on Mars. I don't know what the figure would be doing an all-propulsive approach, but it'll be significantly more than that.
I believe the 4.8 km/s of delta-v that site factored in included some 400 m/s of retro-propulsion into it. If you look at the video of the Constellation lander, I think they're doing more than 50 m/s of retro-propulsion. If you upped that to say 400 m/s and let the parachute(s) and aeroshell/heat shield take care of the rest it should work. I don't believe even Curiosity got down to 50 m/s when it fired up its retro-rockets. It was going at least 80 m/s at just around 2 km up when those fired. So it'd be more of a challenge to fire those up earlier, but I think 300-400 m/s of delta-v from the retro-propulsion is very reasonable. Particularly when the alternative is a full 1000 m/s delta-v for a landing done only with retro-propulsion.
Do the engines have to be at or near the bottom of the lander ?What if they were on aerodynamic pylons at the top of the vehicle and therefore above most of the mass so the lander effectively hangs below instead of sitting on the landing engines ? A variant on the Skycrane idea .Mick.
Jeff Foust @jeff_foustMike Gazarik: interested in supersonic retropropulsion for Mars EDL; talking with SpaceX about what they did on F9 1st stage recovery.
Quote from: MickQ on 11/12/2013 08:59 amDo the engines have to be at or near the bottom of the lander ?What if they were on aerodynamic pylons at the top of the vehicle and therefore above most of the mass so the lander effectively hangs below instead of sitting on the landing engines ? A variant on the Skycrane idea .Mick.Well that's a possibility. Lobo and I have mentioned this possibility to Steven Pietrobon, who keeps pointing out these thrusters being angled out will cut their efficiency and thus the lander's payload mass. However, if we're dealing with expendable landers, the delta-v needed to land really isn't that high. Even an all-propulsive landing of a large 100 t lander requires only about a 1000 m/s. Add some parachutes and you can easily cut that to 400 m/s, and 200 m/s if you want to up landed mass even more. I'm not sure of the merits of top-mounted versus side-mounted thrusters, but they should be better at preventing debris damaging the landers during the last phase of the descent. For this reason I prefer descent engines be on the sides or up top and angled out. It might cost some efficiency but it does help with safety. Safety for me is the top priority in any Mars landing.
LMO Earth return 3.4
{snip}Basically, to prevent damage during the landing of a large craft, robotic rovers would be sent in advance to prep a landing pad.
Use a tethered powered landing like MSL did with a staged disposable lander would probably be the cheapest way to do it. Tethered rockets to slow descent and a two staged lander. First stage provides precise powered landing capability and is left behind while the 2nd stage is used for ascent. Also gives the science geeks lots of different systems they have to design so it keeps the NASA boys gainfully employed. Do it the good old fashioned overly complicated American Way!