[nicolapcweek94]

> You can land on a planet or moon with no atmosphere.

IE: Mars, the ultimate SpaceX goal, which has just enough atmosphere to be annoying but not nearly enough to make parachutes work, so they had to find another way to land a big enough payload to transport 100 tons of stuff there

[Retric]

All methods of landing on Mars require an aeroshell and parachute sequence, but after that there are three choices https://en.m.wikipedia.org/wiki/Mars_landing

The advantage to powered landings is they work well for delicate stuff like people. Or in space X's case they are landing a really light shell and have a really powerful engine already attached.

[phire]

I feel like there is a [Citation Needed] for that Wikipedia quote.

All existing landers used an aeroshell and parachute sequence, but that doesn't mean it's impossible to land with just Rockets. The aeroshell/parachute method is used because it allows landers to come straight in from a Hoffman transfer orbit, bleed off all that extra speed and land with the least amount of fuel required. But it's way to risky for a manned lander, as the landing date (and location) are locked in months ahead of time with no option for an abort or delay.

A manned lander is likely to aerobreak into a stabke Mars parking orbit first. Allowing the crew can check the lander, check the weather at the landing site and detach from a reusable transfer module. From low Mars orbit, they are going much slower and need to disperse much less energy.

You could just throw Rockets and fuel at the problem. Or wings might be far more attractive than a parachute for slowing down in the upper atmosphere (though, any wings would have to be massive to actually glide all the way down to the lower atmosphere or land)

[Retric]

In either case they use heat shields so the post heat shield second stage will be at the same velocity. Mars has ~0.6% of earths atmosphere, and ~40% the gravity so a heat shields going to hit terminal velocity at ~66 times earths terminal velocity. This only get's worse as you scale up due to mass vs surface area issues. Parachute can bleed off 80+% of that speed for little additional weight unlike wings which would need to survive supersonic retry heating making parachutes a no brainier. http://pics-about-space.com/re-entry-nasa-mars-landers?p=3#i... 1km/sec = 2,236.94 MPH.

Edit: Here, they start at well over 10,000mph heat shield drops that to ~1000mph, parashoot drops that to ~200mph. http://i.i.cbsi.com/cnwk.1d/i/tim//2010/08/12/2012MSLSkyCran...

PS: As to just burning fuel getting literally tons of wasted rocket fuel on mars is really really expensive.

PPS: At 0.6% ATM storms are only a viability problem. Only a tiny amount of ultra fine dust can get suspended.

[phire]

Starting from a circular Mars orbit rather than Hoffman transfer would put their initial entry speed closer to 7000mph.

> Parachute can bleed off 80+% of that speed for little additional weight unlike wings which would need to survive supersonic retry heating making parachutes a no brainier.

Sure, Parachutes make a lot of sense for a one way trip.

But Parachutes are a consumable, one which would be really hard to manufacture on Mars (compared to rocket fuel, which just requires water, carbon dioxide and electricity). Also the size of the parachute gets ridiculously large for larger spacecraft.

If you are planning to make a rocket which shuttles people or cargo (or fuel) between Mars' surface and low Mars orbit, then it makes a whole lot more sense to just manufacture the extra fuel on Mars rather than trying to manufacture parachutes on Mars or shipping extra parachutes to Mars.

As for wings, you don't really want to use them for the subsonic phase. I'm not really sure how viable the idea is, but you want to use them to prolong your trip through the upper atmosphere, where the atmosphere is thinner. This allows you to stretch out all that supersonic atmospheric heating over a much longer time period, at a much slower rate than what your heat shield can dissipate.

Retractable or reconfigurable wings might be needed so you can maximize lift in the upper atmosphere then minimise drag through the supersonic to subsonic transition.

[Retric]

If you are sending stuff back up then parachutes are reusable, but heat shields are not. There might be an argument if you where landing thousands of rockets a day and had giant city's on mars. But, by that point you can just make more.

[phire]

Depends on the heat shield. The Space Shuttle's heat shield was designed for reuse (though in practice the tiles were so fragile that they needed extensive checks and occasional replacement before each reentry).

I notice that Spacex have designed their ablative heat shield to which can withstand hundreds of reentries to Earth without any replacement or refurbishment. Still technically a consumable, but I really doubt anyone is happy with re-packing and re-using the same reentry parachutes hundreds of times.

And that's reentries to Earth, where the entry velocity is 17,000mph and the atmosphere is 100 times thicker. Such a heatsheild can probably withstand thousands of Mars reentries at just 7000mph before replacement.

[nicolapcweek94]

Well, they're landing a really light shell with a really powerful engine for now, on Earth.

Mars is gonna be a different story, considering the Red Dragon mission in 2018 (with already existing infrastructure, since the Red Dragon will basically be a Dragon 2 capsule and the S1 for the mission will be a Falcon Heavy) is gonna have the heaviest payload ever landed on mars with a semi-empty Dragon 2, and all the missions after that will probably try to land at least one MCT, which would be heavier of a Dragon 2 by itself, with some cargo...

Luckily the MCT will have an even more powerful engine, and Mars's lighter gravity will probably help!

[andars]

The first stage will not land on Mars.

[JshWright]

No, but the cool thing is, SpaceX is using their first stage landings to do research that will advance their goals on Mars.

In order to land large payloads on Mars, SpaceX is going to have to do something that, up until recently, has never been done before. They're going to have to fire a rocket engine 'backwards' in an atmosphere (albeit a thin one) while travelling at supersonic speeds. This will be necessary in order to slow down enough to actually land (parachutes don't buy you much on Mars).

This 'supersonic retropropulsion' is something that has been modeled a lot, but is really hard to actually test. You would need to get a rocket up to supersonic speeds, in the thin upper stages of Earth's atmosphere (where the conditions are close to that of Mars) and have it fire its engines backwards. As luck would have it, that's exactly what the Falcon 9 first stage does during its reentry burn. The data they are collecting now will be invaluable in designing their Mars bound spacecraft.

NASA is also very interested in this data: https://www.nasa.gov/press/2014/october/new-commercial-rocke...

[nicolapcweek94]

No it won't, but they're definitely gonna reuse the tech they're developing to land the S1 on Earth to land the MCT on Mars

And to land the BFR S1 on Earth, which I'd guess would be a bit too much for parachutes even with Earth's relatively thick atmosphere, if what people have been saying on Reddit is right

September 21st can't come soon enough!