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The Moon is not a super useful analog. It lacks any sort of atmosphere. Mars has just enough atmosphere that you have to deal with it, but not enough of one to

The Moon is not a super useful analog. It lacks any sort of atmosphere. Mars has just enough atmosphere that you have to deal with it, but not enough of one to be especially useful (parachutes buy you a little, but not much). In fact, some of the best data and tests we have about landing big things on Mars come from SpaceX's first stage booster landings. The burn that the first stage does at the edge of space to slow down is in an environment very similar to that of the Martian atmosphere. https://www.youtube.com/watch?v=_UFjK_CFKgA https://www.youtube.com/watch?v=_UFjK_CFKgA

simonh 9y ago
The Moon isn't a useful analogy precisely because the Martian atmosphere does make a significant difference. Just not enough to allow parachute-only soft landings. I've seen the 'thick enough to be annoying, too thin to be useful' meme before but it doesn't really match reality. For example Pathfinder decelerated from 7.3km/s to 0.4km/s purely using it's aeroshell, before it even deployed the parachutes[0]. Now 0.4km/s is still pretty fast, but a saving of 6.9km/s dV is absolutely enormous. Red Dragon was only possible because, with modifications to the capsule aerodynamics, the Martian atmosphere would brake the capsule enough for it to soft-land on the SuperDracos. There's no way a Dragon could usefuly soft-land on the Moon. It's just way too heavy for it's size. All that thick capsule hull is just dead weight for an airless environment. Given it's high dry mass the rocket equation demands huge amounts of fuel to compensate. The parameters are all wrong for the Moon because it's designed to use atmospheric drag to do most of the work. [0]https://en.wikipedia.org/wiki/Mars_atmospheric_entry https://en.wikipedia.org/wiki/Mars_atmospheric_entry
- rtkwe 9y ago
  It's great and useful for smaller probes but as we've sent larger and larger probes the thin Martian atmosphere has been less and less useful. One of the major issues is that the mass keeps going up but the surface area of the heat shields we can reasonably bring along doesn't scale as well. There's some hopes in inflatable technologies though.
  - simonh 9y ago
    That is a good point. Red Dragon was only feasible by adding an asymmetric sled under the capsule to provide lift, so it could spend significantly longer braking through the atmosphere. Also the ITS video and presentation shows it using it's lifting body design to maximise time up in the atmosphere. At that scale straightforward aeroshells don't cut it, but you can still get a useful effect.
- dtparr 9y ago
  Good points. I've always taken the spirit of that saying to be more like "thick enough to have to deal with, too thin to be enough". On the moon, you don't have to worry about reentry at all, so no need for a heat shield or any sort of aerodynamic protection, just some way to do a propulsive landing. On earth, you need the heat shield and some aero considerations, but you can land safely with parachutes alone. On mars, you need to deal with reentry but also some sort of propulsive landing system for anything of reasonable size.

simonh 9y ago

The Moon isn't a useful analogy precisely because the Martian atmosphere does make a significant difference. Just not enough to allow parachute-only soft landings. I've seen the 'thick enough to be annoying, too thin to be useful' meme before but it doesn't really match reality. For example Pathfinder decelerated from 7.3km/s to 0.4km/s purely using it's aeroshell, before it even deployed the parachutes[0]. Now 0.4km/s is still pretty fast, but a saving of 6.9km/s dV is absolutely enormous. Red Dragon was only possible because, with modifications to the capsule aerodynamics, the Martian atmosphere would brake the capsule enough for it to soft-land on the SuperDracos. There's no way a Dragon could usefuly soft-land on the Moon. It's just way too heavy for it's size. All that thick capsule hull is just dead weight for an airless environment. Given it's high dry mass the rocket equation demands huge amounts of fuel to compensate. The parameters are all wrong for the Moon because it's designed to use atmospheric drag to do most of the work. [0]https://en.wikipedia.org/wiki/Mars_atmospheric_entry https://en.wikipedia.org/wiki/Mars_atmospheric_entry

rtkwe 9y ago
It's great and useful for smaller probes but as we've sent larger and larger probes the thin Martian atmosphere has been less and less useful. One of the major issues is that the mass keeps going up but the surface area of the heat shields we can reasonably bring along doesn't scale as well. There's some hopes in inflatable technologies though.
- simonh 9y ago
  That is a good point. Red Dragon was only feasible by adding an asymmetric sled under the capsule to provide lift, so it could spend significantly longer braking through the atmosphere. Also the ITS video and presentation shows it using it's lifting body design to maximise time up in the atmosphere. At that scale straightforward aeroshells don't cut it, but you can still get a useful effect.
dtparr 9y ago
Good points. I've always taken the spirit of that saying to be more like "thick enough to have to deal with, too thin to be enough". On the moon, you don't have to worry about reentry at all, so no need for a heat shield or any sort of aerodynamic protection, just some way to do a propulsive landing. On earth, you need the heat shield and some aero considerations, but you can land safely with parachutes alone. On mars, you need to deal with reentry but also some sort of propulsive landing system for anything of reasonable size.