Author Topic: SpaceX Mars lander dynamics  (Read 2143 times)

Offline Russel

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SpaceX Mars lander dynamics
« on: 11/17/2016 12:14 PM »
SpaceX wants a direct landing on Mars using a ultra-heavy lander with some lift. There are some questions I'd love to see a good answer for.

Does anyone have any basic parameters for this vehicle? Mass, effective drag area, lift to drag ratio.

Has SpaceX revealed what its landing profile will look like? Speed, elevation, g forces.

What will be the speed and elevation at which it will have to turn and use retro-propulsion?


I'm tempted to dive into a simulation, but I'm wondering if there aren't any short cuts to figuring this out. At this stage its unclear to me how close to the surface it needs to be to reach the point where maximum lift (at maximum angle of attack) equals the force of gravity. Then, given a profile that seeks to minimise g forces on the passengers, how long can the vehicle actually glide for given that as speed reduces so too does the elevation needed to maintain enough lift.

Side thought. What are the specs on the space shuttle and what would happen if you tried to land it on Mars? Does the SpaceX vehicle have more effective drag area and lift relative to the space shuttle, or does it have a higher mass to surface area ratio?

I want to leave aside any other issues and just focus on the dynamics. Of concern to me is the forces that passengers are subjected to given a long period in weightlessness but that can be discussed elsewhere. Just the math please.

Offline guckyfan

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Re: SpaceX Mars lander dynamics
« Reply #1 on: 11/17/2016 03:09 PM »
All data known are in this set of slides from the IAC presentation. A lot of data actually.

http://imgur.com/a/20nku

Offline high road

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Re: SpaceX Mars lander dynamics
« Reply #2 on: 11/18/2016 08:55 AM »
Side thought. What are the specs on the space shuttle and what would happen if you tried to land it on Mars? Does the SpaceX vehicle have more effective drag area and lift relative to the space shuttle, or does it have a higher mass to surface area ratio?

Without any hard data at this point, running the numbers isn't straightforward. Although it seems obvious that the mass to surface area of ITS is considerably higher than the Shuttle: SpaceX vehicles are considerably volume constrained, so you'd want to fill them to the max. If you're sending the supplies on the ISS that is, and not on precursor red dragons or whatever. That would put ITS's mass to surface ratio closer to a shuttle without the wings.

But even if ITS lands with only astronauts on board and dumps waste along the way, making it as light as possible on arrival, it's still likely to have a higher mass to surface area ratio than the Space Shuttle. It needs better TPS to compensate for the higher entry speed of interplanetary travel and it has only 0.006 as much atmosphere to achieve lift from, a shorter amount of time to manoeuver in, and astronauts with several months of bone and muscle loss to bring to the surface safely. All these requirements limit the amount of mass and descent time dedicated to achieving lift versus retropropulsion.

Offline Russel

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Re: SpaceX Mars lander dynamics
« Reply #3 on: 11/18/2016 10:33 AM »
I think there were some estimates out there as to the basic mass and size of the vehicle?

As far as working out the dynamics goes, my initial approach would be to find for a given elevation and thus air density what speed would be necessary so that lift force would be equal to local gravity.

The Space Shuttle supposedly has a hypersonic lift to drag ratio of about 1. That would seem a good starting point.

If the SpaceX vehicle had a lift to drag ratio of 1, that would mean that it would have be generating enough drag to be decelerating at about 4m/s/s - about 0.4g. So if you plug in the mass, effective drag area and air density it shouldn't be too hard to make a start.

Offline Russel

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Re: SpaceX Mars lander dynamics
« Reply #4 on: 11/18/2016 10:53 AM »
All data known are in this set of slides from the IAC presentation. A lot of data actually.

http://imgur.com/a/20nku

They're claiming 4-6gs at Mars. That seems in the ball park to me, but I'd still like to see the detail. It just feels like they have to get fairly deep into the atmosphere before the glide.

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