originally posted by: Flyingclaydisk
Full disclosure...I am not a big fan of the whole manned mission(s) to Mars theme; I strongly question the scientific value of such an endeavor , but
that's for another thread. This thread is intended to address some of the difficulties landing on Mars and a possible solution.
Mars is notoriously difficult to land on...successfully. Depending on how you crunch the numbers, upwards of 50% of Mars missions have failed to land
successfully. One of the big challenges is the fact that while Mars does have an atmosphere, that atmosphere is very, very, thin. Consequently, you
can't really slow a craft down enough using the atmosphere as a brake before you deorbit. (that, and parachutes suck on Mars) We are pretty good at
landing on planets with no atmosphere (i.e. the Moon), and we're great at landing on planets with atmosphere like Earth. Mars, not so much.
Now, Elon Musk might not be very good at marketing cars, but SpaceX builds a bang-up rocket in the Falcon 9. It seems to me a Falcon 9 like product
could be adapted to landing on Mars. The current evolution of the Falcon 9 isn't really designed for a Mars mission, but something like it could be
developed using similar technology.
When people think about SpaceX and Mars many immediately jump to the SpaceX Starship, and that's all fine and good for the "getting there" part, but
it doesn't address the "landing on Mars" part. So, why not develop a Falcon 9-like booster stage which would travel along with a Starship like craft?
The way I see it, you could kill two birds with one stone here. First, you could overcome the atmospheric challenges of landing on Mars, and second
you would land with a booster capable of returning occupants back to orbit upon departure (something rarely discussed in Mars mission discussions).
The Falcon 9 doesn't really use the atmosphere for braking. It basically turns around, fires a retro burn and falls out of the sky (controlled of
course) and then fires another retro burn right before touch down. This same concept would seem to be a viable solution for Mars.
What do you think?
Back in 2011, NASA started a couple of studies that went on for a few years and looked at using SpaceX hardware for landing on Mars. I worked on
those studies. The concept was called “Red Dragon”, because it was supposed to take a Dragon capsule of the type used to transport astronauts to
Low Earth Orbit (a “Blue Dragon”) and look at whether it could actually land on Mars. Here is a pointer to the Wikipedia article on the
From that background, I would like to comment on your OP.
First, the statement that “you can’t really slow a craft down enough” using Mars’ atmosphere is not really correct. All Mars missions that
have gone to the surface have used Mars’ atmosphere to slow down one way or another. What is true is that the type of aerodynamic deceleration you
use will be different, depending on the size of the craft that you are trying to get to the surface.
The process of getting a craft that is entering Mars’ atmosphere slowed down and planted on the surface is referred to as Entry, Descent, and
Landing (EDL). As the name implies, there are 3 different phases to the process. The entry phase is where the craft comes into the top of the
atmosphere (typically, at ≈ 6 km/s) and begins slowing down using aerodynamic drag. In the simplest case, that process is completely passive; the
entry vehicle is stable like a shuttlecock and it simply slows down and falls down to lower altitudes (it’s called a ballistic entry). Typically, a
ballistic entry at Mars will enter the top of the atmosphere at about 250 km and travel maybe 150 km downrange before it reaches its terminal velocity
which is usually below about Mach 2. Below Mach 2, a parachute can be deployed which begins the descent phase. That slows the vertical speed even
further, until the landing phase can begin. The Viking landers Jettisoned the descent parachute and used rocket engines to perform the final (soft)
touchdown. All US Mars missions from that time on stayed on the parachute all the way to the ground and used airbags to cushion the final impact, up
until the Mars Science Lander (MSL) mission of 2011.
When the entry vehicle gets big enough, you can no longer use a purely ballistic entry, you have to start using aerodynamic lift. The MSL mission did
that; it entered the atmosphere with a slight positive angle of attack which generated some lift. That allowed the vehicle to actually fly downrange
much farther than previous missions. That allowed the vehicle to spend more time down in the thicker part of the atmosphere where it had time to slow
down enough to deploy the parachute. Like the Viking landers, the MSL lander used rocket thrusters to zero out its vertical sink rate. (Unlike, any
other lander, however, MSL introduced the Sky Crane idea to lower the rover to the ground from hover).
The Red Dragon would have been 3 or 4 times heavier than MSL, and would have to have used lift even more aggressively during entry phase. The optimum
profile is actually to enter the atmosphere with downward lift, to get down to the thickest part of the atmosphere as quickly as possible and then
roll to a lift-up attitude as close to the surface as you can. That gives you a very long glide out at constant altitude in the densest part of the
atmosphere. Even so, a vehicle the size of Red Dragon or larger would never get down below about Mach 3 without propulsion. That’s why parachutes
become useless when the vehicle gets too big. Instead, the Red Dragon would have fired its retropropulsion thrusters at Mach 3 and used them all the
way to the surface for a soft landing.
A vehicle the size of the Starship, however, would have an entry mass of more than 100 tons. That puts it in a whole new category, especially since
it would be carrying humans. When the Space Shuttle returned from orbit it had to limit the deceleration level to no more than 4 g’s to avoid
hurting the astronauts. It would be necessary to do the same thing with the Starship, and you can’t stay below that deceleration level and get
slowed down enough for landing on a single pass through the atmosphere. Fortunately, it’s entirely possible to make a pass through the atmosphere,
skip out, and come back around multiple times. Because the Starship will be made out of Stainless Steel, its surface is reusable probably hundreds of
I think the real problem that a vehicle the size of Starship will have is doing a propulsive tail-first landing on an unprepared surface. Rocket
motors blasting into loose soil at close range are pretty good at digging a hole right under the landing legs. I don’t know how SpaceX plans to
deal with that.