How do aerospace engineers choose a landing system? (Curiosity rover)

Question

The Sojourner rover with the Mars Pathfinder used a entry, descent, and landing system involving airbags to land on Mars. The Spirit and Opportunity rovers each used more-or-less the same system involving airbags to land on Mars.

The Curiosity rover with the Mars Science Laboratory (MSL) used a very different landing system, including "seven minutes of terror".

Why did NASA engineers select such an apparently complicated landing system for MSL?

How do rocket scientists calculate the impact forces for a Mars landing for a given proposed landing system, when no one has ever used that landing system on Mars ever before?

How do aerospace engineers choose a landing system?

Answer 1

At first glance, this is more an engineering question than a physics question, lurscher's comment actually answered it: Engineers think about possible solutions and their implications, test some of them in simulations and finally test some in real life or with models. Note that 'think about their implications' actually means doing tons of calculations, doing basic and detail engineering to etc.

However, there are physical constraints in place, dmckee and aman mentioned some. The issue boils down to scaling. The heavier Curiosity would need far heavier airbags, or far more more fuel (that has to be shipped to mars) to back in on a pillar of fire, or huge parachutes/brake shields for aerobraking.

NOTE: This can be greatly improved by eloborating on the scale issues, maybe I'll be back later. As of now, this is somewhat soft.

Answer 2

My answer should possibly satisfy you, I think. Here is an artist's conception of Mars Science Laboratory Entry, Descent & Landing...

Curiosity is large in size as it has 10 science instruments to find the availability of life in Mars. Also it uses Multi-Mission Radio-isotope Thermal Generator for fuel. Hence, airbags cannot be used to handle such a large weight since more fuel has to be supplied to carry them and also cannot be expected for such a soft touchdown. Even Aerobraking does not help the landing as @dmckee has already told about it. Moreover, three satellites such as European Space Agency's Mars Express Orbiter, NASA's Mars Reconnaissance Orbiter & Viking Orbiter were used to study the environment of Mars for the safe-landing of Curiosity. After the entry of the Rover into Mars' atmosphere, the processes such as Parachute deploy, Separation of Heat-shield, Radar data collection, Separation from the Aeroshell & using the rocket-powered boosters named Sky Crane and even touchdown are all handled by Curiosity. Hence, NASA mentioned this as seven minutes of terror.

The rocket-powered boosters is a new type of landing system which provides enough upthrust to overcome the gravity of Mars and the force exerted on the rover during entry. This is a proposed landing system because three orbiters are out there to update each situation. But even after these precautions, the count-down of Curiosity landing was not appropriate as expected. The Rover landed some minutes later than the expected time due to severe problems in atmosphere during its entry.

And, I'm helpless to answer your 3rd question as others have already commented it. And yes, they're very true. Scientists have done millions of simulations and tests before taking such a risky landing system.

Answer 3

Well one of the factors that has a impact on the landing system is the size of the rover. The airbag system is feasible for rovers of small size. Curiosity rover is the size of a car so it was too heavy for the air bag method.

Integrated landing systems are also a huge design constraint requiring their own sensors, actuators, and power source. Since landing system is used once and only once it can be considered an design advantage to separate it and lose that extra weight and liability.