Uncrewed Missions to Mars Require New Landing Strategy Says Study

Finding a balance between mass and power

To date, the heaviest vehicle to make contact with Mars is the Curiosity Rover which weighs 1 metric ton; future mission are likely to be more in the 5- to 20-ton range. Knowing how to land more mass on Mars safely is critical to any future Mars mission.

In the past, when a space vehicle enters the Mars atmosphere, it is slowed down from its supersonic speed by deploying a parachute, this is then augmented by rocket engines or airbags that help it maneuver at the right speed and angle for a smooth landing.

"Unfortunately, parachute systems do not scale well with increasing vehicle mass. The new idea is to eliminate the parachute and use larger rocket engines for descent," said Zach Putnam, assistant professor in the Department of Aerospace Engineering at the University of Illinois at Urbana-Champaign.

Existing techniques don't scale up

The current method involves a lot of propellant use which adds to vehicle mass. Increased vehicle mass results in a more expensive vehicle as well as the risk it will exceed current launch capabilities. More propellant would also detract from the payload opportunity for humans or scientific equipment.

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"When a vehicle is flying hypersonically, before the rocket engines are fired, some lift is generated, and we can use that lift for steering," Putnam said.

"If we move the center of gravity so that it's not uniformly packaged, but heavier on one side, it will fly at a different angle."

The new research looks at ways to take advantage of the pressure imbalance between the top and bottom of a vehicle and use the vehicle lift to help steer the vehicle. "We have a certain amount of control authority during entry, descent, and landing--that is, the ability to steer," Putnam said.

"Hypersonically, the vehicle can use lift to steer. Once the descent engines are ignited, the engines have a certain amount of propellant. You can fire engines in such a way that you land very accurately, you can forget about accuracy and use it all to land the largest spacecraft possible, or you can find a balance in between.”

"The question is, if we know we're going to light the descent engines at, say, Mach 3, how should we steer the vehicle aerodynamically in the hypersonic regime so that we use the minimum amount of propellant and maximize the mass of the payload that we can land?"

"To maximize the amount of mass we can landing on the surface, the altitude at which you ignite your descent engines is important, but also the angle your velocity vector makes with the horizon--how steep you're coming in," Putnam explained.

The study has shown how to make the best use of the lift vector with optimal control techniques. The study explores how these ideas can be used across a range of interplanetary delivery conditions and vehicle properties.

"Turns out, it is propellant-optimal to enter the atmosphere with the lift vector pointed down, so the vehicle is diving. Then at just the right moment based on time or velocity, switch to lift up, so the vehicle pulls out and flies along at low altitude," Putnam said. 

"This enables the vehicle to spend more time flying low where the atmospheric density is higher. This increases the drag, reducing the amount of energy that must be removed by the descent engines."