Nosedives would make for better landings on weighty Mars missions
Spacecraft destined for Mars keep getting larger and heavier. NASA's Curiosity rover is already nearly one tonne, but payloads will continue to increase, especially if humans become part of the equation. A new study by the University of Illinois at Urbana-Champaign has examined the options for weightier missions in the five- to 20-ton payload range and found that, paradoxically, having heavy spacecraft make hypersonic nosedives into the Martian atmosphere can allow them to be slowed down more efficiently with better control.
When spacecraft return to Earth, the atmosphere acts as a gigantic brake that slows down the craft from hypersonic to supersonic speeds by turning velocity into heat. It's the same for missions to Mars, but the Martian atmosphere is much thinner than Earth's so it's much less effective.
There's enough of an atmosphere on Mars to slow down a lander from Mach 30 (22,253 mph, 35,812 km/h) to a speed where it can be slowed further by means of parachutes and rockets. However, that only works if the lander is relatively light because there isn't enough atmosphere to slow down the lander fast enough. In other words, a heavier probe will run out of atmosphere and slam into the ground before it's slowed down enough to land.
The University of Illinois team's idea is to ditch the parachutes and rely on rockets to slow the lander down after its heat shield decelerates it to Mach 3 (2,225 mph, 3,581 km/h). However, using a rocket means carrying fuel to slow the craft. It also means carrying more fuel to slow down the fuel, and more fuel to slow down the fuel to slow down the fuel, and so on. That means a bigger lander or a smaller payload.
To reduce the fuel load, the team wants to make the aeroshell that protects the lander aerodynamic and with a shifted center of gravity so it can generate lift and be steered. It's the same principle that the Apollo Command Module used for reentering the Earth's atmosphere, and has been used in a limited way by some planetary missions.
What this means for future Mars missions is that instead of trying to slow down high in the Martian atmosphere, it will do a nose dive into the denser layers and be steered horizontally to slow it down in the denser lower atmosphere. In addition, it allows the lander to come down with greater precision instead of in a general area.
"We have a certain amount of control authority during entry, descent, and landing – that is, the ability to steer." says Zach Putnam, assistant professor in the Department of Aerospace Engineering at the University of Illinois at Urbana-Champaign. "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 land 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."
"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. 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.".
The study was published in the Journal of Spacecraft and Rockets.