Soft landings: Valves to help probes reach Mars

On the cold, barren landscape of Mars, violent winds create dust storms that engulf the planet for months at a time. It never rains, temperatures drop to -90°C at night and the atmosphere is 100 times thinner than Earth’s.

But it wasn’t always this hostile. Billions of years ago, the Red Planet may have been a warm paradise of blue skies and lakes. Astronomers claim Mars could unlock the secrets of how our solar system formed, and the race to find signs of life on the Red Planet is now heating up.

Manned missions are planned for the 2030s, but before then both NASA and the European Space Agency (ESA) plan to send probes to the planet’s surface. NASA recently revealed details of its next Mars rover, which will launch in 2020, while ESA is hoping to send the ExoMars rover in 2018.

Both agencies not only have to contend with making the 34-million-mile journey through space, but once there, they will have to somehow land the probes on the planet’s surface. It’s a manoeuvre in which years of planning could go wrong in a matter of seconds — a lesson that Beagle 2 engineers know only too well.

Previous missions have used airbags to soften the landing, but there was a problem with these ‘bouncing ball’ systems. Essentially a set of non-vented cushions that inflated before the lander touched down on the surface, the airbags would protect the payload but would also cause the lander to bounce as many as 10 times before coming to a rest — potentially leaving it upside down and far from the intended landing site.

Aero Sekur believes it has a better solution. Under a £10m contract with ESA, the Anglo-Italian company has developed a parachute and airbag that immediately deflates upon landing to guide the ExoMars rover to the Red Planet’s surface in a more controlled way — and with an unusual parallel. ‘If you remember as a kid, the fart cushions: that is actually an accepted methodology of absorbing shock on landing,’ said Aero Sekur chairman Mark Butler.

‘The trouble is they are very crude. While it absorbs some shock, that is effectively all it does. It still is a largely uncontrolled process,’ he said. Butler explained that Aero Sekur’s system uses vented airbags that deflate to just the right pressure for the probe to land upright. A sophisticated valve system is used within the design, alongside complex algorithms and extremely tough fabrics.

We’ve tested in some extraordinary conditions. It can cope with the ground sloping up 45˚ and the load sloping 15˚ in any other direction

Mark Butler, Aero Sekur

‘What was engineered is effectively a large doughnut,’ said Butler. ‘It’s about 10m [in diameter] and divided up into 12 segments. Sitting in the hole is the payload and the segments go around the outside. Each of those segments has a fabric valve rigged up to a little explosive device that releases them. Those in turn are rigged up to a very complex control system.’

Within each of the segments is a range of sensors linked to a central computer. In the moments before the system comes down to the surface, the airbags are deployed, taking around three seconds to inflate completely. In the nanosecond the lander hits the ground, the sensors calculate the pressure differentials, the attitude of the load and the velocity. Valves then open each of the segments in the correct order to make sure the payload doesn’t bounce.

‘That’s really the key,’ said Butler. ‘It’s not just that the valves all blow open and it absorbs the shock, but blow open in the correct sequence to bring it down absolutely flat… We’ve done a huge amount of testing and through some really extraordinary conditions. It can cope with the ground sloping up 45˚ and the load sloping 15˚ in any other direction.’

The vented airbags are also half the weight of a conventional ‘bouncing ball’ system — a key consideration when every kilogram of weight adds £14,000 to the cost of launch. But designing such a structure was no easy feat, and the engineering team came up against some significant challenges. One of the biggest, according to Butler, was developing the valve and gassing technology to work fast enough for the vented airbags to prove useful.

In many inflatables, explosives are used to blow the head off a valve. ‘Once we started looking at space, [an explosive system] simply wasn’t good enough,’ said Butler. ‘It wasn’t reliable enough. There was nothing to say these caps would even work, never mind work reliably enough under those conditions. Also they require some oxygen to explode.’

Instead, Butler and his team turned to a proven technology used to separate the stages in a rocket. Tiny piezo-electric crystals, which change shape according to the state of charge across them, function as actuators. ‘We engineered that into our own design, which gave us a valve that didn’t need an explosive cap, was fantastically reliable and required almost no power,’ said Butler. The company now uses the same technology in emergency flotation systems for helicopters. Maintenance is far easier and the system can be reset within minutes.

Despite the thousands of hours spent refining and testing the design, Aero Sekur still doesn’t know whether its vented landing system will be chosen for the ExoMars rover. There are a number of competing systems for landing on Mars, including legged landers and mechanisms that act like a crane to lower a rover onto the surface. But as Butler puts it, no one gets into the space industry for the money.

‘One of the reasons that we started the space business is that it acts as a technology driver,’ said Butler. ‘So we are developing systems and capabilities for space that we can then transfer across into our other areas, such as hard landings for helicopters.’ The innovation it brings, he claims, is well worth the months of uncertainty.

Landing on Mars: the options

Legged landing systems

The legs of the 1976 Viking mission lander were the first-generation landing system technology. It was also developed for the lunar Surveyor and Apollo programmes in the early 1960s. The mechanical solution is typically integrated into the same area that houses the scientific and engineering subsystems.

Airbag landing systems

This technology was developed for the Mars Pathfinder mission and improved upon for the Mars Exploration Rover missions. They have a combination of fixed-thrust solid-rocket motors and airbags to perform the touchdown task. The main drawback of early airbag systems was the bounce they inflicted on the probe upon impact.

 

The sky-crane landing system

The idea for a sky crane first appeared in 1999, following the crash of the Mars Polar Lander. The rocket-powered, hovering vehicle lowers a rover on tethers and places it directly onto a planet’s surface. It was used to land the Curiosity Rover on the Martian surface in 2012.