Safely making tracks on Mars after LDS V994 testing

Images courtesy of NASA/JPL-Caltech
04 Oct 2012


To the immense relief of the engineers involved, the most advanced rover ever has arrived safely - rewarding another world-first attempt whose success hinged on surviving the journey.


News


Enlarge image
The third Mars rover, Curiosity

Can you remember what were you doing on 26 November 2011, at about 10:02 AM? The engineers involved with the latest Mars exploration project can. On that cool and slightly cloudy morning, they watched the culmination of six years work endure a violent ordeal atop nearly 280 tonnes of burning rocket fuel, as an Atlas V 541 rocket left the Earth vertically from the ground where it stood.

Now imagine that for eight and a half months since then, until August 2012, you had lived with prolonged uncertainty as a small spacecraft hurtled through the vastness of space, as you waited to see if that stomach-churning November launch had been successful.

Yet still on top of this came the final agony that even after the turbulent descent through the Martian atmosphere had been completed, those engineers had to wait a further seven minutes. Seven minutes to wonder whether a signal from the rover was already racing to Earth or if one of the thousands of components had failed and transformed the rover into a billion-dollar Martian ornament.

Seven minutes of terror























Video: NASA's seven minutes of terror
“From the top of the atmosphere down to the surface it takes it seven minutes. It takes 14 minutes or so for the signal from the spacecraft to make it to Earth – that’s how far Mars is away from us. So when we first get word that we’ve touched the top of the atmosphere, the vehicle has been alive, or dead, on the surface for at least seven minutes,” says Adam Steltzner, Entry, Descent and Landing (EDL) Engineer.

According to another EDL Engineer, Tom Rivellini, “Entry, descent and landing – also known as EDL – is referred to as seven minutes of terror, because we’ve got literally seven minutes to get from the top of the atmosphere to the surface of Mars, going from 13 000 MPH to zero in perfect sequence, perfect choreography, perfect timing; and the computer has to do it all by itself with no help from the ground. If any
one thing doesn’t work just right, it’s
game over.”


"If any one thing doesn’t work just right, it’s game over"
So it is something of an understatement to say that when a safe rover touchdown was confirmed in the early hours of August 6th, by Curiosity beaming back images of its own shadow on the Martian surface, it was a relief.

Entry, descent and landing

The many descent stages end with Curiosity being lowered from a hovering platform, which then flies away to crash clear of the landing zoneEnlarge image


"When it opens at that speed, it’s a neck-snapping 9Gs"
Each of the unique missions to Mars has seen specialised delivery methods being tried for the first time. To understand the demands, imagine the journey and the descent that the spacecraft made with the delicate rover inside it. Like attempting a hole-in-one in golf, once this one-way trip was set in motion it was just a matter of waiting and hoping that everything worked, had been perfectly assembled, and then survived the traumatic launch, descent stages and landing.

“When people look at it, it looks crazy. That’s a very natural thing. Sometimes when we look at it, it looks crazy,” says EDL Engineer Adam Steltzner, about the turbulent descent and the landing sequence.


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Curiosity in the backshell prior to the the heatshield being fitted, and facing the ground as it was on landing

The final sequence of events began 10 minutes before hitting the atmosphere, when the spacecraft’s cruise stage separated and final preparations for entry began, before the spacecraft hit the atmosphere at about 13 000 MPH. Immediately beginning to slow down, the spacecraft used thrusters to help steer it toward the landing target. After slowing to about Mach 2, the largest supersonic parachute ever built opened up to slow it down to about 200 MPH. “When it opens at that speed, it’s a neck-snapping 9Gs,” says EDL Engineer Tom Rivellini. Once below the speed of sound, the spacecraft’s heatshield separated so the spacecraft could look groundward with its landing radar. “It’s like a big lens cap blocking our view of the ground,” says NASA’s Steve Lee.

Speaking before the landing, EDL Systems Engineer Allen Chen described the final touchdown: “Once we’ve reached an altitude of about one mile, the spacecraft drops out of the back shell at about 200 MPH. It then fires up the landing engines to slow it down even further. Once we’ve descended to about 60 feet above the ground, and are going only about 2 MPH, the rover separates from the descent stage. As the rover is lowered, the wheels deploy. The descent stage cuts the rover loose and flies away, leaving rover safe on the surface of Mars. ”

Testing

Coming from NASA, a name that is synonymous with instigating the very best in high technology advances that the world can produce, it comes as no surprise that testing procedures on Curiosity had been exhaustive prior to the November launch.

According to a NASA spokesman, “It’s been put through its paces. It’s been in a thermal vacuum chamber, kept very cold, parts have been in a centrifuge; we’ve done drop tests, pull tests, drive tests, load tests, stress tests, just an amazing amount of testing this vehicle has been through. We’ve done shorting tests – taking the vehicle and shorting electronics – checked the radios all work together and that the rover doesn’t interact with itself in bad ways. And literally thousands and thousands of software testing hours.”


Video: Curiosity undergoing testing on an LDS shaker system
What’s more, the test procedures must stick to a demanding schedule, as Integration Engineer Eric Poole says, “You don’t want any glitches when you get down to the week before launch. The planets only align every two years, and you only get about a three-week window … so it is very critical that we don’t have anything that would delay our schedule.”

Random vibration testing

NASA’s JPL uses an LDS V994 shaker with an optional guided head-expander

Significantly, the vibration testing of Curiosity was performed on the actual flight model that is now on Mars, calling for precise test accuracy so as to stay within test limits.

“Here at the lab we often call the environmental test portion ‘Shake-and-bake’,” says Antony Ganino, Vehicle Integration Lead at the Jet Propulsion Laboratory (JPL) in California. “We only get one chance to get on Mars and drive this vehicle around, so we want to put it in the harsh environment that it is going to see, and make sure that not only do all of the instruments function, but all of the temperatures that we expect to see on the vehicle are accurate to what we have modelled and planned.”

A whole series of random vibration tests were performed on Curiosity to ensure that the hardware was built and assembled correctly, and would survive the launch conditions. “This test is like putting curiosity through a major earthquake, it’s going to shake it both side-to-side and up and down,” said Randy Stark of the Environmental Test Facility at JPL. The testing was performed with Curiosity in its flight configuration, which is upside-down, and involved shaking it at 1.5 G (1.5 times the acceleration force of gravity on Earth), at a frequency of 10- 400 Hz.

Next in Curiosity’s busy test schedule came system thermal vacuum testing, where NASA simulated the hot and cold environments that Curiosity would see during its journey and during its life on Mars, with conditions down to –130ºC and up to 40ºC, and atmospheric pressure at 1/100th of Earth’s.

“It sure seems like we are putting Curiosity through a lot of abuse,” says Mr Stark, “But the more testing we can do here on Earth will ensure a safer journey on the way to Mars, and a longer life once we get to Mars.”

A flight model and an engineering model

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The DTM called ‘Scarecrow’ was tested on all potential surfaces like non-cohesive sand, rocks and slopes up to 20 degrees, in order to avoid the problems encountered by previous rovers of getting stuck in the dry earth of Mars

In common with satellites of many types, the newest rover to grace the surface of Mars has a sibling left on Earth that was never intended to fly. While the flight model is now leaving tracks in the dry Martian dust, the nearly identical engineering model, or ‘Dynamic Test Model’ (DTM) was used solely to develop the systems that are used in the flight model. “The reason we do all of this testing is to prove that what we think will happen does happen when Curiosity gets to Mars, and that we really understand the dynamics of these vehicles,” explains Savannah McCoy, Rover Verification and Validation Lead.

The DTM was used to run system-level tests on the rover’s structure, such as on the deployment of the wheels, which were used to touch down on the surface for the first time in this mission. However, such tests as the ‘Skycrane Full Motion Drop Test’ were still nervous experiences.


"Because this test is so important to the project, almost all of the team gathered to watch it"
“Because this test is so important to the project, almost all of the team gathered to watch it,” explains Mrs McCoy.

Significantly, the vibration testing of Curiosity was performed on the actual flight model that is now on Mars, calling for precise test accuracy to stay within test limits.

Mission


Curiosity's Sample Analysis at Mars Instrument  - just one of the complex systems aboard

Sensitive components
As the temperatures that Curiosity can encounter vary from +30 to −127 °C, a heat rejection system uses fluid pumped through 60 metres of tubing, to keep sensitive components at optimal temperatures.

Meeting the power demands is a Multi-Mission Radioisotope Thermoelectic Generator (MMRTG), which contains a specially produced form of plutonium dioxide. The natural decay of this radioisotope gives off heat, which thermocouples turn into electricity. The generator provides both electrical power and heat to the rover.

After enduring its stressful life prior to its actual mission and successfully negotiating the ‘seven minutes of terror’, Curiosity is now seeking to find out if Mars could ever have supported life - and laying the foundations for future human missions to the red planet. “Curiosity’s scientific mission involves driving around its landing site – perhaps up to 15 or 20 miles, and collecting samples of rocks and soils with a big jack-hammer drill located on the end of a 6-foot robotic arm,” says a NASA spokesman.

Over two times bigger, five times heavier, and carrying 15 times the weight of scientific equipment compared to the previous rovers Spirit and Opportunity, Curiosity contains a complex chemistry kit to zap rocks, drill soil and gather samples. These samples are delivered to the rover and analysed with some sophisticated and power-hungry analytical laboratory instruments. During the mission, the rover is expected to operate for at least 686 days as it explores with greater range than any previous Mars rover and is expected to traverse a minimum of 12 miles in its two-year mission.





You can see more about our vibration test systems here


All quotes are from publicly available NASA videos 
Images courtesy of NASA/JPL-Caltech 



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