The Lad watched the NASA website on the morning of Monday, August 6th 2012. There was a live feed from the Flight Control Room in Pasadena as the Mars lander Curiosity was finally deposited on the surface of Mars.
This was no mean feat, for one reason, as the lander was a vehicle as big as a car. For another reason, several other attempts to put vehicles onto Mars have failed. Students should note that, famously, there was a crash landing rather than a controlled descent cost hundreds of millions of dollars. The cause? The calculations supporting the descent were based on a confusion of metric units with imperial units.
Hasn’t every practising engineer suffered something similar? Luckily he or she will not have done it in such a crucial situation or, if so, will have managed to recover the error before disaster was committed to paper or, worse, hardware. Such a risk is exactly the enemy with which the engineer wrestles every day. She or he tries to haul pride out of the successful design of a large or small project. It is a major part of what every engineer does. Then it is not an error in an exam question; it is real.
One of the most difficult features of the engineering task was communication with the craft as it approached Mars. Quite simply, Odyssey, carrying Curiosity, could hear no radio instruction for 14 mins even at the speed of light and could not return any data, as to what it is doing for example, for the same period. It had to operate automatically, deciding what to do from its own pre-programmed instructions and its own instrumentation resources.
Another remarkable difference between this Martian landing and the Moon landings was the engineering of the descent process. These differences were true engineering and not science. Curiosity was very heavy [nearly 1000kg or one ton] yet needed, with only limited stored energy, to be able to move over the Martian landscape for at least a Martian year. Every part of the vehicle had to be highly efficient in its function with not a gramme to spare.
They set the delivery design criterion to limit the landing shock to be no greater than those which they were expecting as it roved over the boulders and pot holes in the Mars landscape. The vehicle would have, inescapably, to be designed to resist these anyway.
Curiosity was too massive for gas-filled shock absorbing balloons. The design engineers could not justify heavy, mechanical, shock absorbers. They would be used only during the landing and be, ever after, dragged around as so much dead weight. How about retro-rockets firing away merrily right down to the surface under the computer control of a modern radar altimeter? The problem with this is that the violent dust storm kicked up would damage or destroy the vehicle control and scientific systems. Their solution is remarkable.
In the Entry Descent and Landing [EDL] phase, Mars gravity began accelerating the vehicle, Odyssey, towards Mars at many thousands of miles per hour. But then the resistance of the Martian atmosphere was used to slow it down. It converted the vast, kinetic energy of Odyssey into red heat at the face of the conical vehicle. So far so good; we have done it a thousand times before when bringing craft back to Earth. Trouble is that because the Mars atmosphere is much, much thinner than that of the Earth; we can lose only a relatively small amount of the kinetic energy. So, when it gets to be very low and close to the surface it is still going at a hell of a lick. So a giant parachute is then deployed to slow it down some more. That is alright too, for we have done this successfully many times before on Earth. Then though, we were landing in the sea or had heavy shock absorbers.
So what now for the engineers? Now they come to the stickiest bit. Yes, retro-rockets were used to bring the vehicle to zero velocity. The thing is that it was designed so that, when it came to a halt, it was not on the ground. It was several metres above it: here it solemnly hovered too high for the rockets to kick up too much dust. Now break out the Sky Crane. Curiosity left Odyssey and was slowly lowered by nylon rope tethers to the ground.
This is how NASA described the process.
In the depicted scene, the spacecraft’s descent stage, while controlling its own rate of descent with four of its eight throttle-controllable rocket engines, has begun lowering Curiosity on a bridle. The rover is connected to the descent stage by three nylon tethers and by an umbilical providing a power and communication connection. The bridle will extend to full length, about 25 feet (7.5 meters), as the descent stage continues descending. Seconds later, when touchdown is detected, the bridle is cut at the rover end, and the descent stage flies off to stay clear of the landing site.
Engineers realise that even the reliable connection and release of nylon rope cannot be a simple task. The risk of a failure needs to be vanishingly small when you are designing a tether mechanism with a billion dollar payload hanging on it. The mechanism MUST work when instructed by a computer after months in space. How would you do it?
Then what happens when the rockets of the Sky Crane are expended? Does it simply fall on top of Curiosity? Noooo! The engineers have designed it to do the ‘Flyaway’ where it scoots off to one side some way away. It is not clear to The Lad however whether, after this, it resembles anything more than a fast-food packet thrown out of a car window.
Congratulations to the engineers of NASA and its contractors for an excellent job. So far! They are hoping that Curiosity will not impose the proverbial fate on the NASA cat for many months to come and so is The Lad.
The Lad has to thank the NASA website for some of the background. Remember, this post contains only the simplest of discussions of only one of the problems faced by the engineers. Without any exaggeration, there will have been thousands and thousands of other problems and optimisation decisions. This Is Engineering.
Engineering is one of the three drivers advancing the human race. This blog describes real professional engineering as it is in the real world. It is not well served by the current media. An engineer is posting: not a ‘scientist’. Its target is the career seeker and also the general public.