Propelling Perseverance: The legacy of Viking is helping NASA get to Mars
by Joe Cassady
|Engineers at JPL came to Aerojet Rocketdyne with a question: can you build the same descent engines that you built for us in the 1970s for the Viking program?|
Of course, any trip to Mars starts out needing a big push to get free of Earth’s gravity well and on its way into interplanetary space. Back in the Viking days, we had the mighty Titan booster, and today we are fortunate to have the Atlas 541, with four AJ-60 solid rocket boosters and a Centaur upper stage. Interestingly, the Titan also used a Centaur upper stage powered by the RL10 engine. It is hard to beat the performance of the high-energy Centaur for launching spacecraft to interplanetary destinations. That’s probably why this launch of Mars 2020 marked the 502nd flight of the RL10.
That piece of common heritage is not the real story, however. Nor are the myriad small thrusters used to steer and align Mars 2020 as it enters the Martian atmosphere and begins its high-speed dive, travelling at 21,000 kilometers per hour, until it achieves a gentle touchdown on the surface a mere seven minutes later. The one takeaway you might glean, when you get only one chance for success, is that you want to go with tried and true thrusters. And that’s where the real story begins.
Back in the first decade of the 21st century, as NASA was preparing for its most ambitious Mars landing yet, engineers at JPL came to Aerojet Rocketdyne with a question: can you build the same descent engines that you built for us in the 1970s for the Viking program? The reader may pause here and ask themselves, “Why would JPL want or need engines from the 1970s for their new mission?” The answer is the amazing engine known simply as the MR-80.
The MR-80 design is based on the most widely flown type of in-space rocket engine, monopropellant hydrazine. These engines are simple and reliable. The best testament to that is that engines built in Aerojet’s factory in Redmond, Washington, in the mid-1970s are still operating today on the Voyager spacecraft that are flying at this very moment outside our solar system! The real challenge that JPL presented was that the engines needed to be large enough (800 pounds-force, or 3.5 kilonewtons, thrust) and be capable of throttling down to about 10% of that rated thrust. A 10:1 throttle ratio was something that had never been accomplished before. The engineers responded and dealt with the two separate challenges—scale-up and throttling down—with unique and innovative approaches. The result was the threesome of engines that successfully landed both Vikings on the Martian surface in 1976.
Now fast-forward 30 years, and NASA presents a new challenge. It is the landing of an SUV-sized rover on Mars, a mission called the Mars Science Laboratory (MSL) and its Curiosity rover. And to accomplish this major increase in the size of the landed mass, our space agency devised a new approach called the “skycrane”. Basically, rather than having the rover ride on top of the lander and trundle down a ramp after landing , they designed it to be slung underneath and to be lowered gently to the surface with a tether. Of course, this required the skycrane to slow down enough to come to a hover over the surface and to remain there in a stable position long enough for the rover to be lowered down to rest on its wheels. Then the skycrane needed to fly away from the rover and get out of the way. In order to make all this work, JPL concluded that it needed—have you guessed yet?—MR-80 engines with the same throttling capability. So, the phone in Redmond rang again.
|No matter how daunting the challenge, such as getting Perseverance safely to the Martian surface, we can overcome if we help each other and keep moving forward.|
While it may seem like this should have been an easy request, it was not. The MR-80 engines had not been built in more than 30 years. Several of the key suppliers were either out of business or no longer made the particular parts that had been used before. Imagine how terrible it would be if the reply to JPL had to be, “We’re sorry, but we no longer can make those engines you need!” Nobody wanted to make that call, from the assembler in the shop to the CEO. The entire company was determined to find a way. And succeeded. We started out by scrambling to find the remaining engines that were still in existence from the Viking program. After numerous phone calls and personal connections, we were able to gather up an entire set of Viking MR-80 parts. That provided the basis for setting the requirements for the new parts and processes that were needed. We worked to bring new suppliers in, and collaborated with NASA and JPL to determine the exact configuration for the new engines. And the rest is history.
The MR-80 engines were resurrected, tested, and proven capable to meet the needs of the Skycrane. MSL flew in 2012 and, after the “seven minutes of terror” through the Martian atmosphere, Curiosity was gently deposited onto the surface of Mars. Our success was the triumph of dozens of companies and teams, many hundreds of individuals, and literally thousands of hours of hard work. We just had to persevere! That’s what is important to remember as we face challenges as a nation and as a global community. No matter how daunting the challenge, such as getting Perseverance safely to the Martian surface, we can overcome if we help each other and keep moving forward. And this is why I love the challenges of space and especially the goal of getting ever more sophisticated robots and eventually humans to Mars. As President Kennedy said in his speech at Rice University about the Moon landings, it “serves to organize and measure the best of our energies and skills.” Let’s go to Mars!
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