For Safe Landings On Two Planets
The 2013 National Air and Space Museum Trophy Winners.
- By The Editors
- Air & Space magazine, April 2013
NASA / JPL-CalTech
The National Air and Space Museum trophy is an annual award recognizing both past and present achievements in the management or execution of a scientific or technological project, a distinguished career in air and space technology, or a significant contribution in chronicling the history of air and space technology. For outstanding current achievement, this year the Museum recognizes the Mars Science Laboratory Entry, Descent and Landing team, led by Adam Steltzner, which brought the Curiosity rover to a safe landing on Mars last August. For lifetime achievement, the Museum honors Joe Sutter, the chief project engineer for the world’s first wide-body airliner, the Boeing 747.
Mars Science Laboratory Entry, Descent and Landing Team
Adam Steltzner led the 40-member team at NASA’s Jet Propulsion Laboratory that developed the Sky Crane landing system for the Mars Science Laboratory Curiosity rover. The team began work on the entry, descent, and landing procedures and design in 2003; the rover touched down on Mars on August 6, 2012.
How was the Sky Crane developed?
The Sky Crane came into existence over several years, in fits and starts, with several contributions from different people. Its final germ happened in a great brainstorming session in the fall of 2003. In 1999-2000, after the loss of the Mars Polar Lander, there were some teams working to try and understand how we would land the rover for the Mars Sample Return, which then was on the books for 2003 or 2005. It had been planned to use a legged lander, like the one that had failed on MPL, but the mishap investigation helped underscore some of the weaknesses of a legged lander system. So the teams were looking at [other] ways of delivering a large lander.
There were several ideas out there, one of which was called Rover on a Rope. If you were to imagine the Mars Pathfinder or Mars Exploration Rover landing system and just strip away the airbags and have the rover naked at the end of the bridle, it was akin to that. But that idea was discarded by the teams as being too unstable and unusable.
The method that was chosen was something called the Pallet Lander, in which you take the legged lander and you give it six legs instead of three and you spread them out very flat to make the thing very stable. So the MSL went ahead with the Pallet Lander. Unfortunately, we were struggling with the Pallet Lander [because it was too unstable]. We couldn’t use airbags or legged landers, but the experiences from them were there for us to understand how to innovate. We were forced into innovating by the laws of physics, but it was the experience of the past that allowed us to know how to make a system that was as successful as it was.
So in the fall of 2003, we got [about a dozen engineers] together for a big brainstorming session and threw out on the table everything that we’d previously considered, whether we’d rejected it or not. And we tried to work our way through to get past this logjam we were in with the Pallet. It was out of that brainstorming session and some modifications of the ideas from Rover on a Rope but with some very important additions, like leaving the parachute behind when the two bodies [the rover and its rocket-powered descent stage] are still attached, and waiting until the last minute in vertical flight to do the rover deployment. Those ideas really were the germs that made the Sky Crane happen.
What was it like during the Seven Minutes of Terror?
I had the better part of a decade of my life invested in something that would all go down in the span of seven minutes. The number of things that had to go right to see the fruits of my decade ripen was remarkable: Thousands of lines of code, hundreds of devices, almost all of them mission-critical. So it is terrifying. There was an interesting numerology for us when we were landing because Mars was far enough away from the Earth that it took about 14 minutes for the signal to get from Mars to Earth. So it’s not only seven minutes of terror from the top of the atmosphere down to the surface, but it’s also true that when we first see that first signal, the rover’s been alive or dead for seven minutes on the surface. On landing night, everybody in the control room is just a spectator. The vehicle is flying itself and we’re just along for the ride.
Which part of the landing sequence was most worrisome to you?
The thing that we felt, mathematically, was the single lowest reliability element was probably the parachute, and that’s just [because of] the intrinsic uncertainties associated with parachutes. We throw more than 10,000 troops out of airplanes each year, and largely it’s into a very controlled environment, and we have parachutes designed exactly for those conditions. But we still give them a second parachute because even with all those controls, the odds just aren’t good enough. That’s not the case when you’re moving supersonically in an uncertain atmosphere 10 kilometers above the surface of Mars. So when you do the numbers, you end up convincing yourself that the single highest device risk is the parachute.
But that’s not the thing I was worried about. I was worried we could have missed something on the Sky Crane. It was so new and different. It was the unknowns I was most concerned about. So on landing night, as the data clicked by, I became more and more anxious. I said “Oh my God, is it really going to happen just this easily?” I was pretty wound up for those last 20 or 40 seconds.
And your reaction afterward?
Tremendous relief, tremendous exhilaration, and, frankly, a slight sense of surrealness. To work on something for the better part of a decade, and then to have it done—regardless of the outcome. It’s awesome that it was done successfully, but, I mean, all of a sudden it’s over. It was such a build-up, and then, well, it just happened, now we move on.
The Sky Crane will also be used to land a rover similar to Curiosity in 2020. Are you studying any improvements to the system?
We knew going in that we had some points where the design was not all that we would have hoped it could have been. For example, we were measuring only two components of our velocity in the Sky Crane maneuver with our radar. We wanted to measure three, but because of late antenna development challenges, we could not get the right view angles to do that. So we said, “Well, we understand the local gravity of Mars fairly well at the landing site, so we’ll just estimate the third component.” As it turned out, there was a gravity anomaly at Gale Crater. And that meant that we had an error in our estimates, and we landed much more slowly than we’d anticipated. If that error had been flipped around, we might have landed faster than anticipated and we might have hurt the rover.
That’s something that the team flying in 2020 is going to look long and hard at, whether to put on a third antenna. For the layperson, the Sky Crane will look identical, but there will be some subtle changes the team will make to strengthen its reliability.
In his 2006 book 747, written with aerospace historian Jay Spenser, Joe Sutter recalls the struggles, company politics, and flashes of genius that characterized the design of what was then the world’s largest airliner. In this excerpt, he describes a crisis he faced late in the design process.
Like all airplanes in development, the 747 gained weight during its design. In April 1966, when the contract with Pan Am launched the 747 program, the airplane’s projected gross weight was 655,000 pounds. A year later, it had risen to 680,000 pounds and was still climbing.
This weight growth threatened the entire program. A heavier airplane meant Pan Am and other customers wouldn’t be able to carry as much revenue-generating payload. Juan Trippe [the leader of Pan American Airways] was personally monitoring the progress of the 747. Concerned about its weight growth, he spoke directly with [Boeing president] Bill Allen. It was agreed that Trippe, his assistant Sanford Kauffman, chief engineer John Borger, and other Pan Am officials would come out to Seattle so that I could brief them on this issue.
Pulling no punches, I told Trippe to his face that this weight growth wasn’t just a Boeing problem. In front of Bill Allen and the others, I pointed out that more than half the excess weight was the direct result of ongoing changes specified by Pan Am, which had added to the equipment and amenities they wanted aboard the airplane. The upgrades they had specified for the 747’s seating, passenger lounges, lavatories, galleys, cargo systems, and so on made sense, but every bit of it added weight. “It’s just as much your problem as ours,” I concluded.
Juan Trippe obviously didn’t like what he’d heard but he reserved comment. Nobody else was happy with me either as that meeting ended, but on the basis of that key meeting we redefined the 747’s takeoff weight from 680,000 to 710,000 pounds, which was clearly the right thing to do. If we hadn’t taken that painful step in agreement with Pan Am, the airplane would not have met its mission goals.
Airplane design is the ultimate exercise in compromise. If you increase the fuel load, for example, you need a stronger, roomier structure to house it, so airplane weight and drag go up. You also need more powerful engines to lift it all, which means higher fuel consumption. The design team’s job is therefore to define the optimal balance between these elements that yields the best results. The exception is safety, which is never the subject of compromise. Then as now, my guiding belief is that you’re not living up to the faith placed in you if you don’t play things the way you see them. When you’re in a position of responsibility, you need to do what’s right. In the aerospace arena, if you don’t have the courage to face up to difficult situations—and that includes making sure that unwelcome truths are heard and acted on—then you have no business being a chief engineer.
Weight was now the issue that woke me up at night in a cold sweat. The numbers were coming in very, very high. Worse still, we didn’t know precisely where we stood. Our evolving design was so different in scale from anything the industry had built before that we couldn’t estimate its weight with any certainty.
In an effort to slim the 747 down, I gave my project engineers a weight budget and asked them to pursue lower target weights than they had been coming in with. I thought we were doing pretty well when weight again became a focus of concern on the part of upper management. This time it wasn’t just Boeing Commercial Airplanes that was worried; it was T [Thornton] Wilson, president of all Boeing. [Bill Allen was now chairman of the board]. He assembled a team of top engineers and told them to perform a weight audit on the 747. Managing them was Charles Brewster, a hard-nosed old-timer. A confidant of Wilson’s, Brewster had the president’s ear and didn’t hesitate to use the power this gave him.
Resisting the temptation to feel threatened (“Hey, these guys are undercutting my autonomy”), I chose instead to see this management-imposed review as an opportunity (“My team can certainly use the scrutiny of more engineers, so let’s welcome them”). I ordered my people to support this audit to the best of their ability even as they continued their own efforts to slim down the 747.
The inputs we received from the audit team were pretty much the same ideas my team was coming up with. There was only one audit team proposal that I disagreed with and refused out of hand.
We had given the 747 a triple-slotted flap to keep approach speeds low. We knew we had to have high-lift capabilities for the freighter version of the airplane, which would be landing with heavier payloads than passenger 747s. This requirement led us to do some innovative thinking.
The challenge is that jetliner wings are optimized to high-speed flight. This is great when you’re cruising near the speed of sound, but not during takeoffs and landings when you instead want a wing that’s geared to lower speeds. In general, the way to convert a high-speed wing into one suited to lower speeds is to increase its area and camber.
Increasing the wing’s area reduces its wing loading, which is how much weight each square foot of wing supports. This has the effect of reducing the airplane’s stall speed, the speed at which the airplane can no longer fly and instead begins to fall. Lower stall speeds are particularly desirable during landing, because they mean a slower touchdown, shorter stopping distance, and generally safer operations.
Changing the wing’s camber (that is, how arched or curved it is) also helps reduce landing speeds. So how do you change a wing in flight? If you’ve looked out the window during a jetliner flight, you may have seen the answer. Jets have flaps that extend aft and downward from the rear of the wing. In addition, they have “leading-edge devices” that extend from the front of the wing. These two modifications together significantly increase the wing’s area and camber.
The enormous three-segment flap system did the trick, yielding low approach and landing speeds as well as excellent flight characteristics throughout this critical phase of flight.
I felt strongly that even though the articulated flap array we designed was heavy, it was definitely the way to go for overall safety. The audit gang disagreed, saying we should get rid of the third slot of this triple-slotted flap. When I countered that a double-slotted flap would not meet our performance guarantees for approach, landing, and stall speeds, they replied that those promises should be renegotiated.
Our most important commitments were to launch customer Pan Am. I volunteered to go back to Pan Am and ask whether they would settle for a higher approach speed, but Boeing management turned me down. The audit people felt I was biased and would not do an energetic job of selling this proposed change, which was true, so Maynard Pennell [manager of Boeing’s supersonic transport program] instead attended to it during an SST-related trip to Pan Am in New York. Maynard was well respected. I’m sure he presented this proposed design change as well as anybody could. John Borger heard him out and said, “If you want to take the third segment off, be my guest. But you still have to meet the approach-speed guarantee stipulated in our contract with you.”
Going to a double-slotted flap would have dictated an eight-knot rise in approach speed. Eight knots is 9.2 miles per hour, or 15 kilometers per hour. That’s not a very big difference, so why did it matter to Borger and me both? Because braking distance increases with speed and it’s not a linear increase. Borger knew it would be meaningful in real-world operations and he stuck to his guns, holding us to our promise. I am grateful to him because that triple-slotted flap is a key reason the 747 has consistently been so very safe in service.