Free Bold They Rise: The Space Shuttle Early Years, 1972-1986 (Outward Odyssey: A People's History of S) by David Hitt, Heather R. Smith Page A
Authors: David Hitt, Heather R. Smith
Tags: History
flight control system they could build. The whole idea was, after we’ve flown and we have some experience and we know what the real world is, now we can come back and make it better, but the first job is to make ours as tolerant as possible to the things we don’t know.
While Mattingly was working with the computer models of the flight dynamics of the shuttle, astronaut Hank Hartsfield was on the other side of that research, working with the wind tunnel models and encountering the same concerns about the scalability of the data coming out of those tests.
As I recall, the shuttle program had over twenty-two thousand hours of wind tunnel time to try to figure out what it flies like. Because the decision had been made, there are no test flights. We were going to fly it manned the first flight, and an orbital flight at that, which demanded that, the best you can, [we] understand this. Well, hypersonic aerodynamics is difficult to understand, the uncertainty on the aerodynamic parameters that you get out of the tunnel are big. The things that we were looking at in the simulations were if these uncertainties in the different aerodynamic parameters stack in a certain way, the vehicle could be unstable.
What we were looking for, for those combinations, statistically were possible, but hopefully not very probable they’d happen, but if they did, that was the kind of things we had to plan for. It’s just an uncertain world. You can’t predict, because in the wind tunnel, you have to put in scaling factors. If you’re doing wind tunnel things off a small model, it doesn’t really scale to the big model perfectly, and you have to make assumptions when you do that. The scaling ratios have a big factor, a big effect on what the real numbers are. So if you could fly a full-scale orbiter in the wind tunnel and it would go Mach 15 or something, it would be great, but you can’t do that. You have a little-bitty model, and it’s a shock tunnel or something. You’d get a few seconds of runtime at the right Mach numbers and then try to capture the data off of that.
6. Space Shuttle vehicle testing in the fourteen-foot Transonic Wind Tunnel at NASA ’s Ames Research Center. Courtesy NASA .
Astronaut Don Peterson was involved in studying the redundancy of systems on the orbiter, and particularly the flight control computers. In the report he pointed out that failure rates on some of the avionics could be high. On Apollo and earlier vehicles, NASA built “ultra-reliability components,” components that were overdesigned and tested to make failures less likely.
Failures on Apollo, for that reason, were pretty rare. But that’s very expensive. That’s a very difficult thing to do. I was told that after the lunar program ended, MIT had two of the lunar module computers left over, spares. So they just turned them on and programmed them to run cyclically through all the programs. I think they ran one of those computers for, like, fifteen years, and it never failed. It just kept running, and finally they turned it off. They just said, “It’s not ever going to fail.” That’s the way that equipment was built. But that makes it veryexpensive. So when they built the Shuttle, they said, “We can’t do that. So what we’re going to do is, instead of ultrareliability components, we’re going to rely on something called redundancy.” They were going to have four computers, and they were going to have three TACAN s, and they were going to have four of this and two of that and so on. That way, you could tolerate failures. But as a result of that, the failure rate on some of that equipment was fairly high, compared to Apollo.
They also made the multiple units interdependent. “On a typical automobile you have five tires, but that’s not five levels of redundancy because you need four of them,” Peterson explained.
So you can really only tolerate one failure. You can have one tire go bad and you can take care of that. But we got into
While Mattingly was working with the computer models of the flight dynamics of the shuttle, astronaut Hank Hartsfield was on the other side of that research, working with the wind tunnel models and encountering the same concerns about the scalability of the data coming out of those tests.
As I recall, the shuttle program had over twenty-two thousand hours of wind tunnel time to try to figure out what it flies like. Because the decision had been made, there are no test flights. We were going to fly it manned the first flight, and an orbital flight at that, which demanded that, the best you can, [we] understand this. Well, hypersonic aerodynamics is difficult to understand, the uncertainty on the aerodynamic parameters that you get out of the tunnel are big. The things that we were looking at in the simulations were if these uncertainties in the different aerodynamic parameters stack in a certain way, the vehicle could be unstable.
What we were looking for, for those combinations, statistically were possible, but hopefully not very probable they’d happen, but if they did, that was the kind of things we had to plan for. It’s just an uncertain world. You can’t predict, because in the wind tunnel, you have to put in scaling factors. If you’re doing wind tunnel things off a small model, it doesn’t really scale to the big model perfectly, and you have to make assumptions when you do that. The scaling ratios have a big factor, a big effect on what the real numbers are. So if you could fly a full-scale orbiter in the wind tunnel and it would go Mach 15 or something, it would be great, but you can’t do that. You have a little-bitty model, and it’s a shock tunnel or something. You’d get a few seconds of runtime at the right Mach numbers and then try to capture the data off of that.
6. Space Shuttle vehicle testing in the fourteen-foot Transonic Wind Tunnel at NASA ’s Ames Research Center. Courtesy NASA .
Astronaut Don Peterson was involved in studying the redundancy of systems on the orbiter, and particularly the flight control computers. In the report he pointed out that failure rates on some of the avionics could be high. On Apollo and earlier vehicles, NASA built “ultra-reliability components,” components that were overdesigned and tested to make failures less likely.
Failures on Apollo, for that reason, were pretty rare. But that’s very expensive. That’s a very difficult thing to do. I was told that after the lunar program ended, MIT had two of the lunar module computers left over, spares. So they just turned them on and programmed them to run cyclically through all the programs. I think they ran one of those computers for, like, fifteen years, and it never failed. It just kept running, and finally they turned it off. They just said, “It’s not ever going to fail.” That’s the way that equipment was built. But that makes it veryexpensive. So when they built the Shuttle, they said, “We can’t do that. So what we’re going to do is, instead of ultrareliability components, we’re going to rely on something called redundancy.” They were going to have four computers, and they were going to have three TACAN s, and they were going to have four of this and two of that and so on. That way, you could tolerate failures. But as a result of that, the failure rate on some of that equipment was fairly high, compared to Apollo.
They also made the multiple units interdependent. “On a typical automobile you have five tires, but that’s not five levels of redundancy because you need four of them,” Peterson explained.
So you can really only tolerate one failure. You can have one tire go bad and you can take care of that. But we got into
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