Saturday, September 10, 2011
Sunday, July 3, 2011
Thirty Years at the Leading Edge of Spaceflight
Do engineers study history? Back in the 1960's it was assumed that no spacecraft could be winged, because a wing's sharp leading edges would concentrate the heat of re-entry and melt. The only solution was to get rid of the wings. The lifting body was the answer; only large-radius surfaces could be used. The first attempt (I clearly remember this from a NASA video) was to take a cone and cut it in half along the axis, and fly it with the vertex first and the flat side up. The thought was that there would be no wind impacting the flat upper surface. Of course this made no sense aerodynamically since the forces on the body in the cartoon were obviously unbalanced; one has to wonder how much science was behind it.
The problem with lifting bodies was simple. Winged vehicles have enough lift to land on a runway, like an airplane. Lifting bodies without wings don't, at least not at the weight of a real spacecraft in an unpowered glide. The lift-to-drag ration of the M2-F3 was an abysmal 3.1. The HL-10, the basis for the X-38 design, managed 3.6. Over the course of the lifting body program the configuration evolved in a bizarre way. The early lifting body had a rounded bottom and a completely flat top (HL-10). Ironically, by the final version, the X-24B, the shape had been completely inverted, flat on the bottom and rounded on the top, with relatively sharp leading edges(X-24B)very close to today's Space Shuttle. And of all the vehicles flight-tested during the twelve years of the lifting-body program, the shuttle-like X-24B, with its flat bottom and what can only be described as small delta wings, had by far the highest L/D at 4.5.
At the same time the problem that required the lifting body, the impossibility of building a wing that would not melt during entry, had been solved by the development of high-temperature composites. The graphite-reinforced carbon used for the Shuttle leading edges does quite well at 1500C as long as it isn't hit with objects weighting several pounds moving at hundreds of miles an hour. There are many possible improvements but the X-37 has already proven that a modern winged spacecraft can land autonomously on a runway, under precise control.
Wings and fuselage have such different tasks that a single shape simply cannot do both efficiently. The only actual winged vehicle proposed for the CCDev, the OSC Prometheus, was rejected, apparently, in part, because it carried only four people. The HL-20 was chosen instead, with its futuristic appearance and the ability to fit six seats. It isn't clear whether the selection board considered the fact that one can land on a runway and the other cannot.
The X-37 maintained the shuttle's sharp leading edges and separate wing and fuselage but was a significant advance over the shuttle in two ways; the wing was moved amidships and a separate tail was added; this gave much greater pitch trim authority, reducing the Shuttle's sensitivity to CG shifts. The X-37 also deleted the vertical tail, almost useless during entry because of wake shadowing at high angles of attack, and replaced it with a V-tail with two fully movable surfaces. The TPS was also improved and high-temp metallic skin used in some areas. It appears to be close to the optimum shape for a RLV, unfortunately NASA inexplicably abandoned the project to the DOD, where it is now classified. DOD unfortunately has no manned space misison and will likely drop the project when it is under budget pressure.
NASA has taken a strange direction in returning to the lifting body for both the X-38 (the final iteration of the Crew Emergency Return Vehicle program), and the Dream Chaser, which was chosen over OSC's winged Prometheus for the CCDev program. One wonders if an objective aerodynamic tradeoff was performed, particularly considering the excellent performance of the X-37. Eventually, I believe, aerodynamics will prevail over tradition; wings and fuselage do jobs which are so different that it is impossible for one shape to do them both well.
The problem with lifting bodies was simple. Winged vehicles have enough lift to land on a runway, like an airplane. Lifting bodies without wings don't, at least not at the weight of a real spacecraft in an unpowered glide. The lift-to-drag ration of the M2-F3 was an abysmal 3.1. The HL-10, the basis for the X-38 design, managed 3.6. Over the course of the lifting body program the configuration evolved in a bizarre way. The early lifting body had a rounded bottom and a completely flat top (HL-10). Ironically, by the final version, the X-24B, the shape had been completely inverted, flat on the bottom and rounded on the top, with relatively sharp leading edges(X-24B)very close to today's Space Shuttle. And of all the vehicles flight-tested during the twelve years of the lifting-body program, the shuttle-like X-24B, with its flat bottom and what can only be described as small delta wings, had by far the highest L/D at 4.5.
At the same time the problem that required the lifting body, the impossibility of building a wing that would not melt during entry, had been solved by the development of high-temperature composites. The graphite-reinforced carbon used for the Shuttle leading edges does quite well at 1500C as long as it isn't hit with objects weighting several pounds moving at hundreds of miles an hour. There are many possible improvements but the X-37 has already proven that a modern winged spacecraft can land autonomously on a runway, under precise control.
Wings and fuselage have such different tasks that a single shape simply cannot do both efficiently. The only actual winged vehicle proposed for the CCDev, the OSC Prometheus, was rejected, apparently, in part, because it carried only four people. The HL-20 was chosen instead, with its futuristic appearance and the ability to fit six seats. It isn't clear whether the selection board considered the fact that one can land on a runway and the other cannot.
The X-37 maintained the shuttle's sharp leading edges and separate wing and fuselage but was a significant advance over the shuttle in two ways; the wing was moved amidships and a separate tail was added; this gave much greater pitch trim authority, reducing the Shuttle's sensitivity to CG shifts. The X-37 also deleted the vertical tail, almost useless during entry because of wake shadowing at high angles of attack, and replaced it with a V-tail with two fully movable surfaces. The TPS was also improved and high-temp metallic skin used in some areas. It appears to be close to the optimum shape for a RLV, unfortunately NASA inexplicably abandoned the project to the DOD, where it is now classified. DOD unfortunately has no manned space misison and will likely drop the project when it is under budget pressure.
NASA has taken a strange direction in returning to the lifting body for both the X-38 (the final iteration of the Crew Emergency Return Vehicle program), and the Dream Chaser, which was chosen over OSC's winged Prometheus for the CCDev program. One wonders if an objective aerodynamic tradeoff was performed, particularly considering the excellent performance of the X-37. Eventually, I believe, aerodynamics will prevail over tradition; wings and fuselage do jobs which are so different that it is impossible for one shape to do them both well.
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