Wings in space
by James C. McLane III
|One shuttle idea promoted by Max Faget, designer of the Mercury capsule, was a relatively conventional aircraft, a reusable straight-winged vehicle like Sänger’s concept that might be covered with thin insulation or perhaps even an easily renewable ablative material.|
Von Braun’s space plane reflected the idea that with large wings relative to a vehicle’s weight, the tremendous heat of hypersonic atmospheric entry could be handled by metals selected for resistance to high temperature. No special heat shielding, like the ceramic tiles that cover the current Space Shuttle orbiter, would be needed. Was von Braun so naive about how very hot an orbiting object entering the atmosphere would get that he mistakenly believed an uninsulated aircraft could take the heat?
Twenty years earlier, in the 1930s, another German engineer, Eugen Sänger, had recognized that with rocket power, very high speed flight at extreme altitudes would be possible. Sänger designed a straight-winged space plane that might be able to glide from Europe to other points on the globe, skipping into and out of the atmosphere along the way. By the 1940s Sänger’s space glider had progressed to the wind tunnel testing stage. About that same time, the German army made unsuccessful attempts to extend the range of their new V-2 rocket by adding primitive wings based on Sänger’sspace glider concept.
By the late 1960s, the US had revived the idea of a winged spacecraft. During conceptual design of the Space Shuttle, several alternative configurations were proposed. One idea promoted by legendary NASA engineer Maxime (Max) Faget, designer of the Mercury capsule, was a relatively conventional aircraft, a reusable straight-winged vehicle like Sänger’s concept that might be covered with thin insulation or perhaps even an easily renewable ablative material. It would enter the atmosphere at a steep, nose-high angle, virtually in a full stall condition (although strictly speaking, the wake behind the wings at such speeds would not be similar to the air above a stalled airplane wing). When it reached denser air near the ground, the space plane would push its nose down and transition to level gliding flight. It would land at relatively slow speed because of its low wing loading (the vehicle weight divided by the wing lifting area) and also the wing itself would be efficient, straight, and high-lift.
In the 1970s I knew a young mechanical engineer who worked at NASA’s Manned Spacecraft Center (now named the Johnson Space Center). I’d met Billy Campbell years earlier when we were both in college. Billy joined NASA during the Apollo program and designed tools the astronauts used on the moon. I got my engineering degree and went to work for Mooney Aircraft in Kerrville, Texas. Today Mooney is largely forgotten, but back then they were the world’s third-largest builder of private airplanes. They produced one airplane a day until high bank interest rates killed the market for private light planes. Mooney suspended production and I was out of a job and back living in Houston.
One evening I was visiting Billy and he began talking about NASA’s new Space Shuttle project. He was designing parts for a large flying model of Max Faget’s straight wing space plane. Billy was having problems with the control surfaces; getting the profile right and deciding on their size and shape. Since I’d worked on light aircraft at Mooney, Billy asked if I could help him.
|In some ways it’s unfortunate that the shuttle orbiter flew so well, because other potential wing arrangements were soon forgotten.|
We got into his car and drove out to the Manned Spacecraft Center. The gate guard saluted as we passed. We parked near Billy’s building. It was 8 or 9 pm, but folks inside were still bustling around. We proceeded to a large, cluttered room like a workshop. In the center the aluminum frame of an airplane rested on support stands. Parts, tools, and drawings were scattered around on tables. For a model, it was big: nearly big enough for me to sit inside the fuselage. I would guess it might have been 15 feet long, but after more than three decades, my memory is rusty. I recall it had metal spars, ribs, and bulkheads, exactly like a conventional aircraft. This was familiar stuff to me from my work on small planes at Mooney. I looked at the tail surfaces and the long, straight wings, the aileron attachment points, the hinge locations on the control surfaces and drawings of the ribs and airfoil profile. The design Billy wanted to use would be prone to flutter, an oscillating, destructive, flapping. I suggested how he could minimize that disastrous possibility by flattening the slight curvature on the control surfaces and adding balance weights at appropriate places.
In today’s world where we are encouraged to be vigilant and look for a potential terrorist hiding behind every tree, it seems hard to believe the “security” that prevailed at the Manned Spacecraft Center in the early 1970s. In this example, I’d walked into an important research lab without prior approval or badging and examined a unique test model of a vehicle that might prove significant to our country’s future in space.
I was happy to be able to help. Back then, civil service employees like Billy did personal, hands-on work and they exercised more individual freedom of action than many NASA engineers have today. This big flying model was later taken to the western desert and dropped from a helicopter to see how well it would glide.
In the 1970’s, hypersonic flow around large objects like the Space Shuttle was not well understood. The straight winged orbiter championed by Max Faget lost out to a delta wing concept, a shape thought to have fewer stability and control issues. Picking the delta wing was less risky. The Russians must have also thought it had less risk since their Buran space plane copied the same configuration.
In some ways it’s unfortunate that the shuttle orbiter flew so well, because other potential wing arrangements were soon forgotten. Today, designers of new vehicles to carry people to and from space point to limitations and problems with the shuttle as reason to abandon winged vehicles and go back to evolutionary variations of the original Mercury capsule. It’s also likely that lifting bodies with wings are more challenging to develop than capsules with simple geometric shapes. But, maybe it’s time to reconsider winged possibilities.
|A space plane with huge lifting surfaces and a very low wing loading might not require any external thermal insulation at all.|
As shown in Sänger’s 1930s space plane design, then in von Braun’s Collier’s concepts and again in the 1970s when Max Faget nearly got to demonstrate his straight-winged orbiter, a low wing loading with shallow atmospheric entry should result in an extended, moderate heat pulse. The ultimate goal is to have a configuration and entry profile that creates vehicle temperatures low enough to use uninsulated metallic skin, or perhaps a skin surface with an easily renewable thermal-resistant coating. The insulating tiles that protect the Space Shuttle orbiter is a safety-critical system, where failure has the potential to cause loss of the vehicle. They have proven to be delicate, expensive, and high maintenance.
Even before Space Shuttle designers ever decided to glue ceramic tiles onto the surface of a conventional aluminum airplane, temperature-resistant metals had been used for hypersonic vehicles. First there was Inconel alloy on the X-15 rocket plane, then a titanium frame with a beryllium skin appeared on the Mercury capsule. Boeing’s futuristic X-20 DynaSoar space plane (which unfortunately never got much beyond the mockup phase) featured Rene 41 alloy for the structure combined with a columbium/titanium/zirconium mixture for the outer skin.
In the 1980s there was an audacious experiment on the shuttle Columbia. The Shuttle Entry Air Data System drilled 14 large holes in the nose cap of the orbiter to hold sensor ports. Six additional ports were mounted in other nearby locations. These sensor ports were made from an exotic, high temperature metal named, in a strange coincidence, columbium. The ports were directly exposed to hypersonic flow on the hottest part of the vehicle. They connected, via columbium metal tubes, to sensors that recorded dynamic pressure forces on the nose during entry. This experiment flew on five Shuttle missions, beginning with STS-61C in January 1986. The project was not initiated to prove the successful use of high temperature metals on the orbiter, but perhaps it did provide that verification.
Over the past few decades, there have been many unsuccessful efforts to master continuous powered hypersonic flight. The daunting thermal problem is that sharp, thin leading edges, which are needed to minimize drag, get very hot very fast and there is nowhere for the heat to go. Fortunately, a slowing space vehicle entering the atmosphere doesn’t need to reduce drag to a minimum, so it doesn’t require sharp leading edges (the forward edge of the shuttle orbiter’s wings are broadly rounded.) The orbiter’s delta-shaped wings have a very inefficient lift to drag ratio and poor low-speed lift characteristics, which means it touches down on the runway at a high velocity.
Wing loading (the vehicle’s weight divided by its wing surface area) is a prime parameter affecting flight. The antique aluminum Douglas DC-3 airliner had a big wing with a low loading of about 25 psf (pounds per square foot of wing surface). At the other end of the spectrum, the Space Shuttle orbiter has a high wing loading of about 120 psf. This loading, combined with an inefficient delta-shaped wing, makes the orbiter glide like a brick. A little Cessna 152 private plane features a wing loading of about 11 psf and modern gliders operate down around 7 psf. A space plane with huge lifting surfaces and a very low wing loading might not require any external thermal insulation at all. Building a space plane with a wing loading of, say, 10 psf should not be an impossible proposition. Perhaps some day it will be done.