Advanced Reentry Vehicles

Heat sinks and ablative materials, combined with blunt body shapes, were used for early missile and space program vehicle entries during the late 1950s and early 1960s. But aerospace engineers were constantly seeking to improve their performance. They wanted lighter vehicles capable of achieving higher speeds.

Early U.S. ballistic reentry vehicles had a low ballistic coefficient, or beta. The ballistic coefficient is a calculation of weight, drag, and cross-section of a vehicle. Vehicles with a low beta do most of their slowing down in the thin upper atmosphere. They take longer to slow down and generate less heat, experiencing this heat over a longer period of time. But missile designers wanted vehicles with a high beta, which are usually slender and smoother and generate less drag. They zip through the upper atmosphere without decelerating much and reach the ground still traveling very fast. This is desirable for missile reentry vehicles (RVs) because the faster a warhead approaches its target, the harder it is for an enemy to shoot it down.

The Mark 3 reentry vehicle used on the Atlas ICBM was the first ablative vehicle that had the range to reach other continents. It had a higher beta (the calculation of weight, drag, and cross-section of a vehicle) than earlier blunt body designs and was shaped like a cylinder with a nosecone at the front and a flared section at the rear. But the Air Force wanted even higher beta vehicles and so in 1959-1960, it contracted with both the AVCO and General Electric companies to provide new ablative materials for the experimental RVX-2 and RVX-2A reentry vehicles. These materials—different forms of plastic—could handle the higher temperatures of a high beta vehicle. These tests led to General Electric being awarded the contract to develop the Mark 6 reentry vehicle for the large Titan II ICBM. The Mark 6 was a huge RV, more than ten feet (three meters) long and 7.5 feet (2.3 meters) wide at its base—a grown man could stand beneath it. The nose cap used phenolic nylon (a nylon cloth impregnated with a resin), while the rest of the vehicle used a special plastic. The material was so effective at eliminating heat during the short reentry period that it could be very thin. The nose cap was about two inches (five centimeters) thick, whereas the rest of the heat shield was only a quarter of an inch (6.35 millimeters) thick.

But missile reentry vehicles were not the only reentry technology under development. By the early 1960s, the U.S. Air Force was building a space plane known as the X-20 Dyna Soar (for "Dynamic Soaring"). Dyna Soar had to be controllable during reentry and would not be a blunt-body design like the early missile reentry vehicles or the Mercury spacecraft. Because it had to be reusable, it could not use ablative materials, which burned off. Instead, the X-20 would have used a complex metal structure for cooling with a ceramic nose cap. However, Dyna Soar was canceled in 1963 before it ever flew.

Another reentry vehicle challenge was also emerging with the Apollo space program. The Apollo reentry vehicle would be bigger than anything returned from space. More importantly, it would have to survive the extreme reentry temperatures it would encounter during a 25,000 mile-per-hour (40,234 kilometer-per-hour) return from the Moon. After studying several different vehicle shapes by 1962, NASA engineers decided upon a large cone-shaped vehicle with a large rear heatshield that was made of a stainless steel honeycomb with an outer layer of phenolic epoxy resin as an ablative material.

At the same time that the Apollo program was progressing, NASA was flying the X-15 research aircraft at increasingly higher speeds, beyond Mach 6. These flights exposed the aircraft to high temperatures and allowed researchers to test new materials. These tests also provided useful lessons such as the proper design and location of piping and other internal equipment so as to reduce the effect that reentry heating had on the overall spacecraft.

The U.S. military was still busy building increasingly higher beta reentry vehicles, using methods and materials that are still classified. By 1963, this resulted in the Mark 12 reentry vehicle, a pointy cone designed to sit atop the Minuteman ICBM, which was the forerunner to all modern missile warheads. The Soviet Union was also developing both missile reentry vehicles and other heat shielding for their crewed spacecraft the Vostok and Voskhod. Little is known about Soviet research in this field, but it presumably mirrored many U.S. developments.

By the late 1960s and early 1970s, NASA had started preliminary design work on what eventually became the Space Shuttle. The Shuttle was to be a large, flyable spacecraft, far bigger than anything that had ever returned from space. The Shuttle's Thermal Protection System, or TPS, was difficult to perfect but represented a major breakthrough in reentry technology, for it enabled very large vehicles to survive the intense heat of reentry.

By the 1970s and later, there were fewer milestones to be passed in reentry vehicle technology. Spacecraft designers developed reentry vehicles for the Venus Pioneer and Mars Viking spacecraft, both of which used blunt body, low beta configurations, and ablative materials. In the 1980s, they also developed reentry vehicles for entering Jupiter's atmosphere and the atmosphere of Saturn's moon Titan. Other countries also entered the field. China developed recoverable spacecraft that reputedly used wood as an ablative material. While this may seem primitive, wood (in some cases cork) is actually used as an ablative material for some American rocket engine areas and payload shrouds, which heat up as the rocket flies through the atmosphere.

NASA's X-30 National Aerospace Plane (NASP) of the later 1980s would have imposed formidable challenges to materials engineers, for unlike the Space Shuttle, NASP would have experienced extremely high temperatures for very long periods of time. Conventional approaches, such as ablatives or ceramic tiles, would not have worked. Although a great deal of research was conducted on various materials, the NASP was canceled because of its insurmountable engineering hurdles. When NASA began the X-33 experimental rocket in 1995, its manufacturer, Lockheed-Martin, planned to use a metallic thermal protection system. The X-33 was canceled, but even better solutions would have been needed if larger versions of the X-33 were ever built.

In 2000 and 2001, Russian scientists, working with German researchers, experimented with an inflatable reentry vehicle. This was a very low beta design intended to be cheaper than conventional reentry vehicles. One promising idea that has been proposed for the future is the use of a plasma torch to form an artificial shockwave in front of a reentry vehicle. Just as the shockwave generated by a blunt body can protect a spacecraft by keeping hot gasses away from the skin of the vehicle, the plasma shockwave could theoretically protect a vehicle traveling at hypersonic velocity (Mach 6+) for sustained periods of time. But there is as yet no demand for such a thermal protection system and it remains only a laboratory experiment.

--Dwayne A. Day

Sources and further reading:

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