Surfing Hell at Mach Twenty-Five: The Science and Speculation of Atmospheric Reentry

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Paul Lucas

Issue: 2 January 2006

On February 1st, 2003, the United States Space Shuttle Columbia was enjoying the last minutes of a very successful two-week mission in orbit. As the aging vehicle began its descent to Earth, its crew reported being a bit weary but satisfied at a job well done. However, at approximately 9 am, when the Columbia was 207,000 feet over north-central Texas and traveling at over eighteen times the speed of sound, all contact with the vessel was lost. Within hours charred fragments of its hull were being discovered scattered all across the American Southwest.

The Columbia tragedy was one of the most horrific episodes in the history of American space flight. One that underscores just how dangerous orbital travel can be, especially during those final few minutes of atmospheric reentry, when pressures and heat surrounding the spacecraft are more extreme than at any other time in its mission. Getting into orbit can seem relatively straightforward compared to screaming through burning layers of atmosphere at over two dozen times the speed of sound just to return home.

In this article, we'll examine the conditions involved in reentry, how spaceships cope with it, and coming technology that may help future travelers survive the ordeal.

Basics

Orbital reentry has been completed successfully on hundreds of space missions, both by manned vessels and unmanned payloads. Yet it remains the most potentially dangerous part of any trip into space. The spaceship must go from minimal orbital velocity, approximately twenty-five times the speed of sound, to a speed slow enough to make a safe landing, all in less than one hundred vertical miles.

The first and most famous obstacle facing any space vessel is the heat of reentry, caused by extreme air friction as the craft slams through the outer atmosphere twenty times faster than a bullet. The heat on the outside of the vessel can easily exceed 2,500 degrees Celsius, enough to liquefy most metals.

The second deadly hazard is the structural stress the vessel endures. Manned missions usually pull around two to three G's of force during these decelerations, and unmanned payloads have been known to take up to 10 G's and beyond. This combined with the heat and the buffeting the ship takes as it encounters thicker and thicker layers of atmosphere can threaten to rip the craft apart.

There's also the angle of descent to consider. If a spacecraft comes in at too steep an angle, the air friction will become overwhelming and the ship will burn up and break apart no matter how well protected it is. If it comes in at too shallow an angle, its great velocity will "bounce" it off the outer layers of atmosphere much like a stone skipping off the surface of a pond, sending it careening back out into space. Most spacecraft, therefore, have a fairly narrow "window" of reentry they must adhere to in order to avoid disaster.

In designing the reentry craft, engineers must also consider its ballistic coefficient, often referred to as its "beta." The ballistic coefficient is a calculation of the weight, drag, and cross-section of a vehicle. Vehicles with low betas are generally wide, rounded and blunt, and do most of their slowing down in the thin upper atmosphere. They take longer to slow down and generate less heat over a longer period of time. Thus, low beta reentry vehicles are ideal for manned spacecraft. Vehicles with high betas are usually narrow and bullet-shaped. Missiles and ICBM warheads have high betas and zip through the atmosphere to the ground much faster, making them much harder for a potential enemy to detect, intercept, and shoot down.

Proven Reentry Methods

The most common way of dealing with this extreme heat has been with an ablative heat shield on the surface of the vessel expected to take the brunt of reentry. A wide, rounded disk was found to be the most efficient shape for this type of heat shield, as it creates a shockwave just ahead of the vehicle that more readily deflects the heat away from the craft. These shields are usually made of heat-resistant polymer resins saturated with glass fibers or glass microspheres, and are designed to slough away millimeter by millimeter as they are charred or vaporized away by the heat. This is the method used by most past and present space capsules, with the exception of the Project Mercury capsules, which used thick non-ablative beryllium shields.

ceramic Space Shuttle tiles; photo courtesy of NASA

Another tried and true method, first introduced on the Space Shuttle, is to use modular tiles made of tough ceramic insulating material that can withstand extreme temperatures without deforming. These tiles are designed to conduct heat very, very slowly, so much so that only the outer layers of the tiles will become superheated during the brief reentry period. This method is cheaper and much easier to maintain than a single throwaway heat shield, and thus makes more sense for reusable orbital craft.

However, the Columbia disaster was precipitated by some of these tiles becoming damaged from the impact of a fragment of insulating foam during lift-off 16 days earlier. This allowed a spike of superheated air to pierce the left wing, and doomed the vehicle. As this tragedy has shown, even a single displaced tile can lead to disaster, leading to the new practice of in-flight inspections of the tiles before reentry.

Another method used mostly on ICBM warheads is an ablative nylon-plastic shell. Though the nose cones of the warheads typically experienced much greater heat forces during reentry than blunt-shaped manned space capsules, their high beta configuration means that they spend much less time in the atmosphere dealing with it. Ablative heat-dispersing coatings that can be a mere quarter inch thick have therefore proven more than enough reentry protection for these rugged weapons.

Unusual Heat Shields

China has reportedly used ablative reentry shields for some unmanned vehicles made of specially treated wood such a cork. Though this remains unconfirmed, this is definitely a viable technique, as cork is used in some US rocket engines as an ablative heat-absorptive material.

In the '50s and '60s, the US experimented with using active and passive heat sinks, basically advanced refrigeration units, to help dissipate the heat of reentry. However, these systems proved to be too inefficient and heavy to be practical, and became redundant as other more efficient reentry technologies were perfected.

A final method that has been tested in laboratories but has yet to see practical application is using a plasma torch, basically a single powerful retrorocket, to create an artificial shockwave in front of the vehicle, keeping the super hot reentry plasma away from the skin of the vehicle very similarly to the protective shockwave created by a blunt-body, low beta vehicle. This plasma-created shockwave could theoretically protect a vehicle traveling at hypersonic velocity for sustained periods of time.

Inflatable Shields

MOOSE concept drawing; © General Electric

Another heat shield option that has received an unusual amount of attention is the inflatable heat shield. The very nature of such a shield, if proven workable, would greatly reduce weight and bulk of a reentry-capable spacecraft.

The idea originally came about in a design study in the early '60s examining emergency rescue technology that could be developed for astronauts who run into trouble in orbit. In its original conception as MOOSE (Manned Orbital Operations Safety Equipment), it was designed as a small, easily storable means for a stranded astronaut to bail out of a space station in Low Earth orbit.

A space-suited astronaut would exit a station via an airlock and strap a 1.8 meter diameter elastomeric mold onto his back. Pulling a cord on an attached canister would fill the mold with rapidly hardening polyurethane foam, resulting in a large hemispherical heat shield that would mold itself around the contours of the back of the astronaut's body. Using a hand-held retro rocket, the astronaut would align himself for reentry (in the '60s it was assumed some simple helmet sights and radio contact with the Mission Control would help him find the correct reentry window) and use the hand-rocket to break orbit and send himself back toward Earth. Once in the lower atmosphere, the astronaut would use a parachute mounted on MOOSE's forward straps to initiate a soft landing.

The very idea behind MOOSE at first seems insane (reentry without a spaceship!), but the science is sound and the idea has proven popular enough that updates of MOOSE have been seen in science fiction works such as Allen Steele's novel Orbital Decay and in the Traveller RPG. In fact, the Traveller version postulates that MOOSE variants could be used for military purposes, for dropping personnel onto a planet's surface more stealthily than a full dropship, and by extreme sports enthusiasts, who could use different foams that would produce a variety of colored trails during reentry.

More recently, inflatable heat shields have been the focus of a joint venture between the European Space Agency and the International Science and Technology Centre in Moscow. The system was developed to return small payloads from the International Space Station independently of the Space Shuttle.

IRDT module; image courtesy of ESA

The Inflatable Reentry and Descent Technology (IRDT) module, when inflated, looks like a large flattened cone with flanges along its outer rim, similar to a shuttlecock, with its payload just inside the cone's tip. The inflatable material is densely packed with multiple layers under high pressure, with inner layers able to take up the heat burden of the outer layers and help dissipate the accumulated heat. The IRDT module's shape is also designed to act as an aerobrake.

Two modules of the IRDT system were tested in reentry trials using a Russian Soyuz/Fregat launcher in 2000, with modest success. Further development and research continues.

Soft Landings

A soft landing refers to slowing the vehicle down in the lower atmosphere enough for a human crew to touch down safely. Even though by this time the vehicle has shed the vast majority of its orbital velocity, managing a soft landing is much more difficult than it seems, and requires a great deal of precision in design and execution that can at times escape project engineers. For example, on Yuri Gargarin's famed first flight into space in 1961, he had to jettison from his capsule seven miles up and parachute to the ground because Soviet scientists at the time weren't able to come up with a means to effectively soft-land the capsule itself. If Gargarin would have stayed in the capsule when it hit the ground his chances for survival would have been slim. Gargarin's parachute landing was covered up for decades by the Soviet hierarchy and didn't come to light in the West until well after the USSR's dissolution in 1991.

Soft landing techniques can take several different tacts. By far the most prevalent way of landing is with a parachute or a parasail, deployed only after the vehicle reaches the significantly dense parts of the atmosphere. Retro-rockets, so loved by golden age science fiction, are also sometime used, but usually in conjunction with a parachute. A third method, pioneered by the Space Shuttle, is to give the spacecraft an aerodynamic shell and wings and allow it to glide back to Earth much like a conventional aircraft.

A few other soft-landing methods have been experimented with, but have yet to see widespread use. Aerobraking shrouds have been used on Mars missions, and are being developed for Earth-return missions by the ESA and private interests. These shrouds are usually meant to work in concert with retro-rockets or parachutes, however. Also used on recent Mars missions have been inflatable airbags to cushion the final descent. The lander is encased in a series of tough, inflatable high-pressure air bags, and literally bounces to a stop on the surface. Internal instruments right the craft after the bouncing ceases, and the airbags are deflated to allow the probe to carry out its mission.

Roton test vehicle; © Willy Logan; image licensed under Creative Commons Attribution License v2.5

An unusual concept that was experimented with in the 1990s by private interests under NASA contract was the Roton. The Roton was a rotary rocket—a rocket that spins about its central axis during flight, using the centrifugal force of its spin to pump fuel and stabilize itself aerodynamically. On the Roton, this rotational motion was used to spin up a central shaft on the rocket. After undergoing standard reentry and entering the lower atmosphere, the craft would deploy three large helicopter-like blades attached to this shaft via its nose cone. Using the tremendous momentum of the spinning central shaft, the Roton could rotate the blades fast enough to keep the craft aloft without expending much additional power or fuel. It then lands similarly to a conventional helicopter.

The Roton concept underwent three successful full scale in-atmosphere tests until funding dried up in 2000.

The Coming Age of the Dropship

The idea of a dropship should be familiar to many science fiction fans. They've been mentioned or seen in many novels, films, and video games. By simple definition, a dropship is any craft that's carried into space as cargo but whose primary purpose is to return material and/or personnel quickly and safely to a planet's surface. They are distinguished from space capsules (which technically could be classified as very small dropships) both by their larger carrying capacity and by being able to maneuver significantly during descent on their own, instead of just more or less deadfalling into the atmosphere.

The first true dropship may prove be the so-called Crew Exploration Vehicle (CEV), which is scheduled to supercede the Space Shuttle as the US Space Program's primary orbital vehicle. Though NASA is still considering proposals from different contractors (the final design to be chosen in 2006), as of this writing the front-runner currently appears to be Lockheed-Martin's cross between a lifting-body space plane and a space capsule, originally called the Orbital Space Plane (OSP) project. The OSP was in turn based on experiments carried out with the X-38 lifting body concept aircraft.

In many ways resembling a smaller, stubbier Space Shuttle, Lockheed Martin's CEV would be more aerodynamically designed, with its entire fuselage sculpted for optimized airflow. This would allow it to glide and maneuver more easily during descent, and in turn easing some of the G-stresses astronauts typically endure during reentry. Designed to carry up to four astronauts for up to a week, this CEV design would use composite heat shield tiles similar to the Space Shuttle, as well as an inner titanium shell to protect against potentially catastrophic breaches such as the one that doomed the Columbia.

In many ways, dropships would seem to fit much more logically into the evolution of manned space exploration than vehicles such as the Space Shuttle. As launch and reentry both have very different requirements and hazards, it follows that different spacecraft would be developed to specialize in one or the other. This was the case at the dawn of the space age, when we had towering rockets for launch tasks and tiny hardened capsules for reentry tasks. The drive to develop an all-in-one vehicle, of which the Space Shuttle was supposed to be an intermediate step, is still a dream of many engineers. But despite many projects along those lines over the last few decades, this goal has always seemed to be just beyond NASA's grasp.

Though one can imagine dropships over the next few decades eventually being engineered to be larger and more advanced to better accommodate larger space crews and heavier scientific cargo, another specialized use for these vehicles become readily apparent: the rapid transportation of troops and military assets to the ground from either orbit or from vehicles launched in sub-orbital arcs. In fact, in most science fiction works where dropships are featured, this is exactly the role in which they are most often portrayed.

Military Dropships

Deploying troops to distant points in the world can be a long, laborious task involving great expense and resources. If a hot spot that needs to be addressed quickly suddenly pops up, many hours or days can pass before the first vanguard of troops can arrive to take care of it.

Military drop ships can greatly alter this equation. Sub-orbital arcs, in which a vehicle is launched much like a rocket, grazes the edge of space, then deadfalls back into the atmosphere on a parabolic arc, can reach almost anywhere in the world in under an hour. These were the types of trajectories programmed into the US's and USSR's ICBMs during the Cold War, ensuring that the country that launched an attack on the other would learn the meaning of Mutual Assured Destruction in a half hour to forty five minutes at most.

However in this post-Cold War era of terrorism, brushfire wars, and unprecedented natural disasters, deploying troops and their equipment rapidly to a danger zone can greatly mitigate potential crises. In the decades to come, military organizations may take advantage of advanced dropship technology to develop sub-orbital rapid deployment forces.

Such suborbital vehicles would probably be two stage affairs, with each component reusable. The first stage would be composed of a Rapid Ascent Vehicle (RAV), most likely a large advanced rocket-plane or a combined cycle rocket/scramjet hybrid. The RAV would carry the dropship up to sub-orbital space, deploy its subcraft, and then glide back into the atmosphere back to its starting point.

The dropship, however, would re-enter the atmosphere on a trajectory that would take it well into the crisis zone. Lifting-body-configured dropships, such as larger, more rugged versions of Lockheed Martin's CEV, are the most likely designs for such vehicles.

If a large manned presence in space is ever established, true orbit-to-ground military dropships are sure to be developed; any major military powers then extant are sure to take advantage of the new high ground for troop and ground military asset deployment. Advanced orbit-to-ground dropships can be both larger and more rugged, and can take advantage of non-lifting body configurations. For example, a type of heavy dropship popular in science fiction works (such as both the Traveller RPG and the Mechwarrior computer game series) is the spherical drop ship. These vehicles resemble large globes with usually three or four splayed retrorockets attached under one hemisphere, which also act as landing braces. The ship enters the atmosphere with its blunt end forward, bearing the brunt of reentry much like an oversized low beta space capsule. Once enough of its velocity is shed, the drop ship flips over and fires its rockets for a soft landing. As spherical drop ships are usually depicted as heavy haulers, these rockets are usually profoundly over-powered for the vehicle's weight to handle even very massive loads.

Any large orbital military force that needed to enter an in-atmosphere hot zone quickly would likely deploy different stages of dropships and reentry vehicles. First, missiles or deadfall kinetic energy weapons with ICBM-like high beta reentry shields would pepper the target zone, taking out enemy missile emplacements and any other threats to the dropships to follow. Then lifting-body reentry vehicles and small, hardened space capsules would quickly carry troops and light military assets to the target zone, where the soldiers would act quickly to secure the surrounding area. Finally, with the immediate zone cleared, spherical dropships carrying heavier equipment such as armored fighting vehicles and modular bases would make the descent.

As military dropships may come under fire at any time during descent, they may have to use technologies most other types of reentry craft would eschew. Heat sinks and refrigeration systems, spurned by designers early in the space age as too heavy, may see a resurgence on military dropships, in order for the vehicle to take advantage of steeper reentry angles to greatly reduce the amount of time it may be exposed to enemy fire. Military dropships would also have to take advantage of decoys, expendable mock-ups of the dropship with identical sensor profiles, to draw incoming ordinance away from the main reentry craft. This latter tactic was recounted in the classic military SF novel Starship Troopers by Robert Heinlein.

Conclusions

Returning from space, relying upon miles and miles of turbulent superheated atmosphere to slow your vehicle, is without a doubt the most dangerous part of any trip above the atmosphere. As manned initiatives in space by both national governments and private interests are likely to increase in the coming decades, perfecting reentry technology even further will become paramount. After all, there is very little reason to go into space if one can't return home safely to boast about it.