Atmospheric reentry is without a doubt the most challenging phase of spaceflight, for a number of very good reasons. Find out why we have yet to master this regime of flight in this article.
Before the disintegration of OV-102 Columbia highlighted the dangers of atmospheric re-entry, it had seemingly become almost routine; the Columbia Accident Investigation Board noted that NASA had a zero impact specification even though it recorded a multitude of thermal protection system (TPS) strikes by debris in every mission. Unfortunately, a piece of foam insulation fell off the External Tank and impacted the reinforced carbon-carbon leading edge of Columbia's wing, allowing the heat of re-entry to penetrate into the wing and cause it to fail. Since we had never suffered a catastrophic failure in this environment, the situation was assumed to be acceptable. Now, at great cost, we know better.
What exactly makes re-entry so difficult?
Dissipating kinetic energy
For any given orbital altitude, there is a specific velocity required to maintain that orbit. For a standard Space Shuttle LEO (Low-Earth Orbit) profile, the orbital velocity is around 17,500 mph (about 7823 m/s). The landing speed of the Shuttle is around 225 mph or 100 m/s (this velocity is even lower for reentry capsules, for obvious reasons). Given that kinetic energy equals ½*m*v^2, that equates to a factor of 60 difference between the two energy states. Somehow, a reentry vehicle needs to bleed off that excess energy. Enough fuel could be carried for a fully powered reentry, but that would be prohibitive from a mass standpoint and would still require you to kill all of your forward velocity to land. Given our available options, aerobraking, using the atmosphere to slow down was the natural choice. (The Delta Clipper, or DC-X, had it been built, would have been an exception to the rule, since it was to land vertically.)
From an aerodynamic standpoint, a vehicle can be blunt (a ballistic reentry vehicle, or R/V, like the Apollo capsule) or generate lift (like the shuttle). The first benefits from the strong shockwave it generates - this insulates it from the high-temperature airstream to a degree. The corresponding disadvantage is that it decelerates much more strongly, especially in the lower, denser parts of the atmosphere, though it requires less time to reenter. The lifting body slows down much more gradually by shifting its deceleration into the upper atmosphere.
With either approach, you have to deal with the thermal consequences of your choice. Since the ballistic R/V gets to the ground faster, it’s also bleeding off kinetic energy faster, which results in correspondingly higher peak temperatures. The lifting R/V spends more time bleeding energy off in the upper atmosphere, so its peak heating is potentially lower. However, the total amount of heat absorbed can end up being higher. Furthermore, peak temperature and the sharpness of a surface are related, since the sharp edge doesn’t build up that insulating buffer of air that the ballistic R/V does. As a result, temperatures along the nose and leading edge of the lifting R/V can become very high (up to 2400 degrees F in the case of the Shuttle, and neither its leading edge nor its nose are particularly sharp). Constructing materials and structures that can withstand such temperatures is quite a challenge; making them durable and impact-resistant is an even greater challenge.
In my next article, I will discuss some of the technologies used to cope with these conditions.