Next Generation Reentry

A friend of mine and I were talking about the Columbia accident and how a similar situation could be avoided in the future. He said that any new spacecraft should go back to capsules with ablative heat shields that splash down in the ocean beneath a trio of parachutes. His reasoning is that the design is structurally simple, there is a minor ability to control splashdown point through roll maneuvers to adjust entry angle, and the heat shields tend to be more robust than the tiles used on the Space Shuttle.

Of course, I then took the liberty of reminding him of his occupation and the fact that any real pilot would rather fly a spacecraft back to earth rather than dangle beneath nylon chutes. Emotional connection noted, he alerted me that every ounce of aerodynamic concession built into a spacecraft is an ounce not dedicated to the primary mission of being in space. What was I going to do? Lie and say that a wing is useful in an airless vacuum?

After a few more hours of going back and forth with the pros and cons of wings vs capsules, we agreed that there is no single solution that can satisfy all the possible needs of spaceflight. Imagine having one type of automobile to serve all transportation purposes from going to the supermarket to hauling freight cross country. It’d either be really useful for one job and horrible at the others, or mediocre at everything.

Humans are pretty good with building capsules so there’s not much for us to do with regards to their design. Spaceplanes on the other hand have a lot of room for experimentation. The fact that they have wings is the single greatest advantage over a capsule with regards to crew safety, comfort and airframe longevity. After Columbia, many people, (self proclaimed experts and otherwise) said that spaceplanes are not as useful, not as safe and not as efficient as capsules. These statements were reactionary and misguided. One cannot make a blanket conclusion about an entire class of vehicles based on the experiences of the only existing model. It’s not accurate to say that all spaceplanes are going to be problematic just because we flew a very risky system that just happened to have wings for nearly 30 years.

The Space Shuttle was was designed in the 1970s and went through several downgrades in order to keep the price manageable. These changes left the Shuttle that actually flew with a very basic aerodynamic design. The reentry procedures were derived from experience with capsules and computer simulations. Suffice to say, it was a first generation spaceplane and through its entire life was constantly being upgraded, modified and improved. Just as jetliners in the 1960s may physically resemble jetliners of today, the differences in aerodynamics, reliability, avionics and materials make them totally different. The same will hold true for spaceplanes built in the next 50 years when compared with the Shuttle. We should not be discouraged with a system that provided 2012 performance with 1979 technology.

One of the drawbacks to the Shuttle was that it was exposed to very high reentry temperatures due to varying factors including entry angle, angle of attack, entry velocity and wing loading. The Shuttle started entry at an AOA of roughly 40 degrees at Mach 25 and would ramp down to 14 degrees by the time the velocity had bled down to Mach 4. This profile keeps the total reentry time relatively short but drastically increased heating on the vehicle. The shuttle falls to roughly 250,000ft before its entry angle relative to the local horizon begins to shallow out. This is the constant drag portion of the entry. As it slows and descends, the AOA is gradually reduced to more conventional values and it begins to fly like an airplane.

If one were to reverse the angle ratio and have shallower angles early in the sequence, a spaceplane would experience far less severe heating at the expense of time. The Shuttle had APU and radiator limits and could not remain in high speed hypersonic flight indefinitely, so there was a reason to expedite its entry. A shallow initial entry for the Shuttle would have exhausted the APU fuel and potentially overheated its systems due to the cargo bay radiators being shut down. However for a purpose designed spaceplane that could last a longer time in the entry configuration, this would not be a factor.

The shallow entry is really simple as a concept, even easier to practice on a simulator and of course ridiculous to write out in raw math. Enter the atmosphere at a very shallow angle with respect to the local horizon, descend at a very slow rate to maintain a roughly constant temperature and wait for speed to bleed off. For all intents and purposes, AOA will control rate of descent and by association, hull temperature. For this reason, wing loading and 1g stall speed are the most critical aerodynamic considerations.

If a spaceplane has a very high wing loading, it will stall at a higher airspeed (max AOA is tied to wing design and sweep). In order for an spaceplane entering the sky from above to remain at a low temperature, it has to stay high and slow as long as possible. The only way to accomplish this is to utilize a low stall speed so that it skims down into the sky instead of dropping into it. Any new spaceplane will have to have a much lower wing loading than the Shuttle to allow it to “surf” through the upper mesosphere without excessive sink rates.

Once relatively level flight in the upper atmosphere has been established, AOA will vary based on final design but will most likely be less than 25 degrees. A ramp down process will then commence to reduce AOA, thereby maintaining the shallow descent rate and slow deceleration. Once a predetermined speed is reached, AOA is increased to descend at a higher rate to a lower altitude. Once arriving at that lower altitude, AOA will be reduced again to reduce descent rate. The entire process then repeats itself as many times as required to bleed speed from Mach 25 to Mach 4.

The control of AOA at hypersonic speeds will not the difficult part of this exercise. Managing energy properly to arrive at a given point while maintaining docile deceleration rates will be. Advanced planning and real-time monitoring will be the solution to this challenge. Cross-ranging will be enhanced first by virtue of the higher L/D ratio and second by the larger fraction of time spent in high altitude flight. Temperature control will not be an issue provided the craft follows the entry procedures and does not allow extremely high descent rates to develop. Theoretically, the leading edge temperature should not exceed 750 degrees Fahrenheit at the maximal heating stage, while remaining far cooler for the majority of the procedure. This allows for far more flexibility in vehicle construction and much larger safety margins. The lack of vehicle heating will quite literally be a life-saving feature.

The Space Shuttle was infamous for the difficulty experienced in maintaining the heat resistant tiles on its undersurface. Being very delicate and brittle in nature, they would break easily if mishandled. On the first Shuttle (Columbia, before the upgrade) had over 30,000 tiles of various thermal properties protecting its airframe. Later modifications replaced some of the upper surface tiles with thermal blankets and fabrics that reduced maintenance between missions. Any new spaceplane would be able to use similar materials over its entire fuselage while still maintaining a substantial margin for airframe protection in emergency or off-design entry profiles.

So its just an idea, one that deserves a lot more thought and research from various people, companies and agencies. Having the capability to fly back from orbit means the potential to bring back heavy payloads, no need for extensive maritime recovery forces, the ability to land at any runway of adequate length and no requirement for passengers and crew to withstand heavy g forces. Capsules are and will still be useful for certain space activities, most notably lunar or interplanetary travel. But for any flight that is intended to spend most of its time in low earth orbit, a spaceplane will offer far more advantages. Let’s not write off the future because of the past.

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About Christopher Williams
It's easier to lie about being boring than it is to be honest about being extraordinary.

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