Aerocapture is a radical method of "putting on the brakes.'' If an interplanetary spacecraft is approaching a world with an atmosphere, it provides a method of decelerating for planetary capture without the use of fuel.
During an aerocapture pass, a spacecraft grazes through the outer layers of the destination world's atmosphere. During the maneuver, atmospheric drag slows the spacecraft enough to ensure capture into a planet-centered orbit. It is a precise, one-pass affair. Everything must work correctly as the probe approaches the planet on its Sun-centered orbit. It must dip into precisely determined atmospheric layers, and decelerate at one-Earth-gravity or more. If all goes well, the spacecraft emerges from the planet's atmosphere in an eccentric planet-centered orbit. The low-point of the orbit can then be raised by onboard thrusters.
The use of aerocapture can save significant spacecraft mass; which directly translates into a lower mission cost. This is done by building around the spacecraft a structure, called an aeroshell, which will protect it from the intense friction-generated heat it will experience during atmospheric entry. This aeroshell consists of a metallic or composite structure upon which is laid a thermal protection system (TPS), which will, as the name implies, provide protection to the spacecraft from the extreme heat generated during the maneuver.
Developing the "right" TPS is not trivial; and not all TPS materials work at all destinations. First of all, they come in two flavors: ablative and non-ablative. The Apollo capsule is perhaps the best-known example of a spacecraft that used an ablative TPS—one that burns away during entry. The challenge is to have enough material to ensure that it does not all burn away before the heating processes are complete, and not too much to burden the spacecraft with unnecessary weight and volume of thermal protection. The space shuttle uses a non-ablative TPS—one that does not burn up. The shuttle TPS is also designed to be reused—an expense with which missions to the outer planets on one-way trips need not be concerned.
The environment in which the TPS must operate and survive varies from planet to planet. Just as each planet is different, so are their atmospheres. Entry into the nitrogen-rich atmosphere of Earth provides a very different environment from that which would be experienced when entering the methane-laced atmosphere of Titan. Not only are the atmospheric constituents and densities different, but at Titan the interaction of the aeroshell with the atmosphere produces ultraviolet radiation to which many TPS materials are transparent! Using the wrong TPS might expose the spacecraft to mission-killing levels of radiation during the aerocapture maneuver. The heating rates, as well as the total amount of heat generated, are also different for each planet. The bottom line is that there will not likely be a single aeroshell design that will work for all missions or all destinations.
The other aspect of aerocapture that has not yet been demonstrated is the maneuver itself. The spacecraft has to hit its atmospheric destination just right, lest it bounce off into space or crash to the surface. Aerocapture is a maneuver, and the spacecraft will be flying through the atmosphere at speeds of several kilometers per second. In order to attain orbit, it must enter within a well-defined region called the "entry corridor." It is actually more like a virtual atmospheric tunnel into which the supersonic spacecraft plunges, preprogrammed to execute various maneuvers, so that it can exit into a useful orbit. If it makes too shallow an entry, it will skim off the atmosphere and go back into space—probably to be lost forever. If it comes in too steeply, the spacecraft will perform an unplanned entry and landing. (This is otherwise known as a crash!) And all of the maneuvers required to keep the vehicle within the safe corridor must be executed autonomously. The mission is likely to take place far beyond the earth, making two-way radio control impossible, and the maneuver will probably be completed before the signal of its initial entry reaches mission control back on Earth.
The first-generation aerocapture systems will probably use rigid aeroshells similar to those that have been flown for other applications. Figure 10.1 shows the nominal configuration for a first use rigid aeroshell. There is a lot known about their manufacture and performance and it only makes sense to attempt a new type of propulsion in a manner that reduces the amount of "new" technologies or systems that must be demonstrated. These systems are collectively known as rigid aeroshells, so named because they are made of hard, rigid materials. While the use of these systems will provide a tremendous mass savings on future missions, the introduction of lighter weight aerocapture systems may make them truly revolutionary. These lighter weight systems are known as ballutes. The word "ballute" comes from the combination of balloon and parachute—which describes them fairly well.
Ballutes are basically large, inflatable structures that provide the aerodynamic drag required to decelerate a spacecraft coming in from interplanetary space, and the lift necessary for them to maneuver into their final orbits. They are very large and allow the spacecraft to initially attain a much higher orbit than is possible using rigid aeroshells. Figure 10.2 is an artist's conception of an inflatable ballute trailing behind the spacecraft as it enters orbit around Saturn's moon, Titan. The latter reduces the overall heat loads and the former reduces further still the heat load on a given area of the ballute. When aerocapturing with a ballute, no "fireball" is evident. With a rigid aeroshell, the heat load on a square meter of aeroshell is very large due to its high-speed entry deep within a planetary atmosphere. The heat experienced by an inflatable ballute is significantly less due to its higher initial capture altitude and larger surface area (Figure 10.2).
There are attached ballutes and detached trailing ballutes. Attached ballutes look at first glance just like traditional aeroshells. They are simply inflated to encompass the spacecraft and provide the increased drag necessary for aerocapture. Trailing ballutes are just that—a large doughnut-shaped balloon trailing behind the spacecraft and attached to it by long cables.
Another possibility is to combine the functions of a solar sail with those of an aerobrake device. Although there are significant thermal and stress issues, at least some sail configurations are capable of withstanding high temperatures and high accelerations. Such a craft might use the sail for all post-launch interplanetary maneuvers in the inner solar system, and also to affect aerocapture.
Aerocapture Application to Solar-System Resource Surveys
Aerocapture techniques can be utilized in a number of solar-system resource-survey missions. One possibility is short-period-comet sample return.
A spacecraft could be launched from Earth and directed to rendezvous with a comet near its closest approach to the Sun. While flying in formation with the comet, the craft could retrieve samples from the comet nucleus and/or coma. After departing the comet, it could head toward Earth where aerocapture could be used to place the sample container in Earth orbit for later retrieval.
If the solar-photon sail is used as an aerobrake or in conjunction with an aerobrake device, a mission to Mars' two small satellites—Deimos and Phobos—can be conducted with minimal use of chemical propellant. In such a scenario, a probe would utilize aerocapture to decelerate from interplanetary velocities into Mars orbit. The sail would then be utilized to rendezvous successively with both of the Martian satellites. After surface samples have been collected and stored, the sail could be used to insert the probe on a trans-Earth trajectory. Aerocapture could be applied once again to slow the precious payload as it grazes Earth's atmosphere upon its return.
Outer-solar-system resources can conceptually also be surveyed with the assistance of a near-aerocapture maneuver. Distant asteroids suitably close to giant planets can be investigated if a space probe performs an aeropass in the giant's dense outer atmosphere so that it is decelerated but not captured. Such a probe will be capable of spending more time in the vicinity of selected asteroids than a non-decelerated flyby craft. Perhaps penetrator subprobes could be deployed in the upper surface layers of the asteroid as the decelerated probe passes by.
To demonstrate aerocapture feasibility for a selected planetary atmosphere, it is necessary to first develop a mathematical model of that atmosphere's density variation with height. For Earth and Mars, this is not an issue as well-understood and proven models exist for both. Thanks to the success of the Cassini/Huygens mission, Titan's atmosphere is also understood at a level that would permit aerocapture.
The next step is to determine a minimum height above the surface of the destination world for the aerocapture trajectory. This allows the calculation of parameters, including deceleration, aeroshell heating, and planet-centered velocity at the conclusion of the aeropass. Maximum stresses on the aeroshell can thus be estimated and compared with aeroshell-design limitations.
Unfortunately, a planet's atmosphere is not a static envelope. The calculated atmospheric density profile might not apply to an actual aeropass due to seasonal and regional variations. An actual aerocapture mission to another solar-system world might require onboard intelligence to compensate for variations in atmospheric density by minimum aeropass height alterations or aeroshell attitude modification. Perhaps an aeroshell could be designed that could change its shape. More likely, a spacecraft attempting aerocapture would perform last minute course alterations to compensate for whatever atmospheric conditions it finds.
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