Rabu, 10 Mei 2017

Model rockets on Mars redux (1998)

Mars Sample Return (MSR) became a high-priority NASA mission in August 1996, following the discovery of possible traces of past life in martian meteorite ALH 84001. NASA targeted its MSR mission for launch no later than 2005. By late 1997, however, MSR planners in the Mars Exploration Program at the Jet Propulsion Laboratory (JPL) faced daunting technical and fiscal challenges. Specifically, their MSR spacecraft was too massive for launch to Mars on a single low-cost rocket.

JPL's MSR spacecraft, which used the Mars Orbital Rendezvous mission mode, consisted of an orbiter for transporting a lander to Mars and returning the Mars samples to Earth, a rover for sample collection, a Mars Ascent Vehicle (MAV) for boosting the collected samples to Mars orbit for retrieval by the orbiter, and a lander for delivering the rover and MAV to Mars's surface.

In May 1998, JPL rover engineer Brian Wilcox proposed a possible solution: replace the MSR mission design's massive (about 500 kilograms) liquid-propellant MAV with a low-mass solid-propellant MicroMAV. The following month, JPL engineers Duncan MacPherson, Doug Bernard, and Bill Layman began a preliminary study to attempt to validate Wilcox's concept. As part of their effort, they held a "mini-workshop" at which they consulted with space industry propulsion engineers. By September, MacPherson was ready to present his group's findings to the second meeting of the NASA-appointed Mars Architecture Team (MAT).

Wilcox had envisioned an alternative MSR scenario in which a large rover would carry and launch his 20-kilogram MicroMAV. MacPherson, Bernard, and Layman proposed a MAV that burned solid propellants but had a more realistic estimated mass of 110 kilograms. This would, they found, require a return to a more traditional MSR scenario in which the MAV would lift off from a stationary lander. A rover would collect samples and deliver them to the MSR lander, which would load them into a container in the MAV's third stage.

Wilcox assumed that, during first-stage flight, airflow over four canted fins on his MicroMAV's first stage could spin it to provide gyroscopic stability. MacPherson, Bernard, and Layman judged, however, that martian air was not dense enough for canted fins to be effective. Prior to first stage ignition, thus, a spin table on the MSR lander would spin their MAV about its long axis 300 times per minute to provide gyroscopic stability. The first stage motor would then ignite and hurl the MAV skyward at from six to 10 gravities of acceleration.

Industry experts attending the mini-workshop had told MacPherson, Bernard, and Layman that metal-based solid propellant yields molten slag when it burns. In a rapidly spinning motor, the centrifugal force causes the slag to adhere to the nozzle, producing unpredictable mass imbalances. These could destablilize the ascending rocket, causing it to tumble out of control. A high spin rate could also cause uneven solid propellant burning. MacPherson told the MAT that metal-free solid propellant would eliminate both problems, though at the price of reduced motor performance.

After first stage burnout, a small despin motor would slow the MAV's rate of spin to 20 revolutions per minute. The MAV would then coast to an altitude of 90 kilometers. Wilcox assumed no active attitude control during the coast, but MacPherson, Bernard, and Layman invoked cold-gas attitude control thrusters to compensate for winds and to orient the MAV accurately for the second stage burn.

An inertial measurement unit and a sun sensor would provide data to the thruster guidance system and to a timer that would govern subsequent MAV operations. The spent first stage would detach one second after timer activation, then the second stage motor would ignite one second after that. Second stage acceleration would peak at 35 times the pull of Earth's gravity just before burnout. The second stage would boost the MAV's apoapsis (orbit high point) to 300 kilometers above Mars, then would separate two minutes after timer start.

Wilcox gave little attention to the MicroMAV's role in preventing biological contamination of Mars (forward contamination) or Earth (back contamination). MacPherson noted that the second-stage motor's trajectory after separation would take it back into Mars's atmosphere, thus eliminating it as a possible source of back contamination.

As in the Wilcox design, the MacPherson/Bernard/Layman third stage motor nozzle would point forward during first stage and second stage flight, ensuring that it would point aft when the gyro-stabilized MAV attained apoapsis halfway through its first orbit. The timer would ignite the third stage motor 50 minutes after timer start; if all had functioned as planned up to that point, this would coincide with apoapsis. The brief burn would raise the MAV's periapsis (orbit low point) out of the atmosphere to an altitude of at least 300 kilometers.

As its last act, the timer would fire a motor that would halt the MAV's spin so that the orbiter could more easily capture it. The waiting orbiter would then maneuver to retrieve the MAV third stage and the precious Mars samples it carried.

MacPherson, Bernard, and Layman found that minor guidance errors, motor performance variations, and the vagaries of Mars's atmosphere could affect the MAV's final orbital parameters and thus the magnitude of the maneuvers the orbiter would need to perform to rendezvous with it. Wilcox, always optimistic about his MicroMAV's capabilities, had calculated that compensating for orbital uncertainties would require that the orbiter carry only enough propellants to enable velocity changes totaling about 100 meters per second. MacPherson's team, by contrast, estimated a possible MAV periapsis range of 300-to-500 kilometers, an apoapsis range of 600-to-800 kilometers, and an orbital inclination range spanning one degree. In the worst case scenario, this would mean that the MSR orbiter might need to make velocity changes totaling about 260 meters per second.

The MacPherson group's results might have thrown cold water on the MicroMAV concept. A 110-kilogram MAV was, however, an improvement over one with a mass of 500 kilograms. Even before they finished their work, JPL adopted the small solid-propellant MAV as part of its baseline MSR mission design.

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