This paper is one of two in a dual abstract submission to AIAA Aviation 2016, "Air Transportation Integration and Operations -Unique and/or transformational Flight Systems"and details the design, construction, testing and results of a team-devised package pickup system and CG relocation apparatus. Said technology was created in parallel with a tailsitter vehicle, Proteus, whose design, construction and testing are outlined in the companion paper, "An Unmanned VTOL and Fixed Wing Package Retrieval and Delivery Vehicle" [1]. Note that all technology outlined in this paper may be integrated into various vehicle configurations, not just Proteus. The vehicle itself and subsequent technology are the culmination of a project assigned to the 2015 NASA Multidisciplinary Aeronautics Research Team Initiative (MARTI) at Langley Research Center. MARTI was tasked with developing an unmanned vertical takeoff and landing (VTOL) vehicle for the purpose of package identification, pickup and delivery. From conception to completion, the team had 12 weeks. To meet these and other requirements, MARTI constructed a dual-functioning, tail-sitting aircraft, named Proteus, which is capable of transition from traditional quadrotor mode to fixed wing design for drag-efficient forward flight. Consult "An Unmanned VTOL and Fixed Wing Package Retrieval and Delivery Vehicle" for additional vehicle details, as such information will only be referenced in support of the topics discussed in this paper [1].
This paper reports on measurements of freestream nitric oxide (NO) rotational and vibrational temperatures and partial pressures, collected in the Caltech T5 reflected shock tunnel. Quantum cascade lasers, emitting mid-infrared light resonant with fundamental rovibrational NO transitions, were directed through the supersonic (Mach ∼5) freestream flow. Tunable diode laser absorption spectroscopy (TDLAS) was used to measure the path-averaged rotational and vibrational temperatures of NO in the flow, in addition to the NO partial pressure. The temperature measurements demonstrate strong evidence of NO rotational and vibrational equilibrium during the 1 ms test period. Agreement between vibrational and rotational temperatures was observed in all experiments, including one h 0;∞ ≈ 8 MJ∕kg and four h 0;∞ 18 MJ∕kg experiments, during and after the nominal test time. Absorption from CO and H 2 O was also observed in the TDLAS measurements, though their concentrations cannot be accurately estimated. The goal of these and future experiments is to develop and demonstrate TDLAS experimental strategies for high-enthalpy impulse facilities and to help to inform improvements of existing models and solvers used for prediction of freestream conditions.
Small spacecraft autonomous rendezvous and docking is an essential technology for future space structure assembly missions. A novel magnetic capture and latching mechanism is analyzed that allows for docking of two CubeSats without precise sensors and actuators. The proposed magnetic docking hardware not only provides the means to latch the CubeSats but it also significantly increases the likelihood of successful docking in the presence of relative attitude and position errors. The simplicity of the design allows it to be implemented on many CubeSat rendezvous missions. A CubeSat 3-DOF ground demonstration effort is on-going at NASA Langley Research Center that enables hardware-in-the loop testing of the autonomous approach and docking of a follower CubeSat to an identical leader CubeSat. The test setup consists of a 3 meter by 4 meter granite table and two nearly frictionless air bearing systems that support the two CubeSats . Four cold-gas on-off thrusters are used to translate the follower towards the leader, while a single reaction wheel is used to control the attitude of each CubeSat. An innovative modified pseudo inverse control allocation scheme was developed to address interactions between control effectors. The docking procedure requires relatively high actuator precision, a novel miminal impulse bit mitigation algorithm was developed to minimize the undesirable deadzone effects of the thrusters. Simulation of the ground demonstration shows that the Guidance, Navigation, and Control system along with the docking subsystem leads to successful docking under 3-sigma dispersions for all key system parameters. Extensive simulation and ground testing will provide sufficient confidence that the proposed docking mechanism along with the choosen suite of sensors and actuators will perform successful docking in the space environment.
A wealth of literature exists on control allocation algorithms for over-actuated air vehicles, launch vehicles, and spacecrafts. Most of these algorithms focus primarily on minimizing some objective function such as command tracking error and/or control effector usage. Linear allocators (pseudo inverses) are usually the conventional choice due to their simplicity and the ability to achieve a significant portion of the theoretical moment/impulse space. Generally, it is assumed that there exists minimal interaction effects between control effectors. In fact, very few studies address the problem of control effector interactions in the context of control allocation, especially for small spacecrafts with a reaction control system (RCS). This paper presents a CubeSat RCS design with a four thruster tetrahedral layout such that when two or more thrusters fire, the resultant impulse differs noticeably compared to the sum of the contributions from individual thruster firings. This undesirable effect is caused by the design of the propellant tank and regulator. To mitigate this issue, an innovative modified pseudo inverse (MPI) control allocation algorithm was developed that adjusts the pseudo inverse solution based on test data. The algorithm is iteration-free and superior to the standard pseudo inverse in minimizing the command tracking error.
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