Marsbees - bioinspired Mars exploration flight vehicles
Marsbee mission consists of a Mars rover (already existing) that serves as a mobile base for Marsbees - a deployable swarm of small bioinspired flapping wing vehicles. In one ConOps scenario, each Marsbee would carry an integrated stereographic video camera and the swarm could construct a 3D topographic map of the local surface for rover path planning. These flying scouts would provide a “third-dimension” to the rover capabilities. In other scenarios, each part of the swarm of Marsbees could carry pressure and temperature sensors for atmospheric sampling, or small spectral analyzers for identification of mineral outcroppings. In each scenario, the rover acts as a recharging and deployment/return station and data and communication hub.
Marsbees can fly in the Martian atmosphere using bioinspired flight mechanisms that insects use.
Human exploration of Mars is one of the major objectives of NASA and commercial entities such as SpaceX and Boeing. The identified innovations unique to the bioinspired flapping Marsbee provide viable multi-mode flying mobility for Martian atmospheric and terrain exploration. A swarm of Marsbees provides an enhanced reconfigurable Mars exploration system that is resilient to individual component failures. These Marsbees can carry sensors and wireless communication devices in combination with a Mars rover and helicopters. These enhanced sensing and information gathering abilities can contribute to the following NASA Mars mission objectives: i) “Determine the habitability of an environment”, ii) “Obtain surface weather measurements to validate global atmospheric models”, and iii) “Prepare for human exploration on Mars”. Various commercial entities, e.g. SpaceX and Boeing, are investing in technologies to transport humans to Mars.
Background - Mars exploration
There is an on-going interest in Mars exploration and landing humans on Mars. Improved information on the Martian environment will reduce uncertainties and their associated risks in future Mars missions (Pohly et al. 2019). One way to improve our model of the red planet is through aerial surveillance, which provides information that augments the observations made by ground-based exploration and satellite imagery. In particular, flight vehicles capable of near surface hovering are attractive candidates for such information gathering missions. Previous efforts in designing Mars aerial vehicles have yet to yield a realizable solution for accomplishing flight, which is mainly due to the ultra-low-density environment inherent to the Martian atmosphere. Although NASA plans to send a helicopter to Mars during the 2020 rover mission, a rotorcraft solution with over 850 gram vehicle and a 1.21 m rotor diameter results in a relatively large (Balaram et al. 2018a) and potentially low efficiency (Shrestha et al. 2016) platform.
There are numerous challenges associated with flying on Mars (Bluman et al. 2018; Kang 2019). The challenge of flying in an ultra-low density environment can be summarized by the force balance between the wing lift L and weight W in equilibrium flight, expressed as L=1/2ρU2SCL=W=mg, where g and ρ are the Martian gravitational acceleration and atmospheric density, U is the free stream velocity, m is the vehicle mass, and S is planform area of both wings. Although the Martian g is about one-third of the acceleration on Earth, the average Martian atmospheric density is only 1.3% of the air density on Earth (Shrestha et al. 2016; Sullivan et al. 2000). Aerodynamic forces are proportional to the ambient fluid density, implying that conventional flight vehicle designs generate insufficient lift on Mars at subsonic cruise velocities. Another consequence of the low density is that the operational Reynolds number of small flight vehicles is of the order of O(102) to O(103) (Shrestha et al. 2016; Sullivan et al. 2000). The dynamic viscosity coefficient of the Martian atmosphere is 1.5×10-5 kg/(m∙s) (Petrosyan et al. 2011), similar to the value on Earth (1.8×10-5 kg/(m∙s)). In these low Reynolds number regimes, the lift coefficients of traditional fixed wing aircraft are significantly reduced (Shyy et al. 2013). To compensate for the reduced lift coefficient, all conventional aircraft designs must fly faster (higher U) or with a much lower wing loading (mg/S). The near absence of oxygen in the Martian atmosphere prevents the use of air-breathing propulsion. Moreover, high take-off and cruise velocities pose significant operational challenges for air vehicle launch and recovery, as well as potentially complicating mission tasks. Take-offs and landings without any infrastructure will either require a hover-capable flyer or support equipment such as catapults, parachutes, or nets.
Prior and current Mars flight vehicle concepts
Several intriguing aerial vehicle concepts have been proposed to overcome the challenges associated with flying on Mars. Liu et al. (Liu et al. 2013) provide a comprehensive review of these proposed designs. The Aerial Regional-scale Environmental Surveyor (ARES) was a rocket-powered, robotic airplane platform to aid the NASA Mars Exploration Program (Braun et al. 2006). The prototype was designed to fly at Martian altitudes between 1 and 2 km. However, the ARES could not land on Mars’ surface, and the concept was abandoned in favor of an orbiting surveyor. To explicitly tackle the issue of the low-density atmosphere, freely falling concepts and Mars balloons have also been proposed (Liu et al. 2013). Additionally, NASA’s Jet Propulsion Laboratory has considered a Mars Helicopter (Balaram et al. 2018b; Grip et al. 2018; Liu et al. 2013).
Marsbees - bioinspired Mars flight vehicle concept
Bioinspired solutions for lift generation provide another set of Mars flight vehicle designs. Insects rely on these unsteady aerodynamic mechanisms to produce high CL values (Shyy et al. 2013) in low Re environments, such as the Martian atmosphere. Previous bioinspired concepts include the Entomopter, which is a flapping wing vehicle that uses a blown wing concept for lift enhancement, and the Solid State Aircraft, which is a solar-powered ornithopter (Liu et al. 2013). However, both of these concepts suffer from the adverse effects of scaling up the entire vehicle in an effort to increase wing area. Moreover, the analysis of the Entomopter was based on simplified aerodynamic models, not including all the unsteady low Re lift production mechanisms. They were forced to augment lift production with the blown wing in order to achieve the large lift coefficients which are routinely achieved by insect-style flapping wings (Shyy et al. 2013).
Kang (2019) has recently proposed a bioinspired solution, called Marsbees, by determining a dynamically similar solution based on the desired vehicle mass by scaling the wing motion, shape, and size so that the relevant dimensionless parameters for maintaining dynamic similarity with the lift-weight balance given by Eq. 1.1 (Bluman et al. 2018; Pohly et al. 2018, 2019). Insects can hover and maneuver on Earth through the use of low-Reynolds number, unsteady aerodynamic force enhancement mechanisms. Dynamic similarity ensures that the Marsbees can benefit from the same insect unsteady lift enhancement mechanisms. These bioinspired Mars flight vehicle solutions have been verified using a well-validated three dimensional Navier-Stokes equation solver to determine the forces for hovering flight of our insect-inspired flapping wing flyer (Kang 2019; Pohly et al. 2019).
In an aerodynamic context, the payload (Pohly et al. 2018) and power perspectives (Bluman et al. 2018) look very promising when the wings are designed with compliant structures. Marsbees can also be used in a swarm, which offers additional design options and possibly improved efficiencies for these missions. These missions may include surveying remote locations, retrieving samples, efficiently generating topographic models of the surrounding terrain, providing near-surface weather data, and assisting rovers in path planning (Pohly et al. 2019).
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