Revolutionary Entry, Descent, and Deployment Concept for the Mars Balloon Scout Mission

NASA has established the future of space exploration by launching projects that will ultimately lead to human missions to the Moon and eventually to Mars. As a result, the Martian environment must be understood to minimize the risk of placing humans on its surface. Although Mars rovers and orbiting satellites are able to gather a significant amount of planetary data, neither form of exploration has provided adequate data for accessing the full risks to human exploration, nor have they provided a good balance between mobility and precise in-situ measurements. However, these requirements can be fulfilled with a super-pressure balloon system. This idea has driven the University of Michigan team to launch the Mars Balloon Scout (MBS) mission. The objectives of the Mars Balloon Scout mission includes the detection of organic compounds and toxic elements in the atmosphere, measurement of ambient weather patterns and detailed exploration of the local Mars geology. Thus, we have designed a balloon and gondola system to carry the scientific payload along with its associated power and communication equipment. In order to support the proposed payload, we will need a thirty meter diameter spherical superpressure gas cell. The gas cell is composed of a high strength Kevlar and Mylar composite material previously conceived by JPL. With this design, the MBS mission can successfully meet its objectives while surviving the harsh environment of the Mars atmosphere. Our confidence in the super-pressure balloon system results from its simple technology, its ability to survey both the atmosphere and the surface, and its long mission life span. The riskiest part of a Mars balloon mission is the entry, descent, and deployment (EDD) phase. Typical EDD phases of previous missions lasted approximately six minutes from entry to touchdown. In the Mars Balloon Scout, the EDD phase is the time period ranging from the spacecraft entry on the Martian atmosphere to the full inflation of the balloon. In order to increase the balloon inflation time, we propose to replace the subsonic parachute by a revolutionary drogue envelope. The drogue envelope not only serves as a subsonic parachute but also creates a protective “shell” around the superpressure balloon as it inflates. The successful implementation of this design will allow for a low-risk mission that capable of advancing NASA’s goal of sending humans to Mars. 1 Experimental Objectives and Obstacles Entry, descent, and deployment present a unique systems engineering challenge. Many aspects of EDD are poorly understood due to the hostile environment and short time in which it occurs, in addition to its inherently complex physics. The entry, descent, and landing (EDL) of a Mars mission is often referred to as the "six minutes of terror" and the EDD phase of the MBS Mission presents an even larger challenge. Atmospheric entry dissipates 99% of the kinetic energy of the spacecraft entering the atmosphere, slowing it from 5400 m/s to about 430 m/s. After atmospheric entry, the remaining kinetic energy must be dissipated in less than six minutes in order to slow the spacecraft from 430 m/s to about 10 m/s and allow full inflation of the balloon before impact. A balloon mission has not yet been selected for Mars, mainly due to failures of most previous deployment tests. These failures have included tangling of tethers, lack of full inflation of the balloon, rupture of the balloon, lack of stability after inflation, and impact of the balloon into the Mars Balloon Scout Mission Revolutionary Entry, Descent, and Deployment Concept 1 ground before inflation. All previous tests used a supersonic parachute during entry, followed by the use of a subsonic parachute during descent and balloon deployment. Because previous failures have all been due to deployment problems, we believe that the redesign of the subsonic parachute is necessary to solve this problem and decrease the risks to EDD. We have developed a revolutionary drogue envelope concept to be used in lieu of a subsonic parachute. The drogue envelope can be thought of as a hybrid between a subsonic parachute and a zero-pressure natural shape balloon. 2 Design of the Entry, Descent, and Deployment System 2.1 Aeroshell The aeroshell consists of two parts, the heat shield and the backshell. The aeroshell of the MBS Mission is based on the Mars Exploration Rover (MER), Pathfinder, and Viking systems. The aeroshell is the first “brake” for the spacecraft upon Mars atmospheric entry. During entry, the heat shield provides protection and dissipates over 99% of the entry energy through aerothermodynamic heating and drag. Peak temperatures reach 1447 °C while slowing the system from 5400 m/s to 430 m/s. The proposed heat shield is made of an aluminum honeycomb structure inserted between graphite-epoxy sheets. The outside of the aeroshell is coated with a layer of phenolic honeycomb. The phenolic honeycomb is then filled with an ablator: a blend of wood, binder, and tiny silica glass spheres. The backshell is made of similar materials, but has a much thinner layer of ablater than the heat shield. The backshell is covered with a thin layer of aluminized Mylar to protect it from the low temperatures encountered in deep space. The packing of the MBS Mission systems is shown in Figure 2-1. In addition to the MBS Mission systems, electronics and batteries that fire various EDD devices such as separation nuts and the parachute mortar are stored in the aeroshell. Finally, a Litton LN-200 Inertial Measurement Unit that monitors and reports the orientation of the heat shield throughout the entire EDD phase is stored in the aeroshell. Figure 2-1 – Aeroshell Packing Scheme 2.2 Supersonic Parachute The supersonic disk-gap-band parachute is similar to that used in the MER Missions, and is a scaled version of the Pathfinder parachute. Our parachute is a scaled version of these parachutes and will be based on the loads calculated by NASA’s Mars parachute development system. This parachute establishes a stable vertical trajectory for deployment while decelerating the vehicle from supersonic flight to about 200 m/s through aerodynamic drag. The supersonic parachute deploys from a mortar that is interfaced with the backshell structure. The mortar is able to accelerate the parachute to beyond the recirculating wake for controlled and reliable inflation. The disk-gap-band parachute is the only parachute that is qualified for supersonic planetary entry deployment. The disk-gap-band provides more stability than a traditional parachute at the higher velocities and loads encountered during entry and descent. The parachute itself is made of polyester and nylon, which are durable and lightweight fabrics. A triple bridle made of Kevlar constitutes the tethers which are connected to the backshell. When backshell jettisons, the Mars Balloon Scout Mission Revolutionary Entry, Descent, and Deployment Concept 2 supersonic disk-gap-band parachute provides the lift to carry the backshell away from the entry vehicle. 2.3 Drogue Envelope The innovative drogue envelope is composed of 24 half-gores made of a polyethylene film with silicon coating. They are sewn together to form the same natural-shape of a hot-air balloon. The drogue envelope deploys at an altitude of about 10 km and act as a subsonic parachute by taking in ram-air through scoops on each of its gores. The drogue will therefore create both separation drag and ram drag as it provides a protective envelope for the gas cell during inflation. The design of the drogue envelope is illustrated in Figure 2 2. The shape of the envelope was determined using a toolkit developed by Cameron Balloons. The toolkit takes as input basic information about the envelope, such as desired volume and number of gores, and then uses the J.H. Smalley natural shape balloon equations to output points along the curvature of one half-gore. Smalley’s set of equations defining the natural-shape is based on his studies of the equilibrium balance of forces acting on the balloon film. This study was performed in the 1930s and is the current accepted method for determining the ideal shape of hot-air balloons. -