Lily Yang

Use of atmospheric braking during Mars missions

The use of a high-lift, winged atmospheric entry-glide vehicle by an early Mars manned mission lasting 14-16 months allows the effective use of atmospheric braking to decelerate upon arrival at Mars. Following nearly-constant deceleration, the vehicle skips out of the atmosphere into a low planetary orbit. The maximum atmospheric heating rate thus generated is of the order of 100 W/sq cm at the stagnation point for a fully catalytic surface; the corresponding equilibrium wall temperature was 2150 K. The vehicle envisioned could be radiatively cooled to an entry speed of over 8 km/sec.

Mars Environmental Survey Probe Aerobrake Preliminary Design Study

The objective of this study is to design aerobrakes for the Mars Environmental Survey (MESUR) vehicles. To contain cost, existing flight-certified materials are considered for the structure and heatshield. Since the probes enter the atmosphere directly, the heatshields had to survive entry during duststorm conditions. A slightly modified Viking forebody shape, consisting of a blunted 70-deg cone, is used. An aluminum honeycomb shell is used for the structure. For the nominal 7-km/s entry, the heatshield material used on the Vikings (a silicone elastomeric charring ablator, SLA-561) was found to be lightest, yielding an aerobrake mass fraction of 13.2%. For the extreme case of entry at 9 km/s, the stagnation region heatshield consisted of the medium-density ablator AVCOAT-5026, used on the Apollo capsules, and SLA-561 was used on the conical skirt. The aerobrake mass fraction for the 9-km/s entry was 18%. The penalty resulting if a single conservatively designed aerobrake were used for both entry velocities could reach 4.8% of the entry mass at 7 km/s. Therefore, it is recommended that separate aerobrakes be designed for probes entering at 7 km/s and at 9 km/s.

TPS SELECTION AND SIZING TOOL IMPLEMENTED IN AN ADVANCED ENGINEERING ENVIRONMENT

A tool, TPSSIZER, was developed to provide a Thermal Protection System (TPS) analysis and design capability . The tool focuse d on analysis of space vehicles at the conceptual design level and was implemented in a collaborative engineering analysis environment. TPS sizing methodologies and data exchange interfaces with supporting disciplines were developed. Additionally, improv ements were made to prior art by introducing automatic generation of TPS stackups, automatic generation of aerothermal environment files, maintenance of consistent material properties descriptions, and the capability to simultaneously ev aluate multiple nom inal and abort flight trajectories .

Atmospheric environment during maneuvering descent from Martian orbit

This paper presents an analysis of the atmospheric maneuvering capability of a vehicle designated to land on the Martian surface, together with an analysis of the entry environment encountered by the vehicle. A maximum lift/drag ratio of 2.3 was used for all trajectory calculations. The maximum achievable lateral ranges varied from about 3400 km to 2500 km for entry velocities of 5 km/s (from a highly elliptical Martian orbit) and 3.5 km/s (from a low-altitude lower-speed orbit), respectively. It is shown that the peak decelerations are an order of magnitude higher for the 5-km/s entries than for the 3.5-km/s entries. The vehicle entering at 3.5 km/s along a gliding trajectory encountered a much more benign atmospheric environment. In addition, the gliders peak deceleration was found to be only about 0.7 earth g, making the shallow flight path ideal for manned vehicles whose crews might be physically weakened by the long voyage to Mars.

Atmospheric maneuvering during Martian entry

A comparative-advantages study is made of two different Martian atmospheric entry maneuvers, on the basis of calculation results for the case of a vehicle with a maximum L/D ratio of 2.3. Entries from a highly elliptical Martian orbit at 5 km/sec are more difficult than those from a lower altitude and speed orbit at 3.5 km/sec, due to their more stringent guidance requirements. Efforts to reduce the deceleration for the higher speed entry by lift-modulation achieved a 40-percent reduction, but at the cost of a 50-percent decrease in lateral range. The lower-speed entrys gliding trajectory is noted to encounter a far more benign atmospheric environment.

The heating environment during Martian atmospheric descent

It has been shown that a vehicle with a lift/drag ratio of 2.3 entering the Martian atmosphere at parabolic speed of 5 km/sec, or from a low orbit at 3.5 km/sec, has a very large landing footprint. At the 5-km/sec entry speed, the trajectory exhibits large skipping motions; however, a lateral range of up to 3300 km is attainable. The entries from low satellite orbit yield a gliding lateral range of 2500 km. The distances correspond to latitude changes of 57 and 42 deg, respectively. The high-speed, skipping entries were accompanied by the most intense heating. The peak stagnation point convective rates varied from 59 W/sq cm to 88 W/sq cm for partially and fully catalytic walls, respectively; the corresponding equilibrium wall temperatures were 1900 K and 2100 K. The peak heating at a wing leading-edge point reached 50 W/sq cm because of the presence of a transitional boundary layer. The lower-speed, gliding entries experienced much milder heating with a peak stagnation point rate of about 14 W/sq cm, resulting in a wall temperature near 1300 K. However, the longer duration of the gliding entries resulted in comparable heat loads for both entry speeds. The highest heat loads approached values experienced by the Shuttle orbiter stagnation point during a typical entry.