Putnam, Zachary R. | Search NASA

NASA NTRS2018

Supersonic Retropropulsion on Robotic Mars Landers: Selected Design Trades

Many concepts for future robotic Mars lander missions require landing heavier payloads than those landed to date. Mars lander architectures to date have relied on a parachute to help slow the lander; however the effectiveness of a parachute in the thin Martian atmosphere is diminished with heavier payloads unless the diameter of the parachute is increased or it is deployed at a higher Mach number, both of which are significant technical challenges. In addition the parachute can be successfully deployed only within a specific Mach number and dynamic pressure range. Targeting the entry trajectory to hit this “Mach-Q box” imposes constraints on the entry ballistic coefficient, limiting it to ~ 150-200 kg/m^2. Eliminating the parachute from the design requires descent engine ignition at supersonic speeds (Supersonic Retropropulsion, or SRP). SRP increases the propellant requirement, but also allows entry ballistic coefficients of ~600 kg/m^2 or more, with the consequence of significantly increased entry mass and landed payload mass.

Lorenz, Christopher G.↗

NASA NTRS2007

A Comparison of Two Skip Entry Guidance Algorithms

The Orion capsule vehicle will have a Lift-to-Drag ratio (L/D) of 0.3-0.35. For an Apollo-like direct entry into the Earth's atmosphere from a lunar return trajectory, this L/D will give the vehicle a maximum range of about 2500 nm and a maximum crossrange of 216 nm. In order to y longer ranges, the vehicle lift must be used to loft the trajectory such that the aerodynamic forces are decreased. A Skip-Trajectory results if the vehicle leaves the sensible atmosphere and a second entry occurs downrange of the atmospheric exit point. The Orion capsule is required to have landing site access (either on land or in water) inside the Continental United States (CONUS) for lunar returns anytime during the lunar month. This requirement means the vehicle must be capable of flying ranges of at least 5500 nm. For the L/D of the vehicle, this is only possible with the use of a guided Skip-Trajectory. A skip entry guidance algorithm is necessary to achieve this requirement. Two skip entry guidance algorithms have been developed: the Numerical Skip Entry Guidance (NSEG) algorithm was developed at NASA/JSC and PredGuid was developed at Draper Laboratory. A comparison of these two algorithms will be presented in this paper. Each algorithm has been implemented in a high-fidelity, 6 degree-of-freedom simulation called the Advanced NASA Technology Architecture for Exploration Studies (ANTARES). NASA and Draper engineers have completed several monte carlo analyses in order to compare the performance of each algorithm in various stress states. Each algorithm has been tested for entry-to-target ranges to include direct entries and skip entries of varying length. Dispersions have been included on the initial entry interface state, vehicle mass properties, vehicle aerodynamics, atmosphere, and Reaction Control System (RCS). Performance criteria include miss distance to the target, RCS fuel usage, maximum g-loads and heat rates for the first and second entry, total heat load, and control system saturation. The comparison of the performance criteria has led to a down select and guidance merger that will take the best ideas from each algorithm to create one skip entry guidance algorithm for the Orion vehicle.

Rea, Jeremy R.↗