Altair Descent and Ascent Reference Trajectory Design and Initial Dispersion Analyses

Abstract

The Altair Lunar Lander is the linchpin in the Constellation Program (CxP) for human return to the Moon. Altair is delivered to low Earth orbit (LEO) by the Ares V heavy lift launch vehicle, and after subsequent docking with Orion in LEO, the Altair/Orion stack is delivered through translunar injection (TLI). The Altair/Orion stack separating from the Earth departure stage (EDS) shortly after TLI and continues the flight to the Moon as a single stack.Altair performs the lunar orbit insertion (LOI) maneuver, targeting a 100-km circular orbit. This orbit will be a polar orbit for missions landing near the lunar South Pole. After spending nearly 24 hours in low lunar orbit (LLO), the lander undocks from Orion and performs a series of small maneuvers to set up for descending to the lunar surface. This descent begins with a small deorbit insertion (DOI) maneuver, putting the lander on an orbit that has a perilune of 15.24 km (50,000 ft), the altitude where the actual powered descent initiation (PDI) commences.At liftoff from Earth, Altair has a mass of 45 metric tons (mt). However after LOI (without Orion attached), the lander mass is slightly less than 33 mt at PDI. The lander currently has a single descent module main engine, with TBD lb f thrust (TBD N), providing a thrust-to-weight ratio of approximately TBD Earth g's at PDI.LDAC-3 (Lander design and analysis cycle #3) is the most recently closed design sizing and mass properties iteration. Upgrades for loss of crew (LDAC-2) and loss of mission (LDAC-3) have been incorporated into the lander baseline design (and its Master Equipment List). Also, recently, Altair has been working requirements analyses (LRAC-1). All nominal data here are from the LDAC-3 analysis cycle. All dispersions results here are from LRAC-1 analyses. Descent PhaseThere are three subphases comprising the descent phase of the Altair mission: the braking burn (BB), the approach, and terminal (note a short pitch-up maneuver will be executed near the beginning of approach). The descent subphases are depicted shown in Figure 1.Implicit guidance algorithms will be used to design the reference trajectories in all these descent subphases. In this approach, we can define, in advance of the mission, a reference trajectory as a vector polynomial function of time that evolves backward from the target state. But the reference trajectory cannot be expected to intersect the initial state of the vehicle due https://ntrs.nasa.gov/search.jsp?R=20100035768 2018-05-13T06:16:06+00:00Z to navigation and control dispersions. Implicit guidance will generate acceleration commands that consist of that computed using the reference trajectory plus two feedback terms. The first "feedback" acceleration correction is proportional to the difference between the actual and reference vehicle's positions. The second term is proportional to the difference between the actual and reference vehicle's velocities. Implicit guidance algorithm will "drive" the vehicle to achieve the target state in the presences of controller errors, na...

Fuel-efficient Descent and Landing Guidance Logic for a Safe Lunar Touchdown

“…This is obviously not true since fuel must be expended to generate thrust. [17][18] However, for the Altair lunar lander, the ratio of fuel consumed (from low-gate to touchdown) to the lander mass at touchdown is <5%, 1,11 which is small. Finally, g (in m/s 2 ) denotes the constant acceleration due to gravity of the Moon.…”

“…The contingency ∆V budget (for the descent engine) of Altair is 58 m/s (re-designation of the landing site and dispersion of system variables). 11 The ∆V budget for the RCS thrusters for the entire powered descent and landing phase estimated to be 11 m/s. 11…”

Mars Ascent Vehicle Design for Human Exploration

“…In comparison, the Mars Ascent Vehicle is significantly more massive than both lunar ascent vehicles. This is primarily due to the very different propulsion requirements of the Mars vs. Lunar ascent trajectories 8 . Table 2 provides a comparison of requirements and masses.…”

Preliminary Design of the Guidance, Navigation, and Control System of The Altair Lunar Lander

“…Details that are related to specific GN&C functions, including navigation, optical navigation, guidance, interactions between thrust vector control and sloshing liquids, etc., are given in seven companion papers. [4][5][6][7][8][9][10] The GN&C team also provided support for the design teams of various subsystems including Thermal, Power, Avionics, Flight Software, Structures, Commands and Data Handling, Comm. and Tracking, and others.…”