The Flexible Lunar Architecture for Exploration (FLARE) is a concept to deliver four crew to the lunar surface for a minimum of seven days and then return them safely to Earth. FLARE can be implemented whenever the component vehicles are operational. FLARE was developed as an alternative to NASA's Human Landing System (HLS) reference architecture from the Design Analysis Cycle (DAC) #2 created in 2019. The DAC2 guidelines required utilization of the Gateway vehicle in a Near- Rectilinear Halo Orbit (NRHO). Instead, FLARE chooses a Low Lunar Frozen Polar Orbit (LLFPO) for lunar rendezvous of components, and an optional Gateway vehicle. The LLFPO provides a stable orbit that overflies the south pole every 2 h, ensuring easy access to the lunar surface for surface aborts with a much lower propellant requirement than NRHO. The minimum FLARE concept uses one Space Launch System (SLS) launch, one Orion, one European Service Module (ESM), and one human lander (launched on commercial vehicle(s)). FLARE adds the SpaceTug, based upon the mature and successful ULA "Common" Centaur Upper Stage vehicle, with modifications to create an Earth-Moon transfer vehicle. In the FLARE baseline mission, the SpaceTug provides propulsion needed to return the Orion + ESM from LLFPO to Earth. The SpaceTug also provides propulsion to deliver the separate human lander components - the Descent Element (DE) and the Ascent Element (AE) - from Low Earth Orbit (LEO) to LLFPO. The SLS Block 1 then launches the Orion + ESM and completes a rendezvous with the mated DE + AE components in LLFPO. FLARE offers optional phases beyond the baseline mission. The SpaceTug can deliver components of the planned Gateway, including the Power and Propulsion Element (PPE) and the Habitable and Logistics Outpost (HALO), to LLFPO. FLARE provides an option to deliver precursor equipment to the lunar surface to enhance and extend the human mission. With these components, including an inflatable habitation module and airl FLARE concept FLARE is supported with a technical analysis of multiple factors, including mass and change in velocity (ΔV) calculations including crew, cargo, and propulsion systems. FLARE develops a plan for launch and mating of necessary components in Earth and lunar orbit. FLARE provides a reference design for the SpaceTug and a human lander, including both the pressurized Ascent Element (AE) and a “common” Descent Element (DE) capable of delivering either crew or cargo to the lunar surface. One SLS Block 1, capable of lifting 26 mt to TLI [ 20 ], could not lift this integrated NRHO lunar lander. To reduce the lander weight on NASA's currently planned Artemis-3 mission, the HLS plans to only send two of the four astronauts from Orion to the lunar surface and back to NRHO. Two other crew will remain aboard Gateway in NRHO [ 21 ]. With FLARE, however, an integrated LLO human lander has a launch mass of 20.4 mt The entire lunar surface is available for observation and mission access at some point during a lunar month. From this inclination, a small 2.5° plane change during descent provides access to likely landing sites on flat areas near the Persistently Illuminated Regions (PIRs) of the south pole [ 26 , 27 ]. The ΔV cost for this 2.5° plane change maneuver is calculated, based upon orbital velocity for a 100 km lunar altitude, to be 71 m/s (see Appendix B Table B19 ). This is additional to the descent propellant requirement for 2.180 km/s [ 18 ]. During ascent (when the vehicle velocity is low) the 2.5° plane change is not budgeted with additional propellant to the required 1.968 km/s [ 18 ] for ascent to the 100 km altitude. A graphical comparison of NRHO and LLFPO and associated ΔV requirements is shown in Fig. 11 . Fig. 11 Comparison of lunar orbits and change-in-velocity (ΔV) transfers. Fig. 11 4.7. Earth communications and lunar surface navigation The average Earth visibility from possible landing sites at the lunar south pole vary from 30% to 70% during a typical month, and no likely site has 100% coverage [ 26 ]. FLARE, however, chooses to place a satellite in orbit for continuous communications between Earth and the lunar landing site. Co-manifested with various elements of FLARE (possibly with PPE, HALO, DE1, AE, or DE2), the satellite(s) could be deployed after the payload stack achieves sufficient ΔV for TLI. 5. SpaceTug The FLARE SpaceTug is based upon a successful, mature flight-proven upper-stage developed by the United Launch Alliance (ULA). The “Common” Centaur (evolved from the Centaur-III) uses a standard RL-10 engine powered by Liquid Oxygen (LOX) and Liquid Hydrogen (LH 2 ) to deliver payloads to LEO atop an Atlas launch vehicle [ 28 ]. The AE volume is 8.4 m 3 (or approximately 2 m 3 per person) and has a pentagon-shaped outer mold line (see Fig. 13 ). The AE is designed such that the crewmembers stand during descent and ascent. The AE supports crew use of either full xEMU spacesuits or Orion Launch and Entry Suits (LES). The crew has sufficient volume to don/doff their xEMU and/or LES two-at-a-time, and also supports crew sleep periods (by use of hammocks) inside the pressurized volume. The AE is designed to carry 100 kg of lunar surface and crew biological samples (allocated as payload). The AE dry mass includes 20 kg of additional science supporting equipment such as containment boxes within the pressurized volume. Fig. The FLARE Option B extends to a 14-day surface mission with longer science traverses using the prepositioned assets. All four Orion crew descend to the lunar surface inside the pressurized AE1. Two of the crew are dressed in xEMU suits, and two are dressed in the Orion LES. Once the DE2 has landed safely on the Moon, the AE1 is depressurized and the two crew dressed in xEMU suits depart. The two crew remaining in the AE1 - dressed in the LES that can keep the astronauts alive for up to 6 days [ 71 ] - then repressurize the AE1 and wait for the return of their crewmates.