LOTUS: Enabling Human/Robotic Servicing Missions in Earth-Moon System

1. The L1 Orbit Used for Servicing (LOTUS): Enabling Human/Robotic Servicing Missions in the Earth-Moon System Brent Wm. Barbee Future In-Space Operations (FISO) Telecon Colloquium June 16th, 2010

2. Background • NASA-GSFC is currently studying a suite of notional missions to inform a forthcoming congressional report on spacecraft servicing capabilities and concepts • The 5th Notional Mission involves human/robotic servicing of a large Sun-Earth L2 (SEL2) telescope in the Earth-Moon system

3. Mission Profile • A large telescope stationed at SEL2 returns to the Earth-Moon system and rendezvouses with a robotic servicing vehicle in a Lyapunov orbit about Earth-Moon L1 (EML1) • A crew vehicle carrying astronauts launches to rendezvous with the servicer/telescope stack • After servicing is complete, the crew vehicle returns to Earth and the telescope returns to SEL2 • The robotic servicer spacecraft remains in orbit for 25 years

4. Telescope Considerations • Minimize telescope maneuver magnitudes • Conserve telescope propellant • Avoid large thruster-induced accelerations • Minimize telescope down-time • Avoid excessive travel time to/from SEL2 • Telescope is assumed to be a cooperative rendezvous target for the robotic servicer

5. Robotic Servicer Orbit • Robotic servicing vehicle has a 25 year lifetime • The orbit it inhabits in the Earth-Moon system must: • Be easily accessed by both the crew vehicle and the telescope • Require minimal station-keeping ΔV • Remain well clear of the Van Allen Belts and GEO

6. Crew Vehicle Objectives • Crew vehicle trajectory should: • Maximize available time for servicing • Provide a total round-trip flight time (launch to landing) of at most 21 days • Offer a free return from launch if possible • Stay clear of the Van Allen Belts and GEO • Provide safe atmospheric re-entry • Maximum atmospheric re-entry velocity of 11 km/s, as per Apollo 10 • Notional Orion was assumed for crew vehicle

7. Robotic Servicer Trajectories

8. Telescope Trajectories • Telescope can travel relatively easily between its SEL2 halo orbit and the EML1 Lyapunov orbit via low-energy transfers • ΔV from SEL2 to EML1 = 45 – 50 m/s • ΔV from EML1 to SEL2 < 1 m/s • Flight time between EML1/SEL2 = 50 – 130 days • Faster transfers are possible but require considerable ΔV • Some telescope downtime will have to be tolerated in exchange increased lifetime from servicing

9. Rendezvous at EML1 • The robotic servicer can rendezvous with and capture the telescope on the EML1 Lyapunov orbit relatively quickly for modest ΔV costs

10. Example EML1 Rendezvous • The relative motion dynamics between spacecraft on a libration point orbit are completely different from the familiar relative motion dynamics between spacecraft in Earth orbit (LEO, GEO, etc.)

11. Crew Trajectory Alternatives • The first option considered was to send the crew directly to the EML1 orbit and perform servicing there • Crew has a free return from launch if necessary • However, this only offered ~ 11 days for servicing, which was insufficient for the planned activities • Outbound and inbound times are not selectable • Additionally, the EML1 orbit experiences eclipses that can be 9 to 12 hours in length

12. Crew Trajectory Alternatives • The second option considered was to place the crew onto a large 21 day long Highly Elliptical Orbit (HEO) about Earth • Completely free return for the crew • However, bringing the robotic servicer and telescope to this orbit within 1 – 3 days of launch and having them depart within 1 – 3 days of re-entry required > 2,000 m/s of ΔV from the robotic servicer and telescope, which is not permissible

13. Zero-Velocity Curve Analysis • The next approach was to study the restricted three-body dynamics • I noticed that there was a large volume of space around Earth that should be accessible from the EML1 orbit for very little ΔV …

14. The LOTUS • Ultra-low departure ΔV from EML1 orbit is easily achieved by the servicer/telescope stack

15. The LOTUS in the Inertial Frame • The LOTUS is a “HEO” with a high perigee • Eccentricity of 0.54 • Period of ~ 10 days • The LOTUS perigee is 83,777 km, well above the Van Allen Belts and GEO

16. Crew Launch to a LOTUS Apogee

17. Crew Free Return From Launch • The crew always has a free return from launch available from in case LOTUS insertion must be aborted

18. Servicing on the LOTUS • The crew spends 16 days on the LOTUS • 1 day is for AR&B with the servicer/telescope stack • 15 days for servicing • Meets requirements for the notional mission under consideration

19. Crew Return to Earth

20. Return to EML1 • The LOTUS naturally returns to EML1 ~ 98 days after initial departure • The servicer can easily reinsert into the EML1 Lyapunov orbit • The telescope can easily continue past EML1 and transfer back to SEL2

21. Mission Summary • Total crew vehicle ΔV (including 100 m/s for AR&B) is 2120 m/s • Quite reasonable considering crew vehicles for lunar missions historically had a 2800 m/s capability • Well within the notional Orion, Ares I, Ares V capability • Total servicing time of 15 days • Total round-trip time of 20.54 days

22. Trajectories in the Inertial Frame

23. LOTUS Eclipse Analysis • Eclipses on the LOTUS are much reduced compared to the EML1 Lyapunov orbit • Judicious choice of start date completely avoids eclipses during the LOTUS • Worst case eclipse duration on the LOTUS is about 4 hours

24. Servicing Time Flexibility • The 15 day servicing case shown here is only one possibility of many • The advantage of the LOTUS is that the crew can arrive at / depart from any points on the LOTUS, making the servicing time selectable • With a 21 day round-trip flight time limit, the maximum servicing time available is 19 days • Launch into /depart from a LOTUS perigee • 0.6 day flight time from launch to LOTUS insertion, same for de-orbit • Launch C3 is -7.98 km2/s2 • Insertion / De-Orbit ΔV is 1800 m/s • Total crew vehicle ΔV of 3700 m/s, not unreasonable for such a low launch C3 and in light of historical and notional future mission capabilities

25. Summary and Conclusions • The LOTUS offers key advantages for human/robotic servicing missions in the Earth-Moon system • Selectable time on-orbit for servicing • Low ΔV access to/from EML1 and therefore to/from SEL2 • Avoidance of eclipses reduces battery size requirements, saving considerable spacecraft mass • No orbit maintenance/station-keeping maneuvers required on LOTUS • Launch C3 and ΔV requirements consistent with anticipated capabilities • Crew always has a free return from launch if necessary • LOTUS perigee is well above the Van Allen Belts and GEO • Atmospheric re-entry velocity when de-orbiting from any point on the LOTUS is always ≤ 11 km/s