1. Future In-Space Operations (FISO) Telecon Colloquium
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 m/s capability
Well within the notional Orion, Ares I, Ares V capability
Total servicing time of 15 days
Total round-trip time of 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 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