When Rocket Lab’s Electron rocket launches the CAPSTONE spacecraft, it will use a series of precisely targeted orbit raising maneuvers and phasing orbits in low Earth orbit to achieve the final injection conditions required to achieve the Ballistic Lunar Transfer that CAPSTONE requires. These maneuvers will be executed by Rocket Lab’s updated “Photon” stage and the plan for these maneuvers will be developed by Rocket Lab’s flight dynamics team. The Rocket Lab Flight Dynamics (FIDO) team consists of Space Exploration Engineering (SEE) and Rocket Lab FIDO engineers who have been training with SEE over the last 2 years. SEE brings a wealth of experience in planning and executing cislunar trajectories of just this type. A big part of our business model is to bring this expertise to our customers so they can continue do these types of projects when the mission is over.
The expertise of SEE’s team comes from missions such as Clementine (DSPSE) which flew in 1994, ISTP(1994), WMAP(2001), IBEX(2008), LCROSS(2009), LADEE(2013), TESS(2018) and SpaceIL Beresheet(2019). All of these missions used combinations of precisely timed Earth phasing orbits and orbit raising maneuvers to achieve their objectives, just as Photon will do to inject the CAPSTONE onto its desired deep space asymptote.
The CAPSTONE Earth orbit profile builds directly from SEE trajectory design and operational experience from a number of previous missions. Each mission design comes from a series of requirements imposed by the launch vehicle (how much mass it can lift to what orbit), the spacecraft propulsion system (how long can burns be, and how many can be done), the navigation techniques used (how many ground passes needed, how big are the antenna on the ground) and operations (how much time does it take to generate new burn plans, process tracking data, generate and test new command files, etc.).
The Interstellar Boundary Explorer (IBEX) trajectory plan was designed, navigated and executed by SEE personnel in 2008. Figure 1 shows the trajectory in an Earth-Moon rotating frame. IBEX used 4 phasing orbits (or loops) to precisely insert IBEX on the desired trajectory in cislunar space. This orbit was ultimately modified to be a 3:1 Lunar resonant orbit that is stable for decades. IBEX is still operational and continues to return good data.
In the Earth-Moon rotating frame, the Earth is in the middle, the Moon is at the top and we draw a line between the Earth and the Moon for clarity. From this vantage point, the Moon appears stationary with respect to the Earth and the background stars appear to rotate.
This classic video (at about the 17:00 mark) shows such a rotating frame. https://youtu.be/bJMYoj4hHqU
This frame is useful because it shows you where the apogees and perigees are with respect to the location of the Moon. For a resonant orbit, you want the apogees to stay clear of the Moon. For a lunar mission, you’d like the last apogee to encounter the Moon. Lunar encounters at apogee will alter the orbit significantly, so controlling these encounters is key in any cislunar mission.
The final extended mission orbit for IBEX is shown in Figure 3 (another rotating view) which has an orbit period that is 1/3 of the Moon’s period. Note that the final orbit (in blue) has apogees that keep clear of the Moon.
The Lunar Atmosphere and Dust Environment Explorer (LADEE) flew in 2013 and spent 6 months in lunar orbit. The cislunar portion of the mission consisted of 3.5 phasing loops, which extended the launch window and reduced the delta-v requirements of the mission. Figure 4 shows the Earth-Moon rotating view of the trajectory, while Figure 5 shows the inertial view. SEE worked closely with NASA Ames Flight Dynamics personnel to design the trajectory and then execute it in operations.
The Transiting Exoplanet Survey Satellite (TESS) launched in 2018 and used a combination of phasing loops (3.5) and a 2:1 resonant orbit, combining features of both IBEX and LADEE trajectories. SEE personnel worked with GSFC engineers to design and fly the mission. Figures 6 and 7 show the phasing loops once again.
In February of 2019, the SpaceIL “Beresheet” lander used a lower starting point with a super-GEO GTO with an apogee near 60,000 km altitude. A series of initial phasing orbits at lower altitudes then had been used on LADEE or IBEX) were necessary. SEE helped design the trajectory, trained the SpaceIL and Israel Aerospace Industries (IAI) Flight Dynamics engineers, and supported them during operations. Figures 8 and 9 show the cislunar portion of the Beresheet trajectory.
The CAPSTONE launch provides a different challenge, as the Electron rocket leaves the Photon + Capstone pair in a really low Earth orbit. We have to do a series of fairly big maneuvers to raise apogee, while giving ourselves enough time between burns to find out how the maneuvers performed, and plan the next one. All of this goes on while the perigee and RAAN are moving due to the Earth’s gravity field. Note that in Figure 10, the starting orbits are really small, compared to starting orbits we’ve used in previous missions.
Figure 11 shows the 7 levels that are used to get to the final targeted asymptote. Finally, Figure 12 shows the same trajectory in an Earth Inertial frame. While the figures don’t show it, there is typically 12 hours or more between each burn, and the entire sequence takes roughly 6 days after launch to achieve.
Once the CAPSTONE spacecraft is precisely placed on its target asymptote by the Photon stage, it is then on a Ballistic Lunar Transfer from which it will approach the Moon and enter into a Near Rectilinear Halo Orbit (NRHO). Figure 12 shows the Ballistic Lunar Transfer CAPSTONE uses to transit to the Moon in an Earth-Sun rotating frame. Finally, Figure 13 shows the same trajectory in an Earth Inertial frame. The BLT and NRHO portion of the mission were planned by our friends at Advanced Space in Colorado. Please visit their page for extensive details of the CAPSTONE Mission:
The CAPSTONE propulsion system comes from our friends at Stellar Exploration. Please visit their page here:
https://www.stellar-exploration.com/
