News NASA Evaluates EM-2 Launch Options for Deep Space Gateway PPE

Nicholas Kang

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Apr 3, 2016
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NASA recently awarded multiple contracts to industry to do a detailed study of the PPE that would be the first proposed piece of the Deep Space Gateway operating in cislunar space. A total amount of approximately $2.4 million was divided up in awards to Boeing, Lockheed Martin, Orbital ATK, Sierra Nevada Corporation, and Space Systems/Loral.

In a presentation to the NASA Advisory Council (NAC) Human Exploration and Operations Committee on November 30, PPE Program Director Dr. Michele Gates from the Glenn Research Center in Cleveland noted that kickoff meetings between NASA and the awardees to officially start work coincided with the committee meeting, starting the last week of November and continuing into the first week of December.

The kickoff meetings start the contract work for the 120-day study. Each of the awardees will provide an initial 45-day status briefing to NASA, which is planned for the January time-frame. That would be followed by draft study results at the 90-day point, and the final study results at 120 days.


Capabilities envisioned for the PPE include SEP for maneuvering and a hypergolic propulsion system for attitude control. Tanks for both the SEP system, which would use Xenon fuel, and the attitude control system would be refuelable.

The PPE would include two International Docking System Standard (IDSS) docking ports at either end and would also handle power and communications for the gateway.

The control mass for the PPE at launch on EM-2 is 7500 kilograms (kg), which would include the payload attachment hardware and a minimum of 1200 kg of Xenon in the 2000 kg class capacity tank.

Notional PPE launch trajectory:

Analysis of a notional launch, deployment, and insertion of the PPE into cislunar space was recently detailed in a paper prepared for presentation in an American Astronautical Society (AAS) meeting titled “Low Thrust Cis-Lunar Transfers Using a 40 kW-Class Solar Electric Propulsion Spacecraft.

The PPE is planned to fly on EM-2, which will be the first crewed flight of the Orion spacecraft. EM-2 is planned to launch using one of the first SLS Block 1B vehicles, and the PPE will fly as a “co-manifested” payload below Orion on top of the Exploration Upper Stage (EUS), encapsulated by the Universal Stage Adapter.

The EUS will fly a more complicated insertion and deployment sequence for this first Orion mission with crew, which is for all intents and purposes a test flight. In order to mitigate some first-flight risks, Orion will stay in Earth orbit for nearly 24 hours before circumnavigating around the Moon.

In the meantime, EUS will place the PPE on a translunar trajectory that will allow the PPE to insert itself in lunar orbit while also safely disposing of the EUS.

The sequence is called Multi Translunar Injection (MTLI) Free and begins after the EUS finishes inserting the Orion / PPE / EUS stack in low Earth orbit during launch.


After an initial orbit, the EUS will make its second burn to raise the apogee to a high-altitude with a period of approximately 24 hours. Orion will then separate from the PPE and EUS to remain in that high Earth orbit for an extended system checkout with its first crew.

After a day in Earth orbit, Orion will perform its own TLI burn to put it on a “free return” trajectory that will make a single loop around the Moon and return to splashdown on Earth.

Shortly after Orion separates from the EUS and PPE, the upper stage will make its final burn which will serve the dual purpose of a translunar injection burn and a disposal burn.

While Orion stays in Earth orbit, the EUS takes the PPE with it to the Moon, placing both on a polar lunar flyby disposal trajectory. As the name implies, about six days later the vehicle flies over one of the lunar poles at an altitude of about 500 kilometers on a hyperbolic disposal trajectory out of the Earth-Moon system.

Back on launch day shortly after this final EUS burn, the upper stage will deploy the PPE so that the spacecraft can independently maneuver to change its path to stay within cislunar space.

“In order to prevent the PPE from escaping the Earth-Moon system along with the EUS, a small burn needs to be performed within a day of TLI,” the paper explained. “This burn is small enough that it could be performed by either the SEP primary propulsion system or the Reaction Control System (RCS).”

The paper provides a detailed look at one possible scenario for flying the PPE from the Earth to the Moon. The trajectory analyzed in the paper employs a 4.2 meter per second burn of the spacecraft RCS about 12 hours after the EUS TLI burn, placing the spacecraft on a trajectory to transfer to its intended destination halo orbit.

The initial RCS burn using more traditional hypergolic engines allows for an extended checkout of the SEP system before it is used to insert the spacecraft into its destination orbit. After another 14 days and well after the spacecraft has made the first polar lunar flyby, SEP system would make an insertion burn that would last for about 22 days.

As selected for analysis in the paper, the transfer would take about 76 days from launch on SLS to capture in NRHO.

The halo orbit the paper used for analysis is a “L2 Southern 9:2 lunar synodic resonant NRHO,” where the highly elliptical orbit reaches its apolune over the Moon’s South Pole and the period of the orbit is such that there are nine orbits every two lunar months. “It is characterized by a perilune radius of about 3,233 km and an orbital period of 6.7 days,” the paper noted.

“The 9:2 lunar synodic resonant NRHO is selected as the DSG orbit based on favorable transfer characteristics as well as the ability to avoid lengthy eclipses by the Earth. This NRHO exhibits nearly-stable behavior, and station-keeping algorithms enable long-term stays for either crewed or un-crewed assets.”

In another paper prepared for presentation in an earlier AAS meeting, “Targeting Cislunar Near Rectilinear Halo Orbits for Human Space Exploration” notes that the 21-day standalone mission duration capability for Orion will constrain early missions to rendezvous with the PPE where the habitation module is not yet at the facility or fully operational: “We also became interested in “short stay” missions, where Orion would stay only 3-4 days in the NRHO (less than a full rev),” the authors wrote.

“This stay time was thought to be adequate for Orion to perform checkout and deployment operations for new elements on early missions, while reducing mission durations well below the 21 day limitation for the short stay missions examined, this means that there would be multiple sets of launch opportunities each month, with each set spanning 3 to 5 consecutive days.”

Research Papers:

Low Thrust Cis-Lunar Transfers Using a 40 kW-Class Solar Electric Propulsion Spacecraft. (AAS 17-583)

Targeting Cislunar Near Rectilinear Halo Orbits for Human Space Exploration (AAS 17-267)