As part of its initiative to advance exploration in the solar system, NASA has awarded Aerojet Rocketdyne a contract for two crucial elements of an Advanced Electric Propulsion System. The new system will meet enhanced propulsion needs of upcoming robotic and human missions in an effort to increase efficiency over traditional, chemical-based propulsion.
Aerojet Rocketdyne and Advanced Electric Propulsion System:
The desire to shift away from chemical propulsion for space-based vehicles has been in NASA’s sights for over 50 years.
To this end, NASA has selected Aerojet Rocketdyne, Inc. of Redmond, Washington, to design and develop an AEPS that will significantly advance the nation’s commercial space capabilities and enable deep space exploration missions.
One of these deep space exploration missions is NASA’s upcoming flagship robotic Asteroid Redirect Mission (ARM) – set for launch in the early part of the next decade – as well as the agency’s larger Journey to Mars goals.
In terms of the newly announced contract, the AEPS Aerojet Rocketdyne will develop could result in an increased fuel efficiency more than 10 times what is currently available with existing chemical propulsion technology.
Additionally, this AEPS technology could more than double the thrust capability of spacecraft compared to current SEP systems.
“Through this contract, NASA will be developing advanced electric propulsion elements for initial spaceflight applications, which will pave the way for an advanced solar electric propulsion demonstration mission by the end of the decade,” said Steve Jurczyk, associate administrator of NASA’s Space Technology Mission Directorate (STMD).
“Development of this technology will advance our future in-space transportation capability for a variety of NASA deep space human and robotic exploration missions, as well as private commercial space missions.”
Per the contract, Aerojet Rocketdyne will oversee the development and delivery of an integrated electric propulsion system consisting of a High Power Thruster, Power processing Unit (PPU), low-pressure xenon flow controller, and electrical harness.
NASA has already developed and tested a prototype High Power Thruster and PPU that Aerojet Rocketdyne will be able to use for reference design.
Moreover, the company will construct, test and deliver an engineering development unit for testing and evaluation in preparation for producing follow-on flight units.
The contract also includes an “option period,” which, if exercised, will allow Aerojet Rocketdyne to develop, verify, and deliver four integrated flight units.
The work being performed under this contract will be led by a team of NASA Glenn Research Center engineers, with additional technical support by Jet Propulsion Laboratory (JPL) engineers.
This work will also directly complement recent advanced solar array systems work, which also received funding from the STMD.
In this manner, the AEPS is the next step in NASA’s SEP project, which is developing critical technologies to extend the range and capabilities of ambitious new science and exploration missions.
While chemical-based propellant provides an excellent, high thrust-to-weight ratio required for vehicles attempting to escape Earth’s gravity, there is an “inherent energy in the propellant as a limiting factor as the propellant energy defines exhaust velocity and hence ISP (Impulse Specific Thrust),” notes an undated Solar Electric Propulsion presentation from NASA’s Technology Development group.
In this type of propulsion mechanism, chemical propellants are converted from their stored energy into kinetic energy, meaning that any type of propulsive force generated must come from propellants carried aboard the spacecraft from launch through the duration of its mission.
Conversely, SEP “uses solar energy, gathered from solar arrays, [that is then] converted into electricity [that is then used] to ionize and accelerate propellant to produce thrust.”
Under this model, SEP has a much higher ISP than conventional chemical-based propulsion – a higher ISP on the magnitude of 1.5 to 10 times more efficient than chemical ISP.
While the thrust generated from SEP is weaker than chemical alternatives, SEP holds an ability to provide thrust over time for much longer durations than chemical-based propulsion can.
Moreover, for missions that are not time-sensitive, “SEP uses less fuel to achieve the same destination and orbit.”
To this end, less fuel needs to be carried on SEP powered spacecraft and can results in anywhere from a 20% to 50% weight reduction and mass savings at launch – which translates directly to a lower launch cost of the mission.
A further benefit to SEP powered spacecraft is a much longer mission life, which is ultimately ideal for deep-space, long-life missions as well as “station keeping” missions that need to hold relative position with another body for a prolonged period – like the upcoming ARM flight.
Aerojet Rocketdyne: Final two pieces of the AEPS technology investment process
At the beginning of the decade, NASA initiated a technology investment study to advance the concept of SEP to enable more efficient exploration of solar system targets, with an eventual use of AEPS elements during human missions to Mars.
The initial results of that technology investment study resulted in a three-pronged development approach, with the first element being the development of advanced next generation solar arrays for “high power electric propulsion technologies to enable 30-50 kW-class SEP,” notes the undated Solar Electric Propulsion presentation.
This first phase of the process resulted in development contracts for advanced solar array systems to Alliant Techsystems Inc. (ATK, now Orbital ATK) and Deployable Space Systems (DSS).
ATK, which began its new solar array development process in October 2012, decided to explore the idea of megaflex solar arrays, using lightweight materials for a new higher-class solar array that would deploy in a near-circular configuration, providing more power at a lower weight with a greater surface area than conventional arrays.
The 18 month development process culminated in March 2014, with portions of the newly developed solar arrays debuting in December 2015 aboard Orbital ATK’s first Enhanced Cygnus OA-4 mission to the International Space Station.
Meanwhile, a similar 18 month development program began in October 2012 for DSS and resulted in the development of Roll Out Solar Arrays (ROSAs).
Meanwhile, from January 2012 to January 2015, NASA developed an in-house electric propulsion 12-15 kW class HET system.
This marked the completion of the first stage of the technology investment initiative and paved the way for Requests For Proposals (RFPs) and contract bids for the final two phases of the project, a high-power PPU (Power Processing Unit) and a High Power Thruster.
For the RFPs, the high-power PPU needed a high-efficiency operation greater than 96%, a high temperature operation of approximately 100° Celsius, and space-qualified parts for 300V operation.
Additionally, the High Power Thruster required a 100 kW-class thruster with a 20,000 hour to 40,000 hour lifetime, a variable ISP, and alternate propellant use capability.
With the contract for the final two elements of the technology investment initiative now awarded to Aerojet Rocketdyne, the company will have three years to deliver its products to NASA.
This contract further enables NASA to continue its commitment to refining development of spaceflight electric propulsion technology.
The first successful ion electric propulsion thruster was developed at NASA’s Glenn Research Center in the 1950s, and the first operational test of an electric propulsion system in space, also developed by Glenn, launched on 20 July 1964 as part of Space Electric Rocket Test 1.
Since that first flight, NASA has increasingly relied on SEP for long-duration, deep-space robotic science and exploration missions to multiple destinations.
The most recent SEP mission for NASA is the on-going Dawn mission, which launched in 2007, surveyed the asteroid Vesta from 2011 to 2012, and then transferred itself via ion engine propulsion out of Vesta’s orbit and to the dwarf planet Ceres, which it has been exploring in-situ since March 2015.
(Images: Via NASA, Orbital ATK and Aerojet Rocketdyne)