The Future of Deep Space Propulsion May Soon Be Radically Altered
According to several experts who testified before Congress this week, we may be on the cusp of advances that could dramatically transform how we fly through space.
Τhere’s a saying among space exploration enthusiasts that human missions to Mars have always been 20 years ahead of available technology. We’ve never quite had the significant research investment and development needed for propulsion, life support, and the ability to land large payloads — to name just a few critical elements — in order to establish human settlements on Mars.
But according to several experts who testified before Congress this week, we may be on the cusp of advances that could radically alter how we fly through space, with breakthroughs that could allow faster travel, larger payloads, and greater efficiency in propulsion. Space industry leaders discussed recent advances in in-space propulsion that were brought about, in part, by the all-but-canceled Asteroid Redirect Mission (ARM), which may surprise some of the program’s critics.
Participants in the hearing, which was held by the Space Subcommittee of the House Committee on Space, Science, and Technology, were part of the Next Space Technologies for Exploration Partnerships (NextSTEP), a public-private collaborative model that uses commercial development of deep space exploration capabilities to support more extensive human spaceflight missions with NASA.
“The development of our in-space propulsion and power technologies are essential for future exploration,” Rep. Brian Babin (R-Texas), the subcommittee chair, told Seeker following the hearing. “The work that NASA is doing to adapt its current work on solar electric propulsion to a Deep Space Gateway architecture and further pursuit of high-power in-space propulsion for a Deep Space Transport are key to ensuring that human exploration of Mars is affordable and sustainable. Future development of these technologies will be essential to unlocking the secrets of our solar system’s ocean worlds, like Europa.”
“I believe space travel beckons humanity even more today than it did 50 years ago.”
ARM was originally designed as a Mars precursor mission to develop deep space exploration capabilities. ARM would find, capture, and redirect an asteroid robotically to orbit the moon, and then astronauts would visit it for exploration and study. But the technology involved in realizing the feat would also help prepare for human missions to the Red Planet and other destinations within the solar system. The astronauts would have also tested Mars-capable spacesuits, sample harvesting techniques, and docking capabilities that would be critical for operating independently of Earth during long-duration missions to Mars.
Yet the idea of sending humans to an asteroid never really captured the attention of the public — or Congress. The Trump administration’s proposed 2018 budget completely cuts funding for ARM.
There is more to ARM than meets the eye. NASA wanted to use the project to make advancements in solar electric propulsion (SEP) — sometimes called ion propulsion — which works by electrically charging, or ionizing, a gas using power from solar panels and emitting the ionized gas to create thrust to propel the spacecraft. These engines are different than chemical rockets and thrusters that most spacecraft use.
SEP engines are much more efficient than conventional chemical propulsion because they turn electrical energy from solar panels into thrust, meaning they don’t have to carry large amounts of heavy, chemical propellant.
“High power solar electric propulsion capabilities, scalable to handle power and thrust levels needed for deep space human exploration missions, are considered essential to efficiently and affordably perform human exploration missions to distant destinations such as Mars,” Bill Gerstenmaier, associate administrator for the Human Exploration and Operations Directorate at NASA, remarked at the hearing.
The concept of solar electric propulsion has been around for a long time. Robert Goddard discussed it in the early 1900s, but the first spacecraft to use the technology was Deep Space 1 in 1998. A few other robotic solar system missions (ESA’s SMART-1, Japan’s Hayabusa) have used solar electric propulsion, and Boeing recently launched the first commercial Earth orbiting satellites that rely solely on electric propulsion. The Dawn mission to the asteroid belt, which launched in 2007, uses ion propulsion.
The improved SEP design packs three times the power of previous models, is 50 percent more efficient, and uses much less propellant. Although developed for asteroid exploration, the new and improved thruster could one day be used to send large payloads to Mars in support of human settlement.
A typical SEP system schematic. | Aerojet Rocketdyne
“SEP systems under development now by NASA and Aerojet Rocketdyne reduce the amount of propellant needed for deep space missions by a factor of 10,” said Joe Cassady, Executive Director for Space, at Aerojet Rocketdyne. “This is important because it costs as much to launch propellant as it does to launch scientific instruments or other mission critical equipment. SEP makes it possible to launch larger, heavier payloads thereby reducing the number of launches needed and the taxpayer cost for the total mission.”
There’s one downside to SEP engines: They lack sufficient powerful over a short amount of time to lift a spacecraft off of Earth’s surface. For that, you need the sudden, swift acceleration to overcome the pull of our planet’s gravity that currently only chemical rockets can provide. To get humans to Mars, the current plan is to use NASA’s large new rocket currently under development, the Space Launch System (SLS).
While a SEP-powered spacecraft provides low acceleration, when it operates in space, it can fire continuously for many years to thrust a large mass to high speed.
“Compared to chemical propulsion, this approach enhances the efficiency of the thruster by more than an order of magnitude and leads to significant mass reductions — a change that allows us to include more payload mass on the same launch vehicle,” said Mitchell Walker, chair of the American Institute of Aeronautics and Astronautics’s Electric Propulsion Technical Committee. “Thus, electric propulsion systems enable space missions that could never take place with chemical propulsion alone.”
Franklin Chang-Diaz, CEO of the Ad Astra Rocket Company and a former NASA astronaut, said despite decades of advances in space technology, deep challenges remain.
“Our transportation workhorse, the chemical rocket, has reached an exquisite level of refinement,” he said. “It has also reached its performance limit. That technology will not provide us with a sustainable path to deep space. It does not mean we need to discard it. On the contrary, chemical rockets will continue to provide foundational launch and landing capabilities for the foreseeable future and reducing their cost is a worthy goal.”
Chang-Diaz added that the path to sustainable transportation lies in high-power electric propulsion.
“By high-power, I mean power levels in the hundreds of kilowatts and up,” he said. “These rockets will first be solar-electric and later, as we move outwards from the sun, they will transition to nuclear-electric power.”
Ad Astra Rocket Company
The electric ion engine that currently propels the Dawn mission has a nominal operation power of 2.3 kW, and the new Boeing satellites operate at slightly less than 5 kW. Upgraded engines tested for ARM offer electric propulsion devices that could operate at nearly 15 kW. Aerojet Rocketdyne’s Nested Hall Thruster delivers 50-200 kW and the VASMIR VX-200 engine has performed more than 10,000 test firings at power levels of 200 kW.
But none of these engines have yet flown to space.
Cassady put things in perspective. “Today we can land one metric ton on the surface of Mars; for a human mission we need to land 80 metric tons of supplies and equipment,” he said. “Mars missions will also send humans much farther than ever before. This combination of heavier payloads combined with the need to travel over greater distances drives us to seek a solution that takes advantage of strategic logistics planning.”
He added that the best approach might be similar to the way that military deployments are conducted today, where heavy equipment, supplies, and other logistical items are pre-deployed by large cargo ships. Then, once the equipment and habitats are in place, soldiers follow by faster air transport. SEP systems, in other words, could become the cargo ship of deep-space missions.
Gerstenmaier said that NASA is also investing in technologies that will allow for the in-space storage and transfer of cryogenic fuels to meet the needs for future propulsion stages to move crew from Low Earth Orbit to a variety of destinations. “A key goal is to demonstrate these new capabilities in the next few years and infuse them into human missions in the next decade,” he said.
Several committee members and invited speakers echoed Chang-Diaz’s opinion that there is strong public sentiment for continued development for space exploration, and in particular a sustainable human mission to Mars.
“I believe space travel beckons humanity even more today than it did 50 years ago,” said Chang Diaz, “but we need to secure a safe, robust, and fast means of transportation.”
Cassady agreed, saying he thought that we are well on our way to having efficient in-space transportation because of SEP, but for the technology to fully reach its potential, we mustn’t get complacent or distracted.
“We must continue to adequately fund these development efforts to ensure we will have the first human footprints on Mars in the 2030s,” he said. “The primary challenge facing high power SEP development is the risk of losing focus as we go through the critical transition period from development to flight demonstration and subsequently, operational use. This requires a stable budget and a constancy of purpose. Everything we do should be with the goal of landing humans on Mars in the 2030s.”