The X3 Ion Engine Test Break Thrust Records

When it comes to the future of space exploration, a number of new technologies are being investigated. Foremost among these are new forms of propulsion that will be able to balance fuel-efficiency with power. Not only would engines that are capable of achieving a great deal of thrust using less fuel be cost-effective, they will be able to ferry astronauts to destinations like Mars and beyond in less time.

This is where engines like the X3 Hall-effect thruster comes into play. This thruster, which is being developed by NASA’s Glenn Research Center in conjunction with the US Air Force and the University of Michigan, is a scaled-up model of the kinds of thrusters used by the Dawn spacecraft. During a recent test, this thruster shattered the previous record for a Hall-effect thruster, achieving higher power and superior thrust.

Hall-effect thrusters have garnered favor with mission planners in recent years because of their extreme efficiency. They function by turning small amounts of propellant (usually inert gases like xenon) into charged plasma with electrical fields, which is then accelerated very quickly using a magnetic field. Compared to chemical rockets, they can achieve top speeds using a tiny fraction of their fuel.

Artist’s concept of Dawn mission using its blue ion engine to reach Ceres in the distance. Credit: NASA/JPL

However, a major challenge so far has been building a Hall-effect thruster that is capable of achieving high levels of thrust as well. While fuel efficient, conventional ion engines typically produce only a fraction of the thrust produced by rockets that rely on solid-chemical propellants. Hence why NASA has been developing the scaled-up model X3 thruster in conjunction with its partners.

The development of the thruster has been overseen by Alec Gallimore, a professor of aerospace engineering and the Robert J. Vlasic Dean of Engineering at the University of Michigan. As he indicated in a recent Michigan News press statement:

“Mars missions are just on the horizon, and we already know that Hall thrusters work well in space. They can be optimized either for carrying equipment with minimal energy and propellant over the course of a year or so, or for speed—carrying the crew to Mars much more quickly.”

In recent tests, the X3 shattered the previous thrust record set by a Hall thruster, achieving 5.4 newtons of force compared with the old record of 3.3 newtons. The X3 also more than doubled the operating current (250 amperes vs. 112 amperes) and ran at a slightly higher power than the previous record-holder (102 kilowatts vs. 98 kilowatts). This was encouraging news, since it means that the engine can offer faster acceleration, which means shorter travel times.

Scott Hall makes some final adjustments on the thruster before the test begins. Credit: NASA

The test was carried about by Scott Hall and Hani Kamhawi at the NASA Glenn Research Center in Cleveland. Whereas Hall is a doctoral student in aerospace engineering at U-M, Kamhawi is NASA Glenn research scientist who has been heavily involved in the development of the X3. In addition, Kamhawi is also Hall’s NASA mentor, as part of the NASA Space Technology Research Fellowship (NSTRF).

This test was the culmination of more than five years of research which sought to improve upon current Hall-effect designs. To conduct the test, the team relied on NASA Glenn’s vacuum chamber, which is currently the only chamber in the US that can handle the X3 thruster. This is due to the sheer amount of exhaust the thruster produces, which can result in ionized xenon drifting back into the plasma plume, thus skewing the test results.

NASA Glenn’s setup is the only one with a vacuum pump powerful enough to create the conditions necessary to keep the exhaust clean. Hall and Kamhawi also had to build a custom thrust stand to support the X3’s 227 kg (500 pound) frame and withstand the force it generates, since existing stands were not up to the task. After securing a test window, the team spent four weeks prepping the stand, the thruster, and setting up all the necessary connections.

All the while, NASA researchers, engineers and technicians were on hand to provide support. After 20 hours of pumping to achieve a space-like vacuum inside the chamber, Hall and Kamhawi conducted a series of tests where the engine would be fired for 12-hours straight. Over the course of 25 days, the team brought the X3 up to its record-breaking power, current and thrust levels.

A side shot of the X3 firing at 50 kilowatts. Credit: NASA

Looking ahead, the team plans to conduct more tests in Gallimore’s lab at U-M using an upgraded vacuum chamber. These upgrades will are schedules to be completed by January of 2018, and will enable the team to conduct future tests in-house. This upgrade was made possible thanks to a $1 million USD grant, contributed in part by the Air Force Office of Scientific Research, with additional support provided by the Jet Propulsion Laboratory and U-M.

The X3’s power supplies are also being developed by Aerojet Rocketdyne, the Sacramento-based rocket and missile propulsion manufacturer that is also the lead on the propulsion system grant from NASA. By Spring of 2018, the engine is expected to be integrated with these power systems; at which point, a series of 100-hour tests that will once again be conducted at the Glenn Research Center.

The X3 is one of three prototypes that NASA is investigating for future crewed missions to Mars, all of which are intended to reduce travel times and reduce the amount of fuel needed. Beyond making such missions more cost-effective, the reduced transit times are also intended to reduce the amount of radiation astronauts will be exposed to as they travel between Earth and Mars.

The project is funded through NASA’s Next Space Technologies for Exploration Partnership (Next-STEP), which supports not just propulsion systems but also habitat systems and in-space manufacturing.

Further Reading: Michigan News

NASA Reignites Program for Nuclear Thermal Rockets

In its pursuit of missions that will take us back to the Moon, to Mars, and beyond, NASA has been exploring a number of next-generation propulsion concepts. Whereas existing concepts have their advantages – chemical rockets have high energy density and ion engines are very fuel-efficient – our hopes for the future hinge on us finding alternatives that combine efficiency and power.

To this end, researchers at NASA’s Marshall Space Flight Center are once again looking to develop nuclear rockets. As part of NASA’s Game Changing Development Program, the Nuclear Thermal Propulsion (NTP) project would see the creation of high-efficiency spacecraft that would be capable of using less fuel to deliver heavy payloads to distant planets, and in a relatively short amount of time.

As Sonny Mitchell, the project of the NTP project at NASA’s Marshall Space Flight Center, said in a recent NASA press statement:

“As we push out into the solar system, nuclear propulsion may offer the only truly viable technology option to extend human reach to the surface of Mars and to worlds beyond. We’re excited to be working on technologies that could open up deep space for human exploration.”

Nuclear reactors (like the one pictured here) are being considered by NASA’s Marshall Space Flight Center for possible future missions. Credit: NASA

To see this through, NASA has entered into a partnership with BWX Technologies (BWXT), a Virginia-based energy and technology company that is a leading supplier of nuclear components and fuel to the U.S. government. To assist NASA in developing the necessary reactors that would support possible future crewed missions to Mars, the company’s subsidiary (BWXT Nuclear Energy, Inc.) was awarded a three-year contract worth $18.8 million.

During this three years in which they will be working with NASA, BWXT will provide the technical and programmatic data needed to implement NTP technology. This will consist of them manufacturing and testing prototype fuel elements and helping NASA to resolve any nuclear licensing and regulatory requirements. BWXT will also aid NASA planners in addressing the issues of feasibility and affordably with their NTP program.

As Rex D. Geveden, BWXT’s President and Chief Executive Officer, said of the agreement:

“BWXT is extremely pleased to be working with NASA on this exciting nuclear space program in support of the Mars mission. We are uniquely qualified to design, develop and manufacture the reactor and fuel for a nuclear-powered spacecraft. This is an opportune time to pivot our capabilities into the space market where we see long-term growth opportunities in nuclear propulsion and nuclear surface power.”

In an NTP rocket, uranium or deuterium reactions are used to heat liquid hydrogen inside a reactor, turning it into ionized hydrogen gas (plasma), which is then channeled through a rocket nozzle to generate thrust. A second possible method, known as Nuclear Electric Propulsion (NEC), involves the same basic reactor converted its heat and energy into electrical energy which then powers an electrical engine.

Artist’s concept of a Bimodal Nuclear Thermal Rocket in Low Earth Orbit. Credit: NASA

In both cases, the rocket relies on nuclear fission to generates propulsion rather than chemical propellants, which has been the mainstay of NASA and all other space agencies to date. Compared to this traditional form of propulsion, both types of nuclear engines offers a number of advantages. The first and most obvious is the virtually unlimited energy density it offers compared to rocket fuel.

This would cut the total amount of propellant needed, thus cutting launch weight and the cost of individual missions. A more powerful nuclear engine would mean reduced trip times. Already, NASA has estimated that an NTP system could make the voyage to Mars to four months instead of six, which would reduce the amount of radiation the astronauts would be exposed to in the course of their journey.

To be fair, the concept of using nuclear rockets to explore the Universe is not new. In fact, NASA has explored the possibility of nuclear propulsion extensively under the Space Nuclear Propulsion Office. In fact, between 1959 and 1972, the SNPO conducted 23 reactor tests at the Nuclear Rocket Development Station at AEC’s Nevada Test Site, in Jackass Flats, Nevada.

In 1963, the SNPO also created the Nuclear Engine for Rocket Vehicle Applications (NERVA) program to develop nuclear-thermal propulsion for long-range crewed mission to the Moon and interplanetary space. This led to the creation of the NRX/XE, a nuclear-thermal engine which the SNPO certified as having met the requirements for a crewed mission to Mars.

Artist’s concept of a bimodal nuclear rocket slowing down to establish orbit around Mars. Credit: NASA

The Soviet Union conducted similar studies during the 1960s, hoping to use them on the upper stages of  of their N-1 rocket. Despite these efforts, no nuclear rockets ever entered service, owing to a combination of budget cuts, loss of public interest, and a general winding down of the Space Race after the Apollo program was complete.

But given the current interest in space exploration, and ambitious mission proposed to Mars and beyond, it seems that nuclear rockets may finally see service. One popular idea that is being considered is a multistage rocket that would rely on both a nuclear engine and conventional thrusters – a concept known as a “bimodal spacecraft”. A major proponent of this idea is Dr. Michael G. Houts of the NASA Marshall Space Flight Center.

In 2014, Dr. Houts  conducted a presentation outlining how bimodal rockets (and other nuclear concepts) represented “game-changing technologies for space exploration”. As an example, he explained how the Space Launch System (SLS) – a key technology in NASA’s proposed crewed mission to Mars – could be equipped with chemical rocket in the lower stage and a nuclear-thermal engine on the upper stage.

In this setup, the nuclear engine would remain “cold” until the rocket had achieved orbit, at which point the upper stage would be deployed and the reactor would be activated to generate thrust. Other examples cited in the report include long-range satellites that could explore the Outer Solar System and Kuiper Belt and fast, efficient transportation for manned missions throughout the Solar System.

The company’s new contract is expected to run through Sept. 30th, 2019. At that time, the Nuclear Thermal Propulsion project will determine the feasibility of using low-enriched uranium fuel. After that, the project then will spend a year testing and refining its ability to manufacture the necessary fuel elements. If all goes well, we can expect that NASA’s “Journey to Mars” might just incorporate some nuclear engines!

Further Reading: NASA, BWXT News

Cancer Risk for a Human Mars Mission Just Got a Lot Worse

Astronauts hoping to take part in a crewed mission to Mars might want to pack some additional rad tablets! Long before NASA announced their proposal for a “Journey to Mars“, which envisions putting boots on the Red Planet by the 2030s, mission planners have been aware that one of the greatest risks for such a mission has to do with the threat posed by cosmic and solar radiation.

But according to a new study from the University of Nevada, Las Vegas, this threat is even worse than previously thought. Using a predictive model, this study indicates that astronauts that are the surface of Mars for extended periods of time could experience cell damage from cosmic rays, and that this damage will extend to other healthy cells – effectively doubling the risk of cancer!

The study, which was led by UNLV scientist Dr. Francis Cucinotta, was published in the May issue of Scientific Reports – under the title of Non-Targeted Effects Models Predict Significantly Higher Mars Mission Cancer Risk than Targeted Effects Models“. Building on conventional models that predict that DNA damage caused by radiation leads to cancer, their model looked at how such damage could spread throughout the body.

At one time, Mars had a magnetic field similar to Earth, which prevented its atmosphere from being stripped away. Credit: NASA

Galactic cosmic rays (GCRs) are one of the greatest hazards posed by space exploration. These particles, which originate from beyond our Solar System, are basically atomic nuclei that have been stripped of their surrounding electrons, thanks to their high-speed journey through space. In the cases of iron and titanium atoms, these have been known to cause heavy damage to cells because of their very high rates of ionization.

Here on Earth, we are protected from these rays and other sources of radiation thanks to our protective magnetosphere. But with missions that would take astronauts well beyond Earth, they become a much greater threat. And given the long-term nature of a mission to Mars, mitigation procedures and shielding are being investigated quite thoroughly. As Cucinotta explained in a UNLV press statement:

“Exploring Mars will require missions of 900 days or longer and includes more than one year in deep space where exposures to all energies of galactic cosmic ray heavy ions are unavoidable. Current levels of radiation shielding would, at best, modestly decrease the exposure risks.”

Previous studies have indicated that the effects of prolonged exposure to cosmic rays include cancer, central nervous system effects, cataracts, circulatory diseases and acute radiation syndromes. However, until now, the damage these rays cause was thought to be confined to those cells that they actually traverse – which was based on models that deal with the targeted effects of radiation. 

Artist’s impression of astronauts exploring the surface of Mars. Credit: NASA/JSC/Pat Rawlings, SAIC

For the sake of their study, Dr. Cucinotta and Dr. Eliedonna Cacao (a Chemical Engineer at UNLV) consulted the mouse Harderian gland tumor experiment. This is the only extensive data-set to date that deals with the non-targeted effects (NTEs) of radiation for a variety of particles. Using this model, they tracked the effects of chronic exposure to GCRs, and determined that the risks would be twice as high as those predicted by targeted effects models.

“Galactic cosmic ray exposure can devastate a cell’s nucleus and cause mutations that can result in cancers,” Cucinotta explained. “We learned the damaged cells send signals to the surrounding, unaffected cells and likely modify the tissues’ microenvironments. Those signals seem to inspire the healthy cells to mutate, thereby causing additional tumors or cancers.”

Naturally, any indication that there could be an elevated risk calls for additional research. As Cucinotta and Cacao indicated in their study, “The scarcity of data with animal models for tissues that dominate human radiation cancer risk, including lung, colon, breast, liver, and stomach, suggest that studies of NTEs in other tissues are urgently needed prior to long-term space missions outside the protection of the Earth’s geomagnetic sphere.”

These studies will of course need to happen before any long-term space missions are mounted beyond Earth’s magnetosphere. In addition, the findings also raise undeniable ethical issues, such as whether or not these risks could (or should) be waived by space agencies and astronauts. If in fact we cannot mitigate or protect against the hazards associated with long-term missions, is it even right to ask or allow astronauts to take part in them?

In the meantime, NASA may want to have another look at the mission components for the Journey to Mars, and maybe contemplate adding an additional layer or two of lead shielding. Better to be prepared for the worst, right?

Further Reading: UNLV, Nature