Monday, August 18, 2014

Alternative propulsion concepts power debate

There are topics in the space field that inflame passionate debate like few others. Bring up SpaceX, for example, and expect to see a phalanx of fans of the space transportation company square off against a cadre of its critics. Discussion of NASA’s Space Launch System heavy-lift booster, for example, frequently ends up in arguments on whether the vehicle is an essential element of any deep space exploration plans, or a pork barrel project. Middle ground is hard to come by; it is, after all, in the crossfire.

Another topic that, perhaps more surprisingly, also generates polarized debate is work on alternative propulsion systems. These efforts, done on the fringes of the research community, seek to exploit aspects of physics that may seem counterintuitive or simply incomprehensive to the average person to permit radical advances in space transportation. When a new development in this area is announced, as was the case a few weeks ago, some people embrace it without reservation as a breakthrough that will open up the solar system, if not the galaxy, while others reject it as being clearly infeasible, if not in violation of the laws of physics. Reality, though, can be a little more complex.

EmDrives and q-thrusters



The latest example of this arose out of a conference presentation that few, apparently, even saw. Near the end of the AIAA Joint Propulsion Conference in Cleveland on July 30, a team from NASA’s Johnson Space Center (JSC) presented a paper titled “Anomalous Thrust Production from an RF Test Device Measured on a Low-Thrust Torsion Pendulum.” The paper was the last in a session on “Advanced Concepts” that was the last listed on the agenda of the three-day conference.

Although the paper itself is not widely available, the abstract was posted on the NASA Technical Report Server. In it, the JSC researchers discuss tests performed last August of a “quantum vacuum plasma thruster,” designed to generate thrust when excited by microwaves. “Approximately 30-50 micro-Newtons of thrust were recorded from an electric propulsion test article” during those tests, the abstract stated. “Test results indicate that the RF [radio frequency] resonant cavity thruster design, which is unique as an electric propulsion device, is producing a force that is not attributable to any classical electromagnetic phenomenon and therefore is potentially demonstrating an interaction with the quantum vacuum virtual plasma.”

The conference presentation itself may have gone unnoticed, but the abstract became the fodder for dozens of articles, starting the day after the conference with a piece in the UK edition of Wired: “Nasa validates ‘impossible’ space drive.” Its coverage—based primarily on the abstract—was that the test demonstrated that a concept developed by British researcher Roger Shawyer, called the EmDrive, actually works. Microwaves could, Shawyer argued, be converted into thrust without the need for propellant.

Other outlets soon followed up: after all, how can you resist something like an “impossible space drive”? Critics also weighed in, though. “Don’t buy stock in impossible space drives just yet,” cautioned Ars Technica, noting a number of issues with the test. Others were far more critical, saying the device violated the conservation of momentum. John Baez, a professor of physics at the University of California Riverside, dismissed the experiment, and the science behind it, as “baloney.” “There's no such thing as ‘virtual plasma,’” he noted in a Google+ post, referring to the language of the AIAA abstract. “If you want to report experimental results that seem to violate the known laws of physics, fine. But it doesn’t help your credibility to make up goofy pseudo-explanations.”

Others took aim at the experiment as described in the abstract. The test was performed in a vaccum chamber, but at ambient pressure: could the results seen be due to some kind of interaction between the microwaves and air, rather than thrust from the chamber? And why did a “null” test article included in the experiment also produce thrust?

Most of the reporting of the test didn’t go beyond the contents of the conference abstract and reactions, positive and negative, it produced. No one in the media appeared to cover the conference talk itself and few, if any, followed up with the JSC team that performed the experiment. Yet, in the midst of this debate, one of the team members talked publicly about the experiment.

At the 17th Annual International Mars Society Convention held in the Houston suburb of League City, Texas, on August 8, Harold “Sonny” White of JSC’s Eagleworks Laboratories gave a plenary talk on “Highly Advanced Space Propulsion Systems.” White is best known for his studies of warp drive concepts (see “Building a starship’s foundation”, The Space Review, September 24, 2012) that have also generated their share of publicity and criticism. And while much of his talk was about that work, he also devoted time to the experiment on what he called “q-thrusters.”

White talked about the setup of the experiments, including the use of two test articles provided by Cannae LLC, a Pennsylvania-based firm led by Guido Fetta, who has filed patents on q-thruster designs. “There was a conjecture that they had that there is some dependency on the ability of this to generate force as a function of these radial slots,” he said in his presentation. One of the test articles had slots, while the other—the “null” test article described in the conference abstract—did not.

Both test articles, though, generated thrust, although the experiment setup did not detect a thrust when the articles were replaced with an “RF dummy load” that served as the experiment’s control (and not the null test article, to the confusion of many readers of the conference abstract.) “Although we may have cast some doubt on the value of the radial slots,” White said, “there’s certainly some things of interest to us in the throat” of the thruster design.

White, while noting that the experiments generated thrusts in his experiments, stopped short of saying that they “validated” the drive in any way. In a brief interview after his talk, he said his team had additional experiments planned to see if what they measured was real or an effect of the experiment design. For example, they are in the process of replacing the amplifier used in the earlier tests with one that can operate at vacuum. “We want to run it at vacuum to help maybe eliminate some other sources of mimicry that might make you wring your hands,” he said.

He also said that, in addition to testing the thrust chamber in forward and reverse, future tests will also place it orthogonal to the torsion pendulum; in that scenario, the pendulum should record no thrust during the test, providing a better control on the experiment than the RF load previously used. “That way, we’re getting everything the same as the forward and reverse” tests, he said. Future tests also won’t use the slotted designs previous tested, he said, citing the lack of effect they had on the earlier results.

Should q-thrusters survive that experimental gantlet, the propulsion they offer many not be as revolutionary as some of the hype. White, in his talk, sees them as a replacement for other kinds of electric propulsion, like Hall thrusters, offering a somewhat higher thrust per unit power. “It’s definitely not something that does spacelift by any stretch. It can’t lift its own weight,” he said. “It is an in-space propulsion system.”

Debating VASIMRM



The Mars Society conference, later the same day, took on another alternative propulsion concept. The conference’s evening session promised a debate: “Is Electric Propulsion an Enabling Technology for Mars Exploration?” Arguing for the motion, according to the conference schedule, was former astronaut Franklin Chang-Diaz, who has been working on an electric propulsion technology called Variable Specific Impulse Magnetoplasma Rocket (VASIMR). His opponent was Mars Society president Robert Zubrin, a long-time critic of VASIMR.

But when the debate started, only one of the participants was present: Zubrin. “We are here, just a few miles from his house and his company, and he has elected not to come,” Zubrin said. “I told Franklin that if he did not come this time, he would be represented by an empty chair.” And, indeed, an empty chair with a nametag for Chang-Diaz sat next to Zubrin on the conference stage.

The lack of a debate opponent did not deter Zubrin from outlining his issues with electric propulsion, or VASIMR in particular, during the hour that had been reserved for the debate. Zubrin did not take issue with the VASIMR technology itself, which turns cold gas into a superheated plasma directed out of a magnetic nozzle, but with its power requirements. To achieve the short Earth-Mars travel times associated with VASIMR—as short as 39 days—would require nuclear power systems orders of magnitude better than ever flown in space.

“If you compare what is required—2,000 to 20,000 times as much power as what has actually been built and flown—this is pure nonsense,” Zubrin said. “It is a scandal that this kind of thing can be promoted to the level where it is repeated by the NASA administrator and no one in NASA counters this.”

Zubrin noted later in the presentation that he was not opposed to electric propulsion in general, stating that it can be useful for transporting cargo on slower trajectories to Mars. “I’m not saying that electric propulsion is a useless technology,” he said. “What I am saying is that this particular piece of hype is completely false. This technology is not for quick trips to Mars.”

Zubrin argued that there’s no need for quick trips to Mars: chemical propulsion systems in use today can send people to Mars in six months. That duration should be acceptable even when taking into account the hazards posed by extended exposure to microgravity and cosmic radiation.

Reached by phone last Friday, Chang-Diaz confirmed he didn’t respond to the invitation to participate in the debate, likening past criticism he’s received from Zubrin to slander. “I don’t really debate folks that resort to slander,” he said. Chang-Diaz said he’s invited Zubrin to visit the facilities of his company, Ad Astra Rocket Company (AARC) and see their efforts, but Zubrin has never accepted those invitations.

“It’s too bad, it really is,” Chang-Diaz continued. “I felt completely hurt by such commentary” from Zubrin.

Chang-Diaz is, unsurprisingly, not deterred by Zubrin’s criticism as his company continues to develop the VASIMR technology. On a visit earlier this year to AARC’s facilities in Webster, Texas, just a few minutes’ drive from JSC, he and other company officials discussed upgrades they were making to the VASIMR thruster and test facilities there to support the next phase in development of the system.

That next phase is a “steady-state” test of the thruster: a 100-hour run of the engine at 100 kilowatts. “That, hopefully, will probably be starting towards the end of this year,” he said Friday. That requires modifications to the vacuum chamber to handle a long-duration test of the engine.

Eventually, AARC hopes to test VASIMR in space. “The best place to do this is on the ISS,” he said during the visit earlier this year. He hopes to fly a VASIMR thruster on the station as soon as late 2016. If successfully demonstrated there, he foresees a number of applications for the thruster, from reboosting the ISS itself to use in space tugs to, yes, potentially sending humans to Mars. “We will not go very far in space with chemical rockets,” he said.

One thing is clear: debates about both the feasibility and utility of alternative propulsion systems won’t end any time soon. If only there was a way to harness all the heat those debates generate…

Source: http://www.thespacereview.com/article/2580/1