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The Myth of Low-Thrust Propulsion

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by Jeffrey F. Bell
Honolulu - Mar 30, 2004
The announcement of Preident Bush's new space initiative has unleashed a flurry of activity in the space engineering community. Many design groups are drawing up plans for new spacecraft and new technologies that could be used for future manned missions to Mars.

Unfortunately, from the rumors and news stories reaching my computer, it seems that many of these designers are wasting their time on ideas that are fundamentally unworkable and would only wreck Plan Bush in the unlikely event that they were actually adopted by NASA. Prominent among these unworkable notions is the idea of low-thrust Nuclear-Electric Propulsion.

Of course, NASA already has a development program for NEP called Prometheus. But despite what you have read in many news articles, this program is strictly limited to developing a 100kw NEP system for unmanned science missions in the outer solar system. This program predates the Space Initiative and was formerly included in the space science program.

Moving Prometheus into the new Code T funding wedge has not changed its goals at all. In fact the only major change associated with this shift has been an unexplained delay of 3-4 years in the scheduled launch of the first mission to use NEP, the Jupiter Icy Moons Orbiter.

Of course a 100kw reactor does not have enough power to drive a manned spacecraft of useful size. But what about a greatly enlarged NEP, say with a megawatt reactor? The rule of thumb is that large nuclear reactors are more efficient and lighter than small ones, because many parameters like the thickness of the rad-shield and pressure vessel remain constant with size or scale up only slowly. So we might expect a super-Prometheus to have a somewhat higher power/weight ratio than the 100kw version now in development.

The problem here is than any performance increase due to the benefits of scale will start from a ludicrously low base. JIMO will weigh about 20 tons upon injection into LEO, and its thrust is so low that it will take 2.5 years just to climb up out of the Earth's gravity well and pass the Moon! Even if this time were cut in half, it would be too slow for manned missions, and even for delivering supplies to the Moon on a regular basis.

One of the most frequently cited reasons for adopting nuclear rockets is to reduce flight times for the crews and ameliorate the effects of long exposure to radiation and microgravity. But nuclear-ion drive will actually increase flight times, at least to any place we will be sending people to in the near future.

And this severe operational drawback isn't compensated for by any major increase in performance or reduction in cost. The problem with NEP is not with the Nuclear part, but with the Electric part. Ion-drive, or any low-thrust engine, just isn't a very big improvement over chemical rockets for missions in the inner solar system, like a trip to Mars and back.

I learned this dismal fact some years ago when I found myself in charge of a team of bright engineers from a major aerospace corporation. Our job was to design an unmanned spacecraft which was intended to return a certain small object from a certain orbit around Mars.

At this time Solar-Electric Propulsion was just being proven out on the Deep Space 1 technology development mission and NASA had finally declared it acceptable for routine science missions proposals.

The engineers started out convinced that ion drive was the only way to fly. We did detailed modeling of how much weight the DS-1 propulsion system would save on a Mars trip. At first the results looked really good. But then reality started to creep into the picture.

First, it turns out that ion drive can't be used to escape from the Earth or to brake back into a low earth orbit. The spiral orbit involved subjects the spacecraft to a long soak in the Van Allen Belts, so all electronics and instruments must be protected from charged particle radiation. Since Mars has no radiation belts, the weight and expense of rad-hardening would be a dead loss in our mission profile.

So we decided to boost our spacecraft directly into an escape trajectory that would pass quickly through the Belts (The JIMO project is now considering the same change, although the danger of space debris seems to be the motivation in their case).

For the same reason we planned to return our precious object to Utah in a conventional entry capsule on a direct entry trajectory. The main spaceship would burn up instead of being reusable.

Even with these changes, ion-drive still seemed to be better for my Mars mission. The engineers had worked out an optimum trajectory and were designing a spacecraft when another dose of reality struck: The optimum trajectory for low-thrust propulsion happened to reach the Earth at a much higher velocity than the optimum chemical-rocket trajectory, and the RV would have to have a much heavier heat shield to insure its safe arrival at Dugway Proving Ground. When this extra weight was factored into the design, the advantage of ion drive shrank even more.

Furthermore, there is a little problem with ion drive that you don't read about in the NASA press releases: the vital high-voltage parts of the thrusters gradually sputter themselves away into space like the filaments in vacuum tubes.

Even for a Mars mission, we needed to carry duplicate (and possibly triplicate) engines to replace the original engines which had a strictly limited life. It is not clear how the Prometheus Project intends to overcome this problem for the 10-15 years that typical outer system missions will last.

Finally, when all the scales had fallen from our eyes, it became clear that the SEP mission would only save enough weight to push us down to the next lower-priced version of the Delta booster -- and the money saved on launch costs would mostly be spent on the extra complications of the ion engines and the huge solar panels. (I don't even want to think about the extra costs and complications of a nuclear reactor.) So I made a command decision to revert to chemical engines.

That mission died from other causes before it became a formal proposal. I know that many other design teams considered SEP during that early burst of enthusiasm, but few actually proposed it, and only one SEP science mission has actually been selected for flight. This is the DAWN asteroid mission.

The asteroid belt is a uniquely attractive place to use SEP because there are many interesting objects to visit and the sun is still just bright enough to make solar cells useful as the power source. In the inner solar system, the velocities and flight times involved are just too small for ion drive to show its merit, whatever power source is used.

So the vision of huge ion-drive spaceships majestically cruising from Earth to Mars is another of those neat ideas from the 1950s that turns out to be unworkable in the real Solar System that we started to discover with Explorer I.

Project Prometheus is just what NASA says it is: a way to send big science payloads to the outer planets and Kuiper Belt and have lots of electric power to run them when they get there. It is not a faster/better/cheaper to send either people or cargo to the Moon or Mars.

Jeffrey F. Bell is Adjunct Professor of Planetology at the University of Hawai'i at Manoa. All opinions expressed in this article are his own and not those of the University.

And now for some feedback....

Editor,

I don't know why your publication gives so much attention to Mr. Jeff Bell and his opinionated views. In every article he writes, he displays a flagrant lack of real knowledge that is apparent to anyone working on the subjects he describes. His latest - the subject article - proposes to debunk the "myth" of low thrust propulsion. As an engineer trained in both the physics of electric propulsion and low thrust orbit mechanics, I can state unequivocally that Mr. Bell's assertions grossly overstate the negatives and belie the real promise of either NEP or SEP. While it is true that EP is no panacea - and it may not have been appropriate for the particular mission he was proposing - his use of this example to condemn its application for broad classes of missions displays an ignorance of the subject matter that borders on irresponsible. A few facts illustrate my point.

Mr. Bell refers to Code E. (I suppose for "exploration")

FACT: Radm. Craig Steidle (ret.) is in charge of the new NASA Code T, which is where Project Prometheus and Project Constellation both reside. Numerous briefings have been given that make that very clear, although they must not have "reached his computer." Two words - "Google search."

Mr Bell states that SEP cannot be used to spiral up through the Van Allen belts due to radiation degradation of the solar arrays and electronics.

FACT: The ESA SMART-1 spacecraft is doing just that right now and has, in fact, completed its transit of the belts. In a private conversation with the lead engineer from SNECMA, I learned that the degradation was not as severe as the designers had planned for, and SMART-1 now has excess power margin for the remainder of the mission.

Mr. Bell asserts that ion engines have "vital high voltage parts" that sputter away and NASA isn't talking about that.

FACT: NASA currently has four ongoing programs working on just those problems, as well as scale-up to higher power levels. The parts are the "grids" used to accelerate the ion beam and their wear mechanisms are well understood and modeled. The joint AF-NASA Integrated High Performance Rocket Propulsion (IHPRPT) effort is also funding work on improved grids that aim to greatly increase lifetime. Preliminary results show excellent promise for the carbon-carbon or pyrolytic graphite materials, but another solution is simply thicker molybdenum grids. NASA talks plenty about it, if Mr. Bell would bother to do ANY research at all. Grid lifetime does not appear to be a "showstopper" for either future SEP or NEP missions.

Mr. Bell asserts that the SEP solution did not really save that much - and concludes the performance advantages are marginal.

FACT: This is based on one mission design for a sample return from Mars (I gather from what he says). Taking that design experience and extrapolating the outcome to a broad class of missions is downright medieval. You can't generalize based on that one case. I can show Mr. Bell numerous mission designs where EP (both SEP and NEP) not only DO make tremendous differences - but actually enable the missions. Those include missions inside the orbits of the asteroids including Mars and Mercury. If Mr. Bell would like I can provide the references.

Mr. Bell states that designers are "wasting their time on ideas that are fundamentally unworkable" and would "wreck Plan Bush."

FACT: Implicit in this blanket statement are two assumptions, both of which are flawed. One is that Mr. Bell apparently thinks the re-focused NASA vision is all about putting people on Mars. That is a common misperception fed by some media interpretations. If you listen to what the plan really says, it is about cooperative robotic and human exploration of the solar system. Mars is certainly one interesting target, but it is not the only goal of the vision. The second is that any EP design for an Earth-Mars mission is unworkable because Mr. Bell's analysis for his mission showed it to be so.

If you insist on giving this guy who has no credentials that I can see for talking about such subjects this much ink, I'd strongly suggest that you put it at least under the banner of an "op-ed." Everyone is entitled to their own opinion, regardless of how uninformed it may be. But don't present it to an unsuspecting public as gospel truth. You run the risk of alienation of the informed reader and of turning your otherwise fine space coverage into the internet equivalent of the National Enquirer.

Yours truly,
R. Joseph Cassady

The opinions expressed above are my own and do not represent my employer, NASA, or the Air Force.)

Related Links
Project Prometheus
Jupiter Icy Moons Orbiter
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 NASA Looks To Department Of Energy For Nuclear Space Tech
 Washington - Mar 22, 2004
The Department of Energy's (DOE) Naval Reactors (NR) Program has joined NASA in its effort to investigate and develop space nuclear power and propulsion technologies for civilian applications. These activities could enable unprecedented space exploration missions and scientific return unachievable with current technology.



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