Catalyzed Fusion: Tuning Up the Ramjet

by Paul Gilster on March 27, 2012

Long-time Centauri Dreams readers have learned to tolerate my eccentricities (or, at least, they’re kind enough not to dwell on them). One of them is my love of poking around in old books related to space travel, which is how Benjamin Field’s A Narrative of the Travels and Adventures of Paul Aermont Among the Planets (1873) recently caught my eye. I don’t know much about Field other than that he chose to produce this tale of interplanetary wanderings under a pseudonym, but what’s fun about his tale is that after his journeys to Jupiter, Saturn, Mars and Venus are over, Field’s protagonist returns to Earth to find that the planet is fully fifty years older, though he himself has aged hardly at all.

Time dilation, the reader might say, but of course Field wouldn’t have known anything about special relativity. It’s fun to consider, though, how an idea that in 1873 would have been simple fantasy — that someone might travel at high speed and age at a different rate than those he left behind — became in 1905 the logical outgrowth of a breakthrough theory and, by the 1960s, a publicly accepted concept. As we saw yesterday, the propagation of Robert Bussard’s interstellar ramjet ideas, first through scientific papers, then through accounts of popular science and eventually science fiction (think Poul Anderson, or Larry Niven), fueled the public imagination. We’d had relativistic effects in science fiction before (as far back as 1931’s Out Around Rigel, by Robert H. Wilson), but the ramjet gave the idea its vehicle.

Image: The Bussard ramjet ushered in an era of public enthusiasm for relativistic flight concepts, along with a host of unresolved issues. Credit: Adrian Mann.

I haven’t mentioned the name of British researcher Anthony Martin in this context yet, but Martin, a major player in the Project Daedalus work in the 1970s, examined Bussard’s ideas early in that decade and saw the possibilities of the ramscoop for deceleration. He was hardly alone in going to work on the many problems of the ramjet. Bussard spoke about fusing protons in his ramscoop engine, but subsequent analysis showed that the power needed to compress protons to fusion densities far outweighed the power that would be produced — Thomas Heppenheimer went on to demonstrate this in a 1978 paper. But three years before this, Daniel Whitmire had studied how a ramjet could draw its power through the much more energetic CNO bi-cycle (carbon-nitrogen-oxygen), a catalytic cycle that seems to be the dominant energy source in stars more than 1.3 times the mass of the Sun. Thus the ‘catalytic ramjet’ was born.

Adam Crowl describes the CNO bi-cycle this way in a recent post:

Basically a hydrogen fuses to a carbon-12, then another is fused to it to make nitrogen-14, then two more to make oxygen-16, which is then highly ‘excited’ and it spits out a helium nucleus (He-4) to return the nitrogen-14 back to carbon-12. Since the carbon-12 isn’t consumed it’s called a “catalytic” cycle, but it’s not chemical catalysis as we know it. Call it “nuclear chemistry”.

Whitmire was a friend and colleague of Al Jackson, a physicist (and these days a Centauri Dreams regular) who collaborated with Whitmire on still another ramjet idea we’ll be discussing soon, a ramjet/laser hybrid. Tomorrow I’ll be running a reminiscence by Al about Whitmire’s work on the catalytic ramjet, along with some thoughts from a letter Bussard himself wrote to Whitmire. For today, though, we focus on catalytic fusion, in which 12C or 20Ne (isotopes of carbon and neon respectively) are the fusion catalysts. Whitmire worked through reaction sequences for each and found that given sufficient temperatures, the reaction rates were far higher than for uncatalyzed proton/proton fusion (in fact, 1018 to 1019 times higher).

The problem with Bussard’s original idea is that the rate of proton-proton fusion is low. Consider that the Sun generates something less than 1 watt per cubic meter as averaged over its whole volume, considerably weaker than the energy production taking place in a light bulb filament. We get huge energy output because the Sun is vast, occupying a volume more than one million times that of Earth. The need for something more energetic aboard the relatively limited confines of the Bussard starship was clear. Either that or the ship would grow huge: Whitmire calculated that if a working proton-proton reactor were built around Bussard’s ideas, it would have to be on the order of 7000 kilometers across, making for one gigantic vehicle! As Whitmire wrote:

This problem was recognized in Bussard’s original work, but no viable alternative to the PPI [proton-proton] chain has yet been suggested. Here we show that the problem of the slow PPI rate can be resolved in principle by exploiting a proton burning catalytic cycle similar to the well known CNO BiCycle occurring in sufficiently hot main sequence stars. The catalyst “fuel” can be taken along since it is not depleted, but the ultimate source of energy is the interstellar hydrogen. The slowest links in the catalytic chains will be found to be 1018 – 1019 times faster than the PPI rate at an ion temperature of 86 keV and number density of 5 x 1019 cm-3.

We should note that Bussard was impressed with Whitmire’s work and saw catalytic fusion as the solution to the proton-proton problem, a point he makes in the letter I’ll be quoting tomorrow. Even so (and Gregory Matloff makes this point in The Starflight Handbook), these reactions are about a million times slower than the deuterium-deuterium reaction, so ramjet fusion remains more than a little problematic.

There are numerous issues with the Bussard ramjet, and Whitmire would go on to look at how interstellar ions could be collected through the combination of electric and magnetic fields, with the ramscoop’s efficiency enhanced by firing laser beams to ionize hydrogen atoms ahead of the vehicle (Matloff and A.J. Fennelly were working on forward-firing lasers for ramjets at more or less the same time). We’ve seen as well how Robert Zubrin and Dana Andrews analyzed the ramjet and found its scoop actually created more drag than thrust, but there are those who continue to study it, believing with Whitmire that “…it would be premature to discount the fusion ramjet as a potentially viable means of relativistic interstellar spaceflight, especially for technological civilizations within or sufficiently close to nebular regions of the galaxy.”

The paper is Whitmire, “Relativistic Spaceflight and the Catalytic Nuclear Ramjet,” Acta Astronautica 2 (1975), pp. 497-509.

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{ 18 comments }

GreenTom March 27, 2012 at 12:21

“Consider that the Sun generates something less than 1 watt per cubic meter as averaged over its whole volume, considerably weaker than the energy production taking place in a light bulb filament.” File that under understatement–going by the specs Wikipedia article, power through light bulb filament is something like 60GW/m^3. (60W light bulb filament is 580mm long x 0.046 mm diameter ~~ 1 mm^3).

I think the power per volume of the sun is actually lower than a compost heap or resting muscle.

Adam March 27, 2012 at 15:49

Hi All
Anyone wanting to read Whitmire’s paper, he made it available on Askmar.com. Here’s the link… Catalytic Nuclear Ramjet

David Given March 27, 2012 at 16:52

I’m disappointed to see that Gutenberg doesn’t have Benjamin Field’s work online. Does anyone have a link to an etext of it?

Interstellar Bill March 27, 2012 at 17:51

The first significant effects of time dilation occur at 70.7% c,
at which a light year is traversed in a year of ship time.

It’s hard to believe anybody would want to pay the huge price
of going beyond 50% c, which is already a hideous speed:
Each kilogram of ship mass has two Megatons of kinetic energy.
The payload will be cowering behind extensive shielding:
no romantic images of crew looking out forward windows,
when a 1-micron particle has a hand-grenade impact.

At 70.7% c, the kinetic energy is 2.4 times worse yet,
and thus shielding and acceleration/decel distances as well

It seems very unlikely that time dilation will ever be significant.

Ron S March 27, 2012 at 20:39

Then again we can be pedantic by noting that a light bulb does not produce power at all: it converts electrical power into heat and a small quantity of light. At least the Sun (indirectly) produces power from latent nuclear energy. So not quite a fair comparison.

ljk March 27, 2012 at 23:20

So did Benjamin Field explain why his explorer did not age while in space? Especially since he never left the Sol system.

torque_xtr March 28, 2012 at 3:37

If the shield is a thin plate mounted tens of kilometers in front of the ship, then a particle of cosmic dust will punch right through it, but then vaporize and disperse before it could reach the ship. Since the shield with only the same cross-section as the ship itself is needed, it will blot out only a small patch in the sky and not interfere with the sight-seeing… Half-meter thick charged particle stopping illuminators and a pack of extra shields still needed though.

Eniac March 28, 2012 at 5:52

Adam, Thanks for the link.

This is very interesting work. As I understand it, Whitmire addresses both the problem of reaction rates with the catalytic cycle and the drag problem with electrostatic fields. Essentially, the reactor volume is reduced from planet-sized to practical dimensions (~10 m) by the much higher reaction cross sections of the CNO cycle. In addition, by putting the reactor at a positive potential of hundreds of megavolts, the momentum lost to decelerating incoming protons will be recovered by acceleration of the exhaust, leading in principle to drag-free operation.

In my understanding that leaves the problem of bremsstrahlung inside the reactor, which is still formidable and quite fundamental.

One thing I do not understand is the need for ionization. I am under the impression (from Zubrin and Andrews, perhaps?) that neutral hydrogen would be ionized upon impinging on a magnetic field that is moving at high velocity. This would mean measures such as lasers or foils are not needed. Is this perhaps incorrect, or has it not been considered by Whitmire?

Whitmire also appears not to account for erosion of said (hopefully unnecessary) foil. If each incoming atom knocks out even a single foil atom, we might as well take fuel instead of foil and do away with the scoop entirely.

Paul Gilster March 28, 2012 at 9:28

David Given writes:

I’m disappointed to see that Gutenberg doesn’t have Benjamin Field’s work online. Does anyone have a link to an etext of it?

I’ve looked all over the place for a digital copy but find none. Sorry — I’m afraid A Narrative of the Travels and Adventures of Paul Aermont Among the Planets is still available only in used bookshops.

And as to ljk’s question on how Field handles the ‘time dilation,’ he doesn’t explain it — it’s solely used as a plot device so that the narrator can look at 50 years of progress and speculate about the future.

Greg March 28, 2012 at 10:01

Paul, I’ve been enjoying these past couple of weeks with the coverage you’ve had on interstellar travel, specially the Bussard ramjet, it takes me back to when I was kid.
In reference to the energy per volume of sun, I had heard it once said that the sun’s heat energy is less per volume than the heat generated if you had the entire surface populated by people standing shoulder to shoulder.

ljk March 28, 2012 at 10:24

Interstellar Bill said on March 27, 2012 at 17:51:

“It’s hard to believe anybody would want to pay the huge price
of going beyond 50% c, which is already a hideous speed:
Each kilogram of ship mass has two Megatons of kinetic energy.
The payload will be cowering behind extensive shielding:
no romantic images of crew looking out forward windows,
when a 1-micron particle has a hand-grenade impact.”

I sincerely hope I am wrong here, but I can see one big reason why someone might want to build and launch a relativistic starship: As a weapon of mass destruction. Just one such vessel going near the speed of light hitting Earth could sterilize just about every living thing on it from the energy of the impact alone. And unless you have a species with a kamikaze attitude, it would not be necessary to have a crew a board, so you could focus on getting the ship to its target in one piece, no need for life support or scientific exploration, other than maybe collecting data necessary for improving the deadly success of the mission.

Why would we or others want to do such a thing? Well, we might find out via SETI that there are some hostile neighbors in the galaxy and we do not want to wait until they were at our doorstep, or sending a similar weapon at Earth. Or maybe others with a longer and larger perspective on the longevity of their society see us as both a military threat and a competitor for galactic resources and real estate down the road and are not interested in sharing, along with having no moral issues about removing a species alien to them.

I hope these are just the barbaric, paranoid thoughts of a species that has yet to truly understand and appreciate what it means to be a civilized society on a celestial scale. Or maybe it is thinking unique to Earth life. However, while there may be some very exotic beings out there who evolved in ways and places quite different from us and our world, I also doubt that any being no matter how advanced they may be now suddenly appeared whole and sophisticated.

If they have the ability to move through space, did their first instruments capable of such abilities evolve from devices used as weapons of defense and offense such as with our rockets? And were their first sorties into the void inspired by the desire to expand their knowledge about the Universe, or as part of a competitive effort to best their provincial rivals, such as with Apollo?

There is a reason why NASA still gets $17 billion annually, while the Department of Defense gets about $700 billion each year. With human society at least, it is not just physics that has kept us from constructing our first true star vessel. Or even returning humans to the Moon.

I keep wondering when Klaatu and Gort will show up and tell us to knock it off or else.

ljk March 28, 2012 at 10:42

For those who are interested in reading some Nineteenth Century perspectives on their future, check out Looking Backward: 2000-1887 by Edward Bellamy, about a fellow who wakes up in a socialist Boston of the Year 2000. The full novel is online here:

http://xroads.virginia.edu/~hyper/bellamy/header.html

Jules Verne wrote a novel about life in Paris in 1960, which while written in 1863 did not see publication until the 1990s as the original publisher thought it was too pessimistic compared to Verne’s other works about science and technology. I offer it both for historical interest and in contrast to Bellamy’s much more optimistic view of a technological future.

http://en.wikipedia.org/wiki/Paris_in_the_Twentieth_Century

A. A. Jackson March 28, 2012 at 11:13

@Eniac
On the erosion problem, Dan does have a calculation at the end of the paper, it is just not elaborated. Bussard’s comments on the same are very implicit in his paper , tho I knew he was aware of it.
I think both of them were spent by the end of their papers , and just never revisited the problem again. Same goes for bremsstrahlung , they were aware of it, just had to move on to other things.

As far the ionization of neutral hydrogen by a fast moving magnetic field, man!, I never thought of that.
My E&M physics is too rusty.
The real expert around here on that is Greg Benford.
Greg?

John Herlosky March 28, 2012 at 18:40

Regarding the use of relativistic bombs; that issue is nicely illustrated by Charles Pelligrino’s book “Flying to Valhalla” and “The Killing Star”. His design of the ultralight relativistic Valkyrie starship is one that has been thoughtfully worked out and remains a viable design shoukld we decide to pursue antimatter propulsion.

Adam Crowl March 29, 2012 at 4:00

One problem with Charles Pellegrino’s concept is that pion rockets don’t make terribly effective use of the mass-energy in their fuel and their actual specific impulse is considerably lower than the pion-speed. Calculations of the specific impulse vary according to the magnetic nozzle assumed, but in energy terms alone, the top specific impulse is equivalent to ~0.58c even with perfect collimation.

Eniac March 29, 2012 at 8:01

Al,

A rough estimate of the electric field holding an atom together is 10^10 V/m, since ionization potential is around 1 eV and atomic dimensions around 0.1 nm. To ionize, the Lorentz force q v*B must be greater or equal than the electrostatic force qE, which gives the condition E ~ v*B. With the above estimate for E, and v ~ c, we get an ionizing field of about 12 Tesla at speeds near the speed of light.

This is an unrealistic field strength for a ramscoop, I think, and we are interested in velocities considerably less than c, besides. So, it seems, ionization by the magnetic field is not a shortcut that applies here, although it is pretty close. I’d be interested in seeing what Greg thinks about this, too.

I am pretty sure Zubrin and Andrews were counting on this for deceleration by magnetic sail, so there is certainly a need to clarify the issue. It could put a real kink into plans to use magnetic sails with the ISM, since the ISM is not substantially ionized in our vicinity, and most of everywhere, AFAIK.

Stellar wind, which is ionized, is too local in extent to do much deceleration from velocities near c.

Eniac March 29, 2012 at 8:09

Re interstellar missiles: There are substantial drawbacks to weapons that have to be launched decades before they strike and can only be directed at targets light years away. Much easier and “better” to turn the drive into an explosive charge, instead.

ljk March 29, 2012 at 9:24

John Herlosky said on March 28, 2012 at 18:40:

“Regarding the use of relativistic bombs; that issue is nicely illustrated by Charles Pelligrino’s book “Flying to Valhalla” and “The Killing Star”. His design of the ultralight relativistic Valkyrie starship is one that has been thoughtfully worked out and remains a viable design shoukld we decide to pursue antimatter propulsion.”

Winchell Chung’s excellent Web site discusses this ship and the topic here:

http://www.projectrho.com/rocket/aliens.php#id–The_Fermi_Paradox–The_Killing_Star

The starship ISV Venture Star shown at the beginning of the 2009 film Avatar was based on Pellegrino’s design. The details are here:

http://www.projectrho.com/rocket/realdesigns.php#id–Avatar_ISV_Venture_Star

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