Al Jackson: A Laser Ramjet Reminiscence

by Paul Gilster on July 26, 2012

by A. A. Jackson

It’s always good when you can go to the source, which I am delighted to do with this reminiscence by Al Jackson, whose laser-powered ramjet (and laser-powered interstellar rocket) ideas we’ve been looking at for the past few days. Al recalls discovering the work of Eugen Sänger back around 1960 and beginning the study of interstellar propulsion ideas, a passion he continues to this day. His 1975 doctorate from the University of Texas at Austin followed his work for NASA during the heyday of Apollo as astronaut trainer on the Lunar Module Simulator. Here he thinks back to the genesis of his interest in astronautics and reflects on the factors that led to his combining the Bussard ramjet concept with the developing idea of beamed energy.

I don’t think I had ever planned to write a technical paper about interstellar flight. However, as an SF reader I had been entranced by the idea since the early 1950′s. I was already a space cadet! When I got to university in 1959 and started with math and physics I became obsessed with astrodynamics even though I have never taken a course in it. I have a huge collection of celestial mechanics and astrodynamics books, and made my living off that branch of astronautics for nearly 45 years.

My curiosity about interstellar flight (IF) led to haunting libraries for technical works. I had read Les Shepherd’s work on IF and was entranced and it taught me one needed to learn Special Relativity. So I started collecting IF papers. The first thing I learned was the mass-ratio problem, which is why I was thunderstruck by Bussard’s ramjet paper. Lucky for me to work in a library in 1960 at a time when they were throwing out donated journals … Astronautica Acta being one of them! I still have my original hard copy of the issue with Bussard’s paper in it.

It was not hard to collect applied physics IF papers, just not many for a long time. I even joined the British Interplanetary Society to make sure I got the red cover issues [devoted to interstellar flight]. When I was working in the Apollo program two papers really caught my eye, both in Nature, one by G. Marx and the other by J.L. Redding, 1966 and 1967, about a laser pushed IF spacecraft (I don’t think Forward touched on this idea for quite some while later). Papers just accumulated.

I got more and more interested in General Relativity, the 70′s being the great era of applied GR. So I spent 5 years in Austin, in the Relativity Center at UT, more concerned with black hole physics and gravitational radiation. As I have mentioned before, I met Dan Whitmire there. We talked about a lot of things including my interest in IF, which caught his imagination. Soon we were talking and talking about it and I gave him copies of many of my papers. He too was struck by Bussard’s idea , which lead to the catalytic ramjet [See Tuning Up the Interstellar Ramjet].

I puzzled and puzzled over how to get rid of the fusion reactor and the idea struck me in about 1974 to combine a laser sail with a ramjet, but I was writing my dissertation at the time (on gravitational radiation) and didn’t have time for it. After getting my PhD in 1975 I went back into the space program (getting an academic job in physics by the early 70′s proved to be too long a process, which is why I took this course), and that is why the laser-powered ramjet paper has a disclaimer saying my work on it was not connected with my duties at the McDonnell Douglas Corporation.

By this point I knew the Special Relativity, had the concept, and so hobby-worked all the calculations on the paper, after which I sent it to Dan. He suggested a paper. I was very busy but sent him an outline from which he wrote the paper with some improvements and organization and some new ideas (note the ‘runway’ idea, did we originate that?)

[Editor’s note: Let me quote from the Jackson/Whitmire paper on the fusion runway concept, which I haven’t described in the two earlier posts:

Another possibility would be to artificially make a fusion ramjet runway. Micron-size frozen deuterium pellets could be accelerated electrostatically or electromagnetically beginning several years prior to take-off at which time a fusion ramjet with a relatively modest scoop cross section (perhaps a physical structure) would begin acceleration.

I always associate the fusion runway concept with Jordin Kare and his ‘Bussard Buzz Bomb,’ but I’m pretty sure Al’s work pre-dates Kare — PG].

My real focus was on the efficiency problem… Looking back on the paper now I can see we really should have emphasized the technicalities. We should have emphasized more that several different non-fusion systems could be used as ‘engines’ to accelerate mass. The laser beam set-up could have been much the same as what Forward studied a few years later. I think the generic scoop physics problems are there too, though I have a hunch that there are solutions to the Fishback-Heppenheimer-Zubrin ramscoop problems.

And I still regret the blasted primitive plotter I had at work in 1976 when the paper was being written….

Thanks Paul for bringing up this concept again. Over the many years now Dan and I have been somewhat disappointed at its submergence. Thankfully Eugene Mallove and Gregory Matloff did give us a nice treatment in The Starflight Handbook.

Re the posts here on Centauri Dreams, I agree with the comments by Eniac about technical problems of the collector and actually the whole technology of this concept. Yet this is true of all large scale relativistic interstellar vehicles as Esnault-Pelterie, Shepherd and Sänger noted so long ago. The importance is in showing that the physics allows an opening for the engineering physics. There is no exotic physics here only, so to speak, exotic technology.

For a long time no real details were known about the Interstellar Medium. It was not until the 90’s with astrophysical measurements and direct data such as from the Ulysses spacecraft that it became evident that the local ISM is really pretty slim and the distribution of hydrogen in the rest of the Galaxy varies greatly with no definitive global estimate. Any ramjet technology will face this problem.

(PS: That picture of me in the Lunar Module Simulator was only the third time I ever wore it. We had a simulation of the environmental control system in the simulator and the ECS guy was looking for short guys. Most of the astronauts, during Apollo, were under 6 ft. So that’s astronaut Dick Gordon’s practice suit I am wearing, no one touches the mission suits. I have a picture with helmet on, the system worked fine. Alas the crews when training in Houston hated sim training suited. There was an identical set at the Cape, in those days, and they ran suited there, but almost never in Houston. My only real touch with ECS; my subsystems were Guidance, Navigation and Control).

The two papers Al mentions above are Marx, “Interstellar Vehicle Propelled by Terrestrial Laser Beam,” Nature 211 (1966), pp. 22-23, and Redding (with the same title), “Interstellar Vehicle Propelled by Terrestrial Laser Beam,” Nature 212 (1967), pp. 588-589.

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Gerry July 26, 2012 at 12:18

Thanks for this interesting account. I really enjoy this site. I was wondering if this kind of beamed-power technology could be developed at first on a more modest scale for use in interplanetary travel. Obviously a ram-scoop would not be appropriate within the solar system, but otherwise the various implementations of laser-beamed power that have been discussed this week could be quite useful, especially for larger payload manned missions. Has there been study on the physics and engineering challenges at that scale?

Christopher Doll July 26, 2012 at 13:35

As an aside, these articles remind me of the time I got to meet Robert L. Forward a year before he passed away at a conference in Bellingham, Washington. He was giving talks on tethers and laser-powered light sail technologies, but it was a small enough conference where I was able to chat with him over lunch with a few other attendees. His excitement and charm were contagious, and it was exciting being in the company of people who not only enjoy these subjects, but who’ve contributed to the discussions. The first missions to the stars should be named after the pioneers in this field, and I hold out hope that I’ll live to see one of these efforts begin.

Paul Gilster July 26, 2012 at 15:31

Gerry asks about beamed power technologies for use in the Solar System. Jim and Greg Benford have worked for a long time on beamed power to sails (not ramjets), with interplanetary applications. See for example this story:

http://www.centauri-dreams.org/?p=1176

James Benford July 26, 2012 at 16:42

I think first mention of beam-driven propulsion for interstellar purposes in the refereed literature was by Bob Forward, in Missiles and Rockets in 1962. There r he just touched on it, came back with his classic papers on laser & microwave-driven sails 20 years later. That inspired me to think of how to demo it in the lab, which we did in 2000.

Perhaps more people read the Marx paper in Nature. He considered only x-rays to drive the sail, because he flinched from large apertures for the antenna and sail. (Using such energetic photons is a doubtful matter; X-rays reflect only at grazing angles.) Marx didn’t pursue his idea further, at least not in the literature.

Paul Gilster July 26, 2012 at 20:37

More on Forward’s Missiles and Rockets paper soon — it’s been on my list for a long time now.

Eniac July 26, 2012 at 22:30

I always read Al’s contributions here with great pleasure, they strike me as a uniquely powerful combination of sharp reason with seasoned experience. Just as much, I appreciate his (and all of our) intrepid ventures into the far-out land of the physics and engineering of interstellar flight. When I am sceptical, it is because I believe that in finding the right path, more time is often spent avoiding (or back-tracking from) wrong ones than plotting the right one. I hope it is understood that by pointing to adversity, I in no way mean to diminish the work we discussed, which I think is quite brilliant. As is this entire site, by the way.

Thanks Al, and thanks Paul.

A. A. Jackson July 27, 2012 at 7:19

Man, Jim!
I wish I had kept my copies of Missiles and Rockets for the 50′s and 60′s , it was a gift subscription from my parents in my late teens and early 20′s.
I am not surprised Bob Forward first proposed beamed power.
I think Bob was the only person to ask for off prints of our BIS papers!
(Back in those days no e-prints to be had!)
That lead to a long time correspondence with Bob. I can’t say it was a lot of letters. But I did get a bunch of technical publications from him when was revved up about interstellar flight. Most valuable was his single handed bibliography about technical interstellar flight, which others updated later… but , and someone can tell me this, has that ‘Forward’ interstellar bibliography been updated recently? My files don’t seem to contain anything from the 2000′s.
Met Bob about 3 or 4 times. He gave a classified seminar at the Johnson Space Center when he was working antimatter for the DOD , not sure all his technical work on that has been unclassified by now.

James Benford July 27, 2012 at 13:17

Al: Bob Forward describes it thus, in ‘ his autobiography ‘Fast Forward Fifty Years’:
“The first publication I did of the idea was in a popular magazine – “Missiles and Rockets”-April 1962. The article, “Pluto – the Gateway to the Stars”, was reprinted in “Science Digest” in 1962, and that established my claim to having invented the first interstellar vehicle that used known technology.”
The full reference is
“Pluto-Gateway to the Stars,” Missiles and Rockets 10,26 ff. (2 April1962); reprinted Science Digest 52, 70-75 (August 1962).

Gregory Benford July 27, 2012 at 13:22

Al, I recall you, Jim and I discussing star flight when we were both in high school in Dallas. We thought starships would be rockets, hadn’t envisioned lasers and sails.
You and we met in Dallas in 1962, when Jim & I were working summers at Texas Instruments, doing transistor design (drawing by hand the potential surfaces and current flow lines for odd shapes–hopelessly old school, now). I fuzzily recall we did know of Forward’s Science Digest article, published that year. I met Bob in mid-60s, courtesy Jerry Pournelle, and Bob was thinking onward from those ideas already. Great guy! Tragically struck down by a brain tumor in the same year a tumor killed Charles Sheffield.

Bob Andrews July 27, 2012 at 13:34

Another brilliant post Paul and much appreciated by your devotees over the ‘pond’. ‘Our ‘JANUS’ endevours over many years, concentrating on communications, can be accessed in a link from my site. (A new site is under construction) more details anon. Best wishes from us all.

Nathan August 5, 2012 at 20:20

The runway ramjet sounds the most promising. I’m envisaging this pencil thin air bridge extending out of our solar system,

Can anyone tell me some details about it. The deuterium is frozen, how long is it meant to stay frozen, presumably you dont want your spaceship to hit when it is solid, but nor do you wont it to completely diffuse to the background density of interstellar space.

Another question about ramjets in general: how much is the interstellar mass slowed down before fusion? Can one collect the hydrogen for use at the target system?

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