The Enzmann Solution

by Paul Gilster on May 16, 2013

Yesterday I remarked on how many more tools for exoplanet discovery we have today than were available to Harry Stine when he wrote “A Program for Star Flight” in 1973. That same day came the disheartening news that the Kepler mission has been stopped in its tracks by an equipment malfunction. But take heart — a vast amount of data already gathered by Kepler remains to be studied, meaning we’ll be getting Kepler discoveries for some time to come. The Kepler news also sharpens our focus on TESS (Transiting Exoplanet Survey Satellite), which will build our catalog of nearby stars hosting exoplanets, with launch now scheduled for 2017.

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For more on Kepler, see Dennis Overbye’s Breakdown Imperils NASA’s Hunt for Other Earths. But back to Stine, who in 1973 was hunting not only for target exoplanets but also for a propulsion system that would get a human crew to them. He was evidently familiar with Eugen Sänger’s papers on photon rockets, in which the German designer proposed deflecting the gamma rays produced by the annihilation of matter with antimatter to produce thrust. But Sänger’s ideas depended on tuning the gamma ray photons into a directed beam, something that no one could figure out how to do. Stine pondered the idea but rejected it.

Image: Author and rocketeer G. Harry Stine. Credit: New Mexico Museum of Space History.

And although George Marx and Robert Forward had already been examining pushing a large sail with a laser (Forward’s work went back to the early 1960s), Stine seems unaware of it. In any case, he was a rocket man, and it was perhaps inevitable that it would be Project Orion that drew his attention. Nuclear pulse propulsion would detonate a fission bomb behind the vehicle to drive it forward, using enormous shock absorbers to cushion the craft. Theoretical work led by Ted Taylor showed that the principle was sound, and simple tests using chemical explosives were conducted near San Diego, but the Nuclear Test Ban Treaty ended the project in 1963.

I mentioned yesterday that Freeman Dyson, a major player in the Orion research, would go on to publish a 1968 paper that took Orion to the next level, using thermonuclear devices to drive the spacecraft. Dyson’s ultimate craft was capable of speeds of 10,000 kilometers per second, enabling a mission to Alpha Centauri with deceleration at the destination in 130 years. I imagine it was Dyson’s starship that fired the imagination of Robert Duncan-Enzmann, then at Raytheon Corporation, leading to a modified and extended Orion that Stine would use in his article.

Adam Crowl, working with Kelvin Long and Richard Obousy, has produced an excellent overview of the Enzmann design that appeared in the Journal of the British Interplanetary Society last year (reference below). What I’m doing here is looking at Stine’s use of Enzmann as reflected in his Analog article, in which he foresees fleets of Enzmann starships dispatched in a regular pattern of interstellar exploration. The Enzmann vessel is distinctive, as the illustration below shows, a long cylinder capped by a 1000-foot sphere made up of 12 million tons of frozen deuterium, the fuel for its eight Orion-style propulsion modules.

EnzmannHardy

Image: The Enzmann starship as envisioned by the space artist David Hardy. This painting was commissioned by Kelvin Long in 2011 to depict a scene Hardy had first painted in the 1970s.

This was one big vessel, a cylinder 300 feet in diameter and 1000 feet long. Stine points out that a Saturn V without the Apollo escape tower would lie sideways inside this cylinder, which contains nearly a half-million cubic feet of living space. The Empire State Building, New York’s iconic symbol, would fit lengthwise easily enough with just its top tower sticking out the end. But the Enzmann vessel wasn’t, in many ways, a single ship. Stine explains:

The cylindrical portion is made up of three identical cylindrical modules docked end to end. Each module is completely self-sufficient with its own auxiliary nuclear power plant, a closed ecological life support system, living quarters, communication equipment, repair shops, storage holds, and EVA landing craft.

Each drum-like module is built upon a central core 50 feet in diameter and 300 feet long. Covering this backbone are eight decks of sub-modules each measuring 10 feet by 10 feet by 23 feet. These sub-modules are used as living quarters, storerooms, laboratories, and recreational areas by the human crew. Each of the drum-like modules has 700 of the smaller sub-modules.

And so on. The Enzmann starship, during the long coasting part of its journey, would be spun-up around its longitudinal axis to provide artificial gravity for the people on board. Enzmann thought a crew of 200 would be about right, with plenty of room for growth to an optimum population of 2000, a figure that balances against the closed-cycle ecology of the ship. Preserving the balance of population around this figure leads to what Stine calls ‘fascinating problems in applied social engineering.’ Indeed. This is, in fact, perhaps the biggest unknown variable of the mission.

Because Stine thought in terms of ships traveling together, his ultimate expedition would be about the size of a small city of 20,000 or so dispersed through ten starships. Modules and sub-modules could be disassembled during cruise if necessary and attached to another ship, with all parts designed to be interchangeable. Each ‘star fleet’ would launch what Stine called ‘metaprobes’ to move ahead of the main body for advanced reconnoitering of the target.

I mentioned at the outset of this series on 1970s starship projects that among some designers, at least, it was a time of immense optimism. We saw that in Bob Forward’s aggressively ambitious plan for exploration as presented to a subcommittee of the U.S. House. We also see it in spades in Stine’s thinking, making this theme a good note on which to close. Stine believed interstellar travel was possible through the laws of known physics and that it would not involve one-way trips but continued waves of exploration with frequent return to Earth. He goes on:

For expeditions out to about eight light-years, the original crew has a very good chance of returning; with advances in geriatrics and longevity research, we may have a synergistic relationship here that would make star flight out to quite respectable distances something that could be accomplished within a single lifetime. Naturally, some people aren’t ever going to come back to Earth again, but things like that seldom stop motivated people. My own ancestors never saw their native Germany again after making a short 3,000-mile sea voyage a couple of hundred years ago; but I’ve been back several times. In fact, our intrepid interstellar explorers stand a much better chance of getting there and getting back than many terrestrial explorers up to and including Twentieth Century men.

What an era. To Stine, the 1970s in relation to starflight looked like a time that paralleled the 1930s, when the first experiments in rocketry were producing results and we were learning how to reach into the stratosphere. He thought starflight was a mere 40 or so years away, a sentiment that seems all too naïve today given the amounts of energy that would need to be produced, but one that by its fierce commitment to the future can still be inspiring. We will do well to try to keep Stine’s enthusiasm alive even as we tackle the vast propulsion challenges that confront us.

For more on the designs of Robert Duncan-Enzmann, see Crowl, Long and Obousy, “The Enzmann Starship: History & Engineering Appraisal,” JBIS Vol. 65, No. 6 (June, 2012).

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James D. Stilwell May 16, 2013 at 11:43

Reading about the Enzmann starship I have but one comment…Eureka!
The only problem holding us back now ironically is the ‘fascinating applied social engineering problems on Earth’. I’m on it….the problems can be resolved….a little uncomfortable dislocation for several billion people may result….but it can be done….hold on to your chairs….don’t be late for the revolution….I want that starship, not in my next life, but in this one….

Danangel May 16, 2013 at 11:57

Very much enjoyed “Star Trek: Secrets of the Universe” last night. Quite an impressive line up of guests, yourself included. I am presently rereading Caleb Scharf’s book, ‘Gravities Engines’ and enjoyed hearing from him too.
Thanks for the heads up on the airing!

Thomas Mazanec May 16, 2013 at 12:54

I am hurt by the Kepler news too. But IIRC, the 2,xxx planet “candidates” cannot be confirmed by Kepler…it can’t distinguish those from an in-line eclipsing binary. We have to look with ground telescopes for confirmation. That will give us a long stream of Kepler exoplanets to come. I just miss the smaller, longer period, more Earth-like ones the full mission could have found.

Alex Tolley May 16, 2013 at 13:03

I found this link to Winterberg’s ideas and calculations for a D-D fusion pulse engine for interplanetary (interstellar?) travel.
http://arxiv.org/ftp/arxiv/papers/0906/0906.0740.pdf

Alex Tolley May 16, 2013 at 13:32

Working off the Enzmann specs as per kelvin Long, the starship would reach Alpha Centauri in 60 years, traveling most of the way at 0.09c. If that was really possible, that seems to me to be starting to get into the realm of the possible, rather than fantastic. A 60 year flight, in 1 g. might be survivable. A few tons of food and O2 per crew member/year might be an easier way to go than full recycling, if food can be made shelf stable for a century or more.

Question. Is this down scalable to probe size? Instead of a manned 30 kton ship, we use a 30 ton probe, with far less fuel and mass. Or is the scale of the ship a requirement of the propulsion technology? 60 years plus data transmission time at the target might be within the possible horizon of a science/exploration mission for some societies.

What I like about fusion engines is that refueling is relatively easy – mine comets/icy bodies and extract the deuterium, allowing further flights, and/or return to earth.

Another question – if the deuterium is frozen, dies it even need to be enclosed in a “tank”. Would a lightweight mesh do? I assume the deuterium acts as a shield. Would it also not be better to have the crew compartment contained within the tank to act as a cosmic radiation shield, or is the crew compartment mass sufficient for that already?

ljk May 16, 2013 at 14:44

15 May 2013

** Contacts are listed below. **

Text & Image:

http://news.stanford.edu/pr/2013/pr-kepler-hubbard-qanda-051513.html

STANFORD PROFESSOR AND FORMER NASA OFFICIAL
EXPLAINS HOW NASA MIGHT REVIVE THE KEPLER SPACE TELESCOPE

** Synopsis: Scott Hubbard, a consulting professor of aeronautics and astronautics, helped guide the Kepler mission when he served as director of NASA Ames Research Center. He explains how NASA might bring the planet-hunting spacecraft back online. **

NASA officials announced Wednesday, May 15, that the Kepler space telescope — the agency’s primary instrument for detecting planets beyond our solar system — had suffered a critical failure and could soon be shut down permanently.

Scott Hubbard, a consulting professor of aeronautics and astronautics at Stanford’s School of Engineering, served as director of NASA Ames Research Center during much of the building phase of the Kepler space telescope. He also worked on the project alongside William Borucki, the Kepler science principal investigator at Ames and the driving force behind the effort, for the decades leading up to formal approval of the mission.

The Kepler spacecraft’s photo-detector array registers more than 100,000 stars at a time, Hubbard said, and in order to detect exoplanets (planets orbiting stars outside our solar system), the telescope must remain extremely steady so that the stars do not wander across the optics. A series of four gyroscope-like reaction wheels whir within the telescope to hold its gaze. At least three must be functioning to keep Kepler stable. One failed about a year ago and was shut off, and NASA scientists announced Wednesday, May 15, that a second wheel was no longer operating and that Kepler had paused operations.

In a conversation with Stanford News Service, Hubbard explained the possible ways that NASA could bring the spacecraft back online, and what planet hunters will do next if that’s not possible.

Q: How big of a loss will it be if the Kepler space telescope can’t be repaired?

A: The science returns of the Kepler mission have been staggering and have changed our view of the universe, in that we now think there are planets just about everywhere.

It will be very sad if it can’t go on any longer, but the taxpayers did get their money’s worth. Kepler has, so far, detected more than 2,700 candidate exoplanets orbiting distant stars, including many Earth-size planets that are within their star’s habitable zone, where water could exist in liquid form.

Kepler has done what the program managers said it would do, and that is to give us an inventory of extrasolar planets. It completed its primary observation phase, and had entered its extended science phase. We’re already in the gravy train period — there’s still a year and a half’s worth of data in the pipeline that scientists will analyze to identify other candidate planets, and there will continue to be Kepler science discoveries for quite some time.

Q: How might NASA engineers go about getting Kepler functional again?

A: There are two possible ways to salvage the spacecraft that I’m aware of. One is that they could try turning back on the reaction wheel that they shut off a year ago. It was putting metal on metal, and the friction was interfering with its operation, so you could see if the lubricant that is in there, having sat quietly, has redistributed itself, and maybe it will work.

The other scheme, and this has never been tried, involves using thrusters and the solar pressure exerted on the solar panels to try and act as a third reaction wheel and provide additional pointing stability. I haven’t investigated it, but my impression is that it would require sending a lot more operational commands to the spacecraft.

Q: If neither of these options works, Kepler is still an amazing space instrument. Could it conduct other types of experiments?

A: People have asked about using it to find near-Earth objects, or asteroids. Kepler carries a photometer, not a camera, that looks at the brightness of stars, and so its optics deliberately defocus light from stars to create a nice spread of light on the detector, which is not ideal for spotting asteroids.

Whether or not it could function as a detector for asteroids is something that would have to be studied, but since it wasn’t built as a camera, I would say that I’m skeptical. That said, certainly between Ames Research Center and the Jet Propulsion Laboratory, they’ve got the best people in the world working on it.

Q: What’s next for exoplanet hunters?

A: As I said earlier, there is still a year and a half’s worth of data in the pipeline to analyze to identify candidate planets, so there are still discoveries to be made.

It’s important to make clear, though, that in the original queue of missions aimed at finding life elsewhere, a mission like Kepler was a survey mission to establish the statistical frequency of whether these planets are rare or common. It lived the length of its prime mission, and was extremely successful during that time at achieving this goal. It has paved the way for additional missions, such as TESS — Transiting Exoplanet Survey Satellite — and TPF — Terrestrial Planet Finder — which will continue the search for Earth-like exoplanets in the near future.

PIO Contact:

Bjorn Carey
Stanford News Service
+1 (650) 725-1944
bccarey@stanford.edu

Science Contact:

Scott Hubbard
Astronautics and Aeronautics
+1 (650) 498-7077
scott.hubbard@stanford.edu

vincent teofilo May 16, 2013 at 16:27

Reaction Wheel Assemblies are the most crucial but have the lowest reliability of any hardware on a space telescope due to their continuous operation. Hubble Space Telescope was my first program at LM and it had 8 RWAs [2 sets with redundant spares] all of which were replaced twice in the first 20 years of operation. My concern now is who made the RWAs and if they also made the RWAs for IRIS [my last program at LM ] which launches in late June.

Sedjak May 17, 2013 at 12:00

Its just a thought, and probably thought of to some degree by many, but why not send a robotic probe(s) to Kepler with a self-contained reaction wheel(s)/solar panel/sighting/guidance/communications assembly. It could securely latch on (glue, snakebot, insert wild idea here) in the most appropriate configuration and without having to hack into Kepler’s innards, independently correct for the broken reaction wheel. An actual robot that gets inside the scope and swaps out the part is probably out of the question. But if the probe at least got on the same axis as the bad wheel, albeit off-center, it could have a crack at righting the scope.

Engineers of the project would be able to say whether you could do any of this, especially having an independent unit work without some minimal mating with the scope’s electronics. In any case, one could even make a contest for this repair mission if, indeed, Kepler is dead-in-the-water, not re-purposed or fixed. If doable, then perhaps this mode of repair could keep Kepler going for many years. And I bet Musk could get a souped-up Falcon out there on the cheap.

coolstar May 19, 2013 at 1:04

@Thomas Mazanec The false alarm probability for Kepler has been well studied and is not likely to be above 10% for the entire survey (but higher for hot jupiters). So, “confirmation” is not much of a problem. What’s important is that most of the candidates are too faint for atmospheric characterization or for mass (and thus density) determinations (except for masses that can be determined (well) for some multiple systems from transit timing variations and for fewer systems from ellipsoidal variations and doppler boosting (not so well for these techniques)).
The follow-on program to WASP and Super-WASP, Next Generation Transit Survey, should be about finished before TESS launches (they are quite complimentary programs). Here’s a brief description of NGTS:
http://arxiv.org/pdf/1302.6592v1.pdf. This program and MINERVA should keep folks very busy studying brighter systems (and at a fraction of the cost of even an Explorer class mission).

Peter Popov May 19, 2013 at 9:31

@Sedjak “Its just a thought, and probably thought of to some degree by many, but why not send a robotic probe(s) to Kepler with a self-contained reaction wheel(s)/solar panel/sighting/guidance/communications assembly. ” Sending such a device requires R&D for the device plus the launch cost. It is likely much cheaper to produce a second kepler spacecraft, based on the original design. Actually, mass-producing the original spacecraft would have been the cheapest option.

@Alex Tolley. I was also wondering if the fast transit time is due to scaling the starship to a very large size. The article also mentions deceleration at the target star, which should drive the fuel mass an order of magnitude more. This doesn’t look quite right, any comments from rocket engineers? A possible explanation is the lack of fuel tanks – this may decrease the dry mass fraction considerably. And, by the way, I am also wondering if the De ball is used as shield for incoming obstacles/radiation due to interstellar protons.

Eniac May 21, 2013 at 21:53

Did Enzman specify why the fuel was supposed to be Deuterium? As far as I know, D-D fusion is pretty much impossible to achieve. H-bombs use D-T fusion, with either tritium or lithium as fuel, in addition to deuterium. Lithium fissions easily to tritium and helium 3 under fusion conditions, if I am not mistaken, so the two are pretty much interchangeable. Lithium has two key advantages: Lithium deuteride is solid at high temperatures (>600K or so?), so no tanks or freezing are required. Secondly, unlike tritium, lithium is stable and can be found on Earth in decent amounts.

James D. Stilwell May 22, 2013 at 9:15

Eniac’s comment is saved…

ljk May 22, 2013 at 18:42

Kepler’s uncertain future

Last week a reaction wheel on NASA’s Kepler spacecraft failed, putting the future of the extrasolar planet hunting spacecraft into jeopardy. Jeff Foust reports on efforts to rescue or repurpose Kepler, and why, even with the failure, the spacecraft’s exoplanet discoveries will continue.

Monday, May 20, 2013

http://www.thespacereview.com/article/2298/1

Adam May 23, 2013 at 9:26

Hi Guys

Have been getting ready for, and attending, “Starship Century”, so I have missed reading this post.

Design decisions for the Enzmann Starship are hard to dig up. I spoke with Rick Sternbach and Don Davis, both of whom helped depict and design Dr Enzmann’s concepts. The mechanical strength of deuterium is very low – it’s like warm butter – so it need to be enclosed, not “netted” or wrapped in “mesh”. As a result Rick Sternbach developed the vast reflective “tank” which is ultra-thin. Mass-ratio was roughly 100.

As for scaling down, if you ever manage to read the original essay by Stine, he was proposing one stage of the initial probes to have small payloads – but to reach the speeds he wanted, the mass ratio was +1,000. Thus the launch mass was ~1,000,000 tonnes, with multiple stages.

Eniac, deuterium was the fuel of choice because it could be fused without requiring a fission primary to make enough neutrons to get the “dry” fusion reaction started. Dyson’s original paper posited a pure deuterium device, so Stine & Enzmann were following that lead. Of course, as discussed in the paper, Enzmann had been thinking about that design since at least 1949. The necessity of vast mass-ratios led Enzmann to work on his “Echolance” ramjet design, notes for which are available on the Enzmann Starship website.

Eniac May 23, 2013 at 21:52

Andy, if you consider the bremsstrahlung losses, D-D come out as the most difficult of only three realistic fusion reactions (see https://en.wikipedia.org/wiki/Nuclear_fusion#Bremsstrahlung_losses_in_quasineutral.2C_isotropic_plasmas). The energy produced is only 2.9 times that lost to bremstrahlung, vs. 140 for D-T. This leaves very little room for any other losses (of which there are likely plenty), and in my eyes makes it dubious that D-D thermonuclear fusion will ever be achieved.

The fission of Lithium 7 is neutron catalyzed, e.g. no neutron is lost. Together with the extra neutron generated by the D-T reaction, this permits a chain reaction. I don’t know how many neutrons it takes to get started, but I do not think it is a given that only a fission primary can generate them. To me, there is more hope in this than in the only very marginally possible D-D reaction.

James D. Stilwell May 24, 2013 at 21:24

Adam and Eniac comments have been saved….

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