ASPW: A Report from Colorado Springs

by Paul Gilster on December 13, 2010

by Richard Obousy

As project leader for Project Icarus, the ambitious successor to the British Interplanetary Society’s Project Daedalus starship design, Richard Obousy is deeply engaged with the advanced propulsion community. Here he gives us a report on the recent Advanced Space Propulsion Workshop, which he attended in November. It was a sizable gathering, but Richard focuses here on work of particular relevance to Project Icarus and the Tau Zero Foundation, the twin backers of Icarus.

Recently, several members of the Project Icarus team attended the 2010 Advanced Space Propulsion Workshop (ASPW) at the University of Colorado in Colorado Springs. The event ran from from Monday, November 15 through Wednesday, November 17, with over sixty presentations given by a number of researchers. Project Icarus attendees included James French, Rob Adams, Robert Freeland, Andreas Tziolas and myself.

The ASPW is focused on low Technology Readiness Level (TRL) concepts ranging from TRL 1 to 3. A TRL is a measure of the maturity of an evolving technology, with TRL 1 representing the very lowest level of maturity, with only the basic physical principles of an idea demonstrated. At the other end of the spectrum, TRL 9 represents a system that is, for the most part, tested and operational. A detailed breakdown of the TRL levels can be found here.

A TRL 1 to 3 conference typically brings together a good mix of pioneering thinkers who are willing to think outside of the box and explore untested ideas and concepts. Most of the attendees were either NASA scientists, worked at JPL, or were actively affiliated with a university, so the science that was presented was all very plausible and grounded in ideas that could be brought into fruition within a few decades (earlier in some cases), given a sufficient investment of time and funding.

Image: The conference looked out to Pike’s Peak, a 14, 115 ft (4.3 km) elevation mountain. Credit for all images: Richard Obousy.

A number of talks caught our attention due to their relevance to the Icarus Project. One such talk was delivered by John Slough, who gave an intruguing presentation on his research at the University of Washington on Inductively Driven Liner Compression of Fusion Plasmoids. His was the only team at the conference working on pulse propulsion concepts.

The basic concept involves pulsing fusion fuel plasma at high rates into a reaction chamber where it would undergo fusion via use of metal liners to accomplish compression of a magnetized plasmoid. Although remarkable, the only purpose of the fusion would be to drive the next round of plasmoid firing. Propulsion would be achieved through momentum transfer occurring between the electromagnetic gun and the accelerating plasmoid. In other words, all the fusion gain would be put back into driving the next cycle. This differs markedly from the idea behind Daedalus, where a large fraction of the exploding fusion material itself transfers the momentum.

Image: John Slough(University of Washington) speaking on fusion plasmoids.

Rob Adams (Project Icarus) gave a fascinating talk on a conceptual design of a z-pinch fusion propulsion. The results of a detailed study that he had been involved in were presented. These results included the modeling of the z-pinch fusion rocket, the propulsion characteristics, an evaluation of a magnetic nozzle, mission analysis and overall vehicle design.

Image: Rob Adams (Project Icarus) explaining z-pinch fusion propulsion.

Andreas Tziolas (Project Icarus) gave a thoughtful overview of candidate technologies for interstellar exploration and then went on to discuss various aspects of Project Icarus, including ideas that the team has been discussing including vIcarus, satIcarus and others. Andreas received a number of questions from the audience, including one from Sonny White who suggested that the Icarus team put some effort into possible spin-offs that could be realized within two or three decades.

Image: Andreas Tziolas giving his talk on interstellar enabling technologies.

I had the pleasure of presenting a talk on Day 1 that introduced Project Icarus. I spent some time discussing our overall objectives and who makes up our team. I also talked about the original Daedalus propulsion systems and described the essential features, including the cryogenic storage, Deuterium – Helium3 pellet design, injector nozzles, electron beams and the reaction chamber and magnetic nozzles.

Image: Richard Obousy talking about Project Icarus, and the Daedalus propulsion systems.

Day 1 ended in the local student union building with dinner and drinks, which the team enjoyed thoroughly.

The plenary session of Day 2 was opened by Les Johnson, Deputy Manager for the Advanced Concepts Office at NASA MSFC in Huntsville. Les described the limitations of chemical rocket propulsion and illustrated an overall technology roadmap that could provide NASA with the pathways required to meet the space agencies exploration goals for the 21st century. The technologies Les described would enable more effective exploration of our Solar System.

Image: Les Johnson speaking on Day 2 of ASPW 2010.

The renowned Robert Frisbee gave a talk titled ‘To The Stars, One Way or Another,’ which described the details of several studies he had performed while working at JPL over the years. These studies were aimed at identifying propulsion technology requirements for interstellar missions. He explained to us that these studies were made intentionally difficult, and would involve a rendezvous mission as well as a top speed of 0.5c. He also briefly reflected on some breakthrough propulsion ideas, including wormholes and warp drives, and also an analysis of the negative energy requirements for these schemes. This was the first time I have seen Robert speak, and I want to emphasize that he was a thoroughly entertaining and enjoyable presenter. I was surprised at how animated and enthusiastic he was during his presentation, and enjoyed the many clever ‘wise-cracks’ he managed to throw into his talk.

Image: Robert Frisbee giving his talk “To the Stars, One Way or Another.”

Day 3 included a number of fascinating talks, including a talk by David Kirtley (MSNW LLC) who spoke on the concept of ‘Macron Propulsion’, an idea that involves firing small fuel pellets in front of a spacecraft which would then be utilized by that craft for fuel. This ingenious, yet complex approach is attractive, since it overcomes the Rocket Equation as the fuel is not stored onboard the craft. The talk had very interesting parallels to the Daedalus electromagnetic pellet launcher. MSNW has built and is currently testing a 20 Tesla launcher and also a prototype pulsed power bank.

James French (Project Icarus), a veteran rocket scientist who worked on the Saturn V engines, gave a talk on Gas Core Nuclear Rockets. He first gave an overview of the solid and liquid core rockets, and then discussed some of the challenges associated with heat transfer for the working fluid, cooling of the solid parts of the engine and also the problem of how to start and stop a gas-core rocket.

James illustrated the potential use of the Gas Core rocket for Icarus by illustrating a calculation that revealed a potential course correction for a probe released from the main Icarus ‘mother-ship’ traveling at a reasonable fraction of c. The conclusion at which James arrived was that there is potential utility for gas core rockets within Project Icarus.

Image: James French (Project Icarus) talking on Gas Core Nuclear Rockets.

Rob Adams (Project Icarus) gave his second talk of the conference, and explained the Oberth two-burn maneuver. While it was the most amusing talk of the conference (Rob cracks a lot of jokes), it also detailed Rob’s process of rediscovering the Oberth maneuver. This relatively unknown effect is often confused with the gravitational slingshot, but differs markedly in its application. When a spacecraft executes the Oberth maneuver, it is able to obtain far more useful energy for greater delta v than a stationary rocket. Rob believes this maneuver is generally unknown, even among experts, and emphasized its scientific value. He also explained that the maneuver could be used effectively for future missions to obtain higher delta v.

Robert Frisbee gave the final talk of the day, which was largely to encourage all present to ‘think big’, and to explore low TRL technologies so that breakthroughs in understanding can be accomplished. I particularly enjoyed one of Robert’s quotes where he explained that “It’s all science fiction until somebody goes out and does it.” Robert received a standing ovation at the end of his presentation, which was a fitting end to the conference.

Image: From left to right: Andreas Tziolas, Rob Adams, Richard Obousy, Jerry Winchester, Robert Freeland, Jim French.

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Andrew W December 13, 2010 at 14:48

It’s crazy that so much thought is put into interstellar propulsion systems and so little into interplanetary propulsion. What we have now is totally inadequate for manned flight to the planets, which I’d hope would happen this century, I wouldn’t expect interstellar probes to be heading off till the next century.

One interplanetary system that to me seems to get far less study than it deserves is solar thermal electric.
Lots of thought goes into solar PV electric, and also into nuclear thermal electric, I think solar thermal electric picks the best elements of each.
Surely large solar sail like mirrors focusing sunlight onto boilers would be a far cheaper and far lighter option than gas core nuclear reactors? At the low accelerations that can realistically achieved using high Isp electric propulsion (around 0.01g) holding the mirrors in their correct form shouldn’t be a problem.
Another problem I see with nuclear fission reactors is that given the huge energy demands of high Isp propulsion, you actually end up using quite a lot of fission material on even one flight.
(Happy to be corrected if I’ve made an error in the following.)
Nuclear fuel has about a million times the energy density as chemical fuels, but the propellant of an engine with a Isp of 10,000s carries away about 1000 times the energy as the propellant of a chemical rocket, so a nuclear reactor that powered that electric rocket engine would burn 1kg of fissile material/ton of propellant used (actually more allowing for waste heat). That rate of fuel use would soon add up.

Parmanello December 13, 2010 at 22:12

No chance of any videos of the talks I take it?

Eniac December 14, 2010 at 2:10

@Andrew: I think one kg of fissile material per ton of propellant is actually very conservative. The optimal situation is where the fissile material IS the propellant, i.e. the only thing being propelled is the waste from the reaction. That makes for an Isp (or rather exhaust velocity) of ~0.1 c, which would mean complete freedom from the rocket equation in interplanetary flight, and the possibility of reaching a good fraction of c in interstellar flight.

In principle, this can be achieved by operating a nuclear reactor to power a particle accelerator which propels the reactor waste out the back at 0.1 c. More like 0.02-0.05 c, in practice, because of various inefficiencies. Still, thermal efficiency can be 30-50%, and ion accelerators can also be quite efficient, so the nuclear electric option may well beat any direct fission fragment scheme. It does, however, have a weight issue. The reactor must either have a very high burn-up fraction, or operate an entire fuel reprocessing cycle.

Paul Gilster December 14, 2010 at 8:36

Parmanello, good question about videos of these talks. If I find a link to any, I’ll post it — my guess is that none are available, alas.

Andrew W December 14, 2010 at 14:23

Thanks Eniac, one of the problems I thought of was that storing vast quantities of radioactive material in high concentrations wouldn’t be possible, but I suppose that’s easily solved by using a non-fertile fuel like thorium 232.

However, my main point is that at 1AU the sun provides us will a GW of power per square km, and that this can be collected and concentrated by structures as light as soap bubbles to give us huge amounts of thermal energy, the same amounts as can be produced by large nuclear reactors but at a fraction of the weight penalty, a fraction of the cost, without the need to push nuclear technology to the limits, and without the nuclear hazard. In fact we already have solar thermal power stations operating in the 100 MW range, in many respects they’d work a hell of a lot better in interplanetary space.

Material suitable for solar sails has now been produced in the lab, a square km of which would mass just 100kg, so even as far out as Saturn, mirrors made from such a material could in principle concentrate a GW of solar energy for a weight penalty of just 10 tonnes (plus support structures).

So is it rational that this approach isn’t given a higher priority as a power system for interplanetary propulsion? Is there some flaw that I’ve missed? Why is it that at the moment it’s all solar sails, or nuclear thermal, or solar PV rather than solar thermal?

Neal Asher December 15, 2010 at 13:38

Anything on Alcubierre warp drives from Robert Frisbee? And, incidentally, where might I find more information on that subject?

bruce December 15, 2010 at 18:53

Andrew, could you help an old dog out? How does Solar Thermal work? What are you heating and how do you extrude force?

Eniac December 15, 2010 at 19:30

Andrew, I agree that solar thermal is an interesting alternative. However, there are really two independent issues here, whether and how to optically concentrate the sunlight, and how to convert it to useful propulsion. Focusing on the latter, if you want high Isp, a direct thermal rocket will not suffice; you have to use an ion drive. It then comes down to photovoltaic vs. turbine/generator. The efficiencies of these are similar, and photovoltaic wins hands down for practical reasons. Most pertinent of those would be the lack of moving parts, hot fluids, and large radiators.

Nuclear energy holds the promise of a compact device, and operations far from the sun. RTGs are the simplest , in that they need no moving parts and generate electric energy. Reactors are somewhat more complicated, but have higher power densities and can be throttled. In principle, a reactor with thermionic conversion could also have neither moving parts nor liquids. I envision a very hot core, temperature moderated, clad in a thermionic converter and radiating waste heat directly, at high temperature. However, I have not been able to find a description of such a beast, so there must be some serious practical difficulties with that.

Andrew W December 15, 2010 at 20:04

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

Then the electric power generated is used to power a high Isp electric propulsion system (vasimr is just one such system).

It’s similar in many respects to solar PV, and perhaps higher power/wt ratios and therefore performance can be achieved with future PV compared with current PV systems if the solar energy is concentrated by mirrors onto a smaller area of PV cells to reduce the weight of the cells, this would however then require a similarly substantial cooling system as is required with solar thermal anyway.

Andrew W December 15, 2010 at 22:14

When we build ships for manned interplanetary flight, I hope that we don’t follow the lead of Apollo with ships that only last a single mission. Given that, I can’t see how nuclear fission can be competitive against solar power for manned interplanetary flight with the frequent need for nuclear refueling given the huge power demands of high Isp propulsion systems.

If we’re talking solar PV vs solar thermal, I’d expect it to come down to power/wt ratios, turbine systems can produce tens or even hundreds of KW/Kg(think SSME turbo pumps), I doubt any current PV system gets even 1 KW/Kg.

On the other hand the PV option is simpler, doesn’t require generators and depending on how concentrated the light falling on the cells is, mightn’t need fluid cooling.

Someone clever needs to do a study on it all.

Paul Hughes December 16, 2010 at 4:44

Andrew – many of these technologies would once deployed would have an immediate and *dramatic* impact on solar system travel. Initial systems based on John Slough’s DLCFP should be able to achieve 200 km/second speeds and more advanced versions 1000+ km/second. At 1000km/sec, assuming a 1g acceleration would mean:

2 Days to Mars (2 days!)
3-5 Days to the Asteroid Belt
9 Days to Jupiter
16 Days to Saturn

Those are very impressive numbers. With travel times like these the entire Sol System will open up.

Paul

Andrew W December 16, 2010 at 16:04

Paul, in theory any direct fusion propulsion system can achieve remarkably high specific impulse, and therefore high ship velocities, the problem with them is that they’ve all been languishing around TRL 1-3 for a very long time with no one seemingly able to push them any further along.

Eniac December 17, 2010 at 14:55

Andrew, turbines and turbopumps have high power density as long as they are considered open cycle and include no generators. In space, there can be no open-cycle engine, and power to weight ratios will decline precipitously as you add pipes, condensers, pumps and cooling panels. Then there is the generator, which will be difficult to build at better than 1 kW/kg all by itself.

Not all is lost, though. Potentially, we could use sunlight to heat a refractory metal pipe to a very hot temperature, and use thermionic conversion to generate electricity directly between that and a cooler, surrounding pipe. This would really be the same as my envisioned solid state reactor, except for the energy source. The outer pipe (jacket) would have to be sufficiently hot to radiate the waste heat directly, and the inner pipe (core) sufficiently hotter than that to ensure reasonable efficiency. According to http://en.wikipedia.org/wiki/Thermionic_converter, several W/cm^2 could be generated, which would make for a very solid power to weight ratio if the pipe walls can be kept thin.

According to my back-of-the-envelope calculations a jacket temperature of ~1000K should be sufficient to radiate a few W/cm^2, and it seems reasonable that a core temperature of 2000K could be achieved while keeping structural integrity, using, say, tungsten. That should yield an efficiency comparable with photovoltaics, at a potentially much better power to weight ratio.

Andrew W December 17, 2010 at 16:32

I’ve come across suggestions that high speed super conducting generators could achieve 40 – 80 kW/kg. Another option is MHD:
http://en.wikipedia.org/wiki/MHD_generator

Adam Crowl has a post on the nuclear option here that promotes gas core fission – MHD in the links:
http://crowlspace.com/?p=511

I’m not knowledgeable enough in this area to pick which would be the better option in terms of solar to electric, but I’m more certain now than before that solar beats fission.

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