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It was good to chat with you at the meeting on Wednesday.
Further to our chat I think one of the most important parameters to consider is the optimum cruise for any unmanned IS vehicle.
The Daedalus team chose the maximum possible working life of a research team member(s). When I first read the Daedalus report that seemed good to me, but on reflection I think the main IS cruise driver should be the likely “soonest return of data”
What do I mean by that, is that any IS probe must cruise for many decades (perhaps centuries) to reach its target. While it is cruising our technical abilities will (hopefully) improving leading to faster starships. Therefore it would be a poor use of resources to build in 2050AD a 0.02c starship say that will reach a target in 200 plus years only to have it overtaken by a 0.2 starship built 100 years later in 2150AD that can reach the same target and return stellar data back to Earth by 2175AD whereas the 2050AD probe would not return data back until 2255AD.
While I’m not saying that humanity will be able to build a specific speed starship by any specific date. I am saying this kind of analysis (of
possible starship drives) should be used as part the development baseline performance specification of Icarus.
This is why I am not in favor of propulsive braking to achieve orbit of the target star, if it means doubling or greatly extending the flight time. A 2-stage Daedalus probe would then only manage 6% of c resulting in a protracted flight time and in doing so would increase the potential being overtaken by better technology. What a disaster it would be after a centuries of flight, the now orbiting star-probe’s first photograph is of the Starship Enterprise or even worse “The Liberator” from Blakes 7….
I hope this argument make some sense to the team and that it would form some part of the vehicle performance specifications.
Why must Icarus use a fusion/impulse engine? Surely existing technologies can be utilised better? The robustness of the fusion engine means a heavy ship. Perhaps a lighter ship could be built using ion propulsion? Although low thrust, the drive can run for a long time and the mathematics shows that even an acceleration of 0.1g will get you to 0.1c in 10 years or so.
Just a thought.
I think you are quite right that there would be advantages to find a propulsion system that is lighter and requires less fuel/reaction mass than than the 50,000 ton Daedalus needed. Finding a lighter viable alternative to a fusion-pulse concept is likely to be cheaper and bring forward the vehicle’s launch date. These must be good things and the Icarus design team should consider all possible propulsion systems and how they might be integrated and used to produce a robust vehicle concept.
Ion drives offer high specific index’s but at the cost of very low thrusts and the need for exotic propellants such as xenon which at a concentration of 1 part in 20 million in the atmosphere at sea level is perhaps not the ideal propellant choice when vast quantities are required. Ion thrusters also need a good energy source. They will work as far out as Mars on solar panels, beyond that you are back to some kind of thermonuclear device. The RTG powered ion drive concept “TAU” (Thousand Astronomical Unit) would get out out to 0.016 light years after 50 years: not a staggering performance! To get to stellar distances using ion drives would therefore require powerful reactors and lots of thrusters (to get to useful fraction of c) which starts to sound like a heavy vehicle again, that would also be difficult to fuel.
The Helicon “pulsed-plasma” thruster and the related VaSIMR (Variable Specific Impulse Magnetoplasma Rocket) both offer about ten times the performance of conventional Ion thrusters. VaSIMR offers about a pound thrust and (clustered) could offer better vehicle performance than current chemical rockets. VaSIMR also uses hydrogen as a propellant which must be a more attractive option than rare inert gases such as xenon. VaSMIR is to be able to get a crewed vehicle to Mars in 3-months and to Jupiter in 18-months would be a great boost to establishing the interplanetary.. infrastructure and logistics that would need to be in place before any interstellar mission could be launched.
What if pellets of fuel were fired by some manner of canon on a path to the target star system. A fusion power probe could catch the fuel as it accelerated along thus mitigating the requirement to accelerate all of the fuel.
There would be very little flexibility, and one would have to work out the velocity of the probe as it reached the pellet so that it is possible to collect it without damaging either system. The pellets could be fired in rapid succession at different velocities. Also pellets of He3 would sublime.
There are complexities in this approach. Vast amounts would have to be fired out in a short burst. Any canon would have to orbit the sun, as well as be in orbit about the galaxy, sending out a pellet trail to a star would have all sorts of issues. However I think the basic idea is quite elegant. Interstellar probes could be that much cheaper so more could be built, and the same canon used for each mission.
Problematical, but maybe someone else could finesse this. I think it is just too good an idea to waste. I like to think of this as the Pacman project.
Thanks for all the comments. David, great to see you at the conference too.
There seems to be a lot of talk on the propulsion system here. Fusion is an excellent option for propulsion as it liberates the highest energy per unit mass of propellant, other than antimatter of course. This means decent acceleration and a good chance at completeing the mission in the allocated time of a human lifespan.
As pointed out, there are numerous alternatives, but fusion is at a good stage of technological maturity and the process is well understood. We also want to remain faithful to the original Daedalus concept and changing the propulsion system would be quite a radical departure.
I don’t think we’ll ever choose a propulsion system that will make everyone happy, but the important thing at this point is that we focus on one technology and develop it as best we can. Research will occur and the technology will progress.
The original Daedalus configuration of two stages suggested to me that instrumentation could also be installed on the slower first stage which would reach the target system much later than the primary probe. This secondary probe can then be programmed with specific bodies to investigate following analysis of the first fly-by.
If some means of braking (e.g., magsail) is employed, then the above model can extend the mission or place two main probes into the system.
Hi – interesting project. I do think that getting the propulsion system right is key to the project. Fusion is undeniably fantastic but some consideration should be given to key alternatives. Reducuing the mass of that propulsion system is of paramount importance.The primary alternative is beamed energy propulsion. One keeps the heavy power systems on Earth and beams the power up to a collector on the craft which then either directly heats the fuel or converts to energy to power a mega-vasimir. Staging may also be improved by having drop tanks rather than stages. Deceleration via magsail would also save mass.
Hi All, I would like to respond to some of the excellent comments above.
On the 4th November Team Icarus agreed the terms of reference for the project. These are displayed on the HOME page of this web site. This includes considering deceleration into the target system (unlike Daedalus) which the team debated extensively and agreed upon. The other area we agreed upon is that Project Icarus is to be ‘mainly fusion based propulsion’. This means that options such as beamed power, solar sail…being used as the main engine drive are off limits for this particular study. This comes from the need to develop a focussed design study and the task for the team to somehow achieve the initial design requirements – which are our terms of reference for the study. The ‘mainly’ option does allow for assistance from alternative engine drivers, this could be antimatter catalyzed fusion or some nuclear-electric drive or a VASIMR engine for a portion of the mission. If the final design solution does not meet the initial requirements, then Team Icarus would have failed the initial objective set, even if the design is neat.
On another note, good idea from Geoff Hogg on accelerating fusion pellets ahead of the vehicle. This will have to be considered.
Carl and Nathan both mentioned MagSail breaking. Indeed, this is an area we have already decided to look into. It has potential.
A comment on ion engines, VASIMR, or any of the other forms of what propulsion engineers tend to call “electric rockets” that have been proposed above. Those are “electric rockets” because they need electricity. No one appears to have mentioned that these rockets require large power systems that a DAEDALUS class fusion engine does not. In the deep spaces between the stars, solar energy becomes nearly useless due to array size so let’s look at how we can power such a spacecraft. Bottom line up front: fusion rockets are very attractive because they convert fusion reaction products directly to thrust without the energetically expensive step of first converting the energy stored in the fuel to electrical energy and then using that electrical energy to heat a propellant. Still, two alternative energy sources could reasonably be considered:
1. Fission: Efficient use of nuclear fuel in a space reactor requires a balance between radiator area, coolant temperature, and mass. Don’t get me wrong, fission power scales up very well (and a whole heck of a lot better than it scales down) but the mass of solid core fission reactor required to provide the terrawatt level exhaust powers to meet our mission time requirements would be more massive than a DAEDALUS class fusion engine. That’s why you can read papers from the space nuclear community for the last thirty years or more on liquid or gas core nuclear reactors for space applications.
In principle, these are great. Higher temperature means higher power density, smaller radiators, and greater conversion efficiency of nuclear to electrical energy. The down side is that your nuclear fuel (typically highly enriched uranium) is flowing through the system in either solid or liquid form. It’s a strange fact but that we’re probably closer to an electromagnetically or inertially confined fusion engine than either of these types of reactors. Simply put: at least we’ve done EMF or ICF in a laboratory. To the best of my knowledge, no one has ever actually burned liquid or gaseous fission fuel controllably and repeatably. The combination of fluid dynamics, thermodynamics, and nuclear engineering is a formidable energineering task roughly on par with nuclear fusion research.
2. Beamed power: It all sounds so wonderful: don’t carry the energy source with you. Either make a “sail” or just carry the propellant. And, I agree in principle. But practic
The most practical beamed power systems tend to use lasers to beam power. This is because other power beaming options (typically microwaves) suffer from truly huge antenna apertures at both ends. It’s easier to focus a laser at long distances. The problem here is that high power lasers (MW class and above) are not particularly efficient devices. Their conversion efficiencies tend to be on the order of 1% of input electrical power to output light (recirculating free electron lasers a possible exception here). Yes, solid state lasers are getting more efficient all the time, but these are milliwatt class devices. I’ve not seen any indication that they’re giong to yield the gigawatt or terawatt class beams that would be required to move payloads in our timescales here (happy to be contradicted here).
Beamed power is particularly troublesome as a power source for electric rockets, especially during the first leg of outbound flight. This is because most electric rockets create charged particle beams as their exhaust. There’s nothing worse for laser propagation than a charged particle beam. Shooting through your own exhaust, even a little bit, could severely defocus your power beam. I’m not saying that this is unmanagable but I *think* (without reference) that it’s the main reason why most people who talk about “beamed power” over interstellar distances (not interplanetary – that’s a more tractable problem) talk about beaming power directly to a sail. In this case, as with a direct fusion exhaust engine, the energy source is directly converted to momentum at the spacecraft. Just in this case it’s photon-momentum to spacecraft momentum.
In the case of a fusion engine, it’s charged particle fusion “ash” momentum to spacecraft momentum.
I’ve forgone the calculated examples which a proper response to the use of ion engines in interstellar flight requires. But regret that I lack the time to work up a really good set. Matloff and Marlowe’s The Starflight Handbook offers a good basic description. It’s how I got started, at any rate.
Although I have not read it again, I remember the Daedalus report had some thoughts on the interplanetary space-faring civilization that could only bring such a projet to life.
Even though you state that propulsion systems such as VASIMR could be used to ferry parts in and out of outer solar system (Jupiter ?), the task at hand seems tremendeous to me, and now that the Daedalus report has shown that a star probe does not defy the laws of physics, maybe it’s time to look at more practical issues.
Here is one : you stated a 50000 t fueled (?) starship. assuming a 10% dry mass, that’s a 5000t starship you need to ferry to and assemble at some suitable location in the solar system (not that being able to do this alone is a remarkable feat that leaves behind the man vs machine debate, by that time you should be able to get the best of both…).
Even though in-space propulsion failure (VASIMR and the likes) are much less likely than earth to orbit launches, you would still need to lift up many hundreds of tons of pre-assembled hardware before sending them out into the deep.
Chemical, Earth to Orbit launchers have not improved greatly, are reaching there theoretical limits, and reusable designs where high reliability is sought are already massive (bigger than any airliner in service, see Hotol/Skylon vs the A380 to keep british/european examples), even for modest payload : that is heavy lift is most likely to remain for very long confined to expandable rockets and ARES-V-like vehicles the biggest launchers for that kind of “oversized” transport, or they must become “regular” sized payload to justify their added complexity and size.
At 100t(-ish) per launch, that’s about a minimum of 50 launches that would need to be performed just to send the hardware up before the electric-rocket tug takes it into deep space.
Allowing for a launch reliability of 99 % (1 catastrophic failure in 100 flights, nobody has ever done this over a number of time significant to talk about statistics rather than probability), the chances that you simply put into orbit all your hardware without -damaging- interruption is just 0.99^50, ie ~60% only. That’s exactly the situation that happened with the international space station interrupted by the Columbia disaster (the shuttle, STILL being among the most reliable launchers in the world with two losses for 127 flights to date : 98.4% success – proven-, a remarkable achievement). SSMEs are probably the closest rocket engines to theoretical limits, and probably the most reliable pieces of machinery ever built with such shaft horsepower (think of your pellet ejection gun by the way…..)
In my opinion, with no “race” at stake, unlike it was the case for the moon race during the cold war, no well-informed (world ?-)-government would start a project that has no more than 40% of chances of sucesss (assuming that everything else is 100% OK) : people had a better opinion of the space shuttle reliability, based on probabilities rather than facts by the time they designed the ISS.
For the Hermes project, ESA had unrealistic expectations for Ariane V to reach 99.9% reliability (given up since) : that’s the kind of figure needed to start a Daedalus/Icarus-sized endeavour ==> to achieve this, reusability probably is both desirable AND enabling, but yields a massive, complex LV ==> there have to be other uses for such a Mammoth RLV ==> etc…
If you look further, deeply into the logistics aspects of this, are we not even dependant on major breakthrough in energy production (that is mastering controlled fusion on firm ground), space elevators ?
My point is that you can’t just think “top-down” wrt to in-space requirements. You need to relate your technical expectations at some point in time with available technologies supporting your endeavour (TRLs and so on, in plain : system engineering) and work it “bottom-up” for a reality check more that once along the way.
Maybe we can’t say at which point in time we can launch a 50.000t fusion-based starship fueled from the Jovian system, but we could get more confident in saying when we can expect to lauch a 5.000t starship with a different kind of propulsion system (and a much lower payload) from a cis-lunar drydock. You might find in the end that for this interstellar vehicle size, maybe pulsed nuclear fusion no longer is the preferred solution. What about “densifying” hydrogen (see research on metastable metallic hydrogen etc…) and using it in a more conventional “electric” rocket for instance ?
Anyway, I suppose that with all the knowledge we’ve gathered on our nearby stars and the numerous questions raised in doing so (there’s always more questions yielded than answers), thinking at a system level does not reduce to sending off a given payload in a given time to a given star, but requires an all-inclusive galactic exploration strategy (even if it’s a very small portion of the milky way that could be probed initially) : in combination with increasing our earth, near-space or deep-space observation/detection means, their predictible improvements, and even manned/unmanned missions to our closest neighbours (Mars, Gallilean Moons, Titan…), WHAT IS IT THAT WE SEEK IN-SITU AT A GIVEN POINT IN TIME ? Is it a purely science-driven question first ?
Like said about “Starship evolution” in a previous post, on a purely science-based realm, it would even be a shame to send off a starprobe on a 50 years journey that would bring not much more information on the target star system that a 50 year improvement of our Sun-system based (or maybe just beyond Heliopause ?) observation means could !
There’s so much non-engineering issues that would drive such an endeavour that my engineering mind is overwhelmed….
Have fun anyway, glad If I can help !
Re-reading the discussion of helium-3 access choices in the original “Daedalus” study I note an important comment that such a massive retrieval of fusion fuel should be in the context of a vibrant Solar System economy that has more than just a starprobe to fuel.
As for the specifics I think Jupiter as the source of He-3 was a bad idea because of the sheer difficulty of hauling stuff into orbit with a delta-vee of 30-35 km/s. Saturn or Uranus present a much more forgiving gravity well and magnetospheric environment, though Saturn has a denser ring system to avoid.
I’m sure there’ll be more on this topic as discussions progress.
On He-3 mining: in his inspirational book “Mining the Sky”, John S. Lewis argues the case for Uranus. He envisages miners which use gas-core fission engines (refuelling with hydrogen on Uranus) to haul it out of that planet’s gravity well. But whether this is a realistic scenario for before the end of the 21st century, given the current go-slow in space and competing priorities in climate alarmism and sustainable development, is another matter.
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