guest blogger john hare
Anytime a mission, manned or robotic, is proposed to destinations beyond Mars, travel times of years to decades are involved. Efforts to reduce trip times depend on some propulsion system well beyond the capabilities of chemical propulsion. The problem with most proposed systems is either the Isp is still not good enough, or thrust/weight is too low, or both. What we want is some propulsion system that has either relativistic exhaust velocities, or uses no propellant at all while having a high thrust/weight ratio. Other than solar and laser sails, nuclear in some form is required. With fission requiring bomb or at least radioactive type material launched from Earth, it pays to think of fusion solutions that use material that is more benign until triggered in some way.
I seem to recall suggestions that if two pellets of fusible material impact each other at enough velocity, fusion will occur. The problem is that the impact velocities are in the hundreds of km/sec range. Then you don’t get a sustained reaction, you get a really energetic boom. If we can get that energetic boom from a couple of pea sized pellets, Isp goes off the scale, if we can harness it. An Orion variant seems to be the most likely way to harness the boom for propulsion purposes. How do we get the boom?
I suggest that pellets are deployed in the path of a vehicle such that the closure rate is hundreds of km/sec. The external pellets impact the ship carried pellets to create a fusion explosion behind the pusher plate. The navigation and propulsion package for the external pellets becomes the reaction vapor that impacts the pusher plate at relativistic velocities.
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With the external pellets supplying half the fusion mass and all of the vaporised material to drive the ship, effective Isp goes to the millions in a deep space fusion ramjet. Accelerating for a day at one gee gives about 850 km/sec of ship velocity. If Isp is in the millions, then mass ratio becomes less than 1.1. At 73 million kilometers per day, Mars is a day trip and Jupiter, Saturn and company are just weeks. All without launching fission material from Earth.
All we have to do is get the spacecraft up to 200 km/sec or so to get the reactions working. Getting perfect alignment between the ship and pellet stream for perfect impacts will be a bit of a problem. Some people get a bit upset when their vehicle is smacked by an object with TNT equivalent in the thousands, no sense of humor most likely.
We do have the technology to get the ship up to 200 km/sec relative to the pellets. It involves braking from the Earth’s orbit to a very close pass at the sun. Inside the orbit of Mercury, the ship can hit sun relative velocities of over 100 km/sec. The pellets have been braked all the way to a retrograde solar orbit that is the exact counter to the ship’s trajectory. At perihelion, the relative velocities of the ship and pellet stream is over 200 km/sec.
As the ship accelerates up the pellet path, the pellets have to have less velocity delivered from Earth with the ship supplying the impact energy. For interstellar probes, a month of acceleration at one gee gets up to about 8% of light speed. Here’s your safe, simple, and soon fusion drive, see you on Rigel 6.
johnhare
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Funny you should mention fusion propulsion. Was looking at these earlier today:
http://quantumg.net/advancements_in_dense_plasma_focus_for_space_propulsion.pdf
http://www.quantumg.net/propulsion_and_power_generation_capabilities_of_a_dense_plasma_focus_fusion_system.pdf
The first is your typical funding begging conservative scientific paper. The second is a more funny sales pitch to the airforce for an F-302 style vehicle (see http://en.wikipedia.org/wiki/Earth_starships_in_Stargate#F-302 if you don’t know that particular fiction) with “pulsed-train plasmoid weapons, ultrahigh-power lasers, shielding/cloaking devices and gravity or time-distorting devices.” Which just goes to show, the airforce won’t believe *everything*.
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John,
I wonder about the validity of your beginning assumption, which you write as:
“Anytime a mission, manned or robotic, is proposed to destinations beyond Mars, travel times of years to decades are involved. Efforts to reduce trip times depend on some propulsion system well beyond the capabilities of chemical propulsion. The problem with most proposed systems is either the Isp is still not good enough, or thrust/weight is too low, or both. ”
A couple Megawatts of power, plus VASIMR, could give you a spacecraft that can travel beyond Mars in weeks-to-months, not years-to-decades. Charlie Bolden talked about this exact issue before the U.S. Senate in July.
DARPA is working on the power issue as part of their FAST program.
Franklin Chang Diaz is working on VASIMR.
While these technologies are not here yet, compared to fusion they are on the near-term horizon, and are much easier to develop than nuclear fusion engines.
Care to comment or respond?
– Charles
Charles Miller
Charles — the fusion drive is closer than you think.
See Charles Orth’s paper at: http://www.osti.gov/bridge/purl.cover.jsp;jsessionid=6727449C1B281CB91FBB5F0D6ACE0840?purl=/324289-cMnvOZ/webviewable/
Later this year we will achieve thermonuclear burn in the lab for the first time — so we may be on the “near-term horizon” too!
https://lasers.llnl.gov/
You really ought to come out for a visit.
Cheers,
Bill
VASMIR and “not here yet” are synonymous.
The Journal of the British Interplanetary Society (JBIS) has published articles outlining this sort of idea, including fusion explosion concepts and transfer of momentum from a stream of pellets to a plate or magnetic sail. Penn State has a program looking into using antimatter to “catalyze” fusion micro explosions in a drive of this type. I’ll give you points for ambition on the precision needed to get this to work.
I’ve always liked the idea of sending propulsive charges out to Jupiter and returning them in a retrograde orbit. How much extra relative delta v would that give you during acceleration close to the sun?
With the mass of the required power source, VASIMR is exactly what I was talking about. Isp and/or thrust/weight are too low. You really need to double check all the numbers being thrown around on that one
You didn’t think I was suggesting somebody go out and build or even spend study money on this fusion concept now were you?
Bill — Great to know there is progress being made. I grew up on Heinlein, and have wanted fusion spaceships since that time. If we are making progress that is wonderful. Next time you come to WDC, let’s get together.
Trent — People in glass houses (e.g., advocates of investing breakthrough-level technologies) should avoid trashing other people’s technology projects. We need to invest in a broad portfolion of high-risk projects, and it hurts every advanced-technology project to when a circular firing squad is encouraged. Also, I never said VASIMR is “here yet.” But it is making steady and real progress. Real hardware has been built, and is being tested.
Also, note that inertial confinement fusion and VASIMR require some of the very same enabling technology as they both need very powerful magnetic fields. There is synergy.
John —
1) I trust the people who are doing the analysis on the combination of FAST & electric propulsion. It requires LOTS of power to get the short trip times. The true enabling technology to make this happen is the DARPA FAST program. Most people have not recognized what very high amounts of solar-electric power (1+MW) might mean for space development. We have the equivalent of a “Moore’s law” for solar electric technology. For the last 30 years, solar cell efficiency has increased at a level of 1% per year. Combined with concentrator technology, or thin-film, entire new capabilities avail themself.
2) It sounds like Bill may think it is time to spend some new study money on fusion transportation concepts.
Onwards and upwards,
– Charles
I’d love to see a fusion rocket, but the type of precision for dropping pea-sized pellets close to the sun, from earth, and hitting your time window is rather extreme, no?
This certainly has advantages in that the power for ignition doesn’t have to be generated by an onboard reactor, and it meets your “no launching fission material” requirements. I should do some calculations for how long it will take to fall into the sun before you have your pellets impact to create your day trip to Mars. I’m also curious about how many pellets you could miss (due to pelletship problems) before your orbit was off enough that you missed the rest of the pellets.
Day trip to Mars was just refering to the velocity attainable. The drop to the near sun position would take 3-4 months. The only possible use for this concept is for missions to beyond Jupiter. As Charles has brought up, for Mars or the belt, there are some options coming available.
The precision is probably not attainable in anything resembling the near future, and I would hope there are better options for the far future.
I’m sorry John, but I have to find fault with your basic premise. Contrary to popular belief, you can get nuclear fusion by just slamming two objects together at high velocity. The geometry of the collision must be very precisely aligned. I somehow doubt that you are going to get that precision with guided missiles flying around the solar system. I appreciate your imagination, but I think this system is much more complicated, and probably much more difficult to get working, than to just build a bunch of H-bombs and toss them out the back of the spacecraft as per the original Orion propulsion concept. The only advantage of this scheme over the Orion concept is that you would not be penalized for the mass of the ‘fuel’ while the ship is accelerating.
Hmm.. I wonder if there is a government employee who gets to read all of these wonderful blog posts that just happen to contain phrases like ‘guided missiles’ and ‘H-bombs’. If so, I sincerely apologize for the inconvenience.
Hmm.. I wonder if there is a government employee who gets to read all of these wonderful blog posts that just happen to contain phrases like ‘guided missiles’ and ‘H-bombs’. If so, I sincerely apologize for the inconvenience.
I hope it is at least somewhat more entertaining than most of the chaff they have to sift.
Eric,
After a bit more thought, I would say this ‘might’ be possible in fifty to a hundred years if desirable. Look at munitions accuracy today compared to fifty or a hundred years ago. The two satelights destroyed by China and the US involved quite good accuracy at closure rates of 7 km/s. I doubt that would have been considered possible a century, or perhaps thirty years ago. Of course, I don’t advocate spending actual money on the idea.
Ok, another wacky fusion propulsion idea for you John.
Say you have a standard LH2/LOX booster and you replace the LH2 with liquid deuterium. No fusion there right, the chamber temperatures are way too low. Well, there is actually a well known way to do D-D fusion at those temperatures and much much lower temperatures – by bombardment with muons. If you had a cheap light muon generator you’d be set, but alas, we’ve been waiting for one of those for 50 years..
But wait, muons are those nifty particles you might have heard of that are created in the upper atmosphere by cosmic radiation and are detected at 10,000 particles/m^2 at sea level and in caves, etc, even though they have a lifetime of only a few microseconds – thanks to the relativistic effect caused by their high velocity. Such high velocity muons can be made in cyclotrons.
So if you had a facility that could produce a high velocity, high density beam of muons, you could target them on your rocket chamber. This should be relatively easy as muons are charged particles – a beam of them can be targeted much like an electron beam. Also, they are about as small as an electron and pass through matter much more effectively than electrons, so getting them into the rocket chamber to cause D-D fusion would be free.
There are two hiccups though, making beams of muons is a 105.7 MeV operation, so you need a significant ground facility, and because you want the D-D fusions to happen on the gas side of the chamber you wont be able to use regenerative cooling, not that I’m sure you do that with hydrogen boosters anyway, but something to keep in mind.
Trent:
If the half-life of a muon is 1.56 microseconds, you’d also have to accelerate your muons to near light speed in order for relativistic time dilation to preserve them to their target. And then they’d have to be slowed down because they have to be slow-moving enough to become a bound muon in order to increase the nuclear reaction cross section.
Interestingly, it was work in muon-catalyzed fusion that led directly to the work of Jones et al in “cold” fusion. Now *there’s* a field ripe for theoretical work, if you happen to know any condensed matter guys who will touch it. 🙂
Oh hey, long time reader here.
Anyway, the fusion pellet stream thingie has been considered and developed before: its in The Starflight Handbook (don’t have my copy on hand at the moment so I can’t give study names and etc).
http://www.amazon.com/Starflight-Handbook-Pioneers-Interstellar-Editions/dp/0471619124
You might also be interested in pellet stream propulsion – I figure the two would probably mesh quite nicely.
I read the Starflight Handbook many years back. I’ll have to see if I still have it in storage. I vaguely remember it having pellet stream, but don’t remember fusion pellet stream. I hope I didn’t plagarize the thing by selective memory.
This is a neat idea John, very much like a crude or maybe the term is “low resolution” bussard ramjet. My only nit is that you somehow have to preposition the pellets which means instead of waiting for your low Isp spaceship to transit the long miles you have to wait for your pellet prepositioning ship to transit the long miles before it ever leave! Talk about waiting to get off the ground! Oh,and another problem is it doesn’t solve World Poverty. 😉
Of course it solves world poverty, the pellets are manufactured by the Soylent Green corporation. 🙂
I saw this being discussed on Nasaspaceflight.com, and it sounds like it has possibilities. No need for a massively expensive multi-terrawatt energy beaming facility makes it the most attractive.
Most of the legwork could be done by supplying the fuel pellets with small solar sails. The first lot could launched at the sun, where they gain velocity (say about 200 kps) and come straight back up the ramjet’s throat. More sails could be dispatched to varying distances to make up the “runway,” maybe a couple of hundred AU long.
Locating the pellets would be easy; shine a laser on the sails. Shine the laser a lot more and the ramjet could steer the pellets into its throat.
I read another (fairly detailed) paper about using fusion fuel pellet transfer with lightsails and a 2.5 TW laser. Isp was about 150 000 in a Z-pinch thrust chamber.