Comments on: Fusion Drive

By: Troy

Troy — Wed, 12 Jan 2011 12:53:08 +0000

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.

By: john hare

john hare — Thu, 04 Feb 2010 08:37:49 +0000

Of course it solves world poverty, the pellets are manufactured by the Soylent Green corporation.

By: Tom Cuddihy

Tom Cuddihy — Thu, 04 Feb 2010 01:09:44 +0000

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.

By: john hare

john hare — Tue, 02 Feb 2010 10:25:33 +0000

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.

By: derpderp

derpderp — Tue, 02 Feb 2010 02:54:33 +0000

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.

By: Danny

Danny — Mon, 01 Feb 2010 06:29:43 +0000

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.

By: Trent Waddington

Trent Waddington — Sun, 31 Jan 2010 13:03:55 +0000

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.

By: john hare

john hare — Sun, 31 Jan 2010 11:39:52 +0000

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.

By: john hare

john hare — Sat, 30 Jan 2010 10:26:50 +0000

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.

By: Eric Collins

Eric Collins — Sat, 30 Jan 2010 10:02:18 +0000

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.