# Bussard ramjet



## pham (Jun 14, 2006)

One thing I've always wondered - how would it avoid hitting an object in space? Or am I suffering from a fatal misunderstanding?


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## chrispenycate (Jun 14, 2006)

pham said:
			
		

> One thing I've always wondered - how would it avoid hitting an object in space? Or am I suffering from a fatal misunderstanding?


How big an object? A big one, you could  steer round it (a few light minutes should allow you to develope enough delta V. If we assume we're sweeping the area in front with a laser (in order to plasmolise the interstellar hydrogen, so it could be concentrated by the magnetic field), in all probability grains of dust and such would be vaporised, plasmolised and run into the funnel.
Which leaves bits small enough to be difficult to detect, but big enough (and made of something difficult enough to melt) not to be immediately vapourised. These would probably generate quite a jolt, but fortunately matter between stellar systems is likely to be very diffuse, and the statistical chances of hitting something are probably less than your aeroplane being hit by a meteorite (reasonable; but without going there, ou can't ever be sure of density of matter, just that it's sufficiently diffuse that a few light years of it don't diffuse starlight apreciably.


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## pham (Jun 14, 2006)

Hi Chris,  I imagine if you're travelling at a decent percentage of the speed of light, that might be quite a catastrophic jolt?  A couple of questions occur - what are we using as our equivalent of a crows nest; and do you know of any good resources on the www on Bussard ramjets, and other proposed interstellar propulsion systems?  cheers


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## chrispenycate (Jun 14, 2006)

Yes, a considerable jolt; to be avoided if possible. Perhaps you could detect the laser light reflected back from your scanning laser, and concentrate its power on anything solid, so as to almost guarantee vapourisation. Still, a gram of gas would apply a fair amount of momentum, and tend to push things off line.
We did a thread on space propulsion in the lounge
http://www.chronicles-network.com/forum/8336-methods-of-space-travel.html
and I could, if it would help, dig out the pm I did.

I assume youve Googled the Bussard ram jet? I got most of this stuff out of books and new scientists, and although I'd be happy to try and find references, I'm sure you'd be as effective as me.(Oh, now you've set me off on it I'll probably do it anyway, and I'll post anything I find)

A really big telescope would be difficult under constant accelleration, unless you built it all round (it could be done, particularly if you were spinning the ship for gravity (you couldn't expect more than a fraction of a g from the drive), and the magnetic funnel, far bigger than the ship, would protect it from particles, so it looks as if the cylindrical craft  is wearing one of those collars to stop dogs scratching themselves)


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## pham (Jun 15, 2006)

Hi Chris,

Yes I've Googled and found some stuff, none of it particularly long on technical details, but I probably dug no deeper than the first couple of pages - I was being lazy and hoping you had prior knowledge of something!

 I'll have a look at that thread, thanks. To be honest I find myself a little baffled by relativity, but I'm trying my best to come to grips with it!


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## chrispenycate (Jun 20, 2006)

The principal limitation of a reaction drive (the only type of drive we know how to build for spacecraft) is the law on conservation : mass times velocity is a constant. More even than energy limitations, the fact that we have to throw large quantities of mass backwards in order to accelerate yourself forwards is the major source of fuel problems. Of course, greater exhaust velocities demand less mass ; but even if we can eject the reaction mass at the speed of light (the absolute limit  without fundamentally modifying the universe, as our velocity approaches reasonable interstellar levels (something we’re nowhere near achieving yet even for small probes) the amount of reaction mass required limits us to one shot, multistage, single use vehicles. These are not adequate for interstellar exploration ; hardly even for emergencies when going to a known, preset destination. 

So, why don’t aircraft suffer from the same problems ? All their reaction mass is provided by the atmoshere through which they are flying (that same atmosphere that’s reducing their top speed with friction. And, in the sixties, a gentleman named Robert Bussard proposed the same solution for interstellar travel.  “But“ (I hear you say) “there is no atmosphere in space,“ Not quite true ;it’s very, very rarified (a better vacuum than we can make down here, 10-21  grams per cubic meter, and we need tons to make a reasonable propulsion mass) If we can collect all the gas in front over an area of about the size of a planet, the faster the vessel is travelling, the more fuel it’s collecting. Evidently, a scoop this big would be prohibitively massive, so instead, it's built out of magnetic fields, channelling and concentrating the gas, to be heated and re-ejected behind, at the greatest velocity we can manage: optimall as a beam of electromagnetic radiation (we're talking about the equivalent of a hydrogen bomb every half second or so, here - serious attitude. In the more likely case that the hydrogen couldn't be converted entirely into energy, the maximum velocity that could be attained by the vessel (after an infinite acceleration time) would be just slightly lower than  the ejection speed of the plamsolised reaction mass.

All control of this process, from the original collection to is final ejection would be by magnetic fields; and nit produced by a horseshoe magnet, either. Millions and milions of gauss; in fact, an unshielded human nervous system would probably burn out through induction if walking through it. (I see, suddenly, a repair robot, with no metallic parts and no electronics. Built largely out of glass, totally metal (or any other conductor) free his control system a mass of tiny mechanical cogs programmed by perforated tape, a nano Babbage engine: but I diverge) To generate this we propose a spiderweb of superconducting wire, carrying mega-ampères of current on hair thin conductors. Shaped by the forces that it generated, the funnel extends thousands of miles in front of and to the sides of the vessel, and, in concentrating the interstellar medium for use as fuel, it also prevents particles from hitting the vessel itself, reducing the radiation risk inside. In the absence of resistance, the current flows indefinitely, eliminating the need for continuous recharging (which is just as well, cause I don't know how to do it. And we will be losing current by induction to the incoming gas; oh well, minor technical problem)

If the interstellar gas isn't sufficiently ionised for the magnetic field to collect it all, we could excite it with a laser scanning in front, which would also vaporise any dust particles. Certainly, this would apply a slight reverse thrust, but at only a few megawatts, it's negligable.  

There are some drawbacks to the system. Firstly, it has to be moving fast to work at all, and slingshot manouvres and the like ( sending it throug a huge linear accelerator for example) are complicated by the planet sized mesh uf wire in front. Then there's the rather important problem of slowing down. The scoop must be ahead of the craft, at all times (not only that's where the gas is being forced in, but it's the main particle screening. Assuming that your drive unit can be reversed, (not obvious) the reaction mass is now being ejected through the heart of the collection scoop; no problem with a photon drive, but very dangerous with plasma. If the drive can't be reversed, as you dive into your destination system, you turn off propulsion, storing the incoming gas in tanks for your departure, and use the nteraction between your magnetic scoop and the star's natural magnetic field to slow you. (from .9 c. Tricky) (one of my earlier stories had a homecoming starship miscalculate the manouvre, due to unexpected sunspots. and plough a huge trough through the photosphere, revealing the hotter layers beneath. The vessel was evidently totally destroyed, but the daylight side of their home planet was sterilised, atmosphereric disturbances (mega hurricanes and the like), tidal waves and earthquakes put most of the other side back into the stone age; inconvinient)

The energy source for everything (drive, fields, laser and a tiny bit for life support an communications) should come from fusing the incoming gas; still, if you prefer antimatter, that'd work fine too (if you can figure out how to store it. Be careful round the outlet nozzles; whether photon or plasma based, these beams could disrupt a planet from the outside edge of the solar system, What's the fine for accidentally melting Bolivia? (a problem shared with any drive capable of taking a vessel un to interstellar speeds, even a solar sail)


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## pham (Jun 21, 2006)

Hi Chris,

That's great. Thanks. I managed to get hold of a copy of "Entering Space" by Robert Zubrin, which is going to cover the same sort of ground. Also there's a "starflight handbook" by Mallove and Matloff that looks quite good.

So these ramjets would be good weapons as well? I beleive there's a novel on which the shipboard computer murders someoene by forcing their shuttle into the outlet.


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## chrispenycate (Jun 21, 2006)

Good weapons? I hate to think of something like that as a "weapon" but yes, anything controling that amount of energy could do an impressive amount of destruction. Better trust the guy you've got driving it a lot; if he's got grudges, you're never going to be able to catch him. But then, anything driving a starship up to practical, interstellar velocities (in the universe drawn by todays fizzycysts) is going to involve similar energy levels; and, as the World Trade Center indicates, energy can invariably be used for ends other than those intended by its original harnesser.

I hereby apologise for the missed letters, spelling and grammatical errors; I posted it rough, without going over it as intently as I should. The real question is the level to aim it at ;I don't want to talk down to people, but going into equations would seem unwise (I didn't put _any _figures in; perhaps this was exaggerated?)
And I hope your book's got illustrations; it's a real _pig _trying to get some of those concepts over in words.


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## pham (Jun 21, 2006)

Yes, I must admit I'm no mathematician - this week I got a book from the library called "Relativity Demystified"..I thought it might be useful, but was amazed to find that even in a "demystified" form, the equations were chasing each other off every page.


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## dreamwalker (Jul 6, 2006)

chrispenycate said:
			
		

> Then there's the rather important problem of slowing down. The scoop must be ahead of the craft, at all times (not only that's where the gas is being forced in, but it's the main particle screening. Assuming that your drive unit can be reversed, (not obvious) the reaction mass is now being ejected through the heart of the collection scoop; no problem with a photon drive, but very dangerous with plasma.


With a plasma drive, why worry about reversing it? wouldn't you just turn your ship around and hope that the exhust vapories and/or expels debris from your direction of travel?
It probably be more effective than a magneitc scoop at doing that, and if you where still conserned about collecting reaction mass, you'd (simply?) reverse the direction of the funnel and your matter colecting bucket? hmmm
I'll stick with my FTL drives for the time being, far less complicated


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## j d worthington (Jul 6, 2006)

Try turning something that's moving with that inertial momentum. Takes a looooong loop, or you come apart (and not necessarily just at the seams). Reversal would be necessary to bleed that off so that the ship could be turned safely without either coming apart or making anything organic within into a rather nasty paste. Same goes for FTL, I'm afraid. No way to get around inertia save an "inertial drive", which is even more highly speculative than FTL itself.


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## dreamwalker (Jul 6, 2006)

j. d. worthington said:
			
		

> Try turning something that's moving with that inertial momentum. Takes a looooong loop, or you come apart (and not necessarily just at the seams). Reversal would be necessary to bleed that off so that the ship could be turned safely without either coming apart or making anything organic within into a rather nasty paste. Same goes for FTL, I'm afraid. No way to get around inertia save an "inertial drive", which is even more highly speculative than FTL itself.


Not necessary with FTL, depends on which drive you take, but you generally avoid problems with momentum, inertia, realitivity and reaction mass quanties. The ones i'm interested in generally work by manipulating space time, ie warp drives and such. As soon as we figure out gravity and negative energy, they'd be simpler to use than the aformetioned propultion system.


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## j d worthington (Jul 6, 2006)

dreamwalker said:
			
		

> Not necessary with FTL, depends on which drive you take, but you generally avoid problems with momentum, inertia, realitivity and reaction mass quanties. The ones i'm interested in generally work by manipulating space time, ie warp drives and such. As soon as we figure out gravity and negative energy, they'd be simpler to use than the aformetioned propultion system.


There's still the "internal inertia" that would effect the bodies inside the craft. Manipulation of the spacetime continua (or fabric thereof) might possibly avert this, though that's highly debatable. That would mean some serious disruption of said fabric. That energy would have to go somewhere, and effect something, most like. And the most vulnerable things would be the organic matter inside that tin can....


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