A Fusion Ramjet...

[Vega115]

Ok, I'm not sure if all of you get Discover magazine, but if you do, in the August 2003 issue (Can we rocket to another star?), it talks about the idea of a Fusion Ramjet.

Basically, it is an inverted cone, leading to a reactor, the cone being a couple ten thousand miles wide (if a i read correctly), and as the Fusion Ramjet goes through space, it picks up hydrogen particles which are funneled to the reactor, and by some combustion process (dont ask me how...) it shoots the Ramjet to near light speeds due to a lack of fuel tanks to weigh it down..so the article says. (a rough estimate by its creator: it would take 25 years to get to Alpha Centrui and 90 years to 55 Cancri (41 light years away). Kinda fast if you ask me. The pros of the Ramjet: near-light speed;unlimited interstellar travel in any direction

Cons?: requires major advances in physics and engineering knowledge.

Though one problem is drag. With fusion particles shooting out the back, the interstellar particles of hydrogen used as fuel start piling up in front of it, slowing it down. Also, at denser parts of the galaxy, the ship might grind to a stop, but Robert Zubrin (engineer who runs Pioneer Astronautics) suggested using a magnetic field as a brake to slow the ship down without wasting fuel.

The biggest problem? Deuterium and Tritium, used in modern experimental fusion reactors are rare in space. Interstellar hydrogen is the regular variety with a single proton. And no one has a clue how to fuse together pure hydrogen. But we do know it happens everyday: The sun.


So, do you guys think it is possible to use a fusion ramjet to travel between stars if we make the right advances in physics and engineering? :-?

[tracer]

What that article is describing is not a new idea.

It's called a "Bussard Ramjet" or Ramscoop. Unfortunately, according to calculations done shortly after Bussard came up with the idea, it turns out the scoop in front will generate more drag from the interstellar hydrogen it rams into than the amount of thrust it could possibly produce by subjecting all of the hydrogen it collects to nuclear fusion. I'm surprised the Discover article only mentioned the drag problem as a minor drawback, rather than the insurmountable show-stopper it actually is.

[Musashi]

What about a magnetic scoop? Or is that the Bussard Ram as well. It has been a while since I read anything on ramjets....

[Kaptain K]

Magnetic scoop=Bussard ramjet! 8)

[Musashi]

Thanks, I wasn't sure.

[daver]

Bussard seems to have invented lots of things that sounded good on paper but didn't work out so well in practice.

As i recall, he came up with the idea of the ramjet by noting that at relativistic velocities your starship is going to have to find some way of protecting itself from the impact of interstellar hydrogen. So he came up with the bright idea of, instead of diverting it, collecting it and fusing it to produce propulsion for your starship. Simple, elegant.

Unfortunately, we can't do 4H fusion. If we could do H+H+H+H->4He fusion at 100% efficiency, there would be enough energy to accelerate the helium ash to about 11%c (i haven't run the numbers for a long time, but that's what i remember).

Now, the "obvious" scenario for running the starship is to beam a big laser ahead of your starship to ionize the interstellar hydrogen, and then use a huge magnet to collect the hydrogen for fusion. Unfortunately, as you near 11%c, the drag of the hydrogen on your ramscoop is going to approach the thrust of the helium from your reactor, and your ship stops accelerating.

However, all is not lost. As long as we don't know how to achieve 4H fusion anyway, we can pretend that our ramscoop can collect and fuse the hydrogen in place (hey, if you're going to dream, dream big). So we've replaced the interstellar ramjet with something like an interstellar scramjet. And as long as we're wishing, we may as well do away with the ionization laser, and have the scramscoop do all the dirty work.

But it turns out that the sun is in the middle of a vast desert, interstellar-hydrogen-wise.--the local medium is about 0.1 atom/cc, or roughly 1e-13 kg/km**3. At .001 c, a 1 km**2 ramscoop will gather 3e-11 kg of H per second. Even at 3e7 meters/sec exhaust velocity, this is only going to give 1 milli-Newton thrust. So we need 10,000 km**2 ramscoop per kilogram starship for 1 g acceleration. If your starship massed as much as a nuclear sub, your scramscoop radius is going to be on the order of a million km. Note that the scramscoop has to be essentially massless.

There is nothing here that is blatantly impossible, but it looks less likely than the Infinite Improbability Drive.

[pmcolt]

On the other hand, it does make a handy brake, if we can just figure out a good way of getting up to speed to begin with. And travel through normal space saves you from all the improbable things that happen while the Infinite Improbability Drive is running. Not to mention the feeling of being drunk that you get from hyperspace travel...

[mike alexander]

>Bussard seems to have invented lots of things that sounded good on paper but didn't work out so well in practice. <

All I could think about was the joke about the horse breeder who wanted to develop a thoroughbred that could run the mile in one minute.

The story can get as shaggy as you want, but finally he hires a theoretical physicist. The physicist comes back the next day and tells him it should be simple to do, take about a week and cost about $500.

"How can that be possible?" the breeder asks.

"Well," the physicist replies, "You just have to imagine the horse as a frictionless rolling sphere..."

[eburacum45]

I would use Bussard ramscoops for material collection within a solar system, and for interstellar braking at the end of a voyage;
and perhaps they could be used as fuel collectors for antimatter-fusion hybrid spacecraft, particularly in parts of the galaxy that have greater interstellar densities of gas and dust.
But the old concept of the ramjet would not work as such.

[weatherc]

...Not to mention the feeling of being drunk that you get from hyperspace travel...

[obvious set-up for HHGTTG fans]What's so bad about feeling drunk?[/obvious set-up for HHGTTG fans]

[Colt]

-Colt

[wedgebert]

I would use Bussard ramscoops for material collection within a solar system, and for interstellar braking at the end of a voyage;
and perhaps they could be used as fuel collectors for antimatter-fusion hybrid spacecraft, particularly in parts of the galaxy that have greater interstellar densities of gas and dust.
But the old concept of the ramjet would not work as such.

From what I understand, there isn't enough matter in space to justify a bussard ramscoop. You could fly though a nebula all day long and the amount of matter you collect would be minimal.

For one, most interstellar is electrically and magenetically neutral, so you can't use EM fields to collect it unless you find a way to give the atoms a charge.

Also , the drag induced isn't going to have much of an effect for interstellar braking, you'd be better off deploying solar sails.

Finally, most interstellar material isn't the kind you'd use a fuel anyways. If we're travelling across the universe with fusion powered starships, we'll probably be using Deuterium - Helium-3 Fusion and would have no use for normal hydrogen or helium.

I'm kinda curious why you'd want an antimatter/fusion hybrid. Antimatter provides vast amounts more power than fusion, so much that if we had a way to produce antimatter in enough quantities, it would make fusion look as appealing as coal.

However, I doubt we'll do much with antimatter for a long time. It takes many times more energy to make than you can get out of it (antimatter is more of a battery than a fuel), is extremely dangerous to handle, and we'll probably use most of what we make as weapons

[Chip]

On the other hand, it does make a handy brake, if we can just figure out a good way of getting up to speed to begin with...

Maybe we could build a huge solar sail, powered by a distant Earth vicinity based laser. Then, as we began to approach Alpha Centauri, we inverted the sail (like an umbrella turned wrong way round in the wind,) and it becomes a sort of a ram scoop-brake as Zubrin suggests.

I think the biggest theoretical pluses to the ram scoop and solar sail designs are founded in not having to take your fuel with you, thereby saving enormous weight.

[eburacum45]

I am desperately trying to find uses for Dr Bussard's mostly useless ramscoop; braking might be the most efficient it gets.
Anti-matter fusion uses much less antimatter than straight matter/antimatter reactions
http://ffden-2.phys.uaf.edu/213.web....ionfusion.html
http://ffden-2.phys.uaf.edu/213.web....crofusion.html
...
at interstellar speeds you do encounter a lot of matter, but to charge it would need a high powered forward pointing laser- this decreases the efficiency of the drive for acceleration, but not for braking; also the amat/fusion flame could be pointing forwards until the ship slows down so much that the interstellar matter is not worth collecting any more.
Then you fill the magnetic field up with charged dust and it acts as a solar sail for more braking
This is the same magnetic solar sail that you launched the ship with in the first place.

[mike alexander]

What's so bad about feeling drunk?

Okay, I'll go.:

You ask a glass of water.

[wedgebert]

I am desperately trying to find uses for Dr Bussard's mostly useless ramscoop; braking might be the most efficient it gets.
Anti-matter fusion uses much less antimatter than straight matter/antimatter reactions
http://ffden-2.phys.uaf.edu/213.web....ionfusion.html
http://ffden-2.phys.uaf.edu/213.web....crofusion.html
...
at interstellar speeds you do encounter a lot of matter, but to charge it would need a high powered forward pointing laser- this decreases the efficiency of the drive for acceleration, but not for braking; also the amat/fusion flame could be pointing forwards until the ship slows down so much that the interstellar matter is not worth collecting any more.
Then you fill the magnetic field up with charged dust and it acts as a solar sail for more braking
This is the same magnetic solar sail that you launched the ship with in the first place.

It might use less antimatter, but it produces a lot less power or delta-v, and you might as well just use straight fusion, especially Deuterium - Helium3 fusion.

Also if your antimatter/fusion flame is pointing forwards, there's not going to be any interstellar dust for you to collect. The exhaust from your engine is going to blow any that comes near out of your path, thus reducing the drag.

Even at interstellar speeds, the amount of matter you encounter is very low, on the order of 1 atom per cubic meter of space.

Finally, we're not even sure if it's possible to build a laser capable of ionizing the hydrogen and have it use less power than your fusion plant generates.

[eburacum45]

That's right; a long cone of space in front of the ship would be swept free of dust, assuming a fusion motor placed centrally facing directly forward.

So unless we can come up with a geometry which allows the exhaust of the ship to actively aid the collection of interstellar dust and gas, or even use the drive energy as a method to ionise the dust - none of which sounds plausible, but I'll look into it...

you may as well just put the magsail out as a brake by itself- it might not produce much braking, but it is better than nothing.

A damn shame- the bussard ramscoop could collect a lot of mass at 1 molecule per m^3 over a distance of light years, depending on the cross-section of the scoop; I wonder if it might be better to store it somehow.

Antimatter fusion hybrid might be the first practical form of interstellar drive, using stored fuel rather than collected; full antimatter/matter reactions produce a lot of gamma rays which are difficult to extract thrust from directly, without a gamma ray reflector material.

[eburacum45]

The closest to a feasible ramscoop design I have found on the web so far is here
http://scientium.com/diagon_alley/co...ip/rockets.htm
mention is made of antimatter catalysis, as descibed earlier-
but the amount of matter scooped by a ship travelling at 33% c with a 1000km wide scoop will be about 350 tonnes per year...

it does seem a lot of effort to get little return.

(some other quite good designs on that page too)