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How Fusion Propulsion Will Work

By k4gpb in MLP
Tue Mar 06, 2001 at 06:36:52 PM EST
Tags: Science (all tags)
Science

How Fusion Propulsion Will Work In order to go to Mars and other planets that are out of the reach of conventional rocket engines, NASA is developing several advanced propulsion systems, including harnessing the power of the sun. Fusion-powered spacecraft are designed to recreate the same types of high-temperature reactions that occur in the core of the sun. Using this type of propulsion system, a spacecraft could speed to Mars in just about three months. The building of a fusion-powered spacecraft would be the equivalent of developing a car on Earth that can travel twice as fast as any car with a fuel efficiency of 7,000 miles per gallon.


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How Fusion Propulsion Will Work | 33 comments (30 topical, 3 editorial, 0 hidden)
Besides being MLP, (5.00 / 1) (#3)
by bgalehouse on Mon Mar 05, 2001 at 04:06:26 PM EST

There is also the fact that the only real problems with nuclear drives are political. I believe it was Dyson (some decades ago) who came up with the idea of building a spacecraft with a payload at one end, a big plate at the other, and a combination shock absorber/launch tube connecting them.

Launch shaped nukes of the back, let the force accelerate the thing. There were actually miniature atmospheric flight tests run using conventional explosives.

It was even figured that the thing would be able to take off from the surface using the main drive - given a sufficiently remote launch site :-)

Fusion/Fission (none / 0) (#6)
by lmaniac on Mon Mar 05, 2001 at 04:37:18 PM EST

I believe you are confusing the use of Fusion for fuel (ie deuterium -> energy, water) vs Fission (Uranium -> big bang, nuclear waste). If I am mistaken please ignore this. Otherwise FYI

[ Parent ]
No real difference. (none / 0) (#9)
by bgalehouse on Mon Mar 05, 2001 at 05:59:53 PM EST

Both can be continuous or explosive, at least in theory. The practice of continuous fusion is much harder than the practice of continuous fission. So the Dyson idea would work just fine for both types of nuclear explosive. Assuming it ever got off the ground politically - just imagine the contraversy involved in getting a few hundred small atom bombs together to fuel the thing.

[ Parent ]
Pulsed Fusion (none / 0) (#19)
by Blarney on Tue Mar 06, 2001 at 01:15:58 AM EST

Shooting off a bunch of little bomblets would make the driving force pulsate.

Geez. Every minute or so, you'd be slammed back into your seat, then zero G again, keep it up for 3 months. yeah, they could probably build a Dyson-type spacecraft, but who'd want to ride in it?

[ Parent ]

Ever hear of shock absorbers? (none / 0) (#24)
by QuoteMstr on Tue Mar 06, 2001 at 07:16:49 AM EST

It might sound silly, but the laws of inertia still apply in space. I suppose you could mount the payload section on a giant spring or hydraulic cylinder and make that absorb the momentum. It still wouldn't be even, but it would be tolerable. I suspect the effect would be about as annoying as a ship's pitch and roll, and you'd grow used to it after time.

[ Parent ]
Orion drive (none / 0) (#28)
by sigwinch on Tue Mar 06, 2001 at 08:56:26 PM EST

The concept is called the Orion Drive. The bombs would be very small (as far as nukes go ;-) and would be dropped out very fast (about once a second). You'd probably want to put the passenger compartment on springs and use shock absorbers to smooth out the pulses.

The nice thing is that it scales nicely to very large ships. Wanna send an Iowa-class battleship to Mars? No problem with an Orion drive! The not-so-nice thing is the fallout from all the nukes.

--
I don't want the world, I just want your half.
[ Parent ]

very light on details... (4.00 / 1) (#4)
by ana on Mon Mar 05, 2001 at 04:24:56 PM EST

All of this stuff is theoretical, which is not a problem really, except that they don't really say that. Scaling up plasma machines is notoriously difficult; big machines seem to be able to develop whole new categories of instabilities not anticipated in the little ones. I'm not counting on this happening in my lifetime (and it'd be useful for the article to emphasize the history enough so that you don't, either).

Ana

Years go by; will I still be waiting
for somebody else to understand?
--Tori Amos

Can't really blame the author (5.00 / 2) (#5)
by weirdling on Mon Mar 05, 2001 at 04:36:12 PM EST

The original article was sketchy and of the same level of accuracy as would be expected from a 'fusion rockets for dummies' style article. They expect that with a fusion rocket, they can make Mars in just three months. With an Americium fission rocket, you can make it in as little as two weeks.
Of course, theoretically, fusion is more efficient, but fission is far more powerful at the current time. NASA's ongoing ignorance in the realm of nuclear fission is largely political, I realize, but there are several technologies available today that can make mars in four months, and provide the benefit of shielding and centripetal gravity for the trip.
If you want to rewrite this thing because people insist on more of a write-up, feel free to use these links.

I'm not doing this again; last time no one believed it.
americium (none / 0) (#7)
by alprazolam on Mon Mar 05, 2001 at 04:59:02 PM EST

problem with americium is people don't want rockets launched with radioactive isotopes...however it seems to me if you could have 'launch' occur from space, the effects of an explosion would be significantly less severe.

[ Parent ]
Of course (none / 0) (#8)
by weirdling on Mon Mar 05, 2001 at 05:23:15 PM EST

My fondest dream is an Americium rocket actually built in space. One of the big hurdles is building a breeder reactor in space, but this is doable. Then need to go see if we can locate some Uranium and/or Plutonium out there in the asteroid belt. Then make the Americium...

I'm not doing this again; last time no one believed it.
[ Parent ]
available today? (none / 0) (#10)
by delmoi on Mon Mar 05, 2001 at 06:08:36 PM EST

NASA's ongoing ignorance in the realm of nuclear fission is largely political, I realize, but there are several technologies available today

It seems like you have an intresting definition of 'available' and 'today'. Just beacuse these technologies are fesable dosn't mean you can go out an use them
--
"'argumentation' is not a word, idiot." -- thelizman
[ Parent ]
Of course you can't go out and use them (none / 0) (#12)
by weirdling on Mon Mar 05, 2001 at 06:34:45 PM EST

I take your point on the relative readiness of these designs, but in terms of a Mars shot, they are as ready as anything else; they would require the same amount of special-purpose development that any piece of Mars shot equipment would. That being said, compared to the fusion idea, it is much more readily available, with target launch times less than ten years if work started immediately, which is only a few years more than it would take to do something based on the Shuttle motor. It is a comparable 'ready today' in that almost all the research necessary to complete a simple thermo-nuclear rocket has been completed and all that lacks is actually designing and building one. The newer Particulate-Bed slow-neutron reactors are quite a bit further from use, and the Americium rocket is a theory, while the plasma-fission designs are pure conjecture, but all of these designs are more ready than fusion. So, yes, they aren't off-the-shelf, but they are far more ready than fusion...
I'm not doing this again; last time no one believed it.
[ Parent ]
Still several months for humans (none / 0) (#14)
by fsh on Mon Mar 05, 2001 at 08:11:20 PM EST

While the trip can be made in two weeks (or even less eventually), the rocket would have to accelerate between 10-20G's to do it. Humans can barely stand 9G's for short amounts of time, so we'll still be limited to a 2-4 month long trip (depending on the relative positions of Earth & Mars).
-fsh
[ Parent ]
Errr, 12-24 months (none / 0) (#15)
by fsh on Mon Mar 05, 2001 at 08:21:39 PM EST

Sorry, meant 12-24 months. Re: Zubrins' 'Case for Mars'
-fsh
[ Parent ]
True, but think of the possibilities (none / 0) (#23)
by QuoteMstr on Tue Mar 06, 2001 at 07:13:12 AM EST

Imagine how useful this would be for a permanant Martian facility! Sure, it would take a few months for humans to travel to mars, but it would be a huge bonus to be able to send supplies with a two-to-four-week travel time. That's faster than the Atlantic crossing in the 1700s! Besides, in theory, you can embed people in fluid or somesuch to absorb extremely high accelerations. With iv nourishment, a physically fit individual may be able to take a two-week trip with such a mechanism.

[ Parent ]
Yes, but... (none / 0) (#33)
by fluffy grue on Wed Mar 07, 2001 at 06:17:34 PM EST

...if everyone's in suspensory fluid, how do they get OUT when the trip's done? :) Not to mention that their skin would probably slough off in the meantime due to water retention.
--
"Is not a quine" is not a quine.
I have a master's degree in science!

[ Hug Your Trikuare ]
[ Parent ]

Actually, (none / 0) (#26)
by weirdling on Tue Mar 06, 2001 at 12:15:49 PM EST

3 Gs is about as much as a human can take for an extended period of time. However, 3 Gs works out to 28.6 M/s^2 acceleration. Over a day of 3 G acceleration, you would be going (consult HP) 2,488 kps.
Here is a website with backing mathematics that claims that only a 3km/s change is required to achieve an 8-month transfer orbit. That can be achieved by a 10.4 minute burn at half a G. Most of these nuclear rockets are designed to burn at low-G numbers for short times, up to 45 minutes. However, buring harder would result in the elliptical being wider, allowing one to hit Mars sooner. So, half a G for 45 minutes is ~13km/s.
Of course, the actual G number depends on the output of the rocket divided by the weight of the capsule, so it can be tuned to be pretty much whatever you want...

I'm not doing this again; last time no one believed it.
[ Parent ]
Question (none / 0) (#21)
by pwhysall on Tue Mar 06, 2001 at 06:43:24 AM EST

When you're travelling at 80KM/sec, how do you stop?
--
Peter
K5 Editors
I'm going to wager that the story keeps getting dumped because it is a steaming pile of badly formatted fool-meme.
CheeseBurgerBrown
[ Parent ]
Like this (none / 0) (#22)
by QuoteMstr on Tue Mar 06, 2001 at 07:10:02 AM EST

You wouldn't come to a sudden stop --- you'd probably thrust around 60% of the way to mars, stop thrusting, turn the craft around and begin thrusting the other way. You would use aerobraking in the martian atmosphere to absorb the rest of the momentum. One *very* nice effect of this would be that you would actually have gravity on the journey, depending on how much acceleration you used.

[ Parent ]
Skeptical (none / 0) (#11)
by a humble lich on Mon Mar 05, 2001 at 06:25:18 PM EST

I think we should first try to get sustained fusion reactions working on Earth before we think about putting them in space. The article says that the technology is about 25 years away--fusion has been about 25 years away since the fifties. I don't claim to be an expert in plasma physics and the article was rather terse, but I was surprised they they proposed a mirror containment. It was my impression that there were some serious problems with mirror fusion, which is one of the reasons tokemaks (sp?) are popular mow.

Fusion for space propulsion. (5.00 / 1) (#13)
by claudius on Mon Mar 05, 2001 at 07:57:01 PM EST

An advantage that fusion may have over other methods of space propulsion is that the design specifications are different for propulsion vs. power production. In a fusion power plant one wants to balance fusion power production with the power losses due to effects such as resistivity in the coils, imperfect particle confinement, and radiative losses. "Breakeven," or getting as much power out as one puts in, is a necessity in a practical fusion power plant. Propulsion, on the other hand, forces one to optimize different things--high outflow velocities and low mass, for example--so breakeven may not strictly be necessary. In short, there is no intrinsic reason why we couldn't see practical fusion propulsion long before fusion is realized as an energy source.

To address your question, mirror machines have historically suffered from instabilities that trigger rapid cross-field transport and reduced confinement. For space applications, where the plasma density could conceivably be lower, these instabilities may or may not be triggered. (I haven't done the calculation). Also, other confinement schemes, such as "reversed field configurations" and "field-reversed ion ring configurations," have mirror-like geometries in the external coils, yet plasma currents that make the overall magnetic geometries non-mirror-like. The stability and confinement properties of these devices are different and, in many cases, better than mirror machines.

A curious feature of fusion propulsion is the possibility of using exotic fusion reactions such as p + B11 -> 3 He2 + 8.7 MeV. This reaction has the advantage that one of the ingredients is present in copious amounts in the interstellar medium (you don't have to pack it in--just collect it in situ), and the products are charged particles (alphas) which can be steered out the back efficiently using the magnetic fields. (p + Li6 -> He4 + He3 may also be a good candidate reaction for the same reasons).

This is all speculation on my part--though I'm a plasma physicist by training, I am not a specialist in fusion or plasma rockets.

[ Parent ]
Problem with collecting in situ (none / 0) (#25)
by QuoteMstr on Tue Mar 06, 2001 at 07:20:31 AM EST

The problem with doing it that way is that you'd need a *huge* collector --- the interplanetary gas is too diffuse to collect much in a reasonably-sized area. If you're making something that big, you might as well make it a bit bigger and use a solar sail --- it'd be cheaper, anyway.

[ Parent ]
Magnetic ramscoops... (none / 0) (#29)
by locke baron on Wed Mar 07, 2001 at 06:31:24 AM EST

IANAPF (Plasma Physicist), but isn't most of the interplanetary dust and gas ionized anyway (by solar wind, cosmic rays)? And if it is, a magnetic field could be used to rake this matter in. (Yes, this comes straight out of Star Trek, but I can't think of any reason why this wouldn't work.). As a side benefit, the magnetic field would have some limited shielding effect against cosmic rays and solar wind, no? Not to mention, act as a solar sail...
Micro$oft uses Quake clannies to wage war on Iraq! - explodingheadboy
[ Parent ]
ok, but poor comparison (none / 0) (#17)
by nickp on Mon Mar 05, 2001 at 11:03:33 PM EST

You can't really compare space travel to travel on or very near earth. It's a lot easier to travel efficiently in space. No friction, you don't have to struggle with keeping yourself from falling down, getting over obstacles... and there's plenty of help from the sun (I think MIR already used sun-powered propulsion to keep itself in orbit), planet slingshot gravity effects to boost your speed, and other energy sources, a large proportion of which the earth's atmosphere would consume, preventing you from using them on earth.

"Gravitation cannot be held responsible for people falling in love." -- Albert Einstein

space travel is harder (5.00 / 1) (#31)
by esonik on Wed Mar 07, 2001 at 01:28:05 PM EST

It's a lot easier to travel efficiently in space.

No, it's harder because you have nothing to push against. You have to use the rocket principle which is very inefficient. Using solar wind or gravity sling shot is very time consuming



[ Parent ]
fusion idea not new (4.00 / 1) (#18)
by nickp on Mon Mar 05, 2001 at 11:24:35 PM EST

Well, the fusion idea is not new. The Russians had a classified project going on in the 50s, the idea being a creation of a massive fusion-based reactor contained by massive magnetic fields. Unfortunately, I can't remember the code name for this. It's something like "Tokomak". The idea completely fell through once it was clear that the energy spent on magnetic fields to hold the thing and to shield the reactor would outweigh by far the useful amount of energy such a reactor could produce. So just an idea is not going to cut it. We are not closer to achieving fusion power sources than we were in the 50s. Unless, of course, we could get cold fusion in tin soup cans.

"Gravitation cannot be held responsible for people falling in love." -- Albert Einstein

A tokamak is... (4.00 / 1) (#20)
by pwhysall on Tue Mar 06, 2001 at 06:36:39 AM EST

...briefly described here.
--
Peter
K5 Editors
I'm going to wager that the story keeps getting dumped because it is a steaming pile of badly formatted fool-meme.
CheeseBurgerBrown
[ Parent ]
Re: fusion idea not new (none / 0) (#27)
by a humble lich on Tue Mar 06, 2001 at 01:15:08 PM EST

Yes but ...
The US also had quite a fusion program back in the 50s which has continued to this day. The Tokamak idea is far from dead--in fact I believe here are two working tokamaks within two miles of me right now. Looking at the list of physics seminars from last weeks one of the is about modes in a D-III D Tokamak.
   Now it is true that funding for fusion research has gone way down, and it we will achive fusion power I doubt it will be the US that does it unless something changes. Whether we are any closer to achieving fusion power than the 50s ... Well in the 50s they said fusion was about 50 years away. Today we still think it is about 50 years away, but we have learned alot about how foolish we were.




[ Parent ]
Actually at 1G... (none / 0) (#30)
by cwalsh on Wed Mar 07, 2001 at 08:14:19 AM EST

the trip would only take 4 days 6 hours and 28 minutes (so, I was bored :)

This is taking into account the furthest distance apart Mars and the Earth can be (so, you'd technically have to go through the Sun for the trip to take this long :) and for having to decelerate halfway along the trip.

The only problem I'd see from this would be if you hit some micrometeorites head on. That would more than likely suck. (vacuum if you will. *groan*)

Colin

Don't forget the earth's velocity (none / 0) (#32)
by fsh on Wed Mar 07, 2001 at 02:30:40 PM EST

This calculation isn't completely accurate because you're not including the effects of the earth's velocity on the rocket. IE, the rocket would also be moving in the direction of the earth at a tangent to the earth's orbit, so a great deal of extra thrust would have to be used to counteract this effect. This is why most orbits are ellipses, anchored on one end by the earth, and at the other end, on the other side of the sun, anchored by Mars.
-fsh
[ Parent ]
How Fusion Propulsion Will Work | 33 comments (30 topical, 3 editorial, 0 hidden)
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