Nuclear Propulsion - The best way for space travel

soph · 2003-02-22 20:36:56

I see two weaknesses: if they get too big, debris become a major vulnerability, and secondly, for very power intense missions (say a belt mining ship or science ship), the power requirements would be too high!

Also, fusion ships would make better launch vehicles from Earth, and I think launch vehicles, like SSTOs, will always be necessary. So, I think each method will have their applications, as it should be!

Besides, with private space access, you'll see every variation of every ship design possible!

Josh Cryer · 2003-02-22 21:16:48

Actually, no, their strength is that they don't break down as they get bigger. They're more robust. All they are is helium and some exotic particle. Regular solar sails have to be made of reflective material which will break down, over time.

This ship could go through a friggin meteor field without having any issues at all. You couldn't say the same for a reflective, foil-based solar sail which was a couple of thousand miles across (assuming you could even build one!).

Fusion would undoubtedly be good, but for cheap and simple transportation, Plasma Sails would be a million times better. Fusion still has the problem of being complicated.

soph · 2003-02-22 21:25:56

Well, Josh-your crew module might have an issue when it sees its plasma sail flying away as the module is being battered to a pulp in that meteor field!

Josh Cryer · 2003-02-22 21:33:52

The Plasma Sail wouldn't fly away... you have to comprehend how vast space is, man. Space is quite... well... empty. That's why we call it space!

The meteor fields of most movies are non-existant. You only find those sorts of compact fields right after an impact or something. And there aren't any like that in our solar system.

No, degradation is all but non-existant in a Plasma Sail. Fusion is what we'd be using to get out of the solar system. And even then, for mass considerations, we'd use a Plasma Sail to get us moving!

soph · 2003-02-22 21:36:42

Eh, first of all, fusion could get you more bang for your buck-it would be a better launch vehicle, with more thrust. You also have the issue of momentum, which allows rockets to speed up to three times their exhaust velocity. Plasma sails, as I understand, have a finite speed, whereas you can manipulate your speed, and fusion can be made much more efficient than the 5,000 seconds isp figures I've been using as estimations.

I was talking about the physical spacecraft being pummeled, not the sails. It was a joke!

Josh Cryer · 2003-02-22 22:30:23

Oh, well, Plasma Sails can have 30 million+ Isps...

Heh, if you had a Plasma Sail which was a thousand kilometers across, you would have some major pushing power.

With a plasma sail, you could get up to some 100-200km/s, turn of the sail (thus droping the mass which the sail is made of), and turn on your nuclear reactors. Not sure if it would be beneficial, though, but from a carry-on fuel perspective, I'm sure it would save a significant ammount of mass.

soph · 2003-02-23 08:43:07

100 km/s-200 km/s? I think that is rather slow-of course the isp figures are impressive.

What I meant for isp being 5,000+, i was thinking along the lines of exhaust velocity, which I don't think plasma sails have. A higher exhaust velocit=more payload.

A higher end fusion drive ship could probably get payload masses of over 60%, if the structural materials were able to support it.

dicktice · 2003-02-23 11:11:27

Face it, Soph...plasma sails, as canvas sails were and probably will be again on Earth, will (if they turn out to be viable that is...ditto fusion) the "workhorse" means of transporting goods around the Solar System, in straight lines...above and below and everywhere in-between...the Ecliptic Plane. Your fixation with fusion-powered space drives (I'm afraid it's all-to-obvious that's what you have) seems to be preventing a meaningful discussion of plasma sail technology. The mere fact of requiring no reaction mass, and providing continuous as well as constant thrust with distance from the Sun, should be argument enough!

soph · 2003-02-23 11:51:08

dicktice, read me the topic again?

fusion is nuclear!

But if you read what I said, I think all forms of propulsion will be applied, especially with private access to space.

There's a reason canvas sails aren't used for transporting, while fossil fuel combustion and nuclear reactors are, dicktice.

I'm not saying plasma sails aren't viable-I'm saying that fusion also has its benefits, and each system will eventually be used.

Josh Cryer · 2003-02-23 15:50:13

Well, the 100-200km/s figure is actually probably off. Especially if we can get a Plasma Sail with 10-20% opacity, which is a few thousand miles across.

I am not about to do the math (I'll get it wrong, I'm certain), but fusion may not even be able to compete with a Plasma Sail!

soph · 2003-02-23 16:23:40

Upon doing some research, a fusion thruster could have an isp of 1,000,000 seconds. While this isn't the 30+ million of plasma sails, this means that it has an exhaust velocity of nearly 10,000,000 m/s. This means that less than 1% of your mass is fuel.

So, I think that fusion can compete with plasma sails. For a few reasons-as a launch vehicle, plasma sails aren't viable (surface to air), and we will always need launch vehicles. Secondly, spaceplanes can work on fusion as well.

Thirdly, fusion can provide a power source for the ship (yes, i know plasma sails can, but not nearly as much as fusion). You have more energy to send back more data (more power=more and faster data transferred).

Here is a site that describes all of the propulsion systems we are talking about, the benefits and disadvantages of each!

soph · 2003-02-23 17:30:57

oh, and right, a vehicle can get up to 2x its exhaust velocity (3x if it spends a lot of fuel). This means our fusion ship can go over 18,000 km/s, quite a bit faster than a plasma sail.

edit: in Entering Space, Zubrin doubles the exhaust velocity numbers, meaning a possible speed of 40,000 km/s, or about 13% light speed.

Preston · 2003-02-24 15:37:04

Isn't a plasma sail a magnetic field from a superconductor, that rides the solar wind? Either that a plasma in a bottle that makes the field, refresh my memory. That takes power, it isn't a power source.

There is no exhaust, so there can be no specific impulse. So what is "30 million"?

Fusion as a launch vehicle means an orion-type pulsed system (with DT, so there will be neutrons). Not that that is necessarily a problem.

soph · 2003-02-24 16:05:37

Preston, you could have a Tokamak design, where there are no bombs. I know what you're talking about, but it doesn't necessarily have to be done that way. Just as we have NTRs and Orion, we can have a magnetic confinement or plasma focus based fusion drive.

A danger with plasma sails is that the magnetic field itself could pose a radiation danger to the crew and cargo, and background radiation would make the structure less efficient.

Josh Cryer · 2003-02-24 16:20:55

Preston, no, actually, a magsail or a plasma sail need gas to inflate the magnetic bubble (unless we're talking Zubrin's design for a magsail, but we're not). A solar sail is said to have an infinite ISP because it doesn't carry fuel, but magsails or plasma sails do carry fuel- in this case helium and some exotic particle which suspends within the bubble.

Read more about the whole thing here:
http://www.newmars.com/cgi-bin/wiki.cgi?Dusty-M2P2
http://www.newmars.com/cgi-bin/wiki.cgi?M2P2
http://www.newmars.com/cgi-bin/wiki.cgi?MagSails

I really need to get back on building my Wiki database, but I've just considered myself too busy...

Josh Cryer · 2003-02-24 16:23:13

And I don't see how the magnetic field would pose a danger to the crew. It would be as soft as Earth's magnetic field, for the most part. And since I don't think it would need to modulate, it wouldn't pose the same sorts of risk we suspect are around high voltage powerlines (I think).

soph · 2003-02-24 16:25:11

Source

"Radiation

The magnetic field of the MagSail may generate local Van Allen-type radiation belts. These belts may pose a significant radiation hazard for payload or crew in the vicinity of the MagSail, though not at the geometric center of the MagSail hoop. The background solar wind and cosmic-ray radiation may also induce long-term cumulative radiation damage in the superconducting hoop, degrading the superconducting properties of the material."

Josh Cryer · 2003-02-24 16:35:10

Right, but M2P2 doesn't use a loop, and it's much much bigger. Van Allen-type radiation belts aren't caused by the magnetosphere, they're caused by radiation from the sun hittting the magntosphere, or so I thought.

soph · 2003-02-24 16:44:08

But a weaker magnetic field also means more exposure to background radiiation.

It would make sense to surround your ship with a substance like water, which serves as an effective radiation shield.

Josh Cryer · 2003-02-24 16:48:43

Magnetosphers are much stronger than mere magnetic fields. The plasma within it could serve as a better radiation scatterer, etc. Really, the negatives for using a plasma sail are really small.

Low inital thrust is probably the only real issue, and that's assuming we can't make plasma sails which are very big, and perhaps even reflective. Big reflective plasma sails would have major thrust.

Preston · 2003-02-24 16:49:19

No magnetic containment scenario for fusion will give you enough thrust, the power is too low (even if they press it to several GWs). High Isp, but not thrust. Magnetic rockets are for high Isp, and pulsed rockets are better for thrust (but can still have a very high Isp, ~10^5 seconds in efficient mode).

soph · 2003-02-24 16:59:35

Your site estimates using magnetic sails, you can get to Mars in just over a month. Using Zubrin's figures, you could get to Mars in just over a day. Using my figures, just over 2.

Not a downside for plasma sails, just an upside for fusion

soph · 2003-02-24 17:00:57

Preston:

If we use orbital launch, which will probably feasible at the time of fusion, thrust doesn't matter much. Alternatively, we could use a large vehicle like the Saturn V, this shouldn't be an issue, either!

Josh Cryer · 2003-02-24 17:26:17

Source? How much fuel would it take to get there in a day?

soph · 2003-02-24 17:42:28

Zubrin said an exhaust velocity of 20,000 km/s is possible, I cited that already (_Entering Space_). This means you can get a speed of at least 40,000 km/s.

Mars is 1.52 AU from the Sun, 67.6 million milles from Earth. Here's my math:

40,000x60 (seconds)= 2.4 km/minute

2.4 million x 60 Hrm, my math is flawed. You can get to Mars in under a minute, just about 30 seconds in fact (this speed is 13% of the speed of light-take it to Zubrin if you don't like my numbers)!

This would require a great deal of acceleration.

So, let's assume it gets it's orbital velocity. 20,000 km/s, this is just over 1 minute.

Now let's take my original estimation of exhaust velocity-10,000 km/s. You still get there in just over 2 minutes.

Now, let's say you get Preston's estimate, 10^5, or an isp of 100,000 seconds. Your exhaust velocity is now 980 km/s.

The fast way (2x exhaust):

1960X60= 117,600 km/minute
ansX60 minutes=7 million km/h
67.6 million/ans= 9 hours, 30 minutes to Mars.

The slow way:
58,800 km/minute
3,528,000 km/h
19.1 hours->Mars

Your propellant mass depends on the thrust you are trying to achieve. But the initial launch mass (to orbit) is 1% with 10^6 isp, 9% with 10^5 isp.

Your propellant mass would be fairly small, because you are getting so much efficiency out of your propellant.

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Re: Nuclear Propulsion - The best way for space travel

#127 2003-02-22 21:16:48

Re: Nuclear Propulsion - The best way for space travel

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