Project Orion

GCNRevenger · 2008-02-20 21:55:11

Even with a magnetic field to reduce the mass of the pusher plate, you still have a great deal of shock to the spacecraft necessitating a shock absorber mechanism. Also, you will still need the plate, since not all the bombs' products are rendered a plasma and you need to shield the craft from the radiation. How much will the magnet system weigh versus the reduction in plate mass?

Also, a <100m class Orion is not worth building except as an ultra-heavy launch vehicle: http://www.astronautix.com/lvs/oriturnv.htm
http://www.astronautix.com/lvs/orietary.htm
At a mere 2000-3000sec ISP, this performance could be easily matched if not exceeded by a GCNR engine (3000-5000sec ISP) or for long flights blown out of the water by a VASIMR engine (up to 10,000sec ISP). Crazy Bob's (Zubrin) NSWR "salt water" rocket (10,000sec?) would also be equal or better too I bet.

I doubt adding a magnetic field to supplement the pusher plate efficiency will help enough to justify the trouble of Orion except to loft gigantic payloads to orbit, which could be done cheaper/safer/easier with a SeaDragon sized chemical "ultra-HLLV" I bet. http://www.astronautix.com/lvs/searagon.htm

Furthermore, please please remember that Orion's fuel will still not come cheap; I reject the estimation that the pulse units will be "not be that bad," but from a programmatic standpoint where will Orion take us after we reach our destinations? The Tsiolkovsky equation, as so plainly illustrated by Zubrin's MarsDirect, that refueling for operations around and return from your destination at your destination yields huge payload dividends. There will be no bomb factories on Mars or the outer planets for quite some time! GCNR and VASIMR could both refuel with Hydrogen, and NSWR could partially refuel with plain old water.

Orion, for its elegance, is not the only game in town, and given its lengthy list of drawbacks, dooms it.

GCNRevenger · 2008-02-20 22:45:28

If the Orion concept really grips you, take a look into the Nuclear Salt Water Rocket concept that Zubrin came up with, and I bet you will like what you find. It is a lot like Orion, except uses a continuous reaction so there is no shock absorber needed and uses water instead of vaporized atomic bomb casings. It would also be far and away cheaper and easier to fly.

cIclops · 2008-02-21 02:35:10

A magnetic field can't replace a pusher plate. Only the relatively low energy ionized particles from the bomb will spiral around the field lines, most of the rest, including larger fragments, will pass straight through into the unprotected spacecraft.

Later designs used fusion microexplosions. Laser ignited deuterium-tritium pellets apparently can achieve an ISP of about a million. It's possible that these smaller explosions can use magnetic fields to focus the charged particles into an exhaust beam.

Colonizing the Stars

A useful starting point is the propellant tank, which contains a mixture of liquid deuterium and tritium. Fuel is pumped to a pellet-preparation system, where it is formed into a large number of small hollow spheres, resembling bubbles, each the size of a grain of sand. The pellets are frozen solid to aid in their handling and passed on to the injector.
The injector is a rapidly rotating device to accelerate pellets to about 10,000 feet per second and eject them into the engine thrust chamber. The high velocity allows a high frequency of fusion microexplosions, increasing the thrust of the engine. To do this, the injector is a small version of the rotary pellet launcher. It injects pellets at a rate of 500 per second.
Each pellet passes down a tube 300 feet long on its way to the thrust chamber. This allows its trajectory to be examined and corrected. The tube is equipped with optical sensors and lasers, which can be focused on one side or the other of the pellet. This blows off small quantities of its mass, producing a thrust which changes the pellet's direction of flight. In this manner the pellet is guided to the proper direction. At the same time its arrival time is predicted, so the laser can be primed to fire during the interval of 10 nanoseconds when the pellet will be passing the fusion point.
The thrust chamber consists of front and rear superconducting magnetic coils, mirrors, a pick-up coil, radiators, and structural tie bars. The superconducting coils produce strong magnetic fields to redirect the charged particles produced in the microexplosion, forcing them to stream out the back to produce thrust. These coils and other structural components are exposed to intense floods of X rays and neutrons. They can be protected from damage by having their exposed surfaces shielded with layers of beryllium and beryllium oxide. The coils and tie bars also must be cooled and for this there are layers of heat pipes inside the shields. These transport heat to the space radiators, where it is rejected at 2300°F. Insulation and recirculation of liquid helium then are enough to maintain the superconductivity of the coils.
The pellet enters the chamber and the laser fires. Its pulse is split into eight subpulses, which are directed onto eight mirrors mounted on the tie bars. The mirrors also are cooled with radiators. The subpulses converge on the pellet as it passes the fusion point, and about 25 percent of its mass undergoes fusion. About one-third of the energy released is in X rays and fast neutrons, the rest being in charged particles. These form a fusion fireball, a mass of intensely energetic plasma, which expands outward. The plasma soon encounters the magnetic field produced by the front and rear coils and pushes it outward, as if that field were a balloon into which air was being blown.
As it expands outward, the magnetic field is pushed past the pickup coil, which generates an induced voltage. The energy transferred to the pickup coil is about 5 percent of that generated in the microexplosion and it appears as electric energy. It is extracted from the pickup coil, transported down a transmission line, and dumped into an array of capacitors forward of the laser. The capacitors store enough energy to excite eight firings of the laser, so if one or a few pellets fail to undergo fusion, there still will be energy to fire the laser.
The plasma from the explosion is prevented from reaching any part of the thrust chamber structure by the magnetic fields. Instead, it is directed out through the rear coil. The thrust is 15,000 pounds; exhaust velocity is 30,000,000 feet per second, or 3 percent of the speed of light—a speed which is written as 3 psol.
The laser is a gas-filled cylinder 400 feet long by 20 feet in diameter. Passing through the central volume of the cylinder are heat pipes, which extend on either side of the cylinder to form the main starship radiator in the shape of two "wings." These run the length of the laser and operate at 2300°F, to maintain the laser at this temperature. Surrounding the laser are the elements of the electron-beam generator and its power supply fed from the capacitor bank. There is a small nuclear reactor to provide start-up power and to operate other onboard systems.
This is the starship engine.

GCNRevenger · 2008-02-21 08:55:15

A magnetic field can't replace a pusher plate. Only the relatively low energy ionized particles from the bomb will spiral around the field lines, most of the rest, including larger fragments, will pass straight through into the unprotected spacecraft.
Later designs used fusion microexplosions. Laser ignited deuterium-tritium pellets apparently can achieve an ISP of about a million. It's possible that these smaller explosions can use magnetic fields to focus the charged particles into an exhaust beam.

Sounds... complicated. Like, trillion dollar kind of complicated.

cIclops · 2008-02-21 09:20:10

A trillion dollars may be an underestimate, starships are not going to be cheap

Even a few trillion is not beyond possibility; US GDP is over $14 T and world GDP is over $50 T.

Terraformer · 2008-02-21 09:25:32

I didn't realise the USAs GDP made p that much of the worlds GDP!!

GCNRevenger · 2008-02-21 10:58:32

Many things are possible, few things are practical.

cIclops · 2008-02-21 12:11:02

A trillion dollars ain't what it used to be.

For perspective: Apollo took about 4% of the US federal budget during its peak year, today that would be $120 billion. A trillion dollar starship program would take just over 8 years if it were funded at the same level as Apollo. Throw in Europe, Japan, newly rich Russia and China, and a two trillion dollar program would be affordable over say 20 years. It would make a great alternative to building even more weapons.

Terraformer · 2008-03-08 08:36:54

Back to Earth-to-Orbit: The problem seems to be that if we make the radiation level acceptable to the public (by using fusion bombs), the existence of high-yeild, low-radiactive bombs that could be used in warfare would make it unacceptable.

Anyone looked at jumpboy11j's Berrylium-8 (think it's that) fission thread in Science and Technology?

GCNRevenger · 2008-03-08 16:22:18

Be-8 fission? Um, don't you mean fusion?

JoshNH4H · 2008-03-09 06:14:36

No, lithium 7 is hit with a neutron, to make lithium 8, which decays to beryllium 8, and releases 16 MeV, and then the beryllium 8 decays to 2 Heleum 4's. possibly as a 3rd layer on the fission-fusion bomb.

Terraformer · 2008-03-09 06:15:45

No, Fission into 2 He-4.

But I remember reading somewhere that firing a Neutron at Li-7 causes it to undergo fission into 1 Deutranium atom and 2 Tritium atoms.

JoshNH4H · 2008-03-09 06:16:17

maybe a fast neutron?

GCNRevenger · 2008-03-09 11:28:53

No, Fission into 2 He-4.
But I remember reading somewhere that firing a Neutron at Li-7 causes it to undergo fission into 1 Deutranium atom and 2 Tritium atoms.

Yes, I think this is what actually occurs. Thats why the military uses Li-7 to supply Tritium in hydrogen bomb designs.

Besides, where are you going to get a powerful enough neutron source to produce Be-8 from? The only one compact enough is, again, an atomic bomb.

JoshNH4H · 2008-03-09 11:42:30

see the nilfir thread in Sci. and Tech.

Terraformer · 2008-03-09 11:59:08

Radioactive waste? Doesn't that give off Neutrons?

JoshNH4H · 2008-03-09 12:06:10

Maybe, but who really wants to carry around radioactive waste? and that makes nilfir still reliant on uranium fission.

GCNRevenger · 2008-03-09 12:27:26

Radioactive waste? Doesn't that give off Neutrons?

Only a little. Because Be-8 decays so fast, you'd need to hit it with a huge dose of neutron radiation to make it undergo a chain reaction before it blows itself apart.

JoshNH4H · 2008-03-09 14:08:24

no, the reaction is
Li-7+N=> Li-8=> Be-8 + 16 MeV => He-4 (+ 16 MeV)

GCNRevenger · 2008-03-09 14:43:27

I'm pretty sure thats only one of the possible reactions, Lithium-7 does produce tritium upon neutron capture.

JoshNH4H · 2008-03-09 15:12:38

http://en.wikipedia.org/wiki/Thermonuclear_bomb
see tritium production: lithium 6 makes tritium, not lithium 7.

GCNRevenger · 2008-03-09 23:27:25

No that isn't true, look down the page on your link for references to "Castle Bravo," where Li-7 would pick up a neutron, but then decay back down to Li-6.

None of that matters though, since you can't make a Beryllium-based atomic bomb.

JoshNH4H · 2008-03-10 13:53:10

no, I always thought it would be used for power, it could do nothing more than to add heat to the reaction.

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