Orion drive and related physics

[fredinno]

50 minutes ago, Bill Phil said:

Even with tests?

Theoretical means that it's described by a theory. This is fact. It'll work. We know explosions produce energy and it's possible to focus that energy. They used chemical explosives to test the pulsing, they blew up a sphere coated in graphite and only ax small portion ablated away ( it was near to a nuke).

The closest thing to tests we have did not use nukes, prototype pusher plate, shock absorbers,etc.

Until then, it's theoretical.

[damerell]

12 hours ago, lobe said:

Ever since I have looked into the vacuum I have noticed something with the Orion drive: what does it push against? In normal explosions/reactions chemicals form a gas to expand against a suface to propel forward. With a nuclear weapon in deep space they only have particles and radiation off the bomb, or am I missing something?

Much of the thread seems to have devolved into semantics, so very simply; when a bomb goes off, stuff goes backwards but not forwards (because the pusher plate is in the way). By conservation of momentum, the ship must go forwards.

A photon drive doesn't "push against" anything, but if photons go backwards, the drive must go the other way to conserve momentum.

[HebaruSan]

11 hours ago, shynung said:

Imgur mirror:

Does the blue shell casing part (for lack of a better phrase) shoot out away the ship, and if so, approximately how fast does it go? It almost seems like that part's role is more similar to conventional chemical "propellant" than the part listed as propellant in the diagram, which impacts the craft rather than being the equal and opposite reaction.

[sevenperforce]

3 minutes ago, HebaruSan said:

Does the blue shell casing part (for lack of a better phrase) shoot out away the ship, and if so, approximately how fast does it go? It almost seems like that part's role is more similar to conventional chemical "propellant" than the part listed as propellant in the diagram, which impacts the craft rather than being the equal and opposite reaction.

Rockets function based on equal and opposite forces. Fuel has to be burned inside a rocket and then expelled out at high speed; if fuel is ejected unburned and then burns in the exhaust stream, it does nothing for the rocket because it is independent and thus exerts no force. 

Since you can't very well blow up a nuke inside a combustion chamber, you can't get any benefit from the stuff that explodes in the opposite direction, like the blue part of the casing. Instead, you've got to depend on a shockwave to bounce off a pusher plate.

[HebaruSan]

10 minutes ago, sevenperforce said:

Rockets function based on equal and opposite forces. Fuel has to be burned inside a rocket and then expelled out at high speed; if fuel is ejected unburned and then burns in the exhaust stream, it does nothing for the rocket because it is independent and thus exerts no force. 

Since you can't very well blow up a nuke inside a combustion chamber, you can't get any benefit from the stuff that explodes in the opposite direction, like the blue part of the casing. Instead, you've got to depend on a shockwave to bounce off a pusher plate.

Well, suppose instead of releasing the nuke to float freely before detonation, we sent it out on a rail that could survive the explosion and held it there such that the shell casing part stayed stationary relative to the main part of the craft. Wouldn't we then have a rigid frame pushing against itself, for no net acceleration? If some part of the original mass of craft-plus-nuke-charges is going to go forwards, then some other part of it has to go backwards, right?

[sevenperforce]

2 minutes ago, HebaruSan said:

Well, suppose instead of releasing the nuke to float freely before detonation, we sent it out on a rail that could survive the explosion and held it there such that the shell casing part stayed stationary relative to the main part of the craft. Wouldn't we then have a rigid frame pushing against itself, for no net acceleration? If some part of the original mass of craft-plus-nuke-charges is going to go forwards, then some other part of it has to go backwards, right?

I bolded the problem part.

[HebaruSan]

Just now, sevenperforce said:

I bolded the problem part.

That part was just for the sake of argument. I realize no such device could be built.

[sevenperforce]

You could get better momentum exchange from an orion-type device if you built a large indestructible tube and detonated the bomb inside the tube. The tube would fill up with plasma and shoot out the open end; the sum forces on the inside of the tube would push you in the opposite direction.

[magnemoe]

7 minutes ago, HebaruSan said:

Well, suppose instead of releasing the nuke to float freely before detonation, we sent it out on a rail that could survive the explosion and held it there such that the shell casing part stayed stationary relative to the main part of the craft. Wouldn't we then have a rigid frame pushing against itself, for no net acceleration? If some part of the original mass of craft-plus-nuke-charges is going to go forwards, then some other part of it has to go backwards, right?

You shoot the nuke out with an air or coil gun, it does not need much speed, guess a few 100 km/h one important point about an orion is that you want the nuke to explode at the right time and distance just as you want ignition at the right time in an diesel engine the pulse plate is oscillating and you want to cancel the return of it and push it forward, if not you need an variable yield bomb down to half yield for an cold start. 

[sevenperforce]

In a conventional rocket, the converging-diverging nozzle is used to force the high-pressure fluid inside the combustion chamber to exit all along the axial direction at the highest possible speed. But since you can't very well do that with a nuke, you've got to ride the shockwave instead. 

[K^2]

6 hours ago, Bill Phil said:

A plain old nuke would actually be very inefficient. The shaped charge redirects a large portion of the nuclear energy.

"Shaped charge" is a misnomer. Nothing can actually redirect this quantity of energy. What it could do is absorb a lot of gamma and neutron flux that casing didn't and help convert a lot of that into kinetic energy of propellant. Gamma that wasn't directed at propellant is still going to be energy lost. You can't do anything about that.

I'll have to look up how much energy is actually consumed by the casing and other matter that used to make up the bomb before explosion. I have a feeling it will be a large enough fraction for it to not be anywhere near as inefficient as you claim.

[shynung]

1 hour ago, HebaruSan said:

Does the blue shell casing part (for lack of a better phrase) shoot out away the ship, and if so, approximately how fast does it go? It almost seems like that part's role is more similar to conventional chemical "propellant" than the part listed as propellant in the diagram, which impacts the craft rather than being the equal and opposite reaction.

I think the casing doesn't do anything other than holding the bomb together until it detonates. The propellant listed in the diagram actually is the propellant, in the sense that it transfers its kinetic energy to the ship. A chemical propellant does the same by expanding against the combustion chamber and nozzle bell.

[lobe]

After reading through the thread, it seems that the plasma from the propellant end is thrown at the pressure plate with sufficient energy to move the vehicle. Like getting knocked over by a hypersonic feather?

[SomeGuy123]

Fusion drives should do a ton better.  All that bomb casing, explosives, extra pusher plate material that will ablate - instead you just react puffs of the same fuel the fusion bomb uses (deuterium/tritium) or an aneutronic fuel (thrust/weight ratio advantages)

Since everything is controlled you'd expect a lot lower thrust but vastly more dV.

[Temstar]

2 hours ago, SomeGuy123 said:

Fusion drives should do a ton better.  All that bomb casing, explosives, extra pusher plate material that will ablate - instead you just react puffs of the same fuel the fusion bomb uses (deuterium/tritium) or an aneutronic fuel (thrust/weight ratio advantages)

Since everything is controlled you'd expect a lot lower thrust but vastly more dV.

There's the small problem of we don't know how to build a fusion reactor (and therefore fusion rocket engine). A fusion rocket will work very well, in theory. But it's much more theoretical than an Orion drive.

[damerell]

7 hours ago, Temstar said:

There's the small problem of we don't know how to build a fusion reactor (and therefore fusion rocket engine). A fusion rocket will work very well, in theory. But it's much more theoretical than an Orion drive.

To say nothing of the lower thrust. One of the attractive things about Orion is the combination of high TWR and high Isp; you can use it to get off planets as well as to go between them.

[B787_300]

46 minutes ago, damerell said:

To say nothing of the lower thrust. One of the attractive things about Orion is the combination of high TWR and high Isp; you can use it to get off planets as well as to go between them.

Yeah if you dont mind irradiating the planet you are taking off from. 

 

An Orion shapeship would work.  it is just a question of where you would want to use it. as you cannot use it on earth or withing parts of the magnetosphere because of the emp that a nuclear explosion creates. so now you have to get it out of earths gravitational well pretty far before you can use the nuclear pulsed propulsion.  (I believe that you have to get it about 3x as high as the ISS is) 

Also yes the actual mode of propulsion wiith nuclear bombs has not been tested but an explosion is an explosion and the old video where they used c4 to fly a smaller model proves that the idea works. (granted a nuclear explosion has more energy in it and has the wonderful side effect of radioactive wastes)

[damerell]

4 minutes ago, B787_300 said:

Yeah if you dont mind irradiating the planet you are taking off from.

You don't; the net global mortality from an Orion launch is of the order of one death. Or rather, it's politically unfeasible, but it's not medically or technically unfeasible.

[magnemoe]

2 minutes ago, damerell said:

You don't; the net global mortality from an Orion launch is of the order of one death. Or rather, it's politically unfeasible, but it's not medically or technically unfeasible.

An fail during launch would however crashing it down on earth, it would be an giant dirty bomb

[B787_300]

@damerellIf you loook at the studies they assume that the explosions are as clean as theoretically possible for the bombs.  If you include the initial ground bursts to get Orion off the ground you are going to be creating TONS of radioactive particles from the ground  (which they hand wave at  and say they are launching from uninhabited places that are away from everyone (like launching from Midway or bikini atolls).  The study was done in such a way to paint the best possible picture for the funding commitees that were going to fund orion. 

Also The study did not inculde a serious discussion of the EMP effects of the explosions.  Back in that day the only poeple who really used computers and electronics were the military.  IN todays world the effects would be disatrous

[Temstar]

1 minute ago, magnemoe said:

An fail during launch would however crashing it down on earth, it would be an giant dirty bomb

Not really, weapons grade plutonium-239 is not that dangerous. After all crew of ballistic missile submarines have to spend month living with large amounts of it in close proximity. It's only the fissile products that are really dangerous and there won't be much of those around should an Orion drive powered ship crash.

[Buster Charlie]

19 hours ago, K^2 said:

"Shaped charge" is a misnomer. Nothing can actually redirect this quantity of energy. What it could do is absorb a lot of gamma and neutron flux that casing didn't and help convert a lot of that into kinetic energy of propellant. Gamma that wasn't directed at propellant is still going to be energy lost. You can't do anything about that.

I'll have to look up how much energy is actually consumed by the casing and other matter that used to make up the bomb before explosion. I have a feeling it will be a large enough fraction for it to not be anywhere near as inefficient as you claim.

Actually I think shaped charge is 1000% the right word. In warheads,  the explosive itself isn't what penetrates the armor of armor tank, iirc it's something like a copper plate that is vaporized and propelled at high speeds in a controlled fashion.  So in essence, the explosive energy efficency is not important to the idea of a shaped charge, only that the explosive force is used to do work in a controlled fashion.  What made me feel this is a perfect analogy,  is the reading I did on why tungsten  is used, and what would happen if you used a different  material with the exact same setup.

 

In short, tungsten diffuses in a nice workable plasma wave, but other materials could be used that would project in a higher velocity concentrated beam. So in essense, the orion drive could be used to make a atomic bomb direct energy weapon.

 

So once again, the shaged charge is more about using the shock wave of an explosive to propell another object, than it is about utilizing more of the explosive energy.

 

Now I have seen breaching charges that use water filled casings to help reflect some of the explosive energy back into the wall or door being breached, but in that case you could argue the door or wall itself is the shaped charge projectile.

[magnemoe]

3 minutes ago, Temstar said:

Not really, weapons grade plutonium-239 is not that dangerous. After all crew of ballistic missile submarines have to spend month living with large amounts of it in close proximity. It's only the fissile products that are really dangerous and there won't be much of those around should an Orion drive powered ship crash.

True plutonium inside the bombs are not dangerous. if ship crashed down at supersonic speed its however an decent risk that some of the high explosives in the bombs explodes this would not be an atom bomb but would generate some fission and spray plutonium around.
An orbital burn on the other hand is safe here as an fail would not be catastrophic

[Temstar]

Just now, magnemoe said:

but would generate some fission and spray plutonium around

Some plutonium will be spread around by a misfire yes, but not very much and definitely no fission. Bombs are specifically designed to prevent misfire situations from assembling a critical mass. If the explosive lenses does not fire properly (some did not fire, or timing was wrong) what will happen is one or two subcritical spheres of plutonium will be shot out of the bomb casing.

[DerekL1963]

3 hours ago, Temstar said:

Not really, weapons grade plutonium-239 is not that dangerous. After all crew of ballistic missile submarines have to spend month living with large amounts of it in close proximity.

We spend months living near weapons grade materials that we're very carefully kept separate from.*   The material is sealed inside weapon cases, which are sealed inside the re-entry vehicles, which are sealed inside the missile, which are sealed inside the launch tube.  Plutonium spread around the environment after the crash of an Orion is an entirely different matter.
 

3 hours ago, Temstar said:

Some plutonium will be spread around by a misfire yes, but not very much and definitely no fission. Bombs are specifically designed to prevent misfire situations from assembling a critical mass. If the explosive lenses does not fire properly (some did not fire, or timing was wrong) what will happen is one or two subcritical spheres of plutonium will be shot out of the bomb casing.

That's not how it works.   If even one detonator fires, the whole explosive mass is going up.  By design this won't cause a full detonation (which requires all detonators to fire with very precise timing), but it will liquefy (and probably vaporize at least a portion) and disperse the core.

 

*  I'm a former SSBN crewman, and FWIW I worked with the birds.