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> Arguably we've had "fusion drive" for spaceships ever since Project Orion. Very simply, build a ship with a big shield in the back, throw nuclear bombs out be

> Arguably we've had "fusion drive" for spaceships ever since Project Orion. Very simply, build a ship with a big shield in the back, throw nuclear bombs out behind the ship, detonate, the shock and radiation hits the plate (some ablates, but it's still fairly study and safe), and some kind of shock absorber system pushes the spaceship forward. Repeat. You can feasibly get to 0.10c with this, using 1960s technology, and really big ships. I never understood how the blast alone can propel the ship. Don't you also need to eject mass from a nozzle to move forward - in which case, it's just like a regular jet, you just replace part of the fuel with nukes?

cma 14y ago
Part of the mass in the fuel in the bomb is converted to photons; photons are massless but carry momentum.
- hcarvalhoalves 14y ago
  I would never have thought about that. That just this is sufficient to push the ship forward is amazing.
  - zalzane 14y ago
    There's also the fissile material involved, but I couldnt tell you what proportion of the energy is disappated through heat and what proportion is dissapated in light. Everyone has seen the videos of the atomic bomb going off - that massive release of heat and energy that literally vaporizes mass that is close to the epicenter. Now imagine that reaction in space where the heat can't be disappated through atmosphere, water, or earth. All of that energy has to go somewhere, so it's all stored in whatever fissile material is left after the bomb goes off and dissapated through light and radiation. I really wish I had some numbers for how hot that remaining material would get - even thinking about all the heat released in an atomic bomb concentrated into such a small amount of mass absolutely boggles the mind.
Groxx 14y ago
Nozzles are basically just a more efficient version of this, since they don't waste as much energy which would escape out the sides (it's instead contributing to internal pressure, which pushes everything out the open end more quickly). Burning rocket fuel expands it rapidly, which is essentially all a bomb does. The downside to a nozzle is that you have to keep the internal pressure low enough that it doesn't rupture. This is far less of a problem with an Orion-style ship. Think of an explosion. They produce moving walls of compressed air that flings stuff around. Though this is just because it's in air, it's a lot of energy. The same amount of energy is still being released if you blow it up anywhere else. (ignoring bonus 'free' energy from e.g. burning the air itself) Take the explosion out of air, and you still have a 'shock wave' from the material which made up the bomb, and electrons / photons / whatever else materialized from that much energy being released in a tiny space. And it's all moving very quickly. When it hits your ship's back-end, it gives it a kick. Obviously far less than the total energy released (whatever % of the sphere of expanding energy which your ship covers), but still something. If the explosion is big enough, or close enough, you are still talking an immense 'kick'. And since you have no relatively-fragile nozzle (just an arbitrarily big, thick wall), the force can be many many times greater than a normal rocket. So to make it survivable, you probably need a big ship and some kind of impact-absorber to take the shock out of it, and / or lots of small bombs that won't kill everyone due to acceleration. Or just get rid of the squishy humans and crank up the proximity / bomb size to whatever you feel like.

cma 14y ago

Part of the mass in the fuel in the bomb is converted to photons; photons are massless but carry momentum.

hcarvalhoalves 14y ago
I would never have thought about that. That just this is sufficient to push the ship forward is amazing.
- zalzane 14y ago
  There's also the fissile material involved, but I couldnt tell you what proportion of the energy is disappated through heat and what proportion is dissapated in light. Everyone has seen the videos of the atomic bomb going off - that massive release of heat and energy that literally vaporizes mass that is close to the epicenter. Now imagine that reaction in space where the heat can't be disappated through atmosphere, water, or earth. All of that energy has to go somewhere, so it's all stored in whatever fissile material is left after the bomb goes off and dissapated through light and radiation. I really wish I had some numbers for how hot that remaining material would get - even thinking about all the heat released in an atomic bomb concentrated into such a small amount of mass absolutely boggles the mind.

Groxx 14y ago

Nozzles are basically just a more efficient version of this, since they don't waste as much energy which would escape out the sides (it's instead contributing to internal pressure, which pushes everything out the open end more quickly). Burning rocket fuel expands it rapidly, which is essentially all a bomb does. The downside to a nozzle is that you have to keep the internal pressure low enough that it doesn't rupture. This is far less of a problem with an Orion-style ship. Think of an explosion. They produce moving walls of compressed air that flings stuff around. Though this is just because it's in air, it's a lot of energy. The same amount of energy is still being released if you blow it up anywhere else. (ignoring bonus 'free' energy from e.g. burning the air itself) Take the explosion out of air, and you still have a 'shock wave' from the material which made up the bomb, and electrons / photons / whatever else materialized from that much energy being released in a tiny space. And it's all moving very quickly. When it hits your ship's back-end, it gives it a kick. Obviously far less than the total energy released (whatever % of the sphere of expanding energy which your ship covers), but still something. If the explosion is big enough, or close enough, you are still talking an immense 'kick'. And since you have no relatively-fragile nozzle (just an arbitrarily big, thick wall), the force can be many many times greater than a normal rocket. So to make it survivable, you probably need a big ship and some kind of impact-absorber to take the shock out of it, and / or lots of small bombs that won't kill everyone due to acceleration. Or just get rid of the squishy humans and crank up the proximity / bomb size to whatever you feel like.