Project Orion-Like Space Vehicle

[Xforce]

TL;DR Summary

The Orion project was available with current technology... however carry thousands of nukes on board might not be a great idea

Nations prohibit the detonation of nukes in space, but I don’t know there is a better way to use these weapons of mass destruction, than propelling a probe on a interstellar travel at relativistic speeds in the near future.

Also, the origin Orion, where all the nukes are carried onboard, will reduce the top speed because of the rocket equations. What about a nuclear mine field?

My idea is, having a probe less than a ton, attached to something like a light sail, tens of kilometers across, but only tens of microns thick, hold together by nanomaterials.

There are around 10,000 nuclear warheads in the world’s arsenal, mostly from America and Russia. We can arrange them into a slightly curved line (curvature decrease as far away from sun) each time when the probe passes a nuke, the nuke detonates nanoseconds after. The thin sail will ablates each time, providing extra thrust than the light pressure, and prevent overheating... when all the nukes have been detonated, the probe should be able to reach relativistic speeds, even sub-light speeds.

 

[anorlunda]

In the 70s, someone made a daydream proposal.

Make a light sail, only 1 atom thick, but the size of the Moon. The total mass of the vessel including sail and payload was only one or two grams. It would accelerated by lasers in close orbit around the Sun. He calculated that it could achieve 0.3 c before being out of range for the lasers. I got two takeaways from reading that.

1. The number one thing you can do to improve a space travel idea is not a more powerful engine, it is to reduce the total mass.
2. People are more likely to listen to your idea if you can do the calculations; even back-of-the-envelope calculations.

Can you show us the calculations to back up what you said?

having a probe less than a ton
...
the probe should be able to reach relativistic speeds, even sub-light speeds.the probe should be able to reach relativistic speeds, even sub-light speeds

 

[Xforce]

The kinetic energy of photons is small indeed. Since they collide with the sail and reflected back, the energy the photon passed to the spacecraft is twice it’s own kinetic energy. So each photon gives E=2*h*c/λ of energy ...
A ton of stuff traveling at .1c will have momentum of E_k=((1/√(1-0.1c²/c²))-1)*mc^2 which at 0.1 c means the kinetic energy of 1% mass converted to energy (interestingly, a bit more than the energy from fusion) or simply 9*10^17 joules. (At this point, I start doubting the nukes are enough)
Let’s suppose each of those nukes yields a megaton, which is 4.18*10^15J . To make calculations easier I’ll use 4*10^15J. Depends on the distance from the detonation, or the strength of the material, but since the energy is randomly emitted in all directions, it will take a infinitely large sail to let 50% photons impact the sail. So let’s say there is a device that directs the radiation, so exactly 50% energy impact the sail on perpendicular direction (this is very unrealistic too), so the total photonic energy onto that huge sail is 2*10^19J The photonic drive have a terrible ratio of power to thrust. At 300Mw per N, if we times this by a second we get 3*10^8J of total energy for 1J of effective energy... this means only 7*10^10J is put onto the spacecraft . In reality the energy is mostly taken up by neutron radiation and alpha radiation... I’m sure they give off more thrust but I still doubt that is enough. Sorry for my clumsy calculations

 

[anorlunda]

9*10^17 joules

You forgot a factor of 2, but pretty close.

this means only 7*10^10J is put onto the spacecraft .

I didn't check that calculation, but the conclusion is that this scheme is on the order of 10⁷ times too little is the right conclusion. In other words, the mass of the vessel would have to be closer to 1 gram than to 1 ton.

OK, now think the next step. How small could we make the mass of a space probe for it to still be useful? In post #1, you just arbitrarily threw out 1 ton.