Interstellar craft design (split from speck of dust impact discussion)

[Widdekind]

anyway, a nuclear-powered pusher-plate craft would, by assumption, have a pusher-plate capable of absorbing high energy particle radiation. So, a pusher-plate craft could accelerate, then coast in an "end over" (stern first) orientation, using its pusher plate to "snow plow" the ISM. However, even to drive an aircraft-carrier-sized ship (100Ktons) to Mars and back ([math]\Delta v \approx 10 km/s[/math]) would require circa 100Gtons of nuclear warheads for fuel. i think that's about the entire global stockpile of nukes. So, all earth could spend trillions of dollars, to load a huge stockpile of fuel pellets aboard a craft, for one trip. (Moreover, 100Gtons probably masses 100Ktons or more? So, at least on the outbound leg, there'd be no room for cargo.)

 

Edited May 20, 2013 by Widdekind

[EdEarl]

National Geographic made a documentary that includes a nuclear bomb propelled interstellar ship.

 

See: "Nat Geo Evacuate Earth"

Edited May 20, 2013 by EdEarl

[Widdekind]

Please ponder a fusion-powered rocket, of initial mass M, which converts 0.007M into (kinetic) energy. Energies are low w.r.t. rest-mass energy, and (so) velocities are low w.r.t. light-speed. So, classical approximations are accurate (if not precise):

 

[math]M c^2 = M' c^2 + \frac{1}{2} M' v^2[/math]

[math]\frac{\Delta M}{M'} = \frac{1}{2} \beta^2[/math]

[math]\epsilon \approx \frac{1}{2} \beta^2[/math]

[math]\beta \approx \sqrt{2 \epsilon} \approx 0.12 [/math]

 

i.e. [math]\approx \frac{c}{8}[/math]. So, an ideal fusion rocket can only accelerate to an eighth of light-speed. And, if you wanted to be able to decelerate at destination, then you would have to save half the energy, and halving the energy of acceleration from departure would reduce speed by [math]\sqrt{2}[/math] to about 0.08c.

 

So, self-nuclear-fusion-propelled space-craft can only accelerate to about 8% of light-speed; externally accelerated "space slingshotted" space-craft could coast at 12% of light-speed, and still carry enough on-board fusionable energy to decelerate at destination.

 

Anything out there traveling faster than 12% of light-speed was externally accelerated, i.e. a "space bullet" fired from some "space gun". Hypothetically, an externally accelerated "space-bullet-craft" loaded with anti-matter could be accelerated to ... and still have enough on-board energy to decelerate at destination:

 

[math]E_0 = \gamma \left( M_{ship} + m_{fuel} \right) c^2[/math]

[math]c P_0 = \gamma \beta \left( M_{ship} + m_{fuel} \right) c^2[/math]

[math]\Delta E = \gamma m_{fuel} c^2 = c \Delta P = c P_0[/math]

[math]\gamma m_{fuel} c^2 = \gamma \beta \left( M_{ship} + m_{fuel} \right) c^2[/math]

[math]m_{fuel} = \beta \left( M_{ship} + m_{fuel} \right)[/math]

[math]\frac{m_{fuel}}{M_{ship} + m_{fuel}} = \beta \le 1[/math]

 

So, a hypothetical externally accelerated space-craft, loaded with externally-supplied anti-matter (& matter) fuel, could cruise at near-light-speed, and still decelerate at destination, depending upon the ratio of fuel-mass to total-ship-mass. The ship would decelerate, by projecting a high-powered laser-like blast, in the forward direction ("focused pair-instability SNe GRB blast"), which could be aimed at the target world, to annihilate any indigenous lifeforms.