I was away from town for a couple of days, and look at how much stuff there is to respond to! It's not often that I get to geek out about interstellar rocketry.
What? Ion drives can have an ISP on the order of 10 000 seconds, which beats VASIMIR by 100% and NERVA by 1000%.
And it's still too low for anything beyond interstellar precursor missions. 100,000 seconds is a good rule-of-thumb minimum ISP for an engine intended for interstellar travel.
NASA thinks that there might be a possibility to use antimatter or some sort of uber ion drive to achieve ISP of 50 000 seconds, but that seems to the physical upper limit of reaction drives.
50,000 seconds for an antimatter rocket sounds like it might be a reference to the AIMStar concept (or something similar), which used antimatter-initiated fusion to heat propellant instead of using the reaction products directly. It's nowhere near the maximum achievable even with fission
, let alone fusion or antimatter. An ideal fission drive would, IIRC, have a Ve of ~0.055c (an ISP of ~1.7 million seconds). Fusion maxes out at a Ve of 0.089c, or ~2.7 million seconds. Antimatter? An ideal beam-core antimatter rocket can achieve a Ve of 0.58c (~17.7 million seconds), and if you can also utilize the gamma rays then the Ve maxes out at .96c, or a staggering 29.4 million seconds!
Are you actually going to be able to build rockets that can achieve those velocities with those engine types? No, because this is the real world and you can't make an idealized perfectly efficient rocket. But there's a whole three orders of magnitude of potential improvement beyond 50,000s before you hit the physical upper limit of reaction rockets.
But besides that, Orion's only advantage is that the fuel is dense ; But the ISP is only around 3000-4000 so you'll still need an absurd mass of it.
This is true of the in-depth design studies for an interplanetary orion, but those were relatively small vehicles and 50's-era pure-fission pulse unit designs. The maximum specific impulse for an orion increases with size, since it can survive using more powerful pulse units and yield rises a lot faster than mass until you hit the energy density limits of nuclear weapons. Using thermonuclear pulse unit designs will allow for more powerful pulse units at a given weight, increasing the specific impulse considerably.
For reference, the momentum-limited interstellar orion design concept had a specific impulse of ~710,000 seconds.
Just thought of something...when you factor in relativity, would that cause the craft to consume less fuel? Think about humans for instance, at high fractions of lightspeed we age slower, consume less food, etc... Would a mechanical device such as an engine consume less fuel then?
... no. It means you use more fuel. In fact, as you get into the relativistic velocity regime (past .5c or so) the required amounts of fuel start rising a lot
faster than predicted by the regular rocket equation. The idea of using high time dilation factors to get large reductions in apparent trip times is probably not workable without engines based on fantastically high-energy processes unknown to modern physics.
I swear, the relativistic rocket equation hates our guts.
If we want to accomplish something within our grandchildren's lifetimes, antimatter is not something we should put on the list of objectives. THere is no remotely cost-effective way to create it, and making it in particle accelerators a particle at a time is so staggeringly inefficient that we'd be at it all century to get anything done.
A large part of the problem with antimatter is that we currently produce it with ultra-high precision-scientific instruments on the bleeding edge of modern technology and which are optimized for things other than the production of antimatter. Forward estimated that a purpose-built antimatter production facility would be something like 10,000 times more efficient at antimatter production than scientific colliders and would bring the cost of antimatter down to ~10 million dollars per milligram, at which point it is cheap enough to be realistically used for space travel.
The bigger issue is storing antimatter for lengthy periods of time.
You could alternatively go with a laser-propelled optical sail, but that requires some intense infrastructure.
No, at least not at first. You could get away with slowly raising additional power plants and lasers to orbit over time, you would need full power after the probe would have covered a lot of distance.
You're forgetting the lenses.
Forward's original concept used a 1,000-kilometer
zone plate, levitated in place by rockets near the orbit of Neptune. While later research has reduced the necessary lens size (mostly by using materials with much higher thermal limits and therefore allowing for much higher acceleration), you still need optical elements of a size such that they cannot be feasibly lifted into orbit; you need to construct them in space, and they should be far enough out that gravitational distortions don't play merry hell with their shape.
Interstellar travel of any sort is hard
. If you're still at the point where you build your space infrastructure at the bottom of a gravity well, chances are you won't be able to pull off an interstellar mission with a low enough travel time that it won't get passed by later generations of probes.